2012届毕业设计说明书外文文献院、部：学生姓名：指导教师：专业：班级：完成时间：ProgrammablelogiccontrollerAprogrammablelogiccontroller(PLC)orprogrammablecontrollerisadigitalcomputerusedforautomationofelectromechanicalprocesses,suchascontrolofmachineryonfactoryassemblylines,amusementrides,orlightingfixtures.PLCsareusedinmanyindustriesandmachines.Unlikegeneral-purposecomputers,thePLCisdesignedformultipleinputsandoutputarrangements,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-backedornon-volatilememory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinaboundedtime,otherwiseunintendedoperationwillresult.1HistoryThePLCwasinventedinresponsetotheneedsoftheAmericanautomotivemanufacturingindustry.Programmablelogiccontrollerswereinitiallyadoptedbytheautomotiveindustrywheresoftwarerevisionreplacedthere-wiringofhard-wiredcontrolpanelswhenproductionmodelschanged.BeforethePLC,control,sequencing,andsafetyinterlocklogicformanufacturingautomobileswasaccomplishedusinghundredsorthousandsofrelays,camtimers,anddrumsequencersanddedicatedclosed-loopcontrollers.Theprocessforupdatingsuchfacilitiesfortheyearlymodelchange-overwasverytimeconsumingandexpensive,aselectriciansneededtoindividuallyrewireeachandeveryrelay.In1968GMHydramatic(theautomatictransmissiondivisionofGeneralMotors)issuedarequestforproposalforanelectronicreplacementforhard-wiredrelaysystems.ThewinningproposalcamefromBedfordAssociatesofBedford,Massachusetts.ThefirstPLC,designatedthe084becauseitwasBedfordAssociates'eighty-fourthproject,wastheresult.BedfordAssociatesstartedanewcompanydedicatedtodeveloping,manufacturing,selling,andservicingthisnewproduct:Modicon,whichstoodforMOdularDIgitalCONtroller.OneofthepeoplewhoworkedonthatprojectwasDickMorley,whoisconsideredtobethe"father"ofthePLC.TheModiconbrandwassoldin1977toGouldElectronics,andlateracquiredbyGermanCompanyAEGandthenbyFrenchSchneiderElectric,thecurrentowner.Oneoftheveryfirst084modelsbuiltisnowondisplayatModicon'sheadquartersinNorthAndover,Massachusetts.ItwaspresentedtoModiconbyGM,whentheunitwasretiredafternearlytwentyyearsofuninterruptedservice.Modiconusedthe84monikerattheendofitsproductrangeuntilthe984madeitsappearance.TheautomotiveindustryisstilloneofthelargestusersofPLCs.2DevelopmentEarlyPLCsweredesignedtoreplacerelaylogicsystems.ThesePLCswereprogrammedin"ladderlogic",whichstronglyresemblesaschematicdiagramofrelaylogic.Thisprogramnotationwaschosentoreducetrainingdemandsfortheexistingtechnicians.OtherearlyPLCsusedaformofinstructionlistprogramming,basedonastack-basedlogicsolver.ModernPLCscanbeprogrammedinavarietyofways,fromladderlogictomoretraditionalprogramminglanguagessuchasBASICandC.AnothermethodisStateLogic,averyhigh-levelprogramminglanguagedesignedtoprogramPLCsbasedonstatetransitiondiagrams.ManyearlyPLCsdidnothaveaccompanyingprogrammingterminalsthatwerecapableofgraphicalrepresentationofthelogic,andsothelogicwasinsteadrepresentedasaseriesoflogicexpressionsinsomeversionofBooleanformat,similartoBooleanalgebra.Asprogrammingterminalsevolved,itbecamemorecommonforladderlogictobeused,fortheaforementionedreasons.NewerformatssuchasStateLogicandFunctionBlock(whichissimilartothewaylogicisdepictedwhenusingdigitalintegratedlogiccircuits)exi...
