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1、英语原文:ProgrammablelogiccontrollerAprogrammablelogiccontroller(PLC)orsimplyprogrammablecontrollerisadigitalcomputerusedforautomationofindustrialprocesses,suchascontrolofmachineryonfactoryassemblylines.Unlikegeneral-purposecomputers,thePLCisdesignedformultipleinputs
2、andoutputarrangements,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-backedornon-volatilememory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinr
3、esponsetoinputconditionswithinaboundedtime,otherwiseunintendedoperationwillresult..FeaturesThemaindifferencefromothercomputersisthatPLCarearmoredforseverecondition(dust,moisture,heat,cold,etc)andhavethefacilityforextensiveinput/output(I/O)arrangements.Theseconnec
4、tthePLCtosensorsandactuators.PLCreadlimitswitches,analogprocessvariables(suchastemperatureandpressure),andthepositionsofcomplexpositioningsystems.Someevenusemachinevision.Ontheactuatorside,PLCoperateelectricmotors,pneumaticorhydrauliccylinders,magneticrelaysorsol
5、enoids,oranalogoutputs.Theinput/outputarrangementsmaybebuiltintoasimplePLC,orthePLCmayhaveexternalI/OmodulesattachedtoacomputernetworkthatplugsintothePLC.PLCwereinventedasreplacementsforautomatedsystemsthatwouldusehundredsorthousandsofrelays,camtimers,anddrumsequ
6、encers.Often,asinglePLCcanbeprogrammedtoreplacethousandsofrelays.Programmablecontrollerswereinitiallyadoptedbytheautomotivemanufacturingindustry,wheresoftwarerevisionreplacedthere-wiringofhard-wiredcontrolpanelswhenproductionmodelschanged.12ManyoftheearliestPLCex
7、pressedalldecisionmakinglogicinsimpleladderlogicwhichappearedsimilartoelectricalschematicdiagrams.Theelectricianswerequiteabletotraceoutcircuitproblemswithschematicdiagramsusingladderlogic.Thisprogramnotationwaschosentoreducetrainingdemandsfortheexistingtechnicia
8、ns.OtherearlyPLCusedaformofinstructionlistprogramming,basedonastack-basedlogicsolver.ThefunctionalityofthePLChasevolvedovertheyearstoincludesequentialrelaycont