PLC电梯控制英文文献.ppt

1 TheProgrammableLogicController JohnR Hackworth FrederickD Hackworth Jr ProgrammableLogicControllers ProgrammingMethodsand Applications 2 5 2 8 TheProgrammableLogicController JohnR Hackworth FrederickD Hackworth Jr FirstEdition London PrenticeHall 2003 35 50 2 5SystemBlockDiagramAProgrammableControllerisaspecializedcomputer Sinceitisacomputer ithasallthebasiccomponentpartsthatanyothercomputerhas aCentralProcessingUnit Memory InputInterfacingandOutputInterfacing AtypicalprogrammablecontrollerblockdiagramisshowninFigure2 5 Figure2 5 ProgrammableControllerBlockDiagram TheCentralProcessingUnit CPU isthecontrolportionofthePLC Itinterpretstheprogramcommandsretrievedfrommemoryandactsonthosecommands InpresentdayPLC sthisunitisamicroprocessorbasedsystem TheCPUishousedintheprocessormoduleofmodularizedsystems Memoryinthesystemisgenerallyoftwotypes ROMandRAM TheROMmemorycontainstheprograminformationthatallowstheCPUtointerpretandactontheLadderLogicprogramstoredintheRAMmemory RAMmemoryisgenerallykeptalivewithanon boardbatteryso thatladderprogrammingisnotlostwhenthesystempowerisremoved Thisbatterycanbea standarddrycellorrechargeablenickel cadmiumtype NewerPLCunitsarenowavailablewithElectricallyErasableProgrammableReadOnlyMemory EEPROM whichdoesnotrequireabattery Memoryisalsohousedintheprocessormoduleinmodularsystems Inputunitscanbeanyofseveraldifferenttypesdependingoninputsignalsexpectedasdescribedabove Theinputsectioncanacceptdiscreteoranalogsignalsofvariousvoltageandcurrentlevels PresentdaycontrollersofferdiscretesignalinputsofbothACandDCvoltagesfromTTLto250VDCandfrom5to250VAC Analoginputunitscanacceptinputlevelssuchas 10VDC 5VDCand4 20ma currentloopvalues DiscreteinputunitspresenteachinputtotheCPUasasingle1or0whileanaloginputunitscontainanalogtodigitalconversioncircuitryandpresenttheinputvoltagetotheCPUasbinarynumbernormalizedtothemaximumcountavailablefromtheunit Thenumberofbitsrepresentingtheinputvoltageorcurrentdepends2 3 upontheresolutionoftheunit Thisnumbergenerallycontainsadefinednumberofmagnitudebitsandasignbit RegisterinputunitspresentthewordinputtotheCPUasitisreceived BinaryorBCD Outputunitsoperatemuchthesameastheinputunitswiththeexceptionthattheunitiseithersinking supplyingaground orsourcing providingavoltage discretevoltagesorsourcinganalogvoltageorcurrent TheseoutputsignalsarepresentedasdirectedbytheCPU TheoutputcircuitofdiscreteunitscanbetransistorsforTTLandhigherDCvoltageorTriacsforACvoltageoutputs Forhighercurrentapplicationsandsituationswhereaphysicalcontactclosureisrequired mechanicalrelaycontactsareavailable Thesehighercurrents however aregenerallylimitedtoabout2 3amperes Theanalogoutputunitshaveinternalcircuitrywhichperformsthedigitaltoanalogconversionandgeneratesthevariablevoltageorcurrentoutput 2 6Update SolvetheLadder Update Whenpowerisappliedtoaprogrammablelogiccontroller thePLC soperationconsistsoftwosteps 1 updateinputsandoutputsand 2 solvetheladder Thismayseemlikeaverysimplisticapproachtosomethingthathastobemorecomplicatedbuttheretrulyareonlythesetwosteps Ifthesetwostepsarethoroughlyunderstood writingandmodifyingprogramsandgettingthemostfromthedeviceismucheasiertoaccomplish Withthisunderstanding thethingsthatcanbeundertakenarethenuptotheimaginationoftheprogrammer Youwillnoticethatthe update solvetheladder sequencebeginsafterstartup TheactualstartupsequenceincludessomeoperationstransparenttotheuserorprogrammerthatoccurbeforeactualPLCoperationontheuserprogrambegins Duringthisstartuptheremaybeextensivediagnosticchecksperformedbytheprocessoronthingslikememory I Odevices communicationwithotherdevices ifpresent andprogramintegrity Insophisticatedmodularsystems theprocessorisabletoidentifythevariousmoduletypes theirlocationinthesystemandaddress Thistypeofsystemanalysisandtestinggenerallyoccursduringstartupbeforeactualprogramexecution 2 7Update ThefirstthingthePLCdoeswhenitbeginstofunctionisupdateI O Thismeansthatalldiscreteinputstatesarerecordedfromtheinputunitandalldiscretestatestobeoutputaretransferredtotheoutputunit Registerdatagenerallyhasspecificaddressesassociatedwithitforbothinputandoutputdatareferredtoasinputandoutputregisters Theseregistersareavailabletotheinputandoutputmodulesrequiringthemandareupdatedwiththediscretedata Sincethis isinput outputupdating itisreferredtoasI OUpdate TheupdatingofdiscreteinputandoutputinformationisaccomplishedwiththeuseofinputandoutputimageregisterssetasideinthePLCmemory Eachdiscreteinputpointhasassociatedwithitonebitofaninputimageregister Likewise eachdiscreteoutputpointhasonebitofanoutputimageregisterassociatedwithit WhenI Oupdatingoccurs eachinputpointthatisONatthattimewillcausea1tobesetatthebitaddressassociatedwiththatparticularinput Iftheinputisoff a0willbesetintothebitaddress Memoryintoday sPLC sisgenerallyconfiguredin16bitwords Thismeansthatone wordofmemorycanstorethestatesof16discreteinputpoints Therefore theremaybea numberofwordsofmemorysetasideastheinputandoutputimageregisters AtI Oupdate thestatusoftheinputimageregisterissetaccordingtothestateofalldiscreteinputsandthestatus oftheoutputimageregisteristransferredtotheoutputunit Thistransferofinformation typicallyonlyoccursatI Oupdate ItmaybeforcedtooccuratothertimesinPLC swhichhaveanImmediateI OUpdatecommand ThiscommandwillforcethePLCtoupdatetheI Oatothertimesalthoughthiswouldbeaspecialcase Onemajoritemofconcernaboutthefirstoutputupdateistheinitialstateofoutputs This isaconcernbecausetheirmaybeoutputsthatifinitiallyturnedoncouldcreateasafetyhazard particularlyinasystemwhichiscontrollingheavymechanicaldevicescapableofcausingbodilyharmtooperators Insomesystems alloutputsmayneedtobeinitiallysettotheiroffstatetoinsurethesafetyofthesystem However theremaybesystemsthatrequireoutputstoinitiallybesetupinaspecificway someonandsomeoff Thiscouldtaketheformofapredeterminedsetuporcouldbearequirementthattheoutputsremaininthestateimmediatelybeforepower down Morerecentsystemshaveprovisionsforbothsetupoptionsandevenacombinationofthetwo Thisisaprimeconcernoftheengineerandprogrammerandmustbedefinedasthesystemisbeingdevelopedtoinsurethesafetyofpersonnelthatoperateandmaintaintheequipment Safetyasrelatedtosystemandprogramdevelopmentwillbediscussedinalaterchapter 2 8SolvetheLadderAftertheI Oupdatehasbeenaccomplished thePLCbeginsexecutingthecommandsprogrammedintoit Thesecommandsaretypicallyreferredtoastheladderdiagram Theladderdiagramisbasicallyarepresentationoftheprogramstepsusingrelaycontactsandcoils The ladderisdrawnwithcontactstotheleftsideofthesheetandcoilstotheright Thisisa holdoverfromthetimewhencontrolsystemswererelaybased Thistypeofdiagramwasusedfortheelectricalschematicofthosesystems AsampleladderdiagramisshowninFigure2 6 4 Figure2 6 SampleLadderDiagram ThesymbolsusedinFigure2 6maybeforeignatthispoint soashortexplanationwillbenecessary ThesymbolsattherightoftheladderdiagramlabeledCR1 CR2 CR3andCR4andarecircularinshapearethesoftwarecoilsoftherelays Thesymbolsattheleftwhichlooklikecapacitors somewithdiagonallinesthroughthem arethecontactsassociatedwiththecoils Thesymbolsthatlooklikecapacitorswithoutthediagonallinesthroughthemarenormallyopencontacts Theseareanalogoustoaswitchthatisnormallyoff Whentheswitchisturnedon thecontactcloses Thecontactsymbolsattheleftthatlooklikecapacitorswithdiagonallinesthroughthemarenormallyclosedcontacts Anormallyclosedcontactisequivalenttoaswitchthatisnormallyturnedon Itwillturnoffwhentheswitchisactuated AscanbeseeninFigure2 6 contactandcoilpositionisasdescribedabove Also onecanseethereasonforthetermladderdiagramiftherungsofastepladderarevisualized Infact eachcompletelineofthediagramisreferredtoasonerungoflogic Theactualinterpretationofthediagramwillalsobediscussedlateralthoughsomeexplanationisrequiredhere Thecontactconfigurationontheleftsideofeachrungcanbevisualizedasswitchesandthecoilsontherightaslights Iftheswitchesareturnedonandoffintheproperconfiguration thelighttotherightwillilluminate ThePLCexecutesthisprogramfromlefttorightandtoptobottom inthatorder Itfirstlooksattheswitch contact configurationtodetermineifcurrentcanbepassedtothelight coil Thedataforthisdecisioncomesfromtheoutputandinputimageregisters Ifcurrentcanbepassed thelight coil willthenbeturnedon Ifnot thelight coil willbeturned off Thisisrecordedintheoutputimageregister OncethePLChaslookedattheleftsideof therungitignorestheleftsideoftherunguntilthenexttimeitsolvesthatparticularrung Once5 thelight coil hasbeeneitherturnedonoroffitwillremaininthatstateuntilthenexttimethePLCsolvesthatparticularrung Aftersolvingarung thePLCmovesontosolvethenextrunginthesamemannerandsoforthuntiltheentireladderhasbeenexecutedandsolved Onerulethatisdifferentfromgeneralelectricaloperationisthedirectionofcurrentflowintherung Inaladderlogic rungcurrentcanonlyflowfromlefttorightandupanddown neverfromrighttoleft Asanexample intheladdershowninFigure2 7 coilCR1willenergizeifanyofthefollowingconditionsexist Figure2 7 Illustrationofallowedcurrentflowinladderrung 1 CR7isoff CR6ison 2 CR7isoff CR2ison CR5ison 3 CR7isoff CR2ison CR3ison 4 CR1ison CR4ison CR3ison 5 CR1ison CR4ison CR5ison Youwillnoticethatthecurrentflowinthecircuitineachofthecaseslistedaboveisfromlefttorightandupanddown CR1willnotenergizeinthecaselistedbelow CR1ison CR4ison CR2ison CR6ison CR5isoff CR3isoff CR7ison ThisisbecausecurrentwouldhavetoflowfromrighttoleftthroughtheCR2contact Thisisnotallowedinladderlogiceventhoughcurrentcouldflowinthisdirectionifweweretobuilditwithrealrelays Remember weareworkinginthesoftwareworldnotthehardwareworld Toreview aftertheI Oupdate thePLCmovestothefirstrungofladderlogic Itsolvesthecontactconfigurationtodetermineifthecoilistobeenergizedorde energized Itthenenergizesorde energizesthecoil Afterthisisaccomplished itmovestotheleftsideofthenextrungand repeatstheprocedure Thiscontinuesuntilallrungshavebeensolved Whenthisprocedureis completewithallrungssolvedandallcoilsintheladdersetupaccordingtothesolutionofeachrung thePLCproceedstothenextstepofit ssequence theI Oupdate 6 AtI Oupdate thestatesofallcoilswhicharedesignatedasoutputsaretransferredfromtheoutputimageregistertotheoutputunitandthestatesofallinputsaretransferredtotheinputimageregister NotethatanyinputchangesthatoccurduringthesolutionoftheladderareignoredbecausetheyareonlyrecordedatI Oupdatetime Thestateofeachcoilisrecordedtotheoutputimageregisteraseachrungissolved However thesestatesarenottransferredtotheoutputunituntilI Oupdatetime Figure2 8ScanCycleThisprocedureofI OupdateandsolvingtheladderdiagramandI OupdateisreferredtoasscanningandisrepresentedinFigure2 8 TheperiodbetweenoneI OupdateandthenextisreferredtoasoneScan TheamountoftimeittakesthePLCtogetfromoneI OupdatetothenextisreferredtoasScanTime Scantimeistypicallymeasuredinmillisecondsandisrelatedto thespeedoftheCPUandthelengthoftheladderdiagramthathastobesolved Theslowerthe processororthelongertheladderdiagram thelongerthescantimeofthesystem ThespeedatwhichaPLCscansmemoryisreferredtoasScanRate Scanrateunitsareusuallylistedinmsec Kofmemorybeingutilizedfortheprogram Asanexample ifaparticularPLChasaratedscanrateof8msec Kandtheprogramoccupies6Kofmemory itwilltakethePLC48msectocompleteonescanoftheprogram 7 1 1Motivation ProgrammableLogicControllers PLC acomputingdeviceinventedbyRichardE Morley in1968 havebeenwidelyusedinindustryincludingmanufacturingsystems transportation systems chemicalprocessfacilities andmanyothers Atthattime thePLCreplacedthe hardwiredlogicwithsoft wiredlogicorso calledrelayladderlogic RLL aprogramming languagevisuallyresemblingthehardwiredlogic andreducedtherebytheconfigurationtime from6monthsdownto6days MoodyandMorley 1999 AlthoughPCbasedcontrolhasstartedtocomeintoplace PLCbasedcontrolwillremain thetechniquetowhichthemajorityofindustrialapplicationswilladhereduetoitshigher performance lowerprice andsuperiorreliabilityinharshenvironments Moreover accordingto astudyonthePLCmarketofFrostandSullivan 1995 anincreaseoftheannualsalesvolume to15millionPLCsperyearwiththehardwarevalueofmorethan8billionUSdollarshasbeen predicted thoughthepricesofcomputinghardwareissteadilydropping Theinventorofthe PLC RichardEMorley fairlyconsidersthePLCmarketasa5 billionindustryatthepresent time ThoughPLCsarewidelyusedinindustrialpractice theprogrammingofPLCbasedcontrol systemsisstillverymuchrelyingontrial and error Alikesoftwareengineering PLCsoftware designisfacingthesoftwaredilemmaorcrisisinasimilarway Morleyhimselfemphasizedthis aspectmostforcefullybyindicating MoodyandMorley 1999 p 110 Ifhouseswerebuiltlikesoftwareprojects asinglewoodpeckercoulddestroycivilization Particularly practicalproblemsinPLCprogrammingaretoeliminatesoftwarebugsandto reducethemaintenancecostsofoldladderlogicprograms ThoughthehardwarecostsofPLCs aredroppingcontinuously reducingthescantimeoftheladderlogicisstillanissueinindustry sothatlow costPLCscanbeused Ingeneral theproductivityingeneratingPLCisfarbehindcomparedtootherdomains for instance VLSIdesign whereefficientcomputeraideddesigntoolsareinpractice Existent softwareengineeringmethodologiesarenotnecessarilyapplicabletothePLCbasedsoftware designbecausePLC programmingrequiresasimultaneousconsiderationofhardwareand software Thesoftwaredesignbecomes thereby moreandmorethemajorcostdriver Inmany industrialdesignprojects morethanSO0 aofthemanpowerallocatedforthecontrolsystem designandinstallationisscheduledfortestinganddebuggingPLCprograms Rockwell 1999 Inaddition currentPLCbasedcontrolsystemsarenotproperlydesignedtosupportthe growingdemandforflexibilityandreconfigurabilityofmanufacturingsystems Afurther problem impellingtheneedforasystematicdesignmethodology istheincreasingsoftware complexityinlarge scaleprojects 1 2ObjectiveandSignificanceoftheThesis TheobjectiveofthisthesisistodevelopasystematicsoftwaredesignmethodologyforPLC operatedautomationsystems Thedesignmethodologyinvolveshigh leveldescriptionbasedon statetransitionmodelsthattreatautomationcontrolsystemsasdiscreteeventsystems astepwise designprocess andsetofdesignrulesprovidingguidanceandmeasurementstoachievea successfuldesign Thetangibleoutcomeofthisresearchistofindawaytoreducethe uncertaintyinmanagingthecontrolsoftwaredevelopmentprocess thatis reducing programminganddebuggingtimeandtheirvariation increasingflexibilityoftheautomation systems andenablingsoftwarereusabilitythroughmodularity Thegoalistoovercome shortcomingsofcurrentprogrammingstrategiesthatarebasedontheexperienceofthe individualsoftwaredeveloper DesignTheoryDevelopment Today theprimaryfocusofmostdesignresearchisbasedonmechanicalorelectrical products Oneoftheby productsofthisproposedresearchistoenhanceourfundamental understandingofdesigntheoryandmethodologybyextendingittothefieldofengineering systemsdesign Asystemdesigntheoryforlarge scaleandcomplexsystemisnotyetfully developed Particularly thequestionofhowtosimplifyacomplicatedorcomplexdesigntask hasnotbeentackledinascientificway Furthermore buildingabridgebetweendesigntheory andthelatestepistemologicaloutcomesofformalrepresentationsincomputersciencesand operationsresearch suchasdiscreteeventsystemmodeling canadvancefuturedevelopmentin engineeringdesign ApplicationinLogicalHardwareDesign Fromalogicalperspective PLCsoftwaredesignissimilartothehardwaredesignof integratedcircuits ModernVLSIdesignsareextremelycomplexwithseveralmillionpartsanda productdevelopmenttimeof3years Whitney 1996 Thedesignprocessisnormallyseparated intoacomponentdesignandasystemdesignstage Atcomponentdesignstage singlefunctions aredesignedandverified Atsystemdesignstage componentsareaggregatedandthewhole systembehaviorandfunctionalityistestedthroughsimulation Ingeneral acomplete verificationisimpossible Hence asystematicapproachasexemplifiedforthePLCprogram designmayimpactthelogicalhardwaredesign PLC PLC programmablelogicalcontroller faceevermorecomplexchallengethesedays Whereoncetheyquietlyrelaysandgaveanoccasionalreporttoacorporatemainframe theyare nowgroundedintocells givenewjobsandnewlanguages andareforcedtocompeteagainsta growingarrayofcontrolproducts Forthisyear sannualPLCtechnologyupdate wequeried PLCmakersonthesetopicsandmore Programminglanguages HigherlevelPLCprogramminglanguageshavebeenaroundforsometime butlatelytheir popularityhasbeenmushrooming AsRaymondLavelle vicepresidentandgeneralmanager SiemensEnergyandAutomation Inc ProgrammableControlsDivision pointsout As programmablecontrolsarebeingusedformoreandmoresophisticatedoperations languages otherthanladderlogicbecomemorepractical efficient andpowerful Forexample it svery difficulttowriteatrigonometricfunctionusingladderlogic Languagesgainingacceptance includeBoolean controlsystemflowcharting andsuchfunctionchartlanguagesasGraphtec anditsvariations Andthese sincreasinginterestinlanguageslikeCandBASIC PLCsinprocesscontrol Thusfar PLCshavenotbeenusedextensivelyforcontinuousprocesscontrol Willthis continue ThefeelingthatI vegotten saysKenJeannette manager productplanning SeriesOneandSeriesSixproducts atGEFrameNorthAmerica isthatPLCswillbeusedin theprocessindustrybutnotnecessarilyforprocesscontrol Severalvendors obviouslybettingthattheoppositewillhappen haveintroducedPLCs optimizedforprocessapplications RichRyan manager commercialmarketing Allen Bradley ProgrammableControlsDiv citesPLCs increasinguseinsuchindustriesasfood chemicals andpetroleum Ryanfeeltherearetwotypesofapplicationsinwhichthey reappropriate One hesays iswherethesizeoftheprocesscontrolsystemthat sbeingautomateddoesn tjustify DCS distributedcontrolsystems Withthestartingpricetagsofthoseproductsbeingrelatively high aprogrammablecontrollermakessenseforsmall lowloopcountapplications The secondswhereyouhavetointegratetheloopcloselywiththesequentiallogic Batchcontroller sereprimeexamples wherethesequenceandmaintainingtheprocessvariableareintertwinedso closelythatthebenefitsofhavingaprogrammablecontrollertodothesequentiallogic outweighssomeofthedisadvantagesofnothavingadistributedcontrolsystem BillBarkovtz presidentofTriconex predictsthat allfuturecontrollersthatcomeoutinthe processcontrolsystembusinesswillembracealotmorePLCtechnologyandalotmorePLC functionalitythantheyeverdidbefore TheProgrammableLogicController JohnR