自动化专业相关英文文献加翻译0字符知识交流.docx
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自动化专业相关英文文献加翻译0字符知识交流.docx
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自动化专业相关英文文献加翻译0字符知识交流
Thischaptercontinuesfromthepreviouschaptersonprogrammingandintroducesinternalrelays.Avarietyofothertermsareoftenusedtodescribetheseelements,suchasauxiliaryrelays,markers,flags,coils,andbitstorage.Theseareoneoftheelementsincludedamongthespecialbuilt-infunctionswithPLCsandareverywidelyusedinprogramming.AsmallPLCmighthaveahundredormoreinternalrelays,someofthembatterybackedsothattheycanbeusedinsituationswhereitisnecessarytoensuresafeshutdownofaplantintheeventofpowerfailure.Laterchaptersconsiderothercommonbuilt-inelements.
7.1InternalRelays
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
OneofthefunctionsprovidedbysomePLCmanufacturersistheabilitytoprogramaninternalrelaysothatits
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