关于PLC的英文文献.doc

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1、1.PROGRAMMABLELOGICCONTROLLERS1.1INTRODUCTIONControlengineeringhasevolvedovertime.Inthepasthumanswasthemainmethodforcontrollingasystem.Morerecentlyelectricityhasbeenusedforcontrolandearlyelectricalcontrolwasbasedonrelays.Theserelaysallowpowertobeswitch

2、edonandoffwithoutamechanicalswitch.Itiscommontouserelaystomakesimplelogicalcontroldecisions.Thedevelopmentoflowcostcomputerhasbroughtthemostrecentrevolution,theProgrammableLogicController(PLC).TheadventofthePLCbeganinthe1970s,andhasbecomethemostcommonc

3、hoiceformanufacturingcontrols.PLChavebeengainingpopularityonthefactoryfloorandwillprobablyremainpredominantforsometimetocome.Mostofthisisbecauseoftheadvantagestheyoffer..Costeffectiveforcontrollingcomplexsystems..Flexibleandcanbereappliedtocontrolother

4、systemsquicklyandeasily..Computationalabilitiesallowmoresophisticatedcontrol..Troubleshootingaidsmakeprogrammingeasierandreducedowntime..Reliablecomponentsmaketheselikelytooperateforyearsbeforefailure.1.2LadderLogicLadderlogicisthemainprogrammingmethod

5、usedforPLC.Asmentionedbefore,ladderlogichasbeendevelopedtomimicrelaylogic.Thedecisiontousetherelaylogicdiagramswasastrategicone.Byselectingladderlogicasthemainprogrammingmethod,theamountofretrainingneededforengineersandtradespeoplewasgreatlyreduced.Mod

6、erncontrolsystemsstillincluderelays,butthesearerarelyusedforlogic.Arelayisasimpledevicethatusesamagneticfieldtocontrolaswitch,aspicturedinFigure2.1.Whenavoltageisappliedtotheinputcoil,theresultingcurrentcreatesamagneticfield.Themagneticfieldpullsametal

7、switch(orreed)towardsitandthecontactstouch,closingtheswitch.Thecontactthatcloseswhenthecoilisenergizediscallednormallyopen.Thenormallyclosedcontactstouchwhentheinputcoilisnotenergized.Relaysarenormallydrawninschematicformusingacircletorepresenttheinput

8、coil.Theoutputcontactsareshownwithtwoparallellines.Normallyopencontactsareshownastwolines,andwillbeopen(non-conducting)whentheinputisnotenergized.Normallyclosedcontactsareshownwithtwolineswithadiagonallinethroughthem.Whentheinputcoilisn