外文翻译--关于PLC的控制应用.doc

PLCControloftheApplication1MotivationProgrammableLogicControllers(PLC),acomputingdeviceinventedbyRichardE.Morleyin1968,havebeenwidelyusedinindustryincludingmanufacturingsystems,transportationsystems,chemicalprocessfacilities,andmanyothers.Atthattime,thePLCreplacedthehardwiredlogicwithsoft-wiredlogicorso-calledrelayladderlogic(RLL),aprogramminglanguagevisuallyresemblingthehardwiredlogic,andreducedtherebytheconfigurationtimefrom6monthsdownto6days.AlthoughPCbasedcontrolhasstartedtocomeintoplace,PLCbasedcontrolwillremainthetechniquetowhichthemajorityofindustrialapplicationswilladhereduetoitshigherperformance,lowerprice,andsuperiorreliabilityinharshenvironments.Moreover,accordingtoastudyonthePLCmarketofFrostandSullivan1995,anincreaseoftheannualsalesvolumeto15millionPLCsperyearwiththehardwarevalueofmorethan8billionUSdollarshasbeenpredicted,thoughthepricesofcomputinghardwareissteadilydropping.TheinventorofthePLC,RichardEMorley,fairlyconsidersthePLCmarketasa5-billionindustryatthepresenttime.Particularly,practicalproblemsinPLCprogrammingaretoeliminatesoftwarebugsandtoreducethemaintenancecostsofoldladderlogicprograms.ThoughthehardwarecostsofPLCsaredroppingcontinuously,reducingthescantimeoftheladderlogicisstillanissueinindustrysothatlow-costPLCscanbeused.Ingeneral,theproductivityingeneratingPLCisfarbehindcomparedtootherdomains,forinstance,VLSIdesign,whereefficientcomputeraideddesigntoolsareinpractice.ExistentsoftwareengineeringmethodologiesarenotnecessarilyapplicabletothePLCbasedsoftwaredesignbecausePLC-programmingrequiresasimultaneousconsiderationofhardwareandsoftware.Thesoftwaredesignbecomes,thereby,moreandmorethemajorcostdriver.Inmanyindustrialdesignprojects,morethanSO0/aofthemanpowerallocatedforthecontrolsystemdesignandinstallationisscheduledfortestinganddebuggingPLCprogramsRockwell,1999.Inaddition,currentPLCbasedcontrolsystemsarenotproperlydesignedtosupportthegrowingdemandforflexibilityandreconfigureabilityofmanufacturingsystems.Afurtherproblem,impellingtheneedforasystematicdesignmethodology,istheincreasingsoftwarecomplexityinlarge-scaleprojects.1ObjectiveandSignificanceoftheThesisTheobjectiveofthisthesisistodevelopasystematicsoftwaredesignmethodologyforPLCoperatedautomationsystems.Thedesignmethodologyinvolveshigh-leveldescriptionbasedonstatetransitionmodelsthattreatautomationcontrolsystemsasdiscreteeventsystems,astepwisedesignprocess,andsetofdesignrulesprovidingguidanceandmeasurementstoachieveasuccessfuldesign.Thetangibleoutcomeofthisresearchistofindawaytoreducetheuncertaintyinmanagingthecontrolsoftwaredevelopmentprocess,thatis,reducingprogramminganddebuggingtimeandtheirvariation,increasingflexibilityoftheautomationsystems,andenablingsoftwarereusabilitythroughmodularity.Thegoalistoovercomeshortcomingsofcurrentprogrammingstrategiesthatarebasedontheexperienceoftheindividualsoftwaredeveloper.AsystematicapproachtodesigningPLCsoftwarecanovercomedeficienciesinthetraditionalwayofprogrammingmanufacturingcontrolsystems,andcanhavewideramificationsinseveralindustrialapplications.Automationcontrolsystemsaremodeledbyformallanguagesor,equivalently,bystatemachines.Formalrepresentationsprovideahigh-leveldescriptionofthebehaviorofthesystemtobecontrolled.Statemachinescanbeanalyticallyevaluatedastowhetherornottheymeetthedesiredgoals.Secondly,astatemachinedescriptionprovidesastructuredrepresentationtoconveythelogicalrequirementsandconstraintssuchasdetailedsafetyrules.Thirdly,well-definedcontrolsystemsdesignoutcomesareconducivetoautomaticcodegeneration-Anabilitytoproducecontrolsoftwareexecutableoncommercialdistinctlogiccontrollerscanreduceprogramminglead-timeandlaborcost.Inparticular,thethesisisrelevantwithrespecttothefollowingaspects.(1)Customer-DrivenManufacturingInmodernmanufacturing,systemsarecharacterizedbyproductandprocessinnovation,becomecustomer-drivenandthushavetorespondquicklytochangingsystemrequirements.Amajorchallengeisthereforetoprovideenablingtechnologiesthatcaneconomicallyreconfigureautomationcontrolsystemsinresponsetochangingneedsandnewopportunities.Designandoperationalknowledgecanbereusedinreal-time,therefore,givingasignificantcompetitiveedgeinindustrialpractice.(2)HigherDegreeofDesignAutomationandSoftwareQualityStudieshaveshownthatprogrammingmethodologiesinautomationsystemshavenotbeenabletomatchrapidincreaseinuseofcomputingresources.Forinstance,theprogrammingofPLCsstillreliesonaconventionalprogrammingstylewithladderlogicdiagrams.Asaresult,thedelaysandresourcesinprogrammingareamajorstumblingstonefortheprogressofmanufacturingindustry.Testinganddebuggingmayconsumeover50%ofthemanpowerallocatedforthePLCprogramdesign.StandardsIEC60848,1999；IEC-61131-3,1993；IEC61499,1998；ISO15745-1,1999havebeenformedtofixanddisseminatestate-of-the-artdesignmethods,buttheynormallycannotparticipateinadvancingtheknowledgeofefficientprogramandsystemdesign.Asystematicapproachwillincreasethelevelofdesignautomationthroughreusingexistingsoftwarecomponents,andwillprovidemethodstomakelarge-scalesystemdesignmanageable.Likewise,itwillimprovesoftwarequalityandreliabilityandwillberelevanttosystemshighsecuritystandards,especiallythosehavinghazardousimpactontheenvironmentsuchasairportcontrol,andpublicrailroads.(3)SystemComplexityThesoftwareindustryisregardedasaperformancedestructorandcomplexitygenerator.Steadilyshrinkinghardwarepricesspoilstheneedforsoftwareperformanceintermsofcodeoptimizationandefficiency.Theresultisthatmassiveandlessefficientsoftwarecodeononehandoutpacesthegainsinhardwareperformanceontheotherhand.Secondly,softwareproliferatesintocomplexityofunmanageabledimensions；softwareredesignandmaintenance-essentialinmodernautomationsystems-becomesnearlyimpossible.Particularly,PLCprogramshaveevolvedfromacouplelinesofcode25yearsagotothousandsoflinesofcodewithasimilarnumberofI/Opoints.Increasedsafety,forinstancenewpoliciesonfireprotection,andtheflexibilityofmodernautomationsystemsaddcomplexitytotheprogramdesignprocess.Consequently,thelife-cyclecostofsoftwareisapermanentlygrowingfractionofthetotalcost.80-90%ofthesecostsaregoingintosoftwaremaintenance,debugging,adaptationandexpansiontomeetchangingneeds.(4)DesignTheoryDevelopmentToday,theprimaryfocusofmostdesignresearchisbasedonmechanicalorelectricalproducts.Oneoftheby-productsofthisproposedresearchistoenhanceourfundamentalunderstandingofdesigntheoryandmethodologybyextendingittothefieldofengineeringsystemsdesign.Asystemdesigntheoryforlarge-scaleandcomplexsystemisnotyetfullydeveloped.Particularly,thequestionofhowtosimplifyacomplicatedorcomplexdesigntaskhasnotbeentackledinascientificway.Furthermore,buildingabridgebetweendesigntheoryandthelatestepistemologicaloutcomesofformalrepresentationsincomputersciencesandoperationsresearch,suchasdiscreteeventsystemmodeling,canadvancefuturedevelopmentinengineeringdesign.(5)ApplicationinLogicalHardwareDesignFromalogicalperspective,PLCsoftwaredesignissimilartothehardwaredesignofintegratedcircuits.ModernVLSIdesignsareextremelycomplexwithseveralmillionpartsandaproductdevelopmenttimeof3yearsWhitney,1996.Thedesignprocessisnormallyseparatedintoacomponentdesignandasystemdesignstage.Atcomponentdesignstage,singlefunctionsaredesignedandverified.Atsystemdesignstage,componentsareaggregatedandthewholesystembehaviorandfunctionalityistestedthroughsimulation.Ingeneral,acompleteverificationisimpossible.Hence,asystematicapproachasexemplifiedforthePLCprogramdesignmayimpactthelogicalhardwaredesign.