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1、ProgrammablelogiccontrollerAprogrammablelogiccontroller(PLC)orprogrammablecontrollerisadigitalcomputerusedforautomationofelectromechanicalprocesses,suchascontrolofmachineryonfactoryassemblylines,amusementrides,orlightingfixtures.PLCsareusedinmanyindustriesandmachines.Unlikegeneral-purpo
2、secomputers,thePLCisdesignedformultipleinputsandoutputarrangements,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-backedornon-volatilememory.APLCisanexampleofarealtimesystemsinceoutputres
3、ultsmustbeproducedinresponsetoinputconditionswithinaboundedtime,otherwiseunintendedoperationwillresult.1.HistoryThePLCwasinventedinresponsetotheneedsoftheAmericanautomotivemanufacturingindustry.Programmablelogiccontrollerswereinitiallyadoptedbytheautomotiveindustrywheresoftwarerevisionr
4、eplacedthere-wiringofhard-wiredcontrolpanelswhenproductionmodelschanged.BeforethePLC,control,sequencing,andsafetyinterlocklogicformanufacturingautomobileswasaccomplishedusinghundredsorthousandsofrelays,camtimers,anddrumsequencersanddedicatedclosed-loopcontrollers.Theprocessforupdatingsu
5、chfacilitiesfortheyearlymodelchange-overwasverytimeconsumingandexpensive,aselectriciansneededtoindividuallyrewireeachandeveryrelay.In1968GMHydramatic(theautomatictransmissiondivisionofGeneralMotors)issuedarequestforproposalforanelectronicreplacementforhard-wiredrelaysystems.Thewinningpr
6、oposalcamefromBedfordAssociatesofBedford,Massachusetts.ThefirstPLC,designatedthe084becauseitwasBedfordAssociates'eighty-fourthproject,wastheresult.BedfordAssociatesstartedanewcompanydedicatedtodeveloping,manufacturing,selling,andservicingthisnewproduct:Modicon,whichstoodforMOdularDIgita
7、lCONtroller.OneofthepeoplewhoworkedonthatprojectwasDickMorley,whoisconsideredtobethe"father"ofthePLC.TheModiconbrandwassoldin1977toGouldElectronics,andlateracquiredbyGermanCompanyAEGandthenbyFrenchSchneiderElectric,thecurrentowner.Oneoftheveryfirst084modelsbuiltisnowondisplayat
