采用PLC的自动化制造系统 毕业设计(论文)文献翻译.docx.docx

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1、AutomatingManufacturingSystemswithPLCsControlengineeringhasevolvedovertime.Inthepasthumanswerethemainmethodforcontrollingasystem.Morerecentlyelectricityhasbeenusedforcontrolandearlyelectricalcontrolwasbasedonrelays.Theserelaysallowpowertobeswitchedonandoffwithoutamechanicalswitch.Itiscomm

2、ontouserelaystomakesimplelogicalcontroldecisions.Thedevelopmentoflowcostcomputerhasbroughtthemostrecentrevolution,theProgrammableLogicController(PLC).TheadventofthePLCbeganinthe1970s,andhasbecomethemostcommonchoiceformanufacturingcontrols.PLCshavebeengainingpopularityonthefactoryfloorandw

3、illprobablyremainpredominantforsometimetocome.Mostofthisisbecauseoftheadvantagestheyoffer.1.1LadderlogicLadderlogicisthemainprogrammingmethodusedforPLCs.Asmentionedbefore,ladderlogichasbeendevelopedtomimicrelaylogic.logicdiagramswasastrategicone.Byselectingladderlogicasthemainprogrammingm

4、ethod,theamountofretrainingneededforengineersandtradespeoplewasgreatlyreduced.Moderncontrolsystemsstillincluderelays,butthesearerarelyusedforlogic.Arelayisasimpledevicethatusesamagneticfieldtocontrolaswitch,aspicturedinFigure1.1.Whenavoltageisappliedtotheinputcoil,theresultingcurrentcreat

5、esamagneticfield.Themagneticfieldpullsametalswitch(orreed)towardsitandthecontactstouch,closingtheswitch.Figure1.1SimpleRelayLayoutsandSchematicsRelaysareusedtoletonepowersourcecloseaswitchforanother(oftenhighcurrent)powersource,whilekeepingthemisolated.Anexampleofarelayinasimplecontrolapp

6、licationisshowninFigure1.2.Inthissystemthefirstrelayontheleftisusedasnormallyclosed,andwillallowcurrenttoflowuntilavoltageisappliedtotheinputA.ThesecondrelayisnormallyopenandwillnotallowcurrenttoflowuntilavoltageisappliedtotheinputB.Ifcurrentisflowingthroughthefirsttworelaysthencurrentwil

7、lflowthroughthecoilinthethirdrelay,andclosetheswitchforoutputC.Thiscircuitwouldnormallybedrawnintheladderlogicform.ThiscanbereadlogicallyasCwillbeonifAisoffandBison.Figure1.2ASimpleRelayControllerTheexampleinFigure1.2doesnotshowtheentirecontrolsystem,butonlythelogic