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1、10IntroductionofProgrammableControllersFromasimpleheritage,theseremarkablesystemshaveevolvedtonotonlyreplaceelectromechanicaldevices,buttosolveanever-increasingarrayofcontrolproblemsinbothprocessandnonprocessindustries.Byallindications,thesemicroprocessorpowere
2、dgiantswillcontinuetobreaknewground开辟新天土也intheautomatedfactoryintothe1990s.10.1HISTORYInthe1960s,electromechanicaldevicesweretheorderoftheday风尚asfarascontrolwasconcerned.Thesedevices,commonlyknownasrelays,werebeingusedbythethousandstocontrolmanysequential-typem
3、anufacturingprocessandstand-alonemachines・Manyoftheserelayswereinuseinthetransportationindustry,morespecifically,theautomotiveindustry.Theserelaysusedhundredsofwiresandtheirinterconnectionstoeffectacontrolsolution.Theperformanceofarelaywasbasicallyreliable一atle
4、astasasingledevice.Butthecommonapplicationsforrelaypanels（继电器盘,配电盘）calledfor要求300to500ormorerelays,andthereliabilityandmaintenanceissuesassociatedwithsupportingthesepanelsbecameaverygreatchallenge.Costbecameanotherissue,forinspiteofthelowcostoftherelayitself,th
5、einstalledcostofthepanelcouldbequitehigh.Thetotalcostincludingpurchasedparts,wiring,andinstallationlabor,couldrangefrom$30〜$50perrelay.Tomakemattesworse,theconstantlychangingneedsofaprocesscalledforrecurringmodificationsofacontrolpanel.Withrelays,thiswasacostly
6、prospect,asitwasaccomplishedbyamajorrewiringeffortonthepanel.Inaddition,thesechangesweresometimespoorlydocumented（验证），causingasecond-shiftmaintenancenightmaremonthslater.Inlightof（按照）this,itwasnotuncommontodiscard（放弃）anentirecontrolpanelinfavorofanewonewiththea
7、ppropriatecomponentswiredinamannersuitedforthenewprocess.Addtothistheunpredictable,andpotentiallyhigh,costofmaintainingthesesystemsasonhigh-volumemotorvehicleproductionlines,anditbecameclearthatsomethingwasneededtoimprovethecontrolprocess-tomakeitmorereliable,e
8、asiertotroubleshoot故障检修，andmoreadaptabletochangingcontrolneeds.Thatsomething,inthelate1960s,wasthefirstprogrammablecontroller.ThisfirstRevolutionary，systemwasdevelopedasaspe
