自动化专业相关英文文献加翻译0字符Word文档下载推荐.docx

自动化专业相关英文文献加翻译0字符Word文档下载推荐.docx

《自动化专业相关英文文献加翻译0字符Word文档下载推荐.docx》由会员分享，可在线阅读，更多相关《自动化专业相关英文文献加翻译0字符Word文档下载推荐.docx（34页珍藏版）》请在冰豆网上搜索。

InPLCsthereareelementsthatareusedtoholddata,thatis,bits,andbehavelikerelays,beingabletobeswitchedonoroffandtoswitchotherdevicesonoroff.Hencetheterminternalrelay.Suchinternalrelaysdonotexistasreal-worldswitchingdevicesbutaremerelybitsinthestoragememorythatbehaveinthesamewayasrelays.Forprogramming,theycanbetreatedinthesamewayasanexternalrelayoutputandinput.Thusinputstoexternalswitchescanbeusedtogiveanoutputfromaninternalrelay.Thisthenresultsintheinternalrelaycontactsbeingused,inconjunctionwithotherexternalinputswitches,togiveanoutput,suchasactivatingamotor.Thuswemighthave（Figure7.1）

Ononerungoftheprogram:

Inputstoexternalinputsactivatetheinternalrelayoutput.

Onalaterrungoftheprogram:

Asaconsequenceoftheinternalrelayoutput,internalrelaycontactsareactivatedandso

controlsomeoutput.

Inusinganinternalrelay,ithastobeactivatedononerungofaprogramandthenitsoutputusedtooperateswitchingcontactsonanotherrung,orrungs,oftheprogram.Internalrelayscanbeprogrammedwithasmanysetsofassociatedcontactsasdesired.

Todistinguishinternalrelayoutputsfromexternalrelayoutputs,theyaregivendifferenttypesofaddresses.Differentmanufacturerstendtousedifferenttermsforinternalrelaysand

havedifferentwaysofexpressingtheiraddresses.Forexample,MitsubishiusesthetermauxiliaryrelayormarkerandthenotationM100,M101,andsoon.SiemensusesthetermflagandthenotationF0.0,F0.1,andsoon.TelemecaniqueusesthetermbitandthenotationB0,B1,andsoon.ToshibausestheterminternalrelayandthenotationR000,R001,andsoon.Allen-BradleyusesthetermbitstorageandnotationinthePLC-5oftheformB3/001,B3/002,andsoon.

7.2LadderPrograms

Withladderprograms,aninternalrelayoutputisrepresentedusingthesymbolforanoutputdevice,namely,withanaddressthatindicatesthatitisaninternalrelay.Thus,withaMitsubishiPLC,wemighthavetheaddressM100,theMindicatingthatitisaninternalrelayormarkerratherthananexternaldevice.Theinternalrelayswitchingcontactsaredesignatedwiththesymbolforaninputdevice,namely,andgiventhesameaddressastheinternalrelayoutput,suchasM100.

7.2.1ProgramswithMultipleInputConditions

Asanillustrationoftheusethatcanbemadeofinternalrelays,considerthefollowingsituation.Asystemistobeactivatedwhentwodifferentsetsofinputconditionsarerealized.WemightjustprogramthisasanANDlogicgatesystem;

however,ifanumberofinputshavetobecheckedinorderthateachoftheinputconditionscanberealized,itmaybesimplertouseaninternalrelay.Thefirstinputconditionsthenareusedtogiveanoutputtoaninternalrelay.Thisrelayhasassociatedcontactsthatthenbecomepartoftheinputconditionswiththesecondinput

Figure7.2showsaladderprogramforsuchatask.Forthefirstrung,wheninputIn1orinputIn3isclosedandinputIn2closed,internalrelayIR1isactivated.ThisresultsinthecontactsforIR1closing.IfinputIn4isthenactivated,thereisanoutputfromoutputOut1.Suchataskmightbeinvolvedintheautomaticliftingofabarrierwhensomeoneapproachesfromeitherside.InputIn1andinputIn3areinputsfromphotoelectricsensorsthatdetectthepresenceofaperson

approachingorleavingfromeithersideofthebarrier,inputIn1beingactivatedfromonesideofitandinputIn3fromtheother.InputIn2isanenablingswitchtoenablethesystemtobecloseddown.ThuswheninputIn1orinputIn3,andinputIn2,areactivated,thereisanoutputfrominternalrelay1.Thiswillclosetheinternalrelaycontacts.IfinputIn4,perhapsalimitswitch,detectsthatthebarrierisclosed,thenitisactivatedandcloses.Theresultisthenanoutputfrom

Out1,amotorthatliftsthebarrier.Ifthelimitswitchdetectsthatthebarrierisalreadyopen,thepersonhavingpassedthroughit,thenitopensandsooutputOut1isnolongerenergizedandacounterweightmightthenclosethebarrier.Theinternalrelayhasenabledtwopartsoftheprogramtobelinked,onepartbeingthedetectionofthepresenceofapersonandthesecondpartthedetectionofwhetherthebarrierisalreadyupordown.Figure7.3ashowshowFigure7.2wouldappearinMitsubishinotationandFigure7.3bshowshowitwouldappearinSiemensnotation.

Figure7.4isanotherexampleofaladderprograminvolvinginternalrelays.Output1iscontrolledbytwoinputarrangements.ThefirstrungshowstheinternalrelayIR1,whichis

energizedifinputIn1orIn2isactivatedandclosed.ThesecondrungshowsinternalrelayIR2,whichisenergizedifinputsIn3andIn4arebothenergized.ThethirdrungshowsthatoutputOut1isenergizedifinternalrelayIR1orIR2isactivated.Thusthereisanoutputfromthesystemifeitheroftwosetsofinputconditionsisrealized.

7.2.2LatchingPrograms

Anotheruseofinternalrelaysisforresettingalatchcircuit.Figure7.5showsanexampleofsuchaladderprogram.

WhentheinputIn1contactsaremomentarilyclosed,thereisanoutputatOut1.ThisclosesthecontactsforOut1andsomaintainstheoutput,evenwheninputIn1opens.WheninputIn2isclosed,theinternalrelayIR1isenergizedandsoopenstheIR1contacts,whicharenormallyclosed.ThustheoutputOut1isswitchedoffandsotheoutputisunlatched.

Considerasituationrequiringlatchcircuitswherethereisanautomaticmachinethatcanbestartedorstoppedusingpush-buttonswitches.Alatchcircuitisusedtostartandstopthepowerbeingappliedtothemachine.Themachinehasseveraloutputsthatcanbeturnedonifthepowerhasbeenturnedonandareoffifthepowerisoff.Itwouldbepossibletodevisealadderdiagramthathasindividuallylatchedcontrolsforeachsuchoutput.However,asimplermethodistouseaninternalrelay.Figure7.6showssuchaladderdiagram.ThefirstrunghasthelatchforkeepingtheinternalrelayIR1onwhenthestartswitchgivesamomentaryinput.Thesecondrungwillthenswitchthepoweron.ThethirdrungwillalsoswitchonandgiveoutputOut2iftheinput2contactsareclosed.ThethirdrungwillalsoswitchonandgiveoutputOut3iftheinput3contactsareclosed.Thusalltheoutputscanbeswitchedonwhenthestartpushbuttonisactivated.Alltheoutputswillbeswitchedoffifthestopswitchisopened.ThusalltheoutputsarelatchedbyIR1.

7.2.3ResponseTime

Thetimetakenbetweenaninputoccurringandanoutputchangingdependsonsuchfactorsastheelectricalresponsetimeoftheinputcircuit,themechanicalresponseoftheoutput

device,andthescantimeoftheprogram.Aladderprogramisreadfromlefttorightandfromtoptobottom.Thusifanoutputdevice,suchasaninternalrelay,issetinonescancycleandtheoutputhastobefedbacktoearlierintheprogram,itwillrequireasecondscanoftheprogrambeforeitcanbeactivated.Figure7.7illustratesthisconcept.

7.3Battery-BackedRelays

IfthepowersupplyiscutofffromaPLCwhileitisbeingused,alltheoutputrelaysandinternalrelayswillbeturnedoff.Thuswhenthepowerisrestored,allthecontactsassociatedwiththoserelayswillbesetdifferentlyfromwhenthepowerwason.Therefore,ifthePLCwasinthemiddleofsomesequenceofcontrolactions,itwouldresumeatadifferentpointinthesequence.Toovercomethisproblem,someinternalrelayshavebatterybackupsothattheycanbeusedincircuitstoensureasafeshutdownofaplantintheeventofapowerfailureandsoenableittorestartinanappropriatemanner.Suchbattery-backedrelaysretaintheirstateofactivation,evenwhenthepowersupplyisoff.Therelayissaidtohavebeenmaderetentive.Thetermretentivememorycoilisfrequentlyusedforsuchelements.Figure7.8ashowstheIEC1131-3standardsymbolforsuchelements.WithMitsubishiPLCs,battery-backedinternalrelaycircuitsuseM300toM377asaddressesforsuchrelays.Othermanufacturersusedifferentaddressesandmethodsofachievingretentivememory.TheAllen-BradleyPLC-5useslatchandunlatchrungs.Iftherelayislatched,itremainslatchedifpowerislostandisunlatchedwhentheunlatchrelayisactivated.（SeeSection7.5foradiscussionofsuchrelaysinthecontextofsetandresetcoils.）Asanexampleoftheuseofsucharelay,Figure7.8bshowsaladderdiagramforasystemdesignedtocopewithapowerfailure.IR1isabattery-backedinternalrelay.WheninputIn1contactsclose,outputIR1isenergized.ThisclosestheIR1contacts,latchingsothatIR1remainsonevenifinputIn1opens.TheresultisanoutputfromOut1.Ifthereisapowerfailure,IR1stillremainsenergizedandsotheIR1contactsremainclosedandthereisanoutputfromOut1.

7.4One-ShotOperation

OneofthefunctionsprovidedbysomePLCmanufacturersistheabilitytoprogramaninternalrelaysothatitscon