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1、ControlSensors&1INTRODUCTIONInfeedbackcontrolsystems一plant（设备）responseismeasuredandcomparedwithareferenceinputandtheerrorisautomaticallyemployedincontrollingtheplant.Itfollowsthatameasurementsystemisanessentialcomponentinanyfeedbackcontrolsystemandformsavitallinkbe

2、tweentheplantandthecontroller.Measurementsareneededinmanyengineeringapplications.Themeasurementprocesshastobeautomated,however,incontrolsystemsapplications.Atypicalmeasurementsystemconsistsofoneormoresensor-transducerunitsandassociatedsignal-conditioning（信号调理）（andm

3、odification改善）devices（seeFig.8.1）.Filteringtoremoveunwantednoiseandamplificationtostrengthenaneededsignalareconsideredsignalconditioning.Analog-to-digitalconversion（ADC）,digital-to-analogconversion（DAC）,modulation（调制）,anddemodulation（解调）aresignalmodification（信号增强、信

4、号改善）methods.Notethatsignalconditioningcanbeconsideredunderthegeneralheadingofsignalmodification.Eventhoughdatarecordingisanintegralfunctioninatypicaldataacquisition（数据采集）system,itisnotacrucialfunctioninafeedbackcontrolsystem.Forthisreason,weshallnotgointodetailsofd

5、atarecordingdevicesinthisbook.Inamultiplemeasurementenvironment,amultiplexer（多路转换器）couldbeemployedpriortoorfollowingthesignal-conditioningprocess,inordertopickonemeasuredsignalatatimefromabankofdatachannelsforsubsequentprocessing.Inthismanner,oneunitofexpensiveproc

6、essinghardwarecanbetime-shared（时分复用）betweenseveralsignals.Sensor-transducer（传感器-变送器）devicesarepredominantly（主要的）analogcomponentsthatgenerateanalogsignals,eventhoughdirectdigitaltransducersarebecomingincreasinglypopularindigitalcontrolapplications.Whenanalogtransduc

7、ersareemployed,analog-to-digitalconverters(ADCs)havetobeusedtoconvertanalogsignalsintodigitaldatafordigitalcontrolThissignalmodificationprocessrequiressamplingofanalogsignalsatdiscretetimepoints.Onceavalueissampled,itisencodedintoadigitalrepresentationsuchasstraigh

8、tbinarycode,agraycode,binary-codedecimal(BCD)codeorAmericanStandardCodeforInformationInterchange(ASCII).Thechangesinananalogsignalduetoitstransie