SI 4 Unit

MT8591 – Sensors and Instrumentation 2.
Unit – II
Motion, Proximity and Ranging Sensors
2.1 Introduction
A motion sensor (or motion detector) is an electronic device that is designed to detect
and measure movement. Motion sensors are used primarily in home and business security
systems, but they can also be found in phones, paper towel dispensers, game consoles, and
virtual reality systems. Unlike many other types of sensors (which can be handheld and
isolated), motion sensors are typically embedded systems with three major components:
a sensor unit, an embedded computer, and hardware (or the mechanical component). These
three parts vary in size and configuration, as motion sensors can be customized to perform
highly specific functions. For example, motion sensors can be used to activate floodlights,
trigger audible alarms, activate switches, and even alert the police.
A proximity sensor is a non-contact sensor that detects the presence of an object

(often referred to as the “target”) when the target enters the sensor’s field. Depending on the
type of proximity sensor, sound, light, infrared radiation (IR), or electromagnetic fields may be
utilized by the sensor to detect a target. Proximity sensors are used in phones, recycling plants,
self-driving cars, anti-aircraft systems, and assembly lines. There are many types of proximity
sensors, and they each sense targets in distinct ways. The two most commonly used proximity
sensors are the inductive proximity sensor and the capacitive proximity sensor.
Range sensors are devices that capture the three-dimensional (3-D) structure of the
world from the viewpoint of the sensor, usually measuring the depth to the nearest surfaces.
These measurements could be at a single point, across a scanning plane, or a full image with
depth measurements at every point. The benefits of this range data is that a robot can be
reasonably certain where the real world is, relative to the sensor, thus allowing the robot to
more reliably find navigable routes, avoid obstacles, grasp objects, act on industrial parts, etc.
2.1.1 Transduction principles

a. Resistive element
Resistance variation type transducer is one of the important groups of transducers
that are quite popular, simple and versatile. Many system variables like displacement,
acceleration, vibration, force, pressure, temperature, humidity, sound level, light intensity etc.
can be transduced using this category of transducers. The basic principle of this kind of
transducer is very simple. Resistance of a conducting wire is given by the expression.
2.2 Unit 2: Motion, Proximity and Ranging Sensors
Where is the specific resistance of the material of the wire, ‘l’ is the length of the
wire and ‘a’ is the area of cross section of the wire. Any stimulus or measurand or a variable
which changes or affects any one of the quantities like “l ”, “a” or “ , the resistance of the
wire is changed. This change in resistance can be suitably converted by an electrical
circuitry to a change in voltage thus a transduction is achieved.
b. Capacitive element
Capacitive transducers are passive transducers that determine the quantities like
displacement, pressure and temperature etc. by measuring the variation in the capacitance of a
capacitor. As we know that a transducer changes a form of energy into another form. So, in the
capacitive transducer, the change in the capacitance is used to measure the physical quantities.
The operating principle of a capacitive transducer is variable capacitance. Now, the question
arises that how the change in the capacitance of the capacitor occurs? Assume a parallel plate
capacitor, whose capacitance is given by:
Where A is the area of the plates,

ε0 is the permittivity of free space,
εr is the relative permittivity and
d is the distance between the two plates in metre.
From the above equation is it clear that the capacitance of the capacitor is dependent
on the area of the two plates, the distance between two plates and the permittivity of the
material. Hence, by varying either area or distance or permittivity, the non-electrical quantities
can be determined.
c. Inductive element
Inductive transducers work on the principle of inductance change due to any
appreciable change in the quantity to be measured i.e. measured. This can be done by
changing the flux with the help of measured and this changing flux obviously changes the
inductance and this inductance change can be calibrated in terms of measured. Hence
inductive transducers use one of the following principles for its working.
 Change of self inductance
 Change of mutual inductance
 Production of eddy current
MT8591 – Sensors and Instrumentation 2.3
2.2 Displacement & Position sensors

A Displacement Sensor is a device that measures the distance between the sensor
and an object by detecting the amount of displacement through a variety of elements and
converting it into a distance. Depending on what element is used, there are several types of
sensors, such as optical displacement sensors, linear proximity sensors, and ultrasonic
displacement sensors. Position sensors are devices that can detect the movement of an object
or determine its relative position measured from an established reference point. These types of
sensors can also be used to detect the presence of an object or its absence.
2.2.1 Potentiometer
The resistive potentiometer is perhaps the best-known displacement measuring
device. It consists of a resistance element with a movable contact as shown in Figure 2.1. A
voltage, Vs, is applied across the two ends A and B of the resistance element, and an output
voltage, VO, is measured between the point of contact C of the sliding element and the end of
resistance element A. A linear relationship exists between the output voltage, V O, and distance
AC, which can be expressed by
Fig. 2.1 Potentiometer
The body whose motion is being measured is connected to the sliding element of the
potentiometer so that translational motion of the body causes a motion of equal magnitude of
the slider along the resistance element and a corresponding change in the output voltage, VO.
Three different types of potentiometers exist, wire wound, carbon film, and plastic film, so
named according to the material used to construct the resistance element. Wire-wound
potentiometers consist of a coil of resistance wire wound on a non-conducting former. As the
slider moves along the potentiometer track, it makes contact with successive turns of the wire
coil. This limits the resolution of the instrument to the distance from one coil to the next. Much
better measurement resolution is obtained from potentiometers using either a carbon film or a
conducting plastic film for the resistance element. Theoretically, the resolution of these is
limited only by the grain size of the particles in the film, suggesting that measurement
resolutions up to 10_4 should be attainable.
In practice, resolution is limited by mechanical difficulties in constructing the spring

system that maintains the slider in contact with the resistance track, although these types are
still considerably better than wire-wound types. Operational problems of potentiometers all
occur at the point of contact between the sliding element and the resistance track. The most
common problem is dirt under the slider, which increases the resistance and thereby gives a
false output voltage reading or, in the worst case, causes a total loss of output. High-speed
motion of the slider can also cause the contact to bounce, giving an intermittent output.
Friction between the slider and the track can also be a problem in some measurement systems
where the body whose motion is being measured is moved by only a small force of a similar
magnitude to these friction forces.
The life expectancy of potentiometers is normally quoted as a number of reversals,

that is, as the number of times the slider can be moved backward and forward along the track.
The values quoted for wire-wound, carbon film, and plastic film types are, respectively, 1, 5,
and 30 million. In terms of both life expectancy and measurement resolution, therefore, the
carbon and plastic film types are clearly superior, although wire-wound types do have one
advantage in respect of their lower temperature coefficient.
This means that wire-wound types exhibit much less variation in their characteristics
in the presence of varying ambient temperature conditions. A typical inaccuracy value is
quoted for translational motion resistive potentiometer is +1% of full-scale reading.
Manufacturers produce potentiometers to cover a large span of measurement ranges. At the
bottom end of this span, instruments with a range of +2 mm are available, while instruments
with a range of +1 m are produced at the top end. The resistance of the instrument measuring
the output voltage at the potentiometer slider can affect the value of the output reading. As the
slider moves along the potentiometer track, the ratio of the measured resistance to that of the
measuring instrument varies, and thus the linear relationship between the measured
displacement and the voltage output is distorted as well. This effect is minimized when the
potentiometer resistance is small relative to that of the measuring instrument. This is achieved
by
1. Using a very high impedance measuring instrument and

2. Keeping the potentiometer resistance as small as possible.
Unfortunately, the latter is incompatible with achieving high measurement sensitivity

as this requires high potentiometer resistance. A compromise between these two factors is
therefore necessary. The alternative strategy of obtaining high measurement sensitivity by
keeping the potentiometer resistance low and increasing the excitation voltage is not possible
in practice because of the power-rating limitation. This restricts the allowable power loss in the
potentiometer to its heat dissipation capacity.
Loading effect on potentiometer
Fig. 2.2 loading effect of potentiometer
The output of a potentiometer is normally connected to an amplifier or a recorder or a

meter which has definite input impedance and hence a current will be drawn by this meter or
amplifier or recorder. When a meter of input resistance Rm is connected across the
potentiometer then the total resistance across C and B is given by
( ⁄ )
( ⁄ )
Where Rp is the total potentiometric resistance and Rm is the meter resistance, xt is the
total length of travel of the potentiometer and xi is the input displacement.
Let
The voltage across C and B
( )
Where ( ) is the resistance between A and C,
( )
( ) ( )
( )
Now if Rp/Rm becomes very small, never to zero, the second term in the denominator
can be omitted when compared to the first term then
This is the condition, when there is

no loading. In reality Rm is not very large.
Hence eo becomes a non-linear function of
displacement of xi. If the actual plots of
against is obtained, this fact is well
exhibit as shown in figure 2.3. let us
examine the situation when
( )
Fig. 2.3 Linearity Graph of

potentiometer
If there were no loading then the output would have been
( ) [ ]
[ ]
( )
Maximum will occur at

( ) ( )( )
[ ] [ ]
( )
( ) ( )( )
[ ] [ ]
( )
[( )( ) ( )( )]
[ ]
( )
[( )( ) ( )( )]
[ ]
( )
( )
[ ]
( )
[ ]
The above equation gives a value of 0.7 approximately that means, when ratio
becomes 0.7, the error is maximum. The maximum error works out to be
( ) ( )
( )
= 12.14
But for values of Rp/Rm < 0.1, the maximum error is approximately 15 (Rp/Rm) percent of
full scale reading. This fact is proved as follows.
( )
[ ]
( )
( ( ))
[ ]
( )
( )
[ ]
( )
Now is always < 1 and Rp/Rm < 0.1 as per the assumptions
( )
( )
i.e., ⁄
⁄ corresponds to the maximum error point
corresponds to the minimum error point
( ⁄ ) ( ⁄ )
( )
( )
The above equation tell us that the error due to loading depends on the ratio of the
potentiometer to meter resistance.
2.2.2 Resolver
The resolver, also known as a synchro-resolver, is an electromechanical device that
gives an analogue output by transformer action. Physically, resolvers resemble a small AC
motor and have a diameter ranging from 10mm to 100 mm. They are frictionless and reliable
in operation because they have no contacting moving surfaces, and consequently they have a
long life. The best devices give measurement resolutions of 0.1%. Resolvers have two stator
windings, which are mounted at right angles to one another, and a rotor, which can have
either one or two windings. As the angular position of the rotor changes, the output voltage
changes. The simpler configuration of a resolver with only one winding on the rotor is
illustrated in Figure. This exists in two separate forms that are distinguished according to
whether the output voltage changes in amplitude or changes in phase as the rotor rotates
relative to the stator winding.
Fig. 2.4 Resolver
Varying amplitude output resolver

The stator of this type of resolver is excited with a single-phase sinusoidal voltage of
frequency ω, where the amplitudes in the two windings are given by:
( ) ( )
Where ( )
The effect of this is to give a field at an angle of (β + π/2) relative to stator winding 1.
Suppose that the angle of the rotor winding relative to that of the stator winding is given by θ.
Then the magnetic coupling between the windings is a maximum for θ = (β + π/2) and a
minimum for θ = β. The rotor output voltage is of fixed frequency and varying amplitude
given by:
( ) ( )
This relationship between shaft angle position and output voltage is non-linear, but
approximate linearity is obtained for small angular motions where |β - θ|< 15°. An intelligent
version of this type of resolver is now available that uses a microprocessor to process the sine
and cosine outputs, giving a measurement resolution of 2 minutes of arc.
Varying phase output resolver

This is a less common form of resolver but it is used in a few applications. The stator
windings are excited with a two-phase sinusoidal voltage of frequency ω, and the
instantaneous voltage amplitudes in the two windings are given by:
( )
( ⁄ ) ( )
The net output voltage in the rotor winding is the sum of the voltages induced due to
each stator winding. This is given by:
( ) ( ) ( ) ( ⁄ )
[ ( ) ( ) ( ) ( )]
( )
This represents a linear relationship between shaft angle and the phase shift of the
rotor output relative to the stator excitation voltage. The accuracy of shaft rotation
measurement depends on the accuracy with which the phase shift can be measured. This can
be improved by increasing the excitation frequency, ω, and it is possible to reduce inaccuracy
to +0.1%. However, increasing the excitation frequency also increases magnetizing losses.
Consequently, a compromise excitation frequency of about 400 Hz is used.
2.2.3 Encoder
An encoder is a sensor of mechanical motion that generates digital signals in response
to motion. As an electro-mechanical device, an encoder is able to provide motion control
system users with information concerning position, velocity and direction. There are two
different types of encoders: linear and rotary. A linear encoder responds to motion along a
path, while a rotary encoder responds to rotational motion. An encoder is generally
categorized by the means of its output. An incremental encoder generates a train of pulses
which can be used to determine position and speed. An absolute encoder generates unique bit
configurations to track positions directly. According to the applied technologies, encoder
results are divided into:
 Magnetic encoder technology uses magnetic fields to produce results. Magnetic poles
(north and south) are placed on the scale. Special Hall sensor reads passage of such
poles past a certain point.
 Optical encoder technology uses an optical signal that comes from a source to receiver
through a disc. The disc or scale has transparent or opaque marks. A ray from source
passes or does not pass through them and receiver fixes it.
 Inductive encoder technology responds to presence of ferromagnetic or electrically
conductive metal at certain point. Such sensors work via coils and electromagnetic
fields.
 Capacitive encoder technology has a sinusoidal rotor. When it rotates, transmitter

signal is simulated. Its sensor tracks changes in capacitance on the receiver board and
translates them into a signal.
 Resistive encoder technology uses shaded areas of conductive material and unshaded
areas of insulating material on its scale. When sections pass through a certain point
circuits of sliding contacts encounter to conducting and non-conducting areas. The
results are translated into digital or analog signals.
 Mechanical encoder technology uses metal discs and sliding contacts for measurement.
When the disc rotates, some contacts met with metal surface, and some fall into gaps.
Each contact is connected to a separate electrical sensor that generates a signal.
a. Optical encoder
Optical encoders use optical and photoelectric sensor systems for converting linear or
angular movements. These encoders have transparent and opaque areas corresponding to the
conducting and non – conducting areas. The sensing system consists of light source each
provided with focusing lens and an equal number of photoelectric devices to receive the light
from the corresponding source. The change between transparent and opaque area must be
sharp so that the transition between logic “0” and “1” is sure.
Incremental Rotary Encoder

The design of an incremental rotary encoder is as in figure. The visible exterior of the
unit includes the shaft, the flange the rotor housing and output cable. The shaft carries a
graduated glass disc with radial grating. The opaque lines which are wide as the transparent
gap between them consists of chromium applied through vapor deposition process mounted
on the flange at a small distance from the graduated scale is a scanning reticle with four small
fields, each with a grading identical to that on the disc. The four fields of lines are phase
shifted in relation to each other by one fourth of the grating pitch.
On both the graduated disc and the scanning reticle these is an additional reference
mark either in the form of a single line for coarser graduation or a group of lines for finer
graduation. All these fields are pend rated by a beam if collimated light produced by a light
source and condenser les. The beam of light is produced by a light is modulated during
rotation of the graduated disc and falls on to solar cells which generates two sinusoidal output
signals (phase shifted by 90 degrees) and a reference mark signal.
One single cycle of 360 degrees corresponds to the angle of rotation of one pitch of the
radial grating i.e, one line and one space. The sinusoidal output signal is converted into square
wave output signals. With digitizing electronics, these signals are then externally interpolated
to obtain 5, 10, 25 or 50 signal cycles corresponding to one pitch of radial grating.
Fig. 2.4 Incremental rotary encoder

Incremental Linear Encoder
Incremental linear measuring systems use a glass scale with line grating, which
consists of a opaque lines and transparent spaces of equal width, one or more reference marks
constitute a second track, a condenser lens for collimating the light beam, the scanning reticle
with index grating and silicon solar cells, when the scale is moved relative to the scanning unit
the lines and spaces of the scale alternately coincide with those of index grating. The
corresponding fluctuating of light are sensed by solar cells which generates two output singles
(phase shifted by 90 degrees) and a reference mark signal.
Fig. 2.5 Linear encoder

Absolute Rotary encoders

Absolute encoders generally are limited to a single revolution and utilize multiple
tracks and outputs, which are read out in parallel to produce binary representation of the
angular position of the shaft. Since there is a one – to – one correspondence between shaft
position and binary output, position data and recovered when power is restored after an
outage, and transient electric noise causes only transient measurement error. Absolute
encoders can be linear but mostly rotary. In the rotary configuration, the encoder shaft is
turned y the part whose position is being detected. The shaft turns the disk that has a radial
pattern with a unique code provided for each of a finite number of distinct positions. For each
bit or resolution, there is one track of code pattern, for example, for 12 bits of resolution, there
are 12 tracks. The number of distinct position is determined as 2 n, so far 12 bits/track. There are
4096 distinct positions, which results in a resolution of better than 0.025%. On the back of the
disk, there are light Emitting diodes for each rack, allowing phototransistors to read the disk
pattern. The disk track patterns are usually not in naturally binary code but in Gray code is the
elimination of large rollover as only one bit changes for each position step. This reduces the
measurement uncertainty to only count. The conversion from gray code to natural binary code
is a simple logical operation that is performed in software.
Fig. 2.6 Absolute rotary encoder
b. Magnetic encoder
In case of the magnetic encoders, the conducting portions of the contacting type
encoders are replaced by magnetic tape with magnetized portions and non-conducting
portions are represented by non-magnetized portions as shown in figure 2.7. for magnetizing
the portions, a coating of magnetic material powder as made. The sensing section consists of
toroidal cores, each provided with two coils namely, Reading coil (R-Coil) and Interrogate coil
(I-Coil). These sensing coils are placed closer to the pattern of the magnetic encoder but there
is no contact with the encoder.
The detection of the magnetized portions saturates the toroidal core and a suitable
output signal is generated. When the interrogate coil is energized with a constant voltage of
200 kHz, the reading coil generates the output signals as a transformer action. If the toroidal
core is over the magnetized portion, the output signal from the R-Coil is low and when the
core is over the non-magnetized portion, the output signal from the R-Coil is high. Hence
based on the presence and the absence of the magnetized portions, the amplitudes of the
output voltages will vary. If there is low level output voltage, it can be represented by binary
logic ‘0’ and if there is high level output voltage, it can be represented by binary logic ‘1’. This
kind of magnetic encoders are very resistant to dust, Greece, moisture and other contaminants
common in industrial environment and to shock and vibration. Hence its applications in
industries are high.
2.7 Magnetic Encoder

c. Inductive encoder
An inductive encoder measures a displacement relative to a target. The
displacement is translated into a movement. This type of encoder accurately detects a
measurement between 5 and 15 µm (micrometer) contactless and is not sensitive to
external (electro-) magnetic fields. In addition to inductive encoders, other technologies
are also available. It can be quite challenging selecting the right technology.The
inductive encoder is still not yet well known by R&D engineers. This is remarkable
given that this robust technology may be a good choice for your application. The
miniature encoders work well in polluted environments and at high temperatures up to
125 °C. In addition, these have an attractive price advantage in contrast to optical and
magnetic encoders.
An inductive encoder detects a linear displacement or rotation relative to a

target. This type of sensor reacts to the presence of ferromagnetic or electrically
conducting metal in a target. The encoder contains a transmitting coil, which contains
four receiver coils. The transmitting coil generates an electromagnetic field.
This field is manipulated under the encoder when a copper object (an
electrically conductive metal) enters the electromagnetic field. The re ceiver coils detect
this manipulation and converts the measurement into a movement. In other words, it is
a digital output signal. The encoder is moved over a target with copper markings that
are placed in a repetitive pattern with a ratio as follows: the width of each marking is
two thirds and the width of the gap is a third. Together, these dimensions make up the
period length. The system converts the movement into an analog signal in the shape of a
sine and a cosine. The analogue signal is then digitized in the encoder in the shape of a
square wave. Interpolation reduces the relatively large period step (1.2 mm) to multiple
small digital steps. With an inductive encoder, the interpolation can even achieve
resolutions smaller than 1 μm. It is possible to choose between linear and rotating
encoder systems depending on the desired movement.
Fig. 2.8 Inductive Encoder
d. Capacitive encoder
The basic principle behind capacitive encoders is that they detect changes in
capacitance using a high-frequency reference signal. This is accomplished with the three main
parts—a stationary transmitter, a rotor, and a stationary receiver. (Capacitive encoders can
also be provided in a “two-part” configuration, with a rotor and a combined
transmitter/receiver.) The rotor is etched with a sinusoidal pattern, and as it rotates, this
pattern modulates the high-frequency signal of the transmitter in a predictable way. The
receiver disk reads the modulations, and on-board electronics — a proprietary ASIC is used by
the vendor CUI Inc. — translate them into increments of rotary motion. The electronics also
produce quadrature signals for incremental encoding, with resolution ranging from 48 to 2,048
pulses per revolution (PPR).
Fig. 2.9 Capacitive Encoder
Capacitive encoders work by transmitting a high-frequency signal through a rotor

that is etched with a sinusoidal pattern. As the rotor moves, this pattern modulates the signal
in a predictable way. The receiver reads the modulations, and on-board electronics translate
them into increments of rotary motion. In the capacitive Electric Encoder, there are two
operating modes: Coarse Mode and Fine Mode. Coarse Mode is typically used upon system
start-up, to determine the initial position. The encoder is then switched to Fine Mode for
ongoing operation. By breaking up the total measuring range into small, equal, distinct
segments, the scale of each segment can be much finer than if the same scale were used over
the entire measuring range. This enables very high resolution without added expense.
The primary concern when using capacitive encoders is their susceptibility to noise
and electrical interference. To combat this, the ASIC circuitry must be carefully designed and
the algorithms for de-modulation must be fine-tuned. Capacitive technology, however, has
been used for many decades in digital calipers and is well-proven. Now it is making its way
into the encoder product space, where it provides high resolution without sacrificing
robustness. While optical encoders can deliver high resolution, their main components — an
optical disk, an LED light source, and photo-detectors — are fragile and highly sensitive to
dust, dirt, and other environmental contamination. They can also be damaged by vibrations
and require a relatively stable temperature range. Magnetic encoders, on the other hand, are
quite robust, but provide lower resolution than optical encoders. They are also sensitive to
magnetic interference, which is a notable concern when used with stepper motors. Probably
the most important difference between optical and capacitive encoders is that capacitive
encoders don’t require an optical disk. This makes capacitive versions more robust, less
susceptible to contamination and less influenced by temperature variations than optical
encoders are. And with no LED to burn out, capacitive encoders can achieve a much longer life
than optical versions.
They are also more efficient, with current consumption typically less than 10 mA —
as compared to the 20 mA or higher consumption of an optical encoder. This is especially
beneficial in applications where power is supplied via a battery. Another benefit of capacitive
encoders is the ability to change the encoder’s resolution by modifying the line count in the
electronics, without changing components. Compared to magnetic encoders, capacitive
versions simply provide better resolution in most situations, and can be produced at a lower
cost.
2.2.4 Linear Variable Differential Transformer (LVDT)

The most widely used inductive transducer to translate the linear motion into
electrical signals is the Linear Variable Differential Transformer (LVDT). The basic
construction of LVDT is shown in Figure below. The transformer consists of a single primary
winding ‘P’ and two secondary windings S1 and S2 wound on a cylindrical former. A
sinusoidal voltage of amplitude 3 to 15 V and frequency 50 to 20000 Hz is used to excite the
primary. The two secondary windings have equal number of turns and are identically placed
on either side of the primary winding.
Fig. 2.10 Linear Variable Differential Transformer (LVDT)

Construction
The primary winding is connected to an alternating current source. A movable soft-
iron core is placed inside the former. The displacement to be measured is applied to the arm
attached to the soft iron core. In practice the core is made of high permeability, nickel iron.
This is slotted longitudinally to reduce eddy current losses. The assembly is placed in a
stainless steel housing to provide electrostatic and electromagnetic shielding. The frequency of
ac signal applied to primary winding may be between 50 Hz to 20 kHz. Since the primary
winding is excited by an alternating current source, it produces an alternating magnetic field
which in turn induces alternating voltages in the two secondary windings. The output voltage
of secondary S1 is ES1 and that of secondary S2 is ES2. In order to convert the outputs from S1
and S2 into a single voltage signal, the two secondary S1 and S2 are connected in series
opposition as shown in Figure. Differential output voltage E0 = ES1 − ES2
Fig. 2.11 Connection of LVDT circuit

Operation
When the core is at its normal (NULL) position, the flux linking with both the
secondary windings is equal and hence equal emfs are induced in them. Thus, at null position:
ES1 = ES2. Thus, the output voltage E0 is zero at null position. Now if the core is moved to the
left of the null position, more flux links with S1 and less with winding S2. Accordingly, output
voltages ES1 is greater than ES2. The magnitude of output voltage is thus, E0 = ES1 − ES2 and
say it is in phase with primary voltage. Similarly, when the core is moved to the right of the
null position ES2 will be more than ES1. Thus the output voltage is E0 = ES1 − ES2 and 180° out
of phase with primary voltage. The amount of voltage change in either secondary winding is
proportional to the amount of movement of the core. Hence, we have an indication of amount
of linear motion. By noticing whether output voltage is increased or decreased, we can
determine the direction of motion.
Fig. 2.12 LVDT Core positions

To summarize, The LVDT closely models an ideal zeroth-order displacement sensor

structure at low frequency, where the output is a direct and linear function of the input. It is a
variable-reluctance device, where a primary center coil establishes a magnetic flux that is
coupled through a center core (mobile armature) to a symmetrically wound secondary coil on
either side of the primary. Thus, by measurement of the voltage amplitude and phase, one can
determine the extent of the core motion and the direction, that is, the displacement. The figure
shows the linearity of the device within a range of core displacement. Note that the output is
not linear as the core travels near the boundaries of its range. This is because less magnetic flux
is coupled to the core from the primary. However, because LVDTs have excellent repeatability,
nonlinearity near the boundaries of the range of the device can be predicted by a table or
polynomial curve-fitting function, thus extending the range of the device.
Fig. 2.13 Output characteristics of LVDT

Advantages
 Linearity is good up to 5 mm of displacement.
 Output is rather high. Therefore, immediate amplification is not necessary.
 Output voltage is step less and hence the resolution is very good.
 Sensitivity is high (about 40 V/mm).
 It does not load the measurand mechanically.
 It consumes low power and low hysteresis loss also.
Disadvantages
 LVDT has large threshold.
 It is affected by stray electromagnetic fields. Hence proper shielding of the device is
necessary.
 The ac inputs generate noise.
 Its sensitivity is lower at higher temperature.
 Being a first-order instrument, its dynamic response is not instantaneous.
2.2.5 Rotary Variable Differential Transformer (RVDT)

Rotary Variable Differential Transformer or RVDT is an inductive transducer which
converts angular displace to an electrical signal. Unlike LVDT, the input of this transducer is
differential value of rotary variable i.e. angular rotation (dθ) to generate voltage output. Like
every transformer, RVDT has two types of winding i.e. Primary winding and Secondary
winding. The primary and secondary winding are wound on a former. There are two
secondary winding having equal number of turns. These winding are placed on either side of
the primary winding identically. A cam shaped magnetic core made of soft iron is connected
to a shaft. This magnetic core can be thus be rotated in between the winding. Carefully observe
the figure below to understand the construction and working principle.
Fig. 2.14 Rotary Variable Differential transformer (RVDT)
The construction of LVDT and RVDT is almost same. The only difference in their
construction is that in RVDT, the core is cam shaped and may be rotated between the windings
by means of a shaft. You should read LVDT – Construction and Working Principle to
understand the constructional detail. The reluctance disturbed by the primary mmf changes
with the rotation of cam shaft. This results in change in the magnetic flux with rotation of the
cam shaft. Due to this change in magnetic flux with rotation of cam, the flux linkage of
secondary winding also changes. Therefore, as per the transformer action, an emf is induced in
secondary winding. The magnitude of induced emf will depend on the rate of change of
rotation. The more the rate of change of rotation, the more will be the rate of change of flux
w.r.t. and hence more emf will be induced. As can be seen from the figure, the two secondary
winding are connected in series but in phase opposition. This is done to get a single output
voltage from the transducer. If Es1, Es2 and E0 be the emf induced in the two secondary winding
S1 & S2 and output voltage respectively then
E0 = ES1 – ES2
Under normal condition of RVDT, the flux linkage of both the secondary winding are
same due to their symmetrical placing with respect to primary and core. Therefore, the
induced emf Es1 and Es2 are equal and hence output voltage E0 of the transducer in such
condition is zero. Therefore, normal position of RVDT is called NULL position. Clockwise
rotation of cam causes an increasing voltage Es2 in one of the one secondary winding while
counter clockwise rotation leads to increase in voltage Es1 of another secondary winding. Thus
the direction as well as magnitude of angular rotation can be ascertained from the magnitude
and phase of transducer output voltage. Phase of transducer output voltage means whether
(ES1 – ES2) is positive or negative. In case of anti-clockwise rotation, the value of Es1 will be
more than that of Es2 and hence (ES1 – ES2) will be positive. In this case we say that output
voltage E0 is in phase with the primary voltage. With the same logic, when cam is rotated in
clockwise direction, the output voltage will be negative i.e. out of phase with primary voltage.
2.2.6 Synchros and Microsyn

Like the resolver, the synchro is a motor-like, electromechanical device with an
analogue output. Apart from having three stator windings instead of two, the instrument is
similar in appearance and operation to the resolver and has the same range of physical
dimensions. The rotor usually has a dumb-bell shape and, like the resolver, can have either
one or two windings. Synchros have been in use for many years for the measurement of
angular positions, especially in military applications, and achieve similar levels of accuracy
and measurement resolution to digital encoders. One common application is axis
measurement in machine tools, where the translational motion of the tool is translated into a
rotational displacement by suitable gearing. Synchros are tolerant to high temperatures, high
humidity, shock and vibration and are therefore suitable for operation in such harsh
environmental conditions. Some maintenance problems are associated with the slip ring and
brush system used to supply power to the rotor. However, the only major source of error in
the instrument is asymmetry in the windings, and reduction of measurement inaccuracy down
to +0.5% is easily achievable.
Fig. 2.15 Schematic representation of synchro windings

Figure shows the simpler form of synchro with a single rotor winding. If an a.c.
excitation voltage is applied to the rotor via slip rings and brushes, this sets up a certain
pattern of fluxes and induced voltages in the stator windings by transformer action. For a rotor
excitation voltage, Vr, given by:
( )
the voltages induced in the three stator windings are:
( ) ( )
( ) ( ⁄ )
( ) ( ⁄ )
where β is the angle between the rotor and stator windings.
Fig. 2.16 Synchros stator voltage waveform
If the rotor is turned at constant velocity through one full revolution, the voltage
waveform induced in each stator winding is as shown in Figure. This has the form of a carrier-
modulated waveform, in which the carrier frequency corresponds to the excitation frequency,
ω. It follows that if the rotor is stopped at any particular angle, β’, the peak-to-peak amplitude
of the stator voltage is a function of β’. If therefore the stator winding voltage is measured,
generally as its root-mean-squared (r.m.s.) value, this indicates the magnitude of the rotor
rotation away from the null position. The direction of rotation is determined by the phase
difference between the stator voltages, which is indicated by their relative instantaneous
magnitudes. Although a single synchro thus provides a means of measuring angular
displacements, it is much more common to find a pair of them used for this purpose.
When used in pairs, one member of the pair is known as the synchro transmitter and
the other as the synchro transformer, and the two sets of stator windings are connected
together, as shown in Figure. Each synchro is of the form shown in Figure, but the rotor of the
transformer is fixed for displacement-measuring applications. A sinusoidal excitation voltage
is applied to the rotor of the transmitter, setting up a pattern of fluxes and induced voltages in
the transmitter stator windings. These voltages are transmitted to the transformer stator
windings where a similar flux pattern is established. This in turn causes a sinusoidal voltage to
be induced in the fixed transformer rotor winding. For an excitation voltage, Vsin(ωt), applied
to the transmitter rotor, the voltage measured in the transformer rotor is given by:
( ) ( )
where θ is the relative angle between the two rotor windings.
Fig. 2.17 Synchro transmitter transformer Circuit
Apart from their use as a displacement transducer, such synchro pairs are commonly
used to transmit angular displacement information over some distance, for instance to
transmit gyro compass measurements in an aircraft to remote meters. They are also used for
load positioning, allowing a load connected to the transformer rotor shaft to be controlled
remotely by turning the transmitter rotor. For these applications, the transformer rotor is free
to rotate and is also damped to prevent oscillatory motions. In the simplest arrangement, a
common sinusoidal excitation voltage is applied to both rotors. If the transmitter rotor is
turned, this causes an imbalance in the magnetic flux patterns and results in a torque on the
transformer rotor that tends to bring it into line with the transmitter rotor. This torque is
typically small for small displacements, and so this technique is only useful if the load torque
on the transformer shaft is very small. In other circumstances, it is necessary to incorporate the
synchro pair within a servo mechanism, where the output voltage induced in the transformer
rotor winding is amplified and applied to a servomotor that drives the transformer rotor shaft
until it is aligned with the transmitter shaft.
2.3 Proximity sensors

A proximity sensor is an electronic solid-state device used to indicate the presence of
an object without making physical contact. The proximity sensor is a very useful device in
hazardous areas such as oil refineries and not so hazardous areas such as car door detection
systems. Proximity sensors do not use any type of physical moving parts instead they allow
signals to transmit through them when something that is being monitored comes in close
proximity of the sensing area. Despite the process, they are still referred to as proximity
switches. Proximity sensors are to be used when the object that needs to be detected is too
small, lightweight or too soft to operate a mechanical switch. When there is a need for rapid
response and high switching rates such as the counting objects, proximity sensors are ideal for
the task. Proximity sensors should also be used when there’s a need to sense material through
nonmetallic barriers such as glass, bottles, plastic, or paper cartons or when working in hostile
environments that demand electrical isolation from the product being monitored. Proximity
sensors are also needed when there is a need to have a device that provides long life and
reliable service or when there are vast electronic control systems that need to be free of
chattering contacts to produce an accurate analysis of what’s being monitored.
Fig. 2.18 Proximity Sensor
2.3.1 Inductive proximity sensors

The inductive proximity sensor as seen in Figure 1 (a) is used to detect both ferrous
metals that contain iron and can be magnetized and nonferrous metals such as what we use to
conduct electricity and copper. Inductive proximity sensors operate under the electrical
principle of magnetism when a fluctuating current induces the voltage in a target object. The
inductive proximity sensor contains a certain type of solid-state control system. It contains an
oscillator circuit that generates a high-frequency magnetic field. When the metal object enters
the field, it disturbs the magnetic field, this disturbance results in a change of state in the high-
frequency circuit.
Fig. 2.19 Inductive Proximity Switch Circuit Diagram
Inductive proximity sensors are intended to be wired into motor control circuits or
electronic control circuits with a voltage rating of 24 V DC. Most proximity sensors come in
two basic configurations one is 3-wire configuration, while the other one is a 2-wire
configuration. The three wire proximity sensor is required to be powered up at all times so the
sensor must have two wires connected to a constant voltage source to operate the electronic
circuitry of the device. The third wire of the proximity inductive switch is the contact wire
which can come normally open or normally closed. Some proximity switches can contain both
so the total number of wires coming from the unit can total to be 4. The two wire proximity
sensor is intended to be connected in series with the load is to control. In its natural state,
there’s enough current that must flow through the circuit to keep the sensor active. This
current is referred to as the leakage current and typically may range from 1 to 2 mA
Typical Applications
1. Detection of the rotating motion.
2. Zero-speed indication.
3. Speed regulation.
4. Shaft travel limiting.
5. Movement indication.
6. Valve open/closed.
2.3.2 Capacitive proximity sensors

A capacitive proximity sensor senses the presence of an object (usually called the
target) without physical contact. They can detect both metallic and nonmetallic targets. They
are ideally suited for liquid level control and for sensing powdered or granulated materials.
The capacitive proximity sensor consists of a high-frequency oscillator along with a sensing
surface formed by two metal electrodes. When an object comes near the sensing surface, it
enters the electrostatic field of the electrodes and changes the capacitance of the oscillator.
As a result, the oscillator circuit starts oscillating and changes the output state of the
sensor when it reaches up to certain amplitude. As the object moves away from the sensor, the
oscillator’s amplitude decreases, switching the sensor back to its initial state. The larger
the dielectric constant of a target, the easier it is for the capacitive proximity sensor to detect.
This constant makes possible the detection of materials inside nonmetallic containers because
the liquid has a much higher dielectric constant than the vessel, which gives the sensor the
ability to see through the vessel and detect the fluid.
Fig. 2.20 Capacitive type Proximity sensor
They typically have a short sensing range of about 1 inch, regardless of the type of
material being sensed. While dealing with non-conductive targets, sensing distance increases
with an increase in
1. The sensing surface size of the sensor.

2. The surface area of the target.
3. The dielectric constant of the target.
For best operation, we should use them in an environment with relatively constant
temperature and humidity. The point at which the proximity sensor recognizes an incoming
target is known as the operating point. The point at which an outgoing target causes the device
to switch back to its normal state is known as the release point. The area between operating
and release points is called the hysteresis zone. The sensitivity adjustment can be made by
adjusting a potentiometer provided on the sensor. If the sensor does not have an adjustment
potentiometer, then the sensor must physically be moved to get the optimum installation
position. Optimum sensitivity provides a longer operating distance. However, the operation
of the oversensitive sensor is very much affected by temperature, humidity, and dirt, etc. and
may cause false triggering of the sensor. Most proximity sensors are equipped with an LED
status indicator to verify the output switching action.
Capacitive proximity sensors are available in various sizes and configurations to meet
different application requirements. One of the most common shapes is the barrel type, which
houses the sensor in a metal or polymer barrel with threads on the outside of the housing. Due
to the threaded housing, we can easily adjust the sensor on a mounting frame. The
major characteristics of capacitive proximity sensors are as under:
1. They can detect nonmetallic targets.

2. They can detect lightweight or small objects that cannot be detected by
mechanical limit switches.
3. They provide a high switching rate for rapid response in object counting
applications.
4. They can detect liquid targets through nonmetallic barriers (glass, plastic, etc.).
5. They have a long operational life with a virtually unlimited number of operating
cycles.
6. The solid-state output provides a bounce-free contact signal.
Typical Applications
1. High/low liquid level.
2. Dry tank.
3. Material present/absent.
4. Product present.
5. Product count.
2.3.3 Optical proximity sensors

Optical proximity sensors generally cost more than inductive proximity sensors, and
about the same as capacitive sensors. They are widely used in automated systems because they
have been available longer and because some can fit into small locations. These sensors are
more commonly known as light beam sensors of the thru-beam type or of the retro reflective
type. Both sensor types are shown below.
Fig. 2.21 Optical type proximity sensors
A complete optical proximity sensor includes a light source, and a sensor that detects
the light. The light source is supplied because it is usually critical that the light be "tailored" for
the light sensor system. The light source generates light of a frequency that the light sensor is
best able to detect, and that is not likely to be generated by other nearby sources. Infra-red
light is used in most optical sensors. To make the light sensing system more foolproof, most
optical proximity sensor light sources pulse the infra-red light on and off at a fixed frequency.
The light sensor circuit is designed so that light that is not pulsing at this frequency is rejected.
The light sensor in the optical proximity sensor is typically a semiconductor device such as a
photodiode, which generates a small current when light energy strikes it, or more commonly a
phototransistor or a photo-darlington that allows current to flow if light strikes it.
Early light sensors used photoconductive materials that became better conductors,
and thus allowed current to pass, when light energy struck them. Sensor control circuitry is
also required. The control circuitry may have to match the pulsing frequency of the transmitter
with the light sensor. Control circuitry is also often used to switch the output circuit at a
certain light level. Light beam sensors that output voltage or current proportional to the
received light level are also available. Through beam type sensors are usually used to signal
the presence of an object that blocks light. If they have adjustable switching levels, they can be
used, for example, to detect whether or not bottles are filled by the amount of light that passes
through the bottle. Retroflective type light sensors have the transmitter and receiver in the
same package. They detect targets that reflect light back to the sensor. Retroreflective sensors
that are focused to recognize targets within only a limited distance range are also available
2.3.4 Magnetic proximity sensors

Magnetic proximity sensors are used for non-contact position detection beyond the
normal limits of inductive sensors. In conjunction with a separate “damping” magnet,
magnetic sensors offer very long sensing ranges from a small package size and can detect
magnets through walls of non-ferrous metal, stainless steel, aluminum, plastic or wood.
Depending on the orientation of the magnetic field the sensor can be damped from
the front or from the side. Since magnetic fields penetrate all non-magnetisable materials, these
sensors can detect magnets through walls made of non-ferrous metal, stainless
steel, aluminum, plastic or wood. In the food industry, the magnetic sensor is often used in
connection with a “pig” (cleaning devices which pass through the inside of pipes). These
magnetic proximity sensors can detect the exact position of the pig from outside the wall of the
stainless steel pipe. Magnetic proxes are also used in ‘clean in place’ (CIP) systems at “diverter
panels” to detect the position of the diverter pipe through the panel faceplates (typically made
of stainless steel).
Magnetic sensors use GMR (Giant Magneto-Resistive Effect) technology. The

measuring cell consists of resistors with several extremely fine, ferromagnetic and non-
magnetic layers. Two of these GMR resistors are used to form a conventional Wheatstone
bridge circuit which produces a large signal proportional to the magnetic field when
a magnetic field is present. A threshold value is defined and an output signal is switched via a
comparator. The features are
1. Detection through plastic, wood, and any non-magnetisable metals

2. Small housings with very long sensing ranges up to 70 mm
3. Cylinder and rectangular designs satisfy space-dependent applications
4. High mechanical stability in case of shock or vibration
5. Flush or non-flush installation in non-magnetisable metals
Comparison
Inductive Magnetic Capacitive

Types Optical sensor
sensor sensor sensor
Based on an
It uses current An optical Based on a electronic
induced by sensor converts mechanical principle where
Principle magnetic fields light rays into principle an electrical
to detect nearby an electrical detected the field is
metal objects signal magnetic field produced on the
active side
Material
Metallic only All material Magnet All material
detected
Operating Low: Medium: Medium: Low:

distance <50mm <100mm <80mm <50mm
Hall-effect: High
Robustness
High High Reed-techno : High
to vibration
Low
Cost Low Medium Low Medium
Hall effect-
sensitive EMC
Dust, oil, aspect Humidity&
Sensitivity Any Reed techno-
of object vapors
magnetic field
disturbances
1.Machine- tolls, 1.Object detection 1. Object 1. Final inspection
assembly line, on conveyor detection on packaging
automative 2.Carton counting lines
industry 3.Product sorting 2.Measurements
2.Detection of 4.Contrast of the filling level
Applications
metal parts in detection of the liquids or
harsh granuals through
environments the walls of plastic
3.High speed or glass tanks
moving parts
2.4 Motion/Ranging sensors

2.4.1 Accelerometer
An accelerometer is an apparatus, either mechanical or electromechanical, for
measuring acceleration or deceleration - that is, the rate of increase or decrease in the velocity
of a moving object. Accelerometers are used to measure the efficiency of the braking systems
on road and rail vehicles; those used in aircraft and spacecraft can determine accelerations in
several directions simultaneously. There are also accelerometers for detecting vibrations in
machinery.
The measurement of acceleration or one of its derivative properties such as vibration,

shock, or tilt has become very commonplace in a wide range of products. At first you might
think of seismic activity or machinery performance monitoring, but would automotive airbags,
sports training products, or computer peripherals come to mind as well? The technology
behind certain accelerometers has advanced to such a degree that many are now very cost
effective and user friendly for the consumer market (joysticks, for example, and running
shoes). The types of sensor used to measure acceleration, shock, or tilt include piezo film,
electromechanical servo, piezoelectric, liquid tilt, bulk micro-machined piezo-resistive,
capacitive, and surface micro-machined capacitive. Each has distinct characteristics in output
signal, development cost, and type of operating environment in which it best functions.
a. Piezoelectric Accelerometer
Among the desirable features of the piezoelectric (PE) accelerometer are accuracy,
durability, large dynamic range, ease of installation, and long life span. Although these devices
cost more than other types, in many situations their benefits outweigh the higher price. To
provide useful data, PE accelerometers require proper signal conditioning circuitry. We will
briefly review the important characteristics of a PE accelerometer and circuit techniques for
signal conditioning. In particular, we will examine an interface that will allow the
accelerometer output's magnitude and frequency to be measured by a microcontroller unit
(MCU).
Fig. 2.22 Piezoelectric Acclerometer
The PE accelerometer uses an internal PE element coupled with a loading mass to

form a single-degree-of-freedom "mass-spring" system. The accelerometer is a charge-sensitive
device; an instantaneous change in stress on the internal PE element produces a charge at the
accelerometer's output terminals that is proportional to the applied acceleration. For
interfacing purposes, the PE accelerometer can be modelled as a voltage generator, Eg, in
series with an internal capacitance, Ci. The internal capacitance is an important characteristic
because it can have a significant effect on overall system sensitivity. A typical PE
accelerometer's sensitivity is specified in Pico coulombs per g (pC/g). Typical sensitivities are
0.5–1000 pC/g. PE accelerometers can be applied to measure vibration levels ranging from 4 g
to >104 g. The useful measurement range of a given unit is often limited only by its signal
conditioning and measurement systems.
The accelerometers can be used to measure very low frequencies. In practice, the low-
frequency response is usually limited by the signal conditioning electronics in order to
eliminate noise from sources such as thermal effects, strain on the accelerometer base, and
tribo-electric noise generated in the connecting cable. The low-frequency cut-off is typically set
around 2 Hz, but may be set higher if the lowest frequencies are not of interest to the user.The
accelerometer's useful upper frequency limit is dependent on its resonance frequency.
The device will exhibit a sharp peak in its electrical output at the resonance frequency
that must be compensated for. The upper resonance frequency is a function of the unit's
mechanical characteristics and the way it is attached to the test object. As a general rule, the
output sensitivity and upper resonance frequency of a PE accelerometer are dependent on the
size (mass) of the accelerometer. For example, a larger accelerometer will have increased
output sensitivity but a lower resonance frequency.
Surface Micro-machined Accelerometers

In recent years, silicon micro-machined sensors have made tremendous advances in
terms of cost and level of on-chip integration for measurements such as acceleration and/or
vibration. These products provide the sensor and the signal conditioning circuitry on chip, and
require only a few external components. Some manufacturers have taken this approach one
step further by converting the analogue output of the analogue signal conditioning to a digital
format such as duty cycle. This method not only lifts the burden of designing fairly complex
analogue circuitry for the sensor, but also reduces cost and board area. Micro-machined
accelerometers are now being incorporated into products such as joysticks and airbags,
applications that were previously impossible due to sensor price and/or size.
Fig. 2.23 Micro-machined Accelerometers
A surface micro-machined device consists of springs, masses, and motion-sensing

components. These sensors are made with the standard IC processing techniques used in
wafer fabrication facilities. After layers of oxide and poly-silicon, IC photolithography and
selective etching are used to create the sensor as a 3D structure suspended above the substrate
to allow free movement in all directions. The core of the sensor is a surface micro-machined
poly-silicon structure or mass suspended above the substrate with "springs." These springs
hold the mass and provide resistance to movement due to acceleration forces. Both the mass
and the wafer have fixed plates that form a differential capacitor in which the fixed plates on
the wafer are driven 180° out of phase. Any movement of the mass unbalances the capacitor,
resulting in a square wave output with the amplitude proportional to the acceleration.
Each axis has a demodulator that rectifies the signal and determines the direction of
acceleration. This output is fed to a duty cycle modulator (DCM) that incorporates external
capacitors to set the bandwidth of each axis. The DCM filters the analogue signal and converts
it to a duty cycle output whose period is set by an external resistor. A 0 g acceleration
produces a 50% duty cycle output. A low-cost microcontroller can be used to measure
acceleration by timing both the duty cycle and the period of each axis.
2.4.2 Global Positioning System (GPS)

The Global Positioning System (GPS) is a satellite-based navigation system that was
developed by the U.S. Department of Defense (DoD) in the early 1970s. Initially, GPS was
developed as a military system to fulfill U.S. military needs. However, it was later made
available to civilians, and is now a dual-use system that can be accessed by both military and
civilian users. GPS provides continuous positioning and timing information, anywhere in the
world under any weather conditions. Because it serves an unlimited number of users as well
as being used for security reasons, GPS is a one-way-ranging (passive) system. That is, users
can only receive the satellite signals. This chapter introduces the GPS system, its components,
and its basic idea. GPS consists, nominally, of a constellation of 24 operational satellites. This
constellation, known as the initial operational capability (IOC), was completed in July 1993.
The official IOC announcement, however, was made on December 8, 1993.
To ensure continuous worldwide coverage, GPS satellites are arranged so that four
satellites are placed in each of six orbital planes. With this constellation geometry, four to ten
GPS satellites will be visible anywhere in the world, if an elevation angle of 10° is considered.
As discussed later, only four satellites are needed to provide the positioning, or location,
information. GPS satellite orbits are nearly circular (an elliptical shape with a maximum
eccentricity is about 0.01), with an inclination of about 55° to the equator. The semi major axis
of a GPS orbit is about 26,560 km (i.e., the satellite altitude of about 20,200 km above the
Earth’s surface). The corresponding GPS orbital period is about 12 sidereal hours (~11 hours,
58 minutes). The GPS system was officially declared to have achieved full operational
capability (FOC) on July 17, 1995, ensuring the availability of at least 24 operational, non
experimental, GPS satellites. In fact, as shown in Section 1.4, since GPS achieved its FOC, the
number of satellites in the GPS constellation has always been more than 24 operational
satellites.
GPS segments
GPS consists of three segments:
1. The space segment,
2. The control segment
3. The user segment
The space segment consists of the 24-satellite constellation introduced in the previous
section. Each GPS satellite transmits a signal, which has a number of components: two sine
waves (also known as carrier frequencies), two digital codes, and a navigation message. The
codes and the navigation message are added to the carriers as binary biphase modulations.
The carriers and the codes are used mainly to determine the distance from the user’s receiver
to the GPS satellites. The navigation message contains, along with other information, the
coordinates (the location) of the satellites as a function of time. The transmitted signals are
controlled by highly accurate atomic clocks onboard the satellites.
The control segment of the GPS system consists of a worldwide network of tracking
stations, with a master control station (MCS) located in the United States at Colorado Springs,
Colorado. The primary task of the operational control segment is tracking the GPS satellites in
order to determine and predict satellite locations, system integrity, behavior of the satellite
atomic clocks, atmospheric data, the satellite almanac, and other considerations. This
information is then packed and uploaded into the GPS satellites through the S-band link. The
user segment includes all military and civilian users. With a GPS receiver connected to a GPS
antenna, a user can receive the GPS signals, which can be used to determine his or her position
anywhere in the world. GPS is currently available to all users worldwide at no direct charge.
Fig. 2.24 GPS Architecture

The working/operation of the Global positioning system is based on the ‘trilateration’

mathematical principle. The position is determined from the distance measurements to
satellites. From the figure, the four satellites are used to determine the position of the receiver
on the earth. The target location is confirmed by the 4 th satellite. And three satellites are used to
trace the location place. A fourth satellite is used to confirm the target location of each of those
space vehicles. The global positioning system consists of satellite, control station and monitor
station and receiver. The GPS receiver takes the information from the satellite and uses the
method of triangulation to determine a user’s exact position.
Fig. 2.25 GPS structure
GPS is used on some incidents in several ways, such as:

1. To determine position locations; for example, you need to radio a helicopter pilot the
coordinates of your position location so the pilot can pick you up.
2. To navigate from one location to another; for example, you need to travel from a
lookout to the fire perimeter.
3. To create digitized maps; for example, you are assigned to plot the fire perimeter and
hot spots.
4. To determine the distance between two different points.
Advantages of GPS:
1. GPS satellite-based navigation system is an important tool for military, civil and
commercial, users
2. Vehicle tracking systems GPS-based navigation systems can provide us with turn by
turn directions
3. Very high speed
Disadvantages of GPS:
1. GPS satellite signals are too weak when compared to phone signals, so it doesn’t work
as well indoors, underwater, under trees, etc.
2. The highest accuracy requires line-of-sight from the receiver to the satellite, this is
why GPS doesn’t work very well in an urban environment.
2.4.3 Bluetooth
Bluetooth wireless technology was named after a Danish Viking and King,
Harald Blatand; his last name means “Bluetooth” in English. He is credited with uniting
Denmark and Norway, just as Bluetooth wireless technology is credited with uniting two
disparate devices. The Bluetooth technology emerged from the task undertaken by Ericsson
Mobile Communications in 1994 to find alternative to the use of cables for communication
between mobile phones and other devices. In 1998, the companies Ericsson, IBM, Nokia and
Toshiba formed the Bluetooth Special Interest Group (SIG) which published the 1 st version in
1999. The first version was 1.2 standard with a data rate speed of 1Mbps. The second version
was 2.0+EDR with a data rate speed of 3Mbps. The third was 3.0+HS with speed of 24 Mbps.
The latest version is 4.0. A Bluetooth technology is a high speed low powered wireless
technology link that is designed to connect phones or other portable equipment together. It is a
specification (IEEE 802.15.1) for the use of low power radio communications to link phones,
computers and other network devices over short distance without wires. Wireless signals
transmitted with Bluetooth cover short distances, typically up to 30 feet (10 meters). It is
achieved by embedded low cost transceivers into the devices. It supports on the frequency
band of 2.45GHz and can support up to 721KBps along with three voice channels. This
frequency band has been set aside by international agreement for the use of industrial,
scientific and medical devices (ISM).rd-compatible with 1.0 devices. Bluetooth can connect up
to “eight devices” simultaneously and each device offers a unique 48 bit address from the IEEE
802 standard with the connections being made point to point or multipoint.
Working Principle
Bluetooth Network consists of a Personal Area Network or a piconet which contains a
minimum of 2 to maximum of 8 Bluetooth peer devices- Usually a single master and up to 7
slaves. A master is the device which initiates communication with other devices. The master
device governs the communications link and traffic between itself and the slave devices
associated with it. A slave device is the device that responds to the master device. Slave
devices are required to synchronize their transmit/receive timing with that of the masters. In
addition, transmissions by slave devices are governed by the master device (i.e., the master
device dictates when a slave device may transmit). Specifically, a slave may only begin its
transmissions in a time slot immediately following the time slot in which it was addressed by
the master, or in a time slot explicitly reserved for use by the slave device. The frequency
hopping sequence is defined by the Bluetooth device address (BD_ADDR) of the master
device. The master device first sends a radio signal asking for response from the particular
slave devices within the range of addresses. The slaves respond and synchronize their hop
frequency as well as clock with that of the master device. Scatternets are created when a device
becomes an active member of more than one piconet. Essentially, the adjoining device shares
its time slots among the different piconets.
Fig. 2.26 GPS Architecture
Piconets
Piconet is collection of a number of devices that are connected via Bluetooth radio
technology in an ad hoc fashion. To start a piconet requirement is two connected devices, like
laptop and cellular phone which may grow to eight connected devices and all other Bluetooth
devices are peer units and they also have implementations which are identical. However, at
the time of piconet establishment, one unit will be acting as a master and the other units as
slaves for connection duration of the piconet. As shown in figure.
Fig. 2.27 Piconets
Bluetooth is a packet-based protocol which is having a master-slave structure. In a

piconet up to 7 slaves can be communicated by one master. Timing on the network can be
controlled by Bluetooth by designating one device as a slave and the other as a master. The
unit which initiates communication link is master and the other participants are slaves. After
breakdown of link, the designations of master/slave no longer apply.
Fact is that, every Bluetooth device is having both master and slave hardware. The
network is called as piconet, which means a small network. When number of slaves is only
one, then link is called as point-to-point. In a point-to-multipoint configuration up to seven
active slaves can be controlled by a master. Slaves never communicate directly with each other
but instead communicate with the master only. Because of timing piconet members cannot
transmit simultaneously, hence jam problem is not there between these devices. Finally,
realization of communication process across the piconets is done if the Bluetooth radio device
is a slave in one and master in other or slave in more than one piconet. Configurations of
piconet in this manner are called scatternets.
The depiction of these various arrangements is in figure. Master’s clock is shared by

all devices. Basic clock which is defined by the master, ticks at intervals of 312.5 µs. Base of
packet exchange is basic clock. A slot of 625 µs is made up of two clock ticks; a slot pair of 1250
µs is made up two slots. Considering a very simple case of packets which are single-slot, the
transmission of master is in even slots and reception is in odd slots; for the case of slaves,
conversely, its transmission is in odd slots and reception is in even slots. Packet length can be
1, 3 or 5 slots but considering all cases, the transmission of slave is in odd slots and master’s
transmission will begin in even slots. At powering on of a Bluetooth device, its operation may
be as a slave device whose master device already running. After this it starts listening for a
master’s inquiry as if done for the new devices and gives response to it. In the phase of inquiry
the master knows the slave’s address; there is no necessity for this phase for the case of very
simple paired devices that are given permission to know address of each other. Once slave’s
address is known to a master, a connection towards it may be opened, but the condition is that
slave is listening for paging requests.
If the case this is like this, master’s page request is responded by the slave and the
synchronization between two devices is done over the frequency hopping sequence and this
sequence is decided by the master and is unique to each piconet. Several types of connections
are predefined by Bluetooth, each of which is having a different combination of available error
protection, quality of service and bandwidth. Once after establishment of connection, the
devices can then start communicating by first optionally authenticating each other. Devices
that are not transmission engaged can enter one of several bandwidths and power-saving
modes or the connection is teared down. Switching of roles can be done by master and slave,
which becomes a necessity when more than one piconet is participated by a device. The
Bluetooth specifications are as follows.
1. Core specification: It defines the Bluetooth protocol stack and the requirements for
testing and qualification of Bluetooth-based products.
2. The profiles specification: It defines usage models that provide detailed

information about how to use the Bluetooth protocol for various types of
applications.
The core specification consists of 5 layers:

1. Radio: Radio specifies the requirements for radio transmission – including frequency,
modulation, and power characteristics – for a Bluetooth transceiver.
2. Baseband Layer: It defines physical and logical channels and link types (voice or
data); specifies various packet formats, transmit and receive timing, channel control,
and the mechanism for frequency hopping (hop selection) and device addressing. It
specifies point to point or point to multipoint links. The length of a packet can range
from 68 bits (shortened access code) to a maximum of 3071 bits.
3. LMP- Link Manager Protocol (LMP): defines the procedures for link set up and
ongoing link management.
4. Logical Link Control and Adaptation Protocol (L2CAP): is responsible for adapting
upper-layer protocols to the baseband layer.
5. Service Discovery Protocol (SDP): – allows a Bluetooth device to query other
Bluetooth devices for device information, services provided, and the characteristics of
those services.
The 1 three layers comprise the Bluetooth module whereas the last two layers make up the
st
host. The interfacing between these two logical groups is called Host Controller Interface.
Advantages of Bluetooth Technology:

1. It removes the problem of radio interference by using a technique called Speed
Frequency Hopping. This technique utilizes 79 channels of particular frequency
band, with each device accessing the channel for only 625 microseconds, i.e. the
device must toggle between transmitting and receiving data from one time slot to
another. This implies the transmitters change frequencies 1,600 times every second,
meaning that more devices can make full use of a limited slice of the radio spectrum.
This ensures that the interference won’t take place as each transmitter will be on
different frequencies.
2. The power consumption of the chip (consisting of transceiver) is low, at about
0.3mW, which makes it possible for least utilization of battery life.
3. It guarantees security at bit level. The authentication is controlled using a 128bit key.
4. It is possible to use Bluetooth for both transferring of data and verbal communication
as Bluetooth can support data channels of up to 3 similar voice channels.
5. It overcomes the constraints of line of sight and one to one communication as in other
mode of wireless communications like infrared.
2.5 Ranging sensors

2.5.1 Beacons
Imagine! You come back to home after a tiring day from office and are warmly
welcomed with a cup of tea. Your home senses your presence and it switches on the lights as
you enter and turns on the music system to your favorite songs. You, roaming in shopping
mall and just in the proximity of a cafe and get notification about the running discount. You
are in hunt of parking in mall and receive a notification parking is empty 10 meter on the right.
These are not far-fetched ideas, it is Beacon technology. Beacons have now forged their way
into our homes to digitize our personal space, retail, events, museums, and more. Beacons
allow us to create smart home, smart cities and smart things everywhere in cost effective way.
Working Principle
A Beacon is a transmitter at a known location, which continuous or periodic signal
with limited information content. A radio technology can be used to identify location and most
commonly used RF technologies are Wi-Fi, Bluetooth and RF ID. Beacons indicate their
presence with periodic signal so that the enabled device like mobile handset can locate them.
Each beacon is given a unique identifier. When the mobile device detects the beacon signal, it
reads the beacon’s Unique identifier, calculates the distance to the beacon and, based on this
data ( coupon, video, form, URL or other forms of information), triggers an action in a beacon
compatible mobile app.
Most of the beacons today use BLE technology, because of its low power
consumption and implementation costs. The technology only allows for small amounts of data
transmission, which is why most beacons only transmit their IDs. Beacon IDs consists of a
maximum of three values:
1. A universally unique identifier (UUID)

2. A major value (optional)
3. A minor value (optional)
Besides these three values, every beacon also transmits information about its signal
power. This information is used by the app to calculate the distance from the source.
End to End Beacons System

The end to end complete beacon enable system is consist of following
 Beacon enabled Application
 Network of deployed beacons
 Bluetooth enabled Device with relevant permissions
 Management platform
In order to use beacon technology a beacon enabled mobile application is required.

One need to purchase and deploy beacons to one or multiple physical locations. Once
deployed, ensure that mobile app is listening for the unique ID’s (UUID’s) of the beacons in
network as shown in figure.
Fig. 2.28 Beacon technology
In addition to Bluetooth being switched on at the device level, some app-level

permissions also required to use beacon technology. These are;
 Push notifications
 Location services
Once beacons are deployed and app is listening for them, then the easy way to
manage content and assess performance of a beacon. A management platform allows us to set
up locations and zones, manage content, automate processes and view detailed analytics.
Benefits of using beacon technology:

1. Location Accuracy: Beacon technology allows a mobile device to understand it’s exact
position, even indoors where smartphones are typically not able to pick up GPS signals
from satellites. This means beacon technology offers a high level of accuracy when
compared to other geo-location technologies.
2. Long Battery life: Bluetooth technology used in beacon is designed to have very low
power consumption, which means that beacon powered apps have minimal impact on the
devices battery life. GPS on the other hand, demands a significant amount of power to run
and therefore significantly impacts battery life when in use.
3. App Wake-up: Mobile devices automatically wake-up when they come within ranges of
beacons, even if the mobile app that is listening is fully closed. This unique feature of
beacons offers a powerful way to drive engagement with your mobile app at exactly the
right time and place. It’s also a great to drive repeat usage of your app.
4. No Internet Connectivity: Mobile apps can pick up beacon signals without an Internet
connection and store data locally on the device. This means beacons are a great proximity
trigger in areas where a stable Internet connection is not available. It’s worth noting that
typically an Internet connection is required to trigger content such as push notifications.
However, there are ways around this such as developing local notifications and caching
content within the app. It is technically possible to run an entire beacon experience
without an Internet connection.
5. Low cost of entry: Setting up a Bluetooth enabled network is relatively low cost when
compared to other technologies such as Wi-Fi. Unless deploying a large network of
beacons, most of the costs are likely to be associated with the development of a beacon
enabled mobile app. A single beacon transmitter cost about just 5 $.
Challenges with Beacon Technology

1. Manageability: A beacon network may have thousands of hardware devices in hundreds
of stores. So the provisioning, monitoring, maintenance of these many small-2 devices is a
challenge.
2. Privacy and security: “Big Brothers (Hackers) are always in hunt”. Beacons require
location permission to be enabled in app devices so there can be privacy concerns.
Bluetooth is inherently not much secure and beacons does not provides strong encryption,
authentication and authorization procedures makes it less secure
3. Battery: Most of the beacons operates on battery and there is not mechanism to get the
information that how much battery is left in particular beacon and how much more time is
can serve. Beacons transmitting regular and very frequent notification also impact
listener’s mobile phones battery. These all shall requires optimization and control.
2.5.2 Ultrasonic ranging

In daily production and life, Ultrasonic ranging sensors are mainly used in
automotive parking distance control (PDC), obstacle avoidance robotics, construction sites
and industrial work environments where liquid level, well depth, pipe length, etc. are required
with non-contact auto ranging. There are two commonly used methods for ultrasonic distance
measurement. One is an ultrasonic ranging system based on single-chip microcomputer or
embedded device, the other is an ultrasonic ranging system based on CPLD (Complex
Programmable Logic Device). In order to understand designs & applications of ultrasonic
ranging sensors, we must first figure out the working principle of ultrasonic sensor ranging.
Fig. 2. 29 Working principle of ultrasonic sensor ranging

An ultrasonic sensor is a sensor that converts an ultrasonic signal into another energy
signal (usually an electrical signal). Ultrasonic is a mechanical shock wave generated in elastic
media with a frequency greater than 20 kHz. Because of its strong directivity, slow energy
consumption and relatively long propagation distance, it is often used in non-contact ranging.
Ultrasonic has the big ability to penetrate liquid and solid, especially in the sunshine opaque
solid. When an ultrasonic hits an impurity or an interface, it will produce a significant
reflection to form an echo, and when it hits a moving object will cause a phenomenon
called Doppler Effect.
Therefore, ultrasonic ranging has a good adaptability to the environment, and
ultrasonic distance measurement can be well compromised in real time, precision, and price.
At present, there are various methods for ultrasonic ranging: such as round-trip time
detection, phase detection, and acoustic amplitude detection. The principle is that the
ultrasonic sensor emits ultrasonic waves of a certain frequency, propagates through the air
medium, and is reflected back after reaching the measurement target or the obstacle. After
being reflected, the ultrasonic receiver receives the pulses, and the time it takes, is the round-
trip time, which is related to the distance traveled by ultrasonic waves. Measuring the wave
propagation time to get the wave propagation distance: Assuming that s is the distance
between the measured object and the range finder, the time measured is t / s, and the velocity
of ultrasonic propagation is expressed as v/ms-1, then there is a relation
When the accuracy is required, the influence of temperature on the ultrasonic

propagation speed needs to be considered; therefore the ultrasonic propagation speed is
corrected according to relation to reduce the error.
v = 331.4 + 0.607T
Where T is the actual temperature, the unit is °C; v is the propagation speed of
ultrasonic wave in the medium, and the unit is m/s.
Fig. 2. 30 The working principle of ultrasonic ranging sensor

The principle of ultrasonic ranging is to transmit ultrasonic waves in a certain
direction through an ultrasonic transmitter, and start timing at the same time of the emission.
When the ultrasonic waves propagate in the air and hit an obstacle, they will immediately
return and be received by the ultrasonic receiver while stop timing immediately. The
ultrasonic ranging sensor adopts the ultrasonic echo ranging principle and uses the accurate
time difference measurement technology to detect the distance between the sensor and the
target. The ultrasonic sensor with small angle and small blind area has the advantages of
precise & non-contact distance measurement, waterproof, anti-corrosion and low cost, etc.
Ultrasonic distance measuring sensors are often used in such a way that one emitter
corresponds to one receiver, but there are also multiple emitters corresponding to one receiver.
So ultrasonic distance sensor measures the round-trip echo time of ultrasonic sound to
determine how far away an object is. This is how the ultrasonic distance sensor works.
Fig. 2.31 Ultrasonic distance sensor module HC-SR04P

Ultrasonic distance sensor module has two optional transmission modes, one is free
operation mode: when there is power supply, the sensor itself can send trigger and burst
signals and it is usually for basic applications; External trigger mode: the external system
(controller or processor) controls trigger signals for advanced applications. These two modes
are suitable for a variety of purposes.In addition, the sensors also involve the choice of two
input power supplies, one is low voltage (5V) for the processor circuit, the distance to the
obstacle can be measured is 3.5m; the other is high voltage (12V) for the controller circuit, the
distance to the obstacle can be measured is 5m.
The data is transmitted by UART (universal asynchronous receiver-transmitter) with

a resolution of less than 5mm. On the other hand, users can select different setting modes
according to their own environment needs, such as free-running / UART triggering / external
trigger settings, etc. At the same time, on the basis of baud rate of UART communication, the
user can also decide whether to set up the circular buffer or not. The output signal uses high
performance ASIC (application-specific integrated circuit) chip to ensure stable transmission
and sensitive reception, and the communication between sensor and PC uses "interface board"
(RS232, power regulator). The data show that the real received ultrasonic wave can be
amplified in real time by using the monitor program on PC, the distance value can be output
by UART (ASCII, mm), and then the detection signal can be converted into the rectangular
TTL level signal (square wave) in real time.
2.5.3 Reflective beacons

Active beacon navigation systems are the most common navigation aids on ships and
airplanes. Active beacons can be detected reliably and provide very accurate positioning
information with minimal processing. Reflective beacon is one among the beacons technology,
functions to range an object over the work space. Reflective beacon utilizes the concept of retro
reflectors. A retroreflector, or corner cube prism as we sometimes call them, is a device or
surface that reflects light back to its source with a minimum of scattering.
The reflective beacon tracking system employs retroreflective targets distributed
throughout the operating area of a moving object in order to measure range and angular
position. A servo-controlled rotating mirror pans a near-infrared laser beam through a
horizontal arc of 90 degrees at a 20 Hz update rate. When the beam sweeps across a target of
known dimensions, a return signal of finite duration is sensed by the detector. Since the targets
are all the same size, the signal generated by a close target will be of longer duration than that
from a distant one. Angle measurement is initiated when the scanner begins its sweep from
right to left; the laser strikes an internal synchronization photodetector that starts a timing
sequence. The beam is then panned across the scene until returned by a retroreflective target in
the field of view. The reflected signal is detected by the sensor, terminating the timing
sequence
Fig. 2.32 AGV with reflective beacons Fig. 2.33: laser-based scanning beacon
Denning Branch International Robotics [DBIR], Pittsburgh, PA, offers a laser-based

scanning beacon system that computes vehicle position and heading out to 183 meters (600 ft)
using cooperative electronic transponders, called active targets. A range of 30.5 meters (100 ft)
is achieved with simple reflectors (passive targets). The LaserNav Intelligent Absolute
Positioning Sensor, shown in figure, is a non-ranging triangulation system with an absolute
bearing accuracy of 0.03 degrees at a scan rate of 600 rpm. The fan-shaped beam is spread 4
degrees vertically to ensure target detection at long range while traversing irregular floor
surfaces, with horizontal divergence limited to 0.017 degrees. Each target can be uniquely
coded so that the LaserNav can distinguish between up to 32 separate active or passive targets
during a single scan. The vehicle's x-y position is calculated every 100 milliseconds. The sensor
package weighs 4.4 kilograms (10 lb), measures 38 centimeters (15 in) high and 30 centimeters
(12 in) in diameter, and has a power consumption of only 300 mA at 12 V. The eye-safe near-
infrared laser generates a 1 mW output at a wavelength of 810 nanometers.
2.5.4 LASER Range Sensor (LIDAR)

Light detection and ranging (LIDAR) mapping is an accepted method of generating
precise and directly georeferenced spatial information about the shape and surface
characteristics of the Earth. Recent advancements in lidar mapping systems and their enabling
technologies allow scientists and mapping professionals to examine natural and built
environments across a wide range of scales with greater accuracy, precision, and flexibility
than ever before. Lidar has become an established method for collecting very dense and
accurate elevation data across landscapes, shallow-water areas, and project sites. This active
remote sensing technique is similar to radar but uses laser light pulses instead of radio waves.
Lidar is typically “flown” or collected from planes where it can rapidly collect points over
large areas. Lidar is also collected from ground-based stationary and mobile platforms. These
collection techniques are popular within the surveying and engineering communities because
they are capable of producing extremely high accuracies and point densities, thus permitting
the development of precise, realistic, three-dimensional representations of railroads,
roadways, bridges, buildings, breakwaters, and other shoreline structures. Collection of
elevation data using lidar has several advantages over most other techniques. Chief among
them are higher resolutions, centimeter accuracies, and ground detection in forested terrain.
a. Working Principle
Lidar, which is commonly spelled LiDAR and also known as LADAR or laser
altimetry, is an acronym for light detection and ranging. It refers to a remote sensing
technology that emits intense, focused beams of light and measures the time it takes for the
reflections to be detected by the sensor. This information is used to compute ranges, or
distances, to objects. In this manner, lidar is analogous to radar (radio detecting and ranging),
except that it is based on discrete pulses of laser light. The three-dimensional coordinates
(e.g., x,y,z or latitude, longitude, and elevation) of the target objects are computed from
1) The time difference between the laser pulse being emitted and returned
2) The angle at which the pulse was “fired,”
3) The absolute location of the sensor on or above the surface of the Earth.
Fig. 2.34 LIDAR structure
There are two classes of remote sensing technologies that are differentiated by the
source of energy used to detect a target: passive systems and active systems. Passive systems
detect radiation that is generated by an external source of energy, such as the sun, while
active systems generate and direct energy toward a target and subsequently detect the
radiation. Lidar systems are active systems because they emit pulses of light (i.e. the laser
beams) and detect the reflected light. This characteristic allows lidar data to be collected at
night when the air is usually clearer and the sky contains less air traffic than in the daytime.
In fact, most lidar data are collected at night. Unlike radar, lidar cannot penetrate clouds,
rain, or dense haze and must be flown during fair weather.
Lidar instruments can rapidly measure the Earth’s surface, at sampling rates greater
than 150 kilohertz (i.e., 150,000 pulses per second). The resulting product is a densely spaced
network of highly accurate georeferenced elevation points - often called a point cloud—that
can be used to generate three-dimensional representations of the Earth’s surface and its
features. Many lidar systems operate in the near-infrared region of the electromagnetic
spectrum, although some sensors also operate in the green band to penetrate water and
detect bottom features. These bathymetric lidar systems can be used in areas with relatively
clear water to measure seafloor elevations.
Typically, lidar-derived elevations have absolute accuracies of about 6 to 12 inches

(15 to 30 centimeters) for older data and 4 to 8 inches (10 to 20 centimeters) for more recent
data; relative accuracies (e.g., heights of roofs, hills, banks, and dunes) are even better. The
description of accuracy is an important aspect of lidar and will be covered in detail in the
following sections.
The ability to “see under trees” is a recurring goal when acquiring elevation data
using remote sensing data collected from above the Earth’s surface (e.g., airplanes or
satellites). Most of the larger scale elevation data sets have been generated using remote
sensing technologies that cannot penetrate vegetation. Lidar is no exception; however, there
are typically enough individual “points” that, even if only a small percentage of them reach
the ground through the trees, there are usually enough to provide adequate coverage in
forested areas.
In effect, lidar is able to see through holes in the canopy or vegetation. Dense forests
or areas with complete coverage (as in a rain forest), however, often have few “openings”
and so have poor ground representation (i.e., all the points fall on trees and mid-canopy
vegetation). A rule of thumb is that if you can look up and see the sky through the trees, then
that location can be measured with lidar. For this reason, collecting lidar in “leaf off”
conditions is advantageous for measuring ground features in heavily forested areas.
Fig. 2.35 LIDAR Signal responses

b. Lidar Platforms
Airborne topographic lidar systems are the most common lidar systems used for
generating digital elevation models for large areas. The combination of an airborne platform
and a scanning lidar sensor is an effective and efficient technique for collecting elevation data
across tens to thousands of square miles. For smaller areas, or where higher density is needed,
Lidar sensors can also be deployed on helicopters and ground-based (or water-based)
stationary and mobile platforms. Lidar was first developed as a fixed-position ground-based
instrument for studies of atmospheric composition, structure, clouds, and aerosols and
remains a powerful tool for climate observations around the world. NOAA and other research
organizations operate these instruments to enhance our understanding of climate change.
Lidar sensors are also mounted on fixed-position tripods and are used to scan specific targets
such as bridges, buildings, and beaches. Tripod-based lidar systems produce point data with
centimeter accuracy and are often used for localized terrain-mapping applications that require
frequent surveys. Modern navigation and positioning systems enable the use of water-based
and land-based mobile platforms to collect lidar data. These systems are commonly mounted
on sport-utility and all-terrain vehicles and may have sensor-to-target ranges greater than a
kilometer. Data collected from these platforms are highly accurate and are used extensively to
map discrete areas, including railroads, roadways, airports, buildings, utility corridors,
harbors, and shorelines. Airplanes and helicopters are the most common and cost-effective
platforms for acquiring lidar data over broad, continuous areas. Airborne lidar data are
obtained by mounting a system inside an aircraft and flying over targeted areas. Most airborne
platforms can cover about 50 square kilometers per hour and still produce data that meet or
exceed the requirements of applications that demand high-accuracy data. Airborne platforms
are also ideal for collecting bathymetric data in relatively clear, shallow water. Combined
topographic and bathymetric lidar systems on airborne platforms are used to map shoreline
and nearshore areas.
c. Applications
Lidar, as a remote sensing technique, has several advantages. Chief among them are
high accuracies, high point density, large coverage areas, and the ability of users to resample
areas quickly and efficiently. This creates the ability to map discrete changes at a very high
resolution, cover large areas uniformly and very accurately, and produce rapid results. The
applications below are examples of some common uses of lidar.
Updating and Creating Flood Insurance Rate Maps – This application is a major driver in the
development and use of lidar data. The application was largely brought about when
hurricanes hit North Carolina and the existing mapped flood zones were quickly shown to be
inadequate.
Forest and Tree Studies – A very costly and time-consuming aspect of timber management is
the effort spent in the field measuring trees. Typically a sample of trees is measured for a
number of parameters and the results are statistically extrapolated throughout the harvest
area. Trees must be measured to determine how much wood is present, when it is most
appropriate to harvest, and how much to harvest. High-resolution, small-footprint lidar has
been used to count trees and measure tree height, crown width, and crown depth. From these
measurements, the standing volume of timber can be estimated on an individual tree basis, or
on a stand level with larger footprint lidar.
Coastal Change Mapping – Mapping the coastal zone is an application that highlights the use
of lidar data along with GIS layers to increase the utility of both data sets. This highly dynamic
region changes on very short timescales (e.g., waves, tides, storms), contains many natural
habitats that are highly dependent on elevation, and is densely populated. As a result, the
rapid changes can affect significant populations and habitats, both of which are becoming less
tolerant to change (i.e., there is less ability to retreat). Lidar data provide the ability to measure
specific events as well as longer-term trends. This provides information that can be applied to
immediate restoration solutions for critical areas, as well as sustainable planning to minimize
future impacts.
2.5.5 Capacitive Transducer

Transducers based on the principle of changes in the capacitance are generally
termed as capacitive transducers. These kinds of transducers are most common in linear
displacement based applications. Other than displacement, many of the industrial variables
such as pressure, level, moisture, etc. can be translated into an electrical signal by means of
change in the capacitance.
The capacitive transducer is functioning similar to the working of a parallel plate
capacitor. The capacitance is calculated as a function of area between two parallel plates, the
distance between the plates and the dielectric medium in between the plates. It is expressed as:
Where A is the area of the plates,

ε0 is the permittivity of free space,
εr is the relative permittivity and
d is the distance between the two plates in metre.
The working principle of the capacitive transducer is based on the change in

capacitance due to any of the change in area of the parallel plates, the distance between the
plates or the permittivity of free space.
Since the capacitance is a function of A, d and ε, i.e., C = f(A, d, ε) any variable which
changes any one of the quantities, the capacitance of the parallel plate capacitor gets changed.
Further, this change in capacitance can required electrical form. The capacitance is connected
to voltage
Q = CV
where Q is the charge in coulombs

C is the capacitance in farads
V is the voltage in volts
On differentiating the above equation
If the capacitance is affected by any of the above parameters, the output is transducer
into an electrical form proportionally. In most of the cases, the changes are caused by means of
physical variables such as pressure, displacement, force, thickness, etc. If there is change in
dielectric medium between the parallel plates, there will be change in capacitance, and hence it
can be used for the measurement of fluid level. Similarly, the change in dielectric medium due
to change in the composition causes the change in absorption on moisture.
b. Examples of capacitive transducers

There are many transducers working on the principle of change in capacitance:
 Capacitive displacement transducer
 Capacitive thickness transducer
 Capacitive pressure transducer
 Capacitive level transducer
 Capacitive moisture transducer
 Capacitive hygrometer
 Condenser microphone, etc.
c. Variable area based capacitive transducer

From the above equation, it can be noted that the capacitance is directly proportional
to the area of cross-section of the parallel plates. The capacitance changes linearly with change
in area of the plates. Hence, the capacitive transducers based on the variable area are suited for
the measurement of both linear and angular displacements.
Measurement of Linear Displacement

Figure 2.36 shows a parallel plate capacitive transducer for the measurement of linear
displacement. In Figure 2.36, there are two plates. One plate is fixed and another plate is
movable. The capacitance of the parallel plate capacitor is:
where W is the length of overlapping of the parallel plates

x is the width of overlapping of the parallel plates
Fig. 2.36 Parallel plate capacitive transducer
The linear displacement to be measured by the capacitive transducer is applied to the

plate which is movable. Now, there will be overlapping of the plates. Due to this, there will be
change in capacitance. The amount of capacitance change is proportional to the displacement
applied to the movable plate. Sensitivity of the parallel plate capacitor of this type can be
expressed as follows:
( )
( )
For a fractional change in capacitance, the sensitivity of the capacitance transducer is

given as:
The above parallel plate capacitive transducer is suitable for linear displacement
measurements in the range of 1 mm to 10 mm with an accuracy of 0.005%. Instead of a two
parallel plate capacitive transducer, a cylindrical capacitive transducer can also be used for the
measurement of linear displacement. It consists of two cylindrical electrodes. One cylindrical
electrode is a fixed one, whereas the other cylindrical electrode is movable one. Figure 2.37
shows the capacitive transducer of cylindrical type. Fixed cylinder
Fig. 2.37 Capacitive transducer of cylindrical type
The capacitance obtained from the cylindrical capacitive transducer is given as:
( )
where W is the length of overlapping between the cylinders

D1 is the outer diameter of inner cylindrical electrode
D2 is the inner diameter of outer cylindrical electrode
Similar to the parallel plate capacitive transducer, the linear displacement to be

measured is applied to the movable cylinder which results in overlapping of two cylinders
which in turn further makes a change in capacitance between the two cylindrical electrodes.
The amount of capacitance is proportional to the displacement applied to the movable
cylinder. The sensitivity of this kind of transducer is given as:
( )
( )
( )
( )
From the above equations, it is observed that the sensitivity is constant, and the
relationship between the capacitance and linear displacement is linear.
Measurement of Angular Displacement

A capacitive transducer used for the measurement of angular displacement is shown
in Figure 2.38. It consists of a set of semi-circular plates forming the two plates of the capacitor,
out of which one plate is fixed and the other plate is movable.
Fig. 2.38 Capacitive transducer for angular displacement measurement
The angular displacement to be measured is applied to the movable plate of the

transducer assembly. Hence there is a change in effective area between the semi-circular plates
which in turn changes the capacitance. The amount of capacitance change is depending on the
angular displacement applied to the movable plate, and it is observed that it is maximum
when the plates overlap each other which means the angular displacement 0 = 180°. At various
angular displacements of 0, the capacitance is determined by
where r is the radius of the semi-circular plates

θ is the angular displacement
d is the gap between the two plates
The maximum value of the capacitance at the angular displacement of θ = 180° is calculated
The sensitivity of the angular displacement type of transducer is:
( )
( )
From the above equations, it can be observed that the change in capacitance is linear
with respect to the applied angular displacement. This type of transducer is useful for the
measurement of angular displacement of 180° maximum.
b. Variable distance-based capacitive transducer

From the basic equation, the capacitance is inversely proportional to the distance
between the two plates (of the capacitor (i.e., C = 1 /d ). With this principle, Figure 2.39 shows
variable distance based capacitive transducer used for the measurement of linear
displacements.
Fig. 2.39 Variable distance-based capacitive transducer
Variable distance based capacitive transducer consists of two plates among which one
plate is fixed, and the other one is movable. The linear displacement to be measured is applied
to the movable plate. When the displacement is towards left, the distance between the plates
decreases and hence the capacitance increases. Whereas when the displacement is towards
right, the distance between the plates increases and hence the capacitance decreases. Since the
capacitance is inversely proportional to the variation in distance between the plates, the
response of the transducer is found to be non-linear. Due to this non-linearity in response, it is
useful for the measurement of only smaller displacements.
c. Variable dielectric-based capacitive transducer

This kind of capacitive transducer works based on variation in the dielectric material
and its corresponding dielectric constant. Figure 2.40 shows the variable dielectric based
capacitive transducer, and this arrangement is suitable for large linear displacements. Here,
the two capacitor plates are kept in fixed position, whereas the solid dielectric material
available, between the fixed plates is movable thereby varying the capacitance of the
transducer assembly. with the varying dielectric between the parallel plates as shown in
Figure 5.7, the capacitance is calculated as follows:
( )
Fig. 2.40 Variable dielectric-based capacitive transducer
If the dielectric material is moved to a displacement of x, then the capacitance is

calculated and the variation in capacitance with respect to the applied displacement x is:
( )
( )
From the above equation, it can be noted that the change in capacitance will be
directly proportional to the applied displacement to the dielectric material of the transducer.
d. Applications of capacitive transducers

Capacitive transducers are applicable in many measurements like pressure, level,
density, moisture, etc. Various parameter measurements are discussed as follows:
Capacitive-type Pressure Transducer

The measurement of change in capacitance resulting from the movement of the elastic
element present inside the transducer arrangement is the basic principle of capacitance
pressure transducer. The materials preferred for the elastic member is Inconel, Ni-Span C or
Stainless Steel diaphragm or metal coated quartz. Figure 2.41 shows the arrangement of a
capacitive pressure transducer. The elastic member is exposed to the applied pressure on one
side, whereas reference pressure is on the opposite side. Based on the reference pressure used,
the capacitive transducer is useful for the measurement of absolute, gauge or differential
pressure. Figure 2.42 shows the pressure measurement from the capacitance pressure
transducer.
Fig. 2.41 Arrangement of a capacitance pressure transducer
Fig. 2.42 Pressure measurement using capacitance pressure detector
An oscillator of high voltage and high frequency is used to energize the sensing
element called diaphragm. In the pressure sensing assembly, reference pressure is allowed on
one side, and the process pressure is allowed on the opposite side. Based on the change of
pressure between reference pressure and process pressure, there will be deflection of the
diaphragm and due to which the change in capacitance is detected by the bridge circuit. With
the bridge circuit, there are two modes of operation such as balanced mode and unbalanced
mode. In the balanced mode, there will be a null detector in which the output voltage is fed
and to maintain the bride at null position, the capacitor arms are varied. The amount of
capacitance variation to make null position is a measure of process pressure. In the case of
unbalanced mode, the ratio between the output voltage obtained and the applied excitation
voltage is the measure of process pressure. The possible accuracy of the capacitance based
pressure detector is from +0.1 to +0.2%. These detectors are used as secondary standards for
low range of absolute pressure and differential pressure applications.
Capacitance-type Moisture Transducer

The capacitance probe used as a moisture sensor is shown in Figure 2.43 in which
capacitance electrodes made up of stainless steel are imbedded in an insulation sheet of plastic
type to provide the plates of the capacitor. This kind of probe arrangement is inserted into the
sample whose moisture is to be measured. Now, the dielectric material is replaced by the
water content (based on the amount of moisture content in the sample) and hence there is a
change in the capacitance. The capacitance change noted from the measuring probe is a
measure of amount of moisture content present in the sample.
Fig. 2.43 Capacitance probe for moisture measurement
Capacitance-type Hygrometer
There are two electrically insulated concentric metal cylinders which forms the two
capacitor plates in the measuring cell of the capacitance hygrometer. The annulus between the
two cylinders is filled with desiccant of alumina type. Two porous metal discs are used as
support for the cylinders and also to retain the desiccant in the annulus. Figure 2.44 shows a
capacitance type measuring cell as a hygrometer. The sample, whose moisture is to be
measured, is allowed to flow through the annulus, and the water present inside the sample is
either absorbed or desorbed by the desiccant which remains in the equilibrium with the
sample in terms of per cent saturation. Since the saturation level of the desiccant is very much
higher than that of the sample, the moisture content of the sample is thus amplified by the
desiccant. The measuring capacitor is a part of an electrical circuit which includes a reference
capacitor. This kind of circuit is powered by a 15 kilocycle, fixed amplitude sine wave.
The measuring and reference capacitors are switched alternately into the circuit in
such a way that its output voltage is a function of connected capacitance. The amplitude of the
output signal varies with the reference and measuring capacitors as they are switched into the
circuit. This difference in amplitude is related to the measured capacitance which is in terms of
the moisture content present inside the sample.
Fig. 2.44 Capacitance-type measuring cell
2.5.6 Inductive Transducer

Transducers based on the principle of variation in inductance are generally termed as
inductive transducers. Nowadays, these kinds of transducers are most common in many
measurements and instrumentation applications. Generally, in inductive transducers, there are
magnetic materials used in the flux path, and there is also one or more number of air gaps. If
there is any change in air gap, there will be change in the inductance of the circuit. By its
principle, the self-inductance or mutual inductance of a pair of coils or production of eddy
currents is changed whenever there is variation in the quantity to be measured.
If the inductive transducer is functioning based on the self-inductance, the inductance
of the coil can be expressed in terms of reluctance of the magnetic circuit such as:
where N is the number of turns of the coil

R is the reluctance of the magnetic circuit
The reluctance of the magnetic circuit can also be expressed as:
where µ0 is the permeability of air

µr is the relative permeability
A is the area of cross-section of the coil
L is the length of the coil
Let G = A/L
Then
Therefore, the inductance of a coil can be expressed in terms of number of turns (N),
permeability of the material (µ) and geometric factor (G). Since the inductance is a function of
N, µ and G, i.e., L = f(N, µ , G), any variable which changes any one of the above quantities, the
inductance of the coil gets changed. Further, this change in inductance can be further
converted into the required form.
b. Examples of inductive transducers

There are many transducers working on the principle of variation in inductance.
Some of the important transducers are listed as follows:
 Induction potentiometers
 Linear Variable Displacement Transformer (LVDT)
 Variable reluctance accelerometer
 Synchro
 Resolvers, etc.
c. Simple inductance type transducer

In simple inductance type, there are three different constructional arrangements of
the inductive coil such as:
 Inductance coil can be wounded over a rectangular type magnetic material.
 Inductance coil can be wounded on a cylindrical type material.
 Two coils are used.
Figure 2.45 shows the constructional arrangement of a simple inductance type

transducer of first type. In this kind of inductive transducer, a single inductive coil of N turns
is wound around a rectangular type ferromagnetic material. This inductive coil is the source of
magneto-motive force which drives the flux through the magnetic circuit. There is an armature
element placed opposite to the inductive coil wounded.
Whenever there is any movement in the mechanical armature element, there will be
changes in the permeability of the flux path generated which will further change the
inductance of the circuit. Based on the change in the inductance, the corresponding output will
be obtained. This output can be calibrated directly against the change in movement of the
armature element.
Fig. 2.45 Simple inductance type transducer of first type
Figure 2.46 shows the constructional arrangement of a simple inductance type

transducer of second type. Here instead of rectangular type magnetic material, a round hollow
magnetic material is used and over which the inductive coil is wounded.
Fig. 2.46 Simple inductance type transducer of second type
There is a movable magnetic core located inside the hollow tube. Due to the core
movement, there will be change in the inductance which produces a corresponding output in
the output indicator connected across the coil wounded. Figure 2.47 shows the constructional
arrangement of a simple inductance type transducer of two-coil type in which two coils are
used.
Fig. 2.47 Simple inductance type transducer of two-coil type

Whenever there is a movement of the magnetic core located at the centre of two coils,
there will be change in the relative inductance of the two coils. Due to this, the overall
inductance of the circuit will also be changed which is proportional to the change in the ratio
of the two inductive coils.
d. Mutual inductance type transducer

In this kind of transducers, two different coils are used. The input excitation,
provided by the external power source, is given to the first coil and the output is obtained
from the second coil. Figure 2.48 shows the mutual inductance type transducer. In Figure 2.48,
there are two separate coils wounded opposite to each other on a rectangular type magnetic
material, and the excitation coil is represented as A and the output coil is represented as B. An
armature is placed opposite to the input and output inductive coils. If there is any change in
the position of the armature, there will be change in the air gap between the rectangular
inductive base material and the armature element. Hence, the inductance of the output coil B is
changed proportional to the mechanical displacement of the armature.
Fig. 2.48 Mutual inductance type transducer
e. Induction potentiometer
Another important inductive type of transducer used for the measurement of
displacement is called as induction potentiometer. Figure 2.49 shows the arrangement of the
induction potentiometer. In Figure 2.49, there is one primary core called rotor and a secondary
core called stator, There are two concentrated coils and among which one is wounded on the
primary core and the other is wounded on the secondary core. The rotor is preferred usually as
dumbbell shaped If any other shape can provide the uniform air gap over the entire periphery,
it may also be used for the rotor. The operational frequency range of standard commercial
induction pots is between (50 — 400) Hz. Its size is ranging from 1 cm to 6 cm, and the
sensitivity of the inductive potentiometer can be one volt per degree rotation. The range of
induction potentiometers is limited within 60° of rotation. However, it is possible to measure a
linear relation up to +900 rotation with careful distribution of primary and secondary
windings.
The coupling between the primary or rotor winding and secondary or stator winding
is provided in such a way that the orientation of one of them with respect to the other winding
determines the induced emf in one of them. In particular, the rotor winding is excited with an
ac and thus there will be an inducing voltage in the stator winding. These two coils provide an
equivalent of a transformer with variable coupling between the primary and the secondary
winding.
Fig. 2.49 Induction potentiometer
Depending on the mutual inductance between the two coils, the amplitude of the
output voltage is varied. The mutual inductance is dependent on the angle of rotation. If the
induction potentiometer is concentrated coil type, the amplitude can be varied sinusoidal. By
having carefully designed distributed coils, a linear distribution over an angle of 180° can be
obtained. It is also found that the mutual inductance M is maximum when the coils are co-
axial and is zero when they are in Quadrature. For any angle 0; between the coils, the
relationship between the mutual inductance M and its corresponding emf is given as
M = Mmax cos θi
The output voltage E0 from the induction potentiometer is also expressed as
E0 = K(Em sin ωext) cos θi
Where Em sin ωext is the excitation voltage and K is the constant

TWO MARK Q&A

1. Name any two resistive transducers with their measured output variable.
1. Potentiometer - Displacement
2. Thermistor - Temperature
3. Strain gauge - Force
2. Specify any four displacement transducers.

1. Resistive potentiometers
2. Variable inductance transducers
3. Linear variable differential transformers (LVDT)
4. Rotary variable differential transformers (RVDT)
5. Synchro
6. Shaft encoders
3. List the applications of potentiometer.

1. The potentiometer is used as a voltage divider in the electronic circuit.
2. The potentiometer is used in radio and television (TV) receiver for volume
control, tone control and linearity control.
3. The potentiometer is used in medical equipment.
4. It is used in wood processing machine.
5. It is used in injection mold machines.
6. Potentiometers are widely used as user controls, and may control a very wide
variety of equipment functions.
4. What are the advantages and disadvantages of LVDT?

Advantages:
1. High range and ruggedness
2. Friction and electrical Isolation
3. Immunity from external effects
4. High input and high sensitivity
Disadvantages:
1. Relatively large displacements are required for appreciable differential output
2. They are sensitive to stray magnetic fields but shielding is possible
3. Many a times, the transducer performance is affected by vibrations
4. The receiving instrument must be selected to operate on a.c
5. The dynamic response is limited � Temperature affects the performance of the
transducer.
5. Illustrate the principle of accelerometer.

Most accelerometers are Micro-Electro-Mechanical Sensors (MEMS). The basic
principle of operation behind the MEMS accelerometer is the displacement of a small
proof mass etched into the silicon surface of the integrated circuit and suspended by
small beams. Consistent with Newton's second law of motion (F = ma), as an
acceleration is applied to the device, a force develops which displaces the mass. The
support beams act as a spring, and the fluid (usually air) trapped inside the IC acts as a
damper, resulting in a second order lumped physical system. This is the source of the
limited operational bandwidth and non-uniform frequency response of accelerometers.
6. Explain the basic principle of capacitive liquid level sensor.

The capacitance of the probe is measured between the inner rod and the outer
shell with the aid of a capacitance bridge. In the portion out of the liquid, air serves as
the dielectric between the rod and the outer shell. In the immersed section, the
dielectric is that of the liquid that causes a large capacitive change as the level of liquid
changes. If the tank is made of metal it can serve as the outer shell. The capacitance
change is directly proportional to the level of the liquid. The dielectric constant of the
liquid must be known for this type of measurement.
7. Explain how absolute encoder is different from incremental encoder.

Absolute rotary encoders are capable of providing unique position values from
the moment they are switched on. Whereas, Incremental encoders generate an output
signal each time the shaft rotates a certain amount. (The number of signals per turn
defines the resolution of the device.) Each time the encoder is powered on it begins
counting from zero, regardless of where the shaft is.
8. How to minimize null voltage in LVDT?

The null voltage, however, is insignificant after demodulation. When the core
moves to one side of null, the voltage across that coil increases as the other decreases.
This results in a steadily increasing voltage across the output leads. This AC voltage is
usually rectified or demodulated to produce a DC output voltage that increases with
the distance of the core from null, and with a polarity.
9. What is null voltage?

When the physical center of the core is in line with the electrical center of the
coils, the voltage in each secondary is equal in magnitude, but opposite in phase.
Differencing the two secondary voltages results in “zero” output volts. This is the null
position of the LVDT. In reality, there is a small residual voltage left due to factors like
winding capacitance and variances in the magnetic materials. This residual voltage is
called null voltage.
10. What are all the desirable features of capacitive transducers?

1. Its force requirements are very small
2. As the moving plates have very little mass, design of transducer with fast
response characteristics is possible
3. There is no physical between moving and stationary parts
4. Does not depends the conductivity of the metal electrode
5. Shielded against the effect of external electric stray fields.
11. Mention any four applications of LVDT.

1. Displacement measurement and LVDT Gage heads LVDT Load cells
2. LVDT Pressure Transducer
3. It act as a secondary transducer, it is used to measure force, weight and
pressure.
12. Explain the principle of magnetic encoders.

In a magnetic encoder, a large magnetized wheel spins over a plate of magneto-
resistive sensors. Just as the disk spins over the mask to let light through in predictable
patterns, the wheel causes predictable responses in the sensor, based on the strength of
the magnetic field. The magnetic response is fed through a signal conditioning electrical
circuit. The number of magnetized pole pairs on the wheel pole, the number of sensors,
and the type of electrical circuit all work together to determine the resolution of the
magnetic encoder.
13. Mention some applications of accelerometers.

1. Accelerometers can be used to measure vehicle acceleration.
2. Accelerometers can be used to measure vibration on cars, machines,
buildings, process control systems and safety installations.
3. They can also be used to measure seismic activity, inclination, machine
vibration, dynamic distance and speed with or without the influence of gravity.
4. Applications for accelerometers that measure gravity, wherein an accelerometer
is specifically configured for use in gravimetry, are called gravimeters.
14. Illustrate the principle of resolver.

A typical resolver has three windings – a primary winding and two secondary
windings. The windings are created using copper wire and are formed on the resolver’s
stationery element – the stator. The primary wire acts as the input for an AC drive
signal and each of the secondary windings are used as pick up or receive windings. In
the diagram above, the rotor is made from a material such as steel or iron and is
arranged relative to the windings such that it will couple varying amounts of energy
into the secondaries depending on its angle of rotation.
15. Illustrate the principle of capacitive transducer.

Capacitance is defined as the function of the geometric quantities, such as the
area of the conducting plate and the distance between them, and the permittivity of the
dielectric medium between them.
In a capacitance transducer, the change in the overlapping area and the distance
between the plates is attributed to the movement due to external force, typically used to
sense displacement, pressure, force. The change in dielectric medium is often attributed
to the displacement of the original medium which is often used in level measurement
for fluids.
16. Laser range sensors are most preferable in automobile. Justify the statement.
Laser range sensors is a surveying method that measures distance to a target by
illuminating the target with pulsed laser light and measuring the reflected pulses with
a sensor. Differences in laser return times and wavelengths can then be used to make
digital 3-D representations of the target
17. Differentiate the function of resolver and synchros.

Synchros and Resolvers aren’t all that different from electric motors. They share
the same rotor, stator, and shaft components. The primary difference between a
synchro and a resolver is a synchro has three stator windings installed at 120 degree
offsets, while the resolver has two stator windings installed at 90 degree angles.
18. List the various forms of potentiometers.

1. Linear type 2. Rotary type 3. Helix type
19. Mention the applications of RF beacons.

RF beacons have many applications, including air and sea navigation,
propagation research, robotic mapping, radio-frequency identification (RFID) / Near
Field Communication(NFC) and indoor guidance, as with real-time locating
systems (RTLS) like Syledis or simultaneous localization and mapping (SLAM).
20. What are the uses of capacitive transducer?

1. Can be used for measurement of linear and angular displacement
2. Can be used for measurement of force and pressure
3. It can be used as pressure transducer
4. Measurement of humidity in gases
5. Commonly used for measurement of level, density, weight.
Essay Type Questions

1. Explain in detail the construction, operation and characteristics of LVDT. Also explain
how displacement is measured using LVDT.
2. Discuss the construction of an RVDT and explain how the displacement can be
measured from the transducer.
3. Describe the principle of operation, construction details of potentiometer. Tabulate

different forms used for various application based on their construction.
4. Explain the principle and application of following transducer.

1. RF beacons
2. Reflective beacons
3. Laser Range Sensor (LIDAR).
4. Bluetooth
5. Explain the principle of working and construction and characteristics of digital encoders.
6. Explain the construction, working principle and applications of the following sensors.
1. Resolvers
2. Synchros
3. Microsync
4. Acclerometer
7. i. Explain the principle of working and construction and characteristics of

magnetic encoders.
ii. Draw a block diagram to show how ranging sensors interact with the
immediate environment.
Unit – III
Force, Magnetic and Heading Sensors
3.1 Introduction
Force sensors, are devices that are designed to translate applied mechanical forces,
such as tensile and compressive forces, into output signals whose value can be used to reflect
the magnitude of the force. The signals may be sent to indicators, controllers, or computers to
inform operators or serve as inputs to provide control over machinery and processes.
Although strictly speaking force sensors and force transducers differ from each other, the two
terms are most commonly used interchangeably. Force sensors are available in a wide range of
sizes and can be used to detect forces from fractions of an ounce to hundreds of tons. They are
used in a wide range of products and applications such as bathroom scales, musical
instruments, medical applications, automobiles to detect seat occupancy, and process control
in manufacturing facilities, to name a few of the many uses for these devices. This chapter will
provide information on force sensors, including the common types and how they work.
Magnetic sensors are solid state devices that are becoming more and more popular
because they can be used in many different types of application such as sensing position,
velocity or directional movement. They are also a popular choice of sensor for the electronics
designer due to their non-contact wear free operation, their low maintenance, robust design
and as sealed Hall Effect devices are immune to vibration, dust and water. One of the main
uses of magnetic sensors is in automotive systems for the sensing of position, distance and
speed. For example, the angular position of the crank shaft for the firing angle of the spark
plugs, the position of the car seats and seat belts for air-bag control or wheel speed detection
for the anti-lock braking system, (ABS). Magnetic sensors are designed to respond to a wide
range of positive and negative magnetic fields in a variety of different applications and one
type of magnet sensor whose output signal is a function of magnetic field density around it is
called the Hall Effect Sensor.
Heading sensors are of particular importance to positioning systems, because they

can help compensate for the foremost weakness of odometry: in an odometry-based
positioning method, any small momentary orientation error will cause a constantly growing
lateral position error. For this reason it would be of great benefit if orientation errors could be
detected and corrected immediately. In this chapter we discuss inclinometer, gyroscopes and
compasses, the two most widely employed sensors for determining the heading of a moving
system.
3.2 Unit 3: Force, Magnetic and Heading Sensors
3.2 Force sensors

Generally, force is an important parameter in the measurement of pressure, load,
strain, displacement, etc. In automotive industries, force sensors are used to monitor impact
forces. By detecting the forces generated at the end effectors, robotic handling and assembly
tasks are controlled. In many of the mechanical systems, direct measurement of forces is useful
in control. Force is parameter that causes a change of momentum in a body of mass m and is
expressed by
F = ma
where F is force applied to a body

m is mass of the body
a is the acceleration
If any force is applied to a body of mass and which does not change its acceleration,
then the force and the body constitute a system in equilibrium. There are two basic types of
force: static force and dynamic force. If the force does not change with respect to time, then it is
said to be static force, and if it changes with time, it is called as dynamic force.
a. Principles of a force measurement

There are several principles to sense and measure force:
 Use a standard mass and a system of levers as reference or an electrical force and
make it balance with the unknown force or an electrical force.
 Measure the acceleration produced by the unknown force produces on a known
mass.
 Measure the pressure it produces on a known surface.
 Sense and measure the effects that the unknown force produces on various materials.
b. Types
The following are the different force transducers in use:
1. Mechanical type force transducers which include mechanical effects such as strain or
displacement of a body or of a spring. Here the unknown force is balanced against a
standard mass through a system of levers.
2. Optical type force transducers which causes a change in optical properties of optical
fibres.
3. Electrical type force transducers which include :
a) Resistive type in which the change of resistance occurs due to change of distance
between particles and due to change of contact area between particles of
semiconductor materials.
b) Capacitive type in which the application of force causes a change in the distance
between the capacitor plates and hence there is a change in capacitance value.
c) Magnetostrictive type in which the force produces a stress which changes the
permeability of a magnetic material with a force produced stress.
d) Piezoelectric type in which the application of mechanical force changes the
voltage developed between the surfaces of a dielectric material.
3.2.1 Strain gauge

A strain gauge is a device whose electrical resistance varies in proportion to the
amount of amount of deformation of a body due to an applied force. Specifically strain ε is
defined as the fractional change in length, as shown in figure.
Fig. 3.1 Strain
Depending on the application of stress to any resistive wire, strain can be negative.
Strain in the same tensile or compressive direction as the force applied is called longitudinal
strain , and strain in the orthogonal direction to the external force is called lateral strain ,
each material has a certain ratio of lateral strain to longitudinal strain. This ratio is called
Poisson's ratio, which is expressed as v(nu). It is always between 0 and 0.5 for all metals.
| |
Although strain is dimensionless, it is sometimes expressed in units such as in/in or

mm/mm. Practically, the magnitude of measured strain is very small. Hence, it is often
expressed as micro strain µε.
a. Principle of Strain gauge

A strain gauge works on the principle of electrical conductance and its dependence on
the conductor’s geometry. Whenever a conductor is stretched within the limits of its elasticity, it
doesn’t break but, gets narrower and longer. Similarly, when it is compressed, it gets shorter
and broader, ultimately changing its resistance. We know, resistance is directly dependent on
the length and the cross-sectional area of the conductor given by:
The change in the shape and size of the conductor also alters its length and the cross-
sectional area which eventually affects its resistance. Any typical strain gauge will have a long,
thin conductive strip arranged in a zig-zag pattern of parallel lines. The reason behind aligning
them in a zig-zag fashion is that they don’t increase the sensitivity since the percentage change
in resistance for a given strain for the entire conductive strip is the same for any single trace.
Also, a single trace is liable to overheating which would change its resistance and thus, making
it difficult to measure the changes precise.
b. Gauge Factor
It is well known that the resistance of a conductor having uniform cross-sectional area
a, length L, and specific resistivity is is given by
If this conductor is subjected to a stress, there will be deformation in the body. Due to
this dimensional change, the resistance of the wire will also change. By differentiating the
above equation, we get
( )
By dividing the LHS and RHS by
But
( )
( )
Since the second term is very small, it is neglected. Then
Substituting
Dividing the equation by on both sides,
⁄ ⁄
⁄ ⁄
⁄
The LHS of the above equation is called as “gauge factor”.
⁄
Statement:
The gauge factor is defined as the unit resistance change per unit strain. The gauge
factor is dependent on three factors:
1. Resistance change due to length change
2. Resistance change due to area change
3. Resistance change due to piezo-resistance change
Gauge Factor also expressed as:
⁄
⁄
Where Π is the longitudinal piezo-resistance coefficient

E is the modulus of elasticity
Gauge Thermal Nominal

Material Composition
factor Coefficient Resistivity
Constantan,
N1 45%, Cu 55% 2.1 – 2.2 2 x 10-3 0.45 – 0.48
Advance, Ferry
Karma Ni 74%, Cr 20%, Fe 3%, Cu 3% 2.1 2 x 10-3 1.25
Nichrome V Ni 80%, Cr 20% 2.2 – 2.6 10-2 1.00
Ni 36%, Cr 8%, Fe 52%, Mn-Si-
Isoelastic 3.5 – 3.6 1.75 x 10-2 1.05
Mo 4%
Pt-W alloy Pt 92%, W 8% 3.6 – 4.5 2.4 x 10-2 0.62
Nickel Ni 100% 12 0.68 0.65
Manganin Cu 84%, Mn 12%, Ni 4% 0.3 – 0.48 2 x 10 -3 -
Platinum Pt 100% 4.8 0.4 0.1
Table 3.1 Strain gauge materials and their properties
Sometimes, semiconductor materials can also be used as strain gauge material. These
semiconductor strain gauges have very high gauge factor of about 125. This high gauge factor
is due to large change in resistance of the strain gauge material due to piezo resistive effect. In
contrast to semiconductor materials, in metals the resistance change under strain is due to the
dimensional change.
c. Types
The strain gauge is the most important sensing element for many types of sensors
which include pressure sensors, load cells, torque sensors, position sensors, etc. the structure
of a strain gauge is shown below.
Fig. 3.2 Structure of Strain Gauge
There are two major categories of strain gauges:

1. Metallic Strain gauges
2. Semiconductor Strain gauges
Metallic strain gauge
Metallic (Resistive) strain gauges can be divided into two categories (a) un-bonded
and (b) bonded—the Former, being of limited use has received less attention than the latter.
Un-bonded strain gauge consists of a piece of wire stretched in multiple folds between a pair
or more of insulated pins Fixed to movable members of a 'body' or even a single flexible
member whose strain is to be measured. There occurs a relative motion between the two
members on strain and the wire gets strained as well with a corresponding change in its
resistance value. The scheme of such a system is shown in Fig. 3.3 (a)
Fig 3.3 (a) Unbounded (b) Bounded strain gauge
The bonded type is more common and in its simplest form consists of wire/strip of
resistance material arranged usually in the form of a grid for larger length and resistance
value. The grid is bonded to the test specimen with an insulation layer between the gauge
material and the specimen as shown in Fig. 3.3 (b).
If the insulation and the bonding material thickness is h which also is the height of
the wire above the specimen surface and H is the distance of the neutral axis of the specimen
from its surface, then the actual strain , in terms of measured strain , is given by
Depending upon the implementation, the resistance gauges can be classified as:
1. Unbonded metal wire,
2. Bonded metal wire,
3. Bonded metal foil,
4. Thin metal film by vacuum deposition, and
5. Thin metal film by sputter deposition.
Un-bonded strain gauges are used in preloaded conditions not to allow the 'strings' to
go slack. The wires are nickel alloys such as Cu-Ni, Cr-Ni, or Ni-Fe with gauge factor between
2 and 4 and diameters varying from 0.02-0.03 mm. The bonded strain gauges are of a few
types. When wire is used, the possibilities are (i) flat grid type, (ii) wrap around type, and (iii)
woven type, although the flat grid type is more Popular of all the three. Etched foil type
resistance strain gauge is one variety that, in recent years, has most extensively been used.
A gauge consists of the resistance element of proper design/shape, the gauge backing,
cement, connection leads, and often protective coating or other protective means. The
construction of the flat grid bonded strain gauge is shown in Fig. 3.4. Such a construction has
the advantage of better strain transmission from the member to the wire grid, small hysteresis
and creep, and is more accurate when the strain member is thin.
Fig. 3.4 Grid type Gauge
The foil gauges are etched out from deposited films or sheets and have higher surface
area to cross-section ratio than wire gauges, and hence, have better heat transfer property so
that they can handle higher current.
For wire gauges, the wires are usually drawn and often annealed, while bonded foil
gauges consist of sensing elements which are formed from sheets of thickness less than 5 x 10-4
cm by photoetching processes so that any arbitrary shape can be given to these elements.
Because the wire grid in the grid type structure has a finite width, the gauge has sensitivity to
transverse strain which may be as large as 2% of the longitudinal sensitivity. In foil grid
structure, the end turns can be made wider or fat enough so that the transverse strain
sensitivity is lesser. A typical grid structure foil gauge is shown in Fig. 3.5.
Fig. 3.5 Grid structure gauge with reduced transverse strain sensitivity
Semiconductor Strain Gauges

First lot of semiconductor strain gauges were produced early in mid-thirties from
single crystal silicon or germanium by cutting thin strips. Lot of work has since been done and
is still being done on the improvement of their performance and manufacturing ease because it
has been known that although the semiconductor gauges have higher gauge factors, they are
much inferior to the resistance types in so far as linearity and temperature stability are
concerned (specifically the latter). But the discovery of semiconductor strain gauges has
cleared the path of smart sensors, including production of strain sensitive cantilevers and
diaphragms by doping selected small areas of monolithic silicon slice.
Fig. 3.6 Semiconductor strain gauge

Semiconductor strain gauges can be divided into two classes—(i) bonded

semiconductor and (ii) diffused semiconductor—depending on their implementation. Strain
sensitivity of semiconductor material depends, among other things, on the crystal material
such as Si or Ge, doping levels (if any), type of doping materials, crystal cut-axis orientation,
and so on. Because the bandgaps both in intrinsic and extrinsic semiconductors are affected by
temperature variation, semiconductor gauges are more prone to temperature variations For
intrinsic semiconductors, gauge factors are larger decreasing with increasing degrees of
doping, the thermal coefficients of resistivity also decrease correspondingly.
d. Rosette
Gauges are made available in combinations often called 'rosettes' and these are
designed in various configurations for specific stress-strain analysis and/or for transducer
applications. A number of gauges are given relative orientations following certain pattern for
the purpose. Thus, a three-gauge rosette used in stress analysis solves problems of a surface
stress in magnitude and direction. Since the stress/strain is necessary to be measured at a
point, it is best to stack these three gauges to form a rosette on that point. In fact, this sandwich
pattern rosette is available from the manufacturers under the name 'stacked rosette'.
Fig. 3.7 Rosetted strain gauges: (a) three-element stacked type, (b) two-element right-angled,
(c) three elements at 45° to each other, (d) three elements at 120° to each other, (e) gauge
pattern on a diaphragm.
Figure 3.7(a) shows such a three element rosette stacked at 45° to each other. In this,
the topmost gauge is farthest from the specimen and all the gauges are insulated from each
other, the topmost gauge gets heated up more compared to the bottommost which use the
specimen as the heat sink. Two element stack type design is also commercially available. Such
a design has an advantage that the strain/stress at the same point is sensed by all the gauges.
The alternative to the stack type design is the planar design which covers a small area rather
than a point. Rosettes with such a design are available in two element 90° planar—usual and
shear, three element 45°, 60° planar. They can be generated on the specimen as well Figures
3.7(b), (c), and (d) show some of the types.
In fact, the technology of generating gauges on the specimen itself or on substrate as

mentioned earlier by vacuum process has lead to wide scope of gauge pattern variation. It can
be of any type depending on the specific requirements. The number of gauges at a location can
also be changed as per this practice. Figure 3.7(e) shows a gauge pattern variation for
measurement of strain in a diaphragm. Gauges 1 and 3 are subjected to tensile tangential stress
while gauges 2 and 4 are subjected to compressive radial stress. 1', 2', 3', and 4' are contact
terminals.
3.2.2 Load cell

The mass of a body is always quantified in terms of a measurement of the weight of
the body, this being the downward force exerted by the body when it is subject to gravity.
Three methods are used to measure this force. The first method of measuring the downward
force exerted by a mass subject to gravity involves the use of a load cell. The load cell measures
the downward force F, and then the mass M is calculated from the equation:
M = F/g,
where g is acceleration due to gravity.
Because the value of g varies by small amounts at different points around the earth’s
surface, the value of M can only be calculated exactly if the value of g is known exactly.
Nevertheless, load cells are, in fact, the most common instrument used to measure mass,
especially in industrial applications. Several different forms of load cells are available. Most
load cells are now electronic, although pneumatic and hydraulic types also exist. These types
vary in features and accuracy, but all are easy to use as they are deflection-type instruments
that give an output reading without operator intervention. The second method of measuring
mass is to use a spring balance. This also measures the downward force when the measured
mass is subject to gravity. Hence, as in the case of load cells, the mass value can only be
calculated exactly if the value of g is known exactly. Like a load cell, the spring balance is also
a deflection-type instrument and so is easy to use. The final method of measuring mass is to
use some form of mass balance instrument. These provide an absolute measurement, as they
compare the gravitational force on the mass being measured with the gravitational force on a
standard mass.
Because the same gravitational force is applied to both masses, the exact value of g is
immaterial. However, being a null-type instrument, any form of balance is tedious to use. The
following paragraphs consider these various forms of load cells in more detail.
a. Hydraulic Load Cells

Hydraulic load cells convert a load to hydraulic pressure. The measured load is
applied to a load platform attached to a piston. The piston is housed in a closed chamber filled
with fluid. When a load is applied, the action of the piston on the diaphragm pressurizes the
liquid. The change in liquid pressure is directly proportional to the force applied by the load.
The liquid pressure is then read through a bourdon tube pressure gauge. Hydraulic load cells
have the following components:
 An elastic diaphragm
 A piston connected to a load platform
 Hydraulic fluid which is usually oil or sometimes water
 Pressure gauge or gauges
 A tube connecting the chamber to the pressure gauge
 Steel housing for the assembly
Fig. 3.8 Hydraulic Load Cell
Because the hydraulic load cell design contains no electrical components, this type of
load cell lends itself to environments where explosion safety is a concern, or where an outside
power source may be difficult to provide. On the flip side, hydraulic load cells tend to be more
expensive than other types, making them cost-prohibitive for certain applications. Hydraulic
load cells can typically measure up to 5MN and have an accuracy of about 0.25 to 1.0 percent
of full-scale output. Their resolution is typically about 0.02 percent. Because these load cells are
sensitive to ambient pressure, the user must reset the readout to zero before each use.
b. Pneumatic Load Cell

Pneumatic load cells function similarly to their hydraulic counterparts in that they
convert fluid pressure into a load measurement. However, the pressurized fluid in a
pneumatic load cell is a type of gas, often times air.
The force to be measured is applied to a loading platform on one side of a diaphragm,

and a pressure supply regulator introduces a pressurized gas to a chamber on the opposite
side of the diaphragm to balance out the force. A nozzle connected to a pressure gauge allows
some of the pressurized gas to escape the chamber. The pressure of the gas flowing through
this nozzle is measured. This pressure is proportional to the force applied. Pneumatic load
cells have the following components:
 A loading platform to apply the force
 A steel chamber filled with pressurized gas or air
 An elastic diaphragm connected to the loading platform that seals the chamber
 An air supply regulator
 Nozzle (bleed valve)
 Pressure gauge
Fig. 3.9 Pneumatic Load Cell
Like their hydraulic counterparts, pneumatic load cells are explosion resistant and are
generally used in applications with intrinsic safety concerns. The pneumatic load cell is also
tolerant of temperature changes. Finally, this type of load cell is sensitive to small loads. This
makes them practical for systems requiring real-time accuracy with the lightest of loads, such
as dispensing IV fluids.
c. Capacitive Load Cells

Capacitive load cells operate on the ability of a material or system to store a charge.
They consist of two parallel plates with a gap between them. An electric current is supplied to
the plates until a stable charge forms on each: one with a positive charge and the other
negative. When a load is applied to one of the plates, the gap narrows causing a stored charge
(or capacitance) between the plates. This charge creates the output of the load cell, which is
then translated to a load measurement. Capacitive load cells consist of:
 A loading platform external to the housing to apply the force

 An insulated housing containing a free moving and a fixed plate
 A dielectric material between the plates (which may be air)
 Electrical wires to the plates
 A rigid rod or connector between the loading platform and the free moving plate
in the housing
Fig. 3.10 Capacitive Load Cell
Capacitive load cells are highly sensitive and accurate over a wide range of forces,
large and small. They are also rather simple in design, making them more cost-effective than
other load cell types. Their ability to be hermetically sealed without compromising their
operation makes them a good choice for food and medical weighing applications where
hygiene is an issue. Because capacitive load cells operate using an electric charge they may not
be a good choice in flammable environments. Also, some dielectric materials are sensitive to
temperature, which can affect the accuracy of the load cell.
d. Piezoelectric Load Cells

Piezoelectric sensors operate based on the piezoelectric effect. The piezoelectric effect
is a natural property of materials such as quartz crystal and other ceramics. Piezoelectricity is
produced when this polarized crystalline material is stressed or deformed. The stress then
causes a shift in the orientation of the internal dipoles of the material. It is similar to di-
electricity, which occurs when a charge develops from a shift of electrons in an insulator.
Piezoelectric sensors can quantify force, pressure, and displacement. Metallic electrodes
bonded to the surface of the material form a measurable net charge. For proper function, the
design must place these electrodes perpendicular to the applied force. Both compression and
tension forces create this piezoelectric effect. Compression forces create an opposite polarity to
tension forces. The output voltage is directly proportional to the applied force. Piezoelectric
load cells consist of:
 A loading platform or system to apply the force

 Metallic electrodes bonded to the piezoelectric material
 The piezoelectric material
 Output wires to measure a change in voltage caused by the change in charge
Fig. 3.11 Piezoelectric Load Cell
A piezoelectric transducer is an active transducer, meaning it does not require an

external power source to generate an output signal. This characteristic makes this device
desirable in applications where an external power source is inconvenient. However, its output
signal does require amplification as it is very small. The piezoelectric effect happens for
dynamic forces. Once a force becomes static, the output of the sensor returns to zero. Therefore
these transducers lend themselves to applications requiring the measurement of a transient
force. Piezoelectric transducers are more durable than other load cells, and have a high
frequency response.
e. Strain Gauge Load Cells

Strain gauge load cells are the most common. Unlike the hydraulic and pneumatic
designs described below that convert pressure differentials to measurements, the strain gauge
load cell operates through changes in electrical resistance. Inside each strain gauge load cell is
at least one strain gauge device. A strain gauge is basically a thin wire etched in a back-and-
forth pattern onto a non-conductive substrate material with connectors at each end of the wire.
The strain gauge functions on the elastic properties of the wire; that is, when a wire is
stretched, its length increases and cross section decreases, and when compressed, the opposite
occurs. In the tension case, the resistance of the wire will increase, while in the compression
case, the resistance of the wire will decrease. Strain gauge load cells convert this change in
resistance to a load measurement. A system using these types of load cells is designed in such
a way that the force to be measured causes a deflection (tension or compression) in the strain
gauges.
Because of the versatility of strain gauges, the mounting configurations to measure

loads are various. The underlying circuitry (the Wheatstone Bridge) which makes the strain
gauge transducer work is discussed in the article “The Versatile Strain Gauge Load Cell.” This
article also discusses the various configurations of strain gauge load cells. Strain gauge load
cells consist of:
 A loading platform or system to apply the force, $F_{load}$

 One or more strain gauges
 An excitation voltage source
 Output wires to measure a change in voltage caused by the change in
resistance of the strain gauges
Fig. 3.13 Strain Gauge Load Cell
Figure 3.13 shows a single end beam load cell with these components. The strain
gauges at the top of the beam are in tension under the load, while those at the bottom are in
compression under the load. Strain gauge load cells are the most popular due to their high
accuracy, low price point, and general ease of use. They have a high frequency response for
dynamic loads and are not sensitive to temperature variations. Because they can fit into a wide
variety of load-mounting configurations, they lend themselves to almost any industrial
application. Strain gauge load cells are passive transducers meaning they require an excitation
voltage to operate. This can restrict their use in areas of limited electrical supply or in areas
where ignition may be an issue.
3.3 Magnetic sensors

Magnetization serves a strong impact in changing the properties of certain materials.
Magnetization changes or produces effects which are mechanical or electrical in nature and
which are measurable. Also, optical energy may produce changes in magnetization
characteristics of the materials. Not all such changes are easily transducible, perhaps, not at
this stage of the state-of-the-art. Nevertheless quite a few are being conveniently used in
developing sensors. Some of these are discussed here.
 Magnetic field sensors—developed following 'ΔY effect' which, in effect, is observed as

the change in Young's modulus with magnetization. The sensors are often termed as
Acoustic Delay Line Components (ADLC).
 Magneto-elastic sensors—based on the fact that in a longitudinal field, torsion given in a
ferromagnetic rod changes its magnetization. This is known as `Matteucci effect'.
 Magnetic elastic sensors—produced using `Villari effect' in which a tensile or
compressive stress changes magnetization or affects magnetization in some way.
 Torque/force sensors—Wiedemann effect' is used to develop the torque/force sensors. In
such sensors, torsion is produced in a ferromagnetic rod carrying a current when
subjected to a longitudinal field.
 Magnetoresistive sensors—becoming increasingly popular, are developed on the basis of
`Thomson effect' which is basically a change in resistance of specified materials with
magnetic field impressed.
 Hall Effect sensors or magnetogalvanic sensors—perhaps, the most common and widely
used type magnetic sensors. These operate on the fact that a crystal carrying a current
when subjected to a magnetic field perpendicular to the direction of the current, produces
a transverse voltage.
 Distance or proximity sensors—developed based on 'skin effect' in which eddy current
forces the current flowing through the interior of a material to move to its surface level
 Wiegand and pulse wire sensors—a specific type of material when subjected to voltages
under stress shows switching effect which occurs due to Barkhausen jump. This is utilized
to produce such sensors. The effect is called `Sixtus-Tonks effect' after the experimenter
who demonstrated the effect.
 Superconducting Quantum interference Devices (SQUIDS)—used for varying
application areas, are based on the superconducting state specifically, 'flux quantization
and Josephson effect'. These types of sensors have a resolution of the order of a few
femtoTesla (IT).
 Magnetostriction - a phenomenon, known over a century and a half now, has also been
used in combination with piezoelectric elements for field measurements. This effect is also
known as 'Joule effect' in which magnetization changes the shape of a ferromagnetic
material body.
Magnetic sensors utilize the magnetic phenomena of inductance, reluctance, and

eddy currents to indicate the value of the measured quantity, which is usually some form of
displacement. Inductive sensors translate movement into a change in the mutual inductance
between magnetically coupled parts. One example of this is the inductive displacement
transducer shown in Figure 3.14. In this, the single winding on the central limb of an “E”-
shaped ferromagnetic body is excited with an alternating voltage. The displacement to be
measured is applied to a ferromagnetic plate in close proximity to the “E” piece.
Movements of the plate alter the flux paths and hence cause a change in the current
flowing in the winding. By Ohm’s law, the current flowing in the winding is given by
⁄ . For fixed values of ω and V, this equation becomes ⁄ , where K is a constant.
The relationship between L and the displacement, d, applied to the plate is a nonlinear one,
and hence the output-current/displacement characteristic has to be calibrated.
Fig. 3.14 Inductive Displacement Sensor
The inductance principle is also used in differential transformers to measure

translational and rotational displacements.
Variable reluctance type, a coil is wound on a permanent magnet rather than on an

iron core as in variable inductance sensors. Such devices are used commonly to measure
rotational velocities. Figure 3.15 shows a typical instrument in which a ferromagnetic
gearwheel is placed next to the sensor. As the tip of each tooth on the gearwheel moves toward
and away from the pick-up unit, the changing magnetic flux in the pickup coil causes a voltage
to be induced in the coil whose magnitude is proportional to the rate of change of flux. Thus,
the output is a sequence of positive and negative pulses whose frequency is proportional to the
rotational velocity of the gearwheel.
Fig. 3.15 Variable Reluctance Sensor

Eddy current sensors consist of a probe containing a coil, as shown in Figure 3.16,
that is excited at a high frequency, which is typically 1 MHz. This is used to measure the
displacement of the probe relative to a moving metal target. Because of the high frequency of
excitation, eddy currents are induced only in the surface of the target, and the current
magnitude reduces to almost zero a short distance inside the target. This allows the sensor to
work with very thin targets, such as the steel diaphragm of a pressure sensor. The eddy
currents alter the inductance of the probe coil, and this change can be translated into a d.c.
voltage output that is proportional to the distance between the probe and the target.
Measurement resolution as high as 0.1 mm can be achieved. The sensor can also work with a
nonconductive target if a piece of aluminum tape is fastened to it.
Fig. 3.16 Eddy current sensor
3.3.1 Magneto-resistive sensor

During the process of magnetization, ferromagnetic materials have offered a property
that change in the dimensions. The change in magnetization of the materials based on the
applied magnetic field further changes the magnetostrictive strain. This effect was utilized in
the development and applications of magnetostrictive transducers. Simply speaking, a
magnetostrictive transducer is used to convert mechanical energy into magnetic energy and
vice versa. It can be used as a sensor and also be used as a transducer, since it can provide
bidirectional coupling between mechanical and magnetic states of the ferromagnetic materials.
a. Magnetostrictive Effect
Certain ferromagnetic materials like Iron, Nickel, Permalloy, etc. change their
magnetic permeability when they are subjected to mechanical stress. This effect is known as
magnetostrictive effect. It is also called as Villari effect.
The amount of change in permeability is based on the materials, the type of stress
(like compression, tension or torsion) and the magnetic flux density in the sample. This kind of
property is applicable in constructing a transducer to convert an input stress to a variation in
induction.
b. Inverse Magnetostrictive effect

It is also known as magneto elastic effect which states that there is a change of
magnetic susceptibility of a magnetostrictive material when it is subjected to mechanical stress.
The direct magnetostrictive effect will characterize the changes in dimensions of a
ferromagnetic material during the process of magnetization, whereas the inverse
magnetostrictive effect will characterize the change in magnetization of magnetostrictive
material when the material is subjected to mechanical stress.
c. Working principle
The principle of working of a magnetostrictive transducer is explained with Figure
3.16 [(a) to (e)]. It demonstrates that the magnitude of strain produced due to full
magnetization and without magnetization. When no magnetic field is applied to the
magnetostrictive material (H = 0), there will be no change in the length of the material. This is
denoted in Figure 3.16 (c). As the magnitude of the applied magnetic field increases to its
limits ±H„ the elliptical magnet tends to rotate to align with the input magnetic field and for
which the axial strain increases and magnetization of the materials in the axial direction
increases to ±B [Figure 3.16 (e)] or decreases to —B [(Figure 3.16 (a)]. At an applied field
strength, the highest magnitude (saturation) of the strain and magnetic induction have been
attained, that means the strain and magnetization of the material will not change with further
changes in the applied magnetic field. Hence both the strain and magnitude of the
magnetization increase moving from the centre outward as the magnitude of the applied field
increases to its saturation values.
Fig. 3.16 Working principle of Magnetoresistive transducer

Alternately, if the material is subjected to compressive force, both the axial strain and
axial magnetization magnitude in the element will decrease with increase in the compressive
stress. These developed flux fields are measured by calculating the voltage developed in a
conductor which is kept at right angles to the flux produced, and this value is dependent on
the applied force.
d. Magnetostrictive materials
Magnetostrictive materials can convert magnetic energy into kinetic energy or vice
versa. This property can be quantified by means of a constant called magnetostrictive
coefficient. It is the fractional change in length as the magnetization of the material increases
from zero to the maximum (saturation) value. The magnetostrictive effect is significant in
Nickel and Nickel—Iron alloys. Cobalt exhibits maximum magnetostriction at about 60 micro
strains. In case of alloys, the highest magnetostriction is offered by Terfenol—D (Terbium,
Iron, Naval Ordnance Laboratory-Dysprosium) which exhibits about 2000 microstarins at
room temperature. Permalloy is a Nickel alloy with 68% Nickel. Ferroxcube--B is also used as a
magnetostrictive transducer, but it is highly brittle, and hence it is very much used. For
different values of stress, the above magnetostrictive materials like Iron and Nickel show
permeability variations which are illustrated in Figure 3.17.
(c) Fig. 3.17 permeability variations of different magnetostrictive materials

e. Applications
Magnetostrictive Load Cell
With a magnetostrictive material as a basic sensor, load cells are developed. Such a
magnetostrictive load cell is shown in Figure 3.18 [(a) and (b)]. From the figure, there is a
magneto elastic probe surrounded by a yoke assembly. Based on the applied force on the
magneto elastic probe, there will be variation in the permeability in the probe material. Due to
which the output signal can be measured from the output terminals of the magnetic winding
over the probe. There is a possibility of increasing the sensitivity by means of increasing the
magnetic flux over the probe. The yoke will act as a shielding against stray magnetic fields. To
eliminate the effects of eddy currents, the yoke as well as the magneto elastic probe can be
laminated.
Fig. 3.18 Magnetostrictive load cells
Magnetostrictive Accelerometer
The arrangement of a magnetostrictive transducer for acceleration measurement is
shown in Figure 3.19. In the figure, there is a mass of the core over which an additional seismic
mass made up of brass is placed. A thin flexible diaphragm is also located on the top of the
seismic mass. When the mass of the core along with the seismic is subjected to acceleration,
there will be change in permeability of the core which will further induce an output signal V,
in the output coil. It is then integrated with respect to time. When compared with a
piezoelectric transducer used for acceleration measurement, these magnetostrictive
acceleration transducers are of larger size and weight. It also offers lower accuracy. Due to
variations in earth's magnetic field, its sensitivity is affected. Lamination of the core is
necessary so as not to be affected during the measurement of higher acceleration.
Fig. 3.19 Magnetostrictive acceleration transducer
Magnetostrictive Force Transducer

Figure 3.20 shows the arrangement of magnetostrictive transducer for force
measurement. In the Figure, there is a laminated core, and it is formed by stacking the
laminations. There is an iron cored coil provided for the measurement of self-inductance. Due
to the application of force, there will be changes in the core characteristics and hence changes
the permeability of the magnetic core. To make the self inductance as maximum, the current
coil is to be adjusted. With this arrangement, it is possible to measure large force with 10% to
20% of change in the self inductance and the measurement accuracy is observed as 2% to 5% of
full scale reading. The iron cored coil is polarized by means of dc mmf, and hence the
operating point of flux density is kept at a point below saturation. Due to the input stress ,
there will be change in flux density B by ±AB depending on the material. The sensitivity of the
magnetostrictive force transducer can be defined as:
where B0 is operating point of flux density.
The induced emf from the output winding terminals is expressed as:
()
()
where S is the sensitivity of the transducer

A is the area of the coil
N is the number of turns in the winding
a is the applied stress
Fig. 3.20 Magnetostrictive force transducer.
Magnetostrictive Torsion Transducer

The magnetostrictive torsion transducer is shown in Figure 3.21. There is a thin wire
made up of nickel, which is stretched between the two poles of a permanent magnet. A stylus
is attached to the nickel wire at the middle. I3efore stretching between the poles of the magnet,
the torsion wire is pre-stressed by miming it. There are two pick up coils wound around the
two halves of the nickel wire as shown in the arrangement. Any displacement of the stylus to
one side of the torsion wire or the other side will increase the torsion on one side and will
decrease the torsion on the other side by equal and opposite amount. It will result in an
increase in the magnetic flux in one half and a decrease in the next half of the torsion wire.
Hence the corresponding induced emfs are in phase opposition and with suitable devices like
LVDT, they can be further processed. This kind of transducer can be used as a phonographic
pick up which has a flat frequency response over 150 Hz to 15 kHz. For frequency greater than
5 kHz, there is a possibility of generation of eddy currents.
Fig. 3.21 Magnetostrictive torsion transducer

f. Features
The following are the special features of the magnetostrictive transducers.
1. They are capable of measuring large forces up to several tones.
2. They have fast transient performance where the frequency is of the order of several
thousand cycles per second.
3. Magnetostrictive acceleration transducers can measure several thousand g. They are
rugged in nature.
4. Since the characteristic of magnetostrictive transducers is dependent upon the
temperature, they are in need of frequent calibration.
5. The characteristic of the transducers can be changed by the environmental factors.
3.3.2 Hall Effect sensor

Hall Effect transducers are used to convert magnetic energy into electrical energy,
and they found applications in the measurement of displacement, current and power. It
belongs to galvano-magnetic phenomena which states that due to the interaction between the
magnetic field and moving electrical charges, there will be generation forces that will change
the movement of charge. Even though it can be offered by all the metals, semiconductors are
predominantly using this phenomenon. If a strip of conducting material carries current in the
presence of transverse magnetic field, an emf is generated between the two edges of
conducting strip. The amount of generated voltage is based on the current and flux density.
This effect is called Hall Effect.
The Hall Effect is exhibited by certain materials like Arsenic, Bismuth, Iron, Silicon,
etc. When any of these Hall materials is subjected to a magnetic field in one direction, an
electric current in another direction which is perpendicular to the direction of the magnetic
flux is produced, then an electric potential is generated in a third direction which is mutually
perpendicular to the other two directions. It is illustrated in Figure 3.22. The amount of voltage
generated due to Hall Effect is based on the strength of the magnetic field, the strength of the
current and the properties of the conducting material.
Figure 3.22 Principle of Hall effect.

From Figure 3.22, Hall material is subjected to magnetic field B normal to the surfaces
of the Hall element, while it carries a current I along the length of the strip of material but
normal to magnetic field. The magnetic field exerts a force called Lorentz force on the electrons
moving at a velocity v, which results that some of the electrons drift towards the edges of the
Hall element. Due to which a potential difference EH is measured between the two edges of
the Hall element which increases with increase of B and I. If e is the charge of the electron, then
the Lorentz force Bev and the force due to Hall effect are equal to each other. Therefore,
EH = Bbv (volts)
where B is magnetic flux density (T)
v is velocity (m/s)
b is the width of the hall element (m)
Due to the electric field along the direction of the motion, the electrons and free
charge carriers assume a velocity along the length of the Hall element. If the mobility of the
charge carriers is dented as , then the velocity v is given by
where Vi is the input voltage

L is the length of the element
Since Vi = , the velocity becomes
Hence,
By taking ,
where KH is Hall co-efficient

t is the thickness of the element (m)
L is the length of the element (m)
The Hall co-efficient depends on the number of free charge carriers per unit volume.
The coefficient for Hall element is given by
( )
( )
where K is a positive constant

(K = 1, if the Hall element is of metals or
K = 3π/8, if the Hall element is of semiconductors)
n is free electrons
p is free holes
is mobility of the electrons
is mobility of the holes
e is charge of the electron
The nominal values of Hall coefficients for various Hall materials are given in Table 3.2.
Material Flux Density Temperature KH

As 0.4 to 0.8 20 4.52 x 10-9
Bi 0.113 20 --1 x 10-6
C 0.4 to 1.8 30 —11.73 x 10-9
Cu 0.8 to 2.2 20 —50 x 10-12
Fe 1.7 25 1.1 x 10-9
Ge 0.001 to 0.8 25 —8 x 10-3
Si 2.0 25 4.1 x 10-6
Sn 0.4 25 —2 x 10-12
Te 0.3 to 0.9 20 53 x 10-6
Table 3.2 Hall Coefficients for Various Hall Materials
b. Applications
The following are the some of the applications of Hall effect transducers:
1. Hall effect transducer for flux measurement
2. Hall effect transducer for displacement measurement
3. Hall effect transducer for current measurement
4. Hall effect transducer for power measurement
Hall Effect Transducer for Flux Measurement

The Hall Effect transducer is used to convert the magnetic flux into its equivalent
electrical voltage. Figure 3.23 shows the arrangement of Hall flux meter. A Hall element is of
semiconducting material. It is used for the purpose of converting magnetic flux into electric
voltage and which can be inserted into the magnetic field to be measured. A known fixed
current is passed through side of the plates of the Hall element. Then the output voltage
generated across the mutually perpendicular direction is proportional to the magnetic flux
density. This Hall flux meter can obtain a continuous output based on the magnetic flux
variations.
The system has an advantage of requiring a very small space in the direction of the
magnetic field and hence it can be inserted in narrow gaps for magnetic flux measurements.
The drawbacks of the system are:
1. Hall element is sensitive to temperature variations.

2. Hall coefficient changes from element to element and hence it needs individual
calibration.
Fig. 3.23 Hall flux meter
Hall Effect Transducer for Displacement Measurement
Fig. 3.24 Hall Effect transducer for displacement measurement
The arrangement of Hall effect transducer for the measurement of displacement is

shown in Figure 3.24. It can be used to measure the displacement of a structural member
which is fixed with a permanent magnet. The structural member is provided with a gap in
which a Hall element is inserted. A permanent magnet attached with the structural member is
used to produce magnetic field strength. When the structural member is subjected to
displacement, there will be variation in the field strength provided by the magnet.
It is noted that the portion of the Hall element is fixed and it does not move. A fixed
current is passed through the Hall element inserted in the gap. The. output voltage generated
by the Hall element is proportional to the magnetic field lines passing through the Hall
element which is based on the relative position of ferromagnetic plate from the structural
element. This arrangement can be used for displacements ranging from 0.1 mm to 10 mm.
Hall Effect Transducer for Current Measurement

The following arrangement, shown in Figure 3.25, is used for the measurement of
current. It is used to measure current in a conductor without the need for interrupting the
circuit and without making electrical connection between the conductor circuit and the meter.
Fig. 3.25 Hall Effect transducer for current measurement
A ferromagnetic tube is used as a magnetic concentrator which is of slotted type and

through which a Hall element is placed. Current I is passed through a conductor and sets up a
magnetic field surrounding the conductor. The voltage generated at the output terminals is
proportional to the magnetic field strength and hence is proportional to the current flowing in
the conductor. With proper calibration of the device for different values of current, any
unknown current can be measured by passing it through the Hall element. This arrangement
can be used for the measurement of currents from <1 mA to thousands of ampere.
Hall Effect Transducer for Power Measurement

The arrangement used for the measurement of power, called Hall effect multiplier, is
shown in Figure 3.26. Current 1 is passed through the current coil which produces a magnetic
field proportional to current i. This magnetic field is perpendicular to the Hall element. A
current i is proportional to the voltage which is passed through the Hall element in a direction
perpendicular to the field as shown in Figure 3.26. Resistance Rs is used to limit the current.
Hence the output voltage from the Hall multiplier is expressed as:
where KH is Hall coefficient

B is flux density
t is the thickness of Hall element
Fig. 3.26 Hall Effect transducer for power measurement
The magnetic flux density B is proportional to current i.
Hence, the output voltage of the Hall element is proportional to the input voltage
v. Hence the voltmeter connected at the output terminals can be calibrated in terms of power.
The Hall voltage here is the representative of power which can be processed further for
control and other purposes.
3.3.3 Current sensor

Measuring a voltage in any system is a “passive” activity as it can be done easily at
any point in the system without affecting the system performance. However, current
measurement is “intrusive” as it demands insertion of some type of sensor which introduces a
risk of affecting system performance. Current measurement is of vital importance in many
power and instrumentation systems. Traditionally, current sensing was primarily for circuit
protection and control. However, with the advancement in technology, current sensing has
emerged as a method to monitor and enhance performance. Knowing the amount of current
being delivered to the load can be useful for wide variety of applications.
Current sensing is used in wide range of electronic systems, viz., Battery life
indicators and chargers, 4-20 mA systems, over-current protection and supervising circuits,
current and voltage regulators, DC/DC converters, ground fault detectors, programmable
current sources, linear and switch-mode power supplies, communications devices ,
automotive power electronics, motor speed controls and overload protection, etc.
Current sensing principles

A current sensor is a device that detects and converts current to an easily measured
output voltage, which is proportional to the current through the measured path. When a
current flows through a wire or in a circuit, voltage drop occurs. Also, a magnetic field is
generated surrounding the current carrying conductor. Both of these phenomena are made use
of in the design of current sensors. Thus, there are two types of current sensing: direct and
indirect. Direct sensing is based on Ohm’s law, while indirect sensing is based on Faraday’s
and Ampere’s law. Direct Sensing involves measuring the voltage drop associated with the
current passing through passive electrical components. The current sensors are classified
based on their operating principles.
CT Current sensor
A magnetic flux (Φ) is induced in the magnetic core due to the flow of the alternating
current (AC) being measured. A secondary magnetic flux (Φ’) is induced in the secondary coil
(N) as a reaction to this primary flux in an effort to cancel it out. A secondary AC current is
also induced in proportion to the secondary magnetic flux (Φ’). This secondary current flows
through the shunt resistor and voltage difference occurs between both sides of the resistor.
This voltage is proportional to the current flowing through the measured conductor. The
characteristics of this type of sensor is as follows
• No power source required (for current sensing function)
• Affordable
• Dedicated to AC (DC not supported)
• Popular with clamp meters used for low energy management in buildings
Fig. 3.27 CT current sensor

Hall element current sensor

When the measured current (principle current) passes through the magnetic core’s
aperture, a magnetic flux is induced in the core. As this magnetic flux flows through the Hall
element, a voltage generates in proportion to the magnetic flux. This voltage induction is
known as the Hall effect. Since the voltage induced by the Hall effect is small, it is boosted
with an amplifier before being output. The output voltage which is proportional to the
measured current allows for current measurement. The characteristics of this type of sensor is
as follows
 Measure DC to AC (<10 kHz)
 Affordable
 Lacks precision due to the linearity of the Hall element and the B-
H characteristics of the magnetic core
 Not suited for long-term measurement due to drifting caused by humidity
and change over time which is a characteristic caused by the Hall element
Fig. 3.28 Hall element current sensor
Rogowski coil current sensor

A voltage is induced in the air-core coil by interlinking the magnetic field produced
by the AC current flowing in the conductor being measured (the primary side of the circuit)
and the air-core coil. This induced voltage is then output as the time derivative (di/dt) of the
measured current, and an output signal proportional to the constant current is obtained by
passing it through an integrator. The characteristics are as follows
 Large currents can be measured because the coreless structure eliminates
any magnetic saturation
 Magnetic loss allows for:
 No heat generation
 No saturation
 No hysteresis
 Flexible and slim due to the air-core coil
 Small insertion impedance

 Affordable
 Dedicated to AC (DC not supported)
 Not recommended for high precision measurement because of high
susceptibility to noise.
Fig. 3.29 Rogowski coil current sensor
3.4 Heading sensor

3.4.1 Compass
Heading is the most significant of the navigation parameters (x, y, and θ) in terms of
its influence on accumulated dead-reckoning errors. For this reason, sensors which provide a
measure of absolute heading or relative angular velocity are extremely important in solving
the real world navigation needs of an autonomous platform. The most commonly known
sensor of this type is probably the magnetic compass. The terminology normally used to
describe the intensity of a magnetic field is magnetic flux density B, measured in Gauss (G).
Alternative units are the Tesla (T), and the gamma (γ), where 1 Tesla = 10 Gauss = 10 gamma. 4
9 The average strength of the earth’s magnetic field is 0.5 Gauss and can be represented as a
dipole that fluctuates both in time and space, situated roughly 440 kilometers off center and
inclined 11 degrees to the planet’s axis of rotation [Fraden, 1993]. This difference in location
between true north and magnetic north is known as declination and varies with both time and
geographical location. Corrective values are routinely provided in the form of declination
tables printed directly on the maps or charts for any given locale. Instruments which measure
magnetic fields are known as magnetometers. For application to mobile robot navigation, only
those classes of magnetometers which sense the magnetic field of the earth are of interest.
1. Baseplate compass (Orienteering compass): This type of compass was developed by the
Kjellstrom brothers and another keen orienteer, Gunnar Tillander in Sweden in the early
1930's. They combined a liquid-filled compass in a housing which could rotate over a
protractor base. This saved a lot of time transferring bearings from compass to map. It
proved to be a new and greatly successful system of direction- finding for outdoor
activities. These compasses are made of clear plastic or perspex. They have a rectangular
base with a 360° dial mounted upon it. Inside the raised dial is a magnetic needle
suspended in clear fluid. The dial October may be turned to read a correct bearing along
the direction of travel arrow which is clearly marked on the base plate.
2. Needle compass (Pocket compass): This type of compass has a magnetised needle
suspended over a fixed card which has been marked off in a clockwise direction in 360
equal units(degrees). The cardinal compass points, north (0°), east (90°), south (180°) and
west (270°) are always clearly marked. Intercardinal points (NE, SE, SW and NW) are
usually shown plus divisions between these and the cardinal points, thus giving a total of
sixteen compass directions. The needle moves freely to align itself north-south as the
compass is turned.
3. Card compass (Marine compass): In this compass the needle is fixed while the compass
card is mounted in fluid (is damped) and moves. Because the moving card absorbs much
of the motion of a boat it is easier to read than a needle compass. The top of the compass is
typically a hemispherical shape and may provide magnification of the readings on the
card. The card is normally marked to show all readings from 0° through to 360°. These
compasses are usually mounted on the boat near to the steering apparatus in which case
they are called the steering compass. Care must be taken not to allow nearby magnetic
fields (e.g. metal apparatus) to interfere with the working of the compass. Hand held
models can be used to sight bearings of objects in the distance when fixing position.
4. Thumb compass: This is a recently developed (1980's) modification of the baseplate

compass where the baseplate is re-modelled to fit around the thumb so that the map and
compass can be held together in one hand, leaving the other hand free. It is used chiefly
for orienteering and rogaining.
5. Gyrocompass: This is an electric compass and as such is not affected by magnetic fields.
Readings are true bearings and there is no need for adjustment for the Earth's magnetic
field. These compasses are sophisticated, stable and very accurate. Large ocean-going
vessels use a gyrocompass as their steering compass. Due to their size, weight and cost
they are not usually found in use on smaller craft.
6. Prismatic compass: This compass has a glass prism sighting arrangement and a lid with a
hairline for lining up the object to be sighted. The compass card rotates in the base and
when it comes to rest the required bearing is read off, through the prisms. These are quite
sophisticated hand compasses.
3.4.2 Gyroscope
Gyroscopes measure both absolute angular displacement and absolute angular
velocity. Until recently, the mechanical, spinning-wheel gyroscope had a dominant position in
the marketplace. However, this position is now being challenged by optical gyroscopes.
Mechanical gyroscopes
Mechanical gyroscopes consist essentially of a large, motor-driven wheel whose
angular momentum is such that the axis of rotation tends to remain fixed in space, thus acting
as a reference point. The gyro frame is attached to the body whose motion is to be measured.
The output is measured in terms of the angle between the frame and the axis of the spinning
wheel. Two different forms of mechanical gyroscopes are used for measuring angular
displacement—the free gyro and the rate-integrating gyro.
Fig. 3.30 Free gyroscope
The free gyroscope is illustrated in Figure 3.30. This measures the absolute angular
rotation about two perpendicular axes of the body to which its frame is attached. Two
alternative methods of driving the wheel are used in different versions of the instrument.
One of these is to enclose the wheel in stator-like coils that are excited with a
sinusoidal voltage. A voltage is applied to the wheel via slip rings at both ends of the spindle
carrying the wheel. The wheel behaves as a rotor, and motion is produced by motor action.
The other, less common, method is to fix vanes on the wheel, which is then driven by directing
a jet of air onto the vanes. The free gyroscope can measure angular displacements of up to 10˚
with a high accuracy. For greater angular displacements, interaction between the
measurements on the two perpendicular axes starts to cause a serious loss of accuracy. The
physical size of the coils in the motor-action driven system also limits the measurement range
to 10 ˚. For these reasons, this type of gyroscope is only suitable for measuring rotational
displacements of up to 10 ˚. A further operational problem of free gyroscopes is the presence of
angular drift (precession) due to bearing friction torque. This has a typical magnitude of 0.5 ˚
per minute and means that the instrument can only be used over short time intervals of say 5
minutes. This time duration can be extended if the angular momentum of the spinning wheel
is increased.
A major application of the free gyroscope is in inertial navigation systems. Only two
free gyros mounted along orthogonal axes are needed to monitor motions in three dimensions
because each instrument measures displacement about two axes. The limited angular range of
measurement is not usually a problem in such applications, as control action prevents the error
in the direction of motion about any axis ever exceeding one or two degrees. However,
precession is a much greater problem, and, for this reason, the rate-integrating gyro is used
much more commonly.
Fig. 3.31 rate integrating Gyroscope

The rate-integrating gyroscope, or integrating gyro as it is commonly known, is

illustrated in Figure 3.31 It measures angular displacements about a single axis only, and
therefore three instruments are required in a typical inertial navigation system. The major
advantage of the instrument over the free gyro is the almost total absence of precession, with
typical specifications quoting drifts of only 0.01_/hour. In addition to their use as a fixed
reference in inertial guidance systems, integrating gyros are also used commonly within
aircraft autopilot systems and in military applications such as stabilizing weapon systems in
tanks.
Optical gyroscopes
Optical gyroscopes are a relatively recent development and come in two forms—ring laser
gyroscope and fiber-optic gyroscope. The ring laser gyroscope consists of a glass ceramic
chamber containing a helium–neon gas mixture in which two laser beams are generated by a
single anode/twin cathode system, as shown in Figure 3.32. Three mirrors, supported by the
ceramic block and mounted in a triangular arrangement, direct the pair of laser beams around
the cavity in opposite directions. Any rotation of the ring affects the coherence of the two
beams, raising the frequency of one and lowering the frequency of the other. The clockwise
and anticlockwise beams are directed into a photodetector that measures the beat frequency
according to the frequency difference, which is proportional to the angle of rotation. The
advantages of the ring laser gyroscope over traditional, mechanical gyroscopes are
considerable. The measurement accuracy obtained is substantially better than that afforded by
mechanical gyros in a similar price range. The device is also considerably smaller physically,
which is of considerable benefit in many applications.
Fig. 3.32 Optical Gyroscope

3.4.3 Inclinometer
Tilt sensors (also called inclinometer or Tiltmeter) are devices that produce an
electrical signal that varies with an angular movement. These sensors are used to measure
slope and tilt within a limited range of motion. Sometimes, the tilt sensors are referred to as
inclinometers because the sensors just generate a signal but inclinometers generate both
readout and a signal.
Fig. 3. 33Tilt Sensor working
These sensors consist of a rolling ball with a conductive plate beneath them. When
the sensor gets power, the rolling ball falls to the bottom of the sensor to form an electrical
connection. When the sensor is tilted, the rolling ball doesn’t fall to the bottom so that the
current cannot flow the two end terminals of the sensor. A basic circuit that uses a tilt sensor is
shown below.
Fig. 3.34 Tilt Sensor Circuit
When the device gets power and is in its upright position, then the rolling ball settle
at the bottom of the sensor to form an electrical connection between the two end terminals of
the sensor. Next the circuit becomes short circuit and the LED gets sufficient current. If the
circuit gets tilted so that the rolling ball doesn’t settle at the bottom of the sensor with the
electrical conduction path, then the circuit becomes open. This is about the circuit operation.
Tilt Sensor Types

These Sensors are classified into different types and the classification of these sensors
includes different devices and technologies to measure tilt, slope, elevation and inclination.
Force Balance Sensor

These sensors are gravity referenced sensors and are anticipated for DC acceleration
measurements like ships, vehicles, aircraft and seismic events. These sensors are frequently
used in inclinometers and tilt meters. Force balance sensors are capable of measuring levels
from 0.0001g to 200g, and the frequency range is from DC to 1000Hz. The advantages of these
sensors include their high accuracy, a change in broad measurement, insensitivity to
temperature change and their high accuracy. The disadvantage of this sensor is its high cost.
Fig. 3.35 Force Balance Sensor
MEMS Sensor
Solid state MEMS are small sensors as they consist of movable proof mass plates that
are attached to a reference frame through a mechanical suspension system. This is a technique
of combining mechanical and electrical components together on a chip to generate a system of
miniature dimensions. Small means that the dimensions are less than the thickness of human
hair. MEMS sensors are key components in many medical, industrial, aerospace, consumer
and automotive applications. These sensors are used in anything from smart phones, gaming,
medical tests and satellites.
Fig. 3.36 MEMS Sensor
Fluid filled Sensor:

These Sensors can either be capacitive or electrolytic.
Electrolytic Sensor
The electrolytic sensor is used to measure an angle and the angle may be expressed in
degrees, arc minutes, or arc seconds. Electrolytic sensors produce extremely accurate pitch
measurements in many applications. These sensors easily maintain their high accuracy and
small size. These sensors function by using cavity filled with fluid or a glass. The fluid
performs between a common positive and negative electrode. When the electrolytic sensor is
levelled, both the positive and negative electrodes get consistently submerged within the fluid
and produce a balanced signal output. When the sensor is rotated, an imbalance is created
between the two electrodes. So, the imbalance of any one of the electrodes is proportional to
the angle of the rotation.
Fig. 3.37 Electrolyte Sensor

Capacitive Tilt Sensors

These types of sensors are designed to take non-contact measurements of inclination and tilt.
These can operate both as switches and sensors. When the geometry of the capacitor is
changed, the capacitor sensor relies on variation of capacitor. Here, the capacitive sensing is
independent of the base material. These devices consist of a suspension beams, comb drive
capacitors and central proof mass. When a tilt occurs, the central mass moves towards one of
the combs so the capacitance increases at one side and decreases at the other side. The main
advantage of the capacitive sensor is its performance ratio and cost-effectiveness, whereas a
limited response is the main disadvantage of this sensor.
Fig. 3.38 Capacitive tilt sensor

Two Mark Q&A

1. State the principle of resistive transducers.
Transducers based on the principle of variation in resistance are generally
termed as resistance transducers. The resistance of a length of a metallic wire is given by
Here, the resistance is a function of , and ,i.e., ( , ). Hence if is any

change in any one of the above quantities, the resistance of the wire gets changed.
Further, this change in resistance can be converted into a change in electrical voltage.
2. What are the factors that affect the resistance of the material?
a. Length
b. Cross – sectional area
c. Resistivity
d. Temperature
e. Composition
3. Define gauge factor of strain gauge. How does it vary?

The gauge factors defined as the unit resistance change per unit strain. The
gauge factor is dependent on three factors:
(i) Resistance change due to length change
(ii) Resistance change due to area change
(iii) Resistance change to piezo – resistance change
4. Distinguish between bounded and unbounded type strain gauge.

The essential difference is that the bonded strain gauges are bonded on to the
specimen whose strain is being measured whereas the unbonded strain gauges are not
bonded on to the specimen. As the bonded strain gauges are well bonded on to the
specimen, the entire strain being experienced by the specimen is transferred to the strain
gauge. However, the bonded strain gauges are affected by temperature changes and also
due to transverse strains.
5. List the materials used for manufacturing of strain gauge.

a. Nichrome,
b. Isoelastic,
c. Manganin,
d. Iridium – Platinum,
6. Give the significance of rosettes.

A strain gauges rosette is a term for an arrangement of two or more strain
gauges that are positioned closely to measure strains along different directions of the
component under evaluation. Single strain gauges can only measure strain effectively in
one direction, so the use of multiple strain gauges enables more measurements to be
taken, providing a more precise evaluation of strain on the surface being measured.
7. State the concept behind the pneumatic load cell.

There is a chamber used in the pneumatic load cell, and the air under pressure is
supplied to it. At the top of the chamber, a diaphragm is placed and a nozzle is placed at
the bottom of the chamber. There is a load member placed on the diaphragm on which
force is applied. Due to the application of force on the load member, the diaphragm
bends and the gap between the diaphragm and the nozzle decreases. It further increases
pressure P2 inside the chamber. This increase in pressure produces a force tending to a
change the diaphragm back to its original position. For a certain force F, the pneumatic
load cell attains equilibrium and the pressure is displayed in the pressure indicator which
is a measure of force F. up to 20kN of force can be measured by using pneumatic load
cells.
8. How will you do force measurement in hydraulic load cell?

Hydraulic load cell is filled with a fluid which is normally oil, and it has a pre
load pressure. A diaphragm is used as a pressure sensor to detect the pressure
corresponding to the applied force. When a certain amount of force is applied on to the
load member, there will be increase in the fluid pressure, which is detected by the
diaphragm. The detected pressure is displayed on a pressure indicator.
9. Mention various types of elastic members in load cell?

a. Cantilever Type elastic member
b. Ring type elastic member
c. Column type elastic member
d.
10. How a gyroscope load cell does work?
A gyroscopic load cell consists of a dynamically balanced rotor on a spindle
mounted in the inner frame of the structure. The measuring structure consists of three
axes of rotational freedom mutually at right angles. The axis of origin is on the centre of
gravity of the rotor. The force to be measured is applied to the measurement structure
through lower swivel and a couple is developed on the inner frame. The time taken for
the outer frame for the completion of one rotation is then measured which is a measure of
applied force.
11. Mention various applications of hall effect transducer.

(i) Hall effect transducer for flux measurement
(ii) Hall effect transducer for displacement measurement
(iii) Hall effect transducer for current measurement
(iv) Hall effect transducer for power measurement
12. What are the drawbacks of hall flux meter?

a. Hall element is sensitive to temperature variations.
b. Hall coefficient changes from element to element and hence it needs individual
calibration.
13. State the basic principle of hall effect.

The hall effect is exhibited by certain materials like Arsenic, Bismuth, Iron
Silicon, etc. when any of these hall materials I subjected to a magnetic field in one
direction, an electric current I another direction which is perpendicular to the direction of
the magnetic flux is produced, then an electric potential is generated in a third direction
which is mutually perpendicular to the other two directions.
14. What is magnetostrictive effect?

Certain ferromagnetic materials like Iron, Nickel, Permalloy, etc. change their
magnetic permeability when they are subjected to mechanical stress. This effect is known
as magnetostrictive effect. The amount of change in permeability is based on the
materials, the type of stress and the magnetic flux density in the sample.
15. Draw the permeability curve for different magnetostrictive materials.

16. How does a magnetostrictive accelerometer work?

There is a mass of the core over which an additional seismic mass made up of
brass is placed. A thin flexible diaphragm is also located on the top of the seismic mass.
when the mass of the core along with the seismic is subjected to acceleration, there will
be change in permeability of the core which will further induce an output signal Vo in the
output coil. It is then integrated with respect to time.
17. A strain gauge has a resistance of 100Ω unstrained and gauge factor is -15. What is the
resistance value, if the value is 1.5%.
Gauge Factor (GF) =
= -15 X 100 X 1.5/100

= -22.5Ω
18. List the advantages of magnetic sensors.

a. High accuracy
b. High resolution
c. Wear free
d. High sensitivity
e. Reliability
19. Compare gyroscope and inclinometers.

The inclinometer is known as the induction bias Angle, and only the data feedback is not
feedback.
The gyroscope is measuring angular velocity, sensing action variables, and then
controlling the steering gear to repair action commands.
20. State the basic principle of compass.

A Magnetic compass is a critical piece of marine navigational equipment.
Simply put, a magnetised needle, suspended freely, points North because of the forces
caused by the Earth’s magnetic field. Once North is known, the other directions are
easily found. The ship magnetic compass is usually housed on the ‘monkey island’ above
the navigating bridge and reflected into the bridge by means of a periscope like device,
so a helmsman can easily read the compass when he is steering the ship.

1. How a strain gauge does work? Describe about the functioning of its different types.
2. Explain in detail about the measurement of strain using several of rosettes.
3. Briefly explain the various strain gauge circuits involved in instrumentation.
4. Consider a nickel based resistance thermometer which has resistance of 125Ω at 20 °C.
if the diameter of the wire is 0.5 mm, what would be the length of the wire? Also
determine the value of resistance at – 50 °C. The resistivity of the nickel wire is 0.8 mΩ-
m and sensitivity is 0.2 Ω/°C. What would be the length of the nickel wire, if its
diameter is chosen as 3 mm? Also determine the value of resistance at 150 °C
5. i. How do you measure force using magnetostrictive force transducer? Explain.
ii Describe the measurement of acceleration using magnetostrictive accelerometer

. with neat sketch.
6. Explain the following terms with neat diagram

1. Compass
2. Gyroscope
3. Inclinometer
7. Briefly explain about the principle of operation of hydraulic load cell and pneumatic
load cell with their neat sketches.
8. i. Describe hall effect and obtain the expression of its output voltage.
ii. With a neat diagram, describe any two applications of hall effect transducer.
Unit – IV
Optical, Pressure & Temperature Sensors
4.1 Introduction
An optical sensor converts light rays into electronic signals. It measures the physical
quantity of light and then translates it into a form that is readable by an instrument. An optical
sensor is generally part of a larger system that integrates a source of light, a measuring device
and the optical sensor. This is often connected to an electrical trigger. The trigger reacts to a
change in the signal within the light sensor. An optical sensor can measure the changes from
one or several light beams. When a change occurs, the light sensor operates as a photoelectric
trigger and therefore either increases or decreases the electrical output. An optical switch
enables signals in optical fibres or integrated optical circuits to be switched selectively from
one circuit to another. An optical switch can operate by mechanical means or by electro-optic
effects, magneto-optic effects as well as by other methods. Optical switches are optoelectronic
devices which can be integrated with integrated or discrete microelectronic circuits.
A pressure sensor works by converting pressure into an analogue electrical signal.

The demand for pressure measuring instruments increased during the steam age. When
pressure sensing technologies were first manufactured they were mechanical and used
Bourdon tube gauges to move a needle and give a visual indication of pressure. Nowadays we
measure pressure electronically using pressure transducers and pressure switches. Pressure
transducers have a sensing element of constant area and respond to force applied to this area
by fluid pressure. The force applied will deflect the diaphragm inside the pressure transducer.
The deflection of the internal diaphragm is measured and converted into an electrical output.
This allows the pressure to be monitored by microprocessors, programmable controllers and
computers along with similar electronic instruments. Most Pressure transducers are designed
to produce linear output with applied pressure. Pressure sensors are used in a range of
industries, including the automotive industry, Biomedical Instrumentation, aviation and the
marine industry
A temperature sensor is an electronic device that measures the temperature of its

environment and converts the input data into electronic data to record, monitor, or signal
temperature changes. There are many different types of temperature sensors. Some temperature
sensors require direct contact with the physical object that is being monitored (contact
temperature sensors), while others indirectly measure the temperature of an object (non-contact
temperature sensors).
4.2 Unit 4: Optical, Pressure and Temperature Sensors
4.2 Optical sensors

An optical sensor converts light rays into an electronic signal. The purpose of an
optical sensor is to measure a physical quantity of light and, depending on the type of sensor,
then translates it into a form that is readable by an integrated measuring device.
Optical Sensors are used for contact-less detection, counting or positioning of parts. Optical
sensors can be either internal or external. External sensors gather and transmit a required
quantity of light, while internal sensors are most often used to measure the bends and other
small changes in direction. A large number of different optical sensors have been developed
using a variety of techniques. These sensors include
a. Photo-Emissive cells
b. Semiconductor photo-electric cells
4.2.1 Photo-Emissive Cells

They are basically of two types of photo-emissive cells, (i) Vacuum type and (ii) gas
field type.
Vacuum Type Emissive Cell

The vacuum photoelectric cell consists of a curved sheet of thin metal with its
concave surface coated with a photo emissive material, which forms the cathode, and a rod
anode mounted at the centre of curvature of the cathode. The whole assembly is mounted in
an evacuated glass envelope as shown in Figure 4.1. As explained above, the cathode is coated
with emissive materials. These materials emit electrons when light radiation strikes them. The
emitted electrons from the cathode are collected by a positive electrode (anode) forming an
electric current. simple circuit for a photo emissive (or photo electric cell is shown in Figure
4.2. This circuit may be used for measurement of luminous flux or luminous intensity.
Fig. 4.1 Photo-Emissive cell Fig. 4.2. Photo-Emissive cell circuit

The current through the tube depends upon the following (i) intensity of light (ii)
colour of light or its wavelength and (iii) voltage applied between cathode and anode. Figure
4.3 shows the current v/s voltage characteristics of a typical highly evacuated photo tube. A
study of these curves shows that for a constant impressed voltage, the current is proportional
to the luminous flux or intensity provided light of same wavelength is used. The micro-
ammeter may be directly calibrated in terms of luminous flux or intensity. Else a resistor R is
connected in the circuit and the voltage, E0, across the resistor is a measure of luminous flux or
intensity. The output voltage, Eo, may be amplified to drive the subsequent stages of the
measurement system.
Fig. 4.3 VI characteristics of a Photo-Emissive cell.
The photoelectric sensitivity is defined as :
Where , = photoelectric current and = luminous flux. In case luminous flux, is

expressed in terms of watt, the photoelectric sensitivity is called Radiant Sensitivity and is
expressed as A/W. On the other hand if the luminous flux is expressed in terms of lumen, the
photoelectric sensitivity is called Luminous Sensitivity and is expressed as A/lm. The
sensitivity of photoelectric cells is moderate, the luminous sensitivity lying between 10 to 100
µA/lm and the radiant sensitivity lying between 0.002 to 0.1 µA/µW at peak spectral
sensitivity.
The major advantage of photoelectric cells is that they are stable and do not change
their characteristics over long periods of time provided they are operated at low voltages and
are protected against excessive light. The disadvantage of photo emissive cells is that current
through the highly evacuated tubes is very small (of the order µA) which lends low sensitivity.
The sensitivity of photoelectric transducers can be increased by using : (i) gas filled tubes and
(ii) photomultipliers.
a. Gas Filled Photo-emissive Tubes.

The sensitivity of a photo-electric cell can be increased amplifying the number of
electrons produced at the cathode by a gas discharge. There is no difference in construction
between a vacuum photo-electric cell and a gas filled cell except the latter contains an inert fps
at a pressure of 1 mm of Hg introduced into the envelop.
The emitted electrons arc accelerated by the electric field and cause ionization and
thus further electrons ire produced by collusion. This process may provide a gain over the
response of a vacuum cell of about 5 to 10. Voltage-current characteristics of a gas-filled
photoelectric cell are shown in Fig. 4.4. If the applied voltage exceeds a critical value
(depending on tube geometry, gas filling and illumination), a glow discharge may set in which
can damage the cathode. A resistor should always be connected in series with the gas-filled
cell to limit the current in case of an accidental overvoltage. The luminous sensitivity lies
between 40 to 150 µA/lm and the radiant sensitivity lies between 0.01 to 0.15 µA/µW. Gas-
filled photo-electric cells are not as stable as vacuum cells and their characteristics are non-
linear. Also they exhibit a time lag in response to modulated or chopped light at frequencies
about 10 kHz.
Fig. 4.4 Characteristics of a gas filled photoemissive cell.
b. Photomultiplier Tubes.
Extremely low levels of luminous intensity can be measured or detected by means of
a photomultiplier tubes which utilize many successive stages of secondary emission to boost
up the output current from its initial very low value A photomultiplier tube is shown in Fig.
4.5. when light strikes a photosensitive cathode, it emits electrons. The liberated electrons are
accelerated by a voltage and arc focused on to the next electrode called Dyanode. These
electrons striking upon the dyanode cause the emission of secondary electrons.
Fig. 4.5. Photomultiplier tube.
The dynodes are so shaped and placed that the electrons always move in the proper
direction and are accelerated sufficiently between dynodes to cause secondary emission at
each surface. The process done at dynode is repeated at subsequent dynodes. The final
electrode in the series is an anode which collects all the electrons. The electrode A is at a
positive potential and the anode current is measured. Let the number of electrons formed
by secondary emission for each primary electron be g. Therefore, the gain of each stage is g. If
there are 'n' dynodes, the total amplification is
The value of g varies with the voltage between successive dynodes and with the
surface composition and the geometry of dynodes from a value of 0.5 to 10. The value of stages
of commercial multipliers is between 9 and 14. The gain A0 is between 105 and 107. The
sensitivity of a photomultiplier may be as high as 20 A/m when compared with 100 µA/lm for
photoelectric cells. The general comments about the photoemissive transducers are :
(i) Their disadvantage is that they are large sized and expensive and need voltage
between 300 V to 2500 V for their operation.
(ii) Their advantages are that they have a high frequency response and a high
sensitivity. Their spectral response can be varied from 100 nm to 1000 nm by
changing the cathode material. Generally these transducers are not used in
general purpose electronic systems.
4.2.2Semiconductor Photoelectric cells

Figure 4.6 illustrates the general scheme of semiconductor photoelectric transducers.
Optical radiation X-rays and certain corpuscular radiations fall on a semiconductor
photoelectric transducer and are absorbed. The process of energy absorption produces
movable charges or a change of mobility of the charge carriers in the semiconductors thereby
producing one of the following effects depending upon the type of transducer used:
(i) Change in resistance, ΔR,

(ii) Change in current output, ΔI, and
(iii) Change in voltage output, ΔV.
Fig. 4.6 Schematic diagram of a semiconductor photoelectric transducer
a. Photoconductive cells.
Electric conduction in semiconductor materials occurs when free charge carriers, e.g.
electrons, are available in the material when an electric field is applied. In certain
semiconductors, light energy falling on them is of the correct order of magnitude to release
charge carriers which increase flow of current produced by an applied voltage. The increase of
current with increase in light intensity with the applied voltage remaining constant means that
the resistance of semiconductors decreases with increase in light intensity. Therefore, these
semiconductors are called photoconductive cells or Photo Resistors or sometimes Light
Dependent Resistors (LDR), since incident light effectively varies their resistance.
Fig. 4.7 Photoconductive cell
The two most commonly used photoconductive semiconductor materials are

cadmium sulphide (CdS) with a hand gap of 2.42 cV and cadmium selenide (Cd Sc) with a
hand gap of 1.74 eV. On account of the large energy hands, both the materials have a very high
resistivity at ambient temperature which gives a very high value of resistance for practical
purposes. The photoconductive cells use a special type of construction which minimizes
resistance while providing maximum surface.
Photoconductive cells are made by chemically sintering the required powder (Cd S)
or (Cd Se) into tablets of the required shape, and enclosing them in a protective envelope of
glass or plastic. Electrodes arc deposited on the tablet surface and are made of materials which
give an ohmic contact, but with low resistance compared with that of the photoconductor.
Gold is typically used. The electrodes are usually inter-digital i.e. in the form of interlocked
fingers or combs as shown in Fig. 4.7. The characteristics of photoconductive cells vary
considerably depending upon the type of materials used. These characteristics are given in
Table 4.1.
Photoconductor Time Constant Spectral band

CdS 100 ms 0.47 – 0.71 µm
CdSe 10 ms 0.6 – 0.77 µm
PbS 400 µs 1 – 3 µm
PbSe 10 µs 1.5 – 4 µm
Table 4.1 Characteristics of Photoconductive Cells
When the cell is kept in darkness, its resistance is called dark resistance. The dark
resistance may be as high as 10 x 1012 Q. If the cell is illuminated its resistance decreases. The
resistance depends on the physical character of the photoconductive layer as well as on the
dimensions of the cell and its geometric configuration. The current depends upon the d.c.
voltage applied. The current is of the order of mA. The spectral response characteristics of two
commercial cells are shown in Fig. 4.8. There is almost no response to radiation of a
wavelength shorter than 300 nm. Cadmium sulphide cells have a peak response in the green
part of the spectrum at 510 nm and can be used in the near infra-red region up to about 750
nm. The maximum response of cadmium sulpho selenide is in the yellow orange at 615 nm.
This can be used in the infra-red region up to about 1000 nm. The photoconductor device
described above is also called a bulk photoconductor. As described earlier, the photoconductor
has a very high resistance at very low illumination levels, which is of the order of Ma The
higher the intensity of light, the lower is the resistance. The resistance drops to a few kil when
exposed to light. When using a photoresistor for a particular application it is important to
select the proper dark resistance as well as the suitable sensitivity. The sensitivity of photo
resistive transducer is defined as :
where ΔR = change in resistance and ΔH = change in irradiation. The spectral

response of the sensor must match that of the light source. A photoconductor has a relatively
large sensitive area. A small change in light intensity causes a large change in resistance. It is
common for a photoconductive element to exhibit a resistance change of 1000 : 1 for a dark to
light irradiance change of 5 x 10-3 W/m2 to 50 W/m2.
Fig. 4.8 Spectral response characteristics of photoconductive cells
The photoconductive cell suffers from a major disadvantage that temperature

changes cause substantial resistance changes for a particular light intensity. Therefore, this
type, of photoconductor is unsuitable for analog applications.
b. Photodiodes
A special type of PN junction device that generates current when exposed to light is
known as Photodiode. It is also known as photodetector or photosensor. It operates in reverse
biased mode and converts light energy into electrical energy. The figure below shows the
symbolic representation and construction details of a photodiode:
Fig. 4.9 Symbol and construction details of Photodiode
It works on the principle of Photoelectric effect. The operating principle of the

photodiode is such that when the junction of this two-terminal semiconductor device is
illuminated then the electric current starts flowing through it. Only minority current flows
through the device when the certain reverse potential is applied to it.
The PN junction of the device placed inside a glass material. This is done to order to
allow the light energy to pass through it. As only the junction is exposed to radiation, thus, the
other portion of the glass material is painted black or is metallised. The overall unit is of very
small dimension nearly about 2.5 mm. It is noteworthy that the current flowing through the
device is in micro-ampere and is measured through an ammeter. Photodiode basically
operates in two modes:
 Photovoltaic mode: It is also known as zero-bias mode because no external
reverse potential is provided to the device. However, the flow of minority carrier
will take place when the device is exposed to light.
 Photoconductive mode: When a certain reverse potential is applied to the device
then it behaves as a photoconductive device. Here, an increase in depletion
width is seen with the corresponding change in reverse voltage.
In the photodiode, a very small reverse current flows through the device that is
termed as dark current. It is called so because this current is totally the result of the flow of
minority carriers and is thus flows when the device is not exposed to radiation.
Fig. 4.10 Working principle of photodiode
The electrons present in the p side and holes present in n side are the minority
carriers. When a certain reverse-biased voltage is applied then minority carrier, holes from n-
side experiences repulsive force from the positive potential of the battery. Similarly, the
electrons present in the p side experience repulsion from the negative potential of the battery.
Due to this movement electron and hole recombine at the junction resultantly generating
depletion region at the junction. Due to this movement, a very small reverse current flows
through the device known as dark current. The combination of electron and hole at the
junction generates neutral atom at the depletion. Due to which any further flow of current is
restricted. Now, the junction of the device is illuminated with light. As the light falls on the
surface of the junction, then the temperature of the junction gets increased. This causes the
electron and hole to get separated from each other. At the two gets separated then electrons
from n side gets attracted towards the positive potential of the battery.
Similarly, holes present in the p side get attracted to the negative potential of the
battery. This movement then generates high reverse current through the device. With the rise
in the light intensity, more charge carriers are generated and flow through the device. Thereby,
producing a large electric current through the device. This current is then used to drive other
circuits of the system. So, we can say the intensity of light energy is directly proportional to the
current through the device. Only positive biased potential can put the device in no current
condition in case of the photodiode. The figure 4.11 shows the VI characteristic curve and
Current Versus illumination curve of a photodiode:
Fig. 4.11 (a) VI Characteristics curve (b) Current Vs Illumination curve
Here, the vertical line represents the reverse current flowing through the device and
the horizontal line represents the reverse-biased potential. The first curve represents the dark
current that generates due to minority carriers in the absence of light. As we can see in the
above figure that all the curve shows almost equal spacing in between them. This is so because
current proportionally increases with the luminous flux. The figure below shows the curve for
current versus illumination: It is noteworthy here that, the reverse current does not show a
significant increase with the increase in the reverse potential.
Advantages of Photodiode
 It shows a quick response when exposed to light.
 Photodiode offers high operational speed.
 It provides a linear response.
 It is a low-cost device.
Disadvantages of Photodiode
 It is a temperature-dependent device. And shows poor temperature stability.
 When low illumination is provided, then amplification is necessary.
Applications of Photodiode
1. Photodiodes majorly find its use in counters and switching circuits.
2. Photodiodes are extensively used in an optical communication system.
3. Logic circuits and encoders also make use of photodiode.
4. It is widely used in burglar alarm systems. In such alarm systems, until exposure
to radiation is not interrupted, the current flows. As the light energy fails to fall
on the device, it sounds the alarm.
4.3 Pressure sensor

Pressure sensors are instruments or devices that translate the magnitude of the
physical pressure that is being exerted on the sensor into an output signal that can be used to
establish a quantitative value for the pressure. There are many different types of pressure
sensors available, which function similarly but rely on different underlying technologies to
make the translation between pressure and an output signal. One difference to note is that
pressure sensors are different from pressure gauges. Pressure gauges by their design provide a
direct output reading of a pressure value referred to as gauge pressure. This can be in the form
of an analog (mechanical) display using a needle and graduated scale, or via a direct digital
display of the pressure reading. Pressure sensors, on the other hand, do not directly provide a
readable output of pressure, but instead generate an output signal value which is proportional
to the pressure reading, but which first needs to be conditioned and processed to convert the
output signal level to a calibrated pressure reading.
There are several common terms associated with pressure measurement devices that
are often used interchangeably. Those terms are pressure sensors, pressure transducers, and
pressure transmitters. Manufacturers and suppliers of these devices may use one or more of
these terms to describe their product offerings. Generally, the primary difference between
these terms has to do with the electrical output signal that is generated and the output
interface of the device. Be aware that there is variation among suppliers with respect to how
their devices are classified. One way to think about the distinction between pressure sensors
vs. pressure transducers and pressure transmitters is that pressure sensors do not have
electronics built into them to provide signal conditioning and an amplified output, unlike the
other two.
Pressure sensors, while used as an umbrella term for all these three types of devices,
typically produce a millivolt output signal. The relatively low voltage output coupled with the
resistance losses that occur with wiring implies that wire lengths must be kept short, which
limits the utilization of the devices to around 10-20 feet from the electronics before too great of
a signal loss is experienced. The output signal will be proportional to the supply voltage used
with the sensor. So, for example, a sensor that generates a 10mV/V output used with a 5VDC
supply will produce an output signal that ranges from 0-50mV in magnitude.
Millivolt outputs allow the engineer to design signal conditioning as needed for the
application and helps to reduce both the cost and the package size of the sensor. The
limitations of these devices are that regulated power supplies must be used as the full-scale
output is proportional to the supply voltage. Also, the low output signal means that these
devices are less suitable for use in electrically noisy environments. An illustration of a half-
bridge circuit with millivolt output is shown in Figure 4.12 below.
Fig. 4.12 A strain gauge pressure sensor using a Wheatstone bridge
Pressure transducers generate a higher level of voltage or frequency output by having

additional signal amplification capabilities built-in to boost the magnitude of the output signal
to say 5V or 10V, and the frequency output to 1-6 kHz. The increased signal strength allows for
the use of pressure transducers at a greater range from the electronics, say 20 feet. These
devices use a higher supply voltage level such as 8-28VDC. Higher output voltages reduce
current consumption, making pressure transducers useable in applications where the
equipment is battery powered.
While pressure sensors and pressure transducers generate a voltage output, pressure
transmitters produce a low impedance current output, typically used as analog 4-20mA signals
in a 2-wire or 4-wire configuration. Pressure transmitters feature good electrical noise
immunity (EMI/RFI) and are therefore suitable for applications where it is necessary to
transmit signals over longer distances. These devices do not require regulated power supplies,
but the higher current output and power consumption make them unsuitable for applications
with battery-powered equipment when the devices are operated at or near full pressure. For
simplicity in the balance of this article, we shall use the umbrella term pressure sensors rather
than making distinct representations of pressure transducers and pressure transmitters. There
is some key terminology that relates to pressure sensors which is presented in this section.
 Gauge pressure is a measurement of pressure made relative to the ambient pressure. A

common example of this is when a tire pressure gauge is used to measure the air
pressure in an automobile tire, If the gauge reads 35 psi, that indicates that the tire’s
pressure is 35 psi above the local ambient pressure.
 Absolute pressure is a measurement made relative to a pure vacuum condition, such as

the vacuum of space. This type of pressure measurement is important in aerospace
engineering applications as the air pressure changes with altitude.
 Differential pressure is a measurement of the pressure difference between two pressure
values, therefore measuring by how much the two differ from each other, not their
magnitude relative to atmospheric pressure or to another reference pressure.
 Vacuum pressure is the pressure measurement of values that are in a negative direction
with respect to atmospheric pressure.
Figure 4.12 below illustrates these terms on a diagram showing the relative
relationships between each.
Fig. 4.12 Relationships of the different pressure measurements
a. Pressure measurement technologies

There are six primary pressure sensor technologies used to sense pressure. These are:
 Potentiometric pressure sensors
 Inductive pressure sensors
 Capacitive pressure sensors
 Piezoelectric pressure sensors
 Strain gauge pressure sensors
 Variable reluctance pressure sensors
Potentiometric pressure sensors use a Bourdon tube, capsule, or bellows which

drives a wiper arm, providing relatively course pressure measurements.
Inductive pressure sensors use a linear variable differential transformer (LVDT) to vary the
degree of inductive coupling that occurs between the primary and secondary coils of the
transformer.
Capacitive pressure sensors use a diaphragm that is deflected by the applied pressure which
results in a change in the capacitance value, which can then be calibrated to provide a pressure
reading.
Piezoelectric pressure sensors rely on the ability of materials such as ceramic or metalized
quartz to generate an electrical potential when the material is subjected to mechanical stress.
Strain gauge pressure sensors rely on a measurement of the change in resistance that occurs in
a material such as silicon when it is subjected to mechanical stress, known as the piezoresistive
effect.
Variable reluctance pressure sensors make use of a diaphragm that is contained in a magnetic
circuit. When pressure is applied to the sensor, the diaphragm deflection causes a change in
the reluctance of the circuit and that change can be measured and used as an indicator of the
applied pressure.
b. Types of Pressure Sensors

Using a pressure sensor, pressure measurements can be taken to determine a range of
different values and different types of pressure depending on whether the pressure
measurement is performed relative to atmosphere, vacuum conditions, or other pressure
reference levels. Pressure sensors are instruments that can be designed and configured to
detect pressure across these variables. Absolute pressure sensors are intended to measure
pressure relative to a vacuum and they are designed with a reference vacuum enclosed within
the sensor itself. These sensors can also measure atmospheric pressure. Similarly, a gauge
pressure sensor detects values relative to atmospheric pressure, and part of the device is
usually exposed to ambient conditions. This device may be employed for blood pressure
measurements. An important aspect of industrial pressure detection processes involves
comparisons between multiple pressure levels. Differential pressure sensors are used for these
applications, which can be challenging due to the presence of at least two different pressures
on a single mechanical structure. Differential pressure sensors are relatively complex in design
because they are often needed to measure minute pressure differentials across larger static
pressures. The principles of transduction and mechanical pressure sensing are common to
most standard pressure sensing units, regardless of their categorization as differential,
absolute, or gauge pressure instruments. Below we look at the most common type of pressure
sensors.
Aneroid Barometer Sensors

An aneroid barometer device is composed of a hollow metal casing that has flexible
surfaces on its top and bottom. What is the barometric pressure sensor working principle?
Atmospheric pressure changes cause this metal casing to change shape, with mechanical levers
augmenting the deformation in order to provide more noticeable results. The level of
deformation can also be enhanced by manufacturing the sensor in a bellows design. The levers
are usually attached to a pointer dial that translates pressurized deformation into scaled
measurements or to a barograph that records pressure change over time. Aneroid barometer
sensors are compact and durable, employing no liquid in their operations. However, the mass
of the pressure sensing elements limits the device’s response rate, making it less effective for
dynamic pressure sensing projects.
Manometer Sensors
A manometer is a fluid pressure sensor that provides a relatively simple design
structure and an accuracy level greater than that afforded by most aneroid barometers. It takes
measurements by recording the effect of pressure on a column of liquid. The most common
form of the manometer is the U-shaped model in which pressure is applied to one side of a
tube, displacing liquid and causing a drop in fluid level at one end and a correlating rise at the
other. The pressure level is indicated by the difference in height between the two ends of the
tube, and measurement is taken according to a scale built into the device. The precision of
reading can be increased by tilting one of the manometer’s legs. A fluid reservoir can also be
attached to render the height decreases in one of the legs insignificant. Manometers can be
effective as gauge sensors if one leg of the U-shaped tube vents into the atmosphere and they
can function as differential sensors when pressure is applied to both legs. However, they are
only effective within a specific pressure range and, like aneroid barometers, have a slow
response rate that is inadequate for dynamic pressure sensing.
Bourdon Tube Pressure Sensors

Although they function according to the same essential principles as aneroid
barometers, bourdon tubes employ a helical or C-shaped sensing element instead of a hollow
capsule. One end of the bourdon tube is fixed into connection with the pressure, while the
other end is closed. Each tube has an elliptical cross-section that causes the tube to straighten
as more pressure is applied. The instrument will continue to straighten until fluid pressure is
matched by the elastic resistance of the tube. For this reason, different tube materials are
associated with different pressure ranges. A gear assembly is attached to the closed end of the
tube and moves a pointer along a graduated dial to provide readings. Bourdon tube devices
are commonly used as gauge pressure sensors and as differential sensors when two tubes are
connected to a single pointer. Generally, the helical tube is more compact and offers a more
reliable performance than the C-shaped sensing element.
Vacuum Pressure Sensors

Vacuum pressure is below atmospheric pressure levels, and it can be challenging to
detect through mechanical methods. Pirani sensors are commonly used for measurements in
the low vacuum range. These sensors rely on a heated wire with electrical resistance
correlating to temperature. When vacuum pressure increases, convection is reduced, and wire
temperature rises. Electrical resistance rises proportionally and is calibrated against pressure
in order to provide an effective measurement of the vacuum. Ion or cold cathode sensors are
commonly used for higher vacuum range applications. These instruments rely on a filament
that generates electron emissions. The electrons pass onto a grid where they may collide with
gas molecules, thereby causing them to be ionized. A charged collection device attracts the
charged ions, and the number of ions it accumulates directly corresponds to the number of
molecules within the vacuum, thus providing an accurate reading of the vacuum pressure.
Sealed Pressure Sensors

Sealed pressure sensors are used when it is desired to obtain a pressure measurement
relative to a reference value (such as atmospheric pressure at sea level), but where it is not
possible to have the sensor directly open to that reference pressure. For example, on
submersible vehicles, a sealed pressure sensor may be used to establish the depth of the
vehicle by measuring the ambient pressure and comparing it to atmospheric pressure that is
available in the sealed device.
c. Applications of Pressure Sensors

Pressure sensors find wide applications in a range of markets including medical,
general industrial, automotive, HVAC, and energy, to name a few. It is important to realize
that while these devices sense pressure, they can be used to perform other important
measurements since there is a relationship between a recorded pressure and the value of these
other parameters. Some examples of pressure sensor use are summarized below:
 In automotive brake systems, pressure sensors may be used to detect fault conditions
in hydraulic brakes that could impact their ability to function.
 Automobile engines use pressure sensors to optimize the fuel/air mixture as driving
conditions change and to monitor the oil pressure level of the operating engine.
 Pressure sensors in cars can be used to detect collisions and trigger the activation of
safety devices such as airbags.
 In medical ventilators, pressure sensors are used to monitor oxygen pressure and to
help control the mix of air and oxygen supplied to a patient.
 Hyperbaric chambers use pressure sensors to monitor and control the pressure
applied during the treatment process.
d. Types of pressure measurement devices

A number of devices can be used for measurement of pressure as explained in the
following section. The static or steady state pressure is not difficult to measure with good
accuracy. However, measurement of dynamic pressure is more intricate and presents
difficulties in measurement as these measurements are strongly influenced by the
characteristics of the fluid and the construction of measuring device. In many cases a pressure
measuring instrument that gives very accurate results for static measurements may give
highly inaccurate results in case of dynamic measurements. In industrial applications pressure
is normally measured by means of indicating gauges and recorders.
Mechanical Instruments
Pressure can be easily transduced to force by allowing it to act on a known area.
Therefore, basic methods of measuring force and pressure are essentially the same except for
the pressures in the high vacuum region. Mechanical instruments used for pressure
measurement are based on comparison with known dead weights acting on known areas or on
the deflection of elastic elements subjected to unknown pressures. Therefore these instruments
may be classified into two groups. The first group includes those instruments in which
pressure measurement is made by balancing the unknown force produced by pressure under
measurement against a known force. Instruments using this principle include manometers,
piston gauges and ring and hell type of gauges. The second group includes those instruments
which employ the balancing the unknown force through a force produced on a known area
due to stress in an elastic medium.
Electromechanical Instruments
These instruments generally employ mechanical means for detecting pressure and
electrical means for indicating or recording pressure. Electromechanical instruments arc very
well suitable for dynamic measurements as they have an excellent frequency response
characteristics.
Electronic Instruments
These pressure measuring instruments normally depend on some physical change
that can he detected and indicated or recorded through electronic means. These instruments
are used for vacuum measurements.
4.3.1 Diaphragm
The operating principle of diaphragm elements is similar to that of the bellows. The
pressure to be measured is applied to the diaphragm, causing it to deflect, the deflection being
proportional to the applied pressure. The movement of the diaphragm depends on its
thickness and diameter. The movement is small and hence a diaphragm element does not
require any springs as is the case in bellows. The diaphragm element is essentially a flexible
disc which may be either flat or corrugated as shown in Fig. 4.13.
(a) Flat Type (b) Corrugated Type

Fig. 4.13. Single diaphragm elements
For the arrangement of a flat diaphragm shown in Fig. 4.14, the maximum deflection,
, and the deflection at any radius, , are given by following expressions :
( )
( )
( )
Fig. 4.14. Deflection of Flat diaphragm
A diaphragm-has an infinite number of natural frequencies. The lowest natural

frequency of a circular diaphragm fixed at its perimeter is,
√
( )
These relationships are valid for uniform pressure loading conditions over the entire
surface of the disc and also when the deflection is smaller than half the thickness of disc.
Therefore, the response is linear only when d < 5 < 0.5 t. In order to have a linear response over
greater deflections than d > 0.5 t, the diaphragms are corrugated. A corrugated diaphragm is
most suitable for applications where a mechanical device is used for measurement of
deflection.
This is because it (corrugated diaphragm) gives a relatively greater output and if

combined with a high magnification linkage, can be used for direct operation of mechanical
indicators. In some cases, a diaphragm element may consist of a single disc; while in others,
two diaphragms are bonded together at their circumference by soldering or pressure welding
to form a capsule. A diaphragm element may consist of one capsule or two or more capsules
connected together with each capsule deflecting on the application of pressure. The total
deflection is the sum of the deflections of individual capsules. Fig. 4.15 shows a diaphragm
element consisting of three capsules. In this assembly, the individual capsule is connected
axially with the next one and is allowed to expand without any restraints. The element may be
provided with stops for over range and under range protection.
Fig. 4.15. Diaphragm element using three capsules
Two different arrangements are used for capsular elements, the convex and the
nested type as shown in Fig. 4.16. Materials used for diaphragms include phosphor bronze,
stainless steels beryllium copper, Ni-span Inconel, Monel and nickel. Non-metallic materials
are used for some applications. Buna N rubber, nylon, and Tenon are used in environments
that corrode metals.
(a)Convex Type (b) Nested Type

Fig. 4.16. Types of capsules.
The diaphragm elements find extensive use in applications where measurement of

low pressures including vacuum is involved. The ranges are 0 — 50 N/m 2 to 0 — 200 kN/m 2.
Accuracies range from ± 0.5% to ± 1.25% of full span.
4.3.2 Bellows
A metallic bellows is a series of circular parts, resembling the folds in, an accordian as
shown in Fig. 4.17. These parts are formed or joined in such a manner that they are expanded
or contracted axially by changes in pressure. The metals used in the construction of bellows
must be thin enough to be flexible, ductile enough for reasonably easy fabrication, and have a
high resistance to fatigue failure. Materials commonly used are brass, bronze, beryllium
copper, alloys of nickel and copper, steel, and monel. Most of the bellows used in pressure
gauges are seamless and are made from drawn tubing by hydraulic or other methods of rapid
forming. These methods produce uniform walls that give a higher life expectancy. Other
methods such as soldering and welding of annular sections, rolling, spinning and turning from
solid stock may also be used for manufacture of bellows.
Fig. 4.17. Bellows Element.
The displacement of bellows element is given by,
where P = Pressure ; N/m2 , b = radius of each corrugation; m, n = number of semi-circular

corrugations, t = thickness of wall; D = mean diameter; m, E = Modulus of elasticity; N/m2,
v = Poisson's ratio.
Normally a Bellows has the ability to move over a greater distance than required in a
pressure application and, therefore, to give it maximum life and to have better accuracy, its
movement is generally opposed by a calibrated spring so that only a part of the maximum
stroke is used. This system is shown in Fig. 4.18 and is called spring loaded bellows. Deflection
of bellows when opposed by a spring
Where is the effective area of bellows

is the stiffness constants of bellows and spring respectively,
Fig. 4.18 Spring loaded Bellows. Fig. 4.19. Measurement of pressure

with bellows
If the bellows assembly operates an electric switch or some other mechanism, we

have,
( )
where F = force required to operate switch or mechanism ; N,

= deflection required to operate switch or mechanism ; m.
There are three main configurations in which bellows elements are used and these are
for measurement of absolute, gauge and differential pressures. These configurations may be
illustrated with the help of Fig. 4.19. There are two bellows, A and B. The pressure applied to
bellows A is P1 and that to bellows B is P2. In case it is desired to measure the absolute
pressure, bellows B is evacuated and the resultant pressure, P1 is the absolute pressure. When
measurement of gauge pressure is desired, bellows B is opened up to atmosphere with
pressure P2 equal to the atmospheric pressure and the reading of the gauge is the gauge
pressure. The measurement of differential pressure P = P1 - P2 is done when pressure P1 is
applied to bellows A and pressure P2 is applied to bellows B.
The Double Bellows Differential Pressure Gauge as shown in Fig. 4.20 is designed for
low differential pressure measurement of high static pressures. The two bellows are filled with
a liquid. High pressure compresses the high pressure bellows. The liquid is transmitted
through the low pressure passage. This causes the bellows on the low pressure side to expand.
This continues till the force on the span spring equals the difference between the forces acting
on the two bellows. The rate of flow is controlled by the pulsation dampner. The span of the
element is controlled by the span spring used Output motion is transmitted by a pressure tight
torque tube. Liquids having low-coefficient of thermal expansion are used in differential
pressure gauges. The liquid may be ethylene glycol or water. A temperature compensating
bellows is placed in the bellows on the high pressure side. This bellows adjusts the capacity of
the bellows on high pressure side to change of volume of the filling liquid resulting from any
change in ambient temperature. The temperature compensator is correct for all differential
ranges and no zero correction is necessary.
Fig. 4.20 Double bellows differential pressure gauge
The advantages of bellows include their simple and rugged construction, moderate
price, their usefulness for measurement of low, medium and high pressures, and their
applicability for use in measurement of absolute, gauge and differential pressures. Bellows
elements, like many other elements, have been greatly improved over the past few years.
The reduction of drift and hysteresis allows their use in functions requiring ± 0.5% of
full span accuracy. They deliver relatively high forces and are well adapted to vacuum and
low pressure measurements. The disadvantages of bellows are that they are not suited for
dynamic measurements on account of their greater mass and longer relative movement. Also
they need temperature compensating devices to avoid errors resulting from changes in
ambient temperature.
4.4 Piezoelectric sensors

Piezoelectric transducers are used to transduce pressure or force into electric charge.
Natural phenomenon of certain dielectric materials is utilized by these transducers. Its
applications include vibration shakers, sonar systems for acoustic detection, location of
underwater objects and non-destructive equipment.
4.4.1 Piezoelectric effect

When a piezoelectric material is subjected to force or pressure, the dimensions of the
crystal are changed. Due to this, an electric potential appears across the surfaces of a crystal.
This effect is called as piezoelectric effect. This effect is reversible. If an electric potential is
applied on the surface of the crystal, it will change the dimensions of the crystal thereby
deforming it. This effect is called as an inverse piezoelectric effect. Any piezoelectric material
used to convert the mechanical force into an electrical voltage can be considered as a charge
generator and a capacitor. Figure 8.1 shows the piezoelectric crystal subjected to force and an
output voltage is developed across the surfaces of the crystal. The voltage developed is
expressed as:
The magnitude and polarity of the developed voltage are proportional to the
magnitude and direction of applied force F. The polarity of the electrical charge depends on
the direction of the applied force, and it is expressed as follows:
Fig. 4.21 Operation of piezoelectric crystal
Q=dxF
where Q is the charge in coulombs

d is the charge sensitivity of the crystal (constant)
F is the applied force in N.
The applied force on the crystal makes deformation on the crystal thickness,
where A is the area of the crystal in m2

t is the thickness of the crystal in m
E is the Young's modulus in N/m2
Substitute F in Q, we get
Due to induced charge, an output voltage is developed. It is given as:
where Cp is the capacitance between the electrodes in F.
( )
is the voltage sensitivity of the crystal
P is the applied pressure or stress
Electric field intensity is expressed as ⁄
The charge sensitivity of the crystal is written as

4.4.2 Piezoelectric materials

Many of the crystallographic elements exhibit piezoelectric properties. But limited
materials are used in piezoelectric transducers. Most common materials are Quartz, Rochelle
salts, Ammonium dihydrogen phosphate, Lithium sulphate, Dipotassium tartrate, Potassium
dihydrogen phosphate and Barium titanate.
a. Quartz (SiO2)
Quartz is silicon dioxide (SiO2) which is popularly used due to its easy availability as
a natural substance. The atoms are arranged in the Quartz crystal and form a hexagon shape in
the plane of paper with its z-axis perpendicular to x-y axis. The x-axis is meant for referring
electrical and y-axis is for mechanical axis. When there is no force applied on the crystal
surface, all charges are balanced. But when a certain amount of force is applied along x-axis,
there will be unbalancing, due to which an electrical charge is generated. It is said to be
transverse effect. When force is applied along z-axis, no effects are occurred due to the
symmetry along the z-axis. Among the piezoelectric materials mentioned earlier, mechanically
and thermally most stable piezoelectric material is Quartz. It provides small internal electric
losses, higher volume resistivity, high tensile strength and high mechanical stability. The
operating temperature of Quartz crystal is up to 550° C. The volume resistivity of Quartz
crystals is 1014 Ω-cm. Its applications include thickness expansion and transverse compression
operation.
b. Rochelle Salt ( )
Rochelle salt is stable at temperature between 35 and 85% relative humidity. These
crystals will deliquesce at higher range of humidity levels, and they will dehydrate at lower
range of humidity levels. They will disintegrate at a temperature of about 55 °C. For the
compatibility of humidity effects, it is recommended to use the crystal with wax coating. With
respect to the direction of applied force, the sensitivity of the Rochelle salt will vary. In y-
direction, the crystal has variable dielectric constants and its resonant frequency of the crystal
changes with respect to temperature. In x-direction, the crystal is primarily used for face shear
motion and longitudinal expansion. The mechanical strength of the Rochelle salts is lower than
that of Quartz crystals. Its volume resistivity is of the order of 10 12 Ω-cm or high.
c. Ammonium Dihydrogen Phosphate ( )

Thermal stability of Ammonium Dihydrogen Phosphate (ADP) is higher than that of
Rochelle salts. Up to 180 °C temperature, it is found stable. But it is preferably used as
thermally stable crystal at 125 °C. These crystals are stable at temperature between 0 and 44%
relative humidity. The volume resistivity is lower than that of the other crystals such as 10 10 Ω-
cm or less with increasing temperature and impurities.
d. Barium Titanate Ceramics ( )

Barium Titanate crystal is of ferro electric type. If the crystal is exposed to an electric
field, it becomes polarized. It maintains its polarization even though the electric field is
removed. If the electric field is applied in reverse direction, it causes a reverse polarization.
This process follows a hysteresis curve. By the application of polarized electric field, these
crystals become piezoelectric.
4.4.3 Piezoelectric coefficients

Piezoelectric coefficients are used to describe the properties of piezoelectric crystal.
Double subscripted notation is used for the representation of piezoelectric constants. From the
notation, first subscript represents the direction of the electrical effect, and the second
subscript is used to represent the mechanical effect using the axis numbering system given in
Figure 4.22. There are some popular piezoelectric coefficients listed as follows:
a. d-coefficient
b. g-coefficient
c. h-coefficient
d. Coupling coefficient k
Fig. 4.22 Axis numbering system in piezoelectric crystal
a. d-coefficient
The most fundamental coefficient is d-coefficient which is related to the charge
generation from the piezoelectric crystal. A certain amount of force is applied to the crystal
axes, and the direction is represented by double subscripts derived from the tensor
representation and the change in strain in the crystal. Due to the symmetry of the crystal, the
following are the significant coefficients:
d11 = 2.26 x1011 coulombs/kg

= 2.30 x 10-12 coulombs/newton
d14 = - 0.655 x10-11 coulombs/kg
= - 0.670 x 10-12 coulombs/newton
Also d12 = - d11
d25 = - d14
d26 = - 2d11
Under short circuit condition, the d-coefficient gives the charge output per unit force
input or charge density per pressure. It does not have second order effect on the crystal
deformation, if the charge is being led away on developing. In case of direct piezoelectric
effect, the coefficient can be measured in mks system as:
⁄
( )
⁄
In case of indirect piezoelectric effect, the coefficient can be measured as the
deformation obtained per unit applied voltage to the crystal under no load conditions, and it is
expressed as:
⁄
( )
⁄
The piezoelectric sensitivity of the crystal varies with respect to the temperature. For
a temperature range of (20 — 200)°C, the coefficient d11 is about —0.016%/°C.
b. g-Coefficient
The g-coefficient can be obtained by means of dividing the d-coefficient by the
absolute dielectric constant. This coefficient represents the generated emf gradient per unit
pressure input. If additional shunt capacity is not considered, it is also a most convenient
coefficient for getting the output voltage of the piezoelectric transducer. It can be represented
as:
c. h-Coefficient
The d- and h-coefficients are related to the input force applied to the piezoelectric
crystal, whereas the h-coefficient is related to the deformation of the crystal due to the applied
force. It is calculated by multiplying the g-coefficient by Young's modulus suitable for crystal
orientation of the material. It is used to measure the emf gradient per unit mechanical
deformation from the crystal. It can be represented as:
d. Coupling Coefficient k
The coupling coefficient can be calculated by taking the square root of the product of
h - coefficient and d - coefficient
√( ) ( )
It is related to the square root of the ratio of the mechanical energy stored in the
crystal to the supplied electrical energy. Hence it can be used as a measure of the efficiency of
the crystal. It can be employed where the transducer is used as an electro mechanical vibrator
and in crystal type filters. The coupling coefficient refers to short circuit output conditions
without mechanical deformation. The piezoelectric materials such as Quartz and Barium
titanate have low coupling coefficients, and it may be a value of 2 for Rochelle salt.
4.4.4 Stack arrangements in piezoelectric crystal

By the application of force at any point of the surface of the piezoelectric crystal, there
will be stressed condition of the crystal and due to which charge will be generated. To get
more charge sensitivity, it is essential to use multiple stack arrangements in piezoelectric
crystal. It is the trade name of Brush Electronic Company's Product. The stack elements can be
arranged either in series or in parallel. If they are connected in series, there will be higher
output voltage for the same force, and if they are in parallel, the result will be lower output
impedance. If more than two bimorphs are connected, it is called as multimorph or piezopile.
Such series, parallel and multi stack arrangements are shown in Figure 4.23.
Fig. 4.23 Series and parallel arrangements of stack
The following are the two types of bimorphs with respect to the application of force on it.
 Bimorph benders
 Bimorph twisters
a. Bimorph Benders
In a bimorph bender, there are two transverse plates of expanding nature cemented
together in such a manner that one plate contracts and the other plate expands when an input
voltage is given. If the crystal element is freely movable, then it will bend based on the input
voltage as shown in Figure 4.24. The voltage can be transduced into an electrical signal by
using a simple supported beam or cantilever beam. The sensitivity of the bimorph
arrangement is high, and it permits larger deflection in comparison with a single crystal.
Fig. 4.24 Bimorph benders
b. Bimorph Twisters
In bimorph twisters, there are two face shear plates cemented together in order to
obtain a series connection, and hence their expanding diagonals are perpendicular. With the
application of voltage and both plates of the bimorph are movable in condition, there will be
formation of bimorph twisters. Its main advantage is high sensitivity. They are applicable in
transducing the small amount of torque. The most commonly used piezoelectric elements are
Rochelle salts operating at room temperature and Ammonium Dehydrogenate Phosphate
(ADP).
4.4.5 Applications
a. Piezoelectric pressure transducer
A piezoelectric pressure transducer commonly used in microphones is shown in
Figure 8.10. A piezoelectric crystal of bimorph bender is placed on a solid base and a force
summing member called diaphragm. The connection between the crystal and the diaphragm is
carried out by using a spindle. In order to get the output voltage, metal electrodes can be
placed on the faces of bimorph bender. Since these two plates are considered as parallel plates
of a capacitor, it can be called as a charge generator. Hence the output voltage is calculated as
Since piezoelectric effect is direction sensitive, tensile and compressive forces develop
voltage outputs of opposite polarity. Based on the magnitude and polarity of the input force,
the magnitude and polarity of the induced output voltage will vary. The piezoelectric
transducer can be able to produce output voltage of about 1 mV to 30 mV. But it is
temperature dependent.
Fig. 4.24 Piezoelectric pressure transducer
b. Piezoelectric torque transducer

Figure 4.25 shows a piezoelectric cantilever-based bimorph bender used for torque
measurement. Whenever a small amount of force is transmitted through a cantilever, there
will be generation of twisting moment. It can be detected by connecting a driving shaft directly
to the bimorph 'ender. The amount of output voltage is dependent on the amount of the
applied force.
Fig. 4.25 Piezoelectric torque transducer
c. Piezoelectric acceleration transducer

The piezoelectric acceleration transducer converts dynamic acceleration into an
electrical energy. Figure 4.26 shows the arrangement of a piezoelectric acceleration transducer.
From the arrangement, there is a proof mass mounted in direct contact with the bimorph
crystal. By doing the screw downing the cap on the main spring of hemispherical shaped,
preloading can be made. A multimorph or piezopile can be used to replace the single bimorph
assembly in order to increase the charge sensitivity as the desired level. The entire
arrangement can be mounted in a metallic body as a protective base. Guide springs shown in
the arrangement are used to minimize the transverse sensitivity. The proof mass is many times
greater than the mass of the bimorph assembly.
Fig. 4.26 Piezoelectric acceleration transducer
4.5 Tactile sensors

Normally, tactile sensors are preferred for robot-based applications for the purpose of
doing repetitive tasks such as machine loading, assembling of parts, painting of objects,
welding of objects, etc. With the tactile-perception capability, any robot can do effective
grasping of Objects. Tactile sensing means that the detection of some important features or
parameters which are affected by means of direct physical contact. For tactile sensing purpose.
it is essential to provide suitable transducers. But these transducers alone are not sufficient
enough. and it is also important to consider certain conditions through which the sensor
contacts with the external objects. It includes contact forces, position and orientation of end-
effectors. It further leads to proper tactile sensing provided that effective acquisition and
processing of tactile sensing data. Consider an object of rigid type which is displaced over the
compliant cover layer of a tactile sensing array as shown in Figure 16.8. It further makes
indentation due to physical contact which can be analysed from two views: From the
measurement of actual contact stresses or simply distribution of forces in the compliant layer.
Based on the defection strategy of the compliant layer, which is essential for geometric object
feature recognition.
Fig. 4.27 Tactile sensing

The tactile sensing consists of two major steps:

1. Forward analysis: It is related to the data acquisition from the tactile sensor. Due to
the indentation of the rigid object on the compliant cover layer of the tactile sensing
array, there will be changes in the stress or strain.
2. Inverse analysis: it deals with the recovery of the force distribution in the compliant
cover layer. Sometimes, it is related to the recovery of the shape of the indentation in
the layer.
4.4.1 Requirements for Tactile Sensing

As per the survey conducted by Harmon to determine the required specifications for
the tactile sensor, the following specifications are used for the design of tactile sensors.
1. Spatial resolution of 1 mm to 2 mm
2. Tactile sensing array size ranging from 5 x 10 to 10 x 20 points
3. Sensitivity of about 0.5 x 10-2 to 1.0 x 10-2
4. Dynamic range of 1000:1
5. Stable behaviour (with no hysteresis)
6. Sampling rate of 100 Hz to 1000 Hz
7. Monotonic response (not necessarily he linear)
8. Compliant interface
9. Rugged
10. Cheap or inexpensive
4.4.2 Resistive Transducer for Tactile Sensing

The resistive transducer for tactile sensing works on the principle of change in
resistance of the material based on the applied pressure. Figure 16.10 shows the basic
configuration of a resistive transducer for tactile sensing.
Fig. 4.28 Basic configuration of a resistive transducer for tactile sensing

From the resistive transducer, the value of every resistor cell will be varied based on
the magnitude of the applied force on the compliant cover layer. The preferable materials used
for the manufacturing of tactile sensors are conductive elastomers which are of insulating type
or silicone based rubbers. They can be converted as conductive type by means of addition of
semiconducting or conductive materials along with it Based on the applied pressure. the
resistance of the elastomers will he noticed from the deformation of the conducting material
which will alter the particle density of the conducting material. Alternatively, the resistance of
the elastomers material will he varied while it gets compressed. During which there will be an
increase in the area of physical contact between the elastomers material and an electrode, and
therefore, there will be change in the contact resistance. The development of tactile sensors
include an array of pressure transducers used to sense the pressure, and they will measure the
change in contact resistance between the resistive material and polyimide From Figure 4.28,
there is a 3 x 3 array of piezoresistive elements that can be utilized for pressure sensing and the
change in resistance has to be converted into voltage at the output section. The resistance R21
can be calculated from
where Vo is the output voltage, VCC is the bias voltage , Rf is the feedback resistance
4.4.3 Capacitive Transducer for Tactile Sensing

A capacitive transducer is used for tactile sensing from which there is a change in
capacitance due to the applied pressure. The capacitance of a parallel plate capacitor will vary
based on the separation between the plates and their area. An elastomeric separator is used in
between the parallel plates which will serve as a compliance sheath, and there will be change
in capacitance due to the applied pressure or load. The basic configuration of a capacitive
transducer for tactile sensing is shown in Figure 4.29 from which the intersection of every row
and column of the conducting strips will be considered as capacitors. Every individual
capacitance can be calculated by measuring their corresponding output voltage at the selected
row and column. To minimize the problems of cross talk and electromagnetic interference,
unconnected rows and columns are grounded.
Fig. 4.29 Basic configuration of capacitive transducer for tactile sensing

4.4.4 Piezoelectric Transducer for Tactile Sensing

It is well known that, when a piezoelectric material is subjected to stress or
deformation, emf will be produced and this is called piezoelectric effect. From the basic
configuration of piezoelectric tactile transducer, illustrated in Figure 4.30, if the generation of
emf is in the same direction of applied stress, it is said to be longitudinal piezoelectric effect. If
emf is produced in the direction perpendicular to the applied stress, it is said to be transversal
effect. In Figure 4.30, if the applied stress in direction-3, the charge generation on the A
piezoelectric crystal's surface is perpendicular to direction 3.
Fig. 4.30 Basic configuration of piezoelectric tactile transducer
The output voltage V generated across the electrodes due to applied stress a is represented
where is piezoelectric constant

is permittivity
h is thickness of the piezoelectric material
Basically, piezoelectric materials are of insulator type. The transducer, shown in

section 4.3, can be considered as a capacitor, and the output voltage across the capacitor is:
where Q is the charge generated by applied stress

C is the capacitance of the parallel plate capacitor
A is the area of the capacitor electrode
From the above equations,
By knowing the charge Q, the amount of force applied on the material can be determined.
4.4.5 Optical Transducer for Tactile Sensing

Fiber optic technology can be utilized here to develop novel tactile transducer design,
and they consist of flexible membranes with a reflecting surface as shown in Figure 4.31.
Fig. 4.31 Reflective transducer
Input light can be travelled into the sensor through a fiber optic cable. A wide cone of
light propagates out of the fiber cable and gets reflected back from the flexible membrane. It is
then collected by the second fiber. If an applied force is onto the surface of the membrane, it
shortens the distance h between the reflective portion of the membrane and fibers. With
respect to the distance h, the light gathered by the fiber optic cable varies and its plot is
illustrated in Figure 4.31. For maintaining wide dynamic range, h > hmin to h < hmin is preferred.
4.6 Temperature sensors

Temperature measurements are amongst the most common and the most important
measurements made in controlling industrial processes. The precise control of temperature is
key factor in process industry especially those which involve chemical operations.
Consequently, it is most important to make a comprehensive survey of methods of
measurement of temperature and their advantages and disadvantages. It is equally important
to have information about the operating limitations in terms of time of response, temperature
range, distance of operation and compatibility with other control elements. This will permit
one to select the best method of measurement for a particular application.
The measurement of temperature is done in many diverse fields like measuring

temperature of a household oven, of a distant planet, of a red hot blume of iron being rolled, of
parts of human body, of windings of electrical machines, of bearings of steam turbines etc.
Temperature is not measured directly like displacement, pressure or flow are but is
measured through indirect means. Change of temperature of a substance causes a variety of
effects. These effects may be physical, chemical, electrical or optical and they may be used for
the measurement of temperature through use of proper temperature sensing devices.
For example, pressure, volume, electrical resistance, expansion co-efficients, colour of

radiation etc., are all related to temperature through fundamental molecular structure. They
change with temperature and these changes can be used to, measure temperature. Calibration
may be achieved through comparison with established temperature standards.
a. Classification of temperature measuring devices

There are many basis for classification of temperature measuring instruments. These
devices can be classified on the basis of :
1. Nature of change produced in the temperature sensing element or the phenomenon

used for production of a change due to temperature.
2. Electrical and non-electrical operating principles.
3. Temperature range of the instrument.
Classification based upon nature of change produced.

The classification given by ASME code is:
(i) Glass Thermometers: These thermometers work on the principle of expansion of

liquids like mercury, alcohol, pentane and other organic liquids filled in glass
casing.
(ii) Pressure Gauge Thermometers: These instruments produce a pressure output on
account of vapour or liquids which work as actuating fluids. They may be further
classified as vapour pressure type and liquid or gas pressure type.
(iii) Differential Expansion Thermometers: The output of these instruments is on
account of differential expansion of two dissimilar metals produced by the
temperature,
(iv) Electrical Resistance Thermometers: The temperature is indicated by change in
resistance of a conductor in these thermometers.
(v) Thermocouples: In these instruments the temperature is indicated by production
of an emf. when the junction of two dissimilar metals are kept at different
temperatures.
(vi) Optical Pyrometers: The temperature is determined by these instruments by
matching the luminosity of the radiation of the hot body with that of a calibrated
source. These instruments utilize the visible area of electromagnetic radiation.
(vii) Radiation Pyrometers: In these instruments, the temperature is estimated by
absorbing radiation of all wavelengths upon a small body and determining the
temperature of the source from the temperature attained by the absorber.
(viii) Fusion Pyrometers: The temperature in this case is determined by observing which
of a series of materials with graduated fusion materials melt or soften when
subjected to the temperature under measurement.
(ix) Calorimetric Pyrometers: The temperature is determined by measuring quantity of

heat removed when the temperature of a body of known thermal capacity is
brought down from an unknown to a known level.
(x) Colour Temperature Charts: The hot body emanates radiation at different
wavelengths (which represent different colours) depending upon the temperature
of the body. Charts are available which relate the colour of radiation with
temperature. The temperature of the hot body can be estimated from the colour of
its radiation and referring to the colour temperature charts.
4.6.1 Thermistor
Thermistor is a contraction of a term "Thermal resistors". Thermistors are generally
composed of semi-conductor materials. Although positive temperature co-efficient of units
(which exhibit an increase in the value of resistance with increase in temperature) are
available, thermistors have a negative coefficient of temperature resistance i.e. their resistance
decreases with increase of temperature. The negative temperature coefficient of resistance can
be as large as several percent per degree Celsius. This allows the thermistor circuits to detect
very small changes in temperature which could not be observed with an RTD or a
thermocouple, In some cases the resistance of thermistor at room temperature may decrease as
much as 5 percent for each 1˚C rise in temperature. This high sensitivity to temperature
changes makes thermistors extremely useful for precision temperature measurements control
and compensation. Thermistors are widely used in applications which involve measurements
in the range of — 60°C to 15"C. The resistance of thermistors ranges from 0.5 to 0.75 MΩ
Thermistor is a highly sensitive device. The price to be paid off for the high sensitivity is in
terms of linearity. The thermistor exhibits a highly non-linear characteristic of resistance
versus temperature.
Thermistors arc composed of sintered mixture of metallic oxides such as manganese,

nickel, cobalt, copper, iron and uranium. They are available in variety of sizes and shapes. The
thermistors may be in the form of heads, rods and discs. Some of the commercial forms are
shown in Fig. 4.32.
Fig. 4.32 Different forms of construction of thermistors.

A thermistor in the form of a bead is smallest in size and the bead may have a
diameter of 0.015 mm to 1.25 mm. Beads may he sealed in the tips of solid glass rods to form
probes which may he easier to mount than the beads. Glass probes have a diameter of about
2.5 mm and a length which varies from 6 mm to 50 mm. Discs are made by pressing material
under high pressure into cylindrical flat shapes with diameters ranging from 2.5 mm to 25
mm.
a. Resistance-Temperature Characteristics of Thermistors.

The mathematical expression for the relationship between the resistance of a
thermistor and absolute temperature of thermistor is:
[ ( )]
where = Resistance of the thermistor at absolute temperature

= Resistance of the thermistor at absolute temperature
= a constant depending upon the material of thermistor, typically 3500 to 4500 K
The resistance temperature characteristics of a typical thermistor are given in Fig.

4.33. The resistance temperature characteristics of Fig. 4.33 show that a thermistor has a very
high negative temperature co-efficient of resistance, making it an ideal temperature
transducer. Fig. 4.33 also shows the resistance-temperature characteristics of platinum which is
a commonly used material for resistance thermometers. Let us compare the characteristics of
the two materials. Between —100°C and 400°C, the thermistor changes its resistivity from
to Ωm, a factor of , while platinum changes its resistivity by a factor of about 10
within the same temperature range. This explains the high sensitivity of thermistors for 10
measurement of temperature.
Fig. 4.33 Resistance-temperature characteristics of a typical thermistor and platinum

The characteristics of thermistors are no doubt non-linear but a linear approximation

of the resistance-temperature curve can be obtained over a small range of temperatures. Thus,
for a limited range of temperature, the resistance of a thermistor varies as given by equation.
[ ]
A thermistor exhibits a negative resistance temperature co-efficient which is typically about

0.051°C.
b. Applications of Thermistors
Although major applications of thermistors are measurement and control of
temperature, they may be used for a number of other applications. The various applications of
thermistors are :
Measurement of Temperature
A thermistor produces a large change of resistance with a small change in the
temperature being measured. This large sensitivity of thermistor provides good accuracy and
resolution. A typical industrial-type thermistor with a 2000Ω resistance at 25°C and a
resistance temperature co-efficient of 3.9 % per °C exhibits a change of 78 Ω per degree °C
change is temperature. When this thermistor is connected in a simple series circuit consisting
of a battery and micro-ammeter as shown in Fig. 4.34, any change in temperature causes a
change in the resistance of thermistor and corresponding change in the circuit current. The
micro-ammeter may be directly calibrated in terms of temperature. The micro-ammeter may
be able to give a resolution of 0.1°C.
Fig. 4.34 Simple series circuit for measurement of temperature using a thermistor
The use of a bridge circuit as shown in Fig. 4.35 gives higher sensitivities. The 4 kΩ
thermistor used will readily indicate as small changes as 0.005°C in temperature.The high
sensitivity together with high thermistor resistance which may about 100 kΩ, makes the
thermistor ideal for remote measurement or control, as the changes in contact or transmission
line resistances due to change in the ambient temperature have almost a negligible effect on
the accuracy of measurement or control. For example a 150 m long transmission line made of
copper when subjected to change of 25°C will affect the accuracy of measurement or control by
approximately 0.05°C.
Fig. 4.35 Measurement of temperature using a thermistor and a bridge circuit for getting
higher sensitivities
Control of Temperature
A simple temperature control circuit may be constructed by replacing the micro-
ammeter shown in the typical thermistor temperature control circuit of 4.36 with a relay. This
is shown in the typical thermistor temperature control circuit of Fig. 4.36. It uses a 4 kΩ
thermistor connected in an AC excited bridge. The unbalance voltage is fed to an AC amplifier
whose output excites a relay coil. The relay contacts are used to control the current in the
circuit which generates the heat. These circuits can be controlled to a precision of 0.00005°C.
Thermistor control systems are inherently sensitive, stable and fast acting and require
relatively simple circuitry. The voltage output of the standard bridge circuit at 25°C is about 18
mV/°C using 4kΩ thermistor in the above configuration (Fig. 4.35)
Fig. 4.36 Typical thermistor temperature control circuit

Temperature Compensation
Since thermistors have a negative temperature coefficient of resistance—opposite to
the positive coefficient of most electrical conductors and semiconductors—they are widely
used to compensate for the effects of temperature on both component and circuit performance.
Disk-type thermistors are used for this purpose where the maximum temperature does not
exceed 125°C. A properly selected thermistor, mounted against or near a circuit element, such
as a copper meter coil, and experiencing the same ambient temperature changes, can be
connected in such a way that the total circuit resistance is constant over a wide range of
temperatures. This is shown in the curves of Fig. 4.37, which illustrates the effect of a
compensation network. The compensator consists of a thermistor, shunted by a resistor. The
negative temperature coefficient of this combination equals the positive coefficient of the
copper coil. The coil resistance of 5000 Ω at 25°C, varies from approximately 4500 Ω at 0°C to
5700 Ω at 60°C, representing a change of about ± 12 percent. With a single thermistor
compensation network, this variation is reduced to about ± 15 Ω or ±1/4 percent. With double
or triple compensation networks, variations can be reduced even further.
Fig. 4.37 Temperature compensation of a copper conductor by means of a thermistor

network
Other Applications
 The other applications of thermistors include:
 Measurement of power at high frequencies
 Measurement of there al conductivity
 Measurement of level, flow and pressure of liquids
 Measurement of composition of gases
 Vacuum measurements
 Providing time delay
c. Salient Features of Thermistors

1. Thermistors are compact, rugged and inexpensive.
2. Thermistors when properly aged, have good stability.
3. The response time of thermistors can vary from a fraction of a second to minutes,
depending on the size of the detecting mass and thermal capacity of the thermistor. It
varies inversely with the dissipation factor. The power dissipation factor varies with
the degree of thermal isolation of the thermistor element.
4. The upper operating limit of temperature for thermistors is dependent on physical
changes in the material or solder used in attaching the electrical connections and is
usually 400°C or less. The lower temperature limit of temperature is normally
determined by the resistance reaching such a high value that it cannot be measured
by standard methods.
5. The measuring current should be maintained to as low a value as possible so that self-
heating of thermistors is avoided otherwise errors are introduced on account of
change of resistance caused by self-heating. Where it is not possible to avoid self-
heating, thermistor stability can be maintained at given temperature by using an
auxiliary heating element. The average power dissipation can be effectively reduced
and the highest sensitivity retained by energizing the thermistor with pulses of
measuring power.
6. Thermistors can be installed at a distance from their associated measuring circuits if
elements of high resistance are used such that the resistance of leads (even though the
leads may be very long) is negligible. This way the resistance of leads does not affect
the readings and hence errors on this count are negligible.
4.6.2 Thermocouple
The thermocouple is a temperature measuring device. It uses for measuring the
temperature at one particular point. In other words, it is a type of sensor used for measuring
the temperature in the form of an electric current or the EMF. The thermocouple consist two
wires of different metals which are welded together at the ends. The welded portion was
creating the junction where the temperature is used to be measured. The variation in
temperature of the wire induces the voltages. The working principle of the thermocouple
depends on the three effects.
Seebeck Effect – The See back effect occurs between two different metals. When the heat
provides to any one of the metal, the electrons start flowing from hot metal to cold metal.
Thus, direct current induces in the circuit. In short, it is a phenomenon in which the
temperature difference between the two different metals induces the potential differences
between them. The See beck effect produces small voltages for per Kelvin of temperature.
Fig. 4.38 Sample Thermocouple
Peltier Effect – The Peltier effect is the inverse of the Seebeck effect. The Peltier effect state
that the temperature difference can be created between any two different conductors by
applying the potential difference between them.
Thompson Effect – The Thompson effect states that when two dissimilar metals join together
and if they create two junctions then the voltage induces the entire length of the conductor
because of the temperature gradient. The temperature gradient is a physical term which shows
the direction and rate of change of temperature at a particular location.
a. Construction
The thermocouple consist two dissimilar metals. These metals are welded together at
the junction point. This junction considers as the measuring point. The junction point
categorizes into three types.
Fig. 4.39 Thermocouple Junctions
1. Ungrounded Junction – In ungrounded junction, the conductors are entirely isolated

from the protective sheath. It is used for high-pressure application works. The major
advantage of using such type of junction is that it reduces the effect of the stray
magnetic field.
2. Grounded Junction – In such type of junction the metals and protective sheath are
welded together. The grounded junction use for measuring the temperature in the
corrosive environment. This junction provides resistance to the noise.
3. Exposed Junction – Such type of junction uses in the places where fast response
requires. The exposed junction is used for measuring the temperature of the gas.
The material used for making the thermocouple depends on the measuring range of
temperature.
b. Working Principle
The circuit of the thermocouple is shown in the figure below. The circuit consists two
dissimilar metals. These metals are joined together in such a manner that they are creating two
junctions. The metals are bounded to the junction through welding.
Fig. 4.40 Iron Constantan Thermocouple
Let the P and Q are the two junctions of the thermocouples. The T 1 and T2 are the
temperatures at the junctions. As the temperature of the junctions is different from each other,
the EMF generates in the circuit. If the temperature at the junction becomes equal, the equal
and opposite EMF generates in the circuit, and the zero current flows through it. If the
temperatures of the junction become unequal, the potential difference induces in the circuit.
The magnitude of the EMF induces in the circuit depends on the types of material used for
making the thermocouple. The total current flowing through the circuit is measured through
the measuring devices. The EMF induces in the thermocouple circuit is given by the equation
( ) ( )
Where Δθ – temperature difference between the hot thermocouple junction and the
reference thermocouple junction. a, b – constants
c. Measurement of Thermocouple Output

The output EMF obtained from the thermocouples can be measured through the
following methods.
1. Multimeter – It is a simpler method of measuring the output EMF of the
thermocouple. The multimeter is connected to the cold junctions of the
thermocouple. The deflection of the multimeter pointer is equal to the current
flowing through the meter.
2. Potentiometer – The output of the thermocouple can also be measured with the
help of the DC potentiometer.
3. Amplifier with Output Devices – The output obtains from the thermocouples is
amplified through an amplifier and then feed to the recording or indicating
instrument.
d. Advantages and Disadvantages of Thermocouples

Advantages
 Thermocouples are cheaper than the resistance thermometers.
 It has wide temperature range.
 Thermocouples follow the temperature changes with a small time lag and as such are
suitable for recording comparatively rapid changes in temperature. Thermocouples
are very convenient for measuring the temperature at one particular point in a piece
of apparatus.
Disadvantages
 They have a lower accuracy and hence they cannot be used for precision work.
 To ensure long life of thermocouples in their operating environments, they should be

protected in an open or closed-end metal protecting tube or well. To prevent
contamination of the thermo-couple, when precious metals like platinum or its alloys
are being used, the protecting tube has to be made chemically inert and vacuum tight.
 The thermocouple is placed remote from measuring devices. Connections are thus
made by means of wires called extension wires. Maximum accuracy of measurement
is assured only when compensating wires are of the same material as the
thermocouple wires. The circuitry is, thus, very complex.
Nickel-alloy, platinum/rhodium alloy, Tungsten/rhenium-alloy, Chromel-gold, iron-

alloy are the name of the alloys used for making the thermocouple.
e. Thermocouple Laws
The following empirically derived thermocouple laws, are useful to understand, diagnose
and utilise thermocouples.
Law of homogeneous circuits

If two thermocouple junctions are at T1 and T2, then the thermal emf generated
is independent and unaffected by any temperature distribution along the wires. In below
Figure 4.41, a thermocouple is shown with junction temperatures at T1 and T2. Along the
thermocouple wires, the temperature is T3 and T4. The thermocouple emf is, however, still a
function of only the temperature gradient T2 – T1.
Fig. 4.41 Junction of homogeneous circuits
Law of intermediate metals

The law of intermediate metals states that a third metal may be inserted into
a thermocouple system without affecting the emf generated, if, and only if, the junctions with
the third metal are kept at the same temperature.
Fig. 4.42 Junction of intermediate metals
When thermocouples are used, it is usually necessary to introduce additional

metals into the circuit This happens when an instrument is used to measure the emf, and
when the junction is soldered or welded. It would seem that the introduction of other
metals would modify the emf developed by the thermocouple and destroy its
calibration. However, the law of intermediate metals states that the introduction of a third
metal into the circuit will have no effect upon the emf generated so long as the junctions of the
third metal are at the same temperature, as shown in Above Figure 4.42.
Law of intermediate temperatures

The law of intermediate temperatures states that the sum of the emf developed by
a thermocouple with its junctions at temperatures T1 and T2, and with its junctions
at temperatures T2 and T3, will be the same as the emf developed if the
thermocouple junctions are at temperatures T1 and T3.
Fig. 4.43 Junction of intermediate temperatures
This law, illustrated in above Figure, is useful in practice because it helps in giving
a suitable correction in case a reference junction temperature other than 0 °C is employed. For
example, if a thermocouple is calibrated for a reference junction temperature of 0 °C and used
with a junction temperature of 20 °C, then the correction required for the observation would be
the emf produced by the thermocouple between 0 °C and 20 °C.
f. Comparison
Basis For
Thermocouple Thermistor
Comparison
Definition The thermocouple is a type of Thermistor is the thermal
device used for measuring the resistor whose resistance
temperature changes with the temperature.
Symbol
Sensing Voltage generate at the junction. Resistance

Parameter
Material Copper, iron, Constantan, Manganese, nickel or cobalt
Chromel, Alloys of metals like oxides, semiconductor material.
Chrome, chromium and nickel,
platinum and rhodium, tungsten
and rhenium,
Accuracy High Very Low
Temperature -50°C to 250°C -200°C to 1250°C
Range
Response Time 0.12 – 10 Seconds 0.2 – 10 seconds
(Depends on size
and packing)
Characteristic Non-linear for negative Linear
Curve temperature coefficient.
Cost Expensive (because of external Cheap
power source
Applications For controlling temperature, Measuring and controlling
Measurement of temperature, temperature.
thermal conductivity,
temperature compensation etc.
4.6.3 Resistance Temperature Detector (RTD)

The resistance thermometer or resistance temperature detector (RTD) uses
the resistance of electrical conductor for measuring the temperature. The resistance of
the conductor varies with the time. This property of the conductor is used
for measuring the temperature. The main function of the RTD is to give a positive
change in resistance with temperature. The metal has a high-temperature coefficient that
means their temperature increases with the increase in temperature. The carbon and
germanium have low-temperature coefficient which shows that their resistance is inversely
proportional to temperature.
a. Construction
The resistance thermometer is placed inside the protective tube for providing the
protection against damage. The resistive element is formed by placing the platinum wire on
the ceramic bobbin. This resistance element is placed inside the tube which is made up of
stainless steel or copper steel.
Fig. 4.44 Resistance Temperature Detector (RTD)
The lead wire is used for connecting the resistance element with the external lead.
The lead wire is covered by the insulated tube which protects it from short circuit. The ceramic
material is used as an insulator for high-temperature material and for low-temperature fibre or
glass is used. The tip of the resistance thermometer is placed near the measurand heat source.
The heat is uniformly distributed across the resistive element. The changes in the resistance
vary the temperature of the element. The final resistance is measured. The below mention
equations measure the variation in temperature.
Where, R0 – resistance at temperature T = 0 and α1, α2, α3……..αn are constants.

Linear Approximation
The linear approximation is the way of estimating the resistance versus temperature
curve in the form of the linear equation.
( )
where – approximation resistance at θ˚C

– approximation resistance at θ0˚C
– θ – θ0 change in temperature ˚C and the αθ0 – resistance temperature coefficient
at θ0˚C
Quadratic Approximation
The quadratic approximation gives the accurate approximation of the resistance
temperature curve. The approximation is expressed in the form of the quadratic equation.
[ ( ) ]
Where α1 – linear fractional change in resistance
α2 – quadratic function change in resistance.
The resistance thermometer is very less sensitive, and the metal used for making the
resistive element is less expensive.The resistance thermometer uses a sensitive element made
of extremely pure metals like platinum, copper or nickel. The resistance of the metal is directly
proportional to the temperature. Mostly, platinum is used in resistance thermometer. The
platinum has high stability, and it can withstand high temperature. Gold and silver are not
used for RTD because they have low resistivity. Tungsten has high resistivity, but it is
extremely brittles. The copper is used for making the RTD element. The copper has low
resistivity and also it is less expensive. The only disadvantage of the copper is that it has low
linearity. The maximum temperature of the copper is about 120˚C. The RTD material is made
of platinum, nickel or alloys of nickel. The nickel wires are used for a limited temperature
range, but they are quite nonlinear. The following are the requirements of the conductor used
in the RTDs.
1. The resistivity of the material is high so that the minimum volume of conductor is
used for construction.
2. The change in resistance of the material concerning temperature should be as high as
possible.
3. The resistance of the material depends on the temperature.
The resistance versus temperature curve is shown in the figure below. The curves are
nearly linear, and for small temperature range, it is very evident.
Fig. 4.45 Characteristic Curve
b. Measurement of Resistance of Thermometers

The measurement of change of resistance of thermometers due to change in
temperature is measured by a Wheatstone bridge. The following factors must be considered
while measuring the resistance.
(i) The contact resistance of the adjustable standard resistor may be large enough to
produce an error and means should be used to minimize this error.
(ii) The general practice is to use the null type bridge but the deflection type bridge
may also be used. In general null type bridges are used when measurement of
static or slow varying temperatures is involved whereas the deflection type of
bridge is used for more rapidly changing temperatures. Dynamic changes are
most conveniently recorded rather than simply indicated, and for this purpose
either the self-balancing bridge or deflection type of bridges may be used
depending on the time rate change of temperature.
(iii) When a resistance bridge is used for measurement, current will necessarily flow
through each bridge arm. Hence, an error will be introduced because of heating
due to PR loss. For resistance thermometers such an error will be of opposite sign
to that caused by conduction and radiation effects from the element. The error, in
general, is small because the gross effects in individual arms will be largely
balanced by similar effects in the other arms. An estimate of the overall error
resulting from PR heating may be had by taking readings at different current
values and extrapolating to zero current.
The self-heating produced in RTDs due to flow of current alters the temperature of
the element. The importance one must assign to this effect depends upon the thermal
communication between the RTD and the medium whose temperature is to be measured. For
measurement of temperature of a block of metal the communication is good, while for an air
temperature measurement the communication is poor. In still air the error due to self-heating
is about 0.5°C per milliwatt.
c. Salient features of RTD

 Resistance wire thermometers have a high degree of accuracy. Precision laboratory
instruments may be calibrated for measuring temperatures to within ± 0.01°C.
Industrial instruments can be calibrated to detect actual temperatures within ± 0.25°C
upto 120°C, and ± 0.5°C from 120°C to 550'C.
 Resistance thermometers are normally designed for fast response as well as accuracy
to provide close control of processes in which narrow ranges or small spans must be
maintained.
 Each type of resistance thermometer is interchangeable in a process without
compensation or recalibration, because each type is calibrated with reference to a
standard resistance—temperature curve. Thus if a unit is damaged it can be easily
replaced.
Other practical problems which are encountered with the use of RTDs are :
 Errors on account of resistance of leads while measuring their resistance.
 Their bulky size which may sometimes give poor transient response.
 They have a fragile construction.
 There is a possibility of self-heating and consequent change in temperature as current
passes through the element when its resistance is measured using bridge circuits.
 Errors are caused due to conduction effects.
Typical industrial applications of resistance thermometers for measuring temperature

are cooling processes, heating, ovens, drying ovens, kilns, process vessels, baths, quenches,
refining, controlled cold storage plants, steam and power generation condensates, steam
exhausts, pickling and plating plants, injection moulding, impression moulding, transfer
moulding and air inversion measurements.
d. Comparison
RTD (Resistance
Basis For Comparison Thermistor
Temperature Detector)
Definition The device use for measuring It is a thermal resistor whose

the change in temperature is resistance changes with the
known as the RTD or temperature.
Symbol
Material Metals (platinum, nickel, Semiconductor

copper, etc.)
RTD (Resistance
Basis For Comparison Thermistor
Temperature Detector)
Accuracy Less accurate. Their accuracy is high. It can

detect even small changes in
temperature
Response Time Slow Fast
Temperature Range -230°C to 660°C -60°C to 15°C
Characteristic Graph Linear Non-linear
Sensitivity Low High
Size Large Small
Cost Cheap Expensive
Resistivity High Low
Applications In industries for measuring For measuring temperature of

large temperature. home appliances.
4.7 Acoustic sensors

Sound and its measurement is important, since it relates to the sense of hearing, as
well as many industrial applications, such as for the detection of flaws in solids, and for
location and linear distance measurement. Sound pressure waves can induce mechanical
vibration and hence failure.
a. Sound Measurements
Sounds are pressure waves that travel through air, gas, solids, and liquids, but cannot
travel through space or a vacuum, unlike radio (electromagnetic) waves. Pressure waves can
have frequencies up to approximately 50 kHz. Sound waves start at 16 Hz and go up to 20
kHz; above 30 kHz, sonic waves become ultrasonic. Sound waves travel through air at
approximately 340 m/sec, depending on factors such as temperature and pressure. The
amplitude or loudness of sound is measured in phons. The Sound pressure level (SPL) units
are often used in the measurement of sound levels, and are defined as the difference in
pressure between the maximum pressure at a point and the average pressure at that point. The
units of pressure are normally expressed as follows:
1 dyne/cm2 = 1µ bar = 1.45 × 10−5 psi

where 1N = 10 dyn and 1μbar = 0.1 Pa
5
The decibel (dB) is a logarithmic measure used to measure and compare amplitudes
and power levels in electrical units, sound, light, and so forth. The sensitivity of the ears and
eyes are logarithmic. To compare different sound intensities, the following applies:
[ ]
where I1 and I2 are the sound intensities at two different locations, and are scalar units. A
reference level (for I2) is 10−16 W/cm2 (the average level of sound that can be detected by the
human ear at 1 kHz) to measure sound levels.
When comparing different pressure levels, the following is used:
[ ]
where P1and P2 are the pressures at two different locations. (Pressure is a measure of sound
power, hence the use of 20 log.) A value of 20 N/m2 for P2 is accepted as the average pressure
level of sound that can be detected by the human ear at 1 kHz, and is therefore the reference
level for measuring sound pressures. Typical figures for SPL are:
 Threshold of pain : 140 to 150 dB
 Rocket engines : 170 to 180 dB
 Factory : 80 to 100 dB
Fig. 4.46 (a) Dynamic Microphone

b. Sound Measuring Devices

Microphones are pressure transducers, and are used to convert sound pressures into
electrical signals. The following types of microphones can be used to convert sound pressure
waves into electrical signals: electromagnetic, capacitance, ribbon, crystal, carbon, and
piezoelectric. Figure 4.46 (a) shows the cross section of a dynamic microphone, which consists
of a coil in a magnetic field driven by sound waves impinging on a diaphragm. An EMF is
induced in the coil by the movement of the diaphragm. Figure 4.46 (b) shows the cross section
of capacitive microphone, which is an accepted standard for accurate acoustical
measurements. Sound pressure waves on the diaphragm cause variations in the capacitance
between the diaphragm and the rigid plate. The electrical signals then can then be analyzed in
a spectrum analyzer for the various frequencies contained in the sounds, or just to measure
amplitude. Sound level meter is the term given to any of a variety of meters for measuring and
analyzing sounds.
Fig. 4.46 (b) Capacitive microphone

a. Resistive microphones
In the past, resistive pressure converters were used quite extensively in microphones.
The converter consisted of a semi-conductive powder (usually graphite) whose bulk resistivity
was sensitive to pressure. Currently, we would say that the powder possessed piezoresistive
properties. However, these early devices had quite a limited dynamic range, poor frequency
response, and a high noise floor. Presently, the same piezoresistive principle can be employed
in the micromachined sensors, where stress-sensitive resistors are the integral parts of a silicon
diaphragm
Fig. 4.47 (a)Piezoresistive microphones

b. Condenser Microphones
If a parallel-plate capacitor is given an electric charge q, the voltage across its plates is
governed by Eq. (Chapter 2). On the other hand, according to Eq. (Chapter 2) the capacitance
depends on distance d between the plates. Thus, solving these two equations for voltage, we
arrive at
where =8.8542×10−12 C2/Nm2 is the permittivity constant. Equation is the basis for
operation of the condenser microphones, which is another way to say “capacitive”
microphones. Thus, a capacitive microphone linearly converts a distance between the plates
into electrical voltage which can be further amplified. The device essentially requires a source
of an electric charge q whose magnitude directly determines the microphone sensitivity. The
charge can be provided either from an external power supply having a voltage in the range
from 20 to 200 V or from an internal source capable of producing such a charge. This is
accomplished by a built-in electret layer which is a polarized dielectric crystal. Presently, many
condenser microphones are fabricated with silicon diaphragms, which serve two purposes: to
convert acoustic pressure into displacement and to act as a moving plate of a capacitor. Some
promising designs are designed to achieve high sensitivity; a bias voltage should be as large as
possible, resulting in a large static deflection of the diaphragm, which may result in reduced
shock resistivity and lower dynamic range. In addition, if the air gap between the diaphragm
and the back plate is very small, the acoustic resistance of the air gap will reduce the
mechanical sensitivity of the microphone at higher frequencies.
Fig. 4.48 Condenser microphone with a mechanical feedback: (a) a circuit diagram; (b)
Interdigitized electrodes on the diaphragm.
For instance, at an air gap of 2 μm, an upper cutoff frequency of only 2 kHz has been
measured. One way to improve the characteristics of a condenser microphone is to use a
mechanical feedback from the output of the amplifier to the diaphragm. Figure 4.48 (a) shows
a circuit diagram and Fig. 4.48 (b) is a drawing of interdigitized electrodes of the microphone.
The electrodes serve different purposes: One is for the conversion of a diaphragm
displacement into voltage at the input of the amplifier A1 and the other electrode is for
converting feedback voltage. Va into a mechanical deflection by means of electrostatic force.
The mechanical feedback clearly improves the linearity and the frequency range of the
microphone; however, it significantly reduces the deflection, which results in a lower
sensitivity.
c. Fiber-Optic Microphone
Direct acoustic measurements in hostile environments, such as in turbojets or rocket
engines, require sensors which can withstand high heat and strong vibrations. The acoustic
measurements under such hard conditions are required for computational fluid dynamics
(CFD) code validation, structural acoustic tests, and jet noise abatement. For such applications,
a fiber-optic interferometric microphone can be quite suitable. One such design is composed of
a single-mode temperature insensitive Michelson interferometer and a reflective plate
diaphragm. The interferometer monitors the plate deflection, which is directly related to the
acoustic pressure. The sensor is water cooled to provide thermal protection for the optical
materials and to stabilize the mechanical properties of the diaphragm. To provide an effect of
interference between the incoming and outgoing light beams, two fibers are fused together
and cleaved at the minimum tapered region (Fig.4.49). The fibers are incorporated into a
stainless-steel tube, which is water cooled. The internal space in the tube is filled with epoxy,
and the end of the tube is polished until the optical fibers are observed. Next, aluminum is
selectively deposited at one of the fused fiber core ends to make its surface mirror reflective.
This fiber serves as a reference arm of the microphone. The other fiber core is left open and
serves as the sensing arm. Temperature insensitivity is obtained by the close proximity of the
reference and sensing arms of the assembly.
Fig. 4.49 Fiber-optic interferometric microphone.

Light from a laser source (a laser diode operating near 1.3 μm wavelength) enters one
of the cores and propagates toward the fused end, where it is coupled to the other fiber core.
When reaching the end of the core, light in the reference core is reflected from the aluminum
mirror toward the input and output sides of the sensor. The portion of light which goes
toward the input is lost and has no effect on the measurement, whereas the portion which goes
to the output strikes the detector’s surface. That portion of light which travels to the right in
the sensing core, exits the fiber, and strikes the copper diaphragm. Part of the light is reflected
from the diaphragm back toward the sensing fiber and propagates to the output end, along
with the reference light. Depending on the position of the diaphragm, the phase of the
reflected light will vary, thus becoming different from the phase of the reference light. While
traveling together to the output detector, the reference and sensing lights interfere with one
another, resulting in the light-intensity modulation. Therefore, the microphone converts the
diaphragm displacement into a light intensity. Theoretically, the signal-to-noise ratio in such a
sensor is obtainable on the order of 70–80 dB, thus resulting in an average minimum detectable
diaphragm displacement of 1 Å (10−10 m). Figure 4.50 shows a typical plot of the optical
intensity in the detector versus the phase for the interference patterns. To assure a linear
transfer function, the operating point should be selected near the middle of the intensity,
where the slope is the highest and the linearity is the best. The slope and the operating point
may be changed by adjusting the wavelength of the laser diode. It is important for the
deflection to stay within one-quarter of the operating wavelength to maintain a proportional
input. The diaphragm is fabricated from a 0.05-mm foil with a 1.25-mm diameter. Copper is
selected for the diaphragm because of its good thermal conductivity and relatively low
modulus of elasticity. The latter feature allows us to use a thicker diaphragm, which provides
better heat removal while maintaining a usable natural frequency and deflection. A pressure of
1.4 kPa produces a maximum center deflection of 39 nm (390 AA), which is well within a one-
quarter of the operating wavelength (1300 nm). The maximum acoustic frequency which can
be transferred with the optical microphone is limited to about 100 kHz, which is well above
the desired working range needed for the structural acoustic testing.
Fig. 4.50 Intensity plot as function of a reflected light phase.

d. Piezoelectric Microphones
The piezoelectric effect can be used for the design of simple microphones. A
piezoelectric crystal is a direct converter of a mechanical stress into an electric charge. The
most frequently used material for the sensor is a piezoelectric ceramic, which can operate up to
a very high frequency limit. This is the reason why piezoelectric sensors are used for the
transduction of ultrasonic waves. Still, even for the audible range, the piezoelectric
microphones are used quite extensively. Typical applications are voice-activated devices and
blood pressure measurement apparatuses where the arterial Korotkoff sounds have to be
detected. For such acoustically non demanding applications, the piezoelectric microphone
design is quite simple (Fig. 4.51). It consists of a piezoelectric ceramic disk with two electrodes
deposited on each side. The electrodes are connected to wires either by electrically conductive
epoxy or by soldering. Because the output impedance of such a microphone is very large, a
high-input-impedance amplifier is required.
Piezoelectric films [polyvinylidene fluoride (PVDF) and copolymers] were used for
many years as very efficient acoustic pickups in musical instruments. One of the first
applications for piezoelectric film was as an acoustic pickup for a violin. Later, the film was
introduced for a line of acoustic guitars as a saddle-mounted bridge pickup, mounted in the
bridge. The very high fidelity of the pickup led the way to a family of vibration-sensing and
accelerometer applications: in one guitar pickup, a thick film, compressive (under the saddle)
design; another is a low-cost accelerometer, and another is an after-market pickup design that
is taped to the instrument. Because of the low Q of the material, these transducers do not have
the self-resonance of hard ceramic pickups. Shielding can be achieved by a fold over design as
shown in Fig.4.52. The sensing side is the slightly narrower electrode on the inside of the fold.
The fold over technique provides a more sensitive pickup than alternative shielding methods
because the shield is formed by one of the electrodes. For application in water, the film can be
rolled in tubes, and many of such tubes can be connected in parallel (Fig. 4.52).
Fig. 4.51 Piezoelectric microphone.

Fig. 4.52 Foldover piezoelectric acoustic pickup (a) and arrangement of a piezoelectric film
hydrophone (b)
4.8 Flow and Level sensors
A level sensor is a device for determining the level or amount of fluids, liquids or
other substances that flow in an open or closed system. There are two types of level
measurements, namely, continuous and point level measurements. Continuous level sensors
are used for measuring levels to a specific limit, but they provide accurate results. Point level
sensors, on the other hand, only determine if the liquid level is high or low. The level sensors
are usually connected to an output unit for transmitting the results to a monitoring system.
Current technologies employ wireless transmission of data to the monitoring system, which is
useful in elevated and dangerous locations that cannot be easily accessed by common workers.
A flow sensor (more commonly referred to as a “flow meter”) is an electronic device

that measures or regulates the flow rate of liquids and gasses within pipes and tubes. Flow
sensors are generally connected to gauges to render their measurements, but they can also be
connected to computers and digital interfaces. They are commonly used in HVAC systems,
medical devices, chemical factories, and septic systems. Flow sensors are able to detect leaks,
blockages, pipe bursts, and changes in liquid concentration due to contamination or pollution.
Flow sensors can be divided into two groups: contact and non-contact flow sensors. Contact
flow sensors are used in applications where the liquid or gas measured is not expected to
become clogged in the pipe when it comes into contact with the sensor’s moving parts. In
contrast, non-contact flow sensors have no moving parts, and they are generally used when
the liquid or gas (generally a food product) being monitored would be otherwise
contaminated or physically altered by coming into contact with moving parts.
Fig. 4.52 Vortex flow sensor.
The two most common types of contact flow sensors are vortex and mechanical flow
sensors. Vortex flow sensors are comprised of a small latch (known as the “buff body”) that
flexes back and forward when coming into contact with a flowing liquid or gas. The
differences in pressure (i.e. the vortices) generated by the latch are measured to determine the
flow rate. Mechanical flow sensors use propellers that spin at a rate that is directly
proportional to the flow rate. Mechanical flow sensors can also be controlled to cause the flow
rate to increase or decrease.
4.8.1 Venturimeter
Venturimeter is a device used to measure the flow rate or discharge of fluid through a
pipe. Venturimeter is an application of Bernoulli’s equation. Its basic principle is also depends
on the Bernoulli equation i.e. velocity increases pressure decreases. The principle of venture
meter is firstly developed by G.B. Venturi in 1797 but this principle comes into consideration
with the help of C. Herschel in 1887. The principle is that when cross sectional area of the flow
is reduced then a pressure difference is created between the different areas of flow which helps
in measuring the difference in pressure. With the help of this pressure difference we can easily
measure the discharge in flow.
Bernoullis principle states that with the increase in the velocity of the fluid its
pressure decreases (or) decreases the fluid potential energy. Decreasing the fluid pressure in
the areas where flow velocity is increased is called as Bernoulli effect.
a. Construction
Venturimeter is very simple in construction. It has following parts which are
arranged in systematic order for proper operation these are inlet section called as converging
cone, cylindrical throat and gradually diverging cone.
Fig. 4.53 Venturimeter setup
1. Converging section/cone
It is the region where the cross section emerges into conical shape for the connectivity
with the throat region. The converging region is attached to the inlet pipe(flow upstream) and
its cross sectional area decreases from beginning to ending. One side it is attached with inlet
and its other side are attached with the cylindrical throat. The angle of convergence is
generally 20-22 degree and its length is 2.7(D-d). Here D is the diameter of inlet section and d
is the diameter of throat. Due to the decrease in the cross sectional area the fluid accelerates
and static pressure decreases. The maximum cone angle of the converging area is limited to
avoid the vena contracta so the flow area will be minimum at the throat.
2. Cylindrical throat
It is middle part of the venturimeter and has lowest cross sectional area. The length is
equal to diameter of throat. Generally the diameter of the throat is 1/4 to 3/4 of the diameter of
the inlet pipe, but mostly it is ½ of the diameter of the pipe. diameter. The diameter of the
throat remains same through out its length. The diameter of throat cannot reduce to its
minimum suitable value because if cross sectional area decrease velocity increase and pressure
decreases. This decrease in pressure goes below the vapour pressure which results in
cavitation. To avoid cavitation a limited value of diameter is preferred.
3. Diverging sections/cone
Diverging section is the third part of this device. One side it is attached with outlet
pipe. The diameter of this section is gradually increases. The diverging section has an angle 5
to 15 degree. The diverging angle is less than the converging angle due to this length of the
diverging cone is larger than converging cone. The main reason of the small diverging angle to
avoid flow separation from the walls and prevents the formation of eddies because flow
separation and eddies formation will results in large amount of loss in energy. To avoid these
losses proper angle of converging and diverging should be maintained. The reasons for the
limitation of Divergent angle:–
1. To reduce adverse pressure gradient and reverse flow (unsteady flow)

2. To prevent flow separation, which causes frictional drag
3. To avoid head loss and cavitation effect
Differential manometer and pressure gauges

Differential manometers are used to measure the pressure in the flow through pipe
and it is mounted between inlet pipe and throat. We can use different pressure gauges in place
of differential manometer to measure the pressure and different sections. The pressure gauges
are mounted at inlet and throat of the venturimeter. The diverging section is not used for
measuring the discharge because at this section flow separation may occur. When fluid flow
through the venture meter then pressure difference is created which measured by the
differential manometer.
b. Working
Working of venturimeter is so simple. As already explained it works on the principle
of Bernoulli’s equation, i.e. when velocity increases pressure decreases. Same principle is
applicable here. The cross section area of throat is smaller than the cross-section area of the
inlet pipe due to this the velocity of flow at throat section is greater than the inlet section, this
happens according to continuity equation. The increase in the velocity of flow at the throat will
results in decrease in the pressure at this section, due to this a pressure difference is developed
between inlet and throat of the venturimeter. This pressure difference can be easily measured
by using differential manometer between the inlet section and throat or by using two separate
gauges at inlet and throat. By measuring the different pressure at the two different sections we
can easily measure or calculate the flow rate through the pipe. Its working can be describe into
following points.
1. The working of venturimeter is based on the Bernoulli’s principle. As the velocity

increases pressure decreases.
2. In the convergent region as the area and pressure decreases the velocity increases and
has a favourable pressure gradient [ i.e., (dp/dx)<0 ] .
3. In the throat region area and pressure are constant and the velocity is also constant
and pressure gradient is zero [ i.e., (dp/dx) = 0 ] .
4. The decrease in the pressure in between the inlet and throat is measured with the
help of differential manometer.
5. The value of height of mercury in the manometer which is obtained from difference
of pressure heads is used to calculate discharge by using bernoullis equation.
6. As the cone angle of divergent region is limited to 5 – 70 reverse flow is eradicated.
Here the pressure gradient is adverse[ i.e., (dp/dx>0) ] .
Advantages:
1. The main advantage of venturimeter is it has very less losses and high accuracy.
2. It has high coefficient of discharge.
3. Easy to operate.
4. It can be installed in any direction between pipe flow i.e. horizontal, vertical and
inclined.
5. Venturimeter has high accuracy as compare to other flow measuring devices like
orifice meter, pitot tube and nozzles.
Disadvantages:
Venturimeter has some disadvantages also like, it has high initial cost because its
calculation is very complicated.
1. The major drawback of the venture meter is we cannot use it for small diameter size
pipe.
2. It is difficult in maintenance and inspections.
3. Initial cost is high.
c. Applications:
1. As discussed above venturimeter is used to measure the discharge in flow through
pipes.
2. In medical applications it is used to measure the rate of flow in the arteries.
3. It has some other industrial applications like in gas, liquids, oil where pressure loss
should be avoided.
4. It also measures the discharge of fluid which has some slurry or dirt particles
because of its smooth design.
4.8.2 Orifice meter

An Orifice Meter is basically a type of flow meter used to measure the rate of flow of
Liquid or Gas, especially Steam, using the Differential Pressure Measurement principle. It is
mainly used for robust applications as it is known for its durability and is very economical. As
the name implies, it consists of an Orifice Plate which is the basic element of the instrument.
When this Orifice Plate is placed in a line, a differential pressure is developed across
the Orifice Plate. This pressure drop is linear and is in direct proportion to the flow-rate of the
liquid or gas. Since there is a drop in pressure, just like Turbine Flow meter, hence it is used
where a drop in pressure or head loss is permissible. The Orifice plates in the Orifice meter, in
general, are made up of stainless steel of varying grades.
When a liquid / gas, whose flow-rate is to be determined, is passed through an Orifice
Meter, there is a drop in the pressure between the Inlet section and Outlet Section of Orifice
Meter. This drop in pressure can be measured using a differential pressure measuring
instrument. Since this differential pressure is in direct proportion to the flow-rate as per the
Bernoulli's Equation hence the differential pressure instrument can be configured to display
flow-rate instead of showing differential pressure. The working principle of Orifice Meter is
the same, as that of Venturimeter.
Fig. 4.54 Orifice setup
b. Construction
Fig. 4.55 shows the construction details of orifice meter.
Inlet Section
A linearly extending section of the same diameter as the inlet pipe for an end
connection for an incoming flow connection. Here we measure the inlet pressure of the fluid /
steam / gas.
Orifice plate
An Orifice Plate is inserted in between the Inlet and Outlet Sections to create a
pressure drop and thus measure the flow.
Outlet section
A linearly extending section similar to the inlet section. Here also the diameter is the
same as that of the outlet pipe for an end connection for an outgoing flow. Here we measure
the Pressure of the media at this discharge. As shown in the adjacent diagram, a gasket is used
to seal the space between the Orifice Plate and the Flange surface, prevent leakage. Sections 1
& 2 of the Orifice meter, are provided with an opening for attaching a differential pressure
sensor (u-tube manometer, differential pressure indicator).
Fig. 4.55 Orifice construction details
c. Shape and Size of orifice meter

Orifice meters are built in different forms depending upon the application specific
requirement, The shape, size and location of holes on the Orifice Plate describes the Orifice
Meter Specifications as per the following:
1. Concentric Orifice Plate

2. Eccentric Orifice Plate
3. Segment Orifice Plate
4. Quadrant Edge Orifice Plate
Fig. 4.56 Shapes of orifice plate

Concentric orifice plate

It is made up of SS and its thickness varies from 3.175 to 12.70 mm. The plate
thickness at the orifice edge should not be exceeded by any of following parameters:
 1 - D/50 where, D = The pipe inside diameter
 2 - d/8 where, d = orifice bore diameter
 3 - (D-d)/8
Eccentric Orifice Plate

It is similar to Concentric Orifice plate other than the offset hole which is bored
tangential to a circle, concentric with the pipe and of a diameter equal to 98% of that of the
pipe. It is generally employed for measuring fluids containing
 Media having Solid particles
 Oils containing water
 Wet steam
Segment Orifice Plate

It has a hole which is a semi circle or a segment of circle. The diameter is customarily
98% of the diameter of the pipe.
Quadrant Edge Orifice Plate

This type of orifice plate is used for flow such as crude oil, high viscosity syrups or
slurries etc. It is conceivably used when the line Reynolds Numbers* range from 100,000 or
above or in between to 3,000 to 5,000 with a accuracy coefficient of roughly 0.5%.
d. Operation of orifice meter

1. The fluid flows inside the Inlet section of the Orifice meter having a pressure P1.
2. As the fluid proceeds further into the Converging section, its pressure reduces
gradually and it finally reaches a value of P2 at the end of the Converging section
and enter the Cylindrical section.
3. The differential pressure sensor connected between the Inlet and the and the
Cylindrical Throat section of the Orifice meter displays the difference in pressure
(P1-P2). This difference in pressure is in direct proportion to the flow rate of the
liquid flowing through the Orifice meter.
4. Further the fluid passed through the Diverging recovery cone section and the
velocity reduces thereby it regains its pressures. Designing a lesser angle of the
Diverging recovery section helps more in regaining the kinetic energy of the
liquid.
Applications of orifice meter

 Natural Gas
 Water Treatment Plants
 Oil Filtration Plants
 Petrochemicals and Refineries
Advantages of orifice meter

 The Orifice meter is very cheap as compared to other types of flow meters.
 Less space is required to Install and hence ideal for space constrained
applications
 Operational response can be designed with perfection.
 Installation direction possibilities: Vertical / Horizontal / Inclined.
Limitations of orifice meter

 Easily gets clogged due to impurities in gas or in unclear liquids
 The minimum pressure that can be achieved for reading the flow is sometimes
difficult to achieve due to limitations in the vena-contracta length for an Orifice
Plate.
 Unlike Venturimeter, downstream pressure cannot be recovered in Orifice
Meters. Overall head loss is around 40% to 90% of the differential pressure .
 Flow straighteners are required at the inlet and the outlet to attain streamline
flow thereby increasing the cost and space for installation.
 Orifice Plate can get easily corroded with time thereby entails an error.
 Discharge Co-efficient obtained is low.
4.8.3 Level sensors

Liquid level sensors have been around for decades, in markets such as food and
beverage, industrial, medical and domestic, printing, agriculture, automotive a nd white
goods for leak detection or level measurement. We often wonder why customers choose
one technology over the other and it is a common question we are asked. Some
equipment manufacturers may also be surprised at both the variety and intelligence of
level sensing alternatives available on the market. Processes that used to involve
expensive pieces of equipment can now be achieved using creative, innovative and
intelligent technologies that can be cost effective, reliable, robust, highly accurate and
simple to install. Fluids that have historically been known to be extremely challenging to
detect such as soap containing bubbles/foam, milk, and sticky substances such as glue
and ink are now proving possible and easier to detect with the variety of level sensing
technologies available. But is there need for such technology—or any level sensing
device is the question that many people may ask. However, with the competitive nature
of the industry and the consistently wanting to improve quality, reduce costs,
inefficiencies and waste kind of mentality, no company wants to take the risk at offering
solutions that are not performing as best as they could be.
 Optical
 Vibrating or tuning fork
 Ultrasonic
 Float
 Capacitance
 Radar
 Conductivity or resistance
a. Optical Level Switches

There are a range of technical terms used to describe this type of level sensing
technology. Optical prism, electro-optic, single-point optical, optical level switch…the
list goes on. For this purpose, we will use the term Optical Level Switch. The switch
operates very simply. Inside the sensor housing is an LED and a phototransistor. When
the sensor tip is in air, the infrared light inside the sensor tip is reflected back to the
detector. When in liquid, the infrared liquid is refracted out of the sensor tip, causing
less energy to reach the detector. Being a solid-state device, these compact switches are
ideal for a vast range of point level sensing applications, especially when reliability is
essential. Optical liquid level switches are suitable for high, low or intermediate level
detection in practically any tank, large or small. They are also suitable for detecting leaks
preventing costly damage. Reflected light, such as a small reflective tank, mirrored tanks,
bubbles, milk or coating fluids can often cause issues with delayed readings.
 Pros – Compact, no moving parts, high pressure and temperature capability, can
detect tiny amounts of liquids
 Cons – Invasive as the sensor requires contact with the liquid, requires power,
certain thick substances can cause coating on the prism.
 Applications – tank level measurement and leak detection applications
b. Capacitance
Capacitance level sensors operate in the way that process fluids have dielectric
constants, significantly different to air. They measure the change in capacitance between
two plates produced by changes in level. Two versions are available, one for fluids with
high dielectric constants and one with low dielectric constants. Capacitance level sensors
work with a range of solids, liquids, and mixed materials. They are also available in
contact and non-contact configurations meaning some of which can be attached outside
the container/tank.
When selecting a device, it is important to know that not every capacitance

senor works with every type of material or tank. In addition, the sensor needs to be
calibrated to the specific material to excuse the varying dielectric constants and
differences in the tank design. As this type of technology is contact based, the reliability
of these sensors can be heavily influenced by fluids sticking to the probe.
 Pros – Solid-state, can be non-invasive, compact, accurate

 Cons – May require calibration, can only be used in certain liquids
 Applications – Tank level monitoring in chemical, food, water treatment, power
and brewery industries.
c. Ultrasonic
Ultrasonic sensors measure levels by calculating the duration and strength of high
frequency sound waves that are reflected off the surface of the liquid and back to the
sensor – the time taken is relative to the distance between the sensor and the liquid. The
length of time in which the sensor takes to react is affected by various elements in the
atmosphere above the media such as turbulence, foam, temperature etc. Hence why the
mounting position is critical in these devices.
 Pros – No moving parts, compact, reliable, not affected by media properties

 Cons – expensive, invasive, performance can be affected by various elements in
the environment
 Applications – Non contact applications with highly viscous and bulk solids.
Used in systems that require remote monitoring
d. Microwave/Radar
In principle radar works in a similar way to ultrasonic, but the pulses travel at
the speed of light and again; the reliability and repeatability can be affecte d – but this
time by the dielectric constant of the fluid. However, radar can provide very precise level
information and also compensate for fixed structures within the container. The downside
can be that the initial cost of the sensor is relatively high, but several manufacturers are
making this technology more accessible to the wider market. These sensors are among
the handful of technologies that work well in foam and sticky substances.
 Pros – very accurate, no calibration required, multiple output options

 Cons – expensive, can be affected by the environment, limited detection range
 Applications – Moist, vaporous and dusty environments. They are also used in
systems in which temperatures vary
e. Vibrating or Tuning Fork

The vibrating sensor technology is perfect for solid and liquid level control,
including sticky materials and foam, as well as powders and fine grained solids.
However, the types of applications that can use tuning forks is limited to overfill or run
dry type applications and they do not provide continuous process measurement.
However can be used in conjunction with continuous level detection systems, acting as
alarm points for over-filling and leaks.
 Pros – Compact, cost effective

 Cons – Invasive, number of uses are limited
 Applications – level control of liquid, powders and fine grained solids within
mining, chemical processing and food and beverage industries.
f. Conductivity or Resistance
Conductive sensors are used for point-level sensing conductive liquids such as
water and highly corrosive liquids. Simply put, two metallic probes of different lengt hs
(one long, one short) insert into a tank. The long probe transmits a low voltage, the
second shorter probe is cut so the tip is at the switching point. When the probes are in
liquid, the current flows across both probes to activate the switch. One of th e benefits to
these devices is that they are safe due to their low voltages and currents. They are also
easy to use and install but regular maintenance checks must be carried out to ensure
there is no build up on the probe otherwise it will not perform properly.
 Pros – No moving parts, easy to use, low-cost

 Cons – Invasive, liquids need to be conductive, probe erosion
 Applications – Tank level measurement for boiler water, reagent monitoring,
highly corrosive liquids
g. Float Switch
Float switches are one of the most cost effective but also well proven
technologies for liquid level sensing. A float switch includes a magnet within a float and
a magnetic reed switch contained within a secure housing. The float moves with the
change in liquid and will cause the reed switch to either open or close depending on if
it’s in air or liquid. Although simple in design, this technology offers long -term
reliability at an attractive price point. Depending on what mounting style the user
chooses heavily depends on the design and construction of the tank or container the
switch will be situated. Typically, suppliers will offer a range of mounting options, with
the most common being horizontal/side mounting and vertical mounting.
 Pros: Non-powered, direct indication, relatively inexpensive, various outputs

 Cons: Invasive, moving parts, large in size, large amount of liquid has to be
present before the float makes contact.
 Applications: Tank level applications where water, oil, hydraulic fluids and
chemicals are being used.
4.9 Radiation sensors
Fig. 4.57 Spectrum of electromagnetic radiation.
Figure 4.57 shows a spectrum of the electromagnetic waves. On its left hand side,
there is a region of the γ -radiation. However, this is not the shortest possible length of the
electromagnetic waves. In addition, a spontaneous radiation from the matter is not necessarily
electromagnetic: There is the so-called nuclear radiation which is the emission of particles from
the atomic nuclei. It can be of two types: the charged particles (α and β particles and protons)
and uncharged particles, which are the neutrons. Some particles are complex like the α-
particles, which are nuclei of helium atoms consisting of two neutrons; other particles are
generally simpler, like the β-particles, which are either electrons or positrons. The γ - and X-
rays belong to the nuclear type of electromagnetic radiation.
In turn, X-rays depending on the wavelengths are divided into hard, soft, and ultra-
soft rays. Ionizing radiations are given that name because as they pass through various media
which absorb their energy, additional ions, photons, or free radicals are created. Certain
naturally occurring elements are not stable but slowly decompose by throwing away a portion
of their nucleus. This is called radioactivity. It was discovered in 1896 by Henry Becquerel when
he found that uranium atoms (Atomic Number Z =92) give off radiation which fogs
photographic plates. In addition to the naturally occurring radioactivity, there are many man-
made nuclei which are radioactive.
These nuclei are produced in nuclear reactors, which may yield highly unstable
elements. Regardless of the sources or ages of radioactive substances, they decay in accordance
with the same mathematical law. The law is stated in terms of the number N of nuclei still
undecayed and dN, the number of nuclei which decay in a small interval dt. It was proven
experimentally that
dN =−λN dt
where λ is a decay constant specific for a given substance. From Equation, it can be
defined as the fraction of nuclei which decays in unit time:
The SI unit of radioactivity is the Becquerel (Bq) which is equal to the activity of a
radionuclide decaying at the rate of one spontaneous transition per second. Thus, the
Becquerel is expressed in a unit of time: Bq=s−1. To convert to the old historical unit, the curie,
the Becquerel should be multiplied by 3.7×1010. The absorbed dose is measured in grays (Gy). A
gray is the absorbed dose when the energy per unit mass imparted to matter by ionizing
radiation is 1 joule per kilograms; that is, Gy = J/kg. When it is required to measure exposure to
X- and γ -rays, the dose of ionizing radiation is expressed in coulombs per kilogram, which is
an exposure resulting in the production of 1 C of electric charge per 1 kg of dry air. In SI, the
unit C/kg replaces the older unit roentgen. The function of any radiation detector depends on
the manner in which the radiation interacts with the material of the detector itself.
There are three general types of radiation detector: the scintillation detector, the
gaseous detector, and the semiconductor detector. Further, all detectors can be divided into
two groups according to their functionality: the collision detector and the energy detector. The
former merely detect the presence of a radioactive particle, whereas the latter can measure the
radiative energy; that is, all detectors can be either quantitative or qualitative.
a. Scintillating Detectors
The operating principle of these detectors is based on the ability of certain materials
to convert nuclear radiation into light. Thus, an optical photon detector in combination with a
scintillating material can form a radiation detector. It should be noted, however, that despite
the high efficiency of the conversion, the light intensity resulting from the radiation is
extremely small. This demands photomultipliers to magnify signals to a detectable level. The
ideal scintillation material should possess the following properties:
1. It should convert the kinetic energy of charged particles into detectable light with a
high efficiency.
2. The conversion should be linear; that is, the light produced should be proportional to
the input energy over a wide dynamic range.
3. The post luminescence (the light decay time) should be short to allow fast detection.
4. The index of refraction of the material should be near that of glass to allow efficient
optical coupling of the light to the photomultiplier tube.
The most widely used scintillators include the inorganic alkali halide crystals (of
which sodium iodine is the favorite) and organic-based liquids and plastics. The inorganics are
more sensitive, but generally slow, whereas organics are faster, but yield less light. One of the
major limitations of scintillation counters is their relatively poor energy resolution. The
sequence of events which leads to the detection involves many inefficient steps. Therefore, the
energy required to produce one information carrier (a photoelectron) is on the order of 1000 eV
or more, and the number of carriers created in a typical radiation interaction is usually no
more than a few thousand. For example, the energy resolution for sodium iodine scintillators
is limited to about 6% when detecting 0.662-MeV γ -rays and is largely determined by the
photoelectron statistical fluctuations. The only way to reduce the statistical limit on energy
resolution is to increase the number of information carriers per pulse. This can be
accomplished by the use of semiconductor detectors.
Fig. 4.58 Scintillation detector
A general simplified arrangement of a scintillating sensor is shown in Fig. 4.58 in

conjunction with a photomultiplier. The scintillator is attached to the front end of the
photomultiplier (PM). The front end contains a photocathode which is maintained at a ground
potential. There is a large number of special plates called dynodes positioned inside the PM
tube in an alternating pattern, reminding one of the shape of a “venetian blind.” Each dynode
is attached to a positive voltage source in a manner that the farther the dynode from the
photocathode, the higher is its positive potential. The last component in the tube is an anode,
which has the highest positive potential, sometimes on the order of several thousand volts. All
components of the PM are enveloped into a glass vacuum tube, which may contain some
additional elements, like focusing electrodes, shields, and so forth.
Although the PM is called a photomultiplier, in reality it is an electron multiplier, as

there are no photons, only electrons inside the PM tube during its operation. For the
illustration, let us assume that a γ -ray particle has a kinetic energy of 0.5 MeV (mega electron
volt). It is deposited on the scintillating crystal resulting in a number of liberated photons. In
thallium-activated sodium iodine, the scintillating efficiency is about 13%, therefore, a total of
0.5×0.13=0.065 MeV, or 65 keV, of energy is converted into visible light with an average energy
of 4 eV. Therefore, about 15,000 scintillating photons are produced per gamma pulse. This
number is too small to be detected by an ordinary photodetector; hence, a multiplication effect
is required before the actual detection takes place. Of the 15,000 photons, probably about
10,000 reach the photocathode, whose quantum efficiency is about 20%.The photocathode
serves to convert incident light photons into low-energy electrons. Therefore, the
photocathode produces about 2000 electrons per pulse. The PM tube is a linear device; that is,
its gain is almost independent of the number of multiplied electrons.
Because all dynodes are at positive potentials (V1 to V10), an electron released from the
photocathode is attracted to the first dynode, liberating several very low energy electrons at
impact with its surface. Thus, a multiplication effect takes place at the dynode. These electrons
will be easily guided by the electrostatic field from the first to the second dynode. They strike
the second dynode and produce more electrons which travel to the third dynode, and so on.
The process results in an increasing number of available electrons (avalanche effect). An
overall multiplication ability of a PM tube is in the order of 106. As a result, about 2×109
electrons will be available at a high voltage anode (Va) for the production of electric current.
This is a very strong electric current which can be easily processed by an electronic circuit. A
gain of a PM tube is defined as
where N is the number of dynodes, α is the fraction of electrons collected by the PM tube, and
δ is the efficiency of the dynode material (i.e., the number of electrons liberated at impact). Its
value ranges from 5 to 55 for a high yield dynode. The gain is sensitive to the applied high
voltage, because δ is almost a linear function of the inter dynode voltage.
A new design of a photomultiplier is called the channel photomultiplier or CPM for

short. It is the evolution of the classical photomultiplier tube. The modern CPM technology
preserves the advantages of the classical PM while avoiding its disadvantages. Figure 4.59
shows the face plate with a photocathode, the bent channel amplification structure, and the
anode. As in the PM of Fig. 4.58, photons in the CPM are converted inside the photocathode
into photoelectrons and accelerated in a vacuum toward the anode by an electrical field.
Instead of the complicated dynode structure, there is a bent, thin semi-conductive channel
which the electrons have to pass.
Each time the electrons hit the wall of the channel, secondary electrons are emitted
from the surface. At each collision, there is a multiplication of the secondary electrons resulting
in an avalanche effect. Ultimately, an electron multiplication of 109 and more can be obtained.
The resulting current can be read out at the anode. The CPM detector is potted with
encapsulation material and is quite rugged compared to the fragile PM. Magnetic field
disturbance is negligibly small. Figure 4.59 illustrates the CPM: on the left is a potted structure
and on the right is the unpotted structure. An important advantage of the CPM technology is
its very low background noise. The term “background noise” refers to the measured output
signal in the absence of any incident light. With classical PMs, the background noise
originating from the dynode structure is generally a non-negligible part of the total
background. As a result the only effective source of background for the CPM is generated from
the thermal emission of the photocathode. Because the CPM is manufactured in a monolithic
semi-conductive channel structure, no charge-up effects might occur as known from classical
PMs with isolating glass bulbs. As a result, extremely stable background conditions are
observed. No sudden bursts occur. Also, due to the absence of dynode noise, a very clean
separation between an event created from a photoelectron and electronic noise can be
performed. This leads into a high stability of the signal over time.
Fig. 4.59 Channel photomultiplier

b. Ionization Detectors
These detectors rely on the ability of some gaseous and solid materials to produce ion
pairs in response to the ionization radiation. Then, positive and negative ions can be separated
in an electrostatic field and measured. Ionization happens because upon passing at a high
velocity through an atom, charged particles can produce sufficient electromagnetic forces,
resulting in the separation of electrons, thus creating ions. Remarkably, the same particle can
produce multiple ion pairs before its energy is expended. Uncharged particles (like neutrons)
can produce ion pairs at collision with the nuclei.
Ionization Chambers
These radiation detectors are the oldest and most widely used. The ionizing particle
causes ionization and excitation of gas molecules along its passing track. As a minimum, the
particle must transfer an amount of energy equal to the ionization energy of the gas molecule
to permit the ionization process to occur. In most gasses of interest for radiation detection, the
ionization energy for the least tightly bound electron shells is between 10 and 20 eV. However,
there are other mechanisms by which the incident particle may lose energy within gas that do
not create ions (e.g., moving gas electrons to a higher energy level without removing it).
Therefore, the average energy lost by a particle per ion pair formed (called the W value) is
always greater than the ionizing energy. The W value depends on the gas, the type of
radiation, and its energy. In the presence of an electric field, the drift of the positive and
negative charges represented by the ions end electrons constitutes an electric current. In a
given volume of gas, the rate of the formation of the ion pair is constant. For any small volume
of gas, the rate of formation will be exactly balanced by the rate at which ion pairs are lost
from the volume, either through recombination or by diffusion or migration from the volume.
If recombination is negligible and all charges are effectively collected, the steady-state current
produced is an accurate measure of the rate of ion-pair formation.
Fig. 4.60 Ionization Chamber
Figure 4.60 illustrates a basic structure of an ionizing chamber and the current versus
voltage characteristic. A volume of gas is enclosed between the electrodes which produce an
electric field. An electric current meter is attached in series with the voltage source E and the
electrodes. There is no electrical conduction and no current under the no-ionization conditions.
Incoming radiation produces, in the gas, positive and negative ions which are pulled by the
electric field toward the corresponding electrodes, forming an electric current. At relatively
low voltages, the ion recombination rate is strong and the output current is proportional to the
applied voltage, because the higher voltage reduces the number of recombined ions. Sufficient
strong voltage suppresses all recombinations by pulling all available ions toward the
electrodes and the current becomes voltage independent. However, it still depends on the
intensity of irradiation. This is the region called saturation and where the ionization chamber
normally operates.
W Value (in eV/Ion Pair)

Gas Fast Alphas
electrons
A 27.0 25.9
He 32.5 31.7
N2 35.8 36.0
Air 35.0 35.2
CH4 30.2 29.0
Proportional Chambers
The proportional chamber is a type of a gas-filled detector which almost always
operates in a pulse mode and relies on the phenomenon of gas multiplication. This is why
these chambers are called the proportional counters. Due to gas multiplication, the output
pulses are much stronger than in conventional ion chambers. These counters are generally
employed in the detection and spectroscopy of low-energy X-radiation and for the detection of
neutrons. Contrary to the ionization chambers, the proportional counters operate at higher
electric fields which can greatly accelerate electrons liberated during the collision. If these
electrons gain sufficient energy, they may ionize a neutral gas molecule, thus creating an
additional ion pair. Hence, the process is of an avalanche type, resulting in a substantial
increase in the electrode current. The name for this process is the Townsend avalanche. In the
proportional counter, the avalanche process ends when the electron collides with the anode.
Because in the proportional counter, the electron must reach the gas ionization level, there is a
threshold voltage after which the avalanche process occurs.
Fig. 4.61 Various operating voltages for gas-filled detectors

In typical gases at atmospheric pressure, the threshold field level is on the order of
10 V/m. Differences between various gas counters are illustrated in Fig. 4.61 At very low
6
voltages, the field is insufficient to prevent the recombination of ion pairs. In the saturation
level, all ions drift to the electrodes. A further increase in voltage results in gas multiplication.
Over some region of the electric field, the gas multiplication will be linear, and the collected
charge will be proportional to the number of original ion pairs created during the ionization
collision. An even further increase in the applied voltage can introduce nonlinear effects,
which are related to the positive ions, due to their slow velocity.
Geiger–Müller Counters
The Geiger–Müller (G-M) counter was invented in 1928 and is still in use because of
its simplicity, low cost, and ease of operation. The G-M counter is different from other ion
chambers by its much higher applied voltage (Fig. 4.61). In the region of the G-M operation,
the output pulse amplitude does not depend on the energy of ionizing radiation and is strictly
a function of the applied voltage. A G-M counter is usually fabricated in the form of a tube
with an anode wire in the center (Fig. 4.62). The tube is filled with a noble gas, such as helium
or argon. A secondary component is usually added to the gas for the purpose of quenching,
which prevents the retriggering of the counter after the detection. The retriggering may cause
multiple pulses instead of the desired one. The quenching can be accomplished by several
methods, among which are short-time reduction of the high voltage applied to the tube, use of
high impedance resistors in series with the anode, and the addition of the quench gas at
concentrations of 5–10%. Many organic molecules possess the proper characteristics to serve as
a quench gas.
Of these, ethyl alcohol and ethyl formate have proven to be the most popular. In a
typical avalanche created by a single original electron, secondary ions are created. In addition
to them, many excited gas molecules are formed. Within a few nanoseconds, these excited
molecules return to their original state through the emission of energy in the form of
ultraviolet (UV) photons. These photons play an important role in the chain reaction occurring
in the G-M counter. When one of the UV photons interacts by photoelectric absorption in some
other region of the gas, or at the cathode surface, a new electron is liberated which can
subsequently migrate toward the anode and will trigger another avalanche. In a Geiger
discharge, the rapid propagation of the chain reaction leads to many avalanches which initiate,
at random, radial and axial positions throughout the tube. Secondary ions are therefore
formed throughout the cylindrical multiplying region which surrounds the anode wire.
Hence, the discharge grows to envelop the entire anode wire, regardless of the position at
which the primary initiating event occurred. Once the Geiger discharge reaches a certain level,
however, collective effects of all individual avalanches come into play and ultimately
terminate the chain reaction.
This point depends on the number of avalanches and not on the energy of the
initiating particle. Thus, the G-M current pulse is always of the same amplitude, which makes
the G-M counter just an indicator of irradiation, because all information on the ionizing energy
is lost. In the G-M counter, a single particle of a sufficient energy can create about 10 9–1010 ion
pairs. Because a single ion pair formed within the gas of the G-M counter can trigger a full
Geiger discharge, the counting efficiency for any charged particle that enters the tube is
essentially 100%. However, the G-M counters are seldom used for counting neutrons because
of a very low efficiency of counting. The efficiency of G-M counters for γ -rays is higher for
those tubes constructed with a cathode wall of high-Z material. For instance, bismuth (Atomic
number Z =83) cathodes have been widely used for the γ -detection in conjunction with gases
of high atomic numbers, such as xenon and krypton, which yield a counting efficiency up to
100% for photon energies below about 10 keV.
Fig. 4.62 Geiger Muller Counter
c. Semiconductor detectors
A semiconductor detector is a radiation detector which is based on a semiconductor,
such as silicon or germanium to measure the effect of incident charged particles or photons. In
general, semiconductors are materials, inorganic or organic, which have the ability to control
their conduction depending on chemical structure, temperature, illumination, and presence of
dopants. The name semiconductor comes from the fact that these materials have an electrical
conductivity between that of a metal, like copper, gold, etc. and an insulator, such as glass.
They have an energy gap less than 4eV (about 1eV). In solid-state physics, this energy gap or
band gap is an energy range between valence band and conduction band where electron states
are forbidden. In contrast to conductors, electrons in a semiconductor must obtain energy (e.g.
from ionizing radiation) to cross the band gap and to reach the conduction band.
Semiconductor detectors are very similar in operation as photovoltaic panels that generate
electric current. In a similar way a current can be induced by ionizing radiation. As ionizing
radiation enters the semiconductor, it interacts with the semiconductor material. It may excite
an electron out of its energy level and consequently leave a hole. This process is known
as electron–hole pair generation.
In semiconductor detectors, the fundamental information carriers are these electron-

hole pairs, which are produced along the path taken by the charged particle (primary or
secondary) through the detector. By collecting electron-hole pairs, the detection signal is
formed and recorded. Semiconductor detectors are widely used in radiation protection, assay
of radioactive materials and physics research because they have some unique features, can be
made inexpensively yet with good efficiency, and can measure both the intensity and the
energy of incident radiation. These detectors are employed to measure the energy of the
radiation and for identification of particles. Of the available semiconductor materials, silicon is
mainly used for charged particle detectors (especially for tracking charged particles) and soft
X-ray detectors while germanium is widely used for gamma ray spectroscopy. A large, clean
and almost perfect semiconductor is ideal as a counter for radioactivity. However, it is difficult
to make large crystals with sufficient purity. The semiconductor detectors have, therefore, low
efficiency, but they do give a very precise measure of the energy. Semiconductor detectors,
especially germanium based detectors, are most commonly used where a very good energy
resolution is required. In order to achieve maximum efficiency the detectors must operate at
the very low temperatures of liquid nitrogen (-196°C).
Principle of Operation of Semiconductor Detectors

The operation of semiconductor detectors is summarized in the following points:
 Ionizing radiation enters the sensitive volume of the detector and interacts with the
semiconductor material.
 Particle passing through the detector ionizes the atoms of semiconductor, producing
the electron-hole pairs. The number of electron-hole pairs is proportional to the
energy of the radiation to the semiconductor. As a result, a number of electrons are
transferred from the valence band to the conduction band, and an equal number of
holes are created in the valence band.
 Under the influence of an electric field, electrons and holes travel to the electrodes,
where they result in a pulse that can be measured in an outer circuit,
 This pulse carries information about the energy of the original incident radiation. The
number of such pulses per unit time also gives information about the intensity of the
radiation.
The energy required to produce electron-hole-pairs is very low compared to the

energy required to produce paired ions in a gaseous ionization detector. In semiconductor
detectors the statistical variation of the pulse height is smaller and the energy resolution is
higher. As the electrons travel fast, the time resolution is also very good. Compared with
gaseous ionization detectors, the density of a semiconductor detector is very high, and charged
particles of high energy can give off their energy in a semiconductor of relatively small
dimensions.
Silicon-based semiconductor detectors

These detectors are mainly used for charged particle detectors (especially for tracking
charged particles) and soft X-ray detectors while germanium is widely used for gamma ray
spectroscopy. A large, clean and almost perfect semiconductor is ideal as a counter
for radioactivity. However, it is difficult to make large crystals with sufficient purity. The
semiconductor detectors have, therefore, low efficiency, but they do give a very precise
measure of the energy. Detectors based on silicon have sufficiently low noise even by room
temperature. This is caused by the large band gap of silicon (Egap= 1.12 eV), which allows us
to operate the detector at room temperature, but cooling is preferred to reduce noise. The
drawback is that silicon detectors are much more expensive than cloud chambers or wire
chambers and require sophisticated cooling to reduce leakage currents (noise). They also suffer
degradation over time from radiation, however this can be greatly reduced thanks to the
Lazarus effect.
Since silicon-based detectors are very good for tracking charged particles, they
constitute a substantial part of detection system at the LHC in CERN. Most silicon particle
detectors work, in principle, by doping narrow (usually around 100 micrometers wide) strips
of silicon to turn them into diodes, which are then reverse biased. As charged particles pass
through these strips, they cause small ionization currents that can be detected and measured.
Arranging thousands of these detectors around a collision point in a particle accelerator can
yield an accurate picture of what paths particles take. For example, the Inner Tracking System
(ITS) of a Large Ion Collider Experiment (ALICE) contains three layers of silicon-based
detectors:
 Silicon Pixel Detector (SPD)
 Silicon Drift Detector (SDD)
 Silicon Strip Detector (SSD)
Silicon Strip Detectors

Silicon-based detectors are very good for tracking charged particles. A silicon strip
detector is an arrangement of strip like shaped implants acting as charge collecting electrodes.
Silicon strip detectors 5 x 5 cm2 in area are quite common and are used in series (just like
planes of MWPCs) to determine charged-particle trajectories to position-accuracies of the
order of several μm in the transverse direction. Placed on a low doped fully depleted silicon
wafer these implants form a one-dimensional array of diodes. By connecting each of the
metalized strips to a charge sensitive amplifier a position sensitive detector is built. Two
dimensional position measurements can be achieved by applying an additional strip like
doping on the wafer backside by use of a double sided technology. Such devices can be used
to measure small impact parameters and thereby determine whether some charged particle
originated from a primary collision or was the decay product of a primary particle that
traveled a small distance from the original interaction, and then decayed.
Silicon strip detectors constitute a substantial part of detection system at the LHC in
CERN. Most silicon particle detectors work, in principle, by doping narrow (usually around
100 micrometers wide) strips of silicon to turn them into diodes, which are then reverse biased.
As charged particles pass through these strips, they cause small ionization currents that can be
detected and measured. Arranging thousands of these detectors around a collision point in a
particle accelerator can yield an accurate picture of what paths particles take. For example, the
Inner Tracking System (ITS) of a Large Ion Collider Experiment (ALICE) contains three layers
of silicon-based detectors:
 Silicon Pixel Detector (SPD)
 Silicon Drift Detector (SDD)
 Silicon Strip Detector (SSD)
In experimental physics, ΔE-E detectors, known as telescopes, are powerful devices

for charged particles identification. In order to provide charged-particle identification,
telescopes consisting of pairs of thin and thick surface-barrier detectors can be used. These
detectors must be positioned in series. The velocity is deduced from the stopping power
measured in the thin detectors (ΔE detectors). There is a strong correlation between the energy
deposited in each detector. This correlation depends on the mass (A), the charge (Z) and the
kinetic energy (E) of each particle. The mass is deduced from the range or from the total kinetic
energy loss in the thicker detector (E detector). Telescopes can be composed of several
detectors (ionization chambers, silicon detectors and scintillators for instance) stacked in order
to slow down incident charged particles, the first detector being the thinnest and the last one
the thickest. CsI scintillation counters can be, for example used as final E counters. As an
example of telescope, an assembly based on two front ΔE silicon detectors (10 or 30 μm) and
an E silicon counter 1500 µm thick may be used for detection of high-energy charged particles.
Germanium-based semiconductor detectors

These detectors are most commonly used where a very good energy resolution is
required, especially for gamma spectroscopy, as well as x-ray spectroscopy. In gamma
spectroscopy, germanium is preferred due to its atomic number being much higher than
silicon and which increases the probability of gamma ray interaction. Moreover, germanium
has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon
and 2.9 eV for germanium. This also provides the latter a better resolution in energy. A large,
clean and almost perfect germanium semiconductor is ideal as a counter for radioactivity.
However, it is difficult and expensive to make large crystals with sufficient purity. While
silicon-based detectors cannot be thicker than a few millimeters, germanium can have a
depleted, sensitive thickness of centimeters, and therefore can be used as a total absorption
detector for gamma rays up to few MeV.
On the other hand, in order to achieve maximum efficiency the detectors must
operate at the very low temperatures of liquid nitrogen (-196°C), because at room
temperatures the noise caused by thermal excitation is very high. Since germanium detectors
produce the highest resolution commonly available today, they are used to measure radiation
in a variety of applications including personnel and environmental monitoring for radioactive
contamination, medical applications, radiometric assay, nuclear security and nuclear plant
safety.
4.10 Fibre Optic Sensor

Fibre optic sensors could be classified as a separate group of sensors, as, although
such sensors are in their prime, these are considered for sensing different types of variables
such as temperature, liquid level, fluid flow, magnetic field, acoustic parameters, and so on.
However, optical radiation happens to be the energy source in these applications with the fibre
acting as medium as well as a sensor. Optical fibres are basically considered as communication
channels but it has been noticed that the optical transmission is affected by external
parameters/stimuli such as temperature, acoustic vibration, magnetic field and many more.
Study of these 'afflictions' or 'interferences' to the extent of the fibre being utilized as the sensor
of such parameters has now been made and as a sensing device, fibre has been divided into
two groups:
Active—the fibre is exposed to the energy source that affects the measurand and a
consequent change in the optical propagation in the fibre is detected and related to the
measurand. These are discussed in detail in subsequent subsections.
Passive—light transmitted through a fibre, called input fibre, is first modulated by a
conventional optical sensor and this intensity-modulated light is propagated through a second
fibre called the output fibre and then detected and corrected with the measurand.
a. Temperature Sensors
When two identical optical fibres are used to propagate radiation from a source, say,
a laser source, and if one of these fibres is in a medium with temperature different than that of
the other, the optical outputs from the two fibres would have a phase difference which is a
function of the difference of temperature as mentioned. This phase difference is due to optical
path length variations in the two paths occurring due to temperature difference and is so small
that it can only be measured by producing interference patterns. Two schemes are given in
Figs. 4.63 (a) and (b) that use He-Ne laser as source and the first one uses Mach-Zender
interferometer as the detector while the second uses a Michelson interferometer. The beam-
splitter (BS) and mirrors (Mi) in the first case have been dispensed with using fibre couplers in
the second. From laser source Reference path fibre
(a) (b)
Fig. 4.63 Temperature measurement using optical fibres (a) Phase difference method,
(b) Technique avoiding beam splitter and mirror
Another optical fibre temperature sensor is used on the principle that a black body
cavity changes radiance with varying temperature. Thus, at the end of a fibre a black body
cavity is formed. The fibre is a high temperature fibre, usually a sapphire fibre, of diameter
0.25-1.25 mm. A thin film of iridium is sputtered onto the end-surface and a protective cover of
Aluminium oxide (Al2O3) is then provided. This measuring fibre has a length usually within
0.3 m and not less than 5 cm. This propagates the radiation from the formed cavity which is
being heated by the heat of the process. At the propagation end, another fibre, a low
temperature fibre made of glass of about 0.6 mm diameter is coupled that has a length usually
within 10 m. The detector system consists of one lens and two narrow band filters of close
range middle wavelengths, two photomultiplier tubes in two measuring channels fed by a
beam-splitter and a mirror. In fact, the filters have wavelengths of 600 and 700 nm respectively
with a spread at the centre of 0.1 gm. The two channels are used to measure temperature by
comparison over a range 500-2000°C. With an input power of 0.1 µW, for 1°C change there
occurs 2% optical flux change and the system has a resolution of 1 in 108. This system is now
being used as a temperature standard between 630.74 and 1769°C which are aluminium and
platinum points respectively. Figure 4.64 shows one such temperature sensor.
Fig. 4.64 Temperature sensor fibre black body cavity
Optical fibre can he used for distributed temperature sensing. Optical pulse from a
pulsed laser source is sent along a fibre over a distance covering a few kilometres. Any
localized change in temperature somewhere along the fibre changes its backscattered intensity
ratio (Stokes/anti-stokes Raman). This backscattered, light is filtered and Raman components
are detected by photodetectors from which the temperature can he known. From the pulse
delay time, the location can also be identified. Resolution of 1°K and 2-3 metres can be
obtained in this system-A schematic representation of the system is shown in Fig. 4.65.
Fig. 4.65 Temperature sensing using backscatter in optical fibre
b. Liquid Level Sensing

Usually, light propagates through a fibre by total internal reflection with appropriate
cladding or even without that, if the light incidence angle is properly chosen. This is because
the refractive index of air is such, with respect to that of the fibre, that no refraction can take
place. If, however, the fibre is placed in a liquid medium of a different refractive index, it is
possible that light refracts into the liquid and total internal reflection inside the fibre stops,
stopping light propagation in it. This principle is utilized in measuring liquid level at specific
values as shown in Fig. 4.46. The bottom end of the fibre is shaped like a prism so that with
large difference in refractive indices of the fibre and the medium like air, there is internal
reflection and the light travels to be detected as shown in Fig. 4.46(a). When liquid level rises
to cover the bottom of the fibre, light refracts into the liquid and the detector fails to show any
output, as shown by Fig. 4.46(b).
Fig. 4.46 Level detector using optical fibre: (a) level below sensor and (b) level covering
sensor.
This single position level detection has been extended for discrete multistep detection
covering the entire height of the tank. In this, a step-index multimode fibre is used and the
fibre goes down carrying the light but in the return upward path, its cladding is exposed and
the fibre Is also given a zig-zag rise with small bend radius at regular intervals in length. When
no liquid is there, cladding mode operation continues and a detector at the end of the return
path of the fibre shows full intensity. But with liquid rising in the tank, refraction of light into
liquid occurs at each bend and the intensity detected by the detector becomes less. Thus, for n
bends there would be n-stepped intensity of signal, reducing in steps with rising liquid. Figure
4.47 (a) shows the system and Fig. 4.47 (b) depicts the intensity versus height plot.
Fig. 4.47 Liquid level sensing in steps

c. Fluid Flow Sensing
Fluid flow rate has been sensed by an optical fibre mounted transversely in a pipeline
through which it flows. Because of the fibre, mounted across the flow, vortex shedding occurs
in the channel and the fibre vibrates, which in turn, causes phase modulation of the optical
carrier wave propagating through the fibre. The vibration frequency is proportional to the
flow rate. Using multimode fibres of core diameter 0.2-0.3 mm and special detecting
techniques, flow rates over a range of 0.2-3 m/s can be measured. Figure 5.59 shows the
scheme to sense fluid flow. The fibre is kept under tension by a tension adjusting system and a
fibre clamp. Flexible fillers are often used for small adjustment of tension fibre.
Fig. 4.48 Fluid flow sensing using fibre optics

d. Microbend Sensors
Acoustic pressure sensing can be done by the microbending of a multimode fibre.
Figures 4.49 (a) and (b) show how light loss occurs in microbends of a fibre. The technique is
utilized as shown in Fig. 4.50 Optical fibre is placed in two corrugated plates to form a
transducer as shown. Applied force causes microbending in the fibre. Consequently, more
light is lost and the receiver detector indicates less intensity. A calibration of force in terms of
the intensity of detected light may also be made.
Fig. 5.49 Microbend sensors: (a) normal condition; no loss of light, (b) bent
condition; partial loss of light. Restoring spring Force applied Photo Light detector
Fig. 4.50 Microbend force sensor using optical fibre.

4.11 Smart Sensors
A 'SMART' transducer is simply expanded as "Sensing, Monitoring And Remote

Transmission" transducer. It can be of either analog type or digital type which can be
combined with a processing unit and a communication interface. These sensors can provide an
electrical output. When they combined with suitable interfacing devices, these sensors are also
called intelligent sensors. It can be defined as a microprocessor-based sensor which can
perform one or more number of the functions like logical functions, decision making, two-way
communication, etc. It can be simply expressed as: Sensors + Suitable interfacing circuits =
SMART sensors Smart sensors are different from conventional type sensors due to their special
functions such as ranging, calibration, communication with other devices, etc.
a. Features of Smart Sensors

1. Special features of smart sensors arc listed as follows:
2. Automatic ranging
3. Auto calibration of data through an in-built system
4. Automatic data acquisition system and storage of data in local memory of
the field device
5. Auto linearization of non-linear functions
6. Auto correction of offsets, time and temperature drifts
7. Self tuning control algorithms
8. Easy communication through serial bus
b. Need to Adopt Smart Sensors

To utilize the following benefits, it is essential to adopt smart sensors in a control
system:
1. No need of bulk cables and connectors: Since smart sensors are of electronic
circuits, there is no need to use any bulk cabling and connectors, and hence
overall cost of the system gets reduced.
2. Digital communication: Due to the integrated manner, the smart sensors can
provide digital communication. They also have an in-built self test or
diagnostic facility.
3. Enhanced features: Smart sensors are having enhanced features like self
calibration. computation, fault diagnostics, duplex communication, multi
sensing, etc. Hence, they will be preferred in all kinds of control system.
4. Reliability: Reduced wiring and ability to provide self test and diagnostics
make the sensors more reliable to use.
5. Higher SNR: The electrical characteristic problems with the conventional
sensors are overcome by the use of smart sensors. There is no noise
interference in Smart sensors due to no usage of long transmission cables.
6. Improved Characteristics: Improved linearity when compared with
conventional non-linear characteristics, reduced cross-sensitivity, reduced
offset and automatic calibration are some of the important characteristics of
a smart sensor.
c. Architecture of a Smart Sensor
Figure 4.51 Block diagram of a smart sensor

The simple structure of any smart sensor is illustrated in Figure 4.51. The sensing
element and signal conditioning are combined to develop a transduction element. The detailed
architecture of a smart sensor is shown in Figure 16.2.
Figure 4.52 Architecture of a smart sensor
The components present in the architecture of a smart sensor are given as follows:
1. Transduction elements (transducers)
2. Interfacing hardware (data acquisition system)
3. Memory hardware
4. Programming devices
5. Communication facilities
6. Compensation facilities
From the architecture, there are several amplifiers (Al, A2, A3 and A4) and Sample
and Hold Circuits (S/H1, S/H2, S/H3 and S/H4) corresponding to different transducers
(Transducer 1, Transducer 2, Transducer 3 and Transducer 4) respectively. To obtain the
digital signal, the analog signals are sent to ADC via Analog Mux. Any type of ADC like flash
type, successive approximation type or dual slope type ADC can be preferred based on
required conversion time constraint. Offset compensation and correction circuits are also
provided along with the processor. These circuits are useful for the offset correction and zero
compensation purpose against temperature drift. For the data storage and retrieval, memory is
also available in the smart sensor.
Sensing Elements
Sensors along with the signal conditioning circuits or simply transducers are used as
sensing purpose. They are in contact with the real world signals or measuring systems. Any
variables such as temperature, pressure, flow, level, etc. can be measured with them. They are
also called a primary sensing part of any measurement system.
Data Acquisition System (DAS)

It can be used for the measurement and processing of any real world signal, or the
variable which is to be measured, before being monitored, displayed or recorded. It consists of
transducers, amplifiers, sample and hold devices, multiplexer and analog to digital conversion
circuits.
Signal Processing Unit

If sensors are available for transduction purpose, it is necessary to use signal
processing units to process the output signals. They will do the process of modifying or
manipulating the input signal in such a way that they should meet the requirements for
further processing. The process of signal conditioning is carried out by the following steps:
 Excitation: For passive transducers, it is essential to supply the power. Signal

conditioning circuit itself generates excitation or power supply to the passive
transducers such as strain gauge, RTD, and etc. If RTD is used as sensor for
temperature measurement, current excitation source can be used to convert change
in resistance based on temperature variation into measurable voltage.
 Amplification: For the purpose of increasing the resolution and reducing the noise,
amplifiers are used which will do boosting of input signals. Filtering: To remove
the unwanted noise components present in the input signals, filters are required.
Depending on the input signal, LPF or HPF can be preferred.
 Linearization: For better response, non-linear response can be converted into linear
response by means of linearizing circuits.
 Sampling: The process of conversion of continuous signal into discrete signal is
called sampling.
 ADC/DAC: An Analog to Digital Converter (ADC) can be used to convert physical
quantity (continuous nature) into digital quantity, whereas the digital quantity can
be converted into continuous analog quantity by means of a Digital to Analog
Converter (DAC).
 Sample and Hold (S/H) circuit: Sample and hold circuit is a device which samples
the voltage of continuously varying analog nature. Further the value can be hold at
a constant level for a specified period of time. In any ADC circuit, sample and hold
circuit can be used to eliminate the variation in the input signal which corrupts the
conversion process.
Multiplexer
It is selection device which selects one of the several analog or digital input signals
and the selected signal will then he forwarded to a single line. If there are 2" inputs has it
selector lines, a multiplexer is used to select a particular input signal to he sent to a single line.
Programming Devices
After the process of Data Acquisition Process, the processed output signal is fed to
the programming device such as microprocessor unit for the purpose of programming.
Memory
For the purpose of storage and retrieval of programmed data, memory unit can be used.
d. Evolution of Smart Sensors

The following is the organisation of the history of Smart sensors starts from First
generation to the Fifth (Current) generation.
 First generation devices had little, if any, electronics associated with them.
 Second generation devices were part of pure analog systems. They have
virtually connected to its associated electronics available in remote place
from the sensor.
The block diagram of a third generation smart device is shown in Figure 4.53.
Figure 4.53 Third generation smart sensor
In the third generation devices, transducers and their associated signal conditioning
circuits are used as discrete devices. ADC can be used for the conversion of analog input signal
into digital output signals. Microcomputer was used for the programming purpose, and ROM
was used for the storage and retrieval of data. With suitable communication interface facility,
communication with the host computer was carried out. In the fourth generation of smart
sensing devices, the transducer and signal conditioning devices have combined in a monolithic
package. The block diagram of fourth generation device is shown in Figure 4.54
Figure 4.54 Fourth generation smart sensor
The block diagram of fifth generation smart sensor is shown in Figure 16.5 along with
and integrated sensor Analog to Digital Conversion (ADC) device. PROM memory can be
combined with sensing and conversion unit in monolithic form. The functioning of the overall
unit can also be carried out easily.
Figure 4.55 Fifth generation smart sensor
e. Applications of Smart Sensors

1. There are several applications of smart sensors in use. Some of them are
given below:
2. Smart accelerometers for the measurement of acceleration
3. Smart optical sensors for object detection
4. Smart IR detector array for the identification of presence of products
5. Smart sensor for defect or fractures monitoring in structures or
infrastructures
6. Smart sensors to detect the minerals present in geological areas
7. Food processing and preservation
8. Biological hazard detection
9. Health monitoring and medical diagnostics
Smart Accelerometer
Smart sensing accelerometer consists of electronics and sensing element combined
together on a single silicon chip. It has a metal coated SiO2, cantilever beam that is fabricated
on the silicon chip. In this arrangement, the capacitance between the cantilever beam and the
silicon substrate provides the output voltage signal. Figure 4.56 shows the photographic view
of a smart sensing accelerometer.
Figure 4.56 Photographic view of a smart sensing accelerometer
Smart Sensing Optical Sensor

To measure the exposure in cameras, smart sensing based optical sensors such as
Optical angle encoders and optical arrays are used. These sensors are functioning similar to
silicon-based pressure sensors. Figure 4.57 shows the photographic view of a smart sensing
optical sensor.
Figure 4.57 Photographic view of a smart sensing optical sensor
4.12 Film sensors

Basically, such sensors are produced by film deposition of different thicknesses on
appropriate substrates. The deposition techniques used are different for the thick and thin film
sensors. Sensors produced through these techniques have varying electrical and mechanical
properties while a variable is being sensed.
a. Thick Film Sensors

Thick film deposition is a mature technique and there has not been substantial
improvement whilst thin films are being developed almost at the same pace as
microelectronics incorporating latest technology. It is to be noted that thick film process had
been in use for producing capacitor, resistor, and conductors—and has subsequently been
adopted in sensor development. The processing of a sensor can be expressed schematically as
Step 1 : Selection and preparation of a substrate.

Step 2 : Preparation of the initial coating material in paste or paint form.
Step 3 : Pasting or painting the substrate by the coating material or screen
printing it.
Step 4 : Firing the sample produced in step 3 in an oxidising atmosphere at a
programmed temperature format.
The substrates used for developing thick film over them are alumina (96% or 99.5%)
and beryllia (99.5%). These are fired at about 625°C. Others used are enamelled steel which is
low carbon steel coated with low alkali content glass frit that are fired at around 850°C.
Alumina or beryllia have dielectric constants around 9.5 and 7 respectively with dielectric
strength around 5600 V/gm. Thermal expansion coefficients are 6.5 x 10 -6 and 7.5 x 10-6
respectively with bulk resistivity being almost the same for both, at about 1014 Ωcm, thermal
conductivities are 0.36 and 2.5 W/(cmK) respectively. Enamelled steel has better strength and
machinability being almost double for those of alumina or beryllia which have values around
175 MPa. Though it has better machinability and improved thermal conductivity, enamelled
steel is less costly.
For thin film deposition, alumina and beryllia can also be used. Besides, special glass,
quartz, fused silica and sapphire are often used which have similar properties and sometimes
even better. It is to be understood that the compatibility between the substrate and the
transducing element in film sensors is very important. For example, there should not be
difference of thermal expansion coefficients which would induce stress between them and
correspondingly result in zero offset, drift, and instability. Sensors which are produced
through thick film deposition (~20 gm) are used for sensing temperature, pressure, gas
concentration, and humidity.
Temperature: Thick film sensors such as (i) thermopiles (usually of gold and gold-
platinum alloy), (ii) thermistors (usually with oxides of manganese, ruthenium. and cobalt),
and (iii) temperature dependent resistances based on gold, platinum, and nickel are used for
temperature sensing.
Pressure: Sensing pressure is possible by making thick film diaphragms or capacitive

devices made with alumina (A1,03) and Bi2Ru2O7, or piezoresistive devices made of same
materials.
Concentration of gases: Gases such as methane (CH4), CO, and C2H5OH can be
checked for concentration using films of SnO2 + Pd, SnO2/ThO2 + hydrophobic SiO2. H2, CO,
C2H5OH, and isobutane are sensed by SnO2 + Pd, Pt, Ba-, Sr and CaTiO3 (Nasicon). Oxygen
and hydrogen gases also are separately sensed by these types of films
Humidity: It is sensed by (i) resistive films made from RuO2 (spine) type)/glass and
(ii) capacitive films made from glass ceramic/Al,03. On the other hand, dew point is sensed by
films made from (BaTiO3/Ru02)-glass.
Starting from the same basic material, say SnSO4, one can produce SnO2 based
sensors for H2, CO, and NH3, as mentioned in the preceding paragraphs. The host material (1%
by weight), PdC12 mixed with SnO2 as catalyst and Mg(NO3)2 (also 1% by weight) is mixed
presumably for sensitivity range, The combination is fired at about 800°C for one hour.
Selectivity is obtained by a second tiring process at almost the same temperature by adding
different ingredients for different gases. For H2 detection, for example, it is mixed with Rh (6%
by weight) and fired for 1 hour at 800°C. For CO, ThO2 is added (5% by weight) and for NH3,
ZrO2 is added (5% by weight) and processed in the same manner as explained. For control of
the porosity of the films which determine the overall sensor sensitivity, organic materials arc
added in a selective manner. For example, alcohol is added for H2 and sometimes, inorganic
materials work well with appropriate selection. Silica of different varieties is added for CO and
NH3. The materials so produced are now painted on the substrate and dried, then calcined at
controlled temperature for varying times. The other thick film variety is the ceramic-metal or
cemet which consists of gold/silver/ ruthenium/palladium based complex oxides in an
insulating medium, mainly glass (lead borosilicate). There are thick film resistors of the cemet
which require precise control of heat treatment. The resistivity is controlled by the size,
concentration, and distribution of the metallic (conductive) component, that is, their resistive
properties, and the insulating medium. Pure metal powders and resistor pigments differ in so
far as changes in their resistive values are concerned and hence, their embedding in per cent
weight changes the resistivity of the sensor developed. Figure 4.58 shows the difference for
two typical cases.
Fig. 4.58 Resistivity variation with change in pigment

b. Thin Film Sensors
Thin film sensor processing differs from thick film technology mainly in the film
deposition techniques. This technology is similar to that used in silicon micromechanics. A
number of techniques are used for thin film deposition, such as:
a. Thermal evaporation
 Resistive heating
 Electron beam heating
b. Sputter deposition
 DC with magnetron
 RF with magnetron
c. Chemical Vapour deposition (CVD)
d. Plasma Enhanced Chemical Vapour Deposition (PECVD)
e. Metallo-Organic Deposition (MOD)
f. Langmuir—Blodgett technique of monolayer deposition.
Of these, the thermal evaporation and sputter deposition are decades old. However,
in the sputter deposition technique, magnetron sputtering is an improved form where a flow
magnetic field perpendicular to the applied electric field is applied. This increases the
ionization probability of the electrons as the Lorentz force E x B restricts the primary electrons
near the cathode. As a result, sputtering efficiency is also enhanced.
Plasma enhanced chemical vapour deposition (PECVD) has been found to be

particularly suitable for sensor fabrication. This is a low temperature process in which plasma
is introduced into the deposition chamber to enhance the pyrolytic process which in normal
CVD process is performed by thermal decomposition that requires high temperature. In this
process, the volatile compound of the material to be deposited is thus vaporized, decomposed,
and made to react with gaseous species over the substrate to produce a nonvolatile amorphous
product on the surface of the substrate. The deposition level is controlled by controlling the
flow-rates of the vapours. A parallel plate, radial flow type PECVD processing chamber is
shown in Fig. 4.59.
Fig. 4.59 A PECVD processing system

Metallo-organic deposition (MOD) is another very versatile technique which can be

used both for thick and thin film sensor fabrication. It consists of applying ink of metallo-
organic compound to the silicon substrate consisting of silicon wafer coated with silica, then
spinning the assembly at about 3000 rpm and finally heat treating the deposit. Metallo-organic
compounds consist of a central metal ion bonded with a ligand through a heterobridge
containing oxygen, sulphur, nitrogen, phosphorus, arsenic, and so on. It is prepared by
dissolving the compound in organic solvent. Specially prepared thin films, by this technique,
are barium titanates (BaTiO3) and their derivatives that are mostly used in pyroelectric
measurement, tin-oxides for gas sensors, super conducting oxides such as yttrium-barium-
copper oxides (YBaxCuyOz) for high temperature and ZrO2, TiO2, stabilized by yttrium for
oxygen sensors. Thin film sensors measure the same variables as done by thick film
counterparts with variations in principles and materials. Table 8.1 shows the variable, sensing
element, and principle of sensing for certain different variables.
Working principles of the materials

Variable Material Principle
Flow Au Thermoanemometry
Humidity Ta2O5 Capacitance change
Magnetic field Ni81Fe19, NiCo, Co72Fe8B20 Magnetoresistive effect
Oxygen ZnO Variation in electrical
conductivity
Pressure Polysilicon Piezoresistive effect
(Diaphragm)
Radiation Au Bolometry
Strain CrNi Piezoresistive effect
Temperature Pt Resistance variation
Langmuir—Blodgett film can be fabricated from materials which have a polar

hydrophilic head and a hydrphobic tail, that is, an amphiphilic material. Fatty acids such as
palmitic (16), magaric (17), stearic (18), arachidic (20), and so on satisfy the requirement. They
are generally represented by
( )
It is a monolayer film. Fatty acid films are soft with low melting points (<70°) and are
not robust for electronic sensors/devices. Their derivatives which can be polymerized are used
instead. Examples are vinyl stearate and diacetylenic acid which can be polymerized by γ or
UV-radiation usually after deposition. However, polymerization before deposition can also be
done. Solid film generation or deposition on the substrate is done by a process known as
dipping which is a special mechanism and is either vertical or horizontal. However,
orientation of the film on the substrate can be different with respect to hydrophilic and
hydrophobic ends.
Considering the circle-end as hydrophilic and line-end as hydrophobic three different

orientations are obtained which are shown in Fig. 4.60 (a), (b), and (c). These are marked as
types X, Y, and Z respectively.
Fig. 4.60 Type X, Y, and Z orientations
LB films are good as ion-selective membranes when deposited on the insulator face of
the ion-sensitive FET. Majority of LB films of different materials have found convenient use as
biosensors. LB gas sensors are classified as chemiresistors and surface acoustic devices (SAW).
In the former, the change in conductivity of the sensor is related to the concentration of the gas
and the materials used for films are phthalocyanines and porphyrines. Surface acoustic devices
such as general SAW devices sense change in frequency due to absorption of gas by the film
deposited on the piezoelectric substrate. Same materials can also be used for film deposition.
4.13 MEMS sensors

MEMS is a technology that combines computers with tiny mechanical devices such as sensors,
valves, gears, mirrors and actuators embedded in a semiconductor chips. Basically, a MEMS
device contains microcircuitry on a tiny silicon chip into which some mechanical devices such
as mirror or sensor has been manufactured. Potentially, such chips can be built in large
quantities at low cost, making them cost-effective for many uses. There are three generic and
distinct merits for MEMS devices and micro fabrication technologies:
 Miniaturization
 Microelectronics Integration
 Parallel Fabrication with high precision.
MEMS products will compete in the market place on the grounds of functional
richness, small sizes, unique performance characteristics ( e.g., fast speed), and/or low cost.
a. Energy Domains and Transducers

MEMS technology enables revolutionary sensors and actuators. In general
terms, sensors are devices that detect and monitor physical or chemical phenomenon, whereas
actuators are ones that produce mechanical motions, force, or torque. Sensing can be broadly
defined as energy transduction processes that result in perception, whereas actuation is energy
transduction processes that produce actions. Sensors and actuators are collectively referred to
as transducers, which serve the function of transforming signals or power from one energy
domain to another. There are six major energy domains of interests:
 Electrical domain (denoted E)

 Mechanical domain (Mec)
 Chemical domain (C)
 Radiative domain (R)
 Magnetic domain (Mag)
 Thermal domain (T).
Radiative (R): Electromagnetic waves, infrared radiations, UV radiation, X – rays, plasma,

high – energy particles, color, absorption, transmission.
Chemical (C): Chemical concentration, pH, reaction rate, molecule recognition, DNA
sequence, DNA hybridization, protein construct.
Mechanical (Mec): Force, pressure, speed flow rate, viscosity, acceleration, gravity, touch,
acoustic vibration noise, stress, strain, hardness, modulus fracture limit.
Electrical (E): Voltage, current, current density, resistance, capacitance, inductance charge,
pulse width, duty cycle, electrons, semiconductors, bandgap
Thermal (T): Temperature, thermal conductance, heat flux, heat capacity, phase change,
calorimetry
Magnetic (Mag): Magnetic field strength and direction, magnetic force, electromagnetic force,
Lorentz force, induction.
Sensors generally transform stimulus signals in various energy domains to one that is
detected by humans or into the electrical domain for interfacing with electronics controllers,
recorders, or computers. For example, a thermal – couple temperature sensor transforms a
thermal signal, temperature, into an electrical signal (e.g., voltage) that can be read
electronically. Often, perceived via such phenomenon as resistance changes, volume
expansion of fluids, increased radiation power of an object, color change of engineered dyes,
shifted resonance frequency of resonant beams, or greater chemical reactivity.
Energy transduction pathways for particular sensor and actuation tasks do not have
to involve only two domains. Rather, the transduction process may incorporate multiple
domains. Direct transduction pathways that involve the minimal number of domains do not
necessarily translate into simpler device, lower cost, or better performances.
Acceleration sensing (Mec _ E). A micromachined proof mass suspended by

cantilevers will experience an inctial force under an applied acceleration. The force will cause
movement of the suspended proof mass. The movement can be picked up using piezoresistors,
resistor elements who resistance changes under applied stress (Mec -> E). The displacement
can also be sensed with a capacitor (Mec -> E). This is the principle of Analog Device
accelerometers. Can one build accelerometers without moving parts? The answer is yes.
Inertial force can also move a heated mass, whose displacement can be picked by temperature
sensors (Mec ->T -> E) Thermal sensing does not provide as good performance as capacitive
sensing of moving air mass, but the fabrication is readily compatible with integrated circuits.
This designed for low sensitivity applications, no moving mass is required, the device is
compatible with mass batch microelectronics.
Oilfactory sensing (C -> E). A carbon based material can be designed to absorb certain
molecules and alter the electrical resistivity (C -> E). the absorbance of certain molecules in the
path of surface acoustic wave devices can alter mechanical properties such as frequency (C ->
M -> E). can one build oilfactory sensors that are simper, The example is binding of chemical
molecules can also alter the color of a specially designed chemical compound, detected using
optoelectronics diodes (C -> R -> E) or without electronics (C -> R).
DNA sequence identification (C -> E). DNA molecules consist of a chain of base pairs
each with four possible varieties _ A, C. G or T. The sequence of base pair in a DNA chain
determines the code of synthesizing proteins. The ability to decipher base pair sequences of
DNA molecules rapidly, accurately and inexpensively is of critical importance for
pharmaceutical and medical applications. There are a wide variety of innovative methods for
detection of DNA sequence through their tell-tale binding (hybridization) certain DNA
molecules may be chemically modified to incorporate fluorescence reporters that lights or
dims upon binding with another DNA strand. Chemical binding events are turned into optical
signals before transduced to the electrical domain (C -> R -> E). Fluorescent image is captured
using high power fluorescent microscopes.
b. Overview of microfabrication
MEMS and IC devices are generally made on single crystal silicon wafers. The following
figure shows the overall process from the production of such wafers to packaging of
individual chip devices. Bulk silicon with silicon consistency does not exist in nature, and
must be prepared through laborious industrial processes. To make bulk crystal silicon, one
start with perfect single crystal silicon seed. It is dipped in molten silicon pool and slowly
drawn out of the liquid. Silicon crystallizes when drawn into the atmosphere and establishes
crystallinity consistent with that of the initial seed. Rods of single crystal with various
diameters and longitudinal crystal orientation can be formed this way. The rods are sawed
into thin, circular slices and polished to form wafers.
A wafer goes through a multi fabrication process in a clean room, where dust,
particles, and even ions in the water are tightly controllable. The cleanliness of air in a clean
room is classified according to the concentration of air borne particles (size lesser than O.5
µm). According to a standard method for characterizing the cleanliness level of a cleanroom, a
class-I1 cleanroom has fewer than 1 particle and class-1OO cleanroom has fewer than 1OO
particles per cubic feet of air sampled.
Water (for rinsing) and chemical solutions must go through stringent, costly
manufacturing and conditioning processes. Ions in water (e.g. Sodium ion, even in trace
amount, will migrate into silicon and thin film materials upon direct contact. These ions may
become trapped charge in dielectrics and hurt device performance. Deionized water used in
semiconductor manufacturing has resistivity in excess of 18MΩ, compared to a resistivity of
less than 5OΩ for tap waters. Precision patterns are made using photolithographic patterning
method. Collimated light passes through a mask and an image reduction lens before hitting
the wafer, in a process akin to taking a photograph of an object through a telephoto length and
recording the image on a photosensitive film. In this case, the object being photographed is the
mask and the film is the wafer, coated with photosensitive film. Various wavelength of light
can be used. Light with higher energy (and hence smaller wavelength) is capable of producing
smaller line widths. The ultimate resolution is dictated by the diffraction limit.
Fig. 4.61 Microfabrication - Overview
Using a machine-automated photolithography process called step – and – repeat,

many identical units can be made on a same wafer with high line width resolution (O. 1 µm or
smaller in commercial processes). The machine used for performing the step – and – repeat
process is called a stepper. The way a stepper works is described as follows. After a reduced
image of the mask is printed to an area on a wafer, the wafer is translated by a precise
distance, and another exposure is made. Many identical devices, called dies, are made on one
given wafer in a single pass.
This differentiates MEMS process from conventional manufacturing technology,

which generally deals with one part at a time..The larger the wafer, the more dies are made in
a given batch. If the die size is 1 x 1cm2, there could be roughly 21 dies on a 2” wafer, 82 on a
4” wafer, 178 on a 6” wafer, 314 on an 8” wafer, and 77O on a 1O” one. These dies have
spacings between them so that they can be mechanically cut and separated. Each cut die,
called a chip, can then be electrically connected and encapsulated for commercial resale. The
process of incorporating a loose die to a housing and a system is called packaging.
c. Silicon – based mems processes

MEMS devices were first developed on silicon wafers because of the easy availability
of mature processing technologies that had been developed within the microelectronics
industry, and the availability of expertise in process management and quality control. Silicon
actually comes in three general forms: single crystal silicon, polycrystalline silicon, and
amorphous silicon. In a single crystal silicon (SCS) material, the crystal lattice is regularly
organized throughout the entire bulk. Single crystal silicon is often encountered in three cases:
1. Single crystal silicon wafer grown from a high – temperature melt/recrystalization

process;
2. Epitaxially grown silicon thin films;
3. Single crystal silicon obtained from recrystalizing polycrystalline or amorphous
silicon by global or local heat treatment.
A polycrystalline silicon material is made of multiple crystalline domains.

Within each individual domain, the crystal lattice is regularly aligned. However, crystal
orientations are different in neighboring domains. Within each individual domain, the crystal
lattice is regularly aligned. However, crystal orientations are different in neighboring domains.
Domain walls, also referred to as grain boundaries, play important roles in determining
electrical conductivity, mechanical stiffness, and chemical etching characteristics. The
polysilicon material can be grown by low pressure chemical vapor deposition (LPCVD), or by
recrystalizing amorphous silicon through global or local heat treatment. Amorphous silicon,
on the other hand, exhibits no crystalline regularity. Amorphous silicon films can be deposited
by chemical vapor deposition methods (CVD), at a lower temperature than that required to
deposit polysilicon. Due to low temperature,, atoms do not have enough vibrational energy to
align themselves after they are incorporated into the solid. Amorphous silicon can be grown
using the LPCVD method. (In a typical, horizontal, low – pressure reactor, the transition
temperature above which polycrystalline structure forms during deposition is 58OoC.)
Amorphous silicon can be formed by plasma enhanced chemical vapor deposition method as
well.
The two most fundamental classes of fabrication technologies are bulk

micromachining and surface micromachining. Bulk micromachining processes involve
selectively removing the bulk (silicon substrate) material in order to form certain three –
dimensional features or mechanical elements, such as beams and membranes. Bulk
micromachining may be combined with wafer bonding to create even more complex three –
dimensional structures. We use the example of a micromachined pressure sensor to illustrate a
representative MEMS process. The process involves two wafers – a bottom wafer is etched to
form a cavity whereas a top wafer is used to make the membrane. A description for each step
in the diagram follows.
STEP a. The process starts with a bare silicon wafer. To create desired cavity shapes, the wafer
must be of a certain crystallographic orientation. The wafer is cleaned thoroughly to remove
any large particles, dirt particles, and invisible organic residues. A combined mechanical wash
and oxidizing acid bath may be used, followed by a rinse by ultrapure water.
STEP b. The cleaned wafer is placed inside a high – temperature furnace filled with running
oxygen gas or water vapor. Oxygen atoms present in the air or dissociated from the water
molecule will react with silicon to form a protective silicon dioxide thin film.
STEP c. The wafer is removed from the furnace and cooled to room temperature. It will be
very clean, because any organic molecules would have been decomposed in the high –
temperature oxidation step. A layer of thin photoresist is deposited on the front surface of the
wafer. (A chemical called hexamethyldisilazane, or HDMS, is sometimes spin – or vapor –
coated to help increase the adhesion between the photoresist and an oxide surface.) The
photoresist is typically spin coated. Alternatively, photoresist thin film can be deposited by
vapor coating, mist coating, or electroplating. The wafer is baked in a convection oven to
remove some portion of the solvent from the photoresist (PR) layer to establish firmness. This
step is generally called “soft bake”. Alternatively, the moisture can be driven off with infrared
lamp, or vacuum.
STEP d. The photoresist is exposed through a mask with a high – energy radiation (such as
ultraviolet ray, electron beam, or X – ray). The entire wafer is then placed inside a developing
solution (often called developer) that removes loosely bound photosensitive polymer. In the
case of positive photoresist, regions hit by light will be dissolved. In the case of negative
photoresist, regions hit by light will stay. The soft bake process in step censures that the
photoresist will not be indiscriminately stripped by the developer.
STEP e. The photoresist needs to be baked again, this time at a higher temperature and often
for a longer duration than the soft bake. This second baking step, called “hard bake”, removes
remaining solvents and makes photoresist that remains on the wafer stick to the wafer even
stronger. The extent of the hard bake will depend on the nature of the subsequent step. The
photoresist mask is here used to selectively mask the underlying layer, the silicon oxide,
against a hydrofluoric acid etchant bath. A HF etchant attacks oxide within the exposed
window, but has negligible etch rate on the underlying silicon and the photoresist mask.
STEP f. The photoresist is removed using an organic solvent etchant such as acetone (at room
temperature or elevated temperatures). The hard – baked photoresist is chemically resistant to
the HF etchant but not to acetone. The organic solvent does not etch the oxide and the silicon.
Fig. 4.62 Silicon based MEMS process

STEP g. The silicon wafer is immersed in a wet silicon etchant, which does not attack the
silicon oxide. Only the silicon in the open oxide window is etched, resulting in a cavity with
sidewalls defined by crystallographic planes. The cavity may reach the other side of the wafer
if the open window is large enough for the given wafer thickness. The wet silicon etching
involves an elevated temperature (7O-9OoC). The etchant would attack hard – baked
photoresist, hence it is impossible to use photoresist directly as mask in this step.
STEP h. The wafer at the end of stage (g) is tilted to provide a clear view of the through wafer
cavity.
STEP i. A second silicon wafer is firmly bond to the frontside of the bottom wafer processed
through step (g). It is important that the environment in which the wafers are processed be
very clean, because tiny particles adhering to the bonding surfaces of either wafer will prevent
good bond strength from being reached.
STEP j. The bonded top wafer is thinned by using mechanical polishing or chemical etching.
The remaining thickness of the top wafer determines the thickness of the membrane. Thin
membranes are desired to have high sensitivity.
STEP k. Strain sensors are then made on the prepared membrane. A thin film layer (e.g.,
oxide) is deposited and patterned. It serves as a barrier layer to ion implantation. Areas on the
silicon wafer hit directly by energetic dopant ions will become doped and form a piezoresistor,
which changes its resistance upon applied stress due to membrane bending under pressure
difference. The wafer is tilted to present another perspective view of the through – wafer
cavity (m). To keep the description succinct, a few detailed steps are skipped between steps j
and k.
d. Packaging and Integration

The two terms integration and packaging are closely related but they are in fact
different. Integration refers to the act of combining mechanical and electrical functionalities.
Packaging refers to the act of placing loose (diced) chips into human – or machine handlable
modules that can be directly assembled on circuit boards and into systems. Packaging refers to
dicing, die assembly, encapsulation, and testing. Packaging is a crucial technological issues
highly relevant for MEMS device design. Although it is not discussed in detail in this book, it
is important to draw attention of beginning students to this very exciting, important, and
dynamically changing area. “Packaging is part of the process. Packaging is not what you do
after the process” should be the motto learned by all students of MEMS. The cost of packaging
often accounts for 3O-9O% of the total cost of the finished system, and it deeply affect the
performance, cost, reliability, and ultimately, the competitiveness of a MEMS device in the
market place.
e. New materials and fabrication processes

In recent years, silicon micromachining techniques are being rapidly augmented with
new materials and processes. Silicon, a semiconductor material, is mechanically brittle. It is
also expensive or unnecessary for certain applications. New materials such as polymer and
compound semiconductors can fill the gap of performance. Polymer materials are being
incorporated into MEMS because of their unique materials properties (e.g., biocompatibility,
optical transparency), processing techniques, and low costs compared to silicon. Polymer
materials that have been explored in recent years include silicone elastomers, parylene, and
polyimide, among others.
Many sensors and actuators are needed to operate in harsh conditions, such as direct
exposure to environmental elements, high temperature, wide temperature swing, or high
shock. Delicate microstructures made of silicon or inorganic thin film materials are not suited
for such applications. Several inorganic materials are being introduced for MEMS applications
in harsh environments. Silicon carbide, in both bulk and thin film forms, are explored for
applications, including high – temperature solid – state electronics and transducers. Diamond
thin films provide the advantage of the high electrical conductivity and high wear resistance
for potential applications including pressure sensors and scanning electron microscopy
probes.
Other compound semiconductor materials including GaAs are also being

investigated. Other metal elements have also been involved in MEMS devices, including
nickel, titanium, etc. New material processing techniques are being developed for fabrication
both on silicon substrates and other materials. New processes for MEMS include laser –
assisted etching for material removal and deposition, stereo lithograph for rapid prototyping,
local electrochemical deposition, photo – electroforming, high aspect ratio deep reactive ion
etching, micro milling, focused ion beam etching, X – rays etching, micro electro discharge, ink
jet printing (e.g., of metal colloids), micro contact printing, in – situ plasma, molding
(including injection molding), embossing, screen printing, electrochemical welding, chemical
mechanical polishing, micro glass blowing, and guided and self – directed self – assembly in
two or three dimensions.
Microfabrication processes have also been expanded to reach nanoscale resolution to

realize nanoelectromechanical systems (NEMS). Reliable and economical fabrication of
electromechanical elements with nanoscopic feature sizes or spacing represents new
challenges and methodologies.
Traditional lithography does not offer sub – 1OO – nm resolution readily, at low cost,
and with parallelism. A variety of nanostructure patterning techniques, often drastically
different from the photolithographic approach, are developed in the physics and chemistry
communities for producing nano – meter scale patterns. Readers who are interested in
exploring this class of techniques may start by reading literatures on nanoimprint lithography,
nano whittling, and nanosphere lithography. It is noteworthy that the materials and
technologies for microelectronics fabrication have not been standing still either. In fact, the
traditional photolithography techniques and semiconductor materials associated with
integrated circuits are undergoing rapidly changes in the past decade. New processing
techniques such as roll – to – roll printing are being actively pursued for fast production of
large area electronics, photovoltaic generators, and optoelectronics displays. Organic polymer
materials are being used in place of semiconductor materials for logic, storage, and optical
display.
The future of new materials and fabrication methods is bright and exciting.
Fabrication and manufacturing technologies such as micromachining, nanofabrication, and
microelectronics fabrication have historically been developed in different communities with
virtual disregard of each other, on independent sets of materials and substrates. As science
and technology progresses towards the micro and nanoscopic dimensions, these distinct
families of fabrication methods are being connected and hybridized to create powerful and
transcending new fabrication methods which will enable new scientific studies and new
devices.
f. Applications
MEMS sensors are used in different domains which include automotive, consumer,
industrial, military, biotechnology, space exploration, and commercial purposes which include
inkjet printers, accelerometers within modern cars, consumer electronics, in personal
computers, etc. The best examples of MEMS devices mainly include adaptive optics, optical
cross-connects, airbag accelerometers, mirror arrays for TVs & displays, steerable
micromirrors, RF MEMS devices, not reusable medical devices, etc. Thus, this is all about
the MEMS sensor. The main disadvantage of these sensors is, even though the making cost for
each part is extremely low. But there is a huge investment associated while designing,
manufacturing, and succeeding MEMS-based product. Consequently, designers are not likely
to expand components for low volume applications.
4.14 Nano sensors

Microelectronics naturally leads to nanoelectrons for realizing nano-devices which
are expected to create an impact in the enhancement of energy conversion, control of pollution,
production of food, and improvement in the conditions of human health and longevity. At
present, however, the best examples of nano-devices, as far as applications are concerned, are
associated with information technology industry. The development of nanostructures has been
accelerated over the past decade because of unexpected breakthroughs in the synthesis and
assembly of nanometer scale structures.
Besides, capabilities of manipulating matter from the top down have been there
during this period. Some areas that have seen such unexpected developments are:
1. Discovery and controlled production of carbon nanotubes and the use of

proximal probe and lithographic schemes
2. Success in placing engineered individual molecules onto appropriate
electrical contacts and in measuring transport through these molecules
3. Availability and use of proximal probe techniques for fabrication of
nanometer-scale structures
4. Development of chemical synthetic methods to realize nanocrystals and also
implantation technique for putting these crystals into a variety of larger
organized structures and making assembly
5. Implantation/incorporation of biological motors into nonbiological
environments
6. Integration of nanoparticles into gas sensors
7. Fabrication of photonic level band gap structures
8. Development of quantum effect, single electron memory, and logic elements
that can operate at room temperature.
Many other offshoots have also been developed that have enhanced the research and
actual commercialization of nano-devices in general and nano-sensors, in particular. One
important area is the development of the nanometer scale objects which can manipulate and
develop other nanometer scale objects economically and efficiently. Scanning tunnelling
microscopy (STM) or atomic force microscopy (AFM) are the techniques used for the purpose.
Work is in progress to integrate nanometer scale control electronics and micromachines for
developing the ideas postulated.
While progressing towards the development of fast and miniaturized memory

structures, giant magnetoresistance structures have been produced using Thomson effect.
These giant magnetoresistance (GMR) structures consist of layers of magnetic and
nonmagnetic metal films wherein the critical layers have thickness of the order of nanometers.
They are used as extremely sensitive magnetic field sensors. The spin-polarized electrons are
transported between the magnetic layers on the nanometer length scale and this process,
during operation, allows the structure to sense magnetic field. These are already in use in
sensing magnetic bits stored on computer discs. Organic nanostructures have been developed
combining chemical self-assembly as mentioned earlier in the chapter, with a mechanical
device. The organic sample is reduced to a size that consists of a single molecule and this is
connected by two gold leads. This structure has been successfully used to measure the
electrical conductivity of a single molecule. Figure 4.63 shows the microstructure of a GMR.
Fig. 4.63 Microstructure of a GMR
As mentioned already, molecular electronics is receiving a boost in the area of sensor

development. Molecules which are quantum electronic devices and through batch processes of
chemistry such devices are designed and synthesized and assembled in useful circuits. The
final stages, however, require processes of self-organization and self-alignment. Molecular
switches have already been produced by these processing machanisms. The circuits use carbon
nanotubes as connecting wires. These devices can be used as power-efficient memories with
extraordinary high speed, perhaps a billion times faster than the CMOS devices. The operation
of the switches depends on the electronic states of the molecules. In the closed state, current
flows by resonant tunnelling through the states. The switches are opened by applying an
oxidizing voltage across the device. Figure 4.64 shows the voltage—current characteristics of
the switches in the on—off states.
Fig. 4.64 Voltage—current characteristics
Research groups have succeeded in fabricating electronic switches such as FET from
single_ walled carbon nanotubes. Figure 8.16 shows a single-walled carbon nanotube (dia = 1.6
nm) placed on metal contacts using AFM. It then behaves like a HT turning 'on' or 'off'
depending on the applied gate voltage. Silicon substrate is itself used as gate.
Fig. 4.65 Single-walled carbon nanotube FET

4.15 LASER sensors
LASER sensor exists in several configurations with some detecting presence and
others measuring distance. A proximity type laser sensor, also called a laser photoelectric
sensor, is commonly used to detect presence of a part, but is not the focus of this discussion.
The focus, figuratively and literally, is on laser distance sensors, that as the name implies,
measures distance. These LASER distance sensors use a focused, coherent light to measure
distance to a target object. In factory automation applications, the target is usually a product or
a machine element. They detect any solid object and produce an output proportional to the
measured distance—independent of material, color and brightness. The high-resolution output
provided by laser distance sensors is used to provide position or displacement, usually to the
input of some type of industrial controller, such as PLC. The output signal is often highly
accurate, and it includes temperature compensation to enhance stability. There are several
types of laser distance sensors including diffuse, background suppression and retroreflective.
These sensors use CMOS or transit time technologies, providing accurate distance
measurements. Laser light must be focused, and it must stay narrow over a great distance and
at a narrow spectrum (color of light). The emitted light can then be triangulated or pulsed,
with each pulse return measured to create distance readings.
a. Angle Equals Distance

CMOS technology is often used in short-range, high-precision laser sensors. The basic
principle is optical triangulation using a CMOS linear imager. A diffuse triangulating laser
distance sensor transmits a laser through a lens and to the target, which reflects the light back
to the sensor. A lens focuses this reflected light into a small spot onto the CMOS linear imager.
The distance to the target object changes the angle of the reflected light and where the light is
received on the linear imager. These CMOS, diffuse triangulating laser distance sensors are
available in measurement ranges from about 1.5-60 cm. They are packaged in small housings,
about 50 mm x 50 mm, with either analog or discrete outputs—and generally work well
regardless of the material, color or brightness of the target object.
These distance sensors have high-resolution, in the low µm range depending on the
measurement span, and response times less than a millisecond. Class 1 lasers are eye-safe
under all operating conditions. However, Class 2 lasers are visible lasers safe for quick
accidental viewing of less than 0.25 s but may damage the eye if deliberately stared into.
b. Light Speed Measured

Time-of-flight technology is often used in long-range laser distance sensors, also
called rangefinder laser sensors (Figure 1). These types of sensors use a transmitter diode to
generate a very short pulse of narrow-spectrum red or infrared light which reflects from the
target object and back to a sensitive, laser energy detector, also called a receiver diode. The
accurate and precise electronics in the sensor can measure the light transit time and use the
constant for the speed of light to calculate the object’s distance from the sensor.
Fig. 4.66 Range finder sensor
This Wenglor OPT Long Range (Transit Time) Laser Sensor is available from
Automation Direct and uses time-of-flight technology to accurately measure distance. Time-of-
flight laser distance sensors are available in a measurement range from a 1 cm to over 100 m
and are similar in size, output options and laser sensing capabilities to CMOS sensors. These
long-distance sensors have good resolution, from about one mm at close distances, to less than
2.5 cm of error over 100 m. To improve measurement accuracy, multiple measurements are
often made, slowing the response time of these devices to several milliseconds.
c. Laser Applications
Laser sensors generally work well in a dirty environment since the focused light can
“burn” through dust. The focused beam also enables long sensing distances, and detection of
small objects or targets through small openings. Laser sensors are often used in process
monitoring and closed-loop feedback control systems. Material handling is a popular
application to enable positioning of cranes, gantries and automatic guided vehicles (Figure 2).
A few of the many other applications include component alignment, height measurement,
robot positioning and weld head location. At times, shiny or transparent objects can cause
problems. Because the laser distance sensor detects reflected or through-beam light,
transparency and surface reflectivity may cause complications.
Applications requiring bouncing a laser off a shiny surface or looking through a

transparent surface should be carefully tested to make sure the measurement works as
required. For example, the laser may need to be mounted at a slight angle to a shiny surface or
adjusted to a lower intensity to properly detect a shiny object, while the intensity may need to
be increased to burn through a transparent object. Once testing is completed and any required
adjustments make, lase sensors will perform will in industrial applications for years.
Fig. 4.67 Both short-range CMOS and this long-range, time-of-flight Wenglor photoelectric
laser sensor are often used in material handling applications.
TWO MARK Q&A

1. Specify the liquids used in liquid filled thermometers.
a. Acetone d. Mercury
b. Ethyl Alcohol e. Pentane
c. Ethyl Ether f. Toluene
2. Illustrate how orifice meter used for flow measurement.

When a liquid / gas, whose flow-rate is to be determined, is passed through an
Orifice Meter, there is a drop in the pressure between the Inlet section and Outlet Section
of Orifice Meter. This drop in pressure can be measured using a differential pressure
measuring instrument. Since this differential pressure is in direct proportion to the flow-
rate as per the Bernoulli's Equation hence the differential pressure instrument can be
configured to display flow-rate instead of showing differential pressure.
3. Mention the features of smart sensors.

a. Automatic ranging
b. Auto calibration of data through an in-built system
c. Automatic data acquisition system and storage of data in local memory of the
field device.
d. Auto linearization of non-linear functions
e. Auto correction of offsets, time and temperature drifts.
f. Self tuning control algorithms
g. Easy communication through serial bus.
4. What are nano sensors?

Nano sensors are chemical or mechanical sensors that can be used to detect the
presence of chemical species and nanoparticles, or monitor physical parameters such as
temperature, on the nanoscale. Nanosensors aid in the progression of fields such as
medical technology; precision agriculture; urban farming; plant nanobionics; prognostics
and diagnostics; SERS-based sensors; and many industrial applications.
5. What is Geiger counter? State its principle of working.

Geiger counters are used to detect radioactive emissions, most commonly beta
particles and gamma rays. The counter consists of a tube filled with an inert gas that
becomes conductive of electricity when it is impacted by a high-energy particle. When a
Geiger counter is exposed to ionizing radiation, the particles penetrate the tube and
collide with the gas, releasing more electrons. Positive ions exit the tube and the
negatively charged electrons become attracted to a high-voltage middle wire.
When the number of electrons that build up around the wire reaches a
threshold, it creates an electric current. This causes the temporary closing of a switch and
generates an electric pulse that is registered on a meter, either acoustically as a click that
increases in intensity as the ionizing radiation increases, or visually as the motion of a
needle pointer.
6. What are bellows? By what materials they are made up of?

Bellows is a series of circular parts, resembling the folds in an accordion
manner. These parts are formed or joined in such a manner that they are expanded or
contracted axially by changes in pressure. Metal bellows are available in a variety of
materials—with brass, bronze, beryllium copper, Monel and stainless steels.
7. What is the principle of piezoelectric transducer?

Piezoelectric transducers are used to transduce pressure or force into electric
charge. When a piezoelectric material is subjected to force or pressure, the dimensions of
the crystal are changed. Due to this, an electric potential appears across the surfaces of a
crystal. This effect is called as piezoelectric effect.
8. List out the applications of MEMS sensors.

a. Airbag Systems e. Rollover Detection
b. Vehicle Security Systems f. Automatic Door Locks
c. Intertial Brake Lights g. Active Suspension
d. Headlight Leveling
9. State the application of radiation sensors.

a. Scintillator
b. Gaseous Ionization Detectors
c. Geiger Counter
10. Write the principle of venture flow meter.

Venturi meters are flow measurement instruments which use a converging
section of pipe to give an increase in the flow velocity and a corresponding pressure drop
from which the flowrate can be deduced. They have been in common use for many years,
especially in the water supply industry.
11. How pressure is measured using manometer.

The principle behind a manometer gas or liquid pressure gauge is extremely
simple. Hydrostatic equilibrium shows that the pressure when a liquid is at rest is equal
at any point. For example, if both ends of the U-tube are left open to the atmosphere then
the pressure on each side will be equal. As a consequence the level of the liquid on the
left-hand side will be the same as the level of the liquid on the right-hand side –
equilibrium. However, if one end of the U-tube is left open to the atmosphere and the
other connected to an additional gas/liquid supply this will create different pressures.
12. Write the principle of bimetallic thermometer.

The bimetallic strip is constructed by bonding together the two thin strips of
different metals. The metals are joined together at one end with the help of the welding.
The bonding is kept in such a way that there is no relative motion between the two
metals. The physical dimension of the metals varies with the variation in temperature.
Since the bimetallic strip of the thermometer is constructed with different
metals. Thereby, the length of metals changes at different rates. When the temperature
increases, the strip bends towards the metal which has a low-temperature coefficient.
And when the temperature decreases, the strip bends towards the metal which has a
high-temperature coefficient.
13. List the different types of thermocouple.

a. Type K Thermocouple (Nickel-Chromium / Nickel-Alumel)
b. Type J Thermocouple (Iron/Constantan)
c. Type T Thermocouple (Copper/Constantan)
d. Type E Thermocouple (Nickel-Chromium/Constantan)
e. Type N Thermocouple (Nicrosil / Nisil)
f. Type S Thermocouple (Platinum Rhodium - 10% / Platinum)
g. Type R Thermocouple (Platinum Rhodium -13% / Platinum)
h. Type B Thermocouple (Platinum Rhodium – 30% / Platinum Rhodium
– 6%)
14. Write the two steps in tactile sensing
1. Forward analysis: it is related to the data acquisition from the tactile sensor.
Due to the indentation of the rigid object on the compliant cover layer of the tactile
sensing array, there will be changes in the stress or strain.
2. Inverse analysis: it deals with the recovery to the force distribution in the
compliant cover layer. Sometimes, it is related to the recovery of the shape of the
indentation in the layer.
15. What is photo electric effect?

Due to the intensity of the incident light or radiation of suitable wavelength on
the surface of the photo sensitive material, there will be ejection of electrons from the
surface of the material and further generation of electrical signal proportional to the
intensity of light. This effect is called photoelectric effect.
16. How do you classify photo electric transducers?

There are two major types of photoelectric transducers:
1. Passive transducers
(a) Photo emissive cells: based on the incident light on the photo sensitive surface,
electrons are emitted from cathode.
(b) Photoconductive cells: an electrical signal output is generated due to eth change
in resistance of the photo sensitive material when light falls on the surface of the
material.
2. Active transducers
(c) Photo voltaic cells and photo thyristors produce an electrical voltage in
proportional to the intensity of the incident light radiation.
17. Compare and contrast photo diodes and photo transistor.

A phototransistor is a photosensible semiconductor device comprising three
electrodes. Light or ultraviolet light activates this bipolar junction transistor. Illumination
of the base generates carriers which supply the base signal while the base electrode is left
floating.The emitter junction constitutes a diode, and transistor action amplifies the
incident light inducing the signal current.
A photodiode is a semiconductor diode which gives an important photocurrent

under illumination. One type of photodiode uses a p-n reverse biased junction operated
below the breakdown voltage. Excess charge carriers or electron hole pairs are generated
by photoconductivity with exposition to electromagnetic radiation. Carriers usually
recombine quickly, but the ones produced near or in the depletion layer of the junction
can cross the junction and give a photocurrent which is superimposed on the small
reverse saturation current.
18. State the principles of fiber optic transducers

The light transmission through the optical fiber is done based on the principle of
Total Internal Reflections (TIR). It simply states that all the light striking a boundary
between two media will be totally reflected. There is no loss in light energy across the
boundary. For the principle of TIR to take place, the following tow conditions are to be
satisfied:
1. The glass around the centre of the fiber (core) should has higher refractive
index (n1) than that of the material (cladding) surrounding the fibre (n2).
2. The light should be incident at an angle of which will be greater than the
critical angleθc.
19. List the major applications of smart sensors.

1. Smart accelerometers for the measurement of acceleration
2. Smart optical sensors for object detection
3. Smart IR detector array for the identification of presence of products
4. Smart sensor for defect or fractures monitoring in structures or infrastructures
20. List the materials used for manufacturing the MEMS sensors.
a. Silicon d. Metals
b. GaAs e. Ceramics
c. Polymers

1. Explain the following
1. Photo Voltaic cell
2. Photo Conductive cell
2. Suggest a suitable transducer for flow and level measurement and explain its
working principle with neat diagram.
3. Explain the construction and working principle of various temperature sensors.
4. Explain the following

1. Diaphragm
2. LASER sensors
3. Film sensors
5. Explain the construction and working principle of various pressure measuring
transducers.
6. Explain in detail about the principle, types and applications of fiber optic
transducers.
7. With a neat block diagram, explain about the features, components and functioning
of a SMART sensor?
8. i. How does piezoelectric tactile transducer work? Explain.
ii Discuss in brief about any two IC sensors used in temperature monitoring

applications.
Unit – V
Signal Conditioning and DAQ systems
5.1 Introduction
The output signal of transducers contains information which is further processed by
the system. Many transducers develop usually a voltage or some other kind of electrical signal
and quite often the signal developed is of very low voltages, may be of the order of mV and
some even V. This signal could be contaminated by unwanted signals like noise due to an
extraneous source which may interfere with the original output signal.
Another problem is that the signal could also be distorted by processing equipment
itself. If the signal after being sensed contains unwanted contamination or distortion, there is a
need to remove the interfering noise / sources before its transmission to next stage. Otherwise
we may get highly distorted results which are far from its true value. The solution to these
problems is to prevent or remove the signal contamination or distortion. The operations
performed on the signal, to remove the signal contamination or distortion, is called Signal
Conditioning.
The term signal conditioning includes many other functions in addition to variable
conversion and variable manipulation. Many signal conditioning processes may be linear,
such as, amplification, attenuation, integration, differentiation, addition and subtraction. Some
may be non-linear processes, such as, modulation, filtering, clipping, etc. The signal
conditioning processes are performed on the signal to bring it to the desired form for further
transmission to next stage in the system. The element that performs this function in any
instrument or instrumentation system is known as Signal Conditioning Element.
5.2 Amplification
Signal amplification is the process of increasing the signal for processing or
digitization. There are two ways that signal amplification can be performed; by increasing the
resolution of the input signal, or by increasing the signal-to-noise ratio. Signal conditioning
uses a range of different amplifiers for different purposes, including instrumentation
amplifiers, which are optimised for use with DC signals, and are characterized by high input
impedance, high common mode rejection ratio (CMRR), and high gain. Another example of a
signal conditioner used for amplification would be an isolation amplifier, which is designed to
isolate high DC levels from the device while passing small AC or differential signal.
5.2.1 Operational amplifier

The operational amplifier (OPAMP) manufactured with integrated circuit technology
contains transistors, diodes, resistors and capacitors, is an extremely versatile device that does
5.2 Unit 5: Signal Conditioning and DAQ systems
countless jobs in many electronic circuits. We limit our discussions regarding OPAMP to an
external description and to those applications, where it serves as a dependant source for
important electronic circuits as well as a device for performing such important functions as
isolation, inversion, addition, subtraction, multiplication and division. It can also be used for
performing mathematical operations like integration and differentiation which together with
its summing capabilities puts it in a position to model differential equations of physical
systems.
(a) (b)
Fig. 5.1 IC741 – Operational Amplifier (a) DIL Package (b) Pin configuration
One of the most popular OPAMPS is the 741 type, which has been produced by
several manufacturers for many years. Also several improved versions have been developed
having different internal circuitry to the original 741 OPAMPS.
The 741 is a plastic encapsulated device with dual in line (DIL) rows of pins as shown
in Fig. 5.1 (a). Such devices are said to be directly pin compatible. The pin connections are
shown in Fig. 5.1 (b). The 741 is an eight pin device. The operational amplifier has two types of
connections: Inverting and Non-inverting, when a voltage is applied to inverting input pin 2,
the output voltage has a sign which is opposite to that of the input. In case, the input voltage is
applied to pin 3, which is designated as non-inverting, the output voltage has the same sign as
the input.
The operational amplifier is shown by a triangular symbol with inverting and non-
inverting inputs being connected at pins 2 and 3 respectively. The OPAMP output appears at
pin 6. Several other pins are labelled in the diagram. The most important of these are 7 and 4,
where positive and negative constant supply voltages, V+ and V- are connected to supply
energy to the integrated circuit. Usually this supply voltage is fixed at ± 15 V with respect to
ground reference. The OPAMP does not work without the supply voltage. Two other pins (1
and 5) are provided on 741 and are labelled offset null. The offset effects are one of the several
types of non-ideal behaviour of OPAMPS.
a. OPAMP circuits used in instrumentation

Some of the commonly used operational amplifier circuits are described below:
Inverter
The circuit of operational amplifier used as an inverter is shown in Fig. 5.2. The
feedback resistance is made equal to the resistance, connected to the inverting end of the
amplifier. It is clear that the output voltage is 180° out of phase with the input voltage.
Fig. 5.2 Operational amplifier as an inverter
Adder
The circuit that performs the addition of signals with amplification (if desired) is
shown in Fig. 5.2 using the superposition theorem, we have,
( ) ( )
Note that the input signals can be added together with different weighing factors, if
desired. However, if all the resistors are chosen to be equal, the circuit acts as a pure adder and
adds the input voltages together at the output i.e. in case , the circuit acts as
a pure adder and output voltage is,
( )
i.e. sum of the individual input voltages. The inversion that occurs cannot be avoided.
Fig. 5.3 Operational amplifier as an adder

Subtractor
The operational amplifier circuit used for subtraction of two input signals is shown in
Fig. 5.4. The output of the second operational amplifier is,
( ) ( )
In case , the circuit acts as a pure subtractor, with the output being:
Fig. 5.4 Operational amplifier as a subtractor
Another circuit which uses only one operational amplifier for the purposes of
subtraction is shown in Fig. 5.5. This circuit performs the subtraction of one signal from
another with amplification if desired. In order to show that the voltage is proportional to
difference in input voltages, we use superposition theorem since the circuit behaves linearly.
The output voltage due to vi alone (i.e. with v2 = 0) is simply that of an inverting amplifier, and
is equal to,
( )
The contribution to output voltage vo from v2 (with vi = 0) is obtained by first realizing

that the voltage at the output terminals is (for large R i),
( )
The output voltage can be written as,
( )
substituting the value of v+ from above equation, we obtain,
( )( )
Fig. 5.5. Operational amplifier as a subtractor
However, if and
( )( )
( )
Similarly
( )
Hence by using superposition by adding above two equations,
( ) ( )
( )
Now if Rf = R, the circuit will perform subtraction without amplification.
Multiplier and Divider

The output voltage of an OPAMP in the inverting mode is ( ) . In case
the circuit of Fig. 5.6 acts as a multiplier and in case , it acts as a divider.
Suppose Rf = 100 MΩ and R1 = 10 MΩ

( )
= -10 Thus the input gets multiplied by a factor 10.
In case Rf = 10 MΩ and R1 = 100 MΩ

( )
= -10/ Thus, the input gets divided by a factor of 10.
By properly, choosing the values of Rf and R1, the multiplier and the divider circuits
can be designed for multiplication and division by any number.
Fig. 5.6 Operational amplifier as multiplier and Divider
5.2.2 Differential amplifier

The operational amplifier which is very important in an instrumentation system is the
differential amplifier. In its basic form it has two inputs and two outputs. The signal available
to the two outputs is identical except that the two are 180° out of phase with each other. The
output voltage of the amplifier is proportional to the difference between the two input
voltages. Fig. 5.7 shows a differential amplifier. This amplifier produces an output voltage
which is proportional to difference between the two input voltages
( )
Where is the differential gain
Fig. 5.7 Operational amplifier used as a differential amplifier
The operational amplifier is designed to have a very high differential gain. This
means or more. Also this high gain implies that since the output
( )
voltage has limited amplitude and therefore, the term, is very large. The differential
( )
voltage ( ) is very small and hence we can safely assume . The input
impedance of a differential amplifier is quite large. For negative feedback, the path must be
from output to . Now , is shifted by 180˚ with respect to and 0° with respect to
By superposition, we get
Now,
Since,
( )
( )
( )
is called Differential Mode Gain. The signal ( ) is called Differential Mode

Signal or simply Difference Signal,
( )
a. Common Mode Signals.

The output voltage of a differential amplifier is proportional to the difference
between the input voltages. Thus if there is no difference between the two input voltages i.e.
when they are equal, the output voltage is zero. Equal inputs are known as common mode
signals because the input signal is common to both inputs. However, in actual practice when
equal input voltages are applied to the two inputs, the output voltage is not exactly equal to
zero on account of difference in response of the two inputs to common mode signals. The
output of a practical difference amplifier with common mode signals is typically of the order
of several hundred microvolt.
We can now distinguish between the gain for the desired difference signals and the
gain for the common mode signals by referring to the former as the difference mode gain and
the latter as the common mode gain
Common mode gain
where = common mode input signal. In fact is the value of the input signal that
is common to both inputs, and is the output signal resulting from the common mode input
signal.
b. Common Mode Rejection Ratio (CMRR).

The common mode rejection ratio (CMRR) is defined as
The common mode rejection ratio (CMRR) is frequently expressed in db.
In an ideal amplifier, this ratio should be infinity but in practice, it is not, and has a
finite value. CMRR is a measure of ratio of the desired signal to the undesired signal. The
larger the CMRR, the better is the amplifier.
c. Advantages of differential amplifiers

The advantages of differential amplifiers are :
Noise Immunity
The differential amplifier can be used in situations where the operation of single
ended operational amplifiers is impossible on account of interference from pick-up and
ground potential differences. When a difference amplifier is used, ideally it responds only to
the difference signal between input terminals and ignores common mode signals like the noise
pick-up and ground voltages that appear in phase on both signal lines. The differential
amplifiers are extensively used in equipment such as electronic voltmeters and oscilloscopes.
Usually the first stage of such an instrument is known as Differential Input, or Balanced Input
or Push-pull Input.
The advantage of this form of construction is that the two input terminals are located
physically close to each other. Consequently noise pick-up (such as hum from power lines or
electrical equipment, "static" from fluorescent lights etc.) tends to be equal on the two
terminals. Also, the wires can be twisted together to equate the pick-up on both the terminals.
Therefore the noise and hum pick-up will be a common mode signal and so the response of a
differential amplifier to this is negligible on account of high value of CMRR.
On the other hand the desired signal can be arranged to be equal and 180° out of
phase at the two input terminals, so that the difference signal ( ) is additive.
Thus a 50 mV signal is made + 50 mV at one terminal and — 50 mV at the other terminal, with
the result that the difference mode input signal ( ) = 100 mV. When a signal source
provides this type of output signal, it is known as a "balanced output" or "push-pull" source.
Drift Immunity:
One of the major problems in amplifier design is the change in the value of gain and
of voltage levels due to aging and variations in temperature. These changes take place slowly
and are, therefore, called Drift. The problem of drift may assume very serious proportions in
electronic instrumentation systems as even an extremely small change may affect the results
drastically.
The differential amplifier has inherent capabilities of eliminating problem of drift.

The two inputs and two outputs that characterize the amplifier are the result of the fact that it
is made up of two identical amplifiers connected side by side. Thus any changes, due to aging
or variations in temperature which occur on one side are duplicated on the other side.
Therefore, these effects may be considered as two identical signals, or in other words a
common mode signal. Since, a differential amplifier has a high value of CMRR, the net effect of
the drift is almost negligible. In case the two sides of the amplifier are made absolutely
identical the effect on account of aging and temperature changes will affect the two sides
equally and hence there will be no output on account of these changes. Thus, the drift problem
is completely eliminated. On account of features described above the differential amplifier
construction is used for the early stages of oscilloscope and electronic voltmeter amplifiers,
where low drift is extremely important.
5.2.3 Instrumentation amplifier

The instrumentation amplifier is a dedicated differential amplifier with extremely
high input impedance. Its gain can be precisely set by a single internal or external resistor. The
high common mode rejection makes this amplifier very useful in recovering small signals
buried in large common-mode offsets and noise.
Fig. 5.8 Instrumentation amplifier input stage

The instrumentation amplifier (IA) is not a general building block, like the OPAMP.
The instrumentation amplifier (IA) is a closed loop device with carefully set gain. The OPAMP
itself is an open loop device with some very large (but variable) gain. This allows the
instrumentation amplifier (IA) to be optimized for its role as signal conditioner of low level
(often d.c.) signals in large amounts of noise. The OPAMP, in contrast, can be used to build a
wide variety of circuits but does not make as good as a difference amplifier as does an
Instrumentation Amplifier (IA). Instrumentation amplifiers consist of two stages. The first
stage offers very high input impedance to both input signals and allow to set the gain with a
single resistor. The second stage is a differential amplifier with the output, negative feedback,
and ground connections all brought out.
The input stage is shown in Fig. 5.8. It consists of two carefully matched OPAMPS.
Each inptut and is applied to the non-inverting input terminal of its OPAMP. This
OPAMP configured as a voltage follower, produces the instrumentation amplifier's very high
input impedance. The outputs of the OPAMPS are connected together through a string of
resistors. The two resistors are internal to integrated circuit while is the gain setting
resistor. It may be internal or connected externally. The output voltage is taken between
outputs of two OPAMPS.
( )( )
The above equation can be derived first noting that there can be no difference in
potential between the inverting and non-inverting inputs of OPAMPS. This means that is at
the top of and is at the bottom of . That is, voltage across Rg is :
( )
( )
This current must flow through all the three resistors because none of the current can
flow intd OPAMPS's input. So the output voltage is,
( )
( )
( )
( )( )
From the above equation, it is clear that decrease in the value of will increase the
output voltage . Therefore, to increase the value of gain, the value of has to be decreased.
. The second stage of the instrumentation amplifier is a unity gain differential amplifier. The
complete diagram of an instrumentation amplifier (including the first and second stage) is
shown in Fig. 5.9. Three terminals are brought out. The sense terminal gives an access to the
feedback loop. The reference terminal allows to establish d.c. reference i.e. ground potential of
the output. For normal operation the sense terminal is directly connected to output
(completing the feedback loop) and tie the reference to ground. This configures the output of a
standard differential amplifier as shown in Fig. 5.9.
Three Amplifier Configuration

This circuit is being described to show the elimination of a common mode signal. The circuit of
Fig. 5.10 is used for this purpose. This circuit uses three operational amplifiers and hence it is
called "Three amplifier configuration". and are the desired signals and is the
common mode signal.
Fig. 5.9 Instrumentation amplifier with full schematic
For simplicity it is assumed
Therefore, we can write,
( ) ( )
( ) ( )
( )( )
OPAMP 1 and OPAMP 2 act as buffers with unity gain for the common mode signal
and with a gain of ( ) for the differential inputs . The circuit has high input
impedance since the OPAMPS 1 and 2 operate in non-inverting mode for common mode
signal. It is clear from the above equation that there is no output corresponding to the common
mode input signal. This is because OPAMP 3 acts as a difference amplifier with a high value of
CMRR. The gain off course can be changed by changing resistance .
Fig. 5.10. Three amplifier configuration
5.3 Filtering
Another important function of a signal conditioner is filtering, and this is where the
signal frequency spectrum is filtered to only include the valid data and block any noise. The
filters can be made from either passive and active components or digital algorithm. A passive
filter only uses capacitors, resistors, and inductors with a maximum gain of one. An active
filter uses passive components in addition to active components such as operational amplifiers
and transistors. State of the art signal conditioners use digital filters because they are easy to
adjust and no hardware is required. A digital filter is a mathematical filter used to manipulate
a signal, such as blocking or passing a particular frequency range. They use logic components
such as ASICs, FPGAs or in the form of a sequential program with a signal processor.
The signal originating from a transducer is fed to the signal conditioning equipment.
In order to measure the output signal of the transducer originating on account of variations of
a physical change, it is desirable that the output signal be reproduced faithfully. For faithful
reproduction of signal it becomes necessary to eliminate any kind of spurious or unwanted
signals which may get introduced into the system either at the transduction stage or at the
signal conditioning stage. The filters are thus designed to pass the signals of wanted
frequencies and to reject the signals of unwanted frequencies which may be unwanted
harmonics and noise. The harmonics or noise may be due to some form of distortion.
a. Types of filters
Filters may be of any physical form. They may be electrical, mechanical, pneumatic,
hydraulic, acoustical etc. The most commonly used filters are electrical in form. The basic
electrical filters are of two forms as regards the components constituting them. They are
1. Passive filters
2. Active filters
Fig. 5.11 Basic configuration of an electrical filter
Passive Filters only use passive circuit elements like resistors, capacitors and
inductors while Active Filters use active elements like operational amplifiers in addition to
passive elements like resistance, inductance and capacitance. The study of other filters may be
carried out by analogies with electrical filters. Both passive and active filters may classified
further as
1. Low pass filters

2. High pass filters
3. Bandpass filters
4. Band stop filters
Let us consider a basic configuration of an electrical filter as shown in figure. The

source is sinusoidal of variable frequency. The filter circuit may be so designed that some
frequencies are passed from input to output of the filter with very little attenuation while other
frequencies are greatly attenuated. The responses of various filters are shown in figure. These
are ideal responses but cannot be achieved in actual practive.
The ideal response of a Low Pass Filter is shown in Fig. 5.12 (a). The voltage gain is:
| ( )|
This gain is constant over a frequency range starting from 0 to a cut off frequency .
The output of any signal having a frequency greater than will be rejected altogether i.e.
there will not be any output voltage corresponding to signals having frequency greater than
whatever their input magnitude be.
The signals having a frequency range of 0 to will be faithfully reproduced with a

constant gain of A. The output corresponding to signals above frequency is equal to zero
i.e. they are rejected.
Fig. 5.12 Ideal characteristics of Filters
The characteristics of a High Pass filter are shown in Fig. 5.12 (b). The high pass filter
has a gain of zero starting from zero frequency up to cut off frequency co,. Above the cut off
frequency, the gain is constant and is equal to A. Thus signals of any frequency beyond we
will be faithfully reproduced with a constant gain A while signals having frequency below co,.
will not be reproduced at all i.e. rejected.
The Band Pass filter shown in Fig. 5.12 (c), will faithfully reproduce signals falling
within the range and faithfully while signals of frequency between 0 to and of
frequency greater than will be rejected. There will be an output corresponding to signals
having frequencies between and and there will not be any output for signals having
frequencies below and above Thus this filter passes a band of frequencies. Fig. 5.12
(d) represents the characteristics of a Band Stop filter. This filter rejects a particular band of
frequencies from to while passing the signals of other frequencies (with a constant
gain A) starting from 0 to and onwards. This is also called a Notch Filter. The filters
discussed above have ideal characteristics i.e they have a sharp cut off.
5.4 Sample and Hold circuit

A sample - and - hold circuit is used to overcome the time division multiplexing
difficulty. Each signal input to the time division multiplexing system is connected to a circuit
as shown in figure. The first step in this procedure is to open switch S2, and close switch S1.
This places an individual capacitor across each input, and the voltage across the capacitor,
after a very short transient time, is equal to the input voltage. The second step is to open
switch S1, then the voltage across each capacitor is the respective input voltage at the instant S 1
was opened. The third step is to close each S2 switch at a given signal from the time division
multiplexing system to connect each capacitor of the sample - and - hold system to the readout
system at the correct portion of the time-division cycle. Usually, the output of the sample - and
- hold system is applied to an analog - to - digital converter. Then all the digital outputs during
one cycle of the sample - and - hold procedure is the value of the respective inputs at the
instant the switches S1 were opened.
Fig. 5.13 An elementary circuit of a Sample – Hold system
Fig. 5.14 Sample – Hold (S/H) System using operational amplifier
A Sample - Hold system which uses an operational amplifier is shown in fig. this
circuit can take a very fast sample of an input voltage signal and hold this value, even though
the input signal may change until another sample is required. This method uses the storing
capability of a capacitor and the high input impedance of an operational amplifier. When
switch S1 is closed, the capacitor C quickly changes to the input voltage level. Now, if switch
S1 is opened the operational amplifier acting as a voltage follower allows a measure of the
capacitor voltage to be taken at the output without changing the capacitor charge.
When a new sample has to be taken, switch S2 is first closed to discharge the capacitor
and hence reset the circuit. The switches used are electronic switches and are achieved by
digital logic levels. In digital circuits, the data is sampled for a particular interval of time since
the communication channels are many a time shared by a number of variables. The main
purpose of signal sampling is the efficient use of the data processing and the data transmission
units. The sampling operation is shown in Figure.
Fig. 5.15Signal sampling process
This shows that an analog signal and a train of periodic sampling signal whose ‘ON’
time is extremely short as compared with the total period of the signal. The result of the
sampling process is identical to multiplying the analog signal by a train of pulses of unit
magnitude. The resultant modulated signal is shown in figure. This preserves the amplitude of
the analog signal in the modulation envelope of the pulses.
The sampling theorem states that if the highest frequency content in the input signal
is Hz, then input signal can be recovered without distortion if it is sampled at a rate of
atleast samples per second. This is called the Nyquist rate. However, in practice it is
necessary to sample at least samples per second in order to reduce effects of noise and
non-sinusoidal filters. The sample and hold circuit acts as a low pass filter with a cut off
frequency ⁄ where = sampling frequency.
5.5 Data Acquisition (DAQ) Systems

In order to optimize the characteristics of the system in terms of the performance of
the system, data handling capacity and other different relevant subsystems are combined
together. The system used for data processing, data conversion, data transmission, data
storage is called data acquisition system. The typical data acquisition system consists of
sensors with necessary signal conditioning, data conversion, data processing, data handling
and transmission, storage and display system. The modern electronic instrumentation is not
becoming more sophisticated because of tremendous development in micro-electronic devices,
op-amps, multiplexers, data converters, microprocessors and microcontrollers. The data
measurement systems and process controls are very flexible and more programmable. Now a
days, because of development in automation field. The data acquisition system relates to
collection of the input data in the digital form rapidly, economically and accurately as
necessary.
a. Objectives of DAQ Systems

 The data acquisition system must acquire the necessary data at correct speed and at
the correct time.
 It must use all the data efficiently to inform the operator about the state of the plant.
 It must monitor the operation of complete plant, so that optimum online safe
operations are maintained.
 It must provide effective human communication system which helps in identifying
the problem areas. This minimizes unit availability and maximizes the unit output at
lower cost.
 It must be able to collect, summarize and store data properly for diagnosis and record
purpose of any operation.
 It must be able to compute unit performance indices using online real time
communications.
 It must be flexible. Also the expansion facility for the future requirement must be
provided by it.
 It must be reliable and should not have a down time greater than 0.1%
The Data Acquisition Systems are basically used to measure and record the signals
obtained in two ways. Firstly the signal may be originating from direct measurement of an
electrical quantity such as AC or DC voltage, frequency, component vale such as resistance,
capacitance etc. Such signals are always found in electronic component testing, environmental
studies etc. secondly the signal any originate from the transducers such as pressure
transducers, thermocouples. The data processing involves a variety of operations ranging from
simple comparison to complicated arithmetic manipulations. This can be used to collect data
or information performs some operations., if required, convert this information to the suitable
form using converters, perform more number of calculations to remove unwanted noise signal,
gather results to be displayed and so on. The transmission of data can take place over very
long distances or very short distances. The results which are gathered may be displayed
directly on the digital panel or may be on CRT. The data stored may be permanent or
temporary. To collect data rapidly, shift digitizer or some high resolution devices may be used.
For converting analog signal to digital, additional transducers, amplifiers and multiplexers are
used. The use of sample and hold circuit increases the speed with which accurate conversion
of information is possible. The data acquisition system is mainly classified as analog data
acquisition system and digital data acquisition system. The analog data acquisition systems
mainly deal with the measurement information which is in the analog form. An analog signal
is the continuous signal such as voltage versus time or displacement due to the pressure.
While the digital data acquisition system may consists of number of discrete and
discontinuous pulse representing high and low pulses which is in digital form. The
relationship of these pulses with time gives the nature or magnitude of the quantity.
b. Analog Data Acquisition System

The basic components used in the analog data acquisition system are as follows
1. Transducer:
The transducer is used to convert the physical quantity into an electrical signal. The
transducers such as strain gauge, thermocouples, piezoelectric devices, photosensitive are
most widely used. The transducer generates a voltage proportional to the physical quantity
being measured. This voltage is applied as a input to the data acquisition system. Apart from
this some special sensors produce frequency which can be counted by an electronic counter.
This frequency forms the integral part of the quantity being measured. Otherwise the signal
may be modulated then voltage level is reduced with the help of discriminator.
2. Signal conditioner:
This device includes the supporting circuitry for the transducers. It allows the output
voltage of transducer to amplify up to desired level. It also converts the output voltage to the
desired form so that it is accepted by the next stage. It produces the conditions in transducers
so that they work properly. It also provides excitation power and balancing circuits.
3. Multiplexers:
It allows a single channel to share it with more than one input quantity. It accepts
multiple analog inputs. With the help of multiplexer we can transmit more than one quantity
using same channel. The multiplexers are mostly used when many quantities are to be
transmitted. Also when the distance between the transmitting end and receiving end is more,
the multiplexers are used. Multiplexers reduce the cost of installation, maintenance and
periodic replacement of channels if those are used for separate input signals
4. Calibrating equipment:
Before each test, the calibration is carried out. This is called pre-calibration. Similarly
after each test calibration is carnal out and it is called post calibration It usually consist,
millivolt calibration of all input circuits and shunt calibration of all bridge type transducers
5. Integrating equipment:
This block is used for integration or the summation of a quantity. The digital
techniques are normally used for integration purposes.
6. Visual display devices:

These are necessary to monitor the input signal continuously. These devices include
panel meters, numerical displays single or multichannel CRO, storage CRO etc
7. Analog recorders:
These are required to record the output signal The analog recorder include snip chart
recorder, magnetic tape recorder etc.
8. Analog computers:
These are used as data reduction device output voltage of the analog output may be
converted to digital form for further computations. Even though the accuracy' of analog
computations a comparatively less than the digital one the analog computers are wed because
of its less cost.
Fig. 5.16 Analog Data Acquisition System
c. Digital Data Acquisition System

The digital data acquisition system includes all the blocks shown in the figure. It may
use some additional function blocks. The essential functions of a data acquisition system are as
follows,
1. It handles the analog signals.

2. It performs measurement.
3. It converts analog signals into digital data and handles it.
4. It performs internal programming and control.
Fig. 5.17 Generation data acquisition system
The various components of the digital data acquisition system are as follows.
1. Transducers
They convert the physical quantity into a proportional electrical signal which is given
as an input to the digital data acquisition
2. Signal Conditioners
They include supporting circuits for amplifying, modifying or selecting certain
positions of these signals.
3. Multiplexers
The multiplexer accepts multiple analog inputs and connects them sequentially to
one measuring instrument.
4. Signal converters
The signal converters are used to translate analog signal to a form, which is suitable
for the next stage that is analog to digital converter. This block is an optional one.
5. Analog to Digital Converters (A/D Converter)

The analog to digital converter converts the analog voltage to its equivalent digital
form. The output of the analog to digital converter may be led to the digital display devices for
display or to the digital recorders for recording the same signal may be fed to the digital
computer lot data reduction or further processing.
6. Auxillary Equipment
The devices which are used for system programming functions and digital data
processing are included in the Auxiliary equipment. The typical functions of the auxiliary
equipment, includes linearization and limit comparison of the signals. These functions are
performed by the individual instruments or the digital computer.
7. Digital Recorders
They record the information in digital form. The digital information is stored on
punched cards, magnetic tape recorders, type written pages, floppies or combination of these
systems. The digital printer used provides a high quality, hard copy for records minimizing
the operator’s work. The data acquisition systems are used, nowadays in increasing wide
fields. These are becoming very much popular because of simplicity, accuracy and the most
important reliability of the systems. These are widely used in industrial areas, scientific areas,
including aerospace, biomedical and telemetry industry. When the lower accuracy is tolerable
or when wide frequency bandwidth is needed, the analog data acquisition systems are used.
The digital data acquisition systems are used when the physical quantity being measured has
very narrow bandwidth. When the high accuracy with low per channel cost is required, the
ultimate solution is to use the digital data acquisition system.
S. No Analog DAQ System Digital DAQ System

1. It deals with the measurement of a It deals with the measurement of a
signal which is in analog form such signal which is in digital form such
as continuous signal. as discrete signal or discontinuous
signal.
2. Useful for wide bandwidth signals Most effective for narrow bandwidth
only signals
3. Low accuracy Higher accuracy
5.5.1 Single - Channel Data Acquisition System

The basic single channel data acquisition system consists of a signal conditioner
circuit, analog to digital converter, buffer. The analog to digital converter performs the
conversions repetitively at a free running internally determined rate. The basic single channel
data acquisition system is as shown in the figure.
Fig. 5.18 Single Channel DAQ Systems

The digital outputs from the buffer are further fed to either digital computer or
storage or print out device. The most popular example of the single channel data acquisition
system is the Digital Panel Meter (DPM). The digital outputs are obtained from the analog to
digital converter. The A?D converter used for the data acquisition system are designed such
that they can accept external commands to convert and hold operations. A/D converters based
on dual slope techniques are mostly used for the conversion of low frequency data, generally
from thermocouples. The successive approximation technique is most widely used because it
gives high resolution and high speed at moderate cost.
Many times it is observed that the signal level is very low compared to the input
requirement. In such cases, the amplification of the input signal is done to bring its level to
match the input requirement. This is called pre-amplification. If the input signals are to be
isolated from the system physically, the conductive paths are broken by using mostly opto-
coupled isolation amplifier. The pre-amplifier may be coupled with active filters before the
processing of data. These filters minimize the effect of noise carrier and interfering high
frequency components. Sometimes special purpose filter such as tracking filter may be used to
preserve the phase dependent data.
5.5.2 Multi – Channel Data Acquisition System

The Multi-Channel Data Acquisition System can be time shared by two or more input
sources. Depending on the desired properties of the multiplexed system, a number of
techniques are employed for such time shared measurements.
a. Multi-Channel Analog Multiplexed System:

The multi-channel DAS has a single A/D converter preceded by a multiplexer, as
shown in figure. The individual analog signals are applied directly or after amplification
and/or signal conditioning, whenever necessary, to the multiplexer. These are further
converted to digital signals by the use of A/D converters, sequentially. For the most efficient
utilisation of time, the multiplexer is made to seek the next channel to be converted while the
previous data stored in the sample/hold is converted to digital form. When the conversion is
complete, the status line from the converter causes the sample/hold to return to the sample
mode and acquires the signal of the next channel. On completion of acquisition, either
immediately or upon command, the S/H is switched to the hold mode, a conversion begins
again and the multiplexer selects the next channel. This method is relatively slower
than systems where S/H outputs or even A/D converter outputs are multiplexed, but it has the
obvious advantage of low cost due to sharing of a majority of sub-systems. Sufficient accuracy
in measurements can be achieved even without the S/H, in cases where signal variations are
extremely slow.
Fig. 5.19 Multi – Channel DAQ System (A/D Preceded by a Multiplexer)
b. Multiplexing the Outputs of Sample/Hold:

The technique of multiplexing the outputs of the S/H is particularly attractive, when a
large number of channels are to be monitored at the same time (synchronously) but at
moderate speeds. An individual S/H is assigned to each channel as shown in Fig. 17.6, and
they are updated synchronously by a timing circuit.
Fig. 5.20 Simultaneous Sampled System Multiplexer

The S/H outputs are connected to an A/D converter through a multiplexer, resulting
in a sequential readout of the outputs. (Applications that might require this approach include
wind tunnel measurements, seismographic experimentation, radar and fire control systems.
The event to be measured is often a one-shot phenomenon and information is required at a
critical point during a one-shot event.)
c. Multiplexing After A/D Conversion:

It is now economically feasible to employ an A/D converter for each analog input and
multiplex the digital outputs. Since each analog to digital converter (A/D) is assigned to an
individual channel, the conversion rate of the A/D need only be as fast as is needed for that
channel, compared to the higher rates that would be needed if it were used as in a multi-
channel analog multiplexed system. The parallel conversion scheme shown in Figure, provides
additional advantages in industrial data acquisition systems where many strain gauges,
thermocouples and LVDTs are distributed over large plant areas. Since the analog signals are
digitised at the source, the digital transmission of the data to the data centre (from where it can
go on to a communication channel) can provide enhanced immunity against line frequency
and other ground loop interferences. The data converted to digital form is used to perform
logic operations and decisions. Based on the relative speed at which changes occur in the data,
the scanning rate can be increased or decreased.
Fig. 5.21 Multi – Channel DAQ using Digital Multiplexing
Alternatively, input channels having slowly varying data can be pre-multiplexed in

any of the forms suggested earlier, so that a set of sequentially multiplexed sub channels can
then replace one channel of the main digital multiplexed system, as indicated in Figure.
d. Multiplexing Low Level Data:

A low level data multiplexing system, as shown in Figure, enables the use of a single
high quality data amplifier for handling multichannel low level inputs.
Fig. 5.22 Low Level Multiplexing
Individual amplifiers are used for each low level signal. Low level multiplexing can
be attractive when a large number of channels (25), all having low level outputs, need to be
used at moderate speeds. The use of individual channels is possible because of the availability
of high quality amplifiers at moderate cost. (A typical application is a 200 channel stress
measurement system in a transmission tower set up.) Several factors have to be considered to
accomplish low level multiplexing successfully. Guarding may have to be employed for every
channel, and each individual guard may have to be switched, so that the appropriate guard is
driven by the common mode pertaining to that channel. Problems of pickup get more
complicated and have to be taken care of, to preempt the possibility of signal-to-signal, and
even common mode-to differential mode signal cross-talk. Capacitance balance may need to be
carried out. When the number of channels to be multiplexed increases, the problems of stray.
capacitances and capacitive balance are worsened. In the specific case of a 48 channel system,
the input channels are subdivided into groups of eight channels in the first tier. Each of these
six subgroups are in turn multiplexed by a six channel multiplexer on the second tier. The
main advantage of using this is the reduction of capacitance effects.
5.5.3 PC based data acquisition

Nowadays easily available, low cost and wide use of personal computer (PC) led the
various manufacturers to develop interfaces between PC and the outputs of transducers. The
outputs are further processed and analyzed by7 the PC. The advanced techniques allows user
to use a PC ADD-ON card which is available in various configurations. By putting the card in
PC, the analaog signals can be directly interfaced to PC. This gives very powerful
communication and analyses of the multiple measurement data. The main features of PC
based DAQ systems are as follows,
1. The PC based system is used to display the parameters of the system continuously.
This helps the operator in monitoring all the parameters instantaneously and
conveniently.
2. The system parameter are displayed with some display attributes such as blinking,
underline, inverse video extra bright so that the attention of the operator is called.
Sometimes colour graphic display is used to indicate normal operation, close to the
upper limit and out of the limit.
3. Sometimes some man-machine interfaces called MIMIC displays are used. These are
useful in displaying the data measured at any location on the plant near the icon of
that location on the server. This helps operator to take quick corrective action.
4. Several parameters are plotted individually or simultaneously on the screen to show
their characteristics. This helps in pointing out the variations in their parameter
measured at two different forms.
Hence the PC based data acquisition systems goes meaningful and reliable results
even though the large numbers of inputs are measured. The basic block diagram of PC based
data acquisition system is as shown in figure.
Fig. 5.23 PC based data acquisition system

In the PC based data acquisition systems, the personal computer consists of additional
hardware such as data acquisition hardware and data analysis hardware. These are associated
by the software. The software contains the complete programs written for accessing the data,
performing calculations with them and display the result on Video Display Unit (VDU). The
data analysis hardware is used to speed up the computations and analysis mainly in case of
Digital Signal Processing (DSP) applications. The data acquisition hardware generally acts as a
multipurpose, multifunction card. The input signal between the range 10mV to 10V is
amplified by digitally programmable gain amplifier to a standard level.. this amplified input
signal is converted to digital signal with the help of high speed analog to digital converters
interfaced to the PC bus. This signal is recorded, displayed or may be processed further for
analysis by the personal computer.
5.6 Data logger

For proper understanding of a Data Logger Operation, it is essential to understand
the difference between analog and digital signals. For example, measurement of temperature
by a milli voltmeter, whose needle shows a reading directly proportional to the emf generated
by the thermocouple, is an analog signal. However, digital equipment presents a digital output
in terms of pulses and involves an electronic pulse counting equipment which counts the
number of pulses. The pulses are generated such that each pulse corresponds to the smallest
value of the parameter being measured. These digital signals are precise at all times. Consider
the example of temperature. In the case of analog measurements even the accuracy of the
potentiometric method is limited by the precision with which the resistance can be subdivided.
In the digital method, the electrical signal obtained from the thermocouple is subdivided by an
electronic decade circuit and thus the thermocouple voltage can be measured to many places
of decimal. An analog device is capable of measuring with an error of ± 0.5% to ±1%, whereas a
digital device can be obtained with an error of any ± 0.01%. An analog instrument responds to
a change in input levels in times of the order of 0.25 to 1 s while a digital instrument gives
accurate readings in a few hundredths of a second, and often many times faster. One
advantage of a digital instrument is that its reading can be recorded by suitable printer. The
Data Logger Operation senses only digital signals and hence analog signals, if any, have to be
converted to digital signals. The digital technique is employed because it measures very small
(or large) signals accurately and fast. The recording device may be a printed log or a punched
paper tape. The printed output can be either line by line on a paper strip or on a type written
page. Time words are printed at the start of each sequence. Time is recorded in hours, minutes
and seconds. Data Logger Operation consists of the channel identity number, followed by
polarity indication (+ or —), the measured value (4 or 6 digits) and units of measurement.
Sometimes the range may also be indicated.
5.6.1 Block diagram
Fig. 5.24 Block diagram of Data logger

The block diagram of a basic data logger is as shown in figure. The basic blocks of the
data logger system are as follows.
1. Input Signals
The input signals fed to the input scanner of the Data Logger Operation can be of the
following types.
1. High level signals from pressure transducers

2. Low level signals from thermocouples
3. ac signals
4. Pneumatic signals from pneumatic transducers
5. On/off signals from switches, relays, etc.
6. Pulse train from tachometer
7. Digital quantities
The last three signals (5, 6 and 7) are of the digital type and are handled by one set of
input scanners and the remaining signals are of the analog type and are handled by a different
set of input scanner. Low level dc signals are first amplified and then conditioned by the law
network and finally fed to the A/D converter. High level signals are fed straight to law
network and converter. The ac and pneumatic signals are first converted to electrical dc
signals, conditioned and then converted. In this manner, all types of signals are converted to a
form, suitable for handling by the data logger. The purpose of the conditioner is to provide a
linear law for signals from various transducers which do not have linear characteristics. Filters
are used for noise and ripple suppression at the interface of the output of the transducers and
the input of the signal conditioner, since these signals carried by the cables are of very low
magnitude. Digital signals are then fed to the digital interface, whereas analog signals are first
amplified, linearised and then brought to the analog interface. They are then converted into
digital form and finally fed to the digital interface.
2. Input Scanner
Because of the scanner select each input signal. In turn, the Data Logger Operation
requires only one signal amplifier and conditioner, one A/D converter and a single recorder.
Modern scanners have input scanners which can scan at the rate of 150 inputs/seconds, but the
rate of scanning has to be matched with the rate of change of input data, and the time required
by the recorder and the output devices to print one output. Sometimes it is desirable to scan
certain parameters at a faster rate and some others at longer intervals. For such mixed scan
rates, the scanning equipment is designed for an interlaced scan operation, in which it is
possible to log some parameters at 30 — 60 minutes interval, some every 5 minutes, and others
every few seconds. A scanner, in effect, is a multiway switch which is operated by a scanner
drive unit for selecting the circuits. As the switch contacts have to continuously (24 hours/day)
deal with low level signals at very, high frequencies, the following requirements (desired
characteristics) must be considered in the design of the contacts and their operations.
1. Low closed resistance

2. High open circuit resistance
3. Low contact potential
4. Negligible interaction between switch energizing signal and input signals
5. Short operating times
6. Negligible contact bounce
7. Long operation life
Although it may not be possible to achieve all these characteristics in one switch, the
arrangement selected must satisfy the maximum possible conditions. The various switching
elements available commercially are as follows.
1. Rotary selector switch

2. Electromagnetic operated relays
3. Dry Reed type
4. Mercury wetted reed type
5. Solid state switches
3. Signal conditioner
Since Data Logger Operation give their readout in the units of measurements
concerned, there are two requirements: a) Scaling linear transducers, b) Correcting the
curvature of a non-linear transducer, such as a thermocouple. Linear inputs can be dealt with
in two ways.
 The simplest is to provide individual resistance attenuation on each input in order to
reduce the transducer output level, where the scale factor is an integral power of ten.
For example, if a particular transducer has a full scale of 10 mV for a pressure of 500
kg/cm2, we can reduce the value to one half by the use of an attenuator, such that 500
kg/cm2 may be represented by 5 mV. If the system is to have a resolution of 1 kg/cm 2,
the A/D converter must have a resolution of 10 pV. This technique is limited only by
the sensitivity of the A/D converter.
 The second method is to change the sensitivity of the A/D converter. But since each
input may require a different scale factor, this is not convenient as an input
attenuation technique.
The signal can be linearised at any one of the following three places a) In the analog
stage before conversion, b) In the conversion process and c) Digitally after conversion. The first
method is not suited to low level voltages, as it requires some form of amplification. The signal
conditioner may be placed between the scanner and the converter. But, each type of transducer
requires individual linearising circuits. The third method requires a storage capability and a
computer processing technique. The most satisfactory is the second method, whereby
linearisation is built into the conversion process.
4. Analog to Digital Converter

The analog to digital converter converts the analog voltage to its equivalent digital
form. The output of the analog to digital converter may be led to the digital display devices for
display or to the digital recorders for recording the same signal may be fed to the digital
computer lot data reduction or further processing.
5. Recording system
The output from the Data Logger Operation can be printed on any of the following.
1. Typewriter
2. Strip printer and/or digitally recorded on punched tape or magnetic tape for
further analysis in a digital computer.
The typewriter provides a conventional log sheet with tabulated results, and prints in
two colours. The signals obtained from the A/D converters are applied to the electromagnetic
operated levers of a typewriter. Plus, Minus, characters which can be printed one at a time,
decimal point shift, line shift, type colour and spacing are controlled by the EM solenoids
which are energized from the programmer unit. Punched paper tape or magnetic tape is used
when the recorded data is to be further analysed or where the rate of data acquisition is too
great for a printer.
6. Programmer
This can be considered as an automatic sequence switch which controls the operation
of all other units of the data logger. The sequential operations performed by a programmer are
as follows.
 Set amplifier gain for individual input, i.e. gain of the amplifier has to be so adjusted
that for a maximum value of input signal, the A/D converter records a full scale
reading.
 Set linearization factor so that the adjusted output from the signal amplifier is directly
proportional to the measured quantity.
 Set high and low alarm limit
 Initiate alarm for abnormal condition
 Select input signal scanner switching is set normally by a timing pulse to select the
reset input.
 Start A/D conversion and Record reading channel identify and time (in order that the
readings may be identified at a later stage, a number identifying that the input has
been normally recorded, with the actual reading and the time during the beginning of
each complete scan).
 Display reading
 Reset logger. (At the end of cycle the A/D converter sections of the logger are reset to
their initial conditions and the cycle, starts again.)
5.7 Applications of Sensors

Sensors elucidated here are only highlighted in terms of their applications as many of
these are discussed in details in the earlier chapters. Their adaptation to specific applications
has been given due consideration. In doing so, a little analysis of the sensor system has been
made in some cases. Obviously not all fronts of application can be covered in the present text
though the ones presented offer a wider exposure.
5.7.1 Automobile sensors (Automotive sensors)

Sensors for automobiles, that is, automotive on-board sensors come with some special
constraints and features that include environment, reliability, cost, and resources and
innovations. Engine is the heart of the automobile which is exposed to vibration, dust,
electrical noise, extreme temperature variations, and hence, the sensors used too get exposed
to all these conditions even though the performance level has to be kept unabated. One such
sensor is the flow-rate sensor as discussed in the following subsection. The engine
compartment of an automobile has a temperature varying from –40 to 150°C and vibrational
acceleration ranging from 3g-30g. Exposure to water, oil, mud, electromagnetic interference,
and the like are also to be taken into consideration for better performance. Reliability in itself is
important criterion for production to be trouble-free and for maintaining accuracy at least over
ten years. Besides, safety can be ensured in this way—at least partially. Reduced cost reduces
price but it should not be done at the cost of safety and accuracy. Innovative designs are
therefore often required for application in automotive conditions. In present day automobile
systems, sensing is required to be done majorly for
(i) Engine control,

(ii) Manouvering control,
(iii) Room and operational comfort control,
(iv) Safety and reliability, and
(v) Fuel consumption control.
a. Flow-rate Sensors
For the conventional engines with carburetor, such sensors are not necessary as the
air-to-fuel ratio is self-adjustable here. For the upcoming electronic fuel injection engines, these
sensors are made use of as the air volume input to the engine is estimated by flow-rate or
pressure sensing. The estimation is done on the basis of engine revolution and the negative
pressure measured. at its intake. As an advancement, for engines using carburetor also the use
of sensors is being considered now and the proposed sensors for this application is ultrasonic
flowmeters where the flow-rate is measured by measuring the difference between the speeds
of sound both upstream and downstream. But the success of such sensors is yet limited
because of large flow-rate changes and temperature variation. As in other areas, micro-sensors
arc making inroads into the automobile systems as well. Solid state sensors developed through
semiconducting technology are used for sensing air and fuel flows. The underlying principle is
to use a heating clement in the form of a transistor or a `semiconductor' resistance bridge with
the advancement of micromachining technology.
Fig. 5. 25 Semiconductor flow sensor
b. Pressure Sensors
Pressure is a very important parameter in on-board automobiles and pressures of
intake manifold, engine oil, brake oil, tyres, room atmosphere, and so on need be measured.
The conventional diaphragms and bellows elements in association with strain gauge, LVDT,
and capacitive elements are already in use for pressure measurement. The semiconductor
capacitive devices and SAW devices are being increasingly used from which high frequency
output can be derived for easier signal processing. With applied pressure, a crystal oscillator
changes its frequency and is being tried now as also the PZT types devices. Such a pressure
sensor processed by the semiconductor technology and MEMS is shown schematically in
figure. For negative pressure sensing for the intake manifold, a semi-smart sensor is used.
Fig. 5.26 Schematic of a pressure sensor.

Amplification, calibration, and temperature compensation arc internally made for in the
sensor using IC technology. A basic sensor unit uses a silicon diaphragm and a vacuum
chamber is created. The sensing silicon chip is resistive in nature and is obtained by adding
appropriate impurity to the diffused design. Solder Si-sensor chip
c. Temperature Sensors
Temperature sensors for motor vehicles are of various kinds depending on the place
where they are used. Temperature switches are pretty common that must be fast. High
temperature sensitive sensors are also very much in demand and there are traditional types
which are used for oil and water. Temperature ranges from —40-180°C in engine coolant, oil
basin, and gear box while for the exhaust gas at the exhaust pipe it may rise upto about
1200°C; the temperature inside the car is less than 80°C, and so on. Therefore, sensors need to
be carefully chosen to offer maximum output and reliability under these conditions. In engine
coolant and oil, bimetal elements are commonly used as thermos relays having hysteresis <
5°C for a temperature range of —40°-140°C, and switching current 6 A at a rated supply of 12
V. For conversion of coolant, air, oil, and gas temperatures in automobiles into analog signal
output for indication or for use by the computer/processor for regulation purposes, negative
temperature coefficient thermistors are mostly used. The thermistor range lies between 50-
150°C with 1 k12 resistance at 25°C. However, for cylinder head, RTD (platinum resistance)
appears to be a better choice. It is the design in either case that has got to be special to fit into
the feature of the vehicle. Figure 9.4 shows a typical RTD used in the vehicle, it is an RTD 600
variety with a range of —40-300°C that possesses an accuracy of ±2°C.
Fig. 5.27 RTD as used in automobiles. Fig. 5.28 Hybrid IC temperature sensor
For coolant, engine oil, and air experiencing temperature range of 40-140°C, quartz
temperature sensors are ow quite frequently used. These have an accuracy of ±1°C. The
thermometer houses the n e electronic processing unit which is a surface-monitored device
mounted on ceramic hybrid system. It can also be adjusted by laser trimming system. The
frequency of quartz changes with temperature and this property is exploited for detection.
Figure shows the basic schematic of such a system. For exhaust gas temperature sensing, Ni—
Cr/Ni thermocouples are used upto 1200°C which has a sensitivity of about 40 µV/°C.
Thermocouples are gradually getting strong foothold in auto-monitoring because of their
active nature.
d. Oxygen Sensors
Oxygen is a very important parameter for auto exhausts and gas concentration inside
the automobile. Appropriate oxygen sensors are installed in the emission controlled systems to
reduce the toxic exhaust and improve fuel consumption. Zirconia and titania sensors are being
used for detecting air-to-fuel ratio for quite some time now. Niobium oxide sensors have also
recently been introduced and are observed to show better performance. Sensors for checking
smoke, humidity, and odour are fixed inside the automobiles and limiting current oxygen
sensors are still extensively used in automobile exhaust. The saturation current, called the
limiting current in these sensors is proportional to the ambient oxygen.
Fig. 5.29 Oxygen sensors
In Fig the cathode is exposed through a pin hole made inside a cover that limits the
diffusion of gas. The size of the pin hole is so chosen as to limit the rate in the transfer of
oxygen at the cathode. There is another variety of temperature sensors where a thin film
platinum cathode, a zirconia electrolyte slice, and a platinum anode are all deposited on a
porous Al203 substrate by sputtering and on the other side, a platinum is deposited as a
heater. Controlling the substrate porosity during the design stage itself limits oxygen transport
by the diffusion process.
e. Torque and Position Sensors

The power trains by which an automobile is run consists of the engine itself, the
transmission link, differential gear, axle, and wheels. The torque generated in the engine is
distributed to the wheels through power trains. A torque sensor for each component at
appropriate position of the power train Provides quick and precise response to power controls.
Non-contact sensor is found to be suitable for practical adaptability, particularly in its
miniature form. Such a sensor is now available and works on the magnetoresistive effect and
which can be installed in main bearing and hence, mean output torque can be detected for
multi cylinder engine with a single sensor. The design is packaged approximately within 12 x
8 x 16 mm small package in which both exciting and pick-up cores of laminated silicon steel
and permalloy respectively arc housed wound with 200-400 turns of wire. The sensor area
exposed to the shaft at a gap of 0.2 mm is about 12 x 3 mm. Position sensing is another
important aspect in automobiles for detecting shaft position, engine speed, throttle position,
potentiometer position, and so on. Here also non-contact sensors receive preference. The
semiconducting sensors such as Hall and magnetoresistive ones and the other varieties such as
ferromagnetic, electromagnetic pick up, optical modular device, Wiegand wire, and capacitive
modular device are also considered suitable. Of the electromagnetic pick-up variety, proximity
sensors are most common because of high resolution and low cost offered by them although
its output varies with changes in rotational speed. Integrated magnetic sensor using
ferromagnetic resistive element is also being increasingly used. It has high sensitivity at low
magnetic field and is comparatively less sensitive to temperature variations. Figure 9.8
diagrammatically represents such a sensor. The sensing unit is the NiCo thin film deposited by
electron-beam evaporation method. The IC contains a signal processing unit with a differential
amplifier and other processing units leading to digital output. Figure 9.9 shows its mounting
arrangement. Al Si02
Fig. 5.30 Integrated Magnetic sensor

Fig. 5.31 Mounting of proximity sensor.
5.7.2 Home appliance sensors

Home comfort depends to a great extent on the availability of automatic home
appliances such as cleaner, refrigerator, washing machine, microwave ovens, and so forth.
Semiconductor technology has grown fast over a last few decades leading to development \of
microminiaturized processors, circuits, and of course sensors therefore enhancing the
capabilities of the home appliances in terms of automation, safety, and efficiency. Smart
operation of the home appliances depends largely on appropriate sensors which have made
the equipment more convenient, energy-economic, and safe.
Basically, the sensors are used in electronic control of the appliances and when
coupled with microcomputers, all these requirements are almost fully met. Therefore, the basic
requirements for the censors for home appliances can now be revisited as they must have low
cost, small size, light weight, better reliability, and easy handling. The sensors used in home
appliances are nothing new though the tendency is to miniaturize them retaining the reliability
and efficiency. Sensors so used belong to all categories, that is, mechanical, chemical, magnetic,
temperature, and radiation types—the last two types having major applications.
In the mechanical category, Silicon pressure sensors, metal diaphragms, and

potentiometers are used in carpet cleaners, while bellows element types are being used in
refrigerators. Potentiometers are also used in washing machines. In the chemical type,
humidity sensors have considerable applications in microwave ovens, clothes dryers, air
conditioners, dehumidifiers, and also in VCR cameras, while gas sensors are used in ovens,
and exhaust fans. Magnetic sensors are widely used in electronic gadgets in entertainment.
There are Hall sensors in VCR cameras, stereo sets and tape recorders. Magnetoresistive
sensors are used in VCR cameras, and tape recorders. In the temperature sensing category,
thermistors are extensively used in ovens, cooking ranges, refrigerators, dishwashers, dryers,
dehumidifiers, air-conditioners, exhaust fans, CD players, and stereoplayers. They are also
used in electric carpet and blanket, cleaners for temperature control. In contrast, PTC devices
are used in cookers and electric cooking ranges; thermocouples are used in gas ovens;
bimetallic elements find use in gas ovens and rice cookers while infrared sensors are employed
in microwave ovens.
Radiation sensors, that is, photodiodes, and phototransistors are used as the major
elements in refrigerators, washing machines, air-conditioners, TV sets, CD players, stereo
players, and video disc players. Photoresistors such as CdS are used in TV sets while VCR
camera uses charge control device (CCD) image sensors and MOS image sensors.
The pyroelectric IR sensor used in microwave ovens comes, in general, in TO

package. It consists of a LiTaO3 pyroelectric element on a silicon base plate and is irradiated
through a silicon window. The sensor appears as shown in Fig. 9.10. Its responsivity is 200-300
V/W, NEP is less than 2 nW/Hz, response time is around 0.2 s, temperature range is –20°C to
100°C, and with silicon window the spectral response is 2-15 ptm.
Fig. 5.32 Basic pyroelectric IR sensor
In a microprocessor-controlled washing machine, water level is sensed using optics principles

that comprise units like a light emitting diode (LED), a photodiode/phototransistor, and a light
slit. The light slit is moved by the water level. This type of sensor is also used in rinsing
chambers for the detection of degree of rinsing which provides information about the
concentration of residual detergents. Here, two such systems arc coupled—one for reference
and the other for measuring. The outputs from the phototransistors of the two sets are
compared for achieving a standard condition. The sensor is schematically shown in figure.
Fig. 5.33 Optical sensor for water level detection.

The sensor used for spin-dry system in washing machines is a PZT ceramic sensor. It
is based on the principle that when water drips on to the surface of the sensor, voltage
developed in the sensor becomes less with more impinging force of water on it. As the clothes
are dried, voltage also increases. PZT, as discussed in an earlier chapter, is a solid solution of
lead zirconate (PbZrO3) and lead titanate (PbTiO3)—it also belongs to pervoskite structures.
Its piezoelectric property depends on TirZr (T/Z) ratio. Most ceramic piezoelectric transducers
belong to this group. Variations in properties have been obtained by partial replacement of
Pb2+ by other divalent cations (such as Ba, Ca, and Sr) and Ti4+ and Zr4+ by tetravalent
cations. Figure gives the structure of a cubical pervoskite. The PZT ceramics are manufactured
(see Chapter 8) in the same way as thermistors, from mixed oxide powders while sintering is
done at 1200-1300°C at normal atmospheric, pressure. Excess lead oxide (PbO) should be used,
as a precaution, in a refractory enclosure during sintering as PbO has a high partial pressure.
The grain-size and porosity of the ceramic are controlled by hot pressing which in turn
controls the electrical property.
Fig. 5.34 The pervoskite cube
Photodiode—LED assembly has also been used for frost detection in refrigerators.
With frost, the light intensity received by the photodetector is reduced as in the case of a
rinsing system. An alternative system for this use piezocrystal oscillator and a PTC thermistor
system. The crystal in an oscillator circuit vibrates at its natural frequency and with frost
formation, its resonance frequency changes. PTC thermistor heats the crystal for making it
frost free. The video cassette recorders, for precise control of the servomotor that drives the
playback and recording heads, use Hall and magnetoresistive sensors. For control of the
cylinder head and its start and stop operations, photosensors arc made use of. The video tape
has to be protected from dew drops formed on the cylinder head of a VCR for which special
humidity sensors have been developed as dew detectors. The detecting film is a hydrophilic
acrylic polymer on which carbon particles are sprinkled.
Homes are moving towards being automated and for that along with the variety of
sensors available, new sensors need be developed. Fortunately, the development is already
underway. Three important categories in home automation are (i) house control, (ii) energy
control/optimization, and (iii) home security. A block schematic of a home automation system
with various sensors is shown in Figure
Fig.5.35 Block schematic of a house automation system: (1) main controller, (2) and (3)
secondary controllers, (4) air conditioners, (5) current sensor, (6) light control, (7) smoke
control, (8) ventilation, (9) security, (10) gas, (11) thermal/electrical keys, (12) bath etc., (13)
earthquake protection, (14) electrical keys (15) air adaptor, (16) light (secondary).
5.7.3 Aerospace sensors

Sensors in aerospace applications have to withstand wide variation in physical
conditions. Density of air changes from a free molecule to normal state, fluid flow
temperatures vary from 20-350 K. At supersonic speed, gases lose their near ideal behaviour.
In air, sensors are subjected to varying acceleration, rain, ice, and other environmental
changes. Parameters which are to be monitored for optimization are not new and include
pressure, flow and flow direction, temperature, aircraft speed, fluid velocity, strain, thrust,
force, acceleration, and so forth. Sensors that are used are not special though their installation,
data acquisition, and processing are to be carefully done.
a. Static Pressure Sensors

For altitude and time rate of change of altitude, that is, vertical speed static pressure
is to be monitored. For this, probes are used which are Pitot tubes of appropriate design and
require to be aligned accurately. One such probe for static and total pressure measurement is
shown in Figure. Here denotes the static pressure and denotes the total pressure. The
holes for placing probe for and are to be properly chosen with reference to their sizes and
location. Besides length, nose shape and cross-section of the probe are also equally important.
There are differences in the actual pressure and the indicated static pressure specially for
supersonic flow. Correction and evaluation of is possible by Mach number versus ⁄
and ⁄ curves. Figure shows the technique for the type of probe of figure. Using the curve
of the ratio of the indicated pressure versus mach number (M), M is first determined, then
using this M, from curve ⁄ , , is evaluated. The curves are dependent also on y, the ratio
of specific heats or the isentropic exponent. Reduction in γ (by, say, 7%) lowers the upper
curve (by 5%) and raises the ⁄ curve by approximately 1%.
Fig. 5.36 Probe for static and total pressure sensing
Misalignment of the probes incurs considerable error in static pressure. Angle with
the flow channel of a 10° cone Pitot is shown in Figure. Prandtl tube and wedge are two other
pressure probes which also suffer from this error. The errors are also shown for 8° wedge and
30° Prandtl tube in Figure
Fig. 5.37 Technique for evaluation of the desired pressure
Fig. 5.38 Error curves for three different probes
For Pitot and other tubes, the term impact pressure is often used in practice. It is the
difference of and and is thus, identical with the dynamic head or dynamic pressure,
⁄ (see equation for Pitot tube), where v is the velocity and p is the fluid density. Target
meters using strain gauge sensors are also used for dynamic pressure measurement specially
in the studies of turbulence in local flow velocities of jet engine drafts where frequency is of
the order of 5-10 kHz.
b. Temperature Sensing
Static temperature is the simple streamline temperature needed to establish the
acoustic speed and hence, the gas velocity from the knowledge of Mach number M. This is
denoted as On the other hand, total temperature , is the one that the gas acquires if it is
isentropically stagnated. For measurement of temperature, the probes usually chosen are RTD
and/or thermocouple. In the aerospace terminology, these are called “Temperature Sensitive
elements (TSE)”. Here again, if is measured , is deducible from , M and γ.
Obviously for measurement of , stagnation should be complete otherwise what is

( )
measured is called the indicated that is, and the recovery ratio ⁄ , is a measure of
success of stagnating. Another parameter called recovery factor ( )⁄( ) is,
however, the theoretical measure of this success. For high temperature measurement,
thermocouple is the first choice. Type K upto 850°C and types B, R, or S thermocouples upto
about 1250°C are used. For even higher temperature (upto about 2300°C), and
are recommended specially in nonoxidizing atmosphere. As gas density changes with
temperature, a pneumatic probe in the form of an aspirator is used to measure temperature by
measuring gas density specially if the velocity at the aspirator entrance is small. If the flow is
sonic, it measures the product of density and pressure . By stopping aspiration, is
measured and density can then be found out and from that temperature.
Static temperature of hot gases such as the exhaust of a jet engine or hydrogen—
oxygen rocket, is obtained by the optical absorption—emission method. Here, actually the
radiation from is utilized and accuracy is improved by simultaneous measurements at
several wavelengths by spectral scanning. There are other optical methods such as line and
band reversal method, spectral intensity distributions within a molecular band, Raman
spectroscopy, and many more wherein the Temperature is correlated with the spectral
distribution pattern of scattered radiation. In all these techniques, only the measurements of
relative intensities are involved.
For measurement of cryogenic temperatures, around that of liquid hydrogen (20 K),
metallic resistance thermometers are used. Examples are gold—irridium alloy, platinum, and
so on. These are well-known and have been discussed earlier.
For measuring temperatures of solid components of aircrafts and rockets, common

methods such as thermocouple are still resorted to. For measuring turbine blade temperature,
a narrow-band or monochromatic radiation pyrometer is used. This avoids the uncertainty in
blade surface emittance. Two wavelength pyrometry can be used for improving performance
reducing uncertainty.
c. Fluid velocity sensors

Fluid velocity can be divided into two parts (i) local linear velocity and (ii) bulk
velocity or bulk flow-rate. The former is measured by Pitot tube, hot wire anemometer and/or
laser Doppler velocity meter (LDV), where the latter, LDV, is of more interest in aerospace
applications. Bulk mass flow-rate can be measured by gyroscopic, Coriolis, moment of
momentum or calorimetric sensors. Besides, for such flow-rates other sensors such as vortex
type, orifice type, flow-nozzles and turbine types are also used.
A common variety is the orifice type velocity meter, shown in Figure, where it is
shown that the flow is restricted through an orifice creating a differential pressure at two
convenient points, both upstream and downstream which is related to bulk velocity of flow by
simple equation. Often empirical, this equation is obtainable through rigorous analysis. The
relation is given as
where q is the volume flow-rate,

is the differential pressure,
P is the absolute pressure,
R is the gas constant,
T is the absolute temperature.
Fig. 5.39 The orifice flow sensor
The relation follows the energy conservation principle through Bernoulli's equation.
However, k a constant, is seen here which takes care of the losses and is dependent on
dimensions of the pipe and orifice. The shape and size of the orifice and the flow-rate play an
important role in its choice. The value of k is available in charts or tables with d/D as a variable
parameter. Solid state flow-rate sensors have already been considered. These are
manufactured using semiconductor technology.
d. Sensing Direction of Air-flow

It is important to determine the direction of air-flow in aerospace services. In cone
type probes or probes resembling cone or wedge, two holes on the inclined surfaces that lie
diametrically opposite to each other are used. The difference of the pressures of these two taps
is measured which is more or less proportional to the angle between the axis of the probe and
the direction of air-flow. There are variations in the design both of the probe (specially its
nose) and hole positions. Figure shows a typical design of a device used for this purpose.
Fig. 5.40 Air velocity and direction of flow sensor
It can be shown that the pitch and yaw angles and are given respectively by
the average pressure being given by
e. Measuring Air-speed on Aircrafts

Air-speed on aircraft can be computed from the measurement of total pressure ( ),
temperature ( ), and static pressure ( ) using the equations of ideal gas. If m is molecular
weight or relative molar mass, R0 is the gas constant, and γ is the isentropic exponent, then for
subsonic flow velocity
√{ ( ) } { }
( )
and for the normal shockwave upstream of the craft, it is
( ) ⁄
√{ }
( )
( )
{ }
Total and static pressures are measured by a probe by extending it from the aircraft
nose. Sometimes the static pressure is measured by two orifices located on the side of the
fuselage, the two are placed diametrically opposite to each other to give average pressure of
yaw and pitch. Temperature T, is measured by an RTD. At a high Mach number, deviations
from the computed values are observed using Equations, mainly because
a) The equations use ideal gas laws

b) Change in specific heats with temperature occurs at high temperatures due
to increased rotational and vibrational energies
c) A time lag appears for equilibrium between the above energies near the
shock front
d) At very high temperatures, ionization and dissociation of molecules occur,
and so on.
Obviously, adequate correction has to be applied for these changes.
f. Monitoring Strain, Force, Thrust, and Acceleration

Aerospace research and studies involve measurement of strain, force, thrust,
acceleration, and so on for operation, innovation, and safety considerations. Both static and
total pressure, and other parameters are measured using the usual sensors such as strain
gauges specially for strain and force.
Appropriate positioning of the gauges and compensation for temperature variations

are to be taken care of. For dynamic strain alone with fluctuation frequency of several Hertz,
the gauge material is chosen to be fatigue-free or to withstand high fatigue. Load cells are also
made of strain gauges bonded to a spring whose deflection is sensed. Load cells are used for
weighing aircrafts and for measurement of thrust forces. Engine thrust of a rocket is sensed or
determined in the test centre from the integral of dynamic pressure which is measured by an
array of total head (pressure) tubes at the exhaust. Alternatively, the force exerted in a tank
receiving the engine exhaust is measured by load cell; here also dynamic pressure is area-
integrated. Acceleration measurement in an aircraft is very important during acrobatic
movements or gusts when stresses in the structures increase and require to be obtained by
correlating stress with acceleration. Standard seismic element devices are used for the purpose
but the location of the device in the aircraft is carefully chosen. One such point is near the CG
(Centre of gravity) of the aircraft, another is the main spar or rigid airframe.
5.7.4 Sensors for manufacturing

Manufacturing, basically is a controlled process or system—the key to the control
being the sensors used in automated manufacturing. Diagrammatic representation between
sensors used in such manufacturing and various automation levels in a plant is shown in
Figure. Increased interconnection has been employed because of the invasion of the computer
in manufacturing and special software based on process data available from signals delivered
by the sensors at various stages and levels. The concept is known as computer-integrated
manufacturing (CIM). Obviously, the prime task of the sensor in this is to run the automated
production processes smoothly by compensating for disturbances, tolerances of workpieces,
and environmental conditions as and when required.
Quality control is also an important area where sensorial assistance is now largely
sought. Intelligent sensors are gradually becoming essential as they can interface with each
other through organized software processing of electronic signals.
Fig. 5.41Interaction diagram between sensors and levels of operations
a. Sensors
Sensors used in production processes have to perform functions which are not
conventional process control functions. The chart depicts the sensor functions briefly. They are
not always as distinct as indicated in the diagram but may be performing in combination on
demand. Most of the sensors used have been considered earlier but for robotic actions, specific
sensors are applied in production engineering. Sensors used in such actions are discussed in
this subsection with the actions for which they are meant for.
Fig. 5.42 Sensory functions
Distance sensing: This can be done by (i) tactile sensors, (ii) electrical sensors such as
inductive and capacitive, (iii) optical sensors using IR, UV, visible, and laser radiations, and
(iv) acoustic sensors using ultrasonic principle.
Contour-tracking: This is a kind of scanning process and is performed by using (i) electrical
sensors such as inductive and capacitive ones and (ii) optical sensors—mostly laser-based
scanners.
Machine vision/Pattern recognition: Here also, tactile arrays and ultrasonic scanning serve
some useful purpose. Besides optical systems with binary vision, grey level vision and
stereovision are widely used.
Machine diagnosis: Well-known sensors arc used to measure pressure, force, torque, speed
(both linear and rotational), temperature, frequency, and a lot of other electrical parameters for
obtaining indirect diagnostic data.
Process parameters: Parameters as mentioned in the preceding paragraphs are measured

under different environmental conditions.
a. Distance sensing
During processing, the workpiece and the tool face the possibility of collision.
Therefore, the distances between the two for various operations need be monitored. In some
processing operations, the distance between the two should be maintained constant as in laser
cutting. Sensors for distance measurement are of two types, namely (i) contact type and (ii)
non-contact type.
The latter type is gaining ground because the sensors in this type are free from wear
and tear. Contact type distance sensors are common metrological instrument components such
as pins, gauge blocks, dial gauges, and many others. Switches and buttons with potentiometric
or inductive pick-up are also used. In the non-contact type distance sensors, inductive,
capacitive, acoustic, and optical techniques are adopted. The inductive pick-ups are designed
and named proximity sensors. Single coil and multi-coil designs are also common. Multi-coil
designs allow to measure the distance in two coordinates. Example of an inductive proximity
sensor used in distance measurement is schematically shown in Fig.
Fig. 5.43Example of an inductive proximity sensor

The middle coil, coil 2, is fed with ac of appropriate frequency allowing it to produce
an ac magnetic field in its own proximity. Coil 1 and coil 3 symmetrically positioned with
respect to coil 2 are also electrically energized with phase opposition with respect to supply of
coil 2 in absence of any metallic body approaching the set up (coils 1 and 3). With any metallic
body approaching, as shown, the magnetic field distributions to coils 1 and 3 change and a
signal is generated which can be seen to be proportional to the distance and angle between the
body and the coil (s).
Ultrasonic sensors housed in robot gripper utilize the period between the reflected
pulse (echo) and the original pulse sent by transmitter for distance measurement. Figure
shows this scheme. In this, hr refers to the distance of reference plane and h, is the height of
the job attached to it. Two pulses, one from the plane and the other from the top of the job, are
shown in Fig. 9.23. Focussed ultrasonic beam pulse after reflection undergoes change in the
pulse height depending on the distance from where the reflection occurs. Figure shows this
change. For measurement of time, digital counting technique can be used.
Fig. 5.45Evaluation of the distance
Fig. 5.46 Diode array sensor: 1. laser source, 2. focussing lens, 3. job, 4. diode array
In optoelectronic technique of distance sensing, a laser beam is focussed on the

approaching body and its reflection is then detected by a properly aligned photodiode after
being converged by a lens. This can detect either (i) the intensity of light, or (ii) the angle of
approach. The latter technique is known as optical triangulation. The detector system consists
of about 1000 diodes arranged in an array which can help to enhance reso.ution. The scheme is
shown in Figure. Obviously, distance d of the detecting diode is important here. From the
figure,
The angle θ, b, and a are fixed while x is a function of d. For the method to be
successful, the approaching surface should have good polish for reflection. The technique can
have a resolution of the order of tens of gm. Triangulation principle is also used in contour
tracking using preview laser scanner. The job profile is obtained by scanning it with a narrow
laser beam and sensing the reflection from the job-piece with an array of diode detectors.
Machine vision is an intelligent sensing system. It involves scanning the object with a
video camera whose output is converted to digital by an ADC for image processing, and
feature computing and identification. Then comparison with model, called pattern recognition,
is performed for the desired output. The system obviously requires a very sensitive viewing of
the object with adequate resolution and discrimination. Images are obtained by (i) ultrasonic
transducer scanner, (ii) X-ray scanner. However, for robotics, the tactile arrays which generate
electrical signals from pressure sensing have been of special significance. The transducers so
used may be conducting rubber type, the capacitance type, or piezoresistive type. The rubbing
pressures produce change in resistance in the conducting rubber type transducers while
capacitance changes in the second even by touching. In the piezoresistive transducer, the
normal piezoresistive action takes place. The scanned output obtained from a multiplexer may
be stored. Response time of each of such sensors is less than 1 ms per 100 units in an array.
Figure shows such a sensor system. Conducting rubber
Fig. 5.47 Piezoresistive array sensor

For machine diagnosis, the techniques applied are (i) process parameter monitoring,
(ii) power consumption by the machine and edges of work-pieces (their condition), (iii) force
and torque sensing, and (iv) change in the noise of the machine in operation. The first
technique is not very straightforward. In force and torque sensing, strain gauges are
extensively used. Noise sensing, however, has become an important technique with the
advancement of device technology. Noise sensors are, in general, capacitive type. Often,
ceramic pieces of PZT material consisting of lead zirconate and lead titanate are used for the
purpose. One such scheme is shown in Figure. Acoustic pressure is transmitted to ceramic
beam with the conical diaphragm. Holes in the diaphragm are provided for equalizing average
pressure. Back up plate prevents sag. Very small capacitive microphones have also been
developed for sound intensity measurement. Their appropriate placement is a very important
aspect. Figure gives a schematic arrangement of the system where dynamic sound pressure pd
can be measured with static pressure ps acting as 'buffer'.
Fig. 5.48 PZT sensor acoustic pick-up system: 1. conical diaphragm with back-up plate, 2.
PZT ceramic beam, 3. metallic support, 4. insulator.
Fig. 5.49 Acoustic pressure sensor for static and dynamic pressure sensing: 1. diaphragm, 2.
Insulator, 3. Air leak.
5.7.5 Medical diagnostic sensors

With the advent of sensor technology involving different materials, processing, and
size and shape, clinical studies are gradually being replaced by instrumental studies in
medical science with the sensors serving as the interface systems between the instrument
hardware and the biological systems.
This requires that the sensors used have very stringent specifications. The most
important consideration is whether the sensor is invasive or noninvasive. Gradations have
been made here, for example, in invasive type minimally invasive category has been defined
where surgical implantation is not needed. Special sensors may be placed remote from the
body. When sensors are to be implanted and/or made indwelling type, they should have
minimal effect on the biological system. This requires choice of materials for the sensors which
is to be nontoxic and system compatible. It is to be remembered that often the sensor is in a
package, so compatibility means that the packaging should also match the tissues in contact
such that irritation and inflammation do not develop and measurement is not affected by any
means. Noninvasive biosensing electrodes are often used on the skin surface leading to
manifestation of allergy from the adhesive tapes for some patients. This is found to change
with stretch conditions and is considered to be a mechanical mismatch. A very important
consideration for indwelling and implanted sensors packaged generally from ploymers is that
the internal aqueous body environment containing chlorides and enzymes must not affect the
sensor packages and the sterility of the sensors must persist.
a. Sensors
Conventional sensors and microsensors are being adopted more and more now in
biomedical activities. Many of these are based on (i) Radiation—electromagnetic and acoustic,
(ii) Force and Pressure, (iii) Temperature, (iv) Electromagnetic variables, (v) Chemical and
Electrochemical principles, (vi) Variables related to blood flow, and (vii) Kinematic and
geometric etc.
Radiation
In the electromagnetic range of radiation, infrared radiation detection of human body
by scanning is now quite common diagnostic technique for low—deep circulation
abnormalities. These la/ abnormalities produce a thermal image different from the normal
case. This technique, known as lennography, now uses sensors such as photodetectors, LDR's
and bolometers. Photodiode detectors are also used in phototherapy which uses visible light to
convert bilirubin in a new-born baby into naturally excreting materials. In the shorter wave
techniques, X-ray methods are very common and useful. In recent years, conventional
ionization detectors or Geiger—Muller counters are replaced by radiation-to-electrical signal
sensors, specifically Si-based semiconductor sensors, as these can easily be adopted in
tomography, digital subtraction radiography, low intensity fluoroscopy, and so on.
Scintillation detectors of special designs are also used for this purpose. Ultrasonic
sensors, that is, piezoelectric sensors are now quite effectively used in measurement of blood
pressure, heartbeat of foetus, and grown up people. The conventional carbon microphone in
stethoscope is now being replaced with new sensors that convert sound into electrical output.
These are used for sensing breathing sound, gastrointestinal sound, and so forth.
Biomechanics
It is an area where sensors for measurement of force and pressure are required. Force
sensors are basically load cells which are properly adopted to derive data from models that
may provide informations regarding material properties of bone, skin, muscle, tendon, and the
like. In direct measurement of human specimens, these sensors provide data for various
activities like movements in sports, walking, and others. Tactile sensors consisting of arrays of
force sensors are mostly used in robotics. Thin film multi-element force sensors are also being
used for measuring force in patients having difficulty in gripping.
Temperature
Temperature in medical diagnosis is now measured by electronic thermometers using
infrared sensors or non-invasive skin-bound sensors which utilize resistive elements such as
bolometers or pyroelectric devices.
Electromagnetic variables
In the recent past, electromagnetic mapping of the human body has been done quite
extensively from which a lot of information can be extracted. In such mapping, resistance
metal strips are used as electrodes. Basically, chemical gradients and membrane potentials are
converted into electrical voltage signals by these electrodes. The strip is coupled to the body by
an electrolyte layer. Silver—silver chloride form a good combination although toxic effect is
not precluded under certain body conditions. Studies of heart, muscle, brain, stomach, and
position of eye are made possible by measuring bioelectric potentials using electrode systems
which can be implanted in the tissue or tied to it surgically. These results are obtained as
electrocardiograms, electromyograms, electioence-phalograms, nerve action potential
diagrams, electrogashograms, electro optograms and so on. Measurement of resistance of
tissues by measuring the current with a constant voltage applied between a pair of electrodes
has also been used as a diagnostic tool particularly for diagnosis of blood volume in tissue
(plethysmographic) or other fluid volume for monitoring breathing and more.
Chemical and electrochemical sensors

In medical/biomedical applications, these sensors are easily adopted. For example,
enzyme electrodes are used for biochemical substrates, immunosensors for immunological
reactions, polarography for body electrolytes and blood gases, and so on. Electrochemical
sensors are more common.
One such sensor is the Clark cell which consists of two electrodes for amperometric
measurement where an oxygen permeable membrane is used. It is used for the measurement
of partial pressure of oxygen in body fluids as well as tissues. Oxygen diffusing through the
membrane is reduced following the reaction
Enzyme electrodes have received more attention in recent years. The principle is to
immobilize the enzyme in the sensor. Enzyme is a complex of proteins, it must be immobilized
retaining its biochemical activity and kept in that condition for quite some time. Basically, a
particular substrate (glucose, for example) enters a biochemical reaction either to consume or
generate some substance which is detected by a chemical sensor (for example, membrane
sensor).
Variables related to blood flow

Blood pressure and blood flow are two very important biological parameters which
need be accurately determined for different parts of the body (for flow specifically). Blood
pressure is determined by the conventional technique but in recent years, non-invasive
electronic techniques are being used. Microsensor technology has been used to develop
miniature pressure sensors which in probe form are packaged and introduced into the
circulatory system. Comparatively, blood flow measurement is more of an invasive method
and microsensors have been developed which can be used for the purpose. Electromagnetic
flow meters and ultrasonic Doppler methods are also being used for the purpose. In the
ultrasonic Doppler method, the ultrasound is reflected off the cellular component of the blood;
by measuring the Doppler shift in frequency of this reflected sound, both velocity and
direction can be ascertained.
Kinematic and geometric

Sensors of linear and angular displacement, area, and volume related to biological
systems have also been developed. Sensors are based on special type of strain gauges and are
used for various detecting parameters such as position of joints, extra growth, bladder area,
tumour size, and so forth. Kinematic sensors are used to study movements of limbs and gait.
Basically, accelerometers of very small sizes are attached to the limbs and the signals obtained
therefrom are integrated for velocity and displacement and the computer is used for recreating
gait for the patient.
5.7.6 Sensors for environmental monitoring

Entire living world is now at risk on counts of health and normal survivality due to
hazards arising out of biological, chemical, and radiation effects on the environment which not
only work locally but are also likely to affect around the globe through transportation.
These hazards are thus, critical/ serious environmental problems and to assess the
extent to which they can affect human and other living entities, measurement of certain
selective parameters are needed. Environmental monitoring is not possible to be done in a
simple way by measuring temperature of a hot body—in fact, a few steps are involved in the
process of monitoring. As environment is affected by pollution, the pollutants are to be
identified. The quantity/concentration of pollutants in specific collected 'sample' need be
determined. As said, hazard occurring at a place is not endemic to that place alone and it is
spread. The three main ways that cause this spread arc (i) atmosphere, (ii) surface water, and
(iii) ground water. The manner in which these hazards affect human/living being can be given
by simple chart as shown in Figure.
Fig. 5.50 The eco-hazard.
Monitoring the environment pollution again involves three steps, namely the (i)
Collection of sample representative enough of the environmental pollution content, (ii) Pre-
treatment of the sample using extraction, separation and so on, and (iii) Analysis for
identification and quantification of analytic pollutant in sample and expressing it in proper
level of concentration. Sampling (collection and preparation of a sample) is the major player in
the three-stage process. Depending on the time and situation, the sampling techniques vary.
Similarly, analysis techniques differ depending on the type of sample. The
sensors/instrumentation in the analyzers are nothing new in general but their matching with
the pollutant and source characteristics are important.
a. Pollution Hazards
Biological effects of the hazards of pollution on humans are manifested in the
excreted wastes in general that may be considered for analysis. Of these, common ones are
urine, stool, exhaled breath, sputum and so on. However, blood is an important medium and
often nails, hair and fat too get affected. If people are exposed to chemical hazards, by
analyzing the above samples, toxic levels can be found and compared to standard maximum
that can be allowed.
For example, maximum level for lead in blood is 0.5 mg/I and maximum
concentration of creatinine in urine is 0.15 mg/g. The analysis is still done by standard
chemical techniques or in some cases, where samples are properly available, by spectroscopic
techniques. Selective sensors are not yet available widely but microsensors and chemical
sensors using silicon and polymers are recently being experimented with. As has been
mentioned already, hazards evolve from various sources like (i) radiation—both ionizing and
nonionizing, (ii) biological, (iii) chemical. Their monitoring is the first requirement and then
control becomes mandatory. Ionizing radiation includes α particles, β particles, γ rays,
neutrons, gamma-rays which are capable of biological mutation. Nonionizing radiation
includes IR, UV, radiowave and microwave, and extremely low frequency (ELF) (within 300
Hz) radiation. Exposure of these radiations over long periods is also hazardous while IR
radiation is known to cause injury to ophthalmic organs such as cornea, retina (λ < 320 nm),
and to a certain extent skin (760 nm < λ < 1400 nm). UV radiation is known to damage skin
(cancerous), sunburn (erythema) and ocular organs. Radio and microwave exposure may lead
to cardiovascular nervous, and haematopoitic functions. ELF causes disharmony in
reproductive system including cancer. Biological hazards cause physiological and
psychological diseases which are well-known by ROW. Chemical agents affect water, air, and
soil leading to long-term biological afflictions.
b. Sensing Environmental Pollution

For ionizing radiation, different sensors. are used depending on the characteristics of
ionization. For low penetration, a-particles (He ++) ionization and proportional counters are
recommended, scintillation detectors using ZnS as scintillators are also used. Semiconductor
detectors have also begun to be used lately.
For β particles (e+, e-), which are slightly more penetrating than a-particles, Geiger
counters such as proportional counters. Scintillation counters with solid and liquid scintillators
are used besides semiconductor detectors. γ rays and X-rays are even more penetrating
radiations. These are sensed by NaI and/or scintillators. On the higher wavelength
sides, sometimes Geiger—Muller and proportional counters are used. Thermoluminescent and
semiconductor detectors are also being used in increased numbers. Li-loaded
Thermoluminescent detectors and p-n junction diodes are used for neutron radiation
detectors. Gas-filled detectors using 3He and BF3 are also used for detecting X- and γ rays.
For sensing non-ionizing radiations thermopiles, bolometers, and diodes are used.
Thermopiles are used for microwave, the bolometers are used for radio-frequency, and the
diodes are used for ELF, whereas for detecting IR radiation bolometers, photoconductors,
Schottky barrier diodes and pyroelectric (BaTiO3) detectors are extensively used. For visible
and ultraviolet radiations, photovoltaic cells, photodiodes, thermocouples/thermopiles and
Schottky diodes are used.
In semiconductor detectors, it is the charge transport property of the materials such

as Ge, Ge(Li), Si, and Si(Li) that is being utilized. These sensors can be packaged and made
handy for site measurement as portable units. Over the recent years, new scintillation
materials have also been used for the purpose. As such, fluorescent emissions from solid
organic and inorganic phosphors are also used. Photodiode scintillators and planar silicon
detectors have also appeared in the arena. Cryogenic bolometers working in pulse mode are
known to provide high resolution. While biological polluting agents are very difficult to be
sensed and quantify because of the 'omnipresence' of microorganisms in the environment,
special 'culture' and microscopic studies have now become common. Counting 'colonies' after
culture in a prescribed medium is a traditional practice though time consuming. In recent
times, immunoassays have been used with success for microbiological and clinical analysis.
Immunoassays can be performed by microsensor implementation.
c. Ecological Studies of Air

Air pollutants form a major group of identifiable and quantifiable 'parameters'. These
include CO, CO2, Cl, SO2, NO, NO2, NO (in general), H2S, hydrides, NI-I3, mercury vapour, O3
and many more. Besides, suspended particles of duct origin or lir-effluents such as carbon
particles also harm ecology. Sensing techniques vary with pollutants, their amount and state of
occurrence. In many cases, reaction with reagents suggests analysis processes which are purely
chemical techniques. Some of these techniques have been adopted in instrumental form. For
example, many electroactive pollutants such as NOR, I-12S, FICOH, SO2, CO and so on are
detected by amperometric or potentiometric methods when appropriate sensing cells arc
developed with electrode catalysts and membrane permeability. This technique can measure
the presence of reactants upto concentration of a hundred ppb.
Flame ionization and photoionization are commonly used for detection of organic
samples. Burning the sample in flame or photoionization with UV rays produces ions which
are stripped in the form of current. In gas, chromatographic system flame ionization is quite
extensively used for better selectivity. Photoionization produces high sensitivity.
Metal oxide semiconductors like SnOx or metals in filament form such as platinum
produce current when CO, H2S, or hydrocarbons are oxidized on them. Selectivity, however, is
not good with this method, although sensitivity is quite high (upto 100 ppm).
Spectrophotometry is extensively used in measurement and detection of pollutants in

air and water. Absorption spectrometry is more of use than emission type. Absorption is
proportional to gas concentration and the wavelength ranges of the spectral medium change
with materials. Thus, for NO, visible range is quite common. For SO2, O3, and Hg vapour, UV
radiation is recommended while IR radiation is used for organic and dipolar inorganic
samples.
Aerosol photometry is another sensing mechanism used for particulate pollutants.

Forward scattering that causes attenuation of incident radiation is used for the purpose.
Particle size is important for detection, as also is the presence of particles in tg/m3 which
should not exceed 100-150 µg/m3.
Elemental analysis including-emission spectroscopy for metals appears to be a better

choice. Of the various types of such techniques, inductively coupled plasma atomic emission
type spectroscopy is more in use though graphite furnace atomic absorption spectroscopy is
also sometimes used for monitoring ecological pollutants.
TWO MARK Q&A

1. What is meant by data logging
Data logging is the process of collecting and storing data over a period of time in order to
analyze specific trends or record the data-based events/actions of a system, network or IT
environment. It enables the tracking of all interactions through which data, files or
applications are stored, accessed or modified on a storage device or application.
2. Why filtering is required in signal conditioning?

Any system signal might contain unwanted frequency components and require
filtering. Filtering removes undesirable noise that could lead to errors in erroneous
measurements. Slowly varying signals such as temperature may require a low-pass filter to
remove high frequency noise that can reduce the accuracy of your measurement.
3. Under environmental monitoring which are the important parameters accounted?

The parameters are
a. Chemical (Acid Rain, Halogenated Hydrocarbon, Greenhouse Gas)
b. Biological(Water Pollutant, Soil Pollutant, Air Pollutant)
c. Radiological(Radionuclide, Radiation Dose , Ionization Radiation)
d. Microbiological(Sterilization, Pathogen, Sewage Treatment)
4. Name the different types of filters used in measurement system.

The four main types of filters are
a. Low-pass filter
b. High-pass filter
c. Band-pass filter
d. Notch/band-reject filter
5. What is need of sample of sample and hold circuits?

A sample and hold circuit stores the signal level (usually voltage) that is present at the
input to the sampler. This sampled voltage stays constant within the sample and hold
circuit until such time that a new sample is desired. When used with an analog to digital
converter (ADC) the input signal is sampled periodically.
6. Signal conditioning is important in a sensor. Justify the statement.

A signal conditioner is a device that converts one type of electronic signal into another type
of signal. Its primary use is to convert a signal that may be difficult to read by conventional
instrumentation into a more easily read format. In performing this conversion a number of
functions may take place.
7. Why active filters are mostly preferred in measurement system?

Active filters are a group of electronic filters that utilizes active components like an
amplifier for its functioning. Amplifiers are used in filters for designing to enhance the
predictability and performance. This is all finished while keeping away from the need of
the inductors.
8. What is meant by signal conditioning? State its functions.

A signal conditioner is a device that converts one type of electronic signal into another type
of signal. Its primary use is to convert a signal that may be difficult to read by conventional
instrumentation into a more easily read format. In performing this conversion a number of
functions may take place.
9. Enumerate the broad classification of environmental monitoring.

They are classified on the bases of:
a. Chemical
b. Radiological
c. Microbiological
d. Biological
10. Define aperture error in sample and hold circuit.

Aperture error is the difference between the actual value of the input signal, and the flat-
topped sample value. The magnitude of this difference is related to the input frequency
and sampling width.
11. Draw a block diagram of DC signal conditioning system.
12. Explain the function of buffer amplifier.

A buffer is a unity gain amplifier packaged in an integrated circuit. Its function is to
provide sufficient drive capability to pass signals or data bits along to a succeeding stage.
Voltage buffers increase available current for low impedance inputs while retaining the
voltage level
13. How does an OPAMP act as inverter?

An op-amp has two differential input pins and an output pin along with power pins. Those
two differential input pins are inverting pin or Negative and Non-inverting pin or
Positive. An op-amp amplifies the difference in voltage between this two input pins and
provides the amplified output across its Vout or output pin. Depending on the input type,
op-amp can be classified as Inverting or Non-inverting.
14. Describe the terms: 1. Common mode gain 2.Differential mode gain
Common mode gain:
When two terminals of a differential amplifier are connected to the same input signal,
the gain of a differential amplifier is called common-mode gain.
Differential mode gain:
Gain of an amplifier is defined as VOUT/VIN. For the special case of a differential amplifier,
the input VIN is the difference between its two input terminals, which is equal to (V 1-V2) as
shown in the following diagram.
15. Write the expression for the transfer function of a band pass RC filter.
16. Write the types of passive RC filters.

a. Low pass filter
b. High pass filter
c. Band pass filter
d. Band stop filter
17. List out the various sensors involved in automobile engines.

a. Mass flow sensor
b. Oxygen sensor
c. Map sensor
18. Define CMRR.

The common mode rejection ratio (CMRR) of a differential amplifier (or other device) is a
metric used to quantify the ability of the device to reject common-mode signals, i.e. those
that appear simultaneously and in-phase on both inputs.
19. What is amplitude modulation? Why is it used in the field of instrumentation.

Amplitude modulation (AM) is a modulation technique used in electronic communication,
most commonly for transmitting information via a radio carrier wave. In amplitude
modulation, the amplitude (signal strength) of the carrier wave is varied in proportion to
that of the message signal being transmitted used in telephone lines and navy
communication.
20. Define slew rate.

The maximum rate at which an amplifier can respond to an abrupt change of input
level.

1. Explain the following with necessary diagrams.
a) Amplification circuit
b) Filtering circuit
c) Sample and Hold circuit
2. What is the need of sample and hold circuit? Define the term slew rate and aperture
error in sample and hold circuit. Also list out the applications of typical sample and
hold circuits.
3. Give a case study on the application of smart sensors in the field of consumer
electronics/home appliances.
4. Give a case study on the application of various sensors in the field of manufacturing
5. Explain the role of various sensors involved in aerospace systems.

6. Discuss in detail the significance of data acquisition and explain the principle of
operation of single channel and multichannel data acquisition strategies.
7. Explain the applications of nano sensors in medical diagnostics.
8. i. Explain about the functioning of multichannel data acquisition system with
its block diagram.
ii. Discuss about the necessity and applications of data loggers.

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MT8591 – Sensors and Instrumentation 2.

A proximity sensor is a non-contact sensor that detects the presence of an object

2.1.1 Transduction principles

Where A is the area of the plates,

2.2 Displacement & Position sensors

Fig. 2.1 Potentiometer

In practice, resolution is limited by mechanical difficulties in constructing the spring

The life expectancy of potentiometers is normally quoted as a number of reversals,

1. Using a very high impedance measuring instrument and

Unfortunately, the latter is incompatible with achieving high measurement sensitivity

Loading effect on potentiometer

Fig. 2.2 loading effect of potentiometer

The output of a potentiometer is normally connected to an amplifier or a recorder or a

The voltage across C and B

This is the condition, when there is

Fig. 2.3 Linearity Graph of

If there were no loading then the output would have been

Maximum will occur at

Fig. 2.4 Resolver

Varying amplitude output resolver

Varying phase output resolver

 Capacitive encoder technology has a sinusoidal rotor. When it rotates, transmitter

Incremental Rotary Encoder

Fig. 2.4 Incremental rotary encoder

Fig. 2.5 Linear encoder

Absolute Rotary encoders

Fig. 2.6 Absolute rotary encoder

2.7 Magnetic Encoder

An inductive encoder detects a linear displacement or rotation relative to a

Fig. 2.8 Inductive Encoder

Fig. 2.9 Capacitive Encoder

Capacitive encoders work by transmitting a high-frequency signal through a rotor

2.2.4 Linear Variable Differential Transformer (LVDT)

Fig. 2.10 Linear Variable Differential Transformer (LVDT)

Fig. 2.11 Connection of LVDT circuit

Fig. 2.12 LVDT Core positions

To summarize, The LVDT closely models an ideal zeroth-order displacement sensor

Fig. 2.13 Output characteristics of LVDT

2.2.5 Rotary Variable Differential Transformer (RVDT)

Fig. 2.14 Rotary Variable Differential transformer (RVDT)

2.2.6 Synchros and Microsyn

Fig. 2.15 Schematic representation of synchro windings

Fig. 2.16 Synchros stator voltage waveform

where θ is the relative angle between the two rotor windings.

Fig. 2.17 Synchro transmitter transformer Circuit

2.3 Proximity sensors

Fig. 2.18 Proximity Sensor

2.3.1 Inductive proximity sensors

Fig. 2.19 Inductive Proximity Switch Circuit Diagram

2.3.2 Capacitive proximity sensors

Fig. 2.20 Capacitive type Proximity sensor

1. The sensing surface size of the sensor.

1. They can detect nonmetallic targets.

2.3.3 Optical proximity sensors

Fig. 2.21 Optical type proximity sensors

2.3.4 Magnetic proximity sensors

Magnetic sensors use GMR (Giant Magneto-Resistive Effect) technology. The

1. Detection through plastic, wood, and any non-magnetisable metals

Inductive Magnetic Capacitive