Chapter-5

Sensors
In virtually every engineering application there
is the need to measure some physical
quantities, such as displacements, speeds,
forces, pressures, temperatures, stresses,
flows, and so on.
• These measurements are performed
using physical devices called sensors,
which are capable of converting a
physical quantity to a more readily
manipulated electrical quantity.
• Often the active element of a sensor is
referred to as a transducer.
• Most sensors, therefore, convert
the change of a physical quantity
(e.g. pressure, temperature) to a
corresponding and usually
proportional change in an electrical
quantity (e.g. voltage, current,
charge).
• Often the direct output from a sensor
needs additional manipulation before
the electrical output is available to the
user.

• Sensors: Element which produces a signal relating

to the quantity being measured.

E.g. Electrical resistance thermometer

– Quantity being measured – temperature.
– Sensor transforms it to change of resistance
• Sensor is used for an input device
that provides a usable output in
response to a specified physical
input.

• E.g. a thermocouple is a sensor that converts a

temperature difference into an electrical output.

• Transducer: element that when subjected to some physical

change experience a related change.

• It is a device which transforms the input signal of one

energy form into an output of another energy form.
• Example is the mercury-in glass thermometer
in which heat energy is converted into
mechanical energy, forcing the mercury to
move along the glass as it expands or
contracts.
– Thus sensors are transducers. but also other
devices can be transducers, such as a motor that
converts an electrical input into rotation
• The term transducer is generally used
to refer to a device that converts a
signal from one form to a different
physical form.

• The most commonly used sensors

and transducers, which are
specially suitable for automation
and control purposes, make
extensive use of electrical signal.
Transducers may be parts of complex
sensors .

For example, a chemical sensor may have a

part which converts the energy of a chemical
reaction into heat and another part, a
thermopile, which converts heat into an
electrical signal.

The combination of the two makes a chemical

sensor a device which produces an electrical
signal in response to a chemical reaction
 Difference between sensors and
transducers

• Sensor is a device, when exposed to a

physical phenomenon (temperature,
displacement, force, etc.), it produces
a proportional output signal
(electrical).

• However, ideally, a sensor is a device

that responds to a change in the
physical phenomenon.
• On the other hand, a transducer is
a device that converts one form of
energy into another form of
energy.
• Sensors are transducers when they sense one form of
energy input and output in a different form of energy.

• For example, a thermocouple responds to a temperature

change (thermal energy) and outputs a proportional change in
electromotive force (electrical energy). Therefore, a
thermocouple can be called as a sensor and or a transducer.
 Transducer performance
terminology
• Range: is the difference between the minimum (or most
negative) and maximum inputs that will give a valid output.
Range is typically specified by the manufacturer of the sensor.
• Span: maximum value-min. value
– For a load cell measurement of forces might
have a range of 0 to 50 KN and a span of 50KN

• Sensitivity: is defined as the change in output per

change in input.
 Classification of sensors

Sensors can be classified as

 Binary (On-Off) sensors vs.
 Analog (Proportional) Sensors
Binary (or digital) sensors produce on-off signals.
Example, a limit switch contact might close whenever the switch is mechanically
actuated.
 The contact is either open or closed: there is no intermediate
position.
Analog sensors, by comparison, produce so-called proportional or analog
signals.
Example, a linear potentiometer might be used to indicate position motion, with the
voltage measured at the potentiometer slide wire proportional to the motion .
• Sensors can be also classified as
Self-generators
Modulators
• Self-generator Sensors are sensors which do not
require the supply of energy from any source other
than the system under measurement.
• Modulator sensors are which require the supply of
energy from other sources
• Self-generators, which are also sometimes
misleadingly called passive, produce an output by
directly extracting energy from the system under
measurement.
Examples of these are
 the mercury-in-glass thermometer, whose mercury column expands
when exposed to increasing temperature
 the photo-voltaic cell, which gives an electrical output when exposed to
light
 the thermocouple, which generates a voltage proportional to input
temperature

• The other kind of sensor structure, which needs an

excitation or auxiliary supply of energy for its operation,
is called a modulating type. During their operation the
energy flow is modulated by the input measurand. They
are also misleadingly classified as active.
• Examples of the modulating group of sensors are all
resistance based sensors, where electrical energy must
be applied to allow the resistance to be measured
• Depending on the selected reference, sensors can be classified
into
absolute and
 relative.
• An absolute sensor detects a stimulus in reference to an
absolute physical scale that is independent on the
measurement conditions.
 Example of an absolute sensor is a thermistor : a temperature-
sensitive resistor. Its electrical resistance directly relates to the
absolute temperature scale of Kelvin
• A relative sensor produces a signal that relates to some special
case.
 Example: thermocouple is a relative sensor. It produces an electric
voltage that is function of a temperature gradient across the
thermocouple wires. a temperature gradient across the
thermocouple wires.
 Displacement, position & proximity Sensors

• Displacement: concerned with the measurement of the

amount by which the object has moved.
• Position: the determination of the position of some
object w.r.t. some reference position.
• Proximity: used to determine when an object has
moved to within some particular critical distance of the
sensor.
 Potentiometer
• A potentiometer is a variable electrical resistance. A
length of resistance material has a voltage applied over
its ends. A slider moves along it (either linear or rotary)
and picks off the voltage at its position or angle. The
tracks may be made from carbon , resistance wire or
piezo resistive material. The latter is the best because it
gives a good analogue output. The wire wound type
produces small step changes in the output depending on
how fine the wire is and how closely it is coiled on the
track.
 Sensors and its Application
Position sensing-
Potentiometers
• resistive potentiometers are one of
the most widely used forms of
position sensor
• can be angular or linear
• consists of a length of resistive
material with a sliding contact onto
(Europe)
the resistive track (US)
• when used as a position transducer
a potential is placed across the two
end terminals, the voltage on the
sliding contact is then proportional
to its position.
 ... continued

• The voltage across RL can be calculated by:

• If RL is large compared to the other resistances (like the input to an operational

amplifier), the output voltage can be approximated by the simpler equation:
 Example

• Assume, in the above circuit, that:

• Since the load resistance is large compared to the

other resistances, the output voltage V L will be
approximately:

• Due to the load resistance, however, it will actually

be slightly lower: ≈ 6.623 V.
 LVDT
The linear variable differential transformer (LVDT) is a
mechanical displacement transducer. It gives an a.c.
voltage output proportional to the distance of the
transformer core to the windings.
 LVDT
• LVDT (Linear Variable Differential
Transformer):
– Inductance - based electromechanical sensor
– “Infinite” resolution
• limited by external electronics
– Limited frequency bandwidth
• 250 Hz typical for DC-LVDT,
• 500 Hz for AC-LVDT
– No contact between the moving core and coil
structure
• no friction, no wear, very long operating
lifetime
– Models with strokes from mm’s to 1 m
available
 Switches
• simplest form of digital displacement sensor
– many forms: lever or push-rod operated micro-
switches; float switches; pressure switches; etc.

A limit switch A float switch

 Opto-switches

• consist of a light source and a light sensor

within a single unit
– 2 common forms are the reflective and slotted types

A reflective opto-switch A slotted opto-switch

Inductive proximity sensors

• Coil inductance is
greatly affected by the
presence of
ferromagnetic materials
• The proximity of a Inductive proximity sensors
ferromagnetic plate is
determined by
measuring the
inductance of a coil.
 Photoelectric Proximity
Sensor
• There are three basic types of photoelectric
sensors. Transmitted beam, or through-beam,
requires a sender and a receiver. Retro-
reflective senses light returning from a
reflector. Both types switch an output when
the beam is broken. Diffuse sensors sense
light returning from the object to be detected
and switch the output when it senses.
Examples

Automatic Door Opener

 Examples

Production counting Case Sorting -By Size

 DIGITAL OPTICAL ENCODER
• A digital optical encoder is a device that
converts motion in to a sequence of Digital
pulses. By counting a single bit or by decoding
a set of bits, the pulses can be converted to
relative or absolute position measurements.
Absolute position Encoders
– a pattern of light and dark strips is printed on to a
strip and is detected by a sensor that moves along
it
• the pattern takes the form of a series of lines as shown
below
• it is arranged so that the combination is unique at each
point
• sensor is an array of photodiodes
 Incremental position
Encoder
– uses a single line that alternates black/white
• two slightly offset sensors produce outputs as shown
below
• detects motion in either direction, pulses are counted
to determine absolute position (which must be initially
reset)
 Velocity and Motion
Sensors
• Motion sensors measure quantities such as
velocity and acceleration
• Can be obtained by differentiating displacement
– differentiation tends to amplify high-frequency noise
• Alternatively can be measured directly
– some sensors give velocity directly
• e.g. measuring frequency of pulses gives speed rather than
position
– some sensors give acceleration directly
• e.g. accelerometers usually measure the force on a mass
 Velocity sensing
• Tachogenerator :- measure angular velocity
– The amplitude or the frequency of the alternating
emf is a measure of angular velocity of rotor.

A.C. generator form of tachogenerator

Variable reluctance tachogenerator

33
 … continued
• Scanning Laser Vibrometry
– No physical contact with the test object; facilitate remote, mass-
loading-free vibration measurements on targets
– measuring velocity (translational or angular)
– automated scanning measurements with fast scanning speed
– However, very expensive
 Acceleration Sensing
• Piezoelectric accelerometer
– Inside a piezoelectric version, the
sensing element is a crystal which
has the property of emitting a
charge when subjected to a
compressive force.
– In the accelerometer, this crystal
is bonded to a mass such that
when the accelerometer is
subjected to a 'g' force, the mass
compresses the crystal which
emits a signal. This signal value
can be related to the imposed 'g'
force.
 … continued
• Capacitive accelerometer
– Capacitive accelerometers sense a
change in electrical capacitance,
with respect to acceleration, to vary
the output of an energized circuit.
– The sensing element consists of two
parallel plate capacitors acting in a
differential mode.
– These capacitors operate in a bridge
circuit and alter the peak voltage
generated by an oscillator
when the structure undergoes
acceleration. Detection circuits
capture the peak voltage, which is
then fed to a summing amplifier
that processes the final output
signal.
 Force and pressure
sensors
• • Force and Pressure generally measured
indirectly through deflection of an alternate
surface
• Mechanism include:
– Physical motion and measurement using (eg) an
LVDT
– Strain gauges (metal that changes resistance
when stressed)
– Piezoelectric materials that generate a current
when deformed
 Force Sensor
• Spring balance
• Force is proportional to displacement.
– Hence, forces are commonly measured by the measurement of displacement.

• Strain gauge load cells

– Based on electrical resistance strain gauge to monitor strain produced in a member when
stretched, compressed or bent by application of force.

• Arrangement is called load cells.

• Strain gauge load cells based on bending of a strain gauged metal element tend
to be used for smaller forces.(From 0 to 5N to 0 to 50 kN.)

38
 Strain Gauges

Force sensors which usually consists of

fine wires which can measure very small
amounts of motion caused by the flexing
of an object, or manipulator.
Used to measure strains and stresses in
many types of components.
stretching in one direction
increases the resistance of the
device, while stretching in the
other direction has little effect
• can be bonded to a surface to measure strain
– used within load cells and pressure sensors

Direction of sensitivity

A strain gauge
 Strain Gage: Gage Factor
• Remember: for a strained thin wire
– R/R = L/L – A/A + /
• A =  (D/2)2, for circular wire D L
• Poisson’s ratio, : relates change in diameter D to change in length L
– D/D = - L/L
• Thus
– R/R = (1+2) L/L + /
dimensional effect piezoresistive effect
• Gage Factor, G, used to compare strain-gate materials

– G = R/R = (1+2) + /

L/L L/L
 Strain Gauges

• Foil strain gauge

– Least expensive
– Widely used
– Not suitable for long distance
– Electromagnetic Interference
– Sensitive to moisture & humidity

• Vibration wire strain gauge

– Determine strain from freq. of AC signal
– Bulky

• Fiber optic gauge

– Immune to EM and electrostatic noise
– Compact size
– High cost
– Fragile
 Fluid Pressure sensors

• Measured by elastic
deformation of
diaphragms, capsules,
bellows, tubes etc.
• Pressure measured
– Absolute (w.r.t. vacuum)
– Differential
– Gauge (w.r.t barometer)

43
 Application
• Automotive
• Industrial
• Air Flow Analysis
• Medical
• Robotics
• Pumping
• Stress Analysis
 Pressure Sensors
• Used to detect pressure of
fluids or gasses.
• Technologies (many)
– Strain gage
– Piezoresistive
– Micro electro mechanical
systems (MEMS)
• Each sensor has a pressure
range that it works in.
• Most have analog outputs that
need amplification
– Some have built-in amplifiers for
direct connection into
microcontroller
 Pressure Sensors
Types
• Differential Pressure
– Difference between two or more
pressures introduced as inputs to
the sensing unit
– 2 input
• Absolute/Gage Pressure
– The pressure relative to perfect
vacuum pressure or set pressure
(like pressure at sea level)
– 1 input
• 4 strain gauges
used
• 2 in radial
direction
• 2 in
circumferential
direction.

47
• Can measure 103 to 108 Pa 48
• Tube made of stainless steel or
phosphor bronze
• Can measure103 to 108 Pa 49
Piezoelectric sensors

• Piezoelectric material are

ionic crystals, when
stressed or compressed
generate electric charges
with one face of material
becoming +vely charged
and the opposite face –
vely charged, as a result
a voltage is produced.

50
 Tactile sensors

• Form of pressure sensor

• One form of tactile sensor
uses piezoelectric
polyvinylidene fluoride
(PVDF) film.
• Reverse piezoelectric effect
used here.
• Used in fingertip of robotic
hand.
• Also in touch display screen

51
Flow and level sensors
Liquid flow

52
Liquid levels

53
 Temperature sensors
• Resistive thermometers
– typical devices use platinum wire (such a device is
called a platinum resistance thermometers or PRT)
– linear but has poor sensitivity

A typical PRT element A sheathed PRT

• Thermistors
– use materials with a high thermal coefficient of
resistance
– sensitive but highly non-linear

A typical disc thermistor A threaded thermistor

• P n junctions
– a semiconductor device with the
properties of a diode (we will
consider semiconductors and
diodes later)
– inexpensive, linear and easy to use
– limited temperature range (perhaps
-50C to 150 C) due to nature of
semiconductor material

pn-junction sensor
 Temperature Sensors
• Bimetallic strips

57
• Resistance temperature
detectors(RTDs)
• Platinum most widely
used

Rt R0 (1  t )

58
 Thermistors
• Small piece of material made from mixture of
metal oxides, such as those of chromium,
cobalt, iron, manganese and nickel.
• These oxides are semiconductors.
• They give large change in resistance per
degree change in temperature.
• Draw back is non linearity.

59
60
• Thermodiodes and transistors
• When the temperature of doped semiconductor
changes, the mobility of their charge carrier
changes and this affects the rate at which the
electrons and holes diffuse across p-n junction.
• In a thermo transistor the voltage across the
junction between the base and the emitter
depend upon the temperature and can be used
as a measure of temperature.

61
Thermocouple

62
 Light Sensors
• Photovoltaic
– light falling on a pn-junction can
be used to generate electricity
from light energy
(as in a solar cell)
– small devices used as sensors are
called photodiodes
– fast acting, but the voltage
produced is not linearly related
to light intensity A typical photodiode
• Photoconductive
– such devices do not produce
electricity, but simply
change their resistance
– photodiode (as described
earlier) can be used in this
way to produce a linear
device
– phototransistors act like
photodiodes but with greater
sensitivity
– light-dependent resistors
(LDRs) are slow, but respond
like the human eye
A light-dependent resistor (LDR)
 Sound Sensors
• Microphones
– a number of forms are available
• e.g. carbon (resistive), capacitive, piezoelectric and
moving-coil microphones
• moving-coil devices use a magnet and a coil attached to a
diaphragm – we will discuss electromagnetism later
 Sensor Interfacing
• Resistive devices
– can be very simple
• e.g. in a potentiometer, with a fixed voltage across the outer
terminals, the voltage on the third is directly related to position

 where the resistance of the device

changes with the quantity being
measured, this change can be
converted into a voltage signal
using a potential divider – as shown
 the output of this arrangement is not
linearly related to the change in
resistance
• Switches
– switch interfacing is also simple
• can use a single resistor as below to produce a voltage
output
• all mechanical switches suffer from switch bounce
• Capacitive and inductive sensors
– sensors that change their capacitance or
inductance in response to external influences
normally require the use of alternating current
(AC) circuitry
– such circuits need not be complicated
– we will consider AC circuits in later lectures
 Key Points
• A wide range of sensors is available
• Some sensors produce an output voltage related to the
measured quantity and therefore supply power
• Other devices simply change their physical properties
• Some sensors produce an output that is linearly related to the
quantity being measured, others do not
• Interfacing may be required to produce signals in the correct
form
 Sensor Selection
• There is often a wide choice of sensors to monitor a particular
stimulus.
• The choice of the ‘right’ sensor must take into account
– availability
– cost
– power consumption
– environmental conditions
– Reliability and lifetime.
• Therefore the choice is often not black and white and it is
prudent to retain a few alternatives.
 Sensor Characteristics: The Transfer
function
• The transfer function converts from the stimulus, s, to the
electrical output signal, S, ie. S = fn(s)
• Many functions are possible
– Linear: S = a + bs (b = slope or sensitivity)
– Logarithmic: S = a + bln(s)
– Power S = a + bsk
• For nonlinear transfer functions b = dS/ds
• Sensitivity can also be defined as the minimum input (or
change) in the physical stimulus parameter which will create a
detectable output change
 Example Primary Transducers
• Light Sensor
– photoconductor
• light  R

– photodiode
• light  I

– membrane pressure sensor

• resistive (pressure   R)
• capacitive (pressure  C)