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1、ProgrammablelogiccontrollerAprogrammablelogiccontroller(PLC)orprogrammablecontrollerisadigitalcomputerusedforautomationofelectromechanicalprocesses,suchascontrolofmachineryonfactoryassemblylines,amusementrides,orlightingfixtures.PLCsareusedinmanyindus
2、triesandmachines.Unlikegeneral-purposecomputers,thePLCisdesignedformultipleinputsandoutputarrangements,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-b
3、ackedornon-volatilememory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinaboundedtime,otherwiseunintendedoperationwillresult.1.HistoryThePLCwasinventedinresponsetotheneedsoftheAmericanautomotivemanufac
4、turingindustry.Programmablelogiccontrollerswereinitiallyadoptedbytheautomotiveindustrywheresoftwarerevisionreplacedthere-wiringofhard-wiredcontrolpanelswhenproductionmodelschanged.BeforethePLC,control,sequencing,andsafetyinterlocklogicformanufacturing
5、automobileswasaccomplishedusinghundredsorthousandsofrelays,camtimers,anddrumsequencersanddedicatedclosed-loopcontrollers.Theprocessforupdatingsuchfacilitiesfortheyearlymodelchange-overwasverytimeconsumingandexpensive,aselectriciansneededtoindividually
6、rewireeachandeveryrelay.In1968GMHydramatic(theautomatictransmissiondivisionofGeneralMotors)issuedarequestforproposalforanelectronicreplacementforhard-wiredrelaysystems.ThewinningproposalcamefromBedfordAssociatesofBedford,Massachusetts.ThefirstPLC,desi
7、gnatedthe084becauseitwasBedfordAssociates'eighty-fourthproject,wastheresult.BedfordAssociatesstartedanewcompanydedicatedtodeveloping,manufacturing,selling,andservicingthisnewproduct:Modicon,whichstoodforMOdularDIgitalCONtroller.Oneofthepeoplewhoworked
8、onthatprojectwasDickMorley,whoisconsideredtobethe"father"ofthePLC.TheModiconbrandwassoldin1977toGouldElectronics,andlateracquiredbyGermanCompanyAEGandthenbyFrenchSchneiderElectric,thecurrentowner.Oneoftheveryfirst084modelsbuiltisnowondisplayat
