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附录四:外文翻译TT302—FieldbusTemperatureTransmitterOperationTheTT302acceptssignalsfrommVgeneratorssuchasthermocouplesorresistivesensorssuchasRTDs.Thecriterionisthatthesignaliswithintherangeoftheinput.FormV,therangeis-50to500mVandforresistance,0-2000Ohm.FunctionalDescription–HardwareThefunctionofeachblockisdescribedbelow.Figure2.1—TT302BlockDiagramMUXMultiplexerTheMUXmultiplexesthesensorterminalstothesignalconditioningsectionensuringthatthevoltagesaremeasuredbetweenthecorrectterminals.SignalConditionerItsfunctionistoapplythecorrectgaintotheinputsignalstomakethemsuittheA/D-converter.A/DConverterTheA/DconvertstheinputsignaltoadigitalformatfortheCPU.SignalIsolationItsfunctionistoisolatethecontrolanddatasignalbetweentheinputandtheCPU.(CPU)CentralProcessingUnit,RAM,PROMandEEPROMTheCPUistheintelligentportionofthetransmitter,beingresponsibleforthemanagementandoperationofmeasurement,blockexecution,self-diagnosticsandcommunication.TheprogramisstoredinaPROM.FortemporarystorageofdatathereisaRAM.ThedataintheRAMislostifthepowerisswitchedoff.HoweverthereisanonvolatileEEPROMwheredatathatmustberetainedisstored.Examples,ofsuchdataaretrim,calibration,blockconfigurationandidentificationdata.TT302-FieldbusTemperatureTransmitterCommunicationControllerItmonitorslineactivity,modulatesanddemodulatescommunicationsignalsandinsertsanddeletesstartandenddelimiters.PowerSupplyTakespoweroftheloop-linetopowerthetransmittercircuitry.PowerIsolationJustlikethesignalstoandfromtheinputsection,thepowertotheinputsectionmustbeisolated.IsolationisachievedbyconvertingtheDCsupplyintoahighfrequencyACsupplyandgalvanicallyseparatingitusingatransformer.DisplayControllerReceivesdatafromtheCPUinformingwhichsegmentsoftheLiquidCrystalDisplay,shouldbeturnedon.LocalAdjustmentTherearetwoswitchesthataremagneticallyactivated.Theycanbeactivatedbythemagnetictoolwithoutmechanicalorelectricalcontact.Figure2.2-LCDIndicatorTemperatureSensorsTheTT302,aspreviouslyexplained,acceptsseveraltypesofsensors.TheTT302isspeciallydesignedfortemperaturemeasurementusingthermocouplesorResistiveTemperatureDetectors(RTDs).Somebasicconceptsaboutthesesensorsarepresentedbelow.ThermocouplesThermocouplesareconstructedwithtwowiresmadefromdifferentmetalsoralloysjoinedatoneend,calledmeasuringjunctionor"hotjunction".Themeasuringjunctionshouldbeplacedatthepointofmeasurement.Theotherendofthethermocoupleisopenandconnectedtothetemperaturetransmitter.Thispointiscalledreferencejunctionorcoldjunction.Formostapplications,theSeebeckeffectissufficienttoexplainthermocouplebehaviorasfollowing:HowtheThermocoupleWorks(SeebeckEffect)Whenthereisatemperaturedifferencealongametalwire,asmallelectricpotential,uniquetoeveryalloy,willoccur.ThisphenomenoniscalledSeebeckeffect.Whentwowiresofdissimilarmetalsarejoinedatoneend,andleftopenattheother,atemperaturedifferencebetweenthetwoendswillresultinavoltagesincethepotentialsgeneratedbythedissimilarmaterialsaredifferentanddonotcanceleachotherout.Now,twoimportantthingsmustbenoted.First:thevoltagegeneratedbythethermocoupleisproportionaltothedifferencebetweenthemeasuring-junctionandthecoldjunctiontemperatures.Thereforethetemperatureatthereferencejunctionmustbeaddedtothetemperaturederivedfromthethermocoupleoutput,inordertofindthetemperaturemeasured.Thisiscalledcoldjunctioncompensation,andisdoneautomaticallybytheTT302,whichhasatemperaturesensoratthesensorterminalsforthispurpose.Secondly,ifthethermocouplewiresarenotused,allthewaytotheterminalsofthetransmitter(e.g.,copperwireisusedfromsensor-headormarshalingbox)willformnewjunctionswithadditionalSeebeckeffects.Itwillbecreatedandruinthemeasurementinmostcases,sincethecold-junctioncompensationwillbedoneatthewrongpoint.NOTETherelationbetweenthemeasuringjunctiontemperatureandthegeneratedmili-voltageistabulatedinthermocouplecalibrationtablesforstandardizedthermocoupletypes,thereferencetemperaturebeing0oC.Standardizedthermocouplesthatarecommerciallyused,whosetablesarestoredinthememoryoftheTT302,arethefollowing:.NBS(B,E,J,K,N,R,S&T).DIN(L&U)ResistiveTemperatureDetectors(RTDs)ResistanceTemperatureDetectors,mostcommonlyknownasRTD’s,arebasedontheprinciplethattheresistanceofmetalincreasesasitstemperatureincreases.StandardizedRTDs,whosetablesarestoredinthememoryoftheTT302,arethefollowing:.JIS[1604-81](Pt50&Pt100).IEC,DIN,JIS[1604-89](Pt50,Pt100&Pt500)..GE(Cu10)..DIN(Ni120)ForcorrectmeasurementofRTDtemperature,itisnecessarytoeliminatetheeffectoftheresistanceofthewiresconnectingthesensortothemeasuringcircuit.Insomeindustrialapplications,thesewiresmaybehundredsofmeterslong.Thisisparticularlyimportantatlocationswheretheambienttemperaturechangesconstantly.TheTT302permitsa2-wireconnectionthatmaycausemeasuringerrors,dependingonthelengthofconnectionwiresandonthetemperaturetowhichtheyareexposed.(SeeFigure2.3-Two-WireConnection).Ina2-wireconnection,thevoltageV2isproportionaltotheRTDresistanceplustheresistanceofthewires.
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