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1、IntroductionofProgrammablecontrollersFromasimpleheritage,theseremarkablesystemshaveevolvedtonotonlyreplaceelectromechanicaldevices,buttosolveanever-increasingarrayofcontrolproblemsinbothprocessandnonprocessindustries.Byallindications,thesemicroprocessorp
2、oweredgiantswillcontinuetobreaknewgroundintheautomatedfactoryintothe1990s.HISTORYInthe1960s,electromechanicaldevicesweretheorderofthedayassfarascontrolwasconcerned.Thesedevices,commonlyknownasrelays,werebeingusedbythethousandstocontrolmanysequential-type
3、manufacturingprocessesandstand-alongmachines.Manyoftheserelayswereinuseinthetransportationindustry,morespecifically,theautomotiveindustry.Theserelaysusedhundredsofwiresandtheirinterconnectionstoeffectacontrolsolution.Theperformanceofarelaywasbasicallyrel
4、iable-atleastasasingledevice.Butthecommonapplicationsforrelaypanelscalledfor300to500ormorerelays,andthereliabilityandmaintenanceissuesassociatedwithsupportingthesepanelsbecameaverygreatchallenge.Costbecameanotherissue,forinspiteofthelowcostoftherelayitse
5、lf,theinstalledcostofthepanelcouldbequitehigh.Thetotalcostincludingpurchasedparts,wiring,andinstallationlabor,couldrangefrom$30~$50perrelay.Tomakemattersworse,theconstantlychangingneedsofaprocesscalledforrecurringmodificationsofacontrolpanel.Withrelays,t
6、hiswasacostlyprospect,asitwasaccomplishedbyamajorrewiringeffortonthepanel.Inadditionthesechangesweresometimespoorlydocumented,causingasecond-shiftmaintenancenightmaremonthslater.Inlightofthis,itwasnotuncommontodiscardanentirecontrolpanelinfavorofanewonew
7、iththeappropriatecomponentswiredinamannersuitedforthenewprocess.Addtothistheunpredictable,andpotentiallyhigh,costofmaintainingthesesystemsasonhigh-volumemotorvehicleproductionlines,anditbecameclearthatsomethingwasneededtoimprovethecontrolprocess–tomakeit
8、morereliable,easiertotroubleshoot,andmoreadaptabletochangingcontrolneeds.Thatsomething,inthelate1960s,wasthefirstprogrammablecontroller.Thisfirst‘revolutionary’systemwandevelopedasaspecificresponsetotheneedsofthemajorautom
