毕业设计外文翻译CAN总线系统的实时对象调用.docx
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毕业设计外文翻译CAN总线系统的实时对象调用.docx
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毕业设计外文翻译CAN总线系统的实时对象调用
大连交通大学2009届本科生毕业设计外文翻译
InvocationofReal-TimeObjectsinaCANBus-System
Abstract
Thepaperfocusesonmethodinvocationofreal-timeobjectsinaCAN-baseddistributedreal-timesystem.Asimpleobjectmodelisintroduced,whichallowstheconvenientmodellingofhardwareandsoftwarecomponents.Relatedtotheobjectmodel,twoissuesarediscussed.Firstly,amodelisintroducedwhichallowstoformandaddressobjectgroups.Thisreflectsabasicneedinareal-timesystemtodistributeinformationtomultipleclientsefficiently.Secondly,wediscussanapproachtoexpresstimingrequirementsforobjectinvocations.Toachievedistributedconsensusoncommunicationresourceaccess,anEDF-likeapproachisintroduced,whichtakesadvantageofknowledgeaboutdeadlines,thenumberofremainingcommunicationactivities,andtheremainingworst-caseexecutiontimefortheinvokedmethodateachpointoftime.
1.Introduction
Futurecomputersystemswill,toalargeextent,monitorandcontrolreal-worldprocesses.Thisresultsinaninevitabledemandfortimelinessandreliability.Distributedsystemswhichinherentlyprovideimmunityagainstsinglefailuresareanadequatearchitecturetomeetthegoalofreliability.Additionally,becauserealworldapplicationsoftenrequirethespatiallydistributedcontrolofelectromechanicalcomponents,adistributedsystemarchitecturebringscomputingpowertothepointswhereitisneeded.Theavailabilityofinexpensiveyetpowerfulmicrocontrollerssupportsadistributedsolution.Thisresultsinasystemmodelwhichiscomposedfromsmartcomponentsimplementingtheinstrumentationinterfacetotherealworldcomprisingsensorsandactuators.Aconvenientwaytomodelsuchanenvironmentistouseanobject-basedapproachwhereobjectsencapsulateallkindsofentitiesnecessarytocontrolaphysicalprocess.
Wecanidentifyatleasttwolayersinsystemsinteractingwiththerealworld.Alowerresponsivesystemleveltightlycontrollingthesensor/actuatorinterfacetotheworldandahighersystemlevelresponsibleforinterpretationandevaluationoftheperceptionoftheworldascommunicatedbytheinstrumentationinterface.Inthislayer,higherleveldecisionsareperformed.Althoughthesedifferentlevelshavedifferentrequirementsconcerningresponsiveness,theyaresometimesnotseparatedclearlysimplybecauseofthehardwarearchitecture.Weconcentrateonthelowersystemlevelanddescribeanobjectmodelwhichismotivatedbyasystemstructurewhichreliesonamodulardesignandindependentcomputingresourcesforthecomponentsoftheinstrumentationinterface.Ratherthanhavingacentral
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大连交通大学2009届本科生毕业设计外文翻译
powerfulprocessorinwhichelectromechanicalcomponentsareonlymodelledasobjectsfromwhichcontrolsignalsaregenerated,theelectromechanicalcomponentsareobjectsthemselvespoweredbyinexpensivemicrocontrollers.Theobjectsofthislayerhaveawelldefinedinterfacetobeeasilycontrolledbyahigherlevelinstancewhichcanexploitthisabstractionratherthandealingwithlowlevelcontrolsignals.
Thehardwaremodulesareconnectedbyafield-bus.WechoosetheCAN-bus(CAN:
ControllerAreaNetwork),developedbyBOSCH[[28]]becauseitprovidesadvancedtechnicalfeaturesandrepresentsanemergingstandardwithawideapplicability.Moreover,popularmicrocontrollersareavailablewithanon-chipCANcontroller.CANisasharedbusdesignedtoconnectcontrolsystemsinaspatiallyrestrictedarealikecars,robots,toolmachines,andotherautomotiveorindustrialautomationapplications.Itistargetedtooperateinanoisyenvironmentwithspeedsofupto1Mbit/sec,exchangingsmallreal-timecontrolmessages.WeusetheCANmessageformattouniformlyinvokemethodsonourobjects.Thisallowsatransparentobjectinvocationinthesensethataninvokingobjectmustnotknowwheretheinvokedobjectresidesinthesystem.Additionally,wecanaddressgroupsofobjectswithasingleCANmessage.
Asacommonresource,thecommunicationmediumhastobesharedbyallcomputingnodes.Accesstothemediumhastobescheduledinawaythatdistributedcomputationsmeettheirdeadlinesinspiteofcompetitionforthecommunicationline.Sincetheschedulingofthebuscannotbebasedonlocaldecisions,adistributedconsensusaboutthebusreservationhastobeachieved.Clearly,thisisonlyaspecialcaseofthemoregeneralproblemtoscheduleacooperativedistributedcomputation.
Thereexistseveralalternativeapproachestosolvethisproblembasedontheassumptionsaboutthebehaviourofthesystemandtheenvironment.Thefirstapproach,knownastime-triggeredapproach[[18]],[[21]],[[19]]assumesacompleteknowledgeofallfutureactionsofthesystem.Hence,duringoperationthesesystemsexhibitminimaloverheadpairedwithamaximumofpredictabilityandarehighlyappropriateforsafety-criticalapplicationswhichcanbemodelledbyaperiodicbehaviour.
Amoreadaptableandeconomicwaytomanagesystemresourceshastobebasedonrun-timeinformation.Thesereal-timesystemshavetoplantheiroperationatrun-time,atleasttoacertainextent.Althoughthetrade-offgenerallyisbetweenadaptabilityandpredictability,thesesystemsarescalableinthesensethattheyallowacoexistenceofactivitieswithdifferentreal-timerequirements.Thesesystemsrelyonrun-timeguarantees[[30]],[[11]]ratherthanonapreplannedfeasibilityguaranteeonly.Aformofcalendar-basedscheduling[[22]],[[30]]isanapproachtorun-timeguaranteeswherehardreal-timeactivitieshavethe
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大连交通大学2009届本科生毕业设计外文翻译
possibilitytoreserveresourcesinadvance.Thefreeresourcescanbeusedbylessimportanttasks.Itshouldbenotedthatschedulingpolicieslikee.g.earliestdeadlinefirst(EDF),leastlaxityfirst(LLF)orratemonotonicschedulingonlyguaranteeoptimality,i.e.ifascheduleexists,itwillbefound.Butinthedynamicsystem,theremaybeconflictingrequestsforwhichtheseschedulingpolicieswillnotfindasolution.Inthiscasetheyfailtoguaranteeanything.Acalendar,ontheotherhand,isamechanismtoguaranteeresourceavailabilityforreservationswhichhavealreadybeenmade.
2.TheSystemModel
Oursystemmodelisinfluencedbyouranticipatedhardwarebasis.Weassumeanumberofdifferentmicrocontrollerswithdifferentperformanceattributesrangingfrom8Bitto32Bitarchitectures.AllmicrocontrollersareequippedwithaCAN-Businterface.Weexploitthedifferentperformanceandpricecharacteristicsofthemicrocontrollerstostructuretheoveralltaskofthesystemintosmallpackages.Sincethesimplemicrocontrollersareintherangeofafew$s,itmustnolongerbearareresource.Inourapplicationexample,asimpleactivesuspensionsystem(Fig.2-1),weuseasimplemicrocontrollerforeachservoorcomplexsensor.Aservoiscomposedfromamotorandaninternalsensorwhichprovidesfeedbackonthecurrentposition.Thissensor/actuatorsystemisconnectedtotheCAN-bustocooperatewithothersimilarobjectsandreceivecontrolcommandsthroughCANmessages.Nodirect,lowlevelcontrolsignalsarevisibleoutsidethemodule.Thus,encapsulationandawelldefinedinterfacearesupported.Onthelogicallevel,thesensor/actuatorblockisseenasanobjectonwhichcertainmethodscanbeinvoked,e.g.topositionitselftoaspecificangel,tocommunicatetheangel,etc.Asensor/actuatorconstitutesthelowestlevelinthecontrolhierarchy,comparablewithasimplereflexloopinabiologicalsystem.Highercontrolinstancesareavailabletocontrolgroupsofobjectsoreventuallytheentiresystemusingmorepowerfulmicroprocessors.Thus,thehighercontrolinstanceshaveawelldefinedinstrumentationinterfacewhichiscomposedfromobjects,eachencapsulatingacertainfunctionality.
Anobjecthasauniquenameandasetofassociatedoperations.Theuniquenameoftheobjectistranslatedtoashortformsystemnameduringrun-timeandmaintainedbyaconfigurationservice.Objectsinsuchanenvironmentmusthavesomeextensionstothesequential,passivemodelknownfromthefieldofprogramminglanguages.Particularlyourmodelincludes:
1).Active,autonomousobjects.
Sinceobjectsmayhaveadedicatedprocessor,itisstraightforwardtoassumeamodelof
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大连交通大学2009届本科生毕业设计外文翻译
autonomous,activeobjects.Anobjectischaracterizedbyanameandalistofmethodswhichcanbeinvokedontheobject.Becausetheobjectisactive,itcanalsoexportinformation.Thisisequivalenttosendinganinvocationtoanotherobjectortoagroupofobjectswithoutapreviousrequest.Asdescribedinchapter3,weprovideatransparentgroupcommunicationmechanism,i.e.anobjectitselfusuallydoesnotknowwhetheritcommunicateswithanobjectonaremoteoronthelocalnode.Thisgroupcommunicationmechanismisabasisforremotemethodinvocationofobjectgroups.Infact,communicationrelationscandynamicallybedefinedduringruntime.So,objectsmayjoinorleaveagroup,andareautonomousindecidingwhentoprocessarequest.
TheconceptofTMO(time-triggeredmessagetriggeredobject)[[16]]derivedfromtheRTO.K(Real-TimeObject)[[17]]conceptenhancestheconventionalobjectmodelbymechanismstospecifythetemporalbehaviourofanobject.Itprovidesaframeworktospecifyacomplexreal-timesystemtogetherwithitsenvironmentinthesameuniformrepresentation.Theaimisanintegrateddesignofadistributedreal-timesystemandsimulatorsgeneratingreal-timeinputmodellingthetargetapplications.ATMOexhibitsactive,spontaneousbehaviouraswellasprovidingservicesrequestedbyamethodcall.ThreeexecutionmodelsarederivedforexecutingTMOS
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