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1、TheprogrammableLogicController(PLC)AProgrammableLogicController,PLC,orProgrammableControllerisasmallcomputerusedforautomationofindustrialprocesses,suchascontrolofmachineryonfactoryassemblylines.Unlikegeneral-purposecomputers,thePLCisdesignedforextendedtemperatureranges,dirtyordustyconditions
2、,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationarestoredinbattery-backedorread-onlymemory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinastrictlyboundedtime.Themaindifferencefromothercomputersar
3、ethespecialinput/outputarrangements.TheseconnectthePLCtosensorsandactuators.PLCsreadlimitswitches,temperatureindicatorsandthepositionsofcomplexpositioningsystems.Someevenusemachinevision.Ontheactuatorside,PLCsdriveanykindofelectricmotor,pneumaticorhydrauliccylindersordiaphragms,magneticrelay
4、sorsolenoids.Theinput/outputarrangementsmaybebuiltintoasimplePLC,orthePLCmayhaveexternalI/OmodulesattachedtoaproprietarycomputernetworkthatplugsintothePLC.PLCswereinventedasreplacementsforautomatedsystemsthatwouldusehundredsorthousandsofrelaysandcamtimers.Often,asinglePLCcanbeprogrammedtorep
5、lacethousandsofrelays.Programmablecontrollerswereinitiallyadoptedbytheautomotivemanufacturingindustry,wheresoftwarerevisionreplacedthere-wiringofhard-wiredcontrolpanelswhenproductionmodelschanged.。TheearliestPLCsexpressedalldecisionmakinglogicinsimpleladderlogicwhichappearedsimilartoelectric
6、alschematicdiagrams.Theelectricianswerequiteabletotraceoutcircuitproblemswithschematicdiagramsusingladderlogic.Thisprogramnotationwaschosentoreducetrainingdemandsfortheexistingtechnicians.。ThefunctionalityofthePLChasevolvedovertheyearstoincludesequentialrelaycontrol,motioncontrol,processcont
7、rol,distributedcontrolsystemsandnetworking.Thedatahandling,storage,processingpowerandcommunicationcapabilitiesofsomemodernPLCsareapproximatelyequivalenttodesktopcomputers.PLC-likeprogrammingcombinedwithremoteI/Ohardware,allowageneral-purposedesktop
