交通灯毕业论文英文资料翻译文档格式.docx

交通灯毕业论文英文资料翻译文档格式.docx

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@#@@#@ @#@@#@IntroductionofProgrammablecontrollers@#@Fromasimpleheritage,theseremarkablesystemshaveevolvedtonotonlyreplaceelectromechanicaldevices,buttosolveanever-increasingarrayofcontrolproblemsinbothprocessandnonprocessindustries.Byallindications,thesemicroprocessorpoweredgiantswillcontinuetobreaknewgroundintheautomatedfactoryintothe1990s.@#@HISTORY@#@Inthe1960s,electromechanicaldevicesweretheorderofthedayassfarascontrolwasconcerned.Thesedevices,commonlyknownasrelays,werebeingusedbythethousandstocontrolmanysequential-typemanufacturingprocessesandstand-alongmachines.Manyoftheserelayswereinuseinthetransportationindustry,morespecifically,theautomotiveindustry.Theserelaysusedhundredsofwiresandtheirinterconnectionstoeffectacontrolsolution.Theperformanceofarelaywasbasicallyreliable-atleastasasingledevice.Butthecommonapplicationsforrelaypanelscalledfor300to500ormorerelays,andthereliabilityandmaintenanceissuesassociatedwithsupportingthesepanelsbecameaverygreatchallenge.Costbecameanotherissue,forinspiteofthelowcostoftherelayitself,theinstalledcostofthepanelcouldbequitehigh.Thetotalcostincludingpurchasedparts,wiring,andinstallationlabor,couldrangefrom$30~$50perrelay.Tomakemattersworse,theconstantlychangingneedsofaprocesscalledforrecurringmodificationsofacontrolpanel.Withrelays,thiswasacostlyprospect,asitwasaccomplishedbyamajorrewiringeffortonthepanel.Inadditionthesechangesweresometimespoorlydocumented,causingasecond-shiftmaintenancenightmaremonthslater.Inlightofthis,itwasnotuncommontodiscardanentirecontrolpanelinfavorofanewonewiththeappropriatecomponentswiredinamannersuitedforthenewprocess.Addtothistheunpredictable,andpotentiallyhigh,costofmaintainingthesesystemsasonhigh-volumemotorvehicleproductionlines,anditbecameclearthatsomethingwasneededtoimprovethecontrolprocess–tomakeitmorereliable,easiertotroubleshoot,andmoreadaptabletochangingcontrolneeds.@#@Thatsomething,inthelate1960s,wasthefirstprogrammablecontroller.Thisfirst‘revolutionary’systemwandevelopedasaspecificresponsetotheneedsofthemajorautomotivemanufacturersintheUnitedStates.Theseearlycontrollers,orprogrammablelogiccontrollers（PLC）,representedthefirstsystemsthat1couldbeusedonthefactoryfloor,2couldhavethere‘logic’changedwithoutextensiverewiringorcomponentchanges,and3wereeasytodiagnoseandrepairwhenproblemsoccurred.@#@Itisinterestingtoobservetheprogressthathasbeenmadeinthepast15yearsintheprogrammablecontrollerarea.Thepioneerproductsofthelate1960smusthavebeenconfusingandfrighteningtoagreatnumberofpeople.Forexample,whathappenedtothehardwiredandelectromechanicaldevicesthatmaintenancepersonnelwereusedtorepairingwithhandtools?

@#@Theywerereplacedwith‘computers’disguisedaselectronicsdesignedtoreplacerelays.Eventheprogrammingtoolsweredesignedtoappearasrelayequivalentpresentations.Wehavetheopportunitynowtoexaminethepromise,inretrospect,thattheprogrammablecontrollerbroughttomanufacturing.@#@AllprogrammablecontrollersconsistofthebasicfunctionalblocksshowninFig.10.1.We’llexamineeachblocktounderstandtherelationshiptothecontrolsystem.Firstwelookatthecenter,asitistheheart（oratleastthebrain）ofthesystem.Itconsistsofamicroprocessor,logicmemoryforthestorageoftheactualcontrollogic,storageorvariablememoryforusewithdatathatwillordinarilychangeasafunctionpowerfortheprocessorandmemory.NextcomestheI/Oblock.ThisfunctiontakesthecontrollevelsignalsfortheCPUandconvertsthemtovoltageandcurrentlevelssuitableforconnectionwithfactorygradesensorsandactuators.TheI/Otypecanrangefromdigital（discreteoron/off）,analog（continuouslyvariable）,oravarietyofspecialpurpose‘smart’I/Owhicharededicatedtoacertainapplicationtask.Theprogrammerisshownhere,butitisnormallyusedonlytoinitiallyconfigureandprogramasystemandisnotrequiredforthesystemtooperate.Itisalsousedintroubleshootingasystem,andcanprovetobeavaluabletoolinpinpointingtheexactcauseofaproblem.ThefielddevicesshownhererepresentthevarioussensorsandactuatorsconnectedtotheI/O.Thesearethearms,legs,eyes,andearsofthesystem,includingpushbuttons,limitswitches,proximityswitches,photosensors,thermocouples,RTDS,positionsensingdevices,andbarcodereaderasinput;@#@andpilotlights,displaydevices,motorstarters,DCandACdrives,solenoids,andprintersasoutputs.@#@Nosingleat