机械工程毕业设计外文翻译自动化制造系统与PLCWord文档下载推荐.docx

机械工程毕业设计外文翻译自动化制造系统与PLCWord文档下载推荐.docx

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2.1INTRODUCTION

Controlengineeringhasevolvedovertime.Inthepasthumanswerethemainmethodforcontrollingasystem.Morerecentlyelectricityhasbeenusedforcontrolandearlyelectricalcontrolwasbasedonrelays.Theserelaysallowpowertobeswitchedonandoffwithoutamechanicalswitch.Itiscommontouserelaystomakesimplelogicalcontroldecisions.Thedevelopmentoflowcostcomputerhasbroughtthemostrecentrevolution,theProgrammableLogicController（PLC）.TheadventofthePLCbeganinthe1970s,andhasbecomethemostcommonchoiceformanufacturingcontrols.

PLCshavebeengainingpopularityonthefactoryfloorandwillprobablyremainpredominantforsometimetocome.Mostofthisisbecauseoftheadvantagestheyoffer.

•Costeffectiveforcontrollingcomplexsystems.

•Flexibleandcanbereappliedtocontrolothersystemsquicklyandeasily.

•Computationalabilitiesallowmoresophisticatedcontrol.

•Troubleshootingaidsmakeprogrammingeasierandreducedowntime.

•Reliablecomponentsmaketheselikelytooperateforyearsbeforefailure.

2.1.1Ladderlogic

LadderlogicisthemainprogrammingmethodusedforPLCs.Asmentionedbefore,ladderlogichasbeendevelopedtomimicrelaylogic.logicdiagramswasastrategicone.Byselectingladderlogicasthemainprogrammingmethod,theamountofretrainingneededforengineersandtradespeoplewasgreatlyreduced.

Moderncontrolsystemsstillincluderelays,butthesearerarelyusedforlogic.Arelayisasimpledevicethatusesamagneticfieldtocontrolaswitch,aspicturedinFigure2.1.Whenavoltageisappliedtotheinputcoil,theresultingcurrentcreatesamagneticfield.Themagneticfieldpullsametalswitch（orreed）towardsitandthecontactstouch,closingtheswitch.Thecontactthatcloseswhenthecoilisenergizediscallednormallyopen.Thenormallyclosedcontactstouchwhentheinputcoilisnotenergized.Relaysarenormallydrawninschematicformusingacircletorepresenttheinputcoil.Theoutputcontactsareshownwithtwoparallellines.Normallyopencontactsareshownastwolines,andwillbeopen（non-conducting）whentheinputisnotenergized.Normallyclosedcontactsareshownwithtwolineswithadiagonallinethroughthem.Whentheinputcoilisnotenergizedthenormallyclosedcontactswillbeclosed（conducting）.

Figure2.1SimpleRelayLayoutsandSchematics

Relaysareusedtoletonepowersourcecloseaswitchforanother（oftenhighcurrent）powersource,whilekeepingthemisolated.AnexampleofarelayinasimplecontrolapplicationisshowninFigure2.2.Inthissystemthefirstrelayontheleftisusedasnormallyclosed,andwillallowcurrenttoflowuntilavoltageisappliedtotheinputA.ThesecondrelayisnormallyopenandwillnotallowcurrenttoflowuntilavoltageisappliedtotheinputB.Ifcurrentisflowingthroughthefirsttworelaysthencurrentwillflowthroughthecoilinthethirdrelay,andclosetheswitchforoutputC.Thiscircuitwouldnormallybedrawnintheladderlogicform.ThiscanbereadlogicallyasCwillbeonifAisoffandBison.

Figure2.2ASimpleRelayController

TheexampleinFigure2.2doesnotshowtheentirecontrolsystem,butonlythelogic.WhenweconsideraPLCthereareinputs,outputs,andthelogic.Figure2.3showsamorecompleterepresentationofthePLC.Heretherearetwoinputsfrompushbuttons.Wecanimaginetheinputsasactivating24VDCrelaycoilsinthePLC.Thisinturndrivesanoutputrelaythatswitches115VAC,thatwillturnonalight.Note,inactualPLCsinputsareneverrelays,butoutputsareoftenrelays.TheladderlogicinthePLCisactuallyacomputerprogramthattheusercanenterandchange.Noticethatbothoftheinputpushbuttonsarenormallyopen,buttheladderlogicinsidethePLChasonenormallyopencontact,andonenormallyclosedcontact.DonotthinkthattheladderlogicinthePLCneedsomatchtheinputsoroutputs.Manybeginnerswillgetcaughttryingtomaketheladderlogicmatchtheinputtypes.

Figure2.3APLCIllustratedWithRelays

Manyrelaysalsohavemultipleoutputs（throws）andthisallowsanoutputrelaytoalsobeaninputsimultaneously.ThecircuitshowninFigure1.4isanexampleofthis,itiscalledasealincircuit.Inthiscircuitthecurrentcanflowthrougheitherbranchofthecircuit,throughthecontactslabelledAorB.TheinputBwillonlybeonwhentheoutputBison.IfBisoff,andAisenergized,thenBwillturnon.IfBturnsonthentheinputBwillturnon,andkeepoutputBonevenifinputAgoesoff.AfterBisturnedontheoutputBwillnotturnoff.

Figure2.4ASeal-inCircuit

2.1.2Programming

ThefirstPLCswereprogrammedwithatechniquethatwasbasedonrelaylogicwiringschematics.Thiseliminatedtheneedtoteachtheelectricians,techniciansandengineershowtoprogramacomputer-but,thismethodhasstuckanditisthemostcommontechniqueforprogrammingPLCstoday.AnexampleofladderlogiccanbeseeninFigure2.5.Tointerpretthisdiagramimaginethatthepowerisontheverticallineonthelefthandside,wecallthisthehotrail.Ontherighthandsideistheneutralrail.Inthefiguretherearetworungs,andoneachrungtherearecombinationsofinputs（twoverticallines）andoutputs（circles）.Iftheinputsareopenedorclosedintherightcombinationthepowercanflowfromthehotrail,throughtheinputs,topowertheoutputs,andfinallytotheneutralrail.Aninputcancomefromasensor,switch,oranyothertypeofsensor.AnoutputwillbesomedeviceoutsidethePLCthatisswitchedonoroff,suchaslightsormotors.Inthetoprungthecontactsarenormallyopenandnormallyclosed.WhichmeansifinputAisonandinputBisoff,thenpowerwillflowthroughtheoutputandactivateit.AnyothercombinationofinputvalueswillresultintheoutputXbeingoff.

Figure2.5ASimpleLadderLogicDiagram

ThesecondrungofFigure2.5ismorecomplex,thereareactuallymultiplecombinationsofinputsthatwillresultintheoutputYturningon.Ontheleftmostpartoftherung,powercouldflowthroughthetopifCisoffandDison.Powercouldalso（andsimultaneously）flowthroughthebottomifbothEandFaretrue.Thiswouldgetpowerhalfwayacrosstherung,andthenifGorHistruethepowerwillbedeliveredtooutputY.Inlaterchapterswewillexaminehowtointerpretandconstructthesediagrams.

ThereareothermethodsforprogrammingPLCs.Oneoftheearliesttechniquesinvolvedmnemonicinstructions.TheseinstructionscanbederiveddirectlyfromtheladderlogicdiagramsandenteredintothePLCthroughasimpleprogrammingterminal.AnexampleofmnemonicsisshowninFigure2.6.Inthisexampletheinstructionsarereadonelineatatimefromtoptobottom.Thefirstline00000hastheinstructionLDN（inputloadandnot）forinputA..ThiswillexaminetheinputtothePLCandifitisoffitwillremembera1（ortrue）,ifitisonitwillremembera0（orfalse）.ThenextlineusesanLD（inputload）statementtolookattheinput.Iftheinputisoffitremembersa0,iftheinputisonitremembersa1（note:

thisisthereverseoftheLD）.TheANDstatementrecallsthelasttwonumbersrememberedandifthearebothtruetheresultisa1,otherwisetheresultisa0.Thisresultnowreplacesthetwonumbersthatwererecalled,andthereisonlyonenumberremembered.Theprocessisrepeatedforlines00003and00004,butwhenthesearedonetherearenowthreenumbersremembered.TheoldestnumberisfromtheAND,thenewernumbersarefromthetwoLDinstructions.TheANDinline00005combinestheresultsfromthelastLDinstructionsandnowtherearetwonumbersremembered.TheORinstructiontakesthetwonumbersnowremainingandifeitheroneisa1theresultisa1,otherwisetheresultisa0.Thisresultreplacesthetwonumbers,andthereisnowasinglenumberthere.ThelastinstructionistheST（storeoutput）thatwilllookatthelastvaluestoredandifitis1,theoutputwillbeturnedon,ifitis0theoutputwillbeturnedoff.

Figure2.6AnExampleofaMnemonicProgramandEquivalentLadderLogic

TheladderlogicprograminFigure2.6,isequivalenttothemnemonicprogram.EvenifyouhaveprogrammedaPLCwithladderlogic,itwillbeconvertedtomnemonicformbeforebeingusedbythePLC.Inthepastmnemonicprogrammingwasthemostcommon,butnowitisuncommonforuserstoevenseemnemonicprograms.

SequentialFunctionCharts（SFCs）havebeendevelopedtoaccommodatetheprogrammingofmoreadvancedsystems.Thesearesimilartoflowcharts,butmuchmorepowerful.TheexampleseeninFigure2.7isdoingtwodifferentthings.Toreadthechart,startatthetopwhereissaysstart.Belowthisthereisthedoublehorizontallinethatsaysfollowbothpaths.AsaresultthePLCwillstarttofollowthebranchontheleftandrighthandsidesseparatelyandsimultaneously.Onthelefttherearetwofunctionsthefirstoneisthepowerupfunction.Thisfunctionwillrununtilitdecidesitisdone,andthepowerdownfunctionwillcomeafter.Ontherighthandsideistheflashfunction,thiswillrununtilitisdone.Thesefunctionslookunexplained,buteachfunction,suchaspowerupwillbeasmallladderlogicprogram.Thismethodismuchdifferentfromflowchartsbecauseitdoesnothavetofollowasinglepaththroughtheflowchart..

Figure2.7AnExampleofaSequentialFunctionChar

StructuredTextprogramminghasbeendevelopedasamoremodernprogramminglanguage.ItisquitesimilartolanguagessuchasBASIC.AsimpleexampleisshowninFigure2.8