【本科毕业设计方案英文翻译】可编程逻辑控制器Programmablelogiccontroller

1/13ProgrammablelogiccontrollerCynthiaCooperFromWikipedia,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,etcandhavethefacilityforextensiveinput/output(I/Oarrangements.TheseconnectthePLCtosensorsandactuators.PLCsreadlimitswitches,analogprocessvariables(suchastemperatureandpressure,andthepositionsofcomplexpositioningsystems.Someevenusemachinevision.Ontheactuatorside,PLCsoperateelectricmotors,pneumaticorhydrauliccylinders,magneticrelaysorsolenoids,oranalogoutputs.Theinput/outputarrangementsmaybebuiltintoasimplePLC,2/13orthePLCmayhaveexternalI/OmodulesattachedtoacomputernetworkthatplugsintothePLC.PLCswereinventedasreplacementsforautomatedsystemsthatwouldusehundredsorthousandsofrelays,camtimers,anddrumsequencers.Often,asinglePLCcanbeprogrammedtoreplacethousandsofrelays.Programmablecontrollerswereinitiallyadoptedbytheautomotivemanufacturingindustry,wheresoftwarerevisionreplacedthere-wiringofhard-wiredcontrolpanelswhenproductionmodelschanged.ManyoftheearliestPLCsexpressedalldecisionmakinglogicinsimpleladderlogicwhichappearedsimilartoelectricalschematicdiagrams.Theelectricianswerequiteabletotraceoutcircuitproblemswithschematicdiagramsusingladderlogic.Thisprogramnotationwaschosentoreducetrainingdemandsfortheexistingtechnicians.OtherearlyPLCsusedaformofinstructionlistprogramming,basedonastack-basedlogicsolver.ThefunctionalityofthePLChasevolvedovertheyearstoincludesequentialrelaycontrol,motioncontrol,processcontrol,distributedcontrolsystemsandnetworking.Thedatahandling,storage,processingpowerandcommunicationcapabilitiesofsomemodernPLCsareapproximatelyequivalenttodesktopcomputers.PLC-likeprogrammingcombinedwithremoteI/Ohardware,allowageneral-purposedesktopcomputertooverlapsomePLCsincertainapplications.UndertheIEC61131-3standard,PLCscanbeprogrammedusingstandards-basedprogramminglanguages.AgraphicalprogrammingnotationcalledSequentialFunctionChartsisavailableoncertainprogrammablecontrollers.3/13PLCcomparedwithothercontrolsystemsPLCsarewell-adaptedtoarangeofautomationtasks.Thesearetypicallyindustrialprocessesinmanufacturingwherethecostofdevelopingandmaintainingtheautomationsystemishighrelativetothetotalcostoftheautomation.PLCscontaininputandoutputdevicescompatiblewithindustrialpilotdevicesandcontrols.PLCapplicationsaretypicallyhighlycustomizedsystemssothecostofapackagedPLCislowcomparedtothecostofaspecificcustom-builtcontrollerdesign.Ontheotherhand,inthecaseofmass-producedgoods,customizedcontrolsystemsareeconomicduetothelowercostofthecomponents,whichcanbeoptimallychoseninsteadofagenericsolution。Forhighvolumeorverysimplefixedautomationtasks,differenttechniquesareused.Forexample,aconsumerdishwasherwouldbecontrolledbyanelectromechanicalcamtimercostingonlyafewdollarsinproductionquantities.Amicrocontroller-baseddesignwouldbeappropriatewherehundredsorthousandsofunitswillbeproducedandsothedevelopmentcost(designofpowersuppliesandinput/outputhardwarecanbespreadovermanysales,andwheretheend-userwouldnotneedtoalterthecontrol.Automotiveapplicationsareanexample。millionsofunitsarebuilteachyear,andveryfewend-usersaltertheprogrammingofthesecontrollers.However,somespecialtyvehiclessuchastransitbusseseconomicallyusePLCsinsteadofcustom-designedcontrols,becausethevolumesarelowandthedevelopmentcostwouldbeuneconomic.Verycomplexprocesscontrol,suchasusedinthechemicalindustry,mayrequirealgorithmsandperformancebeyondthecapabilityofevenhigh-4/13performancePLCs.Veryhigh-speedorprecisioncontrolsmayalsorequirecustomizedsolutions。forexample,aircraftflightcontrols.PLCsmayincludelogicforsingle-variablefeedbackanalogcontrolloop,aproportional,integral,derivativeorPIDcontroller.APIDloopcouldbeusedtocontrolthetemperatureofamanufacturingprocess,forexample.HistoricallyPLCswereusuallyconfiguredwithonlyafewanalogcontrolloops。whereprocessesrequiredhundredsorthousandsofloops,adistributedcontrolsystem(DCSwouldinsteadbeused.However,asPLCshavebecomemorepowerful,theboundarybetweenDCSandPLCapplicationshasbecomelessclear-cut.DigitalandanalogsignalsDigitalordiscretesignalsbehaveasbinaryswitches,yieldingsimplyanOnorOffsignal(1or0,TrueorFalse,respectively.Pushbuttons,limitswitches,andphotoelectricsensorsareexamplesofdevicesprovidingadiscretesignal.Discretesignalsaresentusingeithervoltageorc