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外文翻译--可编程逻辑控制器.doc

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外文翻译--可编程逻辑控制器.doc

PLCProgrammableLogicControllersPLCs,alsoreferredtoasprogrammablecontrollers,areinthecomputerfamily.Theyareusedincommercialandindustrialapplications.APLCmonitorsinputs,makedecisionsbasedonitsprogram,andcontrolsoutputstoautomateaprocessormachine.ThiscourseismeanttosupplyyouwithbasicinformationonthefunctionsandconfigurationsofPLCs.Inthe1960s,electromechanicaldevicesweretheorderofthedayasfarascontrolwasconcerned.Thesedevicescommonlyknownasrelays,werebeingusedbythethousandstocontrolmanySequentialtypemanufacturingprocessesandStandalonemachines.Manyoftheserelayswereinuseinthetransportationindustry,morespecifically,theautomothveindustry.Theserelaysusedhundredsofwiresandtheirinterconnectionstoaffectacontrolsolution.Theperformanceofarelaywasbasicallyreliableatleastasasingledevice.Butthecommonapplicationsforrelaypanelscalledfor300to500ormorerelays,andthereliabilityandmaintenanceissuesassociatedwithsuppostingthesepenelsbecameaverygreatchallenge.Costbecameantherissue,forinspiteofthelowcostoftherelayitself,theinstalledcostofthepanelcouldbequitehigh.Thetotalcostinduelingpurchasedparts,wiring,andinstallationlabor,couldrangefrom3050perrelay.Tomakemattersworse,theconstantlychangingneedsofaprocesscalledforrecurringmodificationsofaconstantlychangingneedsofaprocesscalledforrecurringmodificationsofacontrolpanel.Withrelays,thiswasacostlyprospect,asitwasaccomplishedbyamajorrewiringeffortonthepanel.Inaddition,thesechangesweresometimespoorlydocumented,causingasecondshiftmaintencancenightmaremonthslater.Inlightofthis,itwasnotuncommontodiscardanentirecontrolpanelinfavorofanewonewiththeappropriatecomponentswiredinamannersuitedforthenewprocess.Addtothistheunpredictable,andpotentiallyhight,costofmaintainingthesesystemsasonhighvolumemotorvehideproductionlines,anditbecameclearthatsomethingwasneededtoimpronethecontrolprocesstomakeitmorereliable,easiertotroubleshoot,andmoreadaptabletochangingcontrolneeds.Thatsomething,inthelate1960s,wasthefirstprogrammablecontroller.ThisfirstrevollutionarysystemwasdevelopedasaspecificresponsetotheneedsofthemajorautomotinemanufacturersintheUnitedStates.theseearlycontrollers,orprogrammablelogiccontrollersPLC,representedthefirstsystemsthat1couldbeusedonthefactoryfloor,2couldhavetherelogicchangewithoutextensiverewiringorcomponentchanges,and3wereeasytodiagnostandrepairwhenproblemsoccurred.Itisinterestingtoobservetheprogressthathasbeenmadeinthepast15yearsinthelate1960smusthavebeenconfusingandfrighteningtoagreatnumberofpeople.Forexample,whathappenedtothehardwiredandeletromechanicaldevicesthatmaintenancepersonnelwereusedtorepairingwithhardtoolsTheywerereplacedwithcomputersdisquisedaselectromicsdesignedtoreplacerelays.Eventheprogrammingtoolsweredesignedtoappearasrelayequivalentpresentations.Wehavetheopportunitynowtoexaminethepromise,inretrospect,whattheprogrammablecontrollerbroughtmanufacturingAllprogrammalecontrollersconsistofthebasicfunctionalblocks.Wewillexamineeachblocktounderstandtherelathonshiptothecontrolsystem.Firstwelookedatthecenter,asitistheheardofthesystem.Itconsistsofamicroprocessor,logicmemoryforthestorageoftheactualcontrollogic,storageorvariablememoryforusewithdatathatwillordimarilychangeasafunctionofthecontrolpropramexecution,andapowersupplytoprovideelectricalpowerfortheprocessorandmemory.NextcomestheI/Oblock.ThisfunctiontakestheconrtollevelsignalsfortheCPUandconvertsthemtovoltageandcurrentlevelssuitable.TheI/Otypecanrangefromdigital,analog,oravarietyofspecialpurposesmartI/Owhicharededicatedtoacertainapplicationtask.Theprogrammingisnormallyusedonlytoinitiallyconfigureandprogramasystemandisnotrequiredforthesystemtooperate.Itisalsousedintroubleshootingasystem,andcanprovetobeavaluabletoolinpinpointingtheexactcauseofaproblem.ThefielddevicesshownhererepresentthevarioussensorsandactuatorsconnectedtotheI/O.Thesearethearms,legs,eyes,andearsofthesystem,includingpushbuttons,limitswitches,proximitysuitches,photosensors,thermocouple,positionsensingdevices,andbarcodereaderasinputandpildtlight,displaydevices,motorstarters,DCandACdrivers,solenlids,andprintersasoutputs.1.BasicPLCOperationPLCsconsistofinputmodules,aCentralProcessingUnitCPU,andoutputmodulesorpoints.AninputacceptsavarietyofdigitaloranalogsignalsfromvariousfielddevicessensorsandconvertsthemintoalogicsignalthatcanbeusedbytheCPU.TheCPUmakesdecisionsandexecutescontrolinstructionsbasedonprograminstructionsinmemory.OutputmodulesconvertcontrolinstructionsfromtheCPUintoadigitaloranalogsignalthatcanbeusedtocontrolvariousfielddevicesactuators.Aprogrammingdeviceisusedtoinputthedesiredinstructions.TheseinstructionsdeterminewhatthePLCwilldoforaspecificinput.Anoperatorinterfacedeviceallowsprocessinformationtobedisplayedandnewcontrolparameterstobeentered.Pushbuttonssensors,inthissimpleexample,connectedtoPLCinputs,canbeusedtostartandstopamotorconnectedtoaPLCthroughamotorstarteractuator.2.HardWiredControlPriortoPLCs,manyofthesecontroltasksweresolvedbycontactorsorrelaycontrols.Thisisoftenreferredtoashardwiredcontrol.Circuitdiagramshadtobedesigned,electricalcomponentsspecifiedandinstalled,andwiringlistscreated.Electricalcomponentsnecessarytoperformaspecifictask.Ifanerrorwasmadethewireshadtobereconnectedcorrectly.Achangeinfunctionorsystemexpansionrequiredextensivecomponentchangesandrewiring.3.AdvantagesofPLCsThesame,aswellasmorecomplextasks,canbedonewithaPLC.WiringbetweendevicesandrelaycontactsisdoneinthePLCprogram.Hardwiring,thoughstillrequiredtoconnectfielddevices,islessintensive.Modifyingtheapplicationandcorrectingerrorsareeasiertohandle.ItiseasiertocreateandchangeaprograminaPLCthanitistowireandrewireacircuit.FollowingarejustafewoftheadvantagesofPLCs1Smallerphysicalsizethanhardwiresolutions.2Easierandfastertomakechanges.3PLCshaveintegrateddiagnosticsandoverridefunctions.4Applicationscanbeimmediatelydocumented.5Applicationscanbeduplicatedfasterandlessexpensively.4.SiemensPLCsSiemensmakesseveralPLCproductlinesintheSIMATICS7family.TheyareS7200,S7300,andS7400.1S7200TheS7200isreferredtoasamicroPLCbecauseofitssmallsize.TheS7200hasabrickdesignwhichmeansthatthepowersupplyandI/Oterminalareonboard.TheS7200canbeusedonsmaller,standaloneapplicationssuchaselevators,carwashes,ormixingmachines.Itcanalsobeusedonmorecomplexindustrialapplicationssuchasbottlingandpackagingmachines.2S7300andS7400TheS7300andS7400PLCsareusedinmorecomplexapplicationsthatsupportagreaterofI/Opoints.BothPLCsaremodularandexpandable.ThepowersupplyandI/OconsistofseparatemodulesconnectedtotheCPU.ChoosingeithertheS7300orS7400dependsonthecomplexityofthetaskandpossiblefutureexpansion.YourSiemenssalesrepresentativecanprovideyouwithadditionalinformationonanyoftheSiemensPLCs.5.CPUCPUisamicroprocessorsystemthatcontainsthesystemmemoryandisthePLCdecisionmakingunit.TheCPUmonitorstheinputsandmakesdecisionsbasedoninstructionsheldintheprogrammemory.TheCPUperformsrelay,countingtiming,datacomparison,andsequentialoperations.6.ProgrammingDevicesTheprogramiscreatedinaprogrammingdevicePGandthentransferredtothePLC.TheprogramfortheS7200canbecreatedusingadedicatedSiemensSIMATICS7programmingdevice,suchasaPG720orPG740,ifSTEP7Micro/WINsoftwareisinstalled.ApersonalcomputerPC,withSTEP7Micro/WINinstalled,canalsobeusedasaprogrammingdevicewiththeS7200.7.SoftwareAsoftwareprogramisrequiredinordertotellthePLCwhatinstructionsitmustfollow.ProgrammingsoftwareistypicallyPLCspecific.AsoftwarepackageforonePLC,oronefamilyofPLCs,suchastheS7family,wouldnotbeusefulonotherPLCs.TheS7200usesaWindowsbasedsoftwareprogramcalledSTEP7Micro/WIN32.ThePG720andPG740haveSTEP7Micro/WIN32softwarepreinstalled.Micro/WIN32isinstalledonapersonalcomputerinasimilarmannertoanyothercomputersoftware.8.ConnectorCablesPPIPointtoPointInterfaceConnectorcablesarerequiredtotransferdatafromtheprogrammingdeviceto

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