Unit v Transducers

8/10/2019 Unit v Transducers

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TRANSDUCERSTRANSDUCERS

ANDANDDATA ACQUISITION SYSTEMSDATA ACQUISITION SYSTEMS

11BY V.BALAJI,BY V.BALAJI, M.TechM.Tech, ( P, ( P h.Dh.D),),

AP/EEE, DCEAP/EEE, DCE


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TRANSDUCERSTRANSDUCERS

Transducers is a device which converts one formTransducers is a device which converts one formof energy into another form of energy.of energy into another form of energy.

Example : Loud speaker converts electricalExample : Loud speaker converts electrical

. . Function of transducer:Function of transducer:

1.It detects and sense the presence,1.It detects and sense the presence,magnitude and physical quantity beingmagnitude and physical quantity beingmeasured.measured.

2.It provides a proportional electrical output2.It provides a proportional electrical outputsignal.signal.

22BY V.BALAJI, M.Tech, ( P h.D),BY V.BALAJI, M.Tech, ( P h.D),



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PARTS OF TRANSDUCERPARTS OF TRANSDUCER

Transducers has two parts namelyTransducers has two parts namely1.1. Sensing elementSensing element

2.2. Transduction elementTransduction element

Sensor:Sensor:

Sensor is a device that is used to sense or detect anySensor is a device that is used to sense or detect anyparticular change in quantity that is being measuredparticular change in quantity that is being measuredand marks appropriate change in its output.and marks appropriate change in its output.




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Detector:

Detector element is that part of transducer

which responds to physical phenomenon orchange in physical phenomenon.

Transduction element:

An element which transforms the output ofsensing element into an electrical signal is known

as Transduction element.




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SELECTION OF TRANSDUCERSELECTION OF TRANSDUCER

The transducer element should recognize and sense the desiredThe transducer element should recognize and sense the desiredinput signal and should be insensitive to other signals presentinput signal and should be insensitive to other signals presentsimultaneously in the measurand.simultaneously in the measurand.

It s ou not a ter t e event to e measure .It s ou not a ter t e event to e measure .

It should have high reproducibilityIt should have high reproducibility

It should have amplitude linearityIt should have amplitude linearity




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SELECTION OF TRANSDUCERSELECTION OF TRANSDUCER

It should be able to withstand hostile instrumentsIt should be able to withstand hostile instrumentswithout damage and should maintain the accuracy withinwithout damage and should maintain the accuracy withinacceptable limits.acceptable limits.

It should have good accuracyIt should have good accuracy

It should have good dynamic responseIt should have good dynamic response

It should not induce phase distortionsIt should not induce phase distortions




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CLASSIFICATION OF TRANSDUCERSCLASSIFICATION OF TRANSDUCERS

Based on principle of Transduction used:Based on principle of Transduction used:

a. Resistive transducera. Resistive transducer

b. Capacitive transducerb. Capacitive transducerc. Inductance transducerc. Inductance transducer

d. Piezoelectric transducerd. Piezoelectric transducer

e. Photo electric transducere. Photo electric transducerf . Electromagnetic transducerf . Electromagnetic transducer

g. Thermo electric transducerg. Thermo electric transducer




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CLASSIFICATION OF TRANSDUCERSCLASSIFICATION OF TRANSDUCERS

Primary and Active transducersPrimary and Active transducers

Ana og an Digita trans ucersAna og an Digita trans ucers

Transducers and inverse transducersTransducers and inverse transducers




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RESISTIVE TRANSDUCERSRESISTIVE TRANSDUCERS

Works on principle of change in ResistanceWorks on principle of change in Resistance

This is achieved by change in either resistivity or lengthThis is achieved by change in either resistivity or length. .

Some of the resistive transducers are potentiometer,Some of the resistive transducers are potentiometer,resistance thermometer , thermistor, resistanceresistance thermometer , thermistor, resistance

hygrometer, Photo conductive cell.hygrometer, Photo conductive cell.




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POTENTIOMETERSPOTENTIOMETERS

A Resistive potentiometer which is used for the purposeA Resistive potentiometer which is used for the purposeof voltage division is called as POT.of voltage division is called as POT.

It is a assive transducer which ives an electricalIt is a assive transducer which ives an electrical

output to mechanical displacement applied on the wiperoutput to mechanical displacement applied on the wiperof the potentiometer.of the potentiometer.

It requires an external power which may be a.c or d.c.It requires an external power which may be a.c or d.c.

so it is called as a passive transducer.so it is called as a passive transducer.




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STRAIN GAUGESSTRAIN GAUGES

It is a resistive transducer.It is a resistive transducer. It operates on the principle of piezo resistiveIt operates on the principle of piezo resistive

effect.effect.

p ezo res s ve e ec .p ezo res s ve e ec .If a metal conductor is stretched orIf a metal conductor is stretched or

compressed or its resistance changes on accountcompressed or its resistance changes on accountof the fact that both length and diameter ofof the fact that both length and diameter ofconductor change. It is used for strainconductor change. It is used for strainmeasurements and associated stress inmeasurements and associated stress inexperimental analysis.experimental analysis.




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TYPES OF STRAIN GAUGESTYPES OF STRAIN GAUGES

1.1. Unbonded metal strain gaugesUnbonded metal strain gauges2.2. Bonded metal wire strain gaugesBonded metal wire strain gauges

3.3. Bonded metal foil strain gaugesBonded metal foil strain gauges

4.4. Vacuum eposite t in meta i m strain gaugesVacuum eposite t in meta i m strain gauges5.5. Sputter deposited thin metal strain gaugesSputter deposited thin metal strain gauges

6.6. Bonded semiconductor strain gaugesBonded semiconductor strain gauges

7.7. Diffused metal strain gaugesDiffused metal strain gauges




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APPLICATIONS OF STRAIN GAUGESAPPLICATIONS OF STRAIN GAUGES

Experimental stress analysis of machines and structures.Experimental stress analysis of machines and structures.

Construction of force, torque, pressure, flow andConstruction of force, torque, pressure, flow and..




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UNBONDED METAL STRAIN GAUGESUNBONDED METAL STRAIN GAUGES

This gauge consists of a wire stretched between twoThis gauge consists of a wire stretched between twopoints in an insulating medium such as air.points in an insulating medium such as air.

he wires ma be made of co er nickel chrome nickelhe wires ma be made of co er nickel chrome nickel

or nickel iron alloys.or nickel iron alloys.

The flexture element is connected via a rod to aThe flexture element is connected via a rod to adiaphragm which is used for sensing of pressure.diaphragm which is used for sensing of pressure.

The wires are tensioned to avoid buckling when theyThe wires are tensioned to avoid buckling when theyexperience a compressive force.experience a compressive force.

1616BY V.BALAJI, M.Tech, ( P h.D),BY V.BALAJI, M.Tech, ( P h.D),AP/EEE, DCEAP/EEE, DCE


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OperationOperation

The four posts as shown in figure., are mountedThe four posts as shown in figure., are mountedon a star spring structure.on a star spring structure. When the centre of the star spring is subjectedWhen the centre of the star spring is subjected

to the force under measurement the starto the force under measurement the star

spring flexes with each pair of the strain gaugespring flexes with each pair of the strain gaugeelements on opposite sides going to strains ofelements on opposite sides going to strains ofopposite polarity.opposite polarity.

The resulting unbalance voltage of the bridgeThe resulting unbalance voltage of the bridge

circuit is proportional to the force and hence thecircuit is proportional to the force and hence thepressure on he diaphragm.pressure on he diaphragm.

They are available power ranges from as low asThey are available power ranges from as low as

00--0.15 to 00.15 to 0--660 bars.660 bars. 1818BY V.BALAJI, M.Tech, ( P h.D),BY V.BALAJI, M.Tech, ( P h.D),AP/EEE, DCEAP/EEE, DCE


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APPLICATION OF UNBONDED STRAIN GAUGESAPPLICATION OF UNBONDED STRAIN GAUGES

Used exclusively in transducer applicationsUsed exclusively in transducer applications Employ preloaded resistance wires connected in aEmploy preloaded resistance wires connected in a

Wheatstone bridge.Wheatstone bridge.



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BONDED STRAIN GAUGESBONDED STRAIN GAUGES

Construction:Construction:

he most common t e is the wire resistancehe most common t e is the wire resistance

gauge with the wire looped back and forth asgauge with the wire looped back and forth asshown in figure 2.1 to form a flat grid in oneshown in figure 2.1 to form a flat grid in oneplane.plane.

The number of loops is limited to 3 or 4 with theThe number of loops is limited to 3 or 4 with theloops as closely spaced as to minimize theloops as closely spaced as to minimize thelength of the wire lying along the transverselength of the wire lying along the transverse

direction OB in comparison to that along thedirection OB in comparison to that along the 2121BY V.BALAJI, M.Tech, ( P h.D),BY V.BALAJI, M.Tech, ( P h.D),AP/EEE, DCEAP/EEE, DCE


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CONSTRUCTION CONTINUED..CONSTRUCTION CONTINUED..

This construction provides insensitive to strainsThis construction provides insensitive to strainsalong the transverse direction and hence itsalong the transverse direction and hence itscross sensitivity becomes very slow.cross sensitivity becomes very slow.

welded at the ends may be preferred for usewelded at the ends may be preferred for usewhere cross sensitivity has to be zero.where cross sensitivity has to be zero.

All of the above resistance elements areAll of the above resistance elements are

cemently secured by nitrocellulose element orcemently secured by nitrocellulose element orphenolic elements , to two sheets of flexiblephenolic elements , to two sheets of flexibleinsulating material ,one on each side of theinsulating material ,one on each side of theelement.element.



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PERFORMANCEPERFORMANCE

For accurate measurement of the strain surface of anyFor accurate measurement of the strain surface of anystructure, it is necessary that the bonded strain gaugestructure, it is necessary that the bonded strain gaugebecome integral with the test surface.become integral with the test surface.

They must possess the merit of having higher surfaceThey must possess the merit of having higher surfacearea or cross section ratio and higher resistance, whicharea or cross section ratio and higher resistance, whichare desirable for faithful transmission of strain from theare desirable for faithful transmission of strain from thestress surface element and strain gauge element andstress surface element and strain gauge element and

slightly higher sensitivity.slightly higher sensitivity.

Improper bonding of the gauge to the surface becomesImproper bonding of the gauge to the surface becomesa source errors due to hysterisis , creep and zero drifta source errors due to hysterisis , creep and zero drift

,apart from lowering the stability for long term,apart from lowering the stability for long termmeasurements.measurements.



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VARIABLE INDUCTANCE TRANSDUCERSVARIABLE INDUCTANCE TRANSDUCERS

This works upon any one of the following principles:This works upon any one of the following principles:

1. ange in se in uctance1. ange in se in uctance

2.Change in mutual inductance2.Change in mutual inductance

3.Production of eddy currents3.Production of eddy currents

Applications:Applications:

It is used to measure displacement and thickness.It is used to measure displacement and thickness.



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TRANSDUCERS WORKING ON THE PRINCIPLE OF SELFTRANSDUCERS WORKING ON THE PRINCIPLE OF SELFINDUCTANCEINDUCTANCE

The self inductance of the coil is L = NThe self inductance of the coil is L = N22

/R/RWhere, N = number of turnsWhere, N = number of turns

R = reluctance of the magnetic circuitR = reluctance of the magnetic circuit

R = AR = A



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LINEAR VARIABLE DIFFERENTIALLINEAR VARIABLE DIFFERENTIALTRANSFORMER(LVDT)TRANSFORMER(LVDT)

LVDT is used to convert linear motion into electrical signals.LVDT is used to convert linear motion into electrical signals.

Principle of operation:Principle of operation:

It works on the principle of mutual induction.It works on the principle of mutual induction.

Advantages:Advantages:

as g range or measur ng sp acemen .as g range or measur ng sp acemen .

Since no physical contact between coil and core, it providesSince no physical contact between coil and core, it providesfrictionless operation and also provides electrical isolation.frictionless operation and also provides electrical isolation.

It gives high output and possesses high sensitivity.It gives high output and possesses high sensitivity.

The are rugged in construction.The are rugged in construction.

Low hysterisisLow hysterisis Power consumption is less than 1 WPower consumption is less than 1 W

Infinite resolutionInfinite resolution



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CHARACTERISTICS OF LVDTCHARACTERISTICS OF LVDT

The output voltage is aThe output voltage is afunction of corefunction of coredisplacement within thedisplacement within thelimit of range of motionlimit of range of motionabout 5 mm from the nullabout 5 mm from the nullposition.position.




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DISADVANTAGESDISADVANTAGES

Relatively large displacements are required forRelatively large displacements are required forappreciable differential output.appreciable differential output.

Sensitive to stray magnetic fields but shielding isSensitive to stray magnetic fields but shielding is..

Performance is affected by vibrations.Performance is affected by vibrations.

Receiving instrument must be selected to operate on a.cReceiving instrument must be selected to operate on a.csignals or demodulator network must be used if d.csignals or demodulator network must be used if d.c

output is required .output is required . Dynamic response is limited mechanically by the mass ofDynamic response is limited mechanically by the mass of

the core and electrically by the frequency of supplythe core and electrically by the frequency of supplyvoltage.voltage.

Temperature also affects the performance of theTemperature also affects the performance of the 3232BY V.BALAJI, M.Tech, ( P h.D),BY V.BALAJI, M.Tech, ( P h.D),



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APPLICATIONSAPPLICATIONS

Used where displacements ranging from fractionUsed where displacements ranging from fractionof mm to few cm have to be measured.of mm to few cm have to be measured.

Acting as secondary transducer it can be used toActing as secondary transducer it can be used to, ., .

Acting as primary transducer converts theActing as primary transducer converts thedisplacement directly into an electrical outputdisplacement directly into an electrical outputproportional to displacement.proportional to displacement.

It can also be used for measurement and controlIt can also be used for measurement and controlof thickness of metal sheet being rolled.of thickness of metal sheet being rolled.

Also used for measurement of tension in a cord.Also used for measurement of tension in a cord.




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CAPACITIVE TRANSDUCERSCAPACITIVE TRANSDUCERS

Capacitive transducers are commonly used forCapacitive transducers are commonly used formeasurement of displacement.measurement of displacement.

Principle of operation is based on capacitor equation ofPrinciple of operation is based on capacitor equation of, ,

C=C=oo rA/drA/d

where,where,

A = overlapping area of plates, m2A = overlapping area of plates, m2

d = distance between two plates, md = distance between two plates, mo=permittivity of free spaceo=permittivity of free space

r = Relative permittivityr = Relative permittivity




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Principle of Change of capacitance may be caused byPrinciple of Change of capacitance may be caused by Change in overlapping area, AChange in overlapping area, A

Change in distance between plate, dChange in distance between plate, d

ange in ie ectric constantange in ie ectric constant




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DISADVANTAGES OF CAPACITIVE TRANSDUCERSDISADVANTAGES OF CAPACITIVE TRANSDUCERS

The metallic parts of the transducers must be insulatedThe metallic parts of the transducers must be insulatedfrom each other. In order to reduce the effects of strayfrom each other. In order to reduce the effects of straycapacitance, the frame must be earthed.capacitance, the frame must be earthed.

times on account of edge effects. guard rings are usedtimes on account of edge effects. guard rings are usedto eliminate this effect.to eliminate this effect.

Output impedance is very high. this leads to loadingOutput impedance is very high. this leads to loadingeffects.effects.

Cable connecting to the measuring point is also a sourceCable connecting to the measuring point is also a sourceof error.of error.




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USES OF CAPACITIVE TRASDUCERSUSES OF CAPACITIVE TRASDUCERS

It is used for measurement of both linear andIt is used for measurement of both linear andangular displacements.angular displacements.

It also used for extremely small displacementsIt also used for extremely small displacements

s use or measuremen o orce ans use or measuremen o orce anpressurepressure

It is also used for measurement of humidity inIt is also used for measurement of humidity ingasesgases

Capacitive transducers are commonly used inCapacitive transducers are commonly used inconjunction with mechanical modifiers forconjunction with mechanical modifiers formeasurement of volume, density, liquid level,measurement of volume, density, liquid level,

weight etc.weight etc. 3939BY V.BALAJI, M.Tech, ( P h.D),BY V.BALAJI, M.Tech, ( P h.D),



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PIEZO ELECTRIC TRANSDUCERSPIEZO ELECTRIC TRANSDUCERS

It works on the principle of piezo electric effect.It works on the principle of piezo electric effect.

Piezo electric effect:Piezo electric effect:

If a varying potential is applied to the proper axisIf a varying potential is applied to the proper axis,,

thee by deforming it.thee by deforming it.

Elements exhibiting piezo electric qualities are called asElements exhibiting piezo electric qualities are called aselectro resistive elements.electro resistive elements.




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OPTICAL TRANSDUERSOPTICAL TRANSDUERS

Transducers that convert light into electrical signal areTransducers that convert light into electrical signal aretermed as optical transducerstermed as optical transducers

Principle:Principle:

produces a change of mobility of charge in theproduces a change of mobility of charge in thesemiconductor there by producing change in capacitancesemiconductor there by producing change in capacitance,change in current and finally change in output voltage.,change in current and finally change in output voltage.

Application:Application: It is used for the measurement of any phenomenonIt is used for the measurement of any phenomenon

which can be made to produce a variation in lightwhich can be made to produce a variation in lightintensity. Sensitivity and speed response of this type ofintensity. Sensitivity and speed response of this type of

transducer is very high.transducer is very high. 4141BY V.BALAJI, M.Tech, ( P h.D),BY V.BALAJI, M.Tech, ( P h.D),



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TYPES OF PHOTO ELECTRIC TRANSDUCERSTYPES OF PHOTO ELECTRIC TRANSDUCERS

Photo emissive typePhoto emissive type Photo conductive typePhoto conductive type

Thermal typeThermal type

Photo diodePhoto diode

Photo transistorPhoto transistor Photo thyristorPhoto thyristor

4242

BY V.BALAJI, M.Tech, ( P h.D),BY V.BALAJI, M.Tech, ( P h.D),



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DIGITAL TRANSDUCERSDIGITAL TRANSDUCERS

Digital transducers are classified into two typesDigital transducers are classified into two types Direct digital transducersDirect digital transducers

Indirect di ital transducersIndirect di ital transducers

Direct digital transducers:Direct digital transducers:It provide output signals in the form ofIt provide output signals in the form ofrectangular pulses of constant duration andrectangular pulses of constant duration and

amplitude, the presence or absence of which inamplitude, the presence or absence of which inits time slot is taken to stand for either 1s orits time slot is taken to stand for either 1s or0s.0s.

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DIGITAL TRANSDUCERSDIGITAL TRANSDUCERS

ADVANTAGES:ADVANTAGES:

1.It is relatively inexpensive1.It is relatively inexpensive

2.It can be made to any degree of accuracy2.It can be made to any degree of accuracy DISADVANTAGES:DISADVANTAGES:

1.due to wear of brushes and contact surface of1.due to wear of brushes and contact surface ofthe encoder, life of the encoder is reduced.the encoder, life of the encoder is reduced.

2. the reliability of indication is limited by2. the reliability of indication is limited byambiguity of indication whenever brushes or sensorsambiguity of indication whenever brushes or sensors

changes from pertaining to 1to regions pertainingchanges from pertaining to 1to regions pertainingto 0 and vice versato 0 and vice versa..

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SHAFT ENCODERSHAFT ENCODER

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DATA ACQUISITION SYSTEMSDATA ACQUISITION SYSTEMS

Data acquisition systems are used to measure and recordData acquisition systems are used to measure and recordanalog signals in basically two different waysanalog signals in basically two different ways

gna s w c or g na e rom rec measuremen o e ec r cagna s w c or g na e rom rec measuremen o e ec r ca

quantities. These signals may be a.c or d.c voltages ,frequency orquantities. These signals may be a.c or d.c voltages ,frequency orresistance etc.resistance etc.

Signals which originate from use of transducersSignals which originate from use of transducers

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COMPONENT OF ANALOG DATA ACQUISITION SYSTEMSCOMPONENT OF ANALOG DATA ACQUISITION SYSTEMS

1.1. TransducersTransducers

2.2. Signal conditioning equipmentSignal conditioning equipment

3.3. MultiplexerMultiplexer

4.4.

Calibrating equipmentCalibrating equipment5.5. Integrating equipmentIntegrating equipment

6.6. Visual display devicesVisual display devices

7.7. Analog recordersAnalog recorders8.8. Analog computersAnalog computers

9.9. High speed cameras and Tv equipmentHigh speed cameras and Tv equipment

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SIGNAL CONDITIONING EQUIPMENTSIGNAL CONDITIONING EQUIPMENT

It transforms the output of transducer to theIt transforms the output of transducer to thedesired magnitude or form required by the nextdesired magnitude or form required by the nextstage of the data acquisition system.stage of the data acquisition system.

Multiplexer:Multiplexer:1.Multiplexing is the process of sharing a1.Multiplexing is the process of sharing a

single channel with more than one output.single channel with more than one output.

2.Multiplexer accepts multiple analog input2.Multiplexer accepts multiple analog inputsignals and connects them sequentially to onesignals and connects them sequentially to onemeasuring input.measuring input.

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Calibrating equipmentCalibrating equipment

Integrating equipmentIntegrating equipment

Visual display devicesVisual display devices

continuous monitoring of input analog signals. These devicescontinuous monitoring of input analog signals. These devicesinclude panel mounted meters ,numerical displays, single or multiinclude panel mounted meters ,numerical displays, single or multi

channel cro and storage type cros.channel cro and storage type cros.

Analog recordersAnalog recorders

Method of recording data in analog form.Method of recording data in analog form.

Analog computersAnalog computers

used as reduction deviceused as reduction device

High speed cameras and Tv equipmentHigh speed cameras and Tv equipment

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DIGITAL DATA ACQUISITION SYSTEMSDIGITAL DATA ACQUISITION SYSTEMS

Components:Components:

1.1. TransducersTransducers

2.2. Signal conditioning equipmentSignal conditioning equipment

3.3.

MultiplexerMultiplexer4.4. Signal converterSignal converter

5.5. Analog to digital converterAnalog to digital converter

6.6. Auxiliary equipmentAuxiliary equipment7.7. Digital recordersDigital recorders

8.8. Digital printersDigital printers

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Uses of digital data acquisition systemsUses of digital data acquisition systems

Data acquisition systems are used in everData acquisition systems are used in everincreasing , large and wide fields inincreasing , large and wide fields in

Industrial and scientific areasIndustrial and scientific areas

AerospaceAerospace BiomedicalBiomedical

TelemetryTelemetry

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RR--2R LADDER NETWORK2R LADDER NETWORK

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Analog to digital converterAnalog to digital converter

Conversion of analog to digital signal.Conversion of analog to digital signal.

Successive approximation method used for conversionSuccessive approximation method used for conversion

Speed of an ADC is expressed as conversion time whichSpeed of an ADC is expressed as conversion time which

conversion is completedconversion is completed

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Unit v Transducers