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外文翻译-- PLC概述.doc

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外文翻译-- PLC概述.doc

英文原文1PLCoverviewProgrammablecontrolleristhefirstinthelate1960sintheUnitedStates,thencalledPLCprogrammablelogiccontrollerProgrammableLogicControllerisusedtoreplacerelays.Fortheimplementationofthelogicaljudgment,timing,sequencenumber,andothercontrolfunctions.TheconceptispresentedPLCGeneralMotorsCorporation.PLCandthebasicdesignisthecomputerfunctionalimprovements,flexible,genericandotheradvantagesandrelaycontrolsystemsimpleandeasytooperate,suchastheadvantagesofcheappricescombinedcontrollerhardwareisstandardandoverall.Accordingtothepracticalapplicationoftargetsoftwareinordertocontrolthecontentoftheuserproceduresmemorycontroller,thecontrollerandconnectingtheaccusedconvenienttarget.Inthemid1970s,thePLChasbeenwidelyusedasacentralprocessingunitmicroprocessor,importexportmoduleandtheexternalcircuitsareused,largescaleintegratedcircuitsevenwhenthePLCisnolongertheonlylogicalICjudgmentfunctionsalsohavedataprocessing,PIDconditioninganddatacommunicationsfunctions.InternationalElectrotechnicalCommissionIECstandardspromulgatedprogrammablecontrollerforprogrammablecontrollerdraftmadethefollowingdefinitionprogrammablecontrollerisadigitalelectroniccomputersoperatingsystem,specificallyforapplicationsintheindustrialdesignenvironment.Itusedprogrammablememory,usedtoimplementlogicintheirinternalstorageoperations,sequencecontrol,timing,countingandarithmeticoperations,suchasoperatinginstructions,andthroughdigitalandanaloginputandoutput,thecontrolofvarioustypesofmachineryorproductionprocesses.Programmablecontrollerandrelatedperipherals,andindustrialcontrolsystemseasilylinkedtoformawhole,toexpanditsfunctionaldesign.Programmablecontrollerfortheuser,isanoncontactequipment,theprocedurescanbechangedtochangeproductionprocesses.Theprogrammablecontrollerhasbecomeapowerfultoolforfactoryautomation,widelypopularreplication.Programmablecontrollerisuserorientedindustriesdedicatedcontrolcomputer,withmanydistinctivefeatures.First,highreliability,antiinterferencecapabilitySecond,programmingvisual,simpleThird,adaptabilitygoodFourthfunctionalimprovements,strongfunctionalinterface.2HistoryofPLCProgrammableLogicControllersPLC,acomputingdeviceinventedbyRichardE.Morleyin1968,havebeenwidelyusedinindustryincludingmanufacturingsystems,transportationsystems,chemicalprocessfacilities,andmanyothers.Atthattime,thePLCreplacedthehardwiredlogicwithsoftwiredlogicorsocalledrelayladderlogicRLL,aprogramminglanguagevisuallyresemblingthehardwiredlogic,andreducedtherebytheconfigurationtimefrom6monthsdownto6daysMoodyandMorley,1999.AlthoughPCbasedcontrolhasstartedtocomeintoplace,PLCbasedcontrolwillremainthetechniquetowhichthemajorityofindustrialapplicationswilladhereduetoitshigherperformance,lowerprice,andsuperiorreliabilityinharshenvironments.Moreover,accordingtoastudyonthePLCmarketofFrostandSullivan1995,anincreaseoftheannualsalesvolumeto15millionPLCsperyearwiththehardwarevalueofmorethan8billionUSdollarshasbeenpredicted,thoughthepricesofcomputinghardwareissteadilydropping.TheinventorofthePLC,RichardEMorley,fairlyconsidersthePLCmarketasa5billionindustryatthepresenttime.ThoughPLCsarewidelyusedinindustrialpractice,theprogrammingofPLCbasedcontrolsystemsisstillverymuchrelyingontrialanderror.Alikesoftwareengineering,PLCsoftwaredesignisfacingthesoftwaredilemmaorcrisisinasimilarway.MorleyhimselfemphasizedthisaspectmostforcefullybyindicatingIfhouseswerebuiltlikesoftwareprojects,asinglewoodpeckercoulddestroycivilization.Particularly,practicalproblemsinPLCprogrammingaretoeliminatesoftwarebugsandtoreducethemaintenancecostsofoldladderlogicprograms.ThoughthehardwarecostsofPLCsaredroppingcontinuously,reducingthescantimeoftheladderlogicisstillanissueinindustrysothatlowcostPLCscanbeused.Ingeneral,theproductivityingeneratingPLCisfarbehindcomparedtootherdomains,forinstance,VLSIdesign,whereefficientcomputeraideddesigntoolsareinpractice.ExistentsoftwareengineeringmethodologiesarenotnecessarilyapplicabletothePLCbasedsoftwaredesignbecausePLCprogrammingrequiresasimultaneousconsiderationofhardwareandsoftware.Thesoftwaredesignbecomes,thereby,moreandmorethemajorcostdriver.Inmanyindustrialdesignprojects,morethanofthemanpowerallocatedforthecontrolsystemdesignandinstallationisscheduledfortestinganddebuggingPLCprograms.Inaddition,currentPLCbasedcontrolsystemsarenotproperlydesignedtosupportthegrowingdemandforflexibilityandreconfigurabilityofmanufacturingsystems.Afurtherproblem,impellingtheneedforasystematicdesignmethodology,istheincreasingsoftwarecomplexityinlargescaleprojects.TheobjectiveofthisthesisistodevelopasystematicsoftwaredesignmethodologyforPLCoperatedautomationsystems.Thedesignmethodologyinvolveshighleveldescriptionbasedonstatetransitionmodelsthattreatautomationcontrolsystemsasdiscreteeventsystems,astepwisedesignprocess,andsetofdesignrulesprovidingguidanceandmeasurementstoachieveasuccessfuldesign.Thetangibleoutcomeofthisresearchistofindawaytoreducetheuncertaintyinmanagingthecontrolsoftwaredevelopmentprocess,thatis,reducingprogramminganddebuggingtimeandtheirvariation,increasingflexibilityoftheautomationsystems,andenablingsoftwarereusabilitythroughmodularity.Thegoalistoovercomeshortcomingsofcurrentprogrammingstrategiesthatarebasedontheexperienceoftheindividualsoftwaredeveloper.3NowofPLCFromthestructureisdividedintofixedPLCandModulePLC,thetwokindsofPLCincludingCPUboard,I/Oboard,displaypanel,memoryblock,power,theseelementsintoadonotremoveoverall.ModuletypePLCincludingCPUmodule,I/Omodules,memory,thepowermodules,bottomoraframe,thesemodulescanbeaccordingtocertainrulescombinationconfiguration.Intheuserview,adetailedanalysisoftheCPUsinternalunnecessary,butworkingmechanismofeverypartofthecircuit.TheCPUcontrolworks,byitreadsCPUinstruction,interpretstheinstructionandexecutesinstructions.Butthepaceofworkbyshocksignalcontrol.Unitworkunderthecontrollercommandusedinadigitalorlogicoperation.Incomputingandstorageregisterofcomputationresult,itisalsoamongthecontrollercommandandwork.CPUspeedandmemorycapacityistheimportantparametersforPLC,itsdeterminesthePLCspeedofwork,IOPLCnumberandsoftwarecapacity,solimitstocontrolsize.CentralProcessingUnitCPUisthebrainofaPLCcontroller.CPUitselfisusuallyoneofthemicrocontrollers.Aforetimethesewere8bitmicrocontrollerssuchas8051,andnowtheseare16and32bitmicrocontrollers.UnspokenruleisthatyoullfindmostlyHitachiandFujicumicrocontrollersinPLCcontrollersbyJapanesemakers,SiemensinEuropeancontrollers,andMotorolamicrocontrollersinAmericanones.CPUalsotakescareofcommunication,interconnectednessamongotherpartsofPLCcontrollers,programexecution,memoryoperation,overseeinginputandsettingupofanoutput.SystemmemorytodaymostlyimplementedinFLASHtechnologyisusedbyaPLCforaprocesscontrolsystem.Asideform.thisoperatingsystemitalsocontainsauserprogramtranslatedformaladderdiagramtoabinaryform.FLASHmemorycontentscanbechangedonlyincasewhereuserprogramisbeingchanged.PLCcontrollerswereusedearlierinsteadofPLASHmemoryandhavehadEPROMmemoryinsteadofFLASHmemorywhichhadtobeerasedwithUVlampandprogrammedonprogrammers.WiththeuseofFLASHtechnologythisprocesswasgreatlyshortened.Reprogrammingaprogrammemoryisdonethroughaserialcableinaprogramforapplicationdevelopment.Usermemoryisdividedintoblockshavingspecialfunctions.Somepartsofamemoryareusedforstoringinputandoutputstatus.Therealstatusofaninputisstoredeitheras1oras0inaspecificmemorybit/eachinputoroutputhasonecorrespondingbitinmemory.Otherpartsofmemoryareusedtostorevariablecontentsforvariablesusedinusedprogram.Forexample,timevalue,orcountervaluewouldbestoredinthispartofthememory.PLCcontrollercanbereprogrammedthroughacomputerusualway,butalsothroughmanualprogrammersconsoles.ThispracticallymeansthateachPLCcontrollercanprogrammedthroughacomputerifyouhavethesoftwareneededforprogramming.TodaystransmissioncomputersareidealforreprogrammingaPLCcontrollerinfactoryitself.Thisisofgreatimportancetoindustry.Oncethesystemiscorrected,itisalsoimportanttoreadtherightprogramintoaPLCagain.ItisalsogoodtocheckfromtimetotimewhetherprograminaPLChasnotchanged.ThishelpstoavoidhazardoussituationsinfactoryroomssomeautomakershaveestablishedcommunicationnetworkswhichregularlycheckprogramsinPLCcontrollerstoensureexecutiononlyofgoodprograms.AlmosteveryprogramforprogrammingaPLCcontrollerpossessesvarioususefuloptionssuchasforcedswitchingonandoffofthesysteminput/outputsI/Olines,programfollowupinrealtimeaswellasdocumentingadiagram.Thisdocumentingisnecessarytounderstandanddefinefailuresandmalfunctions.Programmercanaddremarks,namesofinputoroutputdevices,andcommentsthatcanbeusefulwhenfindingerrors,orwithsystemmaintenance.Addingcommentsandremarksenablesanytechnicianandnotjustapersonwhodevelopedthesystemtounderstandaladderdiagramrightaway.Commentsandremarkscanevenquotepreciselypartnumbersifreplacementswouldbeneeded.Thiswouldspeeduparepairofanyproblemsthatcomeupduetobadparts.Theoldwaywassuchthatapersonwhodevelopedasystemhadprotectionontheprogram,sonobodyasidefromthispersoncouldunderstandhowitwasdone.Correctlydocumentedladderdiagramallowsanytechniciantounderstandthoroughlyhowsystemfunctions.Electricalsupplyisusedinbringingelectricalenergytocentralprocessingunit.MostPLCcontrollersworkeitherat24VDCor220VAC.OnsomePLCcontrollersyoullfindelectricalsupplyasaseparatemodule.ThoseareusuallybiggerPLCcontrollers,whilesmallandmediumseriesalreadycontainthesupplymodule.UserhastodeterminehowmuchcurrenttotakefromI/Omoduletoensurethatelectricalsupplyprovidesappropriateamountofcurrent.Differenttypesofmodulesusedifferentamountsofelectricalcurrent.Thiselectricalsupplyisusuallynotusedtostartexternalinputoroutput.UserhastoprovideseparatesuppliesinstartingPLCcontrollerinputsbecausethenyoucanensuresocalledpuresupplyforthePLCcontroller.Withpuresupplywemeansupplywhereindustrialenvironmentcannotaffectitdamagingly.SomeofthesmallerPLCcontrollerssupplytheirinputswithvoltagefromasmallsupplysourcealreadyincorporatedintoaPLC.4PLCdesigncriteriaAsystematicapproachtodesigningPLCsoftwarecanovercomedeficienciesinthetraditionalwayofprogrammingmanufacturingcontrolsystems,andcanhavewideramificationsinseveralindustrialapplications.Automationcontrolsystemsaremodeledbyformallanguagesor,equivalently,bystatemachines.Formalrepresentationsprovideahighleveldescriptionofthebehaviorofthesystemtobecontrolled.Statemachinescanbeanalyticallyevaluatedastowhetherornottheymeetthedesiredgoals.Secondly,astatemachinedescriptionprovidesastructuredrepresentationtoconveythelogicalrequirementsandconstraintssuchasdetailedsafetyrules.Thirdly,welldefinedcontrolsystemsdesignoutcomesareconducivetoautomaticcodegenerationAnabilitytoproducecontrolsoftwareexecutableoncommercialdistinctlogiccontrollerscanreduceprogrammingleadtimeandlaborcost.Inparticular,thethesisisrelevantwithrespecttothefollowingaspects.Inmodernmanufacturing,systemsarecharacterizedbyproductandprocessinnovation,becomecustomerdrivenandthushavetorespondquicklytochangingsystemrequirements.Amajorchallengeisthereforetoprovideenablingtechnologiesthatcaneconomicallyreconfigureautomationcontrolsystemsinresponsetochangingneedsandnewopportunities.Designandoperationalknowledgecanbereusedinrealtime,therefore,givingasignificantcompetitiveedge

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