3-SIGNAL-CONDITIONING-FOR-RESISTIVE-SENSORS-传感与检测技

3SIGNALCONDITIONINGFORRESISTIVESENSORSThischapterpresents:Methodstoobtainfromresistivesensorsoutputvoltagesinarangesuitedtoanalog-to-digitalconverters.InterferencecompensationSensorlinearization3.1MEASUREMENTOFRESISTANCEThegeneralequationfortheresistancechangeofasensor:ForlinearsensorsTherangeofvaluesforxdependsstronglyonthetypeofsensorandonthemeasurandspan:xfrom0to-1forlinearpotentiometersxfrom10-5to10-2forstraingages……)(0xfRRxRR103.1MEASUREMENTOFRESISTANCETworequirementsforallconditionersforresistivesensors:DrivethesensorwithanelectricvoltageorcurrentThesupplyislimitedbysensorself-heatingunlessthesensingprincipleusessensorself-heatingThepowerdissipatebythesensorisRRRVP200TheveninEquivalentcircuitSensor3.1MEASUREMENTOFRESISTANCEConditionersforremotesensorsmustbeinsensitivetoconnectingleadresistanceorcompensateforit.ThevoltagemeasuredinFigure(a)00021VRRRRIVWWV≈IRiftheoutputimpedanceofthecurrentsourceandtheinputimpedanceofthevoltagemeterarelargeenough.3.1MEASUREMENTOFRESISTANCEMethodsforresistancemeasurement:•Deflection(偏差的)method.Sensethedropinvoltageacrosstheresistancetobemeasuredorthecurrentthroughitorboth.•Nullmethod.Basedonmeasurementbridges.3.1MEASUREMENTOFRESISTANCEThevoltageoutput:xRRVRIvrrr100Itmeansthatv0willconsistofsmallfluctuationsuperimposedonaverylargeoffsetvoltage.3.1MEASUREMENTOFRESISTANCEHowtodeletetheoffsetBysubtractingthedropinvoltageacrossRrdeletetheconstantvoltage.Itcanbeimplementedbyusingtwoidenticalcurrentsources.IfwhereRw1,Rw2andRw3aretheleadsresistance.xIRRRIvZ000RRz3.1MEASUREMENTOFRESISTANCEExample3.1CTRRVRIrrr1.02CRVRrr1.01009301.0405.13855.138010000385.01100100CKmWVRCCKRr3.1MEASUREMENTOFRESISTANCEConsiderthesensitivity,say1mV/℃1925100385.0100510000KmVKVSRVRRRVdTdvSTRVvrrrrrr3.1MEASUREMENTOFRESISTANCEAnotherdeflectionmethodTwo-readingmethod.rrrrVvRRIRvIRV003.2VOLTAGEDIVIDERVoltagedividersiscommonlyusedtomeasurehigh-valueresistance.AssumetheinputresistanceofvoltmetermuchhigherthanR,wehaveIfswapRrandR,wehave000vvVRRRRRVvrrrrr00vVvRRrr3.2VOLTAGEDIVIDERExample3.2PowerdissipationimposestheconditionThemaximaldissipationwillhappenwhenR=RrThesensorresistancerangeisfrom1000kΩtomWRRRVrr1279001.0252maxWVPVRrrkRR3755.2604003.2VOLTAGEDIVIDERThevoltage-resistancerelationinvoltagedividerisnotlinearbecausethecurrentinthecircuitdependsontheunknownresistance.Infigure3.5(b),wehaveRRVvrr0(3.13)Figure3.5(b)3.2VOLTAGEDIVIDERExample3.3From(3.13)SelectVr=-5V,wehaveRr=20kΩVkRVvVkRVvrrrr520455.02000003.2VOLTAGEDIVIDERTheproblemofvoltagedividerusedforstaticmeasurement:Ifx1,theoutputΔv0isalsosmall.Anyerrorpresentwhenmeasuringv0=v0(0)+Δv0willresultinaveryhighpercentageerrorcomparedtoΔv0.Measuresmallchangesinresistance:Placeanothervoltagedividerinparallelwiththeoneincorporatingthesensor–Wheatstonebridge.TheWheatstonebridgeisanullmeasurementmethod.3.2VOLTAGEDIVIDER3.2.1PotentiometersTmrmmmTTrrTTTRRkkVRRRvvRRRVVRRRv111//10000EquivalentThevenincircuitVoltageandresistanceRm:voltagemeterinputresistance3.2VOLTAGEDIVIDERWhen,noerrorattheendsofthescale.Atintermediatepoints,therelativeerrorεisrelativetokThemaximalεα=0.51100kvvvm01212kkdd25.025.0maxkrVv03.2VOLTAGEDIVIDERBecauseαand1-αcanbeinterchangedwithnoeffect,therelativeerrorissymmetricalwithrespecttothecentralpoint.AsimplewaytoreducetheloadingerrorwithoutincreasingRmistoplacearesistorequaltoRmasshowninthefollowingFigure(a).kkVvrm1213.2VOLTAGEDIVIDERExample3.4Definea=RT/R1,b=RT/R2,wehaveAtα=0.25+0.15theoutputshouldbeva=(0.25+0.05)Vr,andatα=0.25-0.15,va=(0.25-0.05)Vr.ThenwehavebaaaVvra1111.1117.49251092.06251063.0babaabaa3.2VOLTAGEDIVIDERAnothermethodtoreducenonlinearityerrorduetoloadingeffectistouseasymmetricalpowersupplyconnectedasshowninFigure.kVvrm11123.2VOLTAGEDIVIDERForremotepotentiometers,three-wirecircuitsyieldzeroandsensitivityerrors.31331310WWTWTrmWWTWrmRRRRRVvRRRRVv3.2VOLTAGEDIVIDERThefour-wirecircuitinFigureavoidstheoffseterror.31100WWTTrmmRRRRVvVv3.2VOLTAGEDIVIDERThepowersupply:•verylowinternalresistance•Verylowtemperaturecoefficient•DCisbestforpotentiometersTheratiobetweentheoutputvoltageandthepowersupplyisinsensitivetopowersupplydrift.1212nrmnVvD3.2VOLTAGEDIVIDER3.2.2ApplicationtoThermistorsAnNTCthermistorcanbemodeledbyatwoparameterequationTheoutputforvoltagedividerinFigureTfReRRTTBT01100RRVRRRVvTrTr103.2VOLTAGEDIVIDERThenwehaveTsfTfRRRRT0TFVTsfVvrr103.2VOLTAGEDIVIDERThesecurvescanbeappliedtolinearizeanNTCthermistorbyashunting(分流)resistorR.IfsischosenforthedesiredtemperaturerangesothatF(T)isapproximatelyastraightline,then1-F(T)willalsobeastraightline.TFRRRRRRRRRTTTp11113.2VOLTAGEDIVIDER3.2.3DynamicMeasurementsIfonlyaccomponentisneeded,wecanjustcoupletheoutputsignaltothemeasuringdevicethroughacapacitorthatformsahigh-p