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外文翻译--PLC控制系统设计的要点.doc外文翻译--PLC控制系统设计的要点.doc

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编号:201311171103585678    类型:共享资源    大小:46.50KB    格式:DOC    上传时间:2013-11-17
  
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PlccontrolsystemdesignelementsOriginally,thePLCwasrepresentedbytheacronymPC.Therewassomeconfusionwithusingthisacronymasitiscommonlyacceptedtorepresentpersonalcomputer.Therefore,PLCisnowcommonlyacceptedtomeanprogrammablelogiccontoller.APLCisauser-friendly,microprocessor-basedspecializedcomputerthatcarriesoutcontrolfunctionsofmanytypesandlevesofcomplexity.Itspurposeistomonitorcrucialprocessparametersandadjustprocessoperationsaccordingly.Itcanbeprogrammed,controlled,andoperatedbyapersonunskilledinoperatingcomputer.Essentially,aPLC’soperatordrawsthelinesanddevicesofladderdiagramswithakeyboardontoadisplayscreen.Theresultingdrawingisconvertedintocomputermachinelanguageandrunasauserprogram.Thecomputertakestheplaceofmuchoftheexternalwiringrequiredforcontrolofaprocess.ThePLCwilloperateanysystemthathasoutputdevicesthatgoonandoff(knownasdiscrete,ordigital,outputs).Itcanalsooperateanysystemwithvariable(analog)outputs.ThePLCcanbeoperatedontheinputsidebyon-offdevices(discrete,ordigital)orbyvariable(analog)inputdevices.Today,thebigunitgrowthinthePLCindustyisatthelowend-wheresmallkeepsgettingsmaller.WhenafewyearsagothemicroPLCenteredthemarket,somethougtthatthesedeviceshad“bottomedout”.Now,nanoPLCs-generallydefinedasthosewith16orfewerI/O-arespreading.Somecanfitintoyourshirtpocket,beingnolargerthanadeckofcardsandatthetimeofthiswriting,PLCDirectplantstointroduceaPLCthesizeofaboxofTic-Taccandythatwillincludemanyfeaturesofcurrentmicromodels.ThefirstPLCsystemsevolvedfromconventionalcomputersinthelate1960sandearly1970s.ThesefirstPLCswereinstalledprimarilyinautomotiveplants.Traditionally,Theautoplantshadtobeshutdownforuptoamonthatmodelchangeovertime.TheearlyPLCswereusedalongwithothernewautomationtechniquestoshortenthechangeovertime.Oneofthemajortime-consumingchangeoverprocedureshadbeenthewiringofneworrevisedrelayandcontrolpanels.ThePLCkeyboardreprogrammingprocedurereplacedtherewiringofapanelfullofwires,relays,timersandothercomponents.ThenewPLCshelpedreducechangeovertimetoamatteroffewdays.Therewasamajorproblemwiththeseearly1970scomputer/PLCreprogrammingprocedures.Theprogramswerecomplicatedandrequiredahighlytrainedprogrammertomakethechanges.Throughthelate1970s,improvementsweremadeinPLCprogramstomakethemsomewhatmoreuserfriendly;in1978,theintroductionofthemicroprocessorchipincreasedcomputerpowerforallkindsofautomotionsystemsandloweredthecomputingcost.Robotics,automotiondevices,andcomputersofalltypes,includingthePLC,consequentlyunderwentmanyimprovements.PLCprograms,writteninhigh-levellanguage,becamemoreunderstandabletomorepeople,andPLCsbecamemoreaffordable.Inthe1980s,withmorecomputerpowerperdollaravailable,thePLCcameintoexponentiallyincreasinguse.SomelargeelectronicsandcomputercompaniesandsomediversecorporateelectronicsdivisionsfoundthatthePLChadbecometheirgreatestvolumeproduct.ThemarketforPLCsgrewfromavolumeof$80millionin1978to$1billionperyearby1990andisstillgrowing.Eventhemachinetoolindustry,wherecomputenumericalcontrols(CNCs)havebeenusedinthepast,isusingPLCs.PLCsarealsousedextensivelyinbuildingenergyandsecuritycontrolsystems.OthernontraditionalusesofPLCs,suchasinthehomeandinmedicalequipment,havedexplodedinthe1990sandwillincreaseasweenterthenewmillennium.ApersonknowledgeableinrelaylogicsystemscanmasterthemajorPLCfunctionsinafewhours,Thesefunctionsmightincludecoils,contacts,timersandcounters.Thesameistrueforapersonwithadigitallogicbackground.Forpersonsunfamiliarwithladderdiagramsordigitalprinciples,howere,thelearningprocesstakesmoretime.ApersonknowledgeableinrelaylogiccanmasteradvancedPLCfunctionsinafewdayswithproperinstruction.Companyschoolsandoperatingmanualsareveryhelpfulinmasteringtheseadvancedfunctions.Advancedfunctionsinorderoflearningmightincludesequence/drumcontroller,registerbituse,andmorefunctions.Followingare8majoradvantagesofusingaprogrammablecontroller.①Flexibility.Inthepast,eachdifferentelectronicallycontrolledproductionmachinerequiredowncontroller;15machinesmightrequire15differentcontrollers.NowitispossibletousejustonemodelofaPLCtorunanyoneofthe15machines.②ImplementingChangeandCorrectingErrors.Withawiredrelay-typepanel,anyprogramalterationsrequiretimeforrewritingofpanelsanddevices.WhenaPLCprogramcircuitorsequencedesignchangeismade,thePLCprogramcanbechangedfromakeyboardsequenceinametterofminutes.NorewiringisrequiredforaPLC-controlledsystem.③Largequantitiesofcontacts.ThePLChasalargenumberofcontactsforeachcoilavailableinitsprogramming.supposethatapanel-wiredrelayhasfourcontactsandallareinusewhenadesignchangerequiringthreemorecontactsismade.Timewouldhavetobetakentoprocureandinstallanewrelayorrelaycontactblock.UsingaPLC,howere,onlythreemorecontactswouldbetypedin.thethreecontactwouldbeautomaticallyavailableinthePLC.Indeed,ahundredcontactscanbeusedfromonerelay-ifsufficientcomputermemoryisavailable.④Lowercost.Increasedtechnologymakesitpossibletocondensemorefunctionsintosmallerandlessexpensivepackages.NowyoucanpurchaseaPLCwithnumerousrelays,timersandcounters,asequencer,andotherfunctionforafewhundreddollars.⑤PilotRunning.APLCprogrammedcircuitcanbeevaluatedintheofficeorlab.Theprogramcanbetypedin,tested,observed,andmodifiedifneeded,savingvaluablefactorytime.Incontrast,conventionalrelaysystemshavebeentestedonthefactoryfloor,whichcanbeverytimeconsuming.⑥VisualObservation.APLCcircuit’soperationcanbeseenduringoperationdirectlyonaCRTscreen.Theoperationormisoperationofacircuitcanbeobservedasithappens.Logicpathslightuponthescreenastheyareenergized.Troubleshootingcanbedonemorequicklyduringvisualobservation.InadvancedPLCsystems,anoperatormessagecanbeprogrammedforeachpossiblemalfunction.InadvancedPLCsystems,anoperatormessagecanbeprogrammedforeachpossiblemalfunction.ThemalfunctiondescriptionappearsonthescreenwhenthemalfunctionisdetectedbythePLClogic.AdvancedPLCsystemsalsomayhavedescriptionsofthefunctionofeachcircuitcomponent.⑦SpeedofOperation.Relayscantakeanunacceptableamountoftimetoactuate.TheoperationalspeedforthePLCprogramisveryfast.ThespeedforthePLClogicoperationisdeterminedbyscantime,whichisamatterofmilliseconds.⑧LadderorBooleanProgrammingMethod.ThePLCprogrammingcanbeaccomplishedintheladdermodebyanelectricianortechnician.Alternatively,aPLCprogrammerwhoworksindigitalorBooleancontrolsystemscanalsoeasilyperformPLCprogramming.Inmodernindustrialproductionequipment,andlargenumbersofsimulationsofthecontroldevices,suchastheceaseelectrical,electromagneticdevicestoturn,thenumberofproducts,temperature,Appower,andsettheflowcontrol,theautomaticcontroloftheindustrialsite,Ifaprogrammablecontroller(PLC)toresolvetheissuehasbecomeacyberneticoneofthemosteffectivetools,thisarticledescribesPlccontrolsystemdesignshouldbenoted.HardwarepurchasemanyproductscurrentlyonthemarketPlc,withtheexceptionofdomesticbrands,foreign:JapanOmron,Mitsubishi,FUJJ,ANASONIC,GermanySiemens,KoreanLG.Inrecentyears,Plcgreaterdeclineinthepricesofproducts,increasingitsperformancecostrate.ThisisatechnicalstaffchosenPlcimportantreasons.Then,howtopurchasePlcproducts?First,thesystemshouldfirstdeterminethesizeofsinglecontrolsystemwithPlcorusingPlcnetworking,thuscalculatingPlcimporting,exporting.Few,andinsomePlctobeinrealneedhaveacertaincushionSecond,determinethetypeofloadunderPlcexport-ledtypeofloadistheDCorexchange-type,bigorsmallcurrents,andPlcexportpointssuchasthefrequencyofmovements,theuseofrelaystodetermineexport-export,ortransistoroutput,orgoodsofexportgates.Differentloadchoosedifferentwaystoexporttotheoperatingsystem'sstabilityisveryimportant.Third,thestoragecapacityandspeedwhileforeignmanufacturersofPlcproductsessentiallythesame,buttherearesomedifferences.Currentlythecompanyhasnotyetfoundfullcompatibilitybetweenproducts.Thedevelopmentofsoftwarecompaniesarenotthesame,andusersofstoragecapacityandprocedurestoimplementtheDirectivespeedaretwoimportantindicators.Generalgreaterstoragecapacity,fasterspeedonthePlchigherprices,butshouldbebasedonsystemsizereasonablechoicePlcproducts.Fourth,programmingdevicesPlcprogrammingcanpurchasethreeways:Usinghand-heldprogrammingdeviceprogrammingingeneral,itcanprovidebusinessesphrasesprogramminglanguagetable.Thisapproachinefficient,butthecapacityofsmallsystems,theproductsmoresuitableforsmallconsumption,andsmallsize,easeofdebuggingscene,butalsolowercosts.Usinggraphicalprogrammingdeviceprogramming,theprogrammingdeviceusedTrapezoidChartprogrammingforvisualandgeneralelectricalpersonnelshortlybeappliedeasily,buttheprogrammingdevicepriceshigher.UsingIBMpersonalcomputersandsoftwarepackagesusingPlcprogramming,thisapproachisthemostefficientway,butmostofthecompany'sPlcdevelopmentsoftwarepackagesexpensive,andthatmeansnotatthescenedebugging.Therefore,thesystemshouldbebasedonthesizeandlevelofdifficulty,thelengthofthedevelopmentcycleandreasonablefundingofthepurchasePlcproducts.Fifth,Andlargecompaniestochoosetheirqualityguaranteedproducts,andgoodtechnicalsupport,aftersalesservicesaregenerallybetter,butalsohelpexpandyourproductandsoftwareupgrades.
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