RTD-Temperature-Sensors热电阻温度传感器英文版

KNE222UniversityofTasmaniaG.VertiganPage12009SchoolOfEngineeringKNE222ElectronicEngineeringOperationalAmplifierApplicationsTheResistiveTemperatureDetector(RTD)Inadditiontothermocouplesformeasuringtemperature,instrumentationengineersfrequentlyuseResistiveTemperatureDetectorsorRTDs.ThesearedeviceswhoseDCresistancevaries(almost)linearlyasafunctionoftemperature.PerhapsthemostcommonoftheseisthePT100,aplatinumbasedsensorwhoseresistanceat0ºCisexactly100Ohms,(seeTable1).Asthesensor’stemperatureincreasessodoesitsresistance,inareasonablylinearmanner.Table1showsthevariationinresistanceofaPT100sensorwithtemperature.Whilethetemperaturecoefficientvariesslightlyoverawiderangeoftemperatures,(typically0.0036to0.0042Ohms/ºC),itcanbeconsideredreasonablyconstantovera50or100ºCrange.Thecommonlyacceptedaveragetemperaturecoefficientis0.00385OhmsperºC.AccordinglythePT100canoftenbeusedwithoutlinearizationoversucharangeprovidedtheappropriatecoefficientisevaluated.Thisdeviceisalsocapableofwithstandingawiderangeoftemperatures,from-200to800ºC,andforsomeapplicationsthevariationsintemperaturecoefficientcanbetolerated.Further,thePT100providesstableandreproducibletemperaturecharacteristics.ForagivenbaseresistanceRo,theresistanceofanRTDatTºCisgivenby:OrooRTRTTTTRTR)())(1()(00…(1)WhereRoisthebaseresistancecorrespondingtoTo,(100Ohmsat0ºC)andisthetemperaturecoefficient,(0.00385OhmsperºC).ThusR(100ºC)=138.5Ohms.Thisapproximationprovidesquiteagoodestimateoftemperatureuptoabout300ºC,asshowninFigure1,thereafterthenonlinearitybecomesevident.Figure1.LinearRTDmodelvs.theactualcharacteristicEquation(1)assumesthatthenonlinearitiesintheRTDcharacteristicarenegligible,iethatthedeviceisentirelylinear,andwhileformanyapplicationsthisapproximationisacceptable,wheremoreprecisionisrequiredanonlinearmodelmustbeused,asoutlinedinEquation(2).))100(1()(32TTCBTATRTRo…(2)Where:A=3.908E-3,B=-5.775E-7andC=-4.183E-12forT0andC=0forT0.KNE222UniversityofTasmaniaG.VertiganPage22009TemperatureinformationcanbeobtainedfromanRTDbymeasuringitsresistance;eitherbyapplyingaknowncurrentandmeasuringtheresultingvoltageorviceversa.CaremusebetakenwhenpassingacurrentthroughanRTDasinternalI2Rheatingwillalsoaffectthedevice’sresistance.ThedegreetowhichthisoccursdependsonthephysicalsizeoftheRTDinquestion,andthereforehowmuchheatitcandissipatebeforeitstemperaturerisessignificantlyaboveambient.Forsmalldevicessensecurrentsmustbekeptquitelow,typicallylessthan3mA.Asmall(thickfilm)PT100deviceappearsinfigure2.Figure2.AThickFilmPT100TemperatureSensorConstructionFigure3.SamplePT100probesRTDsgenerallyhaveasmallthermalmassandthereforecanexhibitafastresponsetorapidchangesintemperature.Thiscanbeusefulinprocesscontrolapplications.InformationCodingTechniques.InstrumentationapplicationsfrequentlyuseProgrammableLogicControllers(PLCs)tostoreandprocessdata,andthereforetheanalogueoutputsignalsofsensingequipmentmustbescaledappropriatelyfortheA-DconverterinputcardofthePLCconcerned.Thisisgenerallyaccomplishedbythesensordrivingcircuitry.Thereareseveralstandardvoltagerangesusedbymanufacturers;theseinclude0to1,0to5and0to10volts,eachcorrespondingtothedesiredrangeoftemperaturesdetectedbytheRTD.Inadditiontothevoltagesourcebasedsignals,itisalsocommontouseacurrentsourcetocarryencodedanalogueinformation.Thismethodofferssignificantnoiseimmunityovervoltagecarriers,sincebothcommonmodeandnormalmodeinducedvoltagescanbetoleratedwithoutsignificantlycorruptingthecurrentflowing.FourtotwentymAcurrentloopsarefrequentlyusedovermoderatetransmissiondistances,forexamplefromonesideofafactorytotheother,toconveyanalogueinformation.KNE222UniversityofTasmaniaG.VertiganPage32009Thelooptransmitterisgenerallysetupsothatthelowerendoftherequiredtemperaturerangecorrespondsto4mAandtheupperendto20mA.Thusshouldtheloopbecomebroken,resultinginatotallossofcurrent,thefaultcanbereadilydetected.Effectivelytheanaloguesignalisencodedasa0-16mA,currentshiftedfromtheoriginby4mA.Therangeoftemperaturesthatcorrespondtothesecurrents(usuallyknownasthespan)isdeterminedbytheuser,whomustprogramthetransmitteraccordingly.Somelooptransmittersarepoweredbythe4mAcurrentcomponent,whileothersrequireanexternalpowersupply.AnRTDDriveCircuit.TheschematicshowninFigure4isdesignedtointerfaceaPT100toaPLCanalogueinputcard.Itofferstwooutputsignals;a0-5voltvoltagesignalanda4-20mAcurrentsignal.ThecircuitusesaWheatstonebridgearrangementtoderiveapositivevoltage,proportionaltotheincreaseinsensorresistancebeyondthebaseresistanceRo,whichcorrespondstothelowerendofthedesiredtemperaturerange,(inthiscase0ºC).Figure4.ATemperatureMeasuringCircuitforthePT100.ThrRTDisincludedinaWheatstonebridgearrangement(sometimesknownasaquarterbridgeconfiguration),whichoperatesfromasplitpowersupply.Howeverinthiscircuitthevoltagesuppliesarenotquiteequal.Thenegativerailisfixedat0.265voltswhilethepositiverailissetsothatthevoltageonthetopsideoftheRTDiszero,i.e.sothatthebridgeisnulled.Thevoltagerequiredtonullthebridgewillvary,dependingonthetemperatureoftheRTD.Thereforetemperatureinformationisencodedintheposit