Development-of-High-precision-Temperature-Measurem

TheNinthInternationalConferenceonElectronicMeasurement&InstrumentsICEMI’2009DevelopmentofHigh-precisionTemperatureMeasurementSystemBasedonARMYiXianjun,LiuCuimeiSchoolofElectricalandInformationEngineering,WuhanInstituteofTechnology693XiongchuAvenue,Wuhan,430073P.R.ChinaEmail:[e-mail]–Thehigh-precisiontemperaturemeasurementsystemwhichadoptsARMprocessorLM3S1138ascontrollingcorewasintroducedinthispaper.Inthetemperaturemeasuringcircuits,thecurrentdrivingcircuitofsensorPt100AandA/Dconvertingcircuitadoptthesamereferencevoltage,whichmakesA/DconversionresultsareonlydependentonthechangingvalueofplatinumresistancewiththetemperaturechangeTR,andwillnotbeaffectedbythestabilityofsensorconstantcurrentdrivecurrentandthereferencevoltageaccuracyofA/Dconverter,thedesignmethodhasimprovedthesamplingaccuracyofthesystemhardwareeffectively.Inthesoftwarecalibrationofmeasurementerror,LeastSquaresMethodwasusedtofitthesamplingvalue-temperaturecurveinconnectionwiththemeasureddataofvarioustemperaturesections,itoffsetsthedeviationwhichoccurredinvariousintermediatelinksofthesignalfromplatinumresistancetotheA/DconversioninthetemperaturemeasurementofPt100A.Inthetemperaturemeasurementprocess,ARMprocessordoreal-timecomputingtogetfinaltemperaturemeasurementresultsaccordingtotheA/DsamplevalueandcombinedwiththepolynomialofSub-fitting.Actualmeasurementresultsshowthat,thesystemtemperaturedeviationislessthan0.05ć.Keywords–ARM,high-precisionmeasurement,temperaturedetectioncircuit,leastsquaresmethod,curvefitting.I.INTRODUCTIONTemperatureisoneofveryimportantparametersintheindustrialproduction,theaccuracyoftemperaturedetectionandcontrolhasimpactonproductionstatusandproductsquality.Therefore,inmanyindustrialfields,theyhavehighdemandsfortheaccuracyoftemperaturemeasurementandcontrol.Inthecombined-sodaprocessproductiontechnologyofthechemicalindustry,thetemperaturedifferencebetweentheimportandexportofoutercoolerliquorsisrequiredatafixedvalue,lowerorhigherthanthefixedvaluewillhaveasignificantnegativeimpact.Obviously,thehighertemperaturemeasurementaccuracycanguaranteetemperaturedifferencecontrolaccuracy.Inviewofthissituation,thesystemadoptsARMprocessorascontrollingcore,usesPlatinumresistanceastemperaturecomponents,doesdetectionfortemperaturesignals.AlthoughthePt100hasmanyadvantages,suchashighprecision,highstabilityetc,inthetemperaturemeasurementsystemwhichtakesPt100asdetectioncomponents,ifitsmeasurementaccuracyisneededtoimprovefurthermore,therearesomepracticaldifficultiesexist.First,inregardtothehardwarecircuit,inadditiontobeabletodetectandamplifytheslightchangesofsignals,italsomustbeabletocompensateprecisionerrorwhichwascausedbycircuitssuchaszero-adjustingcircuit,biascircuitEtcandtemperaturedriftduringtheworkofcircuitcomponents,atthesametime,ithasahighrequirementforsignalconditioningcircuitofplatinumresistanceatalaterstage.Ontheotherhand,inthedesignoftemperaturemeasurementsystem,Temperaturecomponentsofcertainspecificationsweregenerallytakenasanobject,butintheVolumeproductionofPlatinumresistancetemperaturemeasurementcomponents,ProductsErrorofthesamespecificationsiscontrolledwithinacertainrange(SuchasA-gradeplatinumresistancePt100AˈResistanceAdmissibledeviationis±0.06at0ć[1]),thedifferencesofvariousdetectioncomponentsatthesamespecificationswasneglected,whichmakestestresultsoftemperaturemeasurementsystembecomeuncertainintheallowabledeviationscopeofnominaldetectioncomponent,thustheincreaseoftemperaturemeasurementaccuracywaslimited.Basedontheabove,afterthecircuitstructureoftemperaturemeasurementsystemwasintroduced,ahardware-signaldetectioncircuitwhichcanmeettheneedsofhigh-precisionmeasurementwasgiven,andthenforaspecificplatinumresistance,usethemeasureddata,combinewithcurvefittinganalysismethod,cangetdttemperaturemeasurementpolynomial(disA/Dsamplevalue,tistemperature)whichwasappliedtotheeachtemperaturesection.Itsfittingmethodwhichtakesd&tasparameterscanoffsetthedeviationwhichoccurredinvariousintermediatelinksofthesignalfromplatinumresistancetotheA/Dconversioninthe1-795_____________________________978-1-4244-3864-8/09/$25.00©2009IEEETheNinthInternationalConferenceonElectronicMeasurement&InstrumentsICEMI’2009temperaturemeasurementofPt100A,makesthesystem’stemperaturemeasurementmoreaccurate.II.SYSTEMHARDWAREDESIGNA.SystemhardwaretheorychartTemperaturemeasurementsystemadoptsLM3S1138ascontrollingcore.LM3S1138isthefirstprocessorbasedonARM®Cortex™-M3kernel,Theprocessorhasa32bitdata-processingcapacity,itsinteriorintegratesprogrammablesystemtimer(SysTick),systemclockcanbefrequencymultiplyingupto200MHz,whichissufficienttomeettheonline(realtime)computingneedsofthetemperatureerrorsoftwarecompensation.Atthesametime,itcontainsawealthofon-chipperipherals:eightgeneralI/Oports(GPIO)PORTA~PORTG,threeasynchronouscommunicationsports(UART0~UART2),twoI2Cbusinterfaces(I2C0,I2C1),32-bittimerandA/Dconverter,etc,whichbringgreatconveniencetoconnectotherexternalcircuitry.ThehardwarestructurechartoftemperaturemeasurementsystemasshowninFig.1.Temperaturemeasurementpartiscomposedby