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外文翻译--关于PLC的控制应用.doc

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外文翻译--关于PLC的控制应用.doc

PLCControloftheApplication1MotivationProgrammableLogicControllersPLC,acomputingdeviceinventedbyRichardE.Morleyin1968,havebeenwidelyusedinindustryincludingmanufacturingsystems,transportationsystems,chemicalprocessfacilities,andmanyothers.Atthattime,thePLCreplacedthehardwiredlogicwithsoftwiredlogicorsocalledrelayladderlogicRLL,aprogramminglanguagevisuallyresemblingthehardwiredlogic,andreducedtherebytheconfigurationtimefrom6monthsdownto6days.AlthoughPCbasedcontrolhasstartedtocomeintoplace,PLCbasedcontrolwillremainthetechniquetowhichthemajorityofindustrialapplicationswilladhereduetoitshigherperformance,lowerprice,andsuperiorreliabilityinharshenvironments.Moreover,accordingtoastudyonthePLCmarketofFrostandSullivan1995,anincreaseoftheannualsalesvolumeto15millionPLCsperyearwiththehardwarevalueofmorethan8billionUSdollarshasbeenpredicted,thoughthepricesofcomputinghardwareissteadilydropping.TheinventorofthePLC,RichardEMorley,fairlyconsidersthePLCmarketasa5billionindustryatthepresenttime.Particularly,practicalproblemsinPLCprogrammingaretoeliminatesoftwarebugsandtoreducethemaintenancecostsofoldladderlogicprograms.ThoughthehardwarecostsofPLCsaredroppingcontinuously,reducingthescantimeoftheladderlogicisstillanissueinindustrysothatlowcostPLCscanbeused.Ingeneral,theproductivityingeneratingPLCisfarbehindcomparedtootherdomains,forinstance,VLSIdesign,whereefficientcomputeraideddesigntoolsareinpractice.ExistentsoftwareengineeringmethodologiesarenotnecessarilyapplicabletothePLCbasedsoftwaredesignbecausePLCprogrammingrequiresasimultaneousconsiderationofhardwareandsoftware.Thesoftwaredesignbecomes,thereby,moreandmorethemajorcostdriver.Inmanyindustrialdesignprojects,morethanSO0/aofthemanpowerallocatedforthecontrolsystemdesignandinstallationisscheduledfortestinganddebuggingPLCprogramsRockwell,1999.Inaddition,currentPLCbasedcontrolsystemsarenotproperlydesignedtosupportthegrowingdemandforflexibilityandreconfigureabilityofmanufacturingsystems.Afurtherproblem,impellingtheneedforasystematicdesignmethodology,istheincreasingsoftwarecomplexityinlargescaleprojects.1ObjectiveandSignificanceoftheThesisTheobjectiveofthisthesisistodevelopasystematicsoftwaredesignmethodologyforPLCoperatedautomationsystems.Thedesignmethodologyinvolveshighleveldescriptionbasedonstatetransitionmodelsthattreatautomationcontrolsystemsasdiscreteeventsystems,astepwisedesignprocess,andsetofdesignrulesprovidingguidanceandmeasurementstoachieveasuccessfuldesign.Thetangibleoutcomeofthisresearchistofindawaytoreducetheuncertaintyinmanagingthecontrolsoftwaredevelopmentprocess,thatis,reducingprogramminganddebuggingtimeandtheirvariation,increasingflexibilityoftheautomationsystems,andenablingsoftwarereusabilitythroughmodularity.Thegoalistoovercomeshortcomingsofcurrentprogrammingstrategiesthatarebasedontheexperienceoftheindividualsoftwaredeveloper.AsystematicapproachtodesigningPLCsoftwarecanovercomedeficienciesinthetraditionalwayofprogrammingmanufacturingcontrolsystems,andcanhavewideramificationsinseveralindustrialapplications.Automationcontrolsystemsaremodeledbyformallanguagesor,equivalently,bystatemachines.Formalrepresentationsprovideahighleveldescriptionofthebehaviorofthesystemtobecontrolled.Statemachinescanbeanalyticallyevaluatedastowhetherornottheymeetthedesiredgoals.Secondly,astatemachinedescriptionprovidesastructuredrepresentationtoconveythelogicalrequirementsandconstraintssuchasdetailedsafetyrules.Thirdly,welldefinedcontrolsystemsdesignoutcomesareconducivetoautomaticcodegenerationAnabilitytoproducecontrolsoftwareexecutableoncommercialdistinctlogiccontrollerscanreduceprogrammingleadtimeandlaborcost.Inparticular,thethesisisrelevantwithrespecttothefollowingaspects.1CustomerDrivenManufacturingInmodernmanufacturing,systemsarecharacterizedbyproductandprocessinnovation,becomecustomerdrivenandthushavetorespondquicklytochangingsystemrequirements.Amajorchallengeisthereforetoprovideenablingtechnologiesthatcaneconomicallyreconfigureautomationcontrolsystemsinresponsetochangingneedsandnewopportunities.Designandoperationalknowledgecanbereusedinrealtime,therefore,givingasignificantcompetitiveedgeinindustrialpractice.2HigherDegreeofDesignAutomationandSoftwareQualityStudieshaveshownthatprogrammingmethodologiesinautomationsystemshavenotbeenabletomatchrapidincreaseinuseofcomputingresources.Forinstance,theprogrammingofPLCsstillreliesonaconventionalprogrammingstylewithladderlogicdiagrams.Asaresult,thedelaysandresourcesinprogrammingareamajorstumblingstonefortheprogressofmanufacturingindustry.Testinganddebuggingmayconsumeover50ofthemanpowerallocatedforthePLCprogramdesign.StandardsIEC60848,1999IEC611313,1993IEC61499,1998ISO157451,1999havebeenformedtofixanddisseminatestateoftheartdesignmethods,buttheynormallycannotparticipateinadvancingtheknowledgeofefficientprogramandsystemdesign.Asystematicapproachwillincreasethelevelofdesignautomationthroughreusingexistingsoftwarecomponents,andwillprovidemethodstomakelargescalesystemdesignmanageable.Likewise,itwillimprovesoftwarequalityandreliabilityandwillberelevanttosystemshighsecuritystandards,especiallythosehavinghazardousimpactontheenvironmentsuchasairportcontrol,andpublicrailroads.3SystemComplexityThesoftwareindustryisregardedasaperformancedestructorandcomplexitygenerator.Steadilyshrinkinghardwarepricesspoilstheneedforsoftwareperformanceintermsofcodeoptimizationandefficiency.Theresultisthatmassiveandlessefficientsoftwarecodeononehandoutpacesthegainsinhardwareperformanceontheotherhand.Secondly,softwareproliferatesintocomplexityofunmanageabledimensionssoftwareredesignandmaintenanceessentialinmodernautomationsystemsbecomesnearlyimpossible.Particularly,PLCprogramshaveevolvedfromacouplelinesofcode25yearsagotothousandsoflinesofcodewithasimilarnumberofI/Opoints.Increasedsafety,forinstancenewpoliciesonfireprotection,andtheflexibilityofmodernautomationsystemsaddcomplexitytotheprogramdesignprocess.Consequently,thelifecyclecostofsoftwareisapermanentlygrowingfractionofthetotalcost.8090ofthesecostsaregoingintosoftwaremaintenance,debugging,adaptationandexpansiontomeetchangingneeds.4DesignTheoryDevelopmentToday,theprimaryfocusofmostdesignresearchisbasedonmechanicalorelectricalproducts.Oneofthebyproductsofthisproposedresearchistoenhanceourfundamentalunderstandingofdesigntheoryandmethodologybyextendingittothefieldofengineeringsystemsdesign.Asystemdesigntheoryforlargescaleandcomplexsystemisnotyetfullydeveloped.Particularly,thequestionofhowtosimplifyacomplicatedorcomplexdesigntaskhasnotbeentackledinascientificway.Furthermore,buildingabridgebetweendesigntheoryandthelatestepistemologicaloutcomesofformalrepresentationsincomputersciencesandoperationsresearch,suchasdiscreteeventsystemmodeling,canadvancefuturedevelopmentinengineeringdesign.5ApplicationinLogicalHardwareDesignFromalogicalperspective,PLCsoftwaredesignissimilartothehardwaredesignofintegratedcircuits.ModernVLSIdesignsareextremelycomplexwithseveralmillionpartsandaproductdevelopmenttimeof3yearsWhitney,1996.Thedesignprocessisnormallyseparatedintoacomponentdesignandasystemdesignstage.Atcomponentdesignstage,singlefunctionsaredesignedandverified.Atsystemdesignstage,componentsareaggregatedandthewholesystembehaviorandfunctionalityistestedthroughsimulation.Ingeneral,acompleteverificationisimpossible.Hence,asystematicapproachasexemplifiedforthePLCprogramdesignmayimpactthelogicalhardwaredesign.

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