毕业设计论文文献翻译Word格式文档下载.docx

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AutomatingManufacturingSystemswithPLCs

Controlengineeringhasevolvedovertime.Inthepasthumanswerethemainmethodforcontrollingasystem.Morerecentlyelectricityhasbeenusedforcontrolandearlyelectricalcontrolwasbasedonrelays.Theserelaysallowpowertobeswitchedon

andoffwithoutamechanicalswitch.Itiscommontouserelaystomakesimplelogicalcontroldecisions.Thedevelopmentoflowcostcomputerhasbroughtthemostrecentrevolution,theProgrammableLogicController（PLC）.TheadventofthePLCbeganinthe1970s,andhasbecomethemostcommonchoiceformanufacturingcontrols.PLCshavebeengainingpopularityonthefactoryfloorandwillprobablyremainpredominantforsometimetocome.Mostofthisisbecauseoftheadvantagestheyoffer.

1.1Ladderlogic

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

Moderncontrolsystemsstillincluderelays,butthesearerarelyusedforlogic.Arelayisasimpledevicethatusesamagneticfieldtocontrolaswitch,aspicturedinFigure1.1.Whenavoltageisappliedtotheinputcoil,theresultingcurrentcreatesamagneticfield.Themagneticfieldpullsametalswitch（orreed）towardsitandthecontactstouch,closingtheswitch.

Figure1.1SimpleRelayLayoutsandSchematics

Relaysareusedtoletonepowersourcecloseaswitchforanother（oftenhighcurrent）powersource,whilekeepingthemisolated.AnexampleofarelayinasimplecontrolapplicationisshowninFigure1.2.Inthissystemthefirstrelayontheleftisusedasnormallyclosed,andwillallowcurrenttoflowuntilavoltageisappliedtotheinputA.ThesecondrelayisnormallyopenandwillnotallowcurrenttoflowuntilavoltageisappliedtotheinputB.Ifcurrentisflowingthroughthefirsttworelaysthencurrentwillflowthroughthecoilinthethirdrelay,andclosetheswitchforoutputC.Thiscircuitwouldnormallybedrawnintheladderlogicform.ThiscanbereadlogicallyasCwillbeonifAisoffandBison.

Figure1.2ASimpleRelayController

TheexampleinFigure1.2doesnotshowtheentirecontrolsystem,butonlythelogic.WhenweconsideraPLCthereareinputs,outputs,andthelogic.Figure1.3showsamorecompleterepresentationofthePLC.Heretherearetwoinputsfrompushbuttons.Wecanimaginetheinputsasactivating24VDCrelaycoilsinthePLC.Thisinturndrivesanoutputrelaythatswitches115VAC,thatwillturnonalight.Note,inactualPLCsinputsareneverrelays,butoutputsareoftenrelays.TheladderlogicinthePLCisactuallyacomputerprogramthattheusercanenterandchange.Noticethatbothoftheinputpushbuttonsarenormallyopen,buttheladderlogicinsidethePLChasonenormallyopencontact,andonenormallyclosedcontact.DonotthinkthattheladderlogicinthePLCneedsomatchtheinputsoroutputs.Manybeginnerswillgetcaughttryingtomaketheladderlogicmatchtheinputtypes.

Figure1.3APLCIllustratedWithRelays

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

Figure1.4ASeal-inCircuit

1.2Programming

ThefirstPLCswereprogrammedwithatechniquethatwasbasedonrelaylogicwiringschematics.Thiseliminatedtheneedtoteachtheelectricians,techniciansandengineershowtoprogramacomputer-but,thismethodhasstuckanditisthemostcommontechniqueforprogrammingPLCstoday.AnexampleofladderlogiccanbeseeninFigure1.5.Tointerpretthisdiagramimaginethatthepowerisontheverticallineonthelefthandside,wecallthisthehotrail.Ontherighthandsideistheneutralrail.Inthefiguretherearetworungs,andoneachrungtherearecombinationsofinputs（twoverticallines）andoutputs（circles）.Iftheinputsareopenedorclosedintherightcombinationthepowercanflowfromthehotrail,throughtheinputs,topowertheoutputs,andfinallytotheneutralrail.Aninputcancomefromasensor,switch,oranyothertypeofsensor.AnoutputwillbesomedeviceoutsidethePLCthatisswitchedonoroff,suchaslightsormotors.Inthetoprungthecontactsarenormallyopenandnormallyclosed.WhichmeansifinputAisonandinputBisoff,thenpowerwillflowthroughtheoutputandactivateit.AnyothercombinationofinputvalueswillresultintheoutputXbeingoff.

Figure1.5ASimpleLadderLogicDiagram

ThesecondrungofFigure1.5ismorecomplex,thereareactuallymultiplecombinationsofinputsthatwillresultintheoutputYturningon.Ontheleftmostpartoftherung,powercouldflowthroughthetopifCisoffandDison.Powercouldalso（andsimultaneously）flowthroughthebottomifbothEandFaretrue.Thiswouldgetpowerhalfwayacrosstherung,andthenifGorHistruethepowerwillbedeliveredtooutputY.Inlaterchapterswewillexaminehowtointerpretandconstructthesediagrams.

ThereareothermethodsforprogrammingPLCs.Oneoftheearliesttechniquesinvolvedmnemonicinstructions.TheseinstructionscanbederiveddirectlyfromtheladderlogicdiagramsandenteredintothePLCthroughasimpleprogrammingterminal.AnexampleofmnemonicsisshowninFigure1.6.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.

Figure1.6AnExampleofaMnemonicProgramandEquivalentLadderLogic

TheladderlogicprograminFigure1.6,isequivalenttothemnemonicprogram.EvenifyouhaveprogrammedaPLCwithladderlogic,itwillbeconvertedtomnemonicformbeforebeingusedbythePLC.Inthepastmnemonicprogrammingwasthemostcommon,butnowitisuncommonforuserstoevenseemnemonicprograms.

SequentialFunctionCharts（SFCs）havebeendevelopedtoaccommodatetheprogrammingofmoreadvancedsystems.Thesearesimilartoflowcharts,butmuchmorepowerful.TheexampleseeninFigure1.7isdoingtwodifferentthings.Toreadthechart,startatthetopwhereissaysstart.Belowthisthereisthedoublehorizontallinethatsaysfollowbothpaths.AsaresultthePLCwillstarttofollowthebranchontheleftandrighthandsidesseparatelyandsimultaneously.Onthelefttherearetwofunctionsthefirstoneisthepowerupfunction.Thisfunctionwillrununtilitdecidesitisdone,andthepowerdownfunctionwillcomeafter.Ontherighthandsideistheflashfunction,thiswillrununtilitisdone.Thesefunctionslookunexplained,buteachfunction,suchaspowerupwillbeasmallladderlogicprogram.Thismethodismuchdifferentfromflowchartsbecauseitdoesnothavetofollowasinglepaththroughtheflowchart..

Figure1.7AnExampleofaSequentialFunctionChart

StructuredTextprogramminghasbeendevelopedasamoremodernprogramminglanguage.ItisquitesimilartolanguagessuchasBASIC.AsimpleexampleisshowninFigure1.8.ThisexampleusesaPLCmemorylocationi.Thismemorylocationisforaninteger,aswillbeexplainedlaterinthebook.Thefirstlineoftheprogramsetsthevalueto0.Thenextlinebeginsaloop,andwillbewheretheloopreturnsto.Thenextlinerecallsthevalueinlocationi,adds1toitandreturnsittothesamelocation.Thenextlinecheckstoseeiftheloopshouldquit.Ifiisgreaterthanorequalto10,thentheloopwillquit,otherwisethecomputerwillgobackuptotheREPEATstatementcontinuefromthere.Eachtimetheprogramgoesthroughthisloopiwillincreaseby1untilthevaluereaches10.

Figure1.8AnExampleofaStructuredTextProgram

2.1PLCConnections

WhenaprocessiscontrolledbyaPLCitusesinputsfromsensorstomakedecisionsandupdateoutputstodriveactuators,asshownin