plc外文文献译文

1ProgrammablelogiccontrollerAprogrammablelogiccontroller(PLC)isadigitalcomputerusedforautomationofelectromechanicalprocesses,suchascontrolofmachineryonfactoryassemblylines,amusementrides,orlightingfixtures.PLCisusedinmanyindustriesandmachines.1.HistoryThePLCwasinventedinresponsetotheneedsoftheAmericanautomotivemanufacturingindustry.BeforethePLC,control,sequencing,andsafetyinterlocklogicformanufacturingautomobileswasaccomplishedusinghundredsorthousandsofrelays,camtimers,anddrumsequencersanddedicatedclosed-loopcontrollers.In1968GMHydramaticissuedarequestforproposalforanelectronicreplacementforhard-wiredrelaysystems.ThefirstPLC,designatedthe084becauseitwasBedfordAssociates'eighty-fourthproject,wastheresult.BedfordAssociatesstartedanewcompanydedicatedtodeveloping,manufacturing,selling,andservicingthisnewproduct:Modicon,whichstoodforMOdularDIgitalCONtroller.TheModiconbrandwassoldin1977toGouldElectronics,andlateracquiredbyGermanCompanyAEGandthenbyFrenchSchneiderElectric,thecurrentowner.Oneoftheveryfirst084modelsthatwaspresentedtoModiconbyGM,whentheunitwasretiredafternearlytwentyyearsofuninterruptedservice.Modiconusedthe84monikerattheendofitsproductrangeuntilthe984madeitsappearance.2.DevelopmentEarlyPLCsweredesignedtoreplacerelaylogicsystems.ThesePLCswereprogrammedinladderlogic,whichstronglyresemblesaschematicdiagramofrelaylogic.OtherearlyPLCsusedaformofinstructionlistprogramming,basedonastack-basedlogicsolver.ModernPLCscanbeprogrammedinavarietyofways,fromladderlogictomoretraditionalprogramminglanguagessuchasBASICandC.AnothermethodisStateLogic,averyhigh-levelprogramminglanguagedesignedtoprogramPLCsbasedonstatetransitiondiagrams.3.FunctionalityThefunctionalityofthePLChavesequentialrelaycontrol,motioncontrol,processcontrol,distributedcontrolsystemsandnetworking.Regardingthepracticalityofthesedesktopcomputerbasedlogiccontrollers,itisimportanttonotethattheyhavenotbeengenerallyacceptedinheavyindustrybecausethedesktopcomputers2runonlessstableoperatingsystemsthandoPLCs,andbecausethedesktopcomputerhardwareistypicallynotdesignedtothesamelevelsoftolerancetotemperature,Inmorerecentyears,smallproductscalledPLRs(programmablelogicrelays),areusedinlightindustrywhereonlyafewpointsofI/O(i.e.afewsignalscominginfromtherealworldandafewgoingout)areinvolved,andlowcostisdesired.PopularnamesincludePICOController,NANOPLC,andothernamesimplyingverysmallcontrollers.thePLRsareusuallynotmodularorexpandable,buttheirpricecanbetwoordersofmagnitudelessthanaPLCandtheystillofferrobustdesignanddeterministicexecutionofthelogic.4.PLCcomparedwithothercontrolsystemsPLCsarewell-adaptedtoarangeofautomationtasks.Thesearetypicallyindustrialprocessesinmanufacturingwherethecostofdevelopingandmaintainingtheautomationsystemishighrelativetothetotalcostoftheautomation,andwherechangestothesystemwouldbeexpectedduringitsoperationallife.PLCscontaininputandoutputdevicescompatiblewithindustrialpilotdevicesandcontrols;littleelectricaldesignisrequired,andthedesignproblemcentersonexpressingthedesiredsequenceofoperations.PLCapplicationsaretypicallyhighlycustomizedsystemssothecostofapackagedPLCislowcomparedtothecostofaspecificcustom-builtcontrollerdesign.Ontheotherhand,inthecaseofmass-producedgoods,customizedcontrolsystemsareeconomicduetothelowercostofthecomponents,whichcanbeoptimallychoseninsteadofagenericsolution,andwherethenon-recurringengineeringchargesarespreadoverthousandsormillionsofunits.Amicrocontroller-baseddesignwouldbeappropriatewherehundredsorthousandsofunitswillbeproducedandsothedevelopmentcost(designofpowersupplies,input/outputhardwareandnecessarytestingandcertification)canbespreadovermanysales,andwheretheend-userwouldnotneedtoalterthecontrol.Automotiveapplicationsareanexample;millionsofunitsarebuilteachyear,andveryfewend-usersaltertheprogrammingofthesecontrollers.However,somespecialtyvehiclessuchastransitbusseseconomicallyusePLCsinsteadofcustom-designedcontrols,becausethevolumesarelowandthedevelopmentcostwouldbeuneconomic.Programmablecontrollersarewidelyusedinmotioncontrol,positioningcontrolandtorquecontrol.SomemanufacturersproducemotioncontrolunitstobeintegratedwithPLCsothatG-code(involvingaCNCmachine)canbeusedtoinstructmachine3movements.PLCsmayincludelogicforsingle-variablefeedbackanalogcontrolloop,aproportional,integral,derivativeorPIDcontroller.APIDloopcouldbeusedtocontrolthetemperatureofamanufacturingprocess,forexample.HistoricallyPLCswereusuallyconfiguredwithonlyafewanalogcontrolloops;whereprocessesrequiredhundredsorthousandsofloops,adistributedcontrolsystem(DCS)wouldinsteadbeused.AsPLCshavebecomemorepowerful,theboundarybetweenDCSandPLCapplicationshasbecomelessdistinct.PLCshavesimilarfunctionalityasRemoteTerminalUnits.AnRTU,however,usuallydoesnotsupportcontrolalgorithmsorcontrolloops.Ashardwarerapidlybecomesmorepowerfulandcheaper,RTUs,PLCsandDCSsareincreasinglybeginningtooverlapinresponsibilities,andmanyvendorssellRTUswithPLC-likefeaturesandviceversa.TheindustryhasstandardizedontheIEC61131-3functionalblocklanguageforcreatingprogramstorunonRTUsandPLCs,al