1PLC英文資料 1.1Motivation ProgrammableLogicControllers(PLC),acomputingdeviceinventedbyRichardE.Morleyin1968,havebeenwidelyusedinindustryincludingmanufacturingsystems,transportationsystems,chemicalprocessfacilities,andmanyothers.Atthattime,thePLCreplacedthehardwiredlogicwithsoft-wiredlogicorso-calledrelayladderlogic(RLL),aprogramminglanguagevisuallyresemblingthehardwiredlogic,andreducedtherebytheconfigurationtimefrom6monthsdownto6daysMoodyandMorley,1999. AlthoughPCbasedcontrolhasstartedtocomeintoplace,PLCbasedcontrolwillremainthetechniquetowhichthemajorityofindustrialapplicationswilladhereduetoitshigherperformance,lowerprice,andsuperiorreliabilityinharshenvironments.Moreover,accordingtoastudyonthePLCmarketofFrostandSullivan1995,anincreaseoftheannualsalesvolumeto15millionPLCsperyearwiththehardwarevalueofmorethan8billionUSdollarshasbeenpredicted,thoughthepricesofcomputinghardwareissteadilydropping.TheinventorofthePLC,RichardEMorley,fairlyconsidersthePLCmarketasa5-billionindustryatthepresenttime. ThoughPLCsarewidelyusedinindustrialpractice,theprogrammingofPLCbasedcontrolsystemsisstillverymuchrelyingontrial-and-error.Alikesoftwareengineering,PLCsoftwaredesignisfacingthesoftwaredilemmaorcrisisinasimilarway.MorleyhimselfemphasizedthisaspectmostforcefullybyindicatingMoodyandMorley,1999,p.110: Ifhouseswerebuiltlikesoftwareprojects,asinglewoodpeckercoulddestroycivilization.” Particularly,practicalproblemsinPLCprogrammingaretoeliminatesoftwarebugsandtoreducethemaintenancecostsofoldladderlogicprograms.ThoughthehardwarecostsofPLCsaredroppingcontinuously,reducingthescantimeoftheladderlogicisstillanissueinindustrysothatlow-costPLCscanbeused. Ingeneral,theproductivityingeneratingPLCisfarbehindcomparedtootherdomains,forinstance,VLSIdesign,whereefficientcomputeraideddesigntoolsareinpractice.ExistentsoftwareengineeringmethodologiesarenotnecessarilyapplicabletothePLCbasedsoftwaredesignbecausePLC-programmingrequiresasimultaneousconsiderationofhardwareandsoftware.Thesoftwaredesignbecomes,thereby,moreandmorethemajorcostdriver.Inmanyindustrialdesignprojects,morethanSO0/aofthemanpowerallocatedforthecontrolsystemdesignandinstallationisscheduledfortestinganddebuggingPLCprogramsRockwell,1999. Inaddition,currentPLCbasedcontrolsystemsarenotproperlydesignedtosupportthegrowingdemandforflexibilityandreconfigurabilityofmanufacturingsystems.Afurtherproblem,impellingtheneedforasystematicdesignmethodology,istheincreasingsoftwarecomplexityinlarge-scaleprojects. 1.2ObjectiveandSignificanceoftheThesis TheobjectiveofthisthesisistodevelopasystematicsoftwaredesignmethodologyforPLCoperatedautomationsystems.Thedesignmethodologyinvolveshigh-leveldescriptionbasedonstatetransitionmodelsthattreatautomationcontrolsystemsasdiscreteeventsystems,astepwisedesignprocess,andsetofdesignrulesprovidingguidanceandmeasurementstoachieveasuccessfuldesign.Thetangibleoutcomeofthisresearchistofindawaytoreducetheuncertaintyinmanagingthecontrolsoftwaredevelopmentprocess,thatis,reducingprogramminganddebuggingtimeandtheirvariation,increasingflexibilityoftheautomationsystems,andenablingsoftwarereusabilitythroughmodularity.Thegoalistoovercomeshortcomingsofcurrentprogrammingstrategiesthatarebasedontheexperienceoftheindividualsoftwaredeveloper.2PLC英文資料 AsystematicapproachtodesigningPLCsoftwarecanovercomedeficienciesinthetraditionalwayofprogrammingmanufacturingcontrolsystems,andcanhavewideramificationsinseveralindustrialapplications.Automationcontrolsystemsaremodeledbyformallanguagesor,equivalently,bystatemachines.Formalrepresentationsprovideahigh-leveldescriptionofthebehaviorofthesystemtobecontrolled.Statemachinescanbeanalyticallyevaluatedastowhetherornottheymeetthedesiredgoals.Secondly,astatemachinedescriptionprovidesastructuredrepresentationtoconveythelogicalrequirementsandconstraintssuchasdetailedsafetyrules.Thirdly,well-definedcontrolsystemsdesignoutcomesareconducivetoautomaticcodegeneration-Anabilitytoproducecontrolsoftwareexecutableoncommercialdistinctlogiccontrollerscanreduceprogramminglead-timeandlaborcost.Inparticular,thethesisisrelevantwithrespecttothefollowingaspects. Customer-DrivenManufacturing Inmodernmanufacturing,systemsarecharacterizedbyproductandprocessinnovation,becomecustomer-drivenandthushavetorespondquicklytochangingsystemrequirements.Amajorchallengeisthereforetoprovideenablingtechnologiesthatcaneconomicallyreconfigureautomationcontrolsystemsinresponsetochangingneedsandnewopportunities.Designandoperationalknowledgecanbereusedinreal-time,therefore,givingasignificantcompetitiveedgeinindustrialpractice. HigherDegreeofDesignAutomationandSoftwareQuality Studieshaveshownthatprogrammingmethodologiesinautomationsystemshavenotbeenabletomatchrapidincreaseinuseofcomputingresources.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. SystemComplexity Thesoftwareindustryisregardedasaperformancedestructorandcomplexitygenerator.Steadilyshrinkinghardwarepricesspoilstheneedforsoftwareperformanceintermsofcodeoptimizationandefficiency.Theresultisthatmassiveandlessefficientsoftwarecodeononehandoutpacesthegainsinhardwareperformanceontheotherhand.Secondly,softwareproliferatesintocomplexityofunmanageabledimensions;softwareredesignandmaintenance-essentialinmodernautomationsystems-becomesnearlyimpossible.Particularly,PLCprogramshaveevolvedfromacouplelinesofcode25yearsagotothousandsoflinesofcodewithasimilarnumberof1/Opoints.Increasedsafety,forinstancenewpoliciesonfireprotection,andtheflexibilityofmodernautomationsystemsaddcomplexitytotheprogramdesignprocess.Consequently,thelife-cyclecostofsoftwareisapermanentlygrowingfractionofthetotalcost.80-90%ofthesecostsaregoingintosoftwaremaintenance,debugging,adaptationandexpansiontomeetchangingneedsSimmonsetal.,1998. 3PLC英文資料 DesignTheoryDevelopment Today,theprimaryfocusofmostdesignresearchisbasedonmechanicalorelectricalproducts.Oneoftheby-productsofthisproposedresearchistoenhanceourfundamentalunderstandingofdesigntheoryandmethodologybyextendingittothefieldofengineeringsystemsdesign.Asystemdesigntheoryforlarge-scaleandcomplexsystemisnotyetfullydeveloped.Particularly,thequestionofhowtosimplifyacomplicatedorcomplexdesigntaskhasnotbeentackledinascientificway.Furthermore,buildingabridgebetweendesigntheoryandthelatestepistemologicaloutcomesofformalrepresentationsincomputersciencesandoperationsresearch,suchasdiscreteeventsystemmodeling,canadvancefuturedevelopmentinengineeringdesign. ApplicationinLogicalHardwareDesign Fromalogicalperspective,PLCsoftwaredesignissimilartothehardwaredesignofintegratedcircuits.ModernVLSIdesignsareextremelycomplexwithseveralmillionpartsandaproductdevelopmenttimeof3yearsWhitney,1996.Thedesignprocessisnormallyseparatedintoacomponentdesignandasystemdesignstage.Atcomponentdesignstage,singlefunctionsaredesignedandverified.Atsystemdesignstage,componentsareaggregatedandthewholesystembehaviorandfunctionalityistestedthroughsimulation.Ingeneral,acompleteverificationisimpossible.Hence,asystematicapproachasexemplifiedforthePLCprogramdesignmayimpactthelogicalhardwaredesign. 1.3StructureoftheThesis Figure1.1illustratestheoutlineofthefollowingthesis.Chapter2clarifiesthemajorchallengesandresearchissues,anddiscoursestherelevantbackgroundandterminology.ItwillbearguedthatasystematicdesignofPLCsoftwarecancontributetohigherflexibilityandreconfigurabilityofmanufacturingsystems.Theimportantissueofhowtodealwithcomplexityinengineeringdesignwithrespecttodesigningandoperatingasystemwillbedebated.Theresearchapproachappliedinthisthesisisintroducedstartingfromadiscussionofdesigntheoryandmethodologyandwhatcanbelearntfromthatfield. Chapter3coversthestate-of-the-artofcontroltechnologyandthecurrentpracticeindesigningandprogrammingPLCsoftware.Theinfluencesofelectricalandsoftwareengineeringarerevealedaswellasthepotentiallyapplicablemethodsfromcomputersciencearediscussed.ProsandconsareevaluatedandwillleadtotheconclusionthatanewmethodologyisrequiredthatsufficestheincreasingcomplexityofPLCsoftwaredesign. Chapter4representsthemainbodyofthethesisandcapturestheessentialfeaturesofthedesignmethodology.Thoughdesigntheoryisregardedasbeinginapre-scientificstageithasadvancedinmechanical,softwareandsystemengineeringwithrespecttoanumberofproposeddesignmodelsandtheirevaluationthroughoutreal-worldexamples.BasedonaliteraturereviewinChapter2and3potentialapplicabledesignconceptsandapproachesareselectedandappliedtocontextofPLCsoftwaredesign.Axiomaticdesignischosenasunderlyingdesignconceptsinceitprovidesguidanceforthedesignerwithoutrestrictiontoaparticulardesignco