Transducers

Tobeuseful systemsmustinteractwiththeir Tobeuseful,systemsmustinteractwiththeirenvironment.Todothistheyusesensorsandactuatorsactuators Sensorsandactuatorsareexamplesoftransducerstransducers

Atransducerisadevicethatconvertsonephysicalquantityintoanotheronephysicalquantityintoanother

examplesinclude: amercuryinglassthermometer(convertstemperaturey g ( pintodisplacementofacolumnofmercury) amicrophone(convertssoundintoanelectricalsignal).

SensorsSensors

Al h i l f i l Almostanyphysicalpropertyofamaterialthatchangesinresponsetosomeexcitation

b dt d canbeusedtoproduceasensor widelyusedsensorsincludethosethatare: resistive inductive

iti capacitive piezoelectric photoresistive photoresistive elastic thermalthermal

DescribingSensorPerformanceDescribingSensorPerformance

R Range maximumandminimumvaluesthatcanbemeasured

Resolutionordiscrimination smallestdiscerniblechangeinthemeasuredvalue

Error differencebetweenthemeasuredandactualvalues randomerrors systematicerrorsy

Accuracy,inaccuracy,uncertainty accuracyisameasureofthemaximumexpectederroraccuracyisameasureofthemaximumexpectederror

Precision ameasureofthelackofrandomerrors(scatter) ameasureofthelackofrandomerrors(scatter)

Linearity maximumdeviationfromastraight lineresponse maximumdeviationfroma straightline response normallyexpressedasapercentageofthefullscalevaluescalevalue

Sensitivity ameasureofthechangeproducedattheoutputforagivenchangeinthequantitybeingmeasured

TemperaturesensorsTemperaturesensors

Resistivethermometers typicaldevicesuseplatinumwire(suchadeviceis typicaldevicesuseplatinumwire(suchadeviceiscalledaplatinumresistancethermometers orPRT)PRT) linear buthaspoorsensitivity

AtypicalPRTelement AsheathedPRT

Thermistors usematerialswithahighthermalcoefficientof usematerialswithahighthermalcoefficientofresistance sensitive buthighlynon linear sensitive buthighlynonlinear

Atypicaldiscthermistor Athreadedthermistor

pn junctions asemiconductordevicewiththe asemiconductordevicewiththepropertiesofadiode inexpensive linear andeasytouse inexpensive,linear andeasytouse limitedtemperaturerange (perhaps

50Cto150C)duetonatureof50Cto150C)duetonatureofsemiconductormaterial

pn-junction sensor

LightSensorsLightSensors

Photovoltaic lightfallingonapn junction lightfallingonapnjunctioncanbeusedtogenerateelectricityfromlightenergyelectricityfromlightenergy(asinasolarcell) smalldevicesusedassensorssmalldevicesusedassensorsarecalledphotodiodes fastacting butthevoltagefastacting,butthevoltageproducedisnot linearlyrelatedtolightintensity

A typical photodiodeg y

Photoconductive suchdevicesdonotproducepelectricity,butsimplychangetheirresistance photodiode(asdescribed photodiode(asdescribedearlier)canbeusedinthiswaytoproducealineardevicep phototransistorsactlikephotodiodesbutwithgreatersensitivitysensitivity lightdependentresistors(LDRs)areslow,butrespond A light-dependent resistor (LDR)( ) , plikethehumaneye

g p ( )

ForceSensorsForceSensors

St i Straingauge stretchinginonedirectionincreasesthe

i t fth d i hil t t hi i th resistanceofthedevice,whilestretchingintheotherdirectionhaslittleeffect canbebondedtoasurfacetomeasurestrain canbebondedtoasurfacetomeasurestrain usedwithinloadcellsandpressuresensors

Direction of sensitivityDirection of sensitivity

A t iA strain gauge

DisplacementSensorsDisplacementSensors

Potentiometers resistivepotentiometersareoneofthemost resistivepotentiometersareoneofthemostwidelyusedformsofpositionsensor canbeangularorlinear canbeangularorlinear consistsofalengthofresistivematerialwithaslidingcontactontotheresistivetrackslidingcontactontotheresistivetrack whenusedasapositiontransducerapotentialisplacedacrossthetwoendterminals thevoltageplacedacrossthetwoendterminals,thevoltageontheslidingcontactisthenproportionaltoitspositionposition aninexpensiveandeasytousesensor

Inductiveproximitysensors coil inductance is greatly coil inductance is greatly

affected by the presence of ferromagnetic materialsg here the proximity of a

ferromagnetic plate is determined by measuring the inductance of a coil

Inductive proximity sensors

Switches Switches simplestformofdigital displacementsensor

f l h d d manyforms:leverorpushrodoperatedmicroswitches;floatswitches;pressureswitches;etc.

A limit switch A float switch

Optoswitches consistofalightsourceandalightsensorwithinasingleunit 2commonformsarethereflectiveandslottedtypes

A reflective opto-switch A slotted opto-switch

AbsolutepositionencodersAbsolutepositionencoders apatternoflightanddarkstripsisprintedontoastripandisdetectedbyasensorthatmovesalongitandisdetectedbyasensorthatmovesalongit thepatterntakestheformofaseriesoflinesasshownbelowbelow itisarrangedsothatthecombinationisuniqueateachpointp sensorisanarrayofphotodiodes

IncrementalpositionencoderIncrementalpositionencoder usesasinglelinethatalternatesblack/white twoslightlyoffsetsensorsproduceoutputsasshown twoslightlyoffsetsensorsproduceoutputsasshownbelow detectsmotionineitherdirection pulsesarecountedto detectsmotionineitherdirection,pulsesarecountedtodetermineabsoluteposition(whichmustbeinitiallyreset)

Othercountingtechniques severalmethodsusecountingtodetermine severalmethodsusecountingtodetermineposition twoexamplesaregivenbelow twoexamplesaregivenbelow

Opto-switch sensorInductive sensor Opto switch sensorInductive sensor

MotionSensorsMotionSensors

Motionsensorsmeasurequantitiessuchas Motionsensorsmeasurequantitiessuchasvelocityandacceleration

b b d b d ff d l canbeobtainedbydifferentiatingdisplacement differentiationtendstoamplifyhighfrequencynoise

Alternativelycanbemeasureddirectly somesensorsgivevelocitydirectlysomesensorsgivevelocitydirectly e.g.measuringfrequency ofpulsesinthecountingtechniquesdescribedearliergivesspeedratherthanq g pposition

somesensorsgiveaccelerationdirectlyg y e.g.accelerometersusuallymeasuretheforceonamass

SoundSensorsSoundSensors

Mi h Microphones anumberofformsareavailable e.g.carbon(resistive),capacitive,piezoelectricandmovingcoilmicrophones movingcoildevicesuseamagnetandacoilattachedtoadiaphragm

SensorInterfacingSensorInterfacing Resistivedevices canbeverysimple e.g.inapotentiometer,withafixedvoltageacrosstheg p , gouterterminals,thevoltageonthethirdisdirectlyrelatedtoposition

where the resistance of the device changes with the quantity being measured this change can bemeasured, this change can be converted into a voltage signal using a potential divider as shown the output of this arrangement is not

linearly related to the change in resistance

Switches switchinterfacingisalsosimples tc te ac g s a so s p e canuseasingleresistorasbelowtoproduceavoltageoutputg p allmechanicalswitchessufferfromswitchbounce

Capacitiveandinductivesensors sensorsthatchangetheircapacitanceor sensorsthatchangetheircapacitanceorinductanceinresponsetoexternalinfluencesnormallyrequiretheuseofalternatingcurrentnormallyrequiretheuseofalternatingcurrent(AC)circuitry suchcircuitsneednotbecomplicatedsuchcircuitsneednotbecomplicated

KeyPointsKeyPoints

Awiderangeofsensorsisavailable SomesensorsproduceanoutputvoltagerelatedtotheSo e se so s p oduce a output o tage e ated to t e

measuredquantityandthereforesupplypower

Otherdevicessimplychangetheirphysicalpropertiesp y g p y p p Somesensorsproduceanoutputthatislinearlyrelated

tothequantitybeingmeasured,othersdonot

Interfacingmayberequiredtoproducesignalsinthecorrectform