UNIT: 2

REVIEW OF TRANSDUCERS AND SENSORS

Topics Going to be covered
• Definition and classification of transducer.
• Definition and classification of sensors.
• Principles of working and application of light sensor.
• Principles of working and application of proximity sensor.
• Principles of working and application of Hall Effect sensor.
• June 2010
• Define the terms Hysterisis error, Repeatabilty (04)
• Explain with sketch, an eddy current proximity sensor(06)
• Explain the working principle of Hall effect sensor. How it can be used to
determine the level of fuel in an automobile fuel tank.(10)
• June 2009
• Explain how sensing is achieved by an incremental optical encoder(08)
• Explain following performance terminologies of transducers a)Accuracy
b)Repeatabilty c)Drift d) Speed of response.(06)
• Explain the principle of operation of Hall effect sensor(06)
• June July 2014
• Illustrate the following proximity sensors:
• a. Capacitive type. (10 Marks)
• b. Pneumatic type. (10 Marks)
• Dec/jan 2010

• December 2011
Definition and classification of transducer:
• TRANSDUCER:
• Transducer is the heart of any measurement system.

Transducer is device that converts energy from one

form to another.
• The meaning of transducer is induces change.
• Transducer----- Trans (change) + Induce (Provide).
Example of Transducer: Thermocouple
Classification of transducer:
• The transducers are broadly classified on basis of following factors:
1. Based on whether the device sense and converts
2. Based on nature and type of output signal.
3. Based on whether they are self generated or externally powered
4. Based on type of sensing element used.
5. Based on type and nature of measurand to be measured.
6. Based on the purpose in the measurement system.
7. Based on methods of conversion of energy.
1. Based on whether the device sense and
converts:
a. Primary transducer

b. Secondary transducer
Primary transducer:

They are also called as “detectors”.

It sense a physical phenomenon and converts into an
analogous output.

• Example: Thermocouple.
 Secondary transducer:
secondary transducers are those which convert the analogous
output of the primary transducer (detector), which has sensed
the physical phenomenon into an analogous output.
• Example:
1. Pressure measurement with Bourdon tube and LVDT.
2. Load cell with strain gauges.
2. Based on nature and type of output signal.
• a. Analog transducer
• b. Digital transducer

• Analog transducer: are those which convert physical phenomenon into

an analogous output which is a continuous function of time.
Example: Strain gauges, Thermistors, LVDT, Etc.
• Digital transducer: are those which convert physical phenomenon into
an electrical output which is in the form of pulses.
Example: Angular digital encoders and digital level
transducers.
3. Based on whether they are self generated or
externally powered
• a. Active transducer
• b. Passive transducer
• Active transducer: are those which develop their own power to
producing output signal. They are also known as self generating
transducers.
Examples: thermocouples, photo-voltaic cell etc
• Passive transducer: are those which require external power producing
output signal. They are also known as externally powered transducers.
However they absorb some energy from the measurand.
Example: photoemissive cell, Thermistors, resistance
thermometers, etc.
 4. Based on type of sensing element used
• a. Elastic element
• b. Mass sensing element
• c. Thermal sensing elements
• d. Hydropneumatic elements
• Elastic element: most pressure measuring devices use a bourdon tube, a
bellow or diaphragm. The actions of these elements are based on elastic
deformation brought about by the force resulting from pressure
summation.
Example: Springs, Proving ring, bourdon tube etc.
• Mass sensing element: this is based on the inertia of concentrated
mass.
Example: Vibration pickups, accelerometers.
• Thermal sensing elements: these elements sense the heat of a
system by indicating some change in the property of the material
used, which varies with the heat.
Examples: bimetallic thermometer, thermocouples,
Thermistors, optical pyrometer.
• Hydropneumatic elements: the two examples of these are float and
hydrometer, used in static fluid which converts liquid level into
displacement.
5. Based on type and nature of measurand to
be measured.
a. Mechanical transducers
b. Electrical transducers
c. Electromechanical transducers
d. Electromagnetic transducers.
e. Electrochemical transducers.
• Mechanical transducers: are used for measuring quantities such as
position, velocity, force, torque, displacement, time, pressure,
vibration, strain mass etc.
• Electrical transducers: are used for measuring voltage, current and
electrical conductivity.
• Electro mechanical transducers: galvanometers, MEMS,
Potentiometers, load cels etc.
• Electro-magnetic transducers: hall effect sensors, antenna, CRT, LDR
etc.,
• Electro-chemical transducers: pH probes, electro-galvanic fuel cell.
6. Based on the purpose in the measurement
system:
• a. Input transducers
• b. Output transducers
• Input transducers: these transducers convert a non electrical quantity
into an electrical signal .
Example: Strain gauges, photovoltaic cell
• Output transducers: these transducers convert electrical signal back
into non electrical signal according to whether they make physical
contact or not.
Based on method of conversion of energy:
• The energy or signals produced due to physical phenomenon are
converted into another form using mechanical linkages as in the case
of simple dial gauge.
• Example: the property of expansion of liquid due to heat is used in
mercury thermometers to measure temperature.
Advantages of Mechanical transducer:
1. They posses high accuracy.
2. Rugged (rough handling)
3. Relatively low cost.
4. Operate without any external source (Active transducers)
Disadvantages of Mechanical transducer:
• 1. Have poor frequency response.
• 2. Require large force to overcome friction.
• 3. Incompatibility when remote control or indication is required.
Advantages of Electrical transducer:
1. The output can be amplified or attenuated to any desired level.
2. The output can be indicated and recorded remotely.
3. Compact instrumentation
4. Can be controlled with very small power.
5. Friction and mass inertia effects are minimum.
6. Possibility of non-contact measurement.
Disadvantages of Electrical transducer:
1. Expensive.
2. Require external power.
3. Response to external disturbances.
Definitions Sensors:
• Sensor is a device which can respond directly to different physical
attributes such as heat, light, magnets, force related quantities,
radiation etc, or to their vibration.
• A sensor is infact a highly refined transducer provided with signal
conditioning circuit of modifying the signals from the transducer.
• If the sensor itself transduces the physical attributes in addition to
sensing (detecting) is called ‘detector transducer’.
Quality parameters of a sensor system

• Sensitivity :
• It is the ability of the measuring instrument to respond to changes in
measured quantity.
• It is ratio of change of output to change of input.

Magnitude of O/P quantity

• Static Sensitivity = -------------------------------------
Magnitude of I/P quantity

= Slope of O/P V/S I/P curve

• Resolution : It is defined as the smallest increment in the measured
value that can be detected.
• Repeatability : It is the ability to reproduce the output signal exactly
when the same measured quantity is applied repeatedly under the same
environmental conditions
• Accuracy : It is a measure of difference between the measured value and
actual value. Generally defined as % of actual value.
• Accuracy is the extent to which the value indicated by a measured system
might be wrong OR The accuracy of a measurement means conformity
to truth.
• It is thus the summation of all possible errors that are likely to occur.
• Accuracy is expressed as % of full range O/P or % of full - scale deflection.
• Accuracy is the closeness with which an instrument reading
approaches the true value of the quantity being measured.
• Precision : Precision is the ability of an instrument to reproduce a
certain set of readings within a given deviation.
• Range : It is defined as the limits between which inputs can vary.
Range of a transducer defines the limits between which the input can
vary. Range represents the highest possible value that can be
measured by an instrument.
• Span: is maximum value minus the minimum value of the input.
Span = Max. value of I/P - Minimum value of I/P.
• Stability (drift) - It is the ability to give same output when a constant
input is measured over a period of time. Drift is expressed as % of full
range output.
• Hysteresis: Different output for increasing and decreasing value of
input. Transducers can give different outputs from the same value of
quantity being measured according to whether that value has been
reached by a continuously increasing change / or continuously
decreasing change. This effect is called hysteresis.
Static and dynamic characteristics of a sensor
system
• Static characteristics are the values given when steady state
conditions occur. Input is not varying and output is constant. Output
changes only due to drift.
• Dynamic characteristics refer to time varying signal with
corresponding time varying output.
• Response time : This is the time which
Elapses after a step input, when the transducer
gives the output corresponding to some specified
Percentage of steady state value e.g. 95%
• Time constant : This is 63.2 % of response time.
• Rise time : Time taken for the output to rise to
some specified percentage of the steady state output. From 10% to 90%.
• Settling time : This is the time taken for the output to settle to within some
percentage e.g. 2% of steady state value.
Classification of sensors:
• The classification of sensors based on following factors:
1. Based on types of energy transferred
2. Biological sensors.
3. Geodetic sensors.
4. Light sensors.
5. Proximity sensors.
6. Hall Effect sensors.
1.Based on type of energy transferred, we
have
• Thermal: This senses the heat.
• Temperature sensors: thermometers, thermocouples, Thermistors,
bimetal thermometers etc.
• Electromagnetic:
• Electrical resistance sensors: ohm meter, multimeter.
• Electrical current sensors: galvanometer, ammeter.
• Electrical voltage sensors: voltmeter.
• Electrical power sensor: watt-hour meters.
• Metal detector, radar
• Mechanical:
• Pressure sensor: altimeter, barometer, pressure gauge.
• Mechanical sensor: position sensor, acceleration sensor.
• Humidity sensor: hygrometer.
• Chemical sensor: is a device that transforms chemical information,
ranging from the concentration to total composition analysis, into an
analytical useful signal.
• Example: oxygen sensor, pH glass electrodes etc.
• Orientation sensors: gyroscope, artificial horizon distance sensor.
Light sensor:
• Sensor which senses the presence of light are called light sensors of
photo sensors.
• These are also known as photo electric transducers because when
light falls on these sensors, there exists a change in their electrical
property. i.e., light signals induce change in electrical properties of
conductance, resistance, inductance, etc., of the material.
• The most common materials used in manufacturing of light sensors
are cadmium sulphide, cadmium sulpho solenide, and lead telluride
and semi conductors.
 Principles of photo electric transducer:
• Whenever light falls on a material the entire light energy is given to the
electrons of the material, whose kinetic energy increases. The electrical
property of a material depends on its valence electrons and the energy of
these electrons change resulting in change in the electrical property of the
material. This phenomenon is known as photo electric effect.
• There are three such effects are photo emissive, photo conductive, photo
voltaic effect.
• Bases on these effects, there are three transducers namely,
1. Photo emissive transducer or photo tube cells.
2. Photo conductive transducers.
3. Photo voltaic transducers.
Photo emission effect:
• Whenever light fall on a cathode the free valence electrons on the
cathode absorbs the light energy, resulting in increase in kinetic
energy. These electrons ejected from the cathode are attracted
towards the anode constituting anode current which is proportional
to the intensity of light.
Photo tube cell or photo emissive cell:
• This is consists of cathode and anode placed in highly evacuated glass
or quartz tube as shown in figure.
• When light strikes on cathode, the free valence electrons on the
cathode absorb the light energy, resulting in increase in kinetic
energy.
• These electrons ejected from the cathode are attracted towards the
anode constituting anode current.
• There by their electrical properties are changes.
• The electrons travel from cathode to to anode, there by providing a
small current.
• Electrons with sufficient K.E to overcome the forces of attraction
will be emitted.
• The entire light energy is converted into K.E of electrons and is
given by

W = hc / λ Where
h = Plank’s constant
c = speed of light
λ = Wavelength of incident optical radiation
• In the process of emission a part of the energy EW is lost and the
actual K.E of the electrons will be

WACTUAL = hc . eEw / λ
• By adding several successively higher voltage electrodes (or
dynodes) to the envelope, substantial current amplification is
obtained, producing a “photomultiplier tube” (PMT).
Photo multiplier tube

43
USE of Photo emissive cells:

• Can outperform semiconductor device when high gain and fast

response are important light of short wavelength are involved, such
as ultraviolet rays.
• These are used only in special applications when fast response &
high gain is required or when short wave length are involved.
• Semiconductors photo sensors perform best at near infra - red wave
lengths.
• Applications: automatic opening and closing of doors, conveyor belts,
reproduction of motion picture film.
Photoconductive cell:
• Uses a semiconductor material whose resistance changes in
accordance with the radiant energy received.
• Figure shows simplest form of such cell using selenium, they are
provided with two electrodes along with semiconductor material
as soon as light falls on it, resistance decreases and current
through the circuit becomes large.
• The shape of semiconductor material is made as to obtain a large
ratio of dark to light resistance.
• Applications: it is mainly used for detecting ships and aircrafts by
the radiation given out by their exhaust. Street light, clocks, radio
etc.
Photo-detective transducer:
• This works on the principle of change in conductivity of
semiconducting material with change in light intensity.
• The change in conductivity appears as change in resistance and
therefore these device are called photo resistive cell.
• Example: LDR (Light dependent resistor)
Photovoltaic cell:
• Generation of potential difference when the light strikes a junction
of two dissimilar metals is called photovoltaic effect. The cell
which works on this principle is called photovoltaic cell.
• It consists of metal base plate, a non metal semi conductor and
thin transparent metallic layer. When a light falls in between the
junction a potential difference is generating.
• Applications: solar panels/solar cells, television circuits, automatic
control system.
Photo diodes:
• Photo diode is a semiconductor junction diodes bias. Giving very high
resistance, so that when light falls in junction, the diode resistance
decreases and current increases.
WORKING PRINCIPLE of Photo diodes:
• The p-n junction diodes connected in reverse- bias condition. It has
high resistance. Incoming light can excite electrons being bond in the
crystal lattice and will generate free electrons - hole pairs in the
junction.
• As a result resistance (R)↓ drops & current (I)↑ increases current
being proportional to intensity of light radiated.

• Primary disadvantage is low O/P current.

Electrical Comparator (LVDT)
Linear Variable Differential Transformer
• LVDT consists of a cylindrical shell which is wound by a primary
winding and two secondary windings at the sides.
• The number of turns in both the secondary windings is equal, but
the windings are opposite to each other.

• Hence the net output voltages will be the difference in voltages

between the two secondary coils.

• The two secondary coils are represented as V1 and V2.

• A movable iron core is placed in the centre of the cylindrical shell.

Working –
Case 1
If the iron core is in the center, then the voltage induced in both
the secondary windings are equal which results in net output is
equal to zero. Vout = V1 – V2 = 0.
Case 2
When an external force is applied and if the iron core tends to move
out, then the emf voltage induced in the upper secondary coil is
greater when compared to the emf induced in the lower secondary
coil 2. V1 > V2
Case 3
When an external force is applied and if the iron core moves in, then
the emf induced in the lower secondary coil 2 is greater when
compared to the emf induced in the secondary coil 1. V1 < V2
Advantages –

• Infinite resolution can be achieved.

• High sensitivity instrument.
• Good linearity is achieved.
• No friction, inertia, mechanical parts involved.
• Very low power consumption.
 Disadvantages

• It can't be widely used in the area of dynamic measurement since the

core is of appreciable mass compared with the mass of a strain gauge.
• If 60 cps supply voltage is used then it becomes a limiting factor as for as
dynamic measurement is concerned.
Proximity sensors:
• Proximity sensors are devices, they detects the object when the
object approaches within the detection range or boundary of the
sensor.
• It is used in manufacturing process to detect metal and non metal
object.
• They are working based on different principles, namely; variable
reluctance, eddy current loss, saturated core, Hall Effect etc.
• Depending on principle the common type of non-contact proximity
sensors are:
1. Inductive type.
2. Capacitive type.
3. Magnetic type.
4. Optical proximity sensor.
5. Ultrasonic proximity sensor.
6. Fiber optic sensor.
INDUCTIVE TYPE PROXIMITY SENSOR:
• These sensors are used to non contact detection of metallic
objects.
• Principle of operation: inductive proximity sensor detects
magnetic loss due to eddy current that are generated on a
conductive surface by an external magnetic field.
Working:
• The DC supply generates AC in the internal coil, which in turn causes an
alternating magnetic field.
• If no conductive materials are nearer the face of the sensor, the only
impedance to the internal AC is due to the induction of the coil.
• If however, a conductive material enters the magnetic field, eddy current
are generated in the conductor material resulting in impedance to the AC
in the proximity sensor.
• A current sensor present in the sensor, detects when there is a drop in
the internal AC current due to increase in impedance.
• The current sensor controls a switch providing the output.
• These sensors have built in hysteresis.
• The sensor output is ON when the conductive material is inside the
range and will goes OFF of as the material just away from the sensing
region.
• The sensing distance Sn is a function of the diameter of the sensor (i.e.,
diameter of the induction coil).
CAPACITIVE TYPE PROXIMITY SENSOR:
• These sensors are used to detect both metallic and non metallic
object includes plastic, wood, water, liquid etc.
• Principle of operation:
• Capacitive sensors use the variation of capacitance between the
sensor and the object being detected. When the object at a preset
distance from the sensitive side of the sensor, an electronic circuit
inside the sensor begins to oscillate, the rise and fall of such
oscillation is identified by a threshold circuit that drives an amplifier
of an external load.
 Working principle:
• A circuit provided in the sensor uses DC power to generate AC, to measure
the current in the internal AC circuit, and switch the output circuit when
the amount of AC current changes.
• A capacitive proximity sensor detects changes in the capacitance between
the sensing object and the sensor.
• The amount of capacitance varies, depending on the size and distance of
sensing object.
• Capacitor plate holds the positive charge whereas the negative charges
are attracted into the other (object being sensed).
• Only one of the required two capacitor plates is actually built into the
capacitive sensor.
• The changes in the capacitance generated between these two [plates are
detected. The object that can be detected depends on their dielectric
constants.
Eddy current proximity sensor:
• Principle:-
• If a coil is supplied with an A.C supply, an alternating magnetic field is
produced .
• If there is a metal object in close proximity to this alternating magnetic
field, then eddy currents are induced in it.
• The eddy current them selves produce a magnetic field. This distorts the
magnetic field responsible for their production.
• As a result the impedence of the coil changes and so, the amplitude of A.C.
At some preset level, this change can be used to trigger a switch.
• It is used for the detection of non - magnetic but conductive materials.
They have the advantage of being relative inexpensive compact size.
• With high reliability and can have high sensitivity to small displacements.
MAGNETIC TYPE PROXIMITY SENSOR:
• These sensors are used to detect magnetic object.
• Principle of operation:
• Magnetic proximity sensors are actuated by the presence of a
permanent magnet. It uses a reed contact which consists of two low
reluctance Ferro-magnetic thin plates enclosed hermetically in a glass
bulb containing inert gas
Working principle:
• The presence of magnetic field makes the thin plates flex and touch
each other causing an electric contact.
• The plate surface has been treated with a special material particularly
suitable for low current or high inductive circuits.
Hall Effect sensor:
• Hall Effect sensor is a device that detects the presence of magnetic
field is work based on Hall Effect.
• The Hall Effect was discovered by Edwin Hall in 1869.
• Hall Effect: when the current I is passed through the conduction and
the same conductor is placed in magnetic field B perpendicular to the
current flow then a voltage called hall voltage is generated
perpendicular to the both current and magnetic filed. This is known
as Hall Effect.
• The hall voltage Vh is directly proportional to the magnetic field B and
intensity of current I and inversely proportional to the thickness t of the
edge of the conductor.
• Mathematically,
The advantages of Hall Effect sensor are:
1. Non-contact operation so there is no wear and friction.
2. High speed operation.
3. Can measure zero speed.
4. Wide temperature range.
5. Capable measuring large current.
The disadvantages of Hall Effect sensor are:
1. Large temperature drift.
2. Large offset voltage.
Application:
1. Current sensing
2. Power sensing.
3. Fluid level measurement.
4. Speed detector.
5. Position sensing.
Hall Effect sensor used for determining the
level of fluid in automobile fuel tanks:
Working:
• A magnet is attached to a flat and as the level of fuel changes and so
the float distance from the hall sensor changes.
• The result is a hall voltage output which is measure of the distance of
the float from the sensor and hence the level of fuel in the tank is
determined.
•Optical Encoders
• An optical encoder converts linear or rotary motion
into digital signal.
• A rotary encoder translates rotary motion of a device
into digital signal.
• Position encoders can be grouped into two
categories:
1. Incremental encoders

2. Absolute encoders
1. Incremental Encoder:

• Rotary encoder transmits

a specific quantity of pulses
for each revolution of a
device.
• These detect changes in rotation
from some datum position.
• A beam of light passes through slots in a disc
and is detected by a suitable light sensor.
• When the disc is rotated, a pulsed output is
produced by the sensor with the number of
pulses being proportional to the angle through
which the disc rotates.
• Thus the angular position of the disc, and hence
the shaft rotating it, can be determined by the
number of pulses produced since some datum
position.
• In practice ,three concentric tracks with three sensors are used.
• The inner track has just one hole and is used to locate the 'home‘ position
of the disc.
• The other two tracks have a series of equally spaced holes that go
completely round the disc but with the holes in the middle track offset from
the holes in the outer track by one-half the width of a hole.
• This offset enables the direction of rotation to be determined.
• In a clockwise direction the pulses in the outer track lead those in the inner.
• In the anti-clockwise direction they lag.
• The resolution is determined by the number
of slots on the disc.
• With 60 slots occurring with 1 revolution then,
since 1 revolution is a rotation of 360°,
the resolution is 360/60 = 6°.
2. Absolute Encoder:
• It produces a specific binary code for each
angular position of the device.

• These give the actual angular position.

Absolute Encoder:
LEDs Phototransistors

Motor shaft

Decimal Grey

Encoder Disc
•Construction:
• It consists of an encoder disk, an array of optical source
(LED) and an array of photo detector.
• The encoder disk has number of traces on it.
• These traces are arranged in Gray code.
• The Gray Coding has the advantage over other coding that
only one bit (trace) is changed per step.
• This avoids misreading. The numbers of traces correspond
to the number of steps per rotation.
• The optical source is fixed on one side of encoder disk and
on other side of disk, photo detectors are arranged such
that whenever trace of disk comes across the optical
source, light emitted from it can be sensed by photo
detector. Fig. shows this arrangement
• The slots are arranged in such a way that the sequential o/p from the
sensors is a number in the binary code.
• Typical encoders tend to have up to 10 to 12 tracks.
The number of bits in the binary number will be equal to the number
of tracks.
• Thus with 10 tracks there will be 10 bits and so the number of
positions that can be detected is 210 i.e 1024, a resolution of 360/1024
= 0.350.
Binary and Gray Codes

Figure shows the tracks with

normal binary code and the Gray
code.
• The normal form of binary code is not generally
used because changing from one binary number
to the next can result in more than one bit
changing and if , through some misalignment,
one of the bits changes fractionally before the
others.
• This may lead to false counting.
• To overcome this the Gray Code is generally
used.
• With this code only one bit changes in moving
from one number to the next.
Pneumatic Sensors:-
• These involve the use of compressed air.
• The displacement or the proximity of an object being transformed into a
change in air-pressure.
• Low-pressure air allowed to escape through a port in front of the sensor.
• This escaping air, in the absence of any close by object, escapes and in
doing so also reduces the pressure in the nearby sensor O/P port.
• However if there is a close-by object, the air cannot so readily escape
and the result is that the pressure increases in the sensor output port.
• The O/P pressure from the sensor thus depends on the proximity of
objects.
• Such sensors are used for the measurement of displacements of
fractions of mm in ranges of 3 to 12 mm.
 Optical proximity sensor:
• Optical proximity sensors are also called as photoelectric proximity
sensors use light sensitive elements to detect objects.
• A complete optical proximity sensor is consists of light source and light
sensor that detects the light.
• The light source generates light of a frequency that light sensor is best able
to detect, and that is not likely to be generated by other nearby source.
• Infra-red light is the most commonly used light in most optical sensor.
• Most optical proximity sensor light sources pulses the infra-red light on
and off at a fixed frequency.
• The light sensor in the optical proximity sensor is a typically a
semiconductor device such as photodiode, which generates a small current
when light energy strikes it.
Four types of optical proximity sensors are
commonly used are:
1. Direct reflection type (diffused)
2. Retro-reflective type (Reflection with retro reflector)
3. Polarized reflection with reflection.
4. Through-beam type.
1. Direct reflection type (diffused):
• In this type both emitter and receiver are homed together in a single
unit and use the light reflected directly back from the target or object
for detection.
• Distance of sensing is depends upon the color and type of surface of
the object.
2. Retro-reflective type (Reflection with retro
reflector)
• In this type both emitter and receiver are homed together in a single
unit and requires reflector.
• An object detected when it interrupts the light beam between the
sensor and the reflector.
• They detect target that reflect light back to the sensor.
Figure shows the principle of retro reflective
proximity sensor.
3. Polarized reflection with reflection.
• It is similar to Retro-reflective type.
• In this type both emitter and receiver are homed together in a single
unit and requires reflector.
• An object detected when it interrupts the light beam between the
sensor and the reflector but uses an anti-reflex device, which bases
it’s functioning on a polarized band of light, offers considerable
advantages and secure readings even when the object to be sensed
has a very shiny surface.
4. Through-beam type.
• In this type both emitter and receiver are housed separately.
• An object detected when it interrupts the light beam between the
emitter and the receiver.
• These sensors allow for longest distance sensing.
 Ultrasonic proximity sensor:
Working Principle:
• An ultrasonic proximity sensor uses a piezoelectric transducer to send and
detect sound waves.
• Transducer generates high frequency sound waves and evaluates the echo
by the detector which is received back after reflecting off the target.
• Sensors calculate the time interval between sending the signal and
receiving the echo to determine the distance to the target.
• When the target enters the operating range the output switches.
• The ultrasonic proximity switches are equipped with temperature sensors
and a compensation circuit, in order to be able to compensate for changes
in operating distance caused by temperature fluctuations.
• The ultrasonic sensor can work in diffuse (direct reflection type), reflex or
thru-beam mode.
Thru-Beam:
• In this case the emitter and detector are 2 separate units. The emitter
emits the light which is detected by the detector.
• A target is detected when it passes in-between the emitter and
detector.
Diffuse Reflective:
• In this case the emitter and detector are put in the single package in
such a way that their field of view crosses.
• Here the emitter continuously emits the light.
• When the target comes within the operating range of the sensor the
light from the emitter is reflected off the target and detected by the
detector.
Retro-Reflective:
• The main components of this sensor are the emitter, detector and the
Retro-reflector.
• The emitter and the detector are in the same package. The Retro-reflector
is placed little far from the sensor.
• The light from the emitter is reflected off the Retro-reflector and detected
by the detector.
• When the target passes between the sensor and the Retro-reflector the
beam is not reflected back to the detector.
• Here the problem can be that the beam could reflect from the target itself.
For this the polarizing filter is used in the sensor.
• Hence only the light reflected by the retro-reflector is detected by detector
The advantages of an Ultrasonic proximity
sensor are
• No physical contact with the object to be detected, therefore, no friction
and wear
• Unlimited operating cycles since there is no mechanical contact with the
target
• Ultrasonic proximity sensors are not affected by target colour or
atmospheric dust, snow, rain..etc
• Can work in adverse conditions
• Sensing distance is more compared to inductive or capacitive proximity
sensors
• The targets to be detected can be in the solid, liquid, granular or powder
state.