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新疆大学毕业论文（设计）ProgrammablelogiccontrollerCynthiaCooperFromWikipedia,thefreeencyclopediaAprogrammablelogiccontrollerorsimplyprogrammablecontrollerisadigitalcomputerusedforautomationofindustrialprocesses,suchascontrolofmachineryonfactoryassemblylines.Unlikegeneral-purposecomputers,thePLCisdesignedformultipleinputsandoutputarrangements,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-backedornon-volatilememory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinaboundedtime,otherwiseunintendedoperationwillresult.FeaturesControlpanelwithPLC(greyelementsinthecenter).Theunitconsistsofseparateelements,fromlefttoright;powersupply,controller,relayunitsforinputandoutput.ThemaindifferencefromothercomputersisthatPLCsarearmoredforseverecondition(dust,moisture,heat,cold,etc)andhavethefacilityforextensiveinput/output(I/O)arrangements.TheseconnectthePLCtosensorsandactuators.PLCsreadlimitswitches,analogprocessvariables(suchastemperatureandpressure),andthepositionsofcomplexpositioningsystems.Someevenusemachinevision.Ontheactuatorside,PLCsoperateelectricmotors,pneumaticorhydrauliccylinders,magneticrelaysorsolenoids,oranalogoutputs.Theinput/outputarrangementsmaybebuiltintoasimplePLC,orthePLCmayhaveexternalI/OmodulesattachedtoacomputernetworkthatplugsintothePLC.PLCswereinventedasreplacementsforautomatedsystemsthatwouldusehundredsorthousandsofrelays,camtimers,anddrumsequencers.Often,asinglePLCcanbeprogrammedtoreplacethousandsofrelays.Programmablecontrollerswereinitiallyadoptedbytheautomotivemanufacturingindustry,wheresoftwarerevisionreplacedthere-wiringofhard-wiredcontrolpanelswhenproductionmodelschanged.ManyoftheearliestPLCsexpressedalldecisionmakinglogicinsimpleladderlogicwhichappearedsimilartoelectricalschematicdiagrams.Theelectricianswerequiteabletotraceoutcircuitproblemswithschematicdiagramsusingladderlogic.Thisprogramnotationwaschosentoreducetrainingdemandsfortheexisting新疆大学毕业论文（设计）technicians.OtherearlyPLCsusedaformofinstructionlistprogramming,basedonastack-basedlogicsolver.ThefunctionalityofthePLChasevolvedovertheyearstoincludesequentialrelaycontrol,motioncontrol,processcontrol,distributedcontrolsystemsandnetworking.Thedatahandling,storage,processingpowerandcommunicationcapabilitiesofsomemodernPLCsareapproximatelyequivalenttodesktopcomputers.PLC-likeprogrammingcombinedwithremoteI/Ohardware,allowageneral-purposedesktopcomputertooverlapsomePLCsincertainapplications.UndertheIEC61131-3standard,PLCscanbeprogrammedusingstandards-basedprogramminglanguages.AgraphicalprogrammingnotationcalledSequentialFunctionChartsisavailableoncertainprogrammablecontrollers.PLCcomparedwithothercontrolsystemsPLCsarewell-adaptedtoarangeofautomationtasks.Thesearetypicallyindustrialprocessesinmanufacturingwherethecostofdevelopingandmaintainingtheautomationsystemishighrelativetothetotalcostoftheautomation.PLCscontaininputandoutputdevicescompatiblewithindustrialpilotdevicesandcontrols.PLCapplicationsaretypicallyhighlycustomizedsystemssothecostofapackagedPLCislowcomparedtothecostofaspecificcustom-builtcontrollerdesign.Ontheotherhand,inthecaseofmass-producedgoods,customizedcontrolsystemsareeconomicduetothelowercostofthecomponents,whichcanbeoptimallychoseninsteadofagenericsolution。Forhighvolumeorverysimplefixedautomationtasks,differenttechniquesareused.Forexample,aconsumerdishwasherwouldbecontrolledbyanelectromechanicalcamtimercostingonlyafewdollarsinproductionquantities.Amicrocontroller-baseddesignwouldbeappropriatewherehundredsorthousandsofunitswillbeproducedandsothedevelopmentcost(designofpowersuppliesandinput/outputhardware)canbespreadovermanysales,andwheretheend-userwouldnotneedtoalterthecontrol.Automotiveapplicationsareanexample;millionsofunitsarebuilteachyear,andveryfewend-usersaltertheprogrammingofthesecontrollers.However,somespecialtyvehiclessuchastransitbusseseconomicallyusePLCsinsteadofcustom-designedcontrols,becausethevolumesarelowandthedevelopmentcostwouldbeuneconomic.Verycomplexprocesscontrol,suchasusedinthechemicalindustry,mayrequirealgorithmsandperformancebeyondthecapabilityofevenhigh-performancePLCs.新疆大学毕业论文（设计）Veryhigh-speedorprecisioncontrolsmayalsorequirecustomizedsolutions;forexample,aircraftflightcontrols.PLCsmayincludelogicforsingle-variablefeedbackanalogcontrolloop,aproportional,integral,derivativeorPIDcontroller.APIDloopcouldbeusedtocontrolthetemperatureofamanufacturingprocess,forexample.HistoricallyPLCswereusuallyconfiguredwithonlyafewanalogcontrolloops;whereprocessesrequiredhundredsorthousandsofloops,adistributedcontrolsystem(DCS)wouldinsteadbeused.However,asPLCshavebecomemorepowerful,theboundarybetweenDCSandPLCapplicationshasbecomelessclear-cut.DigitalandanalogsignalsDigitalordiscretesignalsbehaveasbinaryswitches,yieldingsimplyanOnorOffsignal(1or0,TrueorFalse,respectively).Pushbuttons,limitswitches,andphotoelectricsensorsareexamplesofdevicesprovidingadiscretesignal.Discretesignalsaresentu