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1、英语原文:ProgrammablelogiccontrollerAprogrammablelogiccontroller(PLC)orsimplyprogrammablecontrollerisadigitalcomputerusedforautomationofindustrialprocesses,suchascontrolofmachineryonfactoryassemblylines.Unlikegeneral-purposecomputers,thePLCisdesignedformultipleinputsandoutputarrangement
2、s,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-backedornon-volatilememory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinaboundedti
3、me,otherwiseunintendedoperationwillresult..FeaturesThemaindifferencefromothercomputersisthatPLCarearmoredforseverecondition(dust,moisture,heat,cold,etc)andhavethefacilityforextensiveinput/output(I/O)arrangements.TheseconnectthePLCtosensorsandactuators.PLCreadlimitswitches,analogproc
4、essvariables(suchastemperatureandpressure),andthepositionsofcomplexpositioningsystems.Someevenusemachinevision.Ontheactuatorside,PLCoperateelectricmotors,pneumaticorhydrauliccylinders,magneticrelaysorsolenoids,oranalogoutputs.Theinput/outputarrangementsmaybebuiltintoasimplePLC,orthe
5、PLCmayhaveexternalI/OmodulesattachedtoacomputernetworkthatplugsintothePLC.PLCwereinventedasreplacementsforautomatedsystemsthatwouldusehundredsorthousandsofrelays,camtimers,anddrumsequencers.Often,asinglePLCcanbeprogrammedtoreplacethousandsofrelays.Programmablecontrollerswereinitiall
6、yadoptedbytheautomotivemanufacturingindustry,wheresoftwarerevisionreplacedthere-wiringofhard-wiredcontrolpanelswhenproductionmodelschanged.12ManyoftheearliestPLCexpressedalldecisionmakinglogicinsimpleladderlogicwhichappearedsimilartoelectricalschematicdiagrams.Theelectricianswerequi
7、teabletotraceoutcircuitproblemswithschematicdiagramsusingladderlogic.Thisprogramnotationwaschosentoreducetrainingdemandsfortheexistingtechnicians.OtherearlyPLCusedaformofinstructionlistprogramming,basedonastack-basedlogicsolver.ThefunctionalityofthePLChasevolvedovertheyearstoinclude
8、sequentialrelaycont