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外文翻译--PLC控制系统设计的要点.doc

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外文翻译--PLC控制系统设计的要点.doc

PlccontrolsystemdesignelementsOriginally,thePLCwasrepresentedbytheacronymPC.Therewassomeconfusionwithusingthisacronymasitiscommonlyacceptedtorepresentpersonalcomputer.Therefore,PLCisnowcommonlyacceptedtomeanprogrammablelogiccontoller.APLCisauserfriendly,microprocessorbasedspecializedcomputerthatcarriesoutcontrolfunctionsofmanytypesandlevesofcomplexity.Itspurposeistomonitorcrucialprocessparametersandadjustprocessoperationsaccordingly.Itcanbeprogrammed,controlled,andoperatedbyapersonunskilledinoperatingcomputer.Essentially,aPLCsoperatordrawsthelinesanddevicesofladderdiagramswithakeyboardontoadisplayscreen.Theresultingdrawingisconvertedintocomputermachinelanguageandrunasauserprogram.Thecomputertakestheplaceofmuchoftheexternalwiringrequiredforcontrolofaprocess.ThePLCwilloperateanysystemthathasoutputdevicesthatgoonandoffknownasdiscrete,ordigital,outputs.Itcanalsooperateanysystemwithvariableanalogoutputs.ThePLCcanbeoperatedontheinputsidebyonoffdevicesdiscrete,ordigitalorbyvariableanaloginputdevices.Today,thebigunitgrowthinthePLCindustyisatthelowendwheresmallkeepsgettingsmaller.WhenafewyearsagothemicroPLCenteredthemarket,somethougtthatthesedeviceshadbottomedout.Now,nanoPLCsgenerallydefinedasthosewith16orfewerI/Oarespreading.Somecanfitintoyourshirtpocket,beingnolargerthanadeckofcardsandatthetimeofthiswriting,PLCDirectplantstointroduceaPLCthesizeofaboxofTicTaccandythatwillincludemanyfeaturesofcurrentmicromodels.ThefirstPLCsystemsevolvedfromconventionalcomputersinthelate1960sandearly1970s.ThesefirstPLCswereinstalledprimarilyinautomotiveplants.Traditionally,Theautoplantshadtobeshutdownforuptoamonthatmodelchangeovertime.TheearlyPLCswereusedalongwithothernewautomationtechniquestoshortenthechangeovertime.Oneofthemajortimeconsumingchangeoverprocedureshadbeenthewiringofneworrevisedrelayandcontrolpanels.ThePLCkeyboardreprogrammingprocedurereplacedtherewiringofapanelfullofwires,relays,timersandothercomponents.ThenewPLCshelpedreducechangeovertimetoamatteroffewdays.Therewasamajorproblemwiththeseearly1970scomputer/PLCreprogrammingprocedures.Theprogramswerecomplicatedandrequiredahighlytrainedprogrammertomakethechanges.Throughthelate1970s,improvementsweremadeinPLCprogramstomakethemsomewhatmoreuserfriendlyin1978,theintroductionofthemicroprocessorchipincreasedcomputerpowerforallkindsofautomotionsystemsandloweredthecomputingcost.Robotics,automotiondevices,andcomputersofalltypes,includingthePLC,consequentlyunderwentmanyimprovements.PLCprograms,writteninhighlevellanguage,becamemoreunderstandabletomorepeople,andPLCsbecamemoreaffordable.Inthe1980s,withmorecomputerpowerperdollaravailable,thePLCcameintoexponentiallyincreasinguse.SomelargeelectronicsandcomputercompaniesandsomediversecorporateelectronicsdivisionsfoundthatthePLChadbecometheirgreatestvolumeproduct.ThemarketforPLCsgrewfromavolumeof80millionin1978to1billionperyearby1990andisstillgrowing.Eventhemachinetoolindustry,wherecomputenumericalcontrolsCNCshavebeenusedinthepast,isusingPLCs.PLCsarealsousedextensivelyinbuildingenergyandsecuritycontrolsystems.OthernontraditionalusesofPLCs,suchasinthehomeandinmedicalequipment,havedexplodedinthe1990sandwillincreaseasweenterthenewmillennium.ApersonknowledgeableinrelaylogicsystemscanmasterthemajorPLCfunctionsinafewhours,Thesefunctionsmightincludecoils,contacts,timersandcounters.Thesameistrueforapersonwithadigitallogicbackground.Forpersonsunfamiliarwithladderdiagramsordigitalprinciples,howere,thelearningprocesstakesmoretime.ApersonknowledgeableinrelaylogiccanmasteradvancedPLCfunctionsinafewdayswithproperinstruction.Companyschoolsandoperatingmanualsareveryhelpfulinmasteringtheseadvancedfunctions.Advancedfunctionsinorderoflearningmightincludesequence/drumcontroller,registerbituse,andmorefunctions.Followingare8majoradvantagesofusingaprogrammablecontroller.①Flexibility.Inthepast,eachdifferentelectronicallycontrolledproductionmachinerequiredowncontroller15machinesmightrequire15differentcontrollers.NowitispossibletousejustonemodelofaPLCtorunanyoneofthe15machines.②ImplementingChangeandCorrectingErrors.Withawiredrelaytypepanel,anyprogramalterationsrequiretimeforrewritingofpanelsanddevices.WhenaPLCprogramcircuitorsequencedesignchangeismade,thePLCprogramcanbechangedfromakeyboardsequenceinametterofminutes.NorewiringisrequiredforaPLCcontrolledsystem.③Largequantitiesofcontacts.ThePLChasalargenumberofcontactsforeachcoilavailableinitsprogramming.supposethatapanelwiredrelayhasfourcontactsandallareinusewhenadesignchangerequiringthreemorecontactsismade.Timewouldhavetobetakentoprocureandinstallanewrelayorrelaycontactblock.UsingaPLC,howere,onlythreemorecontactswouldbetypedin.thethreecontactwouldbeautomaticallyavailableinthePLC.Indeed,ahundredcontactscanbeusedfromonerelayifsufficientcomputermemoryisavailable.④Lowercost.Increasedtechnologymakesitpossibletocondensemorefunctionsintosmallerandlessexpensivepackages.NowyoucanpurchaseaPLCwithnumerousrelays,timersandcounters,asequencer,andotherfunctionforafewhundreddollars.⑤PilotRunning.APLCprogrammedcircuitcanbeevaluatedintheofficeorlab.Theprogramcanbetypedin,tested,observed,andmodifiedifneeded,savingvaluablefactorytime.Incontrast,conventionalrelaysystemshavebeentestedonthefactoryfloor,whichcanbeverytimeconsuming.⑥VisualObservation.APLCcircuitsoperationcanbeseenduringoperationdirectlyonaCRTscreen.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,IfaprogrammablecontrollerPLCtoresolvetheissuehasbecomeacyberneticoneofthemosteffectivetools,thisarticledescribesPlccontrolsystemdesignshouldbenoted.HardwarepurchasemanyproductscurrentlyonthemarketPlc,withtheexceptionofdomesticbrands,foreignJapanOmron,Mitsubishi,FUJJ,ANASONIC,GermanySiemens,KoreanLG.Inrecentyears,Plcgreaterdeclineinthepricesofproducts,increasingitsperformancecostrate.ThisisatechnicalstaffchosenPlcimportantreasons.Then,howtopurchasePlcproductsFirst,thesystemshouldfirstdeterminethesizeofsinglecontrolsystemwithPlcorusingPlcnetworking,thuscalculatingPlcimporting,exporting.Few,andinsomePlctobeinrealneedhaveacertaincushionSecond,determinethetypeofloadunderPlcexportledtypeofloadistheDCorexchangetype,bigorsmallcurrents,andPlcexportpointssuchasthefrequencyofmovements,theuseofrelaystodetermineexportexport,ortransistoroutput,orgoodsofexportgates.Differentloadchoosedifferentwaystoexporttotheoperatingsystemsstabilityisveryimportant.Third,thestoragecapacityandspeedwhileforeignmanufacturersofPlcproductsessentiallythesame,buttherearesomedifferences.Currentlythecompanyhasnotyetfoundfullcompatibilitybetweenproducts.Thedevelopmentofsoftwarecompaniesarenotthesame,andusersofstoragecapacityandprocedurestoimplementtheDirectivespeedaretwoimportantindicators.Generalgreaterstoragecapacity,fasterspeedonthePlchigherprices,butshouldbebasedonsystemsizereasonablechoicePlcproducts.Fourth,programmingdevicesPlcprogrammingcanpurchasethreewaysUsinghandheldprogrammingdeviceprogrammingingeneral,itcanprovidebusinessesphrasesprogramminglanguagetable.Thisapproachinefficient,butthecapacityofsmallsystems,theproductsmoresuitableforsmallconsumption,andsmallsize,easeofdebuggingscene,butalsolowercosts.Usinggraphicalprogrammingdeviceprogramming,theprogrammingdeviceusedTrapezoidChartprogrammingforvisualandgeneralelectricalpersonnelshortlybeappliedeasily,buttheprogrammingdevicepriceshigher.UsingIBMpersonalcomputersandsoftwarepackagesusingPlcprogramming,thisapproachisthemostefficientway,butmostofthecompanysPlcdevelopmentsoftwarepackagesexpensive,andthatmeansnotatthescenedebugging.Therefore,thesystemshouldbebasedonthesizeandlevelofdifficulty,thelengthofthedevelopmentcycleandreasonablefundingofthepurchasePlcproducts.Fifth,Andlargecompaniestochoosetheirqualityguaranteedproducts,andgoodtechnicalsupport,aftersalesservicesaregenerallybetter,butalsohelpexpandyourproductandsoftwareupgrades.

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