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外文翻译--TT302温度变送器.doc

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外文翻译--TT302温度变送器.doc

附录四外文翻译TT302FieldbusTemperatureTransmitterOperationTheTT302acceptssignalsfrommVgeneratorssuchasthermocouplesorresistivesensorssuchasRTDs.Thecriterionisthatthesignaliswithintherangeoftheinput.FormV,therangeis50to500mVandforresistance,02000Ohm.FunctionalDescription–HardwareThefunctionofeachblockisdescribedbelow.Figure2.1TT302BlockDiagramMUXMultiplexerTheMUXmultiplexesthesensorterminalstothesignalconditioningsectionensuringthatthevoltagesaremeasuredbetweenthecorrectterminals.SignalConditionerItsfunctionistoapplythecorrectgaintotheinputsignalstomakethemsuittheA/Dconverter.A/DConverterTheA/DconvertstheinputsignaltoadigitalformatfortheCPU.SignalIsolationItsfunctionistoisolatethecontrolanddatasignalbetweentheinputandtheCPU.CPUCentralProcessingUnit,RAM,PROMandEEPROMTheCPUistheintelligentportionofthetransmitter,beingresponsibleforthemanagementandoperationofmeasurement,blockexecution,selfdiagnosticsandcommunication.TheprogramisstoredinaPROM.FortemporarystorageofdatathereisaRAM.ThedataintheRAMislostifthepowerisswitchedoff.HoweverthereisanonvolatileEEPROMwheredatathatmustberetainedisstored.Examples,ofsuchdataaretrim,calibration,blockconfigurationandidentificationdata.TT302FieldbusTemperatureTransmitterCommunicationControllerItmonitorslineactivity,modulatesanddemodulatescommunicationsignalsandinsertsanddeletesstartandenddelimiters.PowerSupplyTakespowerofthelooplinetopowerthetransmittercircuitry.PowerIsolationJustlikethesignalstoandfromtheinputsection,thepowertotheinputsectionmustbeisolated.IsolationisachievedbyconvertingtheDCsupplyintoahighfrequencyACsupplyandgalvanicallyseparatingitusingatransformer.DisplayControllerReceivesdatafromtheCPUinformingwhichsegmentsoftheLiquidCrystalDisplay,shouldbeturnedon.LocalAdjustmentTherearetwoswitchesthataremagneticallyactivated.Theycanbeactivatedbythemagnetictoolwithoutmechanicalorelectricalcontact.Figure2.2LCDIndicatorTemperatureSensorsTheTT302,aspreviouslyexplained,acceptsseveraltypesofsensors.TheTT302isspeciallydesignedfortemperaturemeasurementusingthermocouplesorResistiveTemperatureDetectorsRTDs.Somebasicconceptsaboutthesesensorsarepresentedbelow.ThermocouplesThermocouplesareconstructedwithtwowiresmadefromdifferentmetalsoralloysjoinedatoneend,calledmeasuringjunctionorhotjunction.Themeasuringjunctionshouldbeplacedatthepointofmeasurement.Theotherendofthethermocoupleisopenandconnectedtothetemperaturetransmitter.Thispointiscalledreferencejunctionorcoldjunction.Formostapplications,theSeebeckeffectissufficienttoexplainthermocouplebehaviorasfollowingHowtheThermocoupleWorksSeebeckEffectWhenthereisatemperaturedifferencealongametalwire,asmallelectricpotential,uniquetoeveryalloy,willoccur.ThisphenomenoniscalledSeebeckeffect.Whentwowiresofdissimilarmetalsarejoinedatoneend,andleftopenattheother,atemperaturedifferencebetweenthetwoendswillresultinavoltagesincethepotentialsgeneratedbythedissimilarmaterialsaredifferentanddonotcanceleachotherout.Now,twoimportantthingsmustbenoted.Firstthevoltagegeneratedbythethermocoupleisproportionaltothedifferencebetweenthemeasuringjunctionandthecoldjunctiontemperatures.Thereforethetemperatureatthereferencejunctionmustbeaddedtothetemperaturederivedfromthethermocoupleoutput,inordertofindthetemperaturemeasured.Thisiscalledcoldjunctioncompensation,andisdoneautomaticallybytheTT302,whichhasatemperaturesensoratthesensorterminalsforthispurpose.Secondly,ifthethermocouplewiresarenotused,allthewaytotheterminalsofthetransmittere.g.,copperwireisusedfromsensorheadormarshalingboxwillformnewjunctionswithadditionalSeebeckeffects.Itwillbecreatedandruinthemeasurementinmostcases,sincethecoldjunctioncompensationwillbedoneatthewrongpoint.NOTETherelationbetweenthemeasuringjunctiontemperatureandthegeneratedmilivoltageistabulatedinthermocouplecalibrationtablesforstandardizedthermocoupletypes,thereferencetemperaturebeing0oC.Standardizedthermocouplesthatarecommerciallyused,whosetablesarestoredinthememoryoftheTT302,arethefollowing.NBSB,E,J,K,N,R,ST.DINLUResistiveTemperatureDetectorsRTDsResistanceTemperatureDetectors,mostcommonlyknownasRTDs,arebasedontheprinciplethattheresistanceofmetalincreasesasitstemperatureincreases.StandardizedRTDs,whosetablesarestoredinthememoryoftheTT302,arethefollowing.JIS160481Pt50Pt100.IEC,DIN,JIS160489Pt50,Pt100Pt500..GECu10..DINNi120ForcorrectmeasurementofRTDtemperature,itisnecessarytoeliminatetheeffectoftheresistanceofthewiresconnectingthesensortothemeasuringcircuit.Insomeindustrialapplications,thesewiresmaybehundredsofmeterslong.Thisisparticularlyimportantatlocationswheretheambienttemperaturechangesconstantly.TheTT302permitsa2wireconnectionthatmaycausemeasuringerrors,dependingonthelengthofconnectionwiresandonthetemperaturetowhichtheyareexposed.SeeFigure2.3TwoWireConnection.Ina2wireconnection,thevoltageV2isproportionaltotheRTDresistanceplustheresistanceofthewires.

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