关于PLC的英文文献.docx

关于PLC的英文文献.docx

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关于PLC的英文文献

关于PLC的英文文献

1.PROGRAMMABLELOGICCONTROLLERS

1.1INTRODUCTION

Controlengineeringhasevolvedovertime.Inthepasthumanswasthemainmethodforcontrollingasystem.Morerecentlyelectricityhasbeenusedforcontrolandearlyelectricalcontrolwasbasedonrelays.Theserelaysallowpowertobeswitchedonandoffwithoutamechanicalswitch.Itiscommontouserelaystomakesimplelogicalcontroldecisions.Thedevelopmentoflowcostcomputerhasbroughtthemostrecentrevolution,theProgrammableLogicController（PLC）.TheadventofthePLCbeganinthe1970s,andhasbecomethemostcommonchoiceformanufacturingcontrols.PLChavebeengainingpopularityonthefactoryfloorandwillprobablyremainpredominantforsometimetocome.Mostofthisisbecauseoftheadvantagestheyoffer..Costeffectiveforcontrollingcomplexsystems.

.Flexibleandcanbereappliedtocontrolothersystemsquicklyandeasily.

.Computationalabilitiesallowmoresophisticatedcontrol..Troubleshootingaidsmakeprogrammingeasierandreducedowntime..Reliablecomponentsmaketheselikelytooperateforyearsbeforefailure.

1.2LadderLogic

LadderlogicisthemainprogrammingmethodusedforPLC.Asmentionedbefore,ladderlogichasbeendevelopedtomimicrelaylogic.Thedecisiontousetherelaylogicdiagramswasastrategicone.Byselectingladderlogicasthemainprogrammingmethod,theamountofretrainingneededforengineersandtradespeoplewasgreatlyreduced.Moderncontrolsystemsstillincluderelays,butthesearerarelyusedforlogic.Arelayisasimpledevicethatusesamagneticfieldtocontrolaswitch,aspicturedinFigure2.1.Whenavoltageisappliedtotheinputcoil,theresultingcurrentcreatesamagneticfield.Themagneticfield

pullsametalswitch（orreed）towardsitandthecontactstouch,closingtheswitch.Thecontactthatcloseswhenthecoilisenergizediscallednormallyopen.Thenormallyclosedcontactstouchwhentheinputcoilisnotenergized.Relaysarenormallydrawninschematicformusingacircletorepresenttheinputcoil.Theoutputcontactsareshownwithtwoparallellines.Normallyopencontactsareshownastwolines,andwillbeopen（non-conducting）whentheinputisnotenergized.Normallyclosedcontactsareshownwithtwolineswithadiagonallinethroughthem.Whentheinputcoilisnotenergizedthenormallyclosedcontactswillbeclosed（conducting）.

Relaysareusedtoletonepowersourcecloseaswitchforanother（oftenhighcurrent）powersource,whilekeepingthemisolated.AnexampleofarelayinasimplecontrolapplicationisshowninFigure2.2.Inthissystemthefirstrelayontheleftisusedasnormallyclosed,andwillallowcurrenttoflowuntilavoltageisappliedtotheinputA.Thesecondrelayisnormallyopenandwillnotallowcurrenttoflowuntila

voltageisappliedtotheinputB.Ifcurrentisflowingthroughthefirsttworelaysthencurrentwillflowthroughthecoilinthethirdrelay,andclosetheswitchforoutputC.Thiscircuitwouldnormallybedrawnintheladderlogicform.ThiscanbereadlogicallyasCwillbeonifAisoffandBison.

1.3Programming

ThefirstPLCwereprogrammedwithatechniquethatwasbasedonrelaylogicwiringschematics.Thiseliminatedtheneedtoteachtheelectricians,techniciansandengineershowtoprogramacomputer-but,thismethodhasstuckanditisthemostcommontechniqueforprogrammingPLCtoday.AnexampleofladderlogiccanbeseeninFigure2.5.Tointerpretthisdiagramimaginesthatthepowerisontheverticallineonthelefthandside,wecallthisthehotrail.Ontherighthandsideistheneutralrail.Inthefiguretherearetworungs,andoneachrungtherearecombinationsofinputs（twoverticallines）and

outputs（circles）.Iftheinputsareopenedorclosedintherightcombinationthepowercanflowfromthehotrail,throughtheinputs,topowertheoutputs,andfinallytotheneutralrail.Aninputcancomefromasensor,switch,oranyothertypeofsensor.AnoutputwillbesomedeviceoutsidethePLCthatisswitchedonoroff,suchaslightsormotors.Inthetoprungthecontactsarenormallyopenandnormallyclosed,whichmeansifinputAisonandinputBisoff,thenpowerwillflowthroughtheoutputandactivateit.AnyothercombinationofinputvalueswillresultintheoutputXbeingoff.

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

TheseinstructionscanbederiveddirectlyfromtheladderlogicdiagramsandenteredintothePLCthroughasimpleprogrammingterminal.AnexampleofmnemonicsisshowninFigure2.6.Inthisexampletheinstructionsarereadonelineatatimefromtoptobottom.Thefirstline00000hastheinstructionLDN（inputloadandnot）forinput00001.ThiswillexaminetheinputtothePLCandifitisoffitwillremembera1（ortrue）,ifitisonitwillremembera0（orfalse）.ThenextlineusesanLD（inputload）statementtolookattheinput.Iftheinputisoffitremembersa0,iftheinputisonitremembersa1（note:

thisisthereverseoftheLD）.TheANDstatementrecallsthelasttwonumbersrememberedandiftheyarebothtruetheresultisa1;otherwisetheresultisa0.Thisresultnowreplacesthetwonumbersthatwererecalled,andthereisonlyonenumberremembered.Theprocessisrepeatedforlines00003and00004,butwhenthesearedonetherearenowthreenumbersremembered.TheoldestnumberisfromtheAND,thenewernumbersarefromthetwoLDinstructions.TheANDinline00005combinestheresultsfromthelastLDinstructionsandnowtherearetwonumbersremembered.TheORinstructiontakesthetwonumbersnowremainingandifeitheroneisa1theresultisa1,otherwisetheresultisa0.Thisresultreplacesthetwonumbers,andthereisnowasinglenumberthere.ThelastinstructionistheST（storeoutput）thatwilllookatthelastvaluestoredandifitis1,theoutputwillbeturnedon;ifitis0theoutputwillbeturnedoff.

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

SequentialFunctionCharts（SFC）havebeendevelopedto

accommodatetheprogrammingofmoreadvancedsystems.Thesearesimilartoflowcharts,butmuchmorepowerful.TheexampleseeninFigure2.7isdoingtwodifferentthings.Toreadthechart,startatthetopwhereissaysstart.Belowthisthereisthedoublehorizontallinethatsaysfollowbothpaths.AsaresultthePLCwillstarttofollowthebranchontheleftandrighthandsidesseparatelyandsimultaneously.Onthelefttherearetwofunctionsthefirstoneisthepowerupfunction.Thisfunctionwillrununtilitdecidesitisdone,andthepowerdownfunctionwillcomeafter.Ontherighthandsideistheflashfunction;thiswillrununtilitisdone.Thesefunctionslookunexplained,buteachfunction,suchaspowerupwillbeasmallladderlogicprogram.Thismethodismuchdifferentfromflowchartsbecauseitdoesnothavetofollowasingle

paththroughthe

flowchart.

StructuredTextprogramminghasbeendevelopedasamoremodernprogramminglanguage.ItisquitesimilartolanguagessuchasBASIC.AsimpleexampleisshowninFigure2.8.ThisexampleusesaPLCmemorylocationN7:

0.Thismemorylocationisforaninteger,aswillbeexplainedlaterinthebook.Thefirstlineoftheprogramsetsthevalueto0.Thenextlinebeginsaloop,andwillbewheretheloopreturnsto.ThenextlinerecallsthevalueinlocationN7:

0,adds1toitandreturnsittothesamelocation.Thenextlinecheckstoseeiftheloopshouldquit.IfN7:

0isgreaterthanorequalto10,thentheloopwillquit,otherwisethecomputerwillgobackuptotheREPEATstatementcontinuefromthere.EachtimetheprogramgoesthroughthisloopN7:

0willincreaseby1untilthevaluereaches10.

N7:

0:

=0;

REPEAT

N7:

0:

=N7:

0+1;

UNTILN7:

0>=10

END_REPEAT;

2.PLCConnections

WhenaprocessiscontrolledbyaPLCitusesinputsfrom

sensorstomakedecisionsandupdateoutputstodriveactuators,as

showninFigure2.9.Theprocessisarealprocessthatwillchange

overtime.Actuatorswilldrivethesystemtonewstates（ormodes

ofoperation）.Thismeansthatthecontrollerislimitedbythe

sensorsavailable,ifaninputisnotavailable,thecontrollerwillhave

nowaytodetectacondition.

ThecontrolloopisacontinuouscycleofthePLCreadinginputs,solvingtheladderlogic,andthenchangingtheoutputs.Likeanycomputerthisdoesnothappeninstantly.Figure2.10showsthebasicoperationcycleofaPLC.WhenpoweristurnedoninitiallythePLCdoesaquicksanitychecktoensurethatthehardwareisworkingproperly.IfthereisaproblemthePLCwillhaltandindicatethereisanerror.Forexample,ifthePLCbackupbatteryislowandpowerwaslost,thememorywillbecorruptandthiswillresultinafault.IfthePLCpassesthesanitychecksitwillthenscan（read）all