Unit 1(part-2)sensors and transducer

UNIT -1
• Sensors & TRANSDUCER
By
V.D.TAMILARASAN
Research associate cum Assistant Professor
Mechanical Engineering Department
Bannari Amman Institute of Technology

DEFINITION
• SENSOR
1. It is defined as an element
which produces signal
relating to the quantity
being measured
2. sensor can be defined as
“A device which provides
a usable output in response
to a specified measured.”
• TRANSDUCER
1. It is defined as an element
when subjected to some
physical change
experiences a related
change or an element
which converts a specified
measured into a usable
output by using a
transduction principle.
2. It can also be defined as a
device that converts a
signal from one form of
energy to another form.

TYPE OF SENSORS AND ITS
APPLICATIONS
• IN MECHATRONICS SYSTEM
WE NEED TO MEASURE THE
FOLLOWING PHYSICAL
QUANTITIES
– SENSORS AND TRANSDUCERS
ARE THE KEY ELEMENT USED
FOR THE MEASUREMENT OF
THE PHYSICAL QUANTITIES
– DISPLACEMENT
– TEMPERATURE
– PRESSURE
– STRESS
– POSITION AND
PROXIMITY
– VELOCITY
– MOTION
– FORCE
– LIQUID FLOW
– LIQUID LEVEL
– LIGHT SENSORS

• SENSORS
– ELEMENT IN A MEASUREMENT SYSTEM THAT ACQUIRES A PHYSICAL
PARAMETER AND CHANGES INTO A SIGNAL(ALSO CAN BE DEFINED AS
PART OF A TRANSDUCER WHICH SENSES OR RESPOND TO A PHYSICAL
QUANTITY OR MEASURAND
SENSOR NORMALLY SENSES THE FOLLOWING PHYSICAL
QUANTITIES
- POSITION
- FORCES
- DISTANCE
- STRAIN
- VIBRATION
- TEMPERATURE
- ACCELERATION ETC.
EXAMPLE OF SENSOR– A THERMOCOUPLE SENSES THE CHANGE IN
TEMPERATURE

• TRANSDUCER
– CONVERTS ENERGY FROM ONE FORM TO ANOTHER
TEMPERATURE, STRAIN --------- ELECTRICAL ENERGY
EXAMPLE- ACCELEROMETER GIVES OUTPUT VOLTAGE PROPORTIONAL TO THE
MECHANICAL MOTION OF THE OBJECT

Active transducers generate electric current or voltage
directly in response to environmental stimulation (Active
transducers are those which do not require any power source for their
operation. They work on the energy conversion principle. They produce an
electrical signal proportional to the input (physical quantity). For example, a
thermocouple is an active transducer.)
Passive transducers produce a change in some passive
electrical quantity, such as capacitance, resistance, or
inductance, as a result of stimulation. These usually
require additional electrical energy for
excitation.(Transducers which require an external power source
for their operation is called as a passive transducer. They
produce an output signal in the form of some variation in
resistance, capacitance or any other electrical parameter, which
than has to be converted to an equivalent current or voltage
signal.)

PERFORMANCE TERMINOLOGY
• 1. RANGE
• 2. SPAN
• 3. ERROR
• 4. ACCURACY
• 5. PRECISION
• 6. SENSITIVITY
• 7. HYSTERISIS
• 8. REPEATABILITY
• 9. REPRODUCIBILITY
• 10. READABILITY
• 11. RESOLUTION
• 12. DEAD ZONE
• 13. DEAD TIME
• 14. BACKLASH
• 15. BIAS
• 16. TOLERANCE
• 17. DRIFT
• 18. UNCERTAINITY

1. RANGE – DIFFERENCE BETWEEN MINIMUM AND MAXIMUM VALUES OF A QUANTITY
EX. 10KN TO 100KN
2. SPAN – IT IS THE DIFFERENCE BETWEEN MAXIMUM AND MINIMUM VALUES OF THE
QUANTITY TO BE MEASURED
EX. 100-10 = 90KN
3. ERROR – DEVIATION OF THE TRUE VALUE FROM THE MEASURED VALUE
ERROR = MEASURED VALUE – TRUE VALUE
4. ACCURACY – IT REPRESENTS HOW CLOSELY THE MEASURED VALUE AGREES WITH THE
TRUE VALUE
5. PRECISION – IT REFERS TO REPEATABILITY OR CONSISTENCY OF MEASUREMENTS WHEN
THE MEASUREMENTS ARE CARRIED OUT UNDER IDENTICAL CONDITIONS AT SHORT
INTERVAL OF TIME
6. SENSITIVITY – IT IS THE RATIO OF THE MAGNITUDE OF THE OUTPUT SIGNAL TO THE
MAGNITUDE OF THE INPUT SIGNAL
SENSITIVITY = OUTPUT/INPUT
7. HYSTERISIS – DIFFERENCE IN THE OUTPUT FOR A GIVEN INPUT WHEN THIS VALUE IS
APPROACHED FROM THE OPPOSITE DIRECTION

8. REPEATABILITY – CLOSENESS OF AGREEMENT AMOUNG NUMBER OF CONSECUTIVE
MEASUREMENTS OF THE OUTPUT FOR THE SAME VALUE OF INPUT UNDER THE SAME
OPERATING CONDITIONS
9. REPRODICIBILITY - CLOSENESS OF AGREEMENT AMOUNG NUMBER OF CONSECUTIVE
MEASUREMENTS OF THE OUTPUT FOR THE SAME VALUE OF INPUT UNDER THE SAME
OPERATING CONDITIONS OVER A PERIOD OF TIME
10. READABILITY – CLOSENESS WITH WHICH THE SCALE OF AN ANALOGUE INSTRUMENT
CAN BE READ
11. RESOLUTION – SMALLEST CHANGE IN A MEASURED VARIABLE TO WHICH AN
INSTRUMENT WILL RESPOND OR
MINIMUM VALUE OF THE INPUT SIGNAL REQUIRED TO CAUSE AN APPRECIABLE
CHANGE OR AN INCREMENT IN THE OUTPUT
12. DEAD ZONE – TIME TAKEN BY AN INSTRUMENT TO BEGIN ITS RESPONSE
13. BACKLASH –IT IS THE LOST MOTION OR FREE PLAY OF THE MECHANICAL ELEMENTS
SUCH AS GEARS, LINKAGES ETC
14. BIAS – THE CONSTANT ERROR THAT EXISTS OVER THE FULL RANGE OF MEASUREMENT
OF AN INSTRUMENT

HYSTERESIS CURVE
• The hysteresis is an error
of a sensor, which is
defined as the maximum
difference in output at
any measurement value
within the sensor’s
specified range when
approaching the point
first with increasing and
then with decreasing the
input parameter

15. TOLERANCE – MAXIMUM ALLOWABLE ERROR IN THE MEASUREMENT
16. DRIFT – THE VARIATION OF CHANGE IN OUTPUT FOR A GIVEN INPUT OVER A
PERIOD OF TIME
17. UNCERTAINITY – DOUBT ABOUT THE UNCERATAINITY OFTHE EXACTNESS OF
MEASURENT RESULTS
18. ZERO DRIFT – CHANGES THAT OCCUR IN THE OUTPUT WHEN THERE IS ZERO INPUT

NONLINEARITY
• The nonlinearity
indicates the maximum
deviation of the actual
measured curve of a
sensor from the ideal
curve
• Nonlinearity (%) =
Maximum deviation in
input ⁄ Maximum full
scale input

Classification of sensors
Detail classification of sensors in view of their applications in manufacturing is as follows.
• A. Displacement, position and proximity sensors
• Potentiometer • Strain-gauged element • Capacitive element • Differential transformers •
Eddy current proximity sensors • Inductive proximity switch • Optical encoders • Pneumatic sensors •
Proximity switches (magnetic) • Hall effect sensors
• B. Velocity and motion
• Incremental encoder • Tachogenerator • Pyroelectric sensors
• C. Force
• Strain gauge load cell
• D. Fluid pressure
• Diaphragm pressure gauge • Capsules, bellows, pressure tubes • Piezoelectric sensors •
Tactile sensor •
• E. Liquid flow
• Orifice plate • Turbine meter F. Liquid level • Floats • Differential pressure
• G. Temperature
• Bimetallic strips • Resistance temperature detectors • Thermistors • Thermo-diodes and
transistors • Thermocouples • Light sensors • Photo diodes • Photo resistors

DISPLACEMENT, POSITION
AND PROXIMITY
SENSORS

DISPLACEMENT SENSORS- MEASURE THE AMOUNT BY WHICH AN OBJECT HAS BEEN
MOVED
POSITION SENSOR- DETERMINE THE POSITION OF THE OBJECT IN RELATION TO SOME
REFERENCE POINT
PROXIMITY SENSOR-IT IS TH EFORM OF POSITION SENSOR AND USED TO DETERMINE
WHEN AN OBJECT HAS MOVED TO WITHIN SOME PARTICULAR CRITICAL DISTANCE
OF THE SENSOR
TYPES OF DISPLACEMENT AND POSITION SENSORS
1. CONTACT SENSOR
2. NON CONTACT SENSOR

POTENTIOMETER SENSOR
A POTENTIOMETER CONSIST OF A RESISTANCE ELEMENT WITH A SLIDING CONTACT
WHICH CAN BE MOVED ALONG THE LENGTH OF THE ELEMENT . SUCH ELEMENT
CAN BE USED FOR LINEAR OR ROTARY DISPLACEMENTS, THE DISPLACEMENT
BEING CONVERTED INTO A POTENTIAL DIFFERENCE

THE ROTARY POTENTIOMETER CONSIST OF CIRCLE WIRE WOUND TRRACK OR A FLIM
OF CONDUCTIVITY PLASTIC WHICH IS ROTATABLE SLIDING CONTACT CAN BE ROTATED

• Relative permittivity of the dielectric
between the plates
• Permittivity of free space
• Area of overlap between the two
plates
• Plate separation
Capacitive sensor for the motion of
linear displacement
a. One of the plate is moved by
displacement so the plate
separation take place
b. The displacement cause that
cause the area of overlap of
change
c. The displacement cause the
dielectric between the plate to
change

FORMS OF CAPACITIVE SENSING ELEMENTS

LVDT (LINEAR VARIABLE DIFFERENTIAL TRANSFORMER)
DIFFERENTIAL TRANSFORMER

LVDT
• CONSIST OF THREE COIL SYMMETERICALLY SPACED ALONG THE INSULATED
TUBES
• THE LEFT SIDE COIL IS PRIMARY COIL AND OTHER TWO COILS ARE
SECONFARY COIL WHICH ARE CONNECTED PARALLEL TO EACH OTHER.
• THE MAGNETIC OR IRON CORE MOVE BETWEEN THIS TWO COIL AS THE
RESULT OF THE DISPLACEMENT BEING MONITORED
• THE AC INPUT TO THE PRIMARY COIL,AC EMF ARE INDUCED IN THE
SEECONDARY COIL
• As the core moves, these mutual inductances change, causing the voltages
induced in the secondary's to change. The coils are connected in reverse
series, so that the output voltage is the difference (hence "differential")
between the two secondary voltages. When the core is in its central
position, equidistant between the two secondary's, equal but opposite
voltages are induced in these two coils, so the output voltage is zero.

The linear variable differential transformer (LVDT) is a type of electrical transformer used for
measuring linear displacement. The transformer has three solenoidal coils placed end-to-end
around a tube. The center coil is the primary, and the two outer coils are the secondaries. A
cylindrical ferromagnetic core, attached to the object whose position is to be measured, slides
along the axis of the tube.
An alternating current is driven through the primary, causing a voltage to be induced in each
secondary proportional to its mutual inductance with the primary. The frequency is usually in the
range 1 to 10 kHz.
As the core moves, these mutual inductances change, causing the voltages induced in the
secondaries to change. The coils are connected in reverse series, so that the output voltage is the
difference (hence "differential") between the two secondary voltages. When the core is in its
central position, equidistant between the two secondaries, equal but opposite voltages are
induced in these two coils, so the output voltage is zero.
When the core is displaced in one direction, the voltage in one coil increases as the other
decreases, causing the output voltage to increase from zero to a maximum. This voltage is in
phase with the primary voltage. When the core moves in the other direction, the output voltage
also increases from zero to a maximum, but its phase is opposite to that of the primary. The
magnitude of the output voltage is proportional to the distance moved by the core (up to its limit
of travel), which is why the device is described as "linear". The phase of the voltage indicates the
direction of the displacement.
Because the sliding core does not touch the inside of the tube, it can move without friction,
making the LVDT a highly reliable device. The absence of any sliding or rotating contacts allows the
LVDT to be completely sealed against the environment.
LVDTs are commonly used for position feedback in servomechanisms, and for automated
measurement in machine tools and many other industrial and scientific applications.

OPTICAL ENCODER
AN ENCODER IS A DEVICE THAT PROVIDES A DIGITAL OUTPUT AS A RESULT
OF LINEAR OR ANGULAR DISPLACEMENT
BASICALLY THEY ARE TWO TYPES
1. INCREMENTAL ENCODER
2. ABSOLUTE ENCODER
INCREMETAL ENCODER DETECTS CHANGES IN ROTATION FROM SOME DATUM
POSITION
ABSOLUTE ENCODER GIVE THE ACTUAL ANGULAR POSITION

NUMBER OF PULSE PROPORTIONAL TO THE ANGLE THROUGH
WHICH DISC IS ROTATED
WITH 60 SLOTS IN ONE REVOLUTION THE RESOLUTION IS 360/60 =6O

ABSOLUTE ENCODER
USED TO MEASURE ANGULAR DISPLACEMENT
GIVES OUTPUT IN THE FORM OF BINARY NUMBER OF SEVERAL DIGITS,EACH
SUCH NUMBER REPRESENTING A PARTICULAR ANGULAR POSITION
TYPICAL ENCODER IS OF 10 TO 12 TRACKS
10 TRACKS =210 =1024 POSITION
RESOLUTION = 360/1024 = 0.35O

INORDER TO PREVENT MISALIGNMENT DUE TO TWO OR MORE BIT CHANGE,
SINGLE BIT CHANGE IS USED

STRAIN GUAGE
Bonded wire strain gauge

Displacement sensor
• Displacement sensors are basically used for
the measurement of movement of an object.
• Potentiometer Sensors
• Strain Gauges
• Capacitive element based sensor
• Linear variable differential transformer (LVDT)

• It works on the principle of conversion of
mechanical displacement into an electrical
signal.
• The sensor has a resistive element and a
sliding contact (wiper). The slider moves along
this conductive body, acting as a movable
electric contact.

• The output voltage is
proportional to the
displacement of the
slider over the wire
• Then the output
parameter
displacement is
calibrated against the
output voltage VA.

APPLICATION
machine-tool controls
Elevators
automobile throttle controls
control of injection molding machines
woodworking machinery
Printing
Spraying
robotics

Strain Gauges
The strain in an element is a ratio of change in length in the direction of
applied load to the original length of an element. The strain changes the
resistance R of the element.
∆R/R α ε; ∆R/R = G ε
G is the constant of proportionality and is called as gauge factor. In general,
the value of G is considered in between 2 to 4 and the resistances are taken of
the order of 100 Ω.

• These foils are made of Constantan alloy (copper-nickel 55-45% alloy) and
are bonded to a backing material plastic (ployimide), epoxy or glass fiber
reinforced epoxy.
Applications of strain gauges
• Strain gauges are widely used in experimental stress analysis and diagnosis
on machines and failure analysis.
• multi-axial stress fatigue testing
• proof testing
• residual stress
• vibration measurement
• torque measurement
• bending
• deflection measurement
• Compression
• tension measurement and strain measurement.

Capacitive element based sensor

• Capacitive sensor is of non-contact type sensor
• primarily used to measure the linear
displacements from few millimeters to hundreds
of millimeters
• It comprises of three plates, with the upper pair
forming one capacitor and the lower pair another
• The linear displacement might take in two forms:
one of the plates is moved by the
displacement so that the plate separation changes
area of overlap changes due to the
displacement.

• The capacitance C of a parallel plate capacitor is given by,
• C = εr εo A / d
• where εr is the relative permittivity of the dielectric between the plates,
εo permittivity of free space, A area of overlap between two plates and d
the plate separation
• Relative permittivity is the factor by which the electric field between the
charges is decreased relative to vacuum
• The permittivity of free space (a vacuum) is a physical constant equal to
approximately 8.85 x 10-12 farad per meter (F/m)
• Capacitive elements can also be used as proximity sensor. The approach of
the object towards the sensor plate is used for induction of change in
plate separation. This changes the capacitance which is used to detect the
object.
• Applications of capacitive element sensors
• • Feed hopper level monitoring • Small vessel pump control • Grease
level monitoring • Level control of liquids • Metrology applications

Linear variable differential transformer
(LVDT)

• Linear variable differential transformer (LVDT) is a
primary transducer used for measurement of linear
displacement with an input range of about ± 2 to ± 400
mm in general.
• It has three coils symmetrically spaced along an
insulated tube.
• The central coil is primary coil and the other two are
secondary coils.
• Secondary coils are connected in series in such a way
that their outputs oppose each other. A magnetic core
attached to the element of which displacement is to be
monitored is placed inside the insulated tube.

• Due to an alternating voltage input to the primary coil,
alternating electromagnetic forces (emfs) are
generated in secondary coils
• When the magnetic core is centrally placed with its
half portion in each of the secondary coil regions then
the resultant voltage is zero
• If the core is displaced from the central position as
shown in Figure 2.2.7, say, more in secondary coil 1
than in coil 2, then more emf is generated in one coil
• If the magnetic core is further displaced, then the
value of resultant voltage increases in proportion with
the displacement

proximity sensors
• Proximity sensors are a type of position sensor
and are used to trace when an object has moved
with in particular critical distance of a transducer
• Eddy current proximity sensors
• Inductive proximity switch
• Optical encoders
• Pneumatic Sensors
• Proximity Switches
• Hall effect sensor

Eddy current proximity sensors

• Eddy current proximity sensors are used to detect non-magnetic but
conductive materials
• They comprise of a coil, an oscillator, a detector and a triggering circuit
• When an alternating current is passed thru this coil, an alternative
magnetic field is generated. If a metal object comes in the close proximity
of the coil, then eddy currents are induced in the object due to the
magnetic field
• These eddy currents create their own magnetic field which distorts the
magnetic field responsible for their generation. As a result, impedance of
the coil(The opposition to current flowing through a coil in an AC circuit is
determined by the AC resistance, more commonly known as Impedance
(Z), of the circuit) changes and so the amplitude of alternating current.
This can be used to trigger a switch at some pre-determined level of
change in current
• Applications of inductive proximity switches • Industrial automation:
counting of products during production or transfer • Security: detection
of metal objects, arms, land mines

Applications of eddy current proximity
sensors
• Automation requiring precise location
• Machine tool monitoring
• Final assembly of precision equipment such as
disk drives
• Measuring the dynamics of a continuously
moving target, such as a vibrating element
• Drive shaft monitoring
• Vibration measurements

• Inductive proximity switches are basically used
for detection of metallic objects
• components; the coil, oscillator, detection
circuit and output circuit
• An alternating current is supplied to the coil
which generates a magnetic field. When, a
metal object comes closer to the end of the
coil, inductance of the coil changes.

• Optical encoders provide digital output as a result of linear /
angular displacement.
• These are widely used in the Servo motors to measure the rotation
of shafts
• It comprises of a disc with three concentric tracks of equally spaced
holes. Three light sensors are employed to detect the light passing
thru the holes. These sensors produce electric pulses which give the
angular displacement of the mechanical element
• The inner track has just one hole which is used locate the ‘home’
position of the disc. The holes on the middle track offset from the
holes of the outer track by one-half of the width of the hole
• With 100 holes in one revolution

• Pneumatic sensors are used to measure the
displacement as well as to sense the proximity of an
object close to it
• The displacement and proximity are transformed into
change in air pressure.
• Low pressure air is allowed to escape through port A.
In the absence of any obstacle / object
• However when an object obstructs the low pressure air
(Port A), there is rise in pressure in output port B. This
rise in pressure is calibrated to measure the
displacement or to trigger a switch

• These are small electrical switches which
require physical contact and a small operating
force to close the contacts

• Magnet based Reed switches are used as
proximity switches. When a magnet attached
to an object brought close to the switch, the
magnetic reeds attract to each other and close
the switch contacts.

• Hall effect sensors work on the principle that
when a beam of charge particles passes
through a magnetic field, forces act on the
particles and the current beam is deflected
from its straight line path
• Thus one side of the disc will become
negatively charged and the other side will be
of positive charge.

application
• The typical application of Hall effect sensor is
the measurement of fluid level in a container.
The container comprises of a float with a
permanent magnet attached at its top. An
electric circuit with a current carrying disc is
mounted in the casing. When the fluid level
increases, the magnet will come close to the
disc and a potential difference generates. This
voltage triggers a switch to stop the fluid to
come inside the container.

Velocity, motion, force and pressure
sensor

• Tachogenerator works on the principle of variable
reluctance
• It consists of an assembly of a toothed wheel and a
magnetic circuit
• Toothed wheel is mounted on the shaft or the element of
which angular motion is to be measured.
• Magnetic circuit comprising of a coil wound on a
ferromagnetic material core.
• As the wheel rotates, the air gap between wheel tooth and
magnetic core changes which results in cyclic change in flux
linked with the coil.
• The alternating emf generated is the measure of angular
motion

Unit 1(part-2)sensors and transducer

• An alternating current (AC) generator can also be
used as a techognerator
• It comprises of rotor coil which rotates with the
shaft
• The rotor rotates in the magnetic field produced
by a stationary permanent magnet or
electromagnet.
• During this process, an alternating emf is
produced which is the measure of the angular
velocity of the rotor

• Strain gauge based sensors work on the principle
of change in electrical resistance
• When, a mechanical element subjects to a
tension or a compression the electric resistance
of the material changes
• This is used to measure the force acted upon the
element.
• Generally strain gauges are used to measure
forces up to 10 MN. The non-linearity and
repeatability errors of this transducer are ±0.03%
and ±0.02% respectively

• Chemical, petroleum, power industry often need to
monitor fluid pressure
• Various types of instruments such as diaphragms,
capsules, and bellows are used to monitor the fluid
pressure
• Specially designed strain gauges doped in diaphragms
are generally used to measure the inlet manifold
pressure in applications such as automobiles
• Specially designed strain gauges doped in diaphragms
are generally used to measure the inlet manifold
pressure in applications such as automobiles

• A stack of capsules is called as ‘Bellows’.
Bellows with a LVDT sensor measures the fluid
pressure in terms of change in resultant
voltage across the secondary coils of LVDT

• In general, tactile sensors are used to sense the contact of fingertips
of a robot with an object
• They are also used in manufacturing of ‘touch display’ screens of
visual display units (VDUs) of CNC machine tools
• the construction of piezo-electric polyvinylidene fluoride (PVDF)
based tactile sensor
• It has two PVDF layers separated by a soft film which transmits the
vibrations. An alternating current is applied to lower PVDF layer
which generates vibrations due to reverse piezoelectric effect.
• These vibrations are transmitted to the upper PVDF layer via soft
film. These vibrations cause alternating voltage across the upper
PVDF layer
• pressure is applied on the upper PVDF layer the vibrations gets
affected and the output voltage changes

• Piezoelectric sensor is used for the measurement of
pressure, acceleration and dynamic-forces such as
oscillation, impact, or high speed compression or tension.
• It contains piezoelectric ionic crystal materials such as
Quartz
• On application of force or pressure these materials get
stretched or compressed.
• During this process, the charge over the material changes
and redistributes
• One face of the material becomes positively charged and
the other negatively charged.
• The net charge q on the surface is proportional to the
amount x by which the charges have been displaced

Liquid flow
1. Liquid flow is generally measured by applying the Bernoulli’s
principle of fluid flow through a constriction
2. The quantity of fluid flow is computed by using the pressure drop
measured. The fluid flow volume is proportional to square root of
pressure difference at the two ends of the constriction
• Orifice plate
• Turbine meter etc.

• It has a disc with a hole at its center, through
which the fluid flows.
• The pressure difference is measured between
a point equal to the diameter of the tube
upstream and a point equal to the half the
diameter downstream.
• It exhibits nonlinear behavior and does not
work with slurries. It has accuracy of ± 1.5%.

• It has a multi blade rotor mounted centrally in
the pipe along which the flow is to be measured
• The fluid flow rotates the rotor. Accordingly the
magnetic pick up coil counts the number of
magnetic pulses generated due to the distortion
of magnetic field by the rotor blades.
• The angular velocity is proportional to the
number of pulses and fluid flow is proportional to
angular velocity.

Temperature and light sensors
• Temperature conveys the state of a
mechanical system in terms of expansion or
contraction of solids, liquids or gases, change
in electrical resistance of conductors,
semiconductors and thermoelectric emfs
• bimetallic strips
• Thermocouples
• thermistors

• Bimetallic strips are used as thermal switch in
controlling the temperature or heat in a
manufacturing process or system
• It contains two different metal strips bonded
together
• The metals have different coefficients of
expansion. On heating the strips bend into curved
strips with the metal with higher coefficient of
expansion on the outside of the curve

• As the strips bend, the soft iron comes in
closer proximity of the small magnet and
further touches.
• Then the electric circuit completes and
generates an alarm.
• In this way bimetallic strips help to protect the
desired application from heating above the
pre-set value of temperature

Thermistors
• Thermistors follow the principle of decrease in resistance with
increasing temperature.
• The material used in thermistor is generally a semiconductor
material such as a sintered metal oxide (mixtures of metal
oxides, chromium, cobalt, iron, manganese and nickel) or
doped polycrystalline ceramic containing barium titanate
(BaTiO3) and other compounds

• As the temperature of semiconductor material increases the
number of electrons able to move about increases which results in
more current in the material and reduced resistance
• Thermistors are available in the form of a bead (pressed disc),
probe or chip
• It has a small bead of dimension from 0.5 mm to 5 mm coated with
ceramic or glass material. The bead is connected to an electric
circuit through two leads. To protect from the environment, the
leads are contained in a stainless steel tube.
Applications of Thermistors
• To monitor the coolant temperature and/or oil temperature inside
the engine
• To monitor the temperature of an incubator
• To control the operations of consumer appliances such as toasters,
coffee makers, refrigerators, freezers, hair dryers, etc.

Thermocouple
• Thermocouple works on the fact that when a
junction of dissimilar metals heated, it produces
an electric potential related to temperature As
per Thomas Seebeck (1821),
• when two wires composed of dissimilar metals
are joined at both ends and one of the ends is
heated, then there is a continuous current which
flows in the thermoelectric circuit
• The net open circuit voltage (the Seebeck
voltage) is a function of junction temperature and
composition of two metals

• Applications of Thermocouples • To monitor
temperatures and chemistry throughout the
steel making process • Testing temperatures
associated with process plants e.g. chemical
production and petroleum refineries • Testing
of heating appliance safety

Light sensors
• A light sensor is a device that is used to detect
light.
• There are different types of light sensors such
as
• photocell/photoresistor
• photo diodes

• Photoresistor is also called as light dependent resistor (LDR).
• It has a resistor whose resistance decreases with increasing incident light intensity.
• It is made of a high resistance semiconductor material, cadmium sulfide (CdS).
• The resistance of a CdS photoresistor varies inversely to the amount of light
incident upon it.
• Photoresistor follows the principle of photoconductivity which results from the
generation of mobile carriers when photons are absorbed by the semiconductor
material.
• The CdS resistor coil is mounted on a ceramic substrate. This assembly is
encapsulated by a resin material. The sensitive coil electrodes are connected to
the control system though lead wires. On incidence of high intensity light on the
electrodes, the resistance of resistor coil decreases which will be used further to
generate the appropriate signal by the microprocessor via lead wires.
•

Applications of photo resistor
• • Computers, wireless phones, and televisions,
use ambient light sensors to automatically control
the brightness of a screen • Barcode scanners
used in retailer locations work using light sensor
technology • In space and robotics: for controlled
and guided motions of vehicles and robots. The
light sensor enables a robot to detect light.
Robots can be programmed to have a specific
reaction if a certain amount of light is detected. •
Auto Flash for camera • Industrial process control

Photo diodes
• Photodiode is a solid-state device which converts incident
light into an electric current
• It is made of Silicon. It consists of a shallow diffused p-n
junction, normally a p-on-n configuration
• When photons of energy greater than 1.1eV (the bandgap
of silicon) fall on the device, they are absorbed and
electron-hole pairs are created
• The depth at which the photons are absorbed depends
upon their energy. The lower the energy of the photons,
the deeper they are absorbed.
• Then the electron-hole pairs drift apart. When the minority
carriers reach the junction, they are swept across by the
electric field and an electric current establishes.

• Photodiodes are one of the types of photodetector,
which convert light into either current or voltage
• It is constructed from single crystal silicon wafers. It is a
p-n junction device. The upper layer is p layer
• It is very thin and formed by thermal diffusion or ion
implantation of doping material such as boron.
Depletion region is narrow and is sandwiched between
p layer and bulk n type layer of silicon
• Light irradiates at front surface, anode, while the back
surface is cathode. The incidence of light on anode
generates a flow of electron across the p-n junction
which is the measure of light intensity

• Applications of photo diodes
• Camera: Light Meters, Automatic Shutter Control, Auto-focus, Photographic Flash Control
•
• Medical: CAT Scanners - X ray Detection, Pulse Oximeters, Blood Particle Analyzers
•
• Industry • Bar Code Scanners • Light Pens • Brightness Controls • Encoders • Position Sensors •
Surveying Instruments • Copiers - Density of Toner Safety Equipment • Smoke Detectors • Flame
Monitors • Security Inspection Equipment - Airport X ray • Intruder Alert - Security System
•
• NPTEL – Mechanical – Mechatronics and Manufacturing Automation
•
• Joint initiative of IITs and IISc – Funded by MHRD Page 39 of 56
• Automotive • Headlight Dimmer • Twilight Detectors • Climate Control - Sunlight Detector
•
• Communications • Fiber Optic Links • Optical Communications • Optical Remote Control

Unit 1(part-2)sensors and transducer

Recommended

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Unit 1(part-2)sensors and transducer (20)

Recently uploaded (20)

Unit 1(part-2)sensors and transducer