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1、ProgrammablelogiccontrollerAprogrammablelogiccontroller(PLC)orprogrammablecontrollerisadigitalcomputerusedforautomationofelectromechanicalprocesses,suchascontrolofmachineryonfactoryassemblylines,amusementrides,orlightingfixtures.PLCsareusedinmanyindustriesandmachines.Unlikegeneral-purposecomput
2、ers,thePLCisdesignedformultipleinputsandoutputarrangements,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-backedornon-volatilememory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproduc
3、edinresponsetoinputconditionswithinaboundedtime,otherwiseunintendedoperationwillresult.1.HistoryThePLCwasinventedinresponsetotheneedsoftheAmericanautomotivemanufacturingindustry.Programmablelogiccontrollerswereinitiallyadoptedbytheautomotiveindustrywheresoftwarerevisionreplacedthere-wiringofhar
4、d-wiredcontrolpanelswhenproductionmodelschanged.BeforethePLC,control,sequencing,andsafetyinterlocklogicformanufacturingautomobileswasaccomplishedusinghundredsorthousandsofrelays,camtimers,anddrumsequencersanddedicatedclosed-loopcontrollers.Theprocessforupdatingsuchfacilitiesfortheyearlymodelcha
5、nge-overwasverytimeconsumingandexpensive,aselectriciansneededtoindividuallyrewireeachandeveryrelay.In1968GMHydramatic(theautomatictransmissiondivisionofGeneralMotors)issuedarequestforproposalforanelectronicreplacementforhard-wiredrelaysystems.ThewinningproposalcamefromBedfordAssociatesofBedford
6、,Massachusetts.ThefirstPLC,designatedthe084becauseitwasBedfordAssociates'eighty-fourthproject,wastheresult.BedfordAssociatesstartedanewcompanydedicatedtodeveloping,manufacturing,selling,andservicingthisnewproduct:Modicon,whichstoodforMOdularDIgitalCONtroller.Oneofthepeoplewhoworkedonthatproject
7、wasDickMorley,whoisconsideredtobethe"father"ofthePLC.TheModiconbrandwassoldin1977toGouldElectronics,andlateracquiredbyGermanCompanyAEGandthenbyFrenchSchneiderElectric,thecurrentowner.Oneoftheveryfirst084modelsbuiltisnowondisplayat
