Basics of Sensors & Transducers

This article provides an introduction to the fundamental of Sensors and Transducers. It illustrates the different classifications of sensors and transducers. Explains capacitive, resistive and inductive transducers in brief. Also shows the examples under these types of transducers.

1. Electrical & Electronic Measurement
Part –IV
Basics of Sensors and Transducers
ER. FARUK BIN POYEN
[email protected]
AEIE, UIT, BU

2. Contents:
Sensors
Types of Sensors
Transducers
Types of Transducers
2

3. Sensors:
Sensorisadevicethatproducesanoutputsignalforthepurposeofsensingofa
physicalphenomenon
Asensorisadevicethatreceivesandrespondstoasignal.
Thissignalmustbeproducedbysometypeofenergy,suchasheat,light,motion,or
chemicalreaction.
Onceasensordetectsoneormoreofthesesignals(aninput),itconvertsitintoan
analogordigitalrepresentationoftheinputsignal.
Basedonthisexplanation,sensorsareusedinallaspectsoflifetodetectand/or
measuredifferentconditions.
Typicallysensorsconvertarecognizedsignalintoanelectrical–analogordigital–
outputthatisreadable.
3

4. Sensors:
Agoodsensorobeysthefollowingrules:
1.Itissensitivetothemeasuredproperty
2.Itisinsensitivetoanyotherpropertylikelytobeencounteredinitsapplication,and
3.Itdoesnotinfluencethemeasuredproperty.
Sensorssufferfromthefollowingdeviations:
1.SensitivityError
2.HysteresisError
3.OffsetErrororBias
4.Non-linearity
5.Quantizationerror
6.AliasingError
7.Noise
8.Drift
4

5. Sensitivity & Hysteresis:
Sensitivityistheabilityofthemeasuringinstrumenttorespondtochangesinthe
measuredquantity.
Itisalsotheratioofthechangeofoutputtothechangeofinput.
E.g.,forasensormeasuringtemperatureandhavingavoltageoutput,thesensitivityis
aconstantwiththeunit[V/K];thissensorislinearbecausetheratioisconstantatall
pointsofmeasurement.
Hysteresisisanerrorcausedwhenthemeasuredpropertyreversesdirection,butthere
issomefinitelagintimeforthesensortorespond,creatingadifferentoffseterrorin
onedirectionthanintheother.
5

6. Offset Error & Non-linearity:
OffsetError:Iftheoutputsignaldiffersfromthecorrectvaluebyaconstant,thesensor
hasanoffseterrororbias.Thisisanerrorinthey-interceptofalineartransfer
function.
Nonlinearityisdeviationofasensor'stransferfunctionfromastraightlinetransfer
function.Usually,thisisdefinedbytheamounttheoutputdifferingfromtheideal
behavioroverthefullrangeofthesensor,oftennotedasapercentageofthefullrange.
6

7. Quantization Error & Aliasing Error:
Ifthesensorhasadigitaloutput,theoutputisessentiallyanapproximationofthe
measuredproperty.Thiserrorisalsocalledquantizationerror.
Ifthesignalismonitoreddigitally,thesamplingfrequencycancauseadynamicerror,or
iftheinputvariableoraddednoisechangesperiodicallyatafrequencynearamultipleof
thesamplingrate,aliasingerrorsmayoccur.
7

8. Noise & Drift:
Noiseisarandomdeviationofthesignalthatvariesintime.
Iftheoutputsignalslowlychangesindependentlyofthemeasuredproperty,thisis
definedasdrift.Longtermdriftovermonthsoryearsiscausedbyphysicalchangesin
thesensor.
8
P.C: ThemistoklisBourdenas; Jan 2009 Architecting Dependable Systems VII

9. Types of Sensors:
ClassificationofSensors
1.ActiveandPassiveSensors
2.AnalogandDigitalSensors
The sensors are also classified into the following criteria:
1.Primary Input quantity (Measurand)
2.Transduction principles (Using physical and chemical effects)
3.Material and Technology
4.Property
5.Application
9

10. Active & Passive Sensors:
ActiveSensors:Thetypeofsensorsthatproducesoutputsignalwithoutthehelpof
externalexcitationsupply.Theownphysicalpropertiesofthesensorvarywithrespect
totheappliedexternaleffect.Therefore,itisalsocalledasSelfGeneratingSensors.
Anysensorwhichrequirestoinputenergytotheenvironmentinordertoretrievethe
measurementisactive.
Examples:Photovoltaiccells,Thermocouples,Piezoelectricdevice.
PassiveSensors:Thetypeofsensorsthatproducesoutputsignalwiththehelpof
externalexcitationsupply.Theyneedanyextrastimulusorvoltage.Sensorsareableto
retrieveameasurementwithoutactivelyinteractingwiththeenvironment.
Example:StrainGauge,Magnetometer,Barometer.
10

11. Analogue & Digital Sensors:
AnalogSensors:Thesensorthatproducescontinuoussignalwithrespecttotimewith
analogoutputiscalledasAnalogsensors.Theanalogoutputgeneratedisproportional
tothemeasuredortheinputgiventothesystem.Generally,analogvoltageintherange
of0to5Vorcurrentisproducedastheoutput.Thevariousphysicalparameterslike
temperature,stress,pressure,displacement,etc.areexamplesforcontinuoussignals.
DigitalSensors:Whendataisconvertedandtransmitteddigitally,itiscalledasDigital
sensors.Digitalsensorsproducediscreteoutputsignals.Discretesignalswillbenon-
continuouswithtimeanditcanberepresentedin“bits”forserialtransmissionandin
“bytes”forparalleltransmission.DigitaloutputcanbeinformofLogic1orLogic0
(ONorOFF).Adigitalsensorconsistsofsensor,cableandatransmitter.Themeasured
signalisconvertedintoadigitalsignalinsidethesensoritselfwithoutanyexternal
component.Cableisusedforlongdistancetransmission.
11

12. Applications of Digital Sensors:
Detectionofleaksingaspipesandcablesusingpressuresensor
Pressuremonitoringintires.
Monitoringairflow
Measuringlevel
Inhalers(medicaldevice)
12

13. Pictorial Depiction of Few Sensors:
FewSensorTypes
13

14. Sensor Types:
Light Sensor
1.IR Sensor (IR Transmitter / IR LED)
2.Photodiode (IR Receiver)
3.Light Dependent Resistor
Temperature Sensor
1.Thermistor
2.Thermocouple
Pressure/Force/Weight Sensor
1.Strain Gauge (Pressure Sensor)
2.Load Cells (Weight Sensor)
Bio-Sensor
Chemical Sensor
14

15. Sensor Types:
Position Sensor
1.Potentiometer
2.Encoder
Hall Sensor (Detect Magnetic Field)
Flex Sensor
Sound Sensor
Microphone
Ultrasonic Sensor
Touch Sensor
PIR Sensor
Tilt Sensor
Accelerometer
Gas Sensor
15

16. Transducers:
Atransducerisadevicethatconvertsenergyfromoneformtoanother.Usuallya
transducerconvertsasignalinoneformofenergytoasignalinanother.
Requirementsofagoodtransducers
1.Smallerinsizeandweight.
2.Highsensitivity.
3.Abilitytowithstandenvironmentalconditions.
4.Lowcost.
16

17. Transducers:
Factortobeconsideredwhileselectingatransducerare:
1.Itshouldhavehighinputimpedanceandlowoutputimpedance,toavoidloadingeffect.
2.Itshouldhavegoodresolutionovertheentireselectedrange.
3.Itmustbehighlysensitivetodesiredsignalandinsensitivetounwantedsignal.
4.Preferablysmallinsize.
5.Itshouldbeabletoworkincorrosiveenvironment.
6.Itshouldbeabletowithstandpressure,shocks,vibrationsetc..
7.Itmusthavehighdegreeofaccuracyandrepeatability.
8.Selectedtransducersmustbefreefromerrors.
9.Thetransducercircuitshouldhaveoverloadprotectionsothatitwillwithstandoverloads.
17

18. Classification of Transducers:
1.Asactiveandpassivetransducer
2.Accordingtotransductionprinciple
3.Asanaloganddigitaltransducer
4.Asprimaryandsecondarytransducer
5.Asforwardtransducerandinversetransducer
18

19. Classification of Transducers:
ForwardTransducersconvertnon-electricalquantityintoelectricalquantitywhereasInverse
Transducerconvertselectricalquantityintonon-electricalquantity.
19

20. Resistive Transducer:
Themostcommonlyusedtypeoftransducerisvariableresistancetransducer.
Itisotherwisecalledasresistivesensors.
Itmeasurestemperature,pressure,displacement,force,vibrations,etc.
Resistivesensorsworksontheprinciplethat,theconductorlengthisdirectlyproportionalto
resistanceoftheconductoranditisinverselyrelatedwithareaoftheconductor.
Ldenotesconductorlength,AforareaoftheconductorandRforresistanceofconductor.ρis
theresistivityanditisconstantforallthematerialsusedforconductorconstruction.
20
�=??????
�
�

21. Resistive Transducer:
Theresistanceofthetransducervariesduetoexternalenvironmentalfactorsandphysical
propertiesoftheconductor.
ACorDCdevicesareusedtomeasuretheresistancechange.
Thistransduceractsasbothprimaryandsecondarytransducer.
Asaprimarytransducer,itconvertsphysicalquantityintomechanicalsignal.
Asasecondarytransducer,theobtainedmechanicalsignalisconvertedintoelectricalsignal.
21

22. Resistive Transducer:
1.ResistanceStrainGauge–Thechangeinvalueofresistanceofmetalsemi-conductordueto
elongationorcompressionisknownbythemeasurementoftorque,displacementorforce.
2.ResistanceThermometer–Thechangeinresistanceofmetalwireduetothechangeintemperature
knownbythemeasurementoftemperature.
3.ResistanceHygrometer–Thechangeintheresistanceofconductivestripduetothechangeof
moisturecontentisknownbythevalueofitscorrespondinghumidity.
4.HotWireMeter–Thechangeinresistanceofaheatingelementduetoconvectioncoolingofaflow
ofgasisknownbyitscorrespondinggasfloworpressure.
5.PhotoconductiveCell–Thechangeinresistanceofacellduetoacorrespondingchangeinlight
fluxisknownbyitscorrespondinglightintensity.
6.Thermistor–Thechangeinresistanceofasemi-conductorisknownbyitscorrespondingmeasure
oftemperature.
7.PotentiometerType–Thechangeinresistanceduetothemovementofthesliderasapartofan
externalforceappliedisknownbyitscorrespondingpressureordisplacement.
22

23. Capacitive Transducer:
Acapacitorconsistsoftwoconductors(plates)thatareelectricallyisolatedfromoneanotherbya
nonconductor(dielectric).
Whenthetwoconductorsareatdifferentpotentials(voltages),thesystemiscapableofstoringan
electriccharge.ThestoragecapabilityofacapacitorismeasuredinFarads.
Theprincipleofoperationofcapacitivetransducersisbasedupontheequationforcapacitanceofa
parallelplatecapacitorasshownbelow:
A = Overlapping area of plates; d = Distance between two plates; ɛ = Permittivity (dielectric constant); F/m.
Thecapacitivetransducersarecommonlyusedformeasurementoflineardisplacement,by
employingthefollowingeffects.
1.Changeincapacitanceduetochangeinoverlappingareaofplates.
2.Changeincapacitanceduetochangeindistancebetweenthetwoplates.
3.Changeincapacitanceduetochangeindielectricbetweenthetwoplates.
23
�??????????????????�??????�????????????���=
??????�
�

24. Capacitive Transducer:
VariableCapacitancePressureGauge:Distancebetweentwoparallelplatesisvariedbyan
externallyappliedforce.
Applications:MeasurementofDisplacement,pressure.
CapacitorMicrophone:Soundpressurevariesthecapacitancebetweenafixedplateanda
movablediaphragm.
Applications:Speech,music,noise.
DielectricGauge:Variationincapacitancebychangesinthedielectric.
Applications:Liquidlevel,thickness.
24

25. Inductive Transducer:
Aninductivesensorisadevicethatusestheprincipleofelectromagneticinductiontodetector
measureobjects.
Aninductordevelopsamagneticfieldwhenacurrentflowsthroughitalternatively;acurrent
willflowthroughacircuitcontaininganinductorwhenthemagneticfieldthroughitchanges.
Thiseffectcanbeusedtodetectmetallicobjectsthatinteractwithamagneticfield.
TheinductivesensorisbasedonFaraday'slawofinduction.
ThetemporalvariationsoftheMagneticFluxɸthroughaNturnscircuitwillinduceavoltagee
whichisdenotedas:
B=InducedMagneticField;S=homogeneousSection;R=Reluctanceofmagneticcircuit.
25
�=−�
�ɸ
��
=−�∗�
��
��
��??????�????????????����????????????���=
�
�
�
;����????????????????????????����????????????���=��
��
�

26. Inductive Transducer:
Oneformofinductivesensordrivesacoilwithanoscillator.Ametallicobjectapproachingthecoil
willaltertheinductanceofthecoil,producingachangeinfrequencyorachangeinthecurrentin
thecoil.Thesechangescanbedetected,amplified,comparedtoathresholdandusetoswitchan
externalcircuit.Thecoilmayhaveaferromagneticcoretomakethemagneticfieldmoreintense
andtoincreasethesensitivityofthedevice.Acoilwithnoferromagneticcore("aircore")canalso
beused,providedtheoscillatorcoilmustcoveralargearea.
Anotherformofinductivesensorusesonecoiltoproduceachangingmagneticfield,andasecond
coil(orotherdevice)tosensethechangesinthemagneticfieldproducedbyanobject,forexample,
duetoeddycurrentsinducedinametalobject.
26

27. Inductive Transducer:
Inductanceisthephenomenonwhereafluctuatingcurrent,whichbydefinitionhasamagnetic
component,inducesanelectromotiveforce(EMF)inatargetobject.
Toamplifyadevice’sinductanceeffect,asensormanufacturertwistswireintoatightcoiland
runsacurrentthroughit.
Aninductiveproximitysensorhasfourcomponents;
A)Coil,B)Oscillator,C)DetectioncircuitandD)Outputcircuit.
Theoscillatorgeneratesafluctuatingmagneticfieldintheshapeofadoughnutaroundthe
windingofthecoilthatislocatedinthedevice’ssensingface.
Whenametalobjectmovesintotheinductiveproximitysensor’sfieldofdetection,Eddy
circuitsbuildupinthemetallicobject,magneticallypushback,andfinallyreducetheInductive
sensor’sownoscillationfield.
Thesensor’sdetectioncircuitmonitorstheoscillator’sstrengthandtriggersanoutputfromthe
outputcircuitrywhentheoscillatorbecomesreducedtoasufficientlevel.
27

28. Inductive Transducer:
MagneticCircuitTransducer:SelfinductanceormutualinductanceofAC-excitedcoilisvaried
bychangesinthemagneticcircuit.Applications:Pressure,displacement.
ReluctancePickup:Reluctanceofthemagneticcircuitisvariedbychangingthepositionofthe
ironcoreofacoil.Applications:Pressure,displacement,vibration,position.
DifferentialTransformer:Thedifferentialvoltageoftwosecondarywindingsofatransformeris
variedbypositioningthemagneticcorethroughanexternallyappliedforce.Applications:
Pressure,force,displacement,position.
EddyCurrentGauge:Inductanceofacoilisvariedbytheproximityofaneddycurrentplate.
Applications:Displacement,thickness.
Magneto-strictionGauge:Magneticpropertiesarevariedbypressureandstress.Applications:
Force,pressure,sound.
28

29. Voltage and Current Transducer:
HallEffectPickup:Apotentialdifferenceisgeneratedacrossasemiconductorplate
(germanium)whenmagneticfluxinteractswithanappliedcurrent.
Applications:Magneticflux,current.
IonizationChamber:Electronflowinducedbyionizationofgasduetoradioactiveradiation.
Applications:Particlecounting,radiation.
Photo-emissiveCell:Electronemissionduetoincidentradiationonphoto-emissivesurface.
Applications:Lightandradiation.
PhotomultiplierTube:Secondaryelectronemissionduetoincidentradiationonphotosensitive
cathode.
Applications:Lightandradiation,photo-sensitiverelays.
29

30. Self –Generating Transducer:
Theydonotrequireanexternalpower,andproduceananalogvoltageorcurrentwhen
stimulatedbysomephysicalformofenergy.
1.ThermocoupleandThermopile:AnEMFisgeneratedacrossthejunctionoftwodissimilar
metalsorsemiconductorswhenthatjunctionisheated.
Applications:Temperature,heatflow,radiation.
2.Moving-CoilGenerator:Motionofacoilinamagneticfieldgeneratesavoltage.
Applications:Velocity,vibration.
3.PiezoelectricPickup:AnEMFisgeneratedwhenanexternalforceisappliedtocertain
crystallinematerials,suchasquartz.
Applications:Sound,vibration.acceleration,pressurechanges.
4.PhotovoltaicCell:Avoltageisgeneratedinasemi-conductorjunctiondevicewhenradiant
energystimulatesthecell.
Applications:Lightmeter,solarcell.
30

31. Difference between Active & Passive Transducers:
ActiveTransducer PassiveTransducer
Theactivetransducerisalsocalledasselfgenerating
typetransducer.
Thepassivetransducerisalsocalledasexternally
poweredtransducer.
Theactivetransducerdoesnotrequireanyauxiliary
(external)powersupply.
Thepassivetransducerrequiresauxiliary(external)
powersupplyfortransduction.
Thesignalconversionissimpler. Thesignalconversionismorecomplicated.
Theenergyrequiredtoproduceoutputisobtainedfrom
thephysicalquantity.
Theyalsoderivedpartofthepowerrequiredfor
conversionfromphysicalquantityunder
measurement.
Exampleofactivetransducerisbourdontube.
ExampleofpassivetransducerisLVDT(linear
variabledifferentialtransformer).
Itgenerateselectriccurrentorvoltagedirectlyin
responsetoenvironmentalstimulation.
Itgivesachangeinsomepassiveelectrical
quantity,suchascapacitance,resistanceor
inductance,asaresultofstimulation.
31

32. References:
http://www.ieec.uned.es/investigacion/Dipseil/PAC/archivos/More%20on%20Transducer
s%20Sensors%20and%20Actuators.pdf
Dincer,Can;Bruch,Richard;Costa‐Rama,Estefanía;Fernández‐Abedul,MariaTeresa;
Merkoçi,Arben;Manz,Andreas;Urban,GeraldAnton;Güder,Firat(2019-05-15).
https://www.electricaltechnology.org/2018/11/types-sensors-applications.html
https://www.electricaltechnology.org/2019/04/resistive-sensors.html
https://www.polytechnichub.com/difference-active-transducer-passive-transducer/
WinncyY.Du,Resistive,Capacitive,Inductive,andMagneticSensorTechnologies,CRC
Press,2014ISBN1439812446,Chapter4InductiveSensors
http://www.nskelectronics.com/inductive_proximity_sensor.html
http://www.gvpcew.ac.in/unit%202.pdf
32