2-RESISTIVE-SENSORS-传感与检测技术

2RESISTIVESENSORS2.1POTENTIOMETERSTheresistancebetweenthatcontactandthebottomterminal(接线端)isApotentiometerisazero-ordersystem.AlxAR(2.1)2.1POTENTIOMETERSEquation(2.1)implies(意味)thefollowingsimplifications:1.theresistanceisuniform(相同的)alongthelength.2.theslidingcontactgivesasmoothresistancevariation.For(2.1)tobevalid(正确)1.inductance(电感)andcapacitance(电容)canbeignoredwhensuppliedbyAC2.resistancechangesduetotemperatureareuniform3.Friction(摩擦)andinertia(惯性)ofwiper4.noiseassociationwiththewipercontactlimitsresolution.2.1POTENTIOMETERSAdvantages:•Simple•Robust(结实)•High-levelvoltagewithhighaccuracyrelativetoitscost.2.1POTENTIOMETERSDualpotentiometerswithasinglecontrolstick:moveinfourquadrants(象限)tolocate(定位)apointinaplane.Outputvoltage:2121ryrxVvVv2.1POTENTIOMETERSLiquidPotentiometer–fortilt(倾斜)measurementStructure:ahermetic(密封的),curvedvial(小瓶)partiallyfilledwithaconductive(导电的)liquid,threemetalelectrodes(电极).Range:±0.5°~±60°Resolution:upto10-82.1POTENTIOMETERSRollingBallPotentiometer2.1POTENTIOMETERSLevelSensor2.1POTENTIOMETERSHydrocarbon(碳氢化合物)Sensor2.2STRAINGAGES2.2.1Fundamentals:PiezoresistiveEffectBasedonthevariationofresistanceofaconductororsemiconductorwhensubjectedto(受到)amechanicalstress.Whenthewireisstressedlongitudinally(纵向的)AlRAdAldldRdR2.2STRAINGAGESThechangeinlengthisgivenbyHooke’slaw:ldlEEAF2.2STRAINGAGESForawirethatinadditiontoalengthlhasatransverse(横断的)dimensiont,alongitudinalstresschangesbothlandt.ByPoisson’slaw,wehaveldltdt2.2STRAINGAGESForawireofcircularcrosssectionofdiameterD,wehavePiezoresistiveeffect:thechangeinresistivityasaresultofamechanicalstress.ldlDdDAdADA22422.2STRAINGAGESP.W.Bridgmanshowedthatthepercentofresistivityandvolumeareproportionalformetal:WhereCistheBridgmanconstant.Forusualalloys,1.13C1.15Byapplied(2.10)VdVCd2242lldlDdDldlVdVlDV2.2STRAINGAGESIfthematerialisisotropic(各向同性的),wehaveWhereGisthegagefactor.Forsmallvariations,theresistanceofthemetallicwireisGldlGCldlRdR2121xRGRRdRRdRRR111000002.2STRAINGAGESExample2.1From(2.15)Fromgeometry(几何学),theareasupportingtheforceisWehaveEAFRGRGR22219145.25.974mmmmmmdDA52.0731019198001.235026GPamNR2.2STRAINGAGESUnlikemetal,whensemiconductorisstressed,inadditiontoitsdimensionalchange,boththenumberofcarriers(载流子)andtheiraveragemobility(迁移率)change.Forsimpletension(拉力)orcompression(压力),ifelectronsflowalongthestressaxis,therelativechangeinresistivityisproportionaltotheappliedstressL02.2STRAINGAGESTheresultinggagefactorisbetween40and200.Thechangeintheelectricresistanceofametalorasemiconductorisrelatedtoitsstrain(应变).0RRG2.2STRAINGAGESThereareseverallimitationsforobtainingvalidinformation:•Theappliedstressshouldnotexceed(超过)theelasticlimitofthegage.•Themeasurementwillbecorrectonlyifallthestrainistransmitted(传输)tothegage.•Temperaturewillinferthemeasurement.Temperaturecompensation:Usedummygage2.2STRAINGAGES2.2STRAINGAGESThecurrentpassingthroughgagecausesheating.Thermoelectromotion(热电运动)isalsoaninterferencefactor,whichappearsatthejunction(连接点)betweendissimilarmetals.Byreversingthesupplypolaritydetectthermoelectricvoltagebecausetheirpolaritydoesnotchange.2.2STRAINGAGES2.2.2TypesandApplicationsStraingagesaremadeofdifferentmetalsandsemiconductors.Allhavelowtemperaturecoefficients.Constantan(康铜)isthemostcommongagealloy(合金).Karma(卡玛合金)issuitableforstaticmeasurement.Isoelastichasarelativelyhightemperaturecoefficient,suitablefordynamicmeasurement.Platinum-tungsten(铂—钨)gageshaveG=4.5andwidertemperaturerange.2.2STRAINGAGESConductiveelastomer(人造橡胶)gage:tactile(触觉的)sensorsinrobots(机器人).Liquidstraingages:largestraininbiologicaltissue.2.2STRAINGAGESThegagefactorisdeterminedbysamplingbecausestraingagescannotbereused.Straingagescanmeasureanyquantitythatbytheuseofanappropriate(合适的)primarysensorweconvertintoaforcecapableofproducingdeformations(变形)of10umandevensmall.2.3RESISTIVETEMPERATUREDETECTORS(RTDS)AnRTDisatemperaturedetectorbaseduponavariationinelectricresistance.00020201011TTRRTTTTTTRRnn(2.20)2.3RESISTIVETEMPERATUREDETECTORS(RTDS)Dynamically,anRTDbehavesasafirst-orderlow-passsystem.Acoveredsensorhasasecond-orderlow-passoverdampedresponse.Restrictions:ItisnotpossibletomeasuretemperaturenearthemeltingpointofconductorMustavoidanyself-heatingduetothemeasurementcircuitMechanicalstrainTemperaturegradient(梯度)2.3RESISTIVETEMPERATUREDETECTORS(RTDS)Example2.2ThetemperatureincrementaboveambienttemperaturewhendissipatingapowerPDwillbeHence,themaximalcurrentforagiventemperatureincrementisWhenthesensorisimmersedinair,weobtainRIPTD2RTImAKWCI4.210006.01.02.3RESISTIVETEMPERATUREDETECTORS(RTDS)Whenthesensorisimmersedinwater,wehavemAKWCI101001.01.02.3RESISTIVETEMPERATUREDETECTORS(RTDS)Toevaluatethepossibilityoftemperaturegradients,useBiot’smodulus(模数)WherehistheheattransmissioncoefficientlistheminordimensionofthemeasuredobjectkisitsthermalconductivityRules:if,temperaturegradientisprobable.if,temperaturegr