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文献翻译
南昌工程学院
07级毕业(设计)论文文献翻译
机械与电气工程系(院)电气工程及其自动化专业
题目基于PLC污水处理控制系统
学生姓名杨佳明
班级电气工程及其自动化
(1)班
学号2007100038
指导教师代芳
日期2011年01月06日
南昌工程学院教务处订制
评语:
指导教师:
年月日
PLC
Abstract:
Asakindofindustrialequipmentpowerhouses,withitsstraightawayprogramminganditsstablejobperformanceinbadenvironment,PLCwonthegeneralcustomers.PLChasbeenwidelyusedinindustrialareas,includingmachinerymanufacturing,transportationsystem,andchemicalprocessequipment,suchasmanyotherfields.Althoughcomputercontroltechnologyhascreated,itshighperformanceandlowcost,andthebadenvironmenthavestrongadaptability,soPLCcontroliswidelyusedinindustrialcontrol.
Keywords:
ProgrammableLogicController(PLC);software
1.ThedevelopmentofPLC
Programmablecontrollerappearedinthelate1960sintheUnitedStates,andthePLC(ProgrammableLogicController)isusedtoreplacerelays.Fortheimplementationofthelogicaljudgment,timing,sequencenumber,andothercontrolfunctions.TheconceptofPLCispresentedbyGeneralMotorsCorporation.ThebasicdesignofPLCisthoseoverallscombinedthecomputerfunctionalimprovements,suchasflexible,generic,relaycontrolsystemsimpleandeasytooperate,andwiththeadvantagesofcheappricescombinedcontrollerhardware.Accordingtothepracticalapplicationoftargetsoftware,thecontentwritestotheuserproceduresmemorycontroller.Thecontrollercanbeconnectedeasilywiththeaccusedconvenienttarget.
Inthemid-1970s,thePLChasbeenwidelyusedasacentralprocessingunitmicroprocessor,importexportmoduleandtheexternalcircuitsareused,large-scaleintegratedcircuitsevenwhenthePLCisnolongertheonlylogical(IC)judgmentfunctionsalsohavedataprocessing,PIDconditioninganddatacommunicationsfunctions.InternationalElectrotechnicalCommission(IEC)standardspromulgatedprogrammablecontrollerforprogrammablecontrollerdraftmadethefollowingdefinition:
programmablecontrollerisadigitalelectroniccomputersoperatingsystem,specificallyforapplicationsintheindustrialdesignenvironment.Itusedprogrammablememory,usedtoimplementlogicintheirinternalstorageoperations,sequencecontrol,timing,countingandarithmeticoperations,suchasoperatinginstructions,andthroughdigitalandanaloginputandoutput,thecontrolofvarioustypesofmachineryorproductionprocesses.Programmablecontrollerandrelatedperipherals,andindustrialcontrolsystemseasilylinkedtoformawhole,toexpanditsfunctionaldesign.Programmablecontrollerfortheuser,isnoncontactequipment,theprocedurescanbechangedtochangeproductionprocesses.Theprogrammablecontrollerhasbecomeapowerfultoolforfactoryautomation,widelypopularreplication.Programmablecontrollerisuser-orientedindustriesdedicatedcontrolcomputer,withmanydistinctivefeatures.
①Highreliability,anti-interferencecapability;
②Programmingvisual,simple;
③Adaptabilitygood;
④Functionalimprovements,strongfunctionalinterface.
2.ThestructureofthePLC
CentralProcessingUnit(CPU)isthebrainofaPLCcontroller.CPUitselfisusuallyoneofthemicrocontrollers.Aforetimethesewere8-bitmicrocontrollerssuchas8051,andnowtheseare16-and32-bitmicrocontrollers.Unspokenruleisthatyou’llfindmostlyHitachiandFujicumicrocontrollersinPLCcontrollersbyJapanesemakers,SiemensinEuropeancontrollers,andMotorolamicrocontrollersinAmericanones.CPUalsotakescareofcommunication,interconnectednessamongotherpartsofPLCcontrollers,programexecution,memoryoperation,overseeinginputandsettingupofanoutput.PLCcontrollershavecomplexroutinesformemorycheckupinordertoensurethatPLCmemorywasnotdamaged(memorycheckupisdoneforsafetyreasons).Generallyspeaking,CPUunitmakesagreatnumberofcheck-upsofthePLCcontrolleritselfsoeventualerrorswouldbediscoveredearly.YoucansimplylookatanyPLCcontrollerandseethatthereareseveralindicatorsintheform.Lightdiodesforerrorsignalization.
Systemmemory(todaymostlyimplementedinFLASHtechnology)isusedbyaPLCforaprocesscontrolsystem.Asideform.Thisoperatingsystemitalsocontainsauserprogramtranslatedformaladderdiagramtoabinaryform.FLASHmemorycontentscanbechangedonlyincasewhereuserprogramisbeingchanged.PLCcontrollerswereusedearlierinsteadofPLASHmemoryandhavehadEPROMmemoryinsteadofFLASHmemorywhichhadtobeerasedwithUVlampandprogrammedonprogrammers.WiththeuseofFLASHtechnologythisprocesswasgreatlyshortened.Reprogrammingaprogrammemoryisdonethroughaserialcableinaprogramforapplicationdevelopment.
Usermemoryisdividedintoblockshavingspecialfunctions.Somepartsofamemoryareusedforstoringinputandoutputstatus.Therealstatusofaninputisstoredeitheras“1”oras“0”inaspecificmemorybit/eachinputoroutputhasonecorrespondingbitinmemory.Otherpartsofmemoryareusedtostorevariablecontentsforvariablesusedinusedprogram.Forexample,timevalue,orcountervaluewouldbestoredinthispartofthememory.
PLCcontrollercanbereprogrammedthroughacomputer(usualway),butalsothroughmanualprogrammers(consoles).ThispracticallymeansthateachPLCcontrollercanprogrammedthroughacomputerifyouhavethesoftwareneededforprogramming.Today’stransmissioncomputersareidealforreprogrammingaPLCcontrollerinfactoryitself.Thisisofgreatimportancetoindustry.Oncethesystemiscorrected,itisalsoimportanttoreadtherightprogramintoaPLCagain.ItisalsogoodtocheckfromtimetotimewhetherprograminaPLChasnotchanged.Thishelpstoavoidhazardoussituationsinfactoryrooms(someautomakershaveestablishedcommunicationnetworkswhichregularlycheckprogramsinPLCcontrollerstoensureexecutiononlyofgoodprograms).
AlmosteveryprogramforprogrammingaPLCcontrollerpossessesvarioususefuloptionssuchas:
forcedswitchingonandoffofthesysteminput/outputs(I/Olines),programfollowupinrealtimeaswellasdocumentingadiagram.Thisdocumentingisnecessarytounderstandanddefinefailuresandmalfunctions.Programmercanaddremarks,namesofinputoroutputdevices,andcommentsthatcanbeusefulwhenfindingerrors,orwithsystemmaintenance.Addingcommentsandremarksenablesanytechnician(andnotjustapersonwhodevelopedthesystem)tounderstandaladderdiagramrightaway.Commentsandremarkscanevenquotepreciselypartnumbersifreplacementswouldbeneeded.Thiswouldspeeduparepairofanyproblemsthatcomeupduetobadparts.Theoldwaywassuchthatapersonwhodevelopedasystemhadprotectionontheprogram,sonobodyasidefromthispersoncouldunderstandhowitwasdone.Correctlydocumentedladderdiagramallowsanytechniciantounderstandthoroughlyhowsystemfunctions.
Electricalsupplyisusedinbringingelectricalenergytocentralprocessingunit.MostPLCcontrollersworkeitherat24VDCor220VAC.OnsomePLCcontrollersyou’llfindelectricalsupplyasaseparatemodule.ThoseareusuallybiggerPLCcontrollers,whilesmallandmediumseriesalreadycontainthesupplymodule.UserhastodeterminehowmuchcurrenttotakefromI/Omoduletoensurethatelectricalsupplyprovidesappropriateamountofcurrent.Differenttypesofmodulesusedifferentamountsofelectricalcurrent.
Thiselectricalsupplyisusuallynotusedtostartexternalinputoroutput.UserhastoprovideseparatesuppliesinstartingPLCcontrollerinputsbecausethenyoucanensuresocalled“pure”supplyforthePLCcontroller.Withpuresupplywemeansupplywhereindustrialenvironmentcannotaffectitdamagingly.SomeofthesmallerPLCcontrollerssupplytheirinputswithvoltagefromasmallsupplysourcealreadyincorporatedintoaPLC.
3.TheproblemsofPLC
PLC(programmablelogicalcontroller)faceevermorecomplexchallengethesedays.Whereoncetheyquietlyrelaysandgaveanoccasionalreporttoacorporatemainframe,theyarenowgroundedintocells,givenewjobsandnewlanguages,andareforcedtocompeteagainstagrowingarrayofcontrolproducts.
HigherlevelPLCprogramminglanguageshavebeenaroundforsometime,butlatelytheirpopularityhasbeenmushrooming.AsRaymondLavelle,vicepresidentandgeneralmanager,Siemensenergyandautomation.Inc,ProgrammableControlsDivision,pointsout:
”Asprogrammablecontrolsisbeingusedformoreandmoresophisticatedoperations,languagesotherthanladderlogicbecomemorepractical,efficient,andpowerful.Forexample,it'sverydifficulttowriteatrigonometricfunctionusingladderlogic.”LanguagesgainingacceptanceincludeBoolean,controlsystemflowcharting,andsuchfunctionchartlanguagesasGraphtecanditsvariations.AndtheseareincreasinginterestinlanguageslikeCandBASIC.
Thesoftwareindustryisregardedasaperformancedestructorandcomplexitygenerator.Steadilyshrinkinghardwarepricesspoilstheneedforsoftwareperformanceintermsofcodeoptimizationandefficiency.Theresultisthatmassiveandlessefficientsoftwarecodeononehandoutpacesthegainsinhardwareperformanceontheotherhand.Secondly,softwareproliferatesintocomplexityofunmanageabledimensions;softwareredesignandmaintenance-essentialinmodernautomationsystems-becomesnearlyimpossible.Particularly,PLCprogramshaveevolvedfromacouplelinesofcode25yearsagotothousandsoflinesofcodewithasimilarnumberof1/Opoints.Increasedsafety,forinstancenewpoliciesonfireprotectionandtheflexibilityofmodernautomationsystemsaddcomplexitytotheprogramd