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TheprogrammableLogicController(PLC)AProgrammableLogicController,PLC,orProgrammableControllerisasmallcomputerusedforautomationofindustrialprocesses,suchascontrolofmachineryonfactoryassemblylines.Unlikegeneral-purposecomputers,thePLCisdesignedforextendedtemperatureranges,dirtyordustyconditions,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationarestoredinbattery-backedorread-onlymemory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinastrictlyboundedtime.Themaindifferencefromothercomputersarethespecialinput/outputarrangements.TheseconnectthePLCtosensorsandactuators.PLCsreadlimitswitches,temperatureindicatorsandthepositionsofcomplexpositioningsystems.Someevenusemachinevision.Ontheactuatorside,PLCsdriveanykindofelectricmotor,pneumaticorhydrauliccylindersordiaphragms,magneticrelaysorsolenoids.Theinput/outputarrangementsmaybebuiltintoasimplePLC,orthePLCmayhaveexternalI/OmodulesattachedtoaproprietarycomputernetworkthatplugsintothePLC.PLCswereinventedasreplacementsforautomatedsystemsthatwouldusehundredsorthousandsofrelaysandcamtimers.Often,asinglePLCcanbeprogrammedtoreplacethousandsofrelays.Programmablecontrollerswereinitiallyadoptedbytheautomotivemanufacturingindustry,wheresoftwarerevisionreplacedthere-wiringofhard-wiredcontrolpanelswhenproductionmodelschanged.。TheearliestPLCsexpressedalldecisionmakinglogicinsimpleladderlogicwhichappearedsimilartoelectricalschematicdiagrams.Theelectricianswerequiteabletotraceoutcircuitproblemswithschematicdiagramsusingladderlogic.Thisprogramnotationwaschosentoreducetrainingdemandsfortheexistingtechnicians.。ThefunctionalityofthePLChasevolvedovertheyearstoincludesequentialrelaycontrol,motioncontrol,processcontrol,distributedcontrolsystemsandnetworking.Thedatahandling,storage,processingpowerandcommunicationcapabilitiesofsomemodernPLCsareapproximatelyequivalenttodesktopcomputers.PLC-likeprogrammingcombinedwithremoteI/Ohardware,allowageneral-purposedesktopcomputertooverlapsomePLCsincertainapplications.UndertheIEC61131-3standard,PLCscanbeprogrammedusingstructuredprogramminglanguages.AgraphicalprogrammingnotationcalledSequentialFunctionChartsisavailableoncertainprogrammablecontrollers.PLCcomparedwithothercontrolsystemsPLCsarewell-adaptedtoacertainrangeofautomationtasks.Thesearetypicallyindustrialprocessesinmanufacturingwherethecostofdevelopingandmaintainingtheautomationsystemishighrelativetothetotalcostoftheautomation,andwherechangestothesystemwouldbeexpectedduringitsoperationallife.PLCscontaininputandoutputdevicescompatiblewithindustrialpilotdevicesandcontrols;littleelectricaldesignisrequired,andthedesignproblemcentersonexpressingthedesiredsequenceofoperationsinladderlogic(orfunctionchart)notation.PLCapplicationsaretypicallyhighlycustomizedsystemssothecostofapackagedPLCislowcomparedtothecostofaspecificcustom-builtcontrollerdesign.Ontheotherhand,inthecaseofmass-producedgoods,customizedcontrolsystemsareeconomicduetothelowercostofthecomponents,whichcanbeoptimallychoseninsteadofagenericsolution,andwherethenon-recurringengineeringchargesarespreadoverthousandsofsales.SomemodernPLCswithfullcapabilitiesareavailableforafewhundredUSD.Thisallowsthemtobeeconomicallyappliedonverysmallcontrolproblems.Forhighvolumeorverysimplefixedautomationtasks,differenttechniquesareused.Forexample,aconsumerdishwasherwouldbecontrolledbyanelectromechanicalcamtimercostingonlyafewdollarsinproductionquantities.Amicrocontroller-baseddesignwouldbeappropriatewherehundredsorthousandsofunitswillbeproducedandsothedevelopmentcost(designofpowersuppliesandinput/outputhardware)canbespreadovermanysales,andwheretheend-userwouldnotneedtoalterthecontrol.Automotiveapplicationsareanexample;millionsofunitsarebuilteachyear,andveryfewend-usersaltertheprogrammingofthesecontrollers.(However,somespecialtyvehiclessuchastransitbusseseconomicallyusePLCsinsteadofcustom-designedcontrols,becausethevolumesarelowandthedevelopmentcostwouldbeuneconomic.)Verycomplexprocesscontrol,suchasusedinthechemicalindustry,mayrequirealgorithmsandperformancebeyondthecapabilityofevenhigh-performancePLCs.Veryhighspeedcontrolsmayalsorequirecustomisedsolutions;forexample,aircraftflightcontrols.PLCsmayincludelogicforsingle-variablefeedbackanalogcontrolloop,aproportional,integral,derivativeorPIDcontroller.APIDloopcouldbeusedtocontrolthetemperatureofamanufacturingprocess,forexample.HistoricallyPLCswereusuallyconfiguredwithonlyafewanalogcontrolloops;whereprocessesrequiredhundredsorthousandsofloops,adistributedcontrolsystem(DCS)wouldinsteadbeused.However,asPLCshavebecomemorepowerful,theboundarybetweenDCSandPLCapplicationshasbecomelessclear-cut..Digitalordiscretesignalsbehaveasswitches,yieldingsimplyanOnorOffsignal(1or0,TrueorFalse,respectively).Pushbuttons,limitswitches,andphoto-eyesareexamplesofdevicesprovidingadiscretesignal.DiscretesignalsareSentusingeithervoltageorcurrent,whereaspecificrangeisdesignatedasOnandanotherasOff.APLCmightuse24VDCI/O,withvaluesabove22VDCrepresentingOnandva