文献翻译——热电偶冷端补偿

第1页外文文献资料ImplementingCold-JunctionCompensationinThermocouple1.ApplicationsAbstract:Thermocouplesareoneofthemostwidelyusedtemperature-measurementdevicesbecauseoftheirruggedness,repeatability,andfastresponsetime.Thisapplicationnotediscussesthebasicoperationofathermocouple,whichincludesthedefinitionandfunctionofareference(cold)junction.Thenotealsogivesguidelinesforselectingadeviceforcold-junctiontemperaturemeasurementbasedonapplicationneeds.Threecircuitexamplesareshown.2.IntroductionOfthemanytransducersavailablefortemperature-measurementapplications,thermocouplesareamongthemostcommon.Thermocouplesarefoundineverydaysystemssuchasautomobilesandhomeappliances.Theyofferacost-effectivemeansformeasuringamuchwiderrangeoftemperaturesthanothercommonsolutionslikeresistancetemperaturedevices(RTDs),thermistors,andtemperature-sensingintegratedcircuits(ICs).Inaddition,ruggedness,repeatability,andfastresponsetimemakethermocouplesapopularchoiceinmanyenvironments.Thereare,however,somedisadvantagestousingthermocouples,notablyitslackoflinearity.AlthoughthermocouplescanbeusedoverawiderrangeoftemperaturesthanRTDsandtemperature-sensingICs,theyarefarlesslinear.Also,RTDsandtemperature-sensingICstypicallyofferbettersensitivityandaccuracy,twodesirablecharacteristicsformorepreciseapplications.Thermocouplesignalsareverylow-levelandoftenrequireamplificationorahigh-resolutiondataconvertertoprocessthesignals.Despitetheabovedisadvantages,overallcost,easeofuse,andwide第2页temperaturerangestillmakethermocouplespopular.3.ThermocoupleBasicsThermocouplesaredifferentialtemperature-measurementdevices.Theyareconstructedwithtwowiresmadefromdissimilarmetals.Onewireispredesignatedasthepositiveside,andtheotherasthenegative.Table1liststhefourmostcommonthermocoupletypes,themetalsoralloysused,andthetemperaturerangeallowedforeachtype.Eachthermocoupletypeoffersauniquethermoelectriccharacteristicoveritsspecifiedtemperaturerange.Table1.BasicThermocoupleCharacteristicsTypePositiveMetal/AlloyNegativeMetal/AlloyTemperatureRange(C)TCopperConstantan-200to+350JIronConstantan0to+750KChromelAlumel-200to+1250EChromelConstantan-200to+900Ifthetwodifferentmetalsarejoined(i.e.,weldedorsoldered)toformtwojunctions,asinFigure1a,thevoltagegeneratedbytheloopisafunctionofthetemperaturedifferencebetweenthetwojunctions.ThisphenomenonisknownastheSeebeckeffect,generallydescribedastheprocessinwhichthermalenergyisconvertedintoelectricalenergy.TheSeebeckeffectistheoppositeofthePeltiereffect,inwhichelectricalenergyisconvertedintothermalenergy,asobservedinapplicationssuchasthermoelectriccoolers.Figure1ashowsthatthemeasuredoutputvoltage,VOUT,isthedifferencebetweenthemeasuring(hot)junctionvoltageandthereference(cold)junctionvoltage.SinceVHandVCaregeneratedbyatemperaturedifferencebetweenthetwojunctions,VOUTisalsoafunctionof第3页thistemperaturedifference.Thescalefactor,whichrelatesthevoltagedifferencetothetemperaturedifference,isknownastheSeebeckcoefficient.Figure1a.TheloopvoltagegeneratedbythetemperaturedifferencebetweentwojunctionsinthethermocoupleistheresultoftheSeebeckeffect.Figure1b.Themostcommonthermocoupleconfigurationhasthetwowiresofathermocouplejoinedatoneend.Theopenendofeachwireisconnectedtoanisothermicconnectormadeofcopper.Figure1billustratestheconfigurationmostcommonlyusedinthermocoupleapplications.Thisconfigurationintroducesathirdmetal(alsoknownasanintermediatemetal)intotheloopandtwoadditionaljunctions.Inthisexample,theopenendsofeachwireareelectricallyconnectedtowiresortracesmadeofcopper.Theseconnectionsintroducetwoadditionaljunctionsintothesystem.Aslongasthesetwojunctionsareatthesametemperature,theintermediatemetal(copper)hasnoeffectontheoutputvoltage.Thisconfigurationallowsthethermocoupletobeusedwithoutaseparatereferencejunction.VOUTisstillafunctionofthedifferencebetweenhot-andcold-第4页junctiontemperatures,relatedbytheSeebeckcoefficient.However,sincethethermocouplemeasurestemperaturedifferentially,thecold-junctiontemperaturemustbeknowninordertodeterminetheactualtemperaturemeasuredatthehotjunction.Thesimplestcaseoccurswhenthecoldjunctionisat0C,alsoknownasanice-bathreference.IfTC=0C,thenVOUT=VH.Inthiscase,thevoltagemeasuredatthehotjunctionisadirecttranslationoftheactualtemperatureatthatjunction.TheNationalBureauofStandards(NBS)provideslookuptablescontainingthecharacterizationdataofthermocouplevoltagesvs.temperaturesforvarioustypesofthermocouples.Thedataareallbasedonacold-junctiontemperatureof0C.Withanice-bathreference,youcandeterminethetemperatureofthehotjunctionbylookingforVHintheappropriatetable.Intheearlydaysofthermocouples,theice-bathreferenceservedasthestandardinthermocoupleapplications.Implementinganicebathtodayisimpracticalinmostsituations.Therefore,whenthecoldjunctionisnotat0C,thetemperatureofthisjunctionmustbeknowninordertodeterminetheactualhot-junctiontemperature.Theoutputvoltageofthethermocouplemustalsobecompensatedtoaccountforthevoltagecreatedbythenonzerocold-junctiontemperature.Thisprocessisknownascold-junctioncompensation.4.SelectingaDeviceforCold-JunctionTemperatureMeasurementAsexplainedabove,toimplementcold-junctioncompensation,thetemperatureofthecoldjunctionmustbedetermined.Thiscalculationcanbeaccomplishedwithanytypeoftemperature-measurementdevice.Amongthemorepopulardevicesaretemperature-sensingICs,thermistors,andRTDs.Eachfamilyofdevicesoffersadvantagesanddisadvantagesovertheothers,sotherequirementsofaspecificapplicationwilldeterminewhichtypetouse.Forapplicationsrequiringextremeaccuracy,acalibratedplatinumRTDoffersthebestperformanceacrossthewidesttemperaturerange.This第5页performancecomes,however,withhighcost.Thermistorsandsilicontemperature-sensingICsarecost-effectivealternativestoRTDsforapplicationsthatdonotrequiresuchhighaccuracy.ThermistorsoperateacrossawidertemperaturerangethansiliconICs.Nonetheless,temperature-sensingICsareoftenpreferredoverthermistorsbecauseoftheirlinearity;correctingathermistorsnonlinearitycanrequiretoomuchworkfromthesystemsmicrocontroller.Thetemperature-sensingICsprovideexcellentlinearitybutoperateacrossanarrowertemperaturerange.Insummary,acold-junctiontemperature-measurementdevicemustbeselectedtomatchtherequirementsofthesystem.Aswithanytemperature-measurementapplication,accuracy,temperaturerange,cost,andlinearityareallsignificantconsiderationsintheselectionprocess.Eachrequirementmustbeweighedcarefullyinordertoselecttheoptimalcombinationofcostandperformance.5.CrunchingtheNumbersOnceyouestablishamethodofcold-junctioncompensation,thecompensatedoutputvoltagemustbetranslatedintothecorrespondingtemperature.AsimpletranslationmethodusesthelookuptablesfromtheNBS.Implementinglookuptablesinsoftwarerequiresmemoryforstorage,butthetablesprovideaquick,accuratesolutionwhenmeasurementsmustberepeatedcontinuously.Twoothermethodsfortranslatingthermocouplevoltagestotemperaturerequiresomewhatmoreworkthanlookuptables:1)linearapproximationusingpolynomialcoefficients;and2)analoglinearizationofthethermocoupleoutputsignal.Softwarelinearapproximationispopularbecausenostorageisnecessaryexceptforthepredeterminedpolynomialcoefficients.Thereis,however,adisadvantagetothismethod:theprocessingtimeassociatedwithsolvingmultiple-orderpolynomials.Theprocessingtimeincreasesforhigherorderpolynomials,whicharetypicallyrequiredwhendealingwithawiderrangeoftemperatures.Fortemperatureswherehigherorderpolynomialsarerequired,第6页lookuptablescanprovebothmoreaccurateandmoreefficientthanlinearapproximation.Beforethedaysofmodernsoftware,analoglinearizationwascommonlyusedtoconvertthemeasuredvoltagetoatemperature(inadditiontomanuallysearchingthroughlookuptables).Thishardware-basedmethodusesanalogcircuitrytocorrectforthenonlinearityinathermocouplesresponse.Itsaccuracydependsontheorderofthecorrectionapproximationused.Thisapproachisstillcommonlyusedinmultimetersthatacceptthermocouplesignals.6.ApplicationCircuitsThefollowingexamplesshowthreemethodsofcold-junctioncompensationthatusesilicontemperature-sensingICs.Thethreecircuitsfocusonsimplesolutionsforapplicationsrequiringonlyanarrowcold-junctiontemperaturerange(0Cto+70Cand-40Cto+85C)andaccuracytowithinafewdegrees.Circuit1includesalocaltemperature-sensingICnearthecoldjunctiontodetermineitstemperature.Circuit2includesaremotediodetemperaturesensorthatisfedbyadiode-connectedtransistormounteddirectlyonthethermocoupleconnector.Circuit3includesananalog-to-digitalconverter(ADC)withon-chipcold-junctioncompensation.AllthreeexamplesusetheK-typethermocouple,whichisconstructedwithchromelandalumel.6.1.Example#1InthecircuitshowninFigure2,a16-bitsigma-deltaADCconvertsthelow-levelthermocouplevoltageintoa16-bitserialdigitaloutput.Anintegratedprogrammable-gainamplifierincreasestheADCsresolution,whichisoftenneededwhenworkingwithlow-levelthermocouplesignals.Atemperature-sensingIC,locatedincloseproximitytothethermocoupleconnector,measuresthetemperatureinthevicinityofthecoldjunction.ThismethodisbasedontheassumptionthatthetemperatureattheICis第7页approximatelythesameasthetemperatureatthecoldjunction.Theoutputvoltagefromthecold-junctiontemperaturesensorisconvertedbychannel2oftheADC.Thetemperaturesensorson-chip2.56VreferenceeliminatestheneedforaseparatereferenceIC.Figure2.Alocaltemperature-sensingIC(MAX6610)determinesthecold-junctiontemperature.Thetemperature-sensingICislocatednearthethermocoupleconnector(coldjunction).Theoutputvoltagesforthethermocoupleandthecold-junctiontemperaturesensorareconvertedbya16-bitADC(MX7705).Whenoperatedinbipolarmode,theADCcanconvertpositiveandnegativelevelsofthethermocouplevoltagepresentatchannel1.Channel2oftheADCconvertsthesingle-endedvoltageoutputoftheMAX6610intoadigitalsignalforthemicro-controllertoprocess.Thetemperature-sensingICsvoltageoutputisproportionaltothemeasuredcold-junctiontemperature.Todeterminetheactualhot-junctiontemperature,youmustfirstdeterminethecold-junctiontemperature.ThenusethelookuptablesprovidedbytheNBSforK-typethermocouplestotranslatethecold-junctiontemperatureintoitscorrespondingthermoelectricvoltage.AftercorrectingforthePGAsgain,addthisvoltagetothedigitizedthermocouplereading.Thentranslatethesumintoatemperature,againusingthelookuptables.Theresultistheactualtemperatureatthehot-junction.Table2displaysmeasurementstakenwhilesweepingthecold第8页junctionfrom-40Cto+85Cinanovenandkeepingthehotjunctionat+100Cinaseparateoven.Theaccuracyofthemeasurementsdependsgreatlyontheaccuracyofthelocaltemperature-sensingICandtheoventemperature.Table2.SampleMeasurementsfortheFigure2CircuitwiththeColdJunctionandHotJunctioninSeparateOvensCold-JunctionTemperature(C)MeasuredHot-JunctionTemperature*(C)Measurement#1-39.9+101.4Measurement#20.0+101.5Measurement#3+25.2+100.2Measurement#4+85.0+99.0*ThevaluesappearinginthecolumnlabeledMeasuredHot-JunctionTemperaturearethecompensated,hot-junctiontemperaturemeasurementstakenfromthecircuit.6.2.Example#2InFigure3,aremote-diodetemperature-sensingICmeasuresthecircuitscold-junctiontemperature.Unlikealocaltemperature-sensingIC,theremote-diodetemperaturesensorneednotbenearthecoldjunctionsinceitusesanexternaldiode-connectedNPNtransistortomeasurethecold-junctiontemperature.Thistransistorismounteddirectlyonthethermocoupleconnectorscopperretainerclip.Thetemperature-sensingICconvertsthesignalfromthisdiode-connectedtransistorintoadigitaloutput.TheADCschannel1convertsthethermocouplevoltageintoadigitaloutput.Channel2oftheADCisunusedandconnectedtoground.TheADCsreferenceinputisfedbyastable2.5VreferenceIC.第9页Figure3.Aremote-diodetemperature-sensorICneednotbenearthecoldjunction,sinceitusesanexternaldiodetosensethetemperature.Thisdiodecanbemounteddirectlyontheoptionalretainerclipofthethermocoupleconnector.TheMAX6002providesastable2.5VreferencevoltagefortheADC.Table3displaysthemeasurementstakenwiththecold-junctiontemperaturesweptfrom-40Cto+85Cwhilekeepingthehot-junctiontemperatureat+100C.Theaccuracyofthemeasurementsdependsontheaccuracyofboththeremote-diodetemperature-sensingICandtheoventemperature.Table3.SampleMeasurementsfortheFigure3CircuitwiththeColdJunctionandHotJunctioninSeparateOvens.Cold-JunctionTemperature(C)MeasuredHot-JunctionTemperature*(C)Measurement#1-39.8+99.1Measurement#2-0.3+98.4Measurement#3+25.0+99.7Measurement#4+85.1+101.5第10页*ThevaluesappearinginthecolumnlabeledMeasuredHot-JunctionTemperaturearethecompensated,hot-junctiontemperaturemeasurementstakenfromthecircuit.6.3.Example#3Figure4includesanICthatcombinesa12-bitADCwithatemperature-sensingdiode.Thetemperature-sensingdiodeconvertstheambienttemperatureintoavoltage.TheICtakesthisvoltageandthethermocouplevoltageandcalculatesthecompensatedhot-junctiontemperature.Thedigitaloutputisthecompensatedhot-junctiontemperaturemeasuredbythethermocouple.Theguaranteedtemperatureerrorofthisdeviceiswithin9LSBsforhot-junctiontemperaturesbetween0Cto+700C.Althoughthisdevicecanmeasureawiderangeofhot-junctiontemperatures,itcannotmeasuretemperaturesbelow0C.Figure4.AnADCwithintegratedcold-junctioncompensationconvertsthethermocouplevoltagewithouttheneedforexternalcompensation.Table4displaysmeasurementstakenfromthecircuitofFigure4withthecold-junctiontemperaturesweptfrom0Cto+70Cwhilekeepingthehotjunctionat+100C.Table4.SampleMeasurementsfortheFigure4CircuitwiththeColdJunctionandHotJunctioninSeparateOvensCold-JunctionTemperature(C)MeasuredHot-JunctionTemperature*(C)Measurement#10.0+100.25Measurement#2+25.2+100.25第11页Measurement#3+50.1+101.0Measurement#4+70.0+101.25*ThevaluesappearinginthecolumnlabeledMeasuredHot-JunctionTemperaturearethedecimalrepresentationofthedigitaloutputsprovidedbythecircuit.7.ConclusionWhenworkingwiththermocouples,youmustestablishareferencepointbecausethermocouplesaredifferentialtemperature-measurementdevices.Athermocoupleprovidesavoltagethatrepresentsthetemperaturedifferencebetweenthehotandcoldjunctions.Ifyouknowboththetemperatureofthecoldjunctionandthetemperatureofthehotjunctionrelativetothecold-junctiontemperature,youcandeterminetheactualhot-junctiontemperature.Themainselectioncriteriafortheappropriatecold-junctioncompensationdeviceareaccuracy,cost,linearity,andtemperaturerange.SomeplatinumRTDsofferthebestaccuracy,butathighcost.Thermistorsareinexpensiveandoperateoverawidetemperaturerange,buttheirlackoflinearitycanbeproblematic.Silicontemperature-sensingICsoperateoveranarrowtemperaturerange,butofferreasonableaccuracy,linearity,andlowcost,thusmakingthemasuitablechoiceformanythermocouplecold-junctioncompensationapplications.第12页中文翻译稿热电偶冷端补偿1.应用摘要：温度测量应用中，热电偶因其坚固性、可靠性以及较快的响应速度得到了普遍应用。本应用笔记讨论了热电偶的基本工作原理，包括参考端(冷端)的定义和功能。本文还给出了按照具体应用选择冷端温度测量器件的注意事项，并给出了三个设计范例。2.概述温度测量应用中有多种类型的变送器，热电偶是最常用的一种，可广泛用于汽车、家庭等领域。与RTD、电热调节器、温度检测集成电路(IC)相比，热电偶能够检测更宽的温度范围，具有较高的性价比。另外，热电偶的牢固、可靠性和快速响应时间使其成为各种工作环境下的首要选择。当然，热电偶在温度测量中也存在一些缺陷，例如，线性特性较差。虽然它们与RTD、温度传感器IC相比可以测量更宽的温度范围，但线性度却大打折扣。除此之外，RTD和温度传感器IC可以提供更高的灵敏度和精度，可理想用于精确测量系统。热电偶信号电平很低，常常需要放大或高分辨率数据转换器进行处理。如果排除上述问题，热电偶的低价位、易使用、宽温度范围使其得到广泛使用。3.热电偶基础热电偶是差分温度测量器件，由两段不同的金属/合金线构成，一段用作正端，另一段用作负端。表1列出了四种最常用的热电偶类型、所用金属以及对应的温度测量范围。每种热电偶在其规定的温度范围内具有独特的热电特性。第13页表1.常用的热电偶类型正型金属/合金负极金属/合金温度范围(C)T铜康铜-200to+350J铁康铜0to+750K铬铝镍合金-200to+1250E铬康铜-200to+900两种不同类型的金属接(焊接)在一起后形成两个结点，如图1a所示，环路电压是两个结点温差的函数。这种现象称为Seebeck效应，用于解释热能转换为电能的过程。Seebeck效应相对于Peltier效应，Peltier效应用于解释电能转换成热能的过程，典型应用有电热致冷器。图1a所示，测量电压VOUT是检测端(热端)结电压与参考端(冷端)结电压之差。因为VH和VC是由两个结的温度差产生的，VOUT也是温差的函数。定标因数，对应于电压差与温差之比，称为Seebeck系数。图1a.环路电压由热电偶两个结点之间的温差产生，是Seebeck效应的结果。第14页图1b.常见的热电偶配置由两条线连接在一端，每条线的开路端与铜恒温线连接。图1b所示是一种最常见的热电偶应用。该配置中引入了第三种金属(中间金属)和两个额外的节点。本例中，每个开路端与铜线电气连接，这些连线为系统增加了两个额外节点，只要这两个节点温度相同，中间金属(铜)不会影响输出电压。这种配置允许热电偶在没有独立参考结点的条件下使用。VOUT仍然是热端与冷端温度之差的函数，与Seebeck系数有关。然而，由于热电偶测量的是温度差，为了确定热端的实际温度，冷端温度必须是已知的。冷端温度为0C(冰点)时是一种最简单的情况，如果TC=0C，则VOUT=VH。这种情况下，热端测量电压是结点温度的直接转换值。美国国家标准局(NBS)提供了各种类型热电偶的电压特征数据与温度对应关系的查找表。所有数据均基于0C冷端温度。利用冰点作为参考点，通过查找适当表格中的VH可以确定热端温度。在热电偶应用初期，冰点被当作热电偶的标准参考点，但在大多数应用中获得一个冰点参考温度不太现实。如果冷端温度不是0C，那么，为了确定实际热端温度必须已知冷端温度。考虑到非零冷端温度的电压，必需对热电偶输出电压进行补偿，既所谓的冷端补偿。4.选择冷端温度测量器件如上所述，为了实现冷端补偿，必须确定冷端温度，这可以通过任何类型的温度检测器件实现。在通用的温度传感器IC、电热调节器和RTD中，不同类型的器件具有不同的优、缺点，需根据具体应用进行选择。对于精度要求非常高的器件，经过校准的铂RTD能够在很宽的温度范围内保持较高精度，但其成本很高。精度要求不是很高时，热敏电阻和硅温度传感器IC能够提供较高的性价比，热敏电阻比硅IC具有更宽的测温范围，而传感器IC具有更高的线性度，因而性能指标更好一第15页些。修正热敏电阻的非线性会占用较多的微控制器资源。温度传感器IC具有出色的线性度，但测温范围很窄。总之，必需根据系统的实际需求选择冷端温度测量器件，需要仔细考虑精度、温度范围、成本和线性指标，以便得到最佳的性价比。5.考虑因素一旦建立了冷端补偿方法，补偿输出电压必须转换成相应的温度。一种简单的方法既是使用NBS提供的查找表，用软件实现查找表需要存储器，但查找表对于连续的重复查询提供了一种快速、精确的测量方案。将热电偶电压转换成温度值的另外两种方案比查找表复杂一些，这两种方法是：1)利用多项式系数进行线性逼近，2)对热电偶输出信号进行模拟线性化处理。软件线性逼近只是需要预先确定多项式系数，不需要存储，因而是一种更通用的方案。缺点是需要较长时间解多阶多项式，多项式阶数越高，处理时间越长，特别是在温度范围较宽的情况下。多项式阶数较高时，查找表相对提供了一种精度更高、更有效温度测量方案。出现软件测试方案之前，模拟线性化常被用来将测量电压转换成温度值(除了人工查找表检索外)。这种基于硬件的方法利用模拟电路修正热电偶响应的非线性。其精度取决于修正逼近多项式的阶数，在目前能够测试热电偶信号的万用表中仍采用这种方法。6.电路中的应用下面讨论了三种利用硅传感器IC进行冷端补偿的典型应用，三个电路均