可编程控制器的数据库系统毕业论文外文翻译

DatabaseSystemsforProgrammableLogicControllersABSTRACTInthispaper,weidentifythedatabaseissuesassociatedwithprogrammablelogiccontrollers(PLC),special-purposecomputersusedinscientificandindustrialapplications,e.g.infactoriesinmanufacturingenvironments.WeproposeasaPLCdatabasesystemasingle-user,real-time,scalablemain-memory-onlyrelationaldatabasesystemwithatwo-levelarchitecturehavinghistoricaldatamodelingandmanipulationcapabilities,andqueryprocessingtechniquesincorporatingtime-and/orerror-constrainedqueryevaluation.Werevisetheladderlogiclanguage,themostcommonPLClanguage,toincorporatedatamanipulationlanguageinstructions.WeaddaseparatetimecomponentintothePLCprocessorscantimetohandledatabaseupdates,backup,integrityenforcementanddataarchivalissues.1.IntroductionAprogrammablelogiccontroller(PLC)isaspecial-purposecomputerusedwithinreal-timescientificcomputingsystems,andindustrialcontrolsystems,say,theautomatedcontrolofafactory'smachinery-therunningexampleusedinthispaper.ThispaperisapositionpaperthatproposesaPLCdatabasesystemanddiscussesitsfeatures.Indoingso,wetouchbaseswithanumberofbasicdatabasetopics,and,thus,frequentlyrefertootherworkfordetails.PLCsaremostlyusedinmanufacturingenvironments-hence,thechoiceofourrunningexample.However,PLCsarealsousedinscientificapplicationsforsignaldatagatheringandpreliminarydataprocessing.Thus,wethinkthatforsomescientificapplications,aPLCdatabasemayalsoserveasalocal/transientpartofalargerscientificdatabase.Withtherapidadvancesincomputerhardwareandfallingmemoryprices,inrecentyears,thecapabilitiesofthenewPLCsinthemarketplacehavebeenincreasingdramatically.ThispaperisapositionpaperthatarguesthataPLCsoftwarecannowcontainadatabasesystemtogreatlyincreaseitsfunctionality.WeproposethearchitectureinFigure1asthearchitectureofanenvironmentwherereal-timedatagathering(frommultiplesensors)andreal-timedatamanipulationtakesplace.WenowlisttheadvantagesofhavingadatabasesystemdirectlyinsideaPLC.(1)DataModelingTechniques:Theinputandoutputbuffersrepresentaratherunorganizedtransientmodeloftherealworld,andhence,carlbemodeledbetterusingthetraditionaldatamodelingtechniquesofdatabases.(2)HistoricalDatabases:PLCsroutinelydealwithdifferentversionsofdataovertime.Therefore,historicaldatamodelingtechniquesaswellashistoricaldatamanipulationtechniquescanreplacetheadhocwaysofmanipulatinghistoricaldatainPLCs.(3)User-FriendlyInterfaces:Presentlyinthemarketplace,thePLCsoftwareandindustrialterminalsallowalimiteddisplayofmessagesandvariable-datainformationinmemory.Forexample,thecontacthistogramfunctiondisplaystheon/offhistoryofaspecificmainmemory.(4)HandlingLargeVolumesofData:Withtheaddedcapabilitiesofadatabaseandaquerylanguage,thePLCmayanalyzemuchlargervolumesofdata.(5)DataReductionandCompactionatthePLCLevel:Presently,forfurtheranalysisorsimplyduetovariousregulations,datacollectedbyPLCsgettransmittedandstoredintoahostcomputeranarchitectureshown..SincethepresentPLCscannotsatisfactorilyanalyzemostdata,theysimplytransmitdatatothehostcomputer.Insomescientificexperimentsandapplications,thedatagatheredissolargethatargumentshavebeenraisedfor"processingthedataon-the-fly"duringtheexecutionofanexperiment/transaction[SSDB86](a)Real-TimeDatabase:Thedataintheinputbuffermustbescannedwithinreasonablyshort"real-time"intervalsrangingfrommicrosecondstoseconds.Therefore,responsestoqueriesmustbeguaranteedtobelessthanacertain"realtime"timebound,almostalwayslessthan5to10seconds.(b)MainMemoryDatabase:Microseconds/secondsqueryresponserestrictionsnecessitatemain-memory-onlydatabases.(c)ScalableDatabase:OncetheenvironmentofaPLCandtherequirementsoftheassociatedapplicationprogramaredetermined,thepossiblequerytypestothedatabasestayfixedforareasonablylongperiodoftime.Sincetheresponsetimeofqueriesisofutmostimportance,theDBMSshouldbescaledsothatonlytheneededroutines/functions(e.g.,accessmethods,datastructures,etc.)areincorporated.Insection2,wediscussthegeneralcharacteristicsPLCs,andbrieflypresenttheladderlogiclanguage.Section3discussesthefeaturesoftheproposeddatabasesystemforPLCs.Ingeneral,thePLChardwareismostlycustom-builtwithoccasionaloff-the-shelfhardware,andconsistsofaCPU(ormultipleCPUs),mainmemory,an"industrialterminal",andhigh-andmedium-speeddatacommunicationshardware.Thesizeofthemainmemoryrangesfrom16Kbytes(of5to10yearsago)to8Mbytes(ofthepresenttime).AlthoughtheCPUhasaninstructionsetsimilartothosefoundinCPUsof16-bitand32-bitmachines,itisespeciallyequippedwithfastbitmanipulationinstructions.TheindustrialterminalcomeswithaspecialkeyboardtomaketheprogrammingofthePLCeasierand/ortointervenewiththeapplicationprogram.ThePLCsoftwareconsistsofanoperatingsystem(rangingfromaverysimplisticmonitor(oftenyearsago)toasophisticatedreal-timeoperatingsystem(oftherecenttime)),high-speedcommunicationssoftwareforcommunicatingwithI/Oprocessors,medium-speedcommunicationssoftwaretotheindustrialterminalandtoother"intelligent"devices.Bothgeneral-purposecomputersandPLCsareusedforindustrialcontrol[Star87].However,theydifferintheprogramminglanguagesthattheyuse,environmentalspecifications,andtheirusertypes.PLCsarerugged,andworkinhostileenvironmentswithnospecialclimatecontrols,toleratingextremesoftemperature(60°C),humidity(95%)andaircontamination.UsersofPLCsincludetheoriginalprogrammersoftheapplicationprograms,aswellasplantelectriciansandmaintenancepersonnel,whoareaccustomedtorelay-typecontrollingenvironments.ArungisanorderedsetofPLCinstructionsdrawnonasingleline.Instructionsonarungareclassifiedasinputinstructions(thosethatmonitortheinputbuffer)andoutputinstructions(thosethatsettheoutputbuffer),andareexecutedfromlefttoright,sequentially(Pleaseseefigure4).APLCapplicationprogramconsistsofamainprogramandasetofsubroutines,eachofwhichcontaininganorderedsetofrungs。Tosummarize,theapplicationprogrammerdealswithactual(realtime)clocktimes,andneedstohavepreciseestimatesforprogramscantimesandI/Oscantimes.Fortimeestimations,thePLCmanufacturerssupplyinformationsuchas4msecondsfor1000ladderlogicinstructions,and1msecondsforcopying256wordsintoaninputbufferduringtheI/Oscan.Inmostapplications,theprocessorscantimeiskeptbelow10seconds.Thus,databasemanipulationinstructionsalsoneedtohaveprecisetimelimitsavailableto(orsetby)users.2.ArchitectureWeproposeatwo-level,single-userdatabasesystemarchitectureasshowninfigure6.WehaveomittedfromourarchitecturetheexternalmodelofthetraditionaldatabasearchitecturenotbecausePLCsarenotpowerful,butbecauseconcurrentlyrunningapplicationprogramsusingdifferentviewscreateproblemsinaccuratelyestimatingtheapplicationprogramscantimes.Thatis,inamultitaskingenvironmentwheretaskscompetefortheresourcessuchasdatabaserelationsandcommunicationlines,decidingasingletop-tobottomexecutiontimeofataskinactualtimeisratherdifficult(ifatallpossible).Asfarasthehardwarecomputingpowerisconcerned,thepresentdayPLCsareaspowerfulaspersonalcomputers(and,indeed,insomerecentproducts,PLCsarepersonalcomputers),andcancertainlysupportmultipleindustrialterminalsanddatasharingamongtheapplicationprograms.3.DataModelingIssuesThetraditionaldatamodelingtechniquesdirectlyapplytoPLCdatabases.Thereisnoreasonwhy,say,theEntity-RelationshipModel[Chen76]ofthedatainthePLCdatabasecannotbedesigned.Allthewell-knownadvantagesofdatamodeling[ToeF82]directlycarryovertothePLCdatabaseenvironment,andwillnotbeelaboratedhere.Asfortheconceptualmodelofourprototypeeffort,wehavechosentherelationalmodel.ThePLCenvironmentnaturallydealswithhistoricaldata,e.g.,thelastreadingofagrindingmachinesensor,itsvalueyesterday,etc..Again,therearevarioushistoricaldatamodelingapproaches[SnoA85]intheliteratureand,theoretically,anyoneofthemisacceptable.Example.Considerasetoffurnacesthatproducehigh-precisionairplaneparts.Therearetwoentityrelations,FURNACEandPART,andonetime-varyingrelationshiprelationPRODUCESasdescribedinfigure7.PleasenotethatPRODUCESrelationhastupletimestampingwithBEGIN-TIMEandEND-TIMEattributes.4.DBMSIssuesThereareanumberofissuesthatneedtoberesolvedinatime-constrained,singleuserDBMSenvironment.Theseare(a)DataArchival.(b)DatabaseBackup.(c)DatabaseIntegrityenforcement.5.QueryProcessingIssuesTheconventionaldatabasemanagementsystems(DBMS)donothavethecapabilityofdynamicallymeetingthetimeconstraintswhentheamountofdataistoolargetoprocesswithinagiventimequota.Ourapproachis,foragiventimequota,totakeanappropriateamountofsampledatasuchthatthesetofsampleddataisguaranteedtobeprocessiblebytheDBMSwithinthegiven.SomebasicPLCinstructionshavealsobeenextendedtoincreasetheirfunctionality.Forexample,wehaveextendedthe"examinelogicswitch"instructions,theExamineInputClosed(XIC)andtheExamineInputOpen(XIO)[AB84,AB85],totestthelogicalvalueofapropositionalcalculusformula,ratherthantestingabitvaluecorrespondingtotheconditionofaphysicalI/O.An"examineF"instructioncausestheformulaFtobeevaluatedandtherolevalueisthenexaminedasinthebasicexamineinstructions.Ingeneral,theformulaFmaycontainaconstant,avariable,acomponentofatuplebeingscannedbythepointer,andfiE)wherefisanaggregatefunctionandEisarelationalalgebraexpression.ThefunctionalityofthePLCTimerandCounterinstructionshavealsobeenenhanced.Usuallyconditionedby"examine"instructions,timersandcounterskeeptrackoftimedintervalsorevents;thenumberoftimedintervalsoreventstobecountedissetinthepresetvaluevariables[AB85,AB84].Withtheintroductionofatimedimensionintothedatabase,eventsandintervalscanalsobe"counted"usingdatabasequeries.6.QueryProcessingIssuesTheconventionaldatabasemanagementsystems(DBMS)donothavethecapabilityofdynamicallymeetingthetimeconstraintswhentheamountofdataistoolargetoprocesswithinagiventimequota.Ourapproachis,foragiventimequota,totakeanappropriateamountofsampledatasuchthatthesetofsampleddataisguaranteedtobeprocessiblebytheDBMSwithinthegiventimequota,and,exacttime-costformulascanbederived.Clearly,themorestagesthequeryprocessorgoesthrough,themoreoverheadisinvolvedintherun-timeestimationapproach.Thisimpliesthat,ateachstage,aslargeanamountoftimeaspossibleshouldbeallocatedtoreducethenumberofstages.Ontheotherhand,allocatinglargeamountsoftimehasahigherriskofoverspendingthetimequotaandmayendupwastingalargeamountoftime,especiallyinahardtimeconstrainedenvironment[AbGM88,StZa88].Thehardtimeconstrainedenvironmentsdenotethoseenvironmentswhereoverspendingthetimequotaisstrictlynotallowed.Therefore,whenoverspendinghappens,thequeryhastobeabortedprematurelyandtheamountoftimeusedinthelaststageisconsideredwasted.7.OtherIssuesTheissueofincompleteinformationinthePLCdatabaseisalsobeinginvestigated.Quiteoften,thesensorsgiveincompleteinformation,usuallyarangeforareading.Onthoseoccasions,theincompletedataisonlyknowntobewithinsomerangeofvalues.Werepresentanincompletedataitemasanintervalwhichcontainstheunknownvalue.WehavefinishedtheimplementationofthefirstversionofaPLCdatabase[-Liu89]havingsomeofthefeaturessummarizedinsection3.ThesystemhasbeendevelopedonSUNworkstationsusingtheClanguage.WeareplanningtotransportitintoaPLC,andevaluateitsperformance.References[AB84]PLC-3ProgrammableControllerProgrammingManual,Allen-BradleyCo.,1984.[AB85]PLC-5/15ProgrammableControllerProcessorManual,Allen-BradleyCo.,1985.[Chri83]Christodoulakis,S.,"EstimatingRecordSelectivities",InformationSystems,Vol8,1983.[HP88]Hewlett-PackardAnnouncementoftheHPRTDBS,Sept.1988.[Klug81]A.Klug,"ABE-AQueryLanguageforConstructingAggregate-by-Example",1stLBLWorkshoponStatisticalDatabaseManagement,Dec.1981.[Ullm88]J.DUllman,"PrinciplesofDatabaseandKnowledge-BaseSystems",ComputerSciencePress,1988.[SSDB86]PanelonScientificDatabases,ThirdInt.WorkshoponStatisticalandSCientificDatabaseManagement,1986.[Star87]R.J.Staron,"AToolsettoDevelopProgrammableControllerSystems",unpublishedmanuscript,Allen-BradleyCo.,March1987.

可编程控制器的数据库系统文章摘要在这篇文章中，我们确定一种在科学和工业上有分外应用目的的计算机——可编程控制器(PLC)数据库系统的相关问题。例如：在制造环境的工厂里。我们建议作为数据库系统的PLC的那些单用户，实时可调整的主记忆体。只有关系数据库系统有一个双级结构，具有历史数据建模和操作能力，查询处理技术。区别于阶梯逻辑语言，最常用的PLC语言，融合数据操作语言指令。我们放入一个单独组成的PLC处理器的扫描时间，处理数据库更新，备份简单算法和数据档案等问题。1.简介可编程控制器(PLC)是一个专用计算机,用来实施科学计算系统和工业控制系统。在本文中举一个自动化控制工厂的机器运行的例子。本文是一份立场文章，提出了PLC的数据库系统,并讨论其特征。这样，我们接触基地的一批基础数据库,因为经常提及其他工作细节而使内容很丰富。可编轨范控制器大多用于制造的环境，所以我们选择了运行的例子。不过，也有的可编轨范控制器用在科学应用的信号数据采集和初步数据处理。因此，我们认为对一些科学应用，可编程数据库,也可以作为本地较大的科学数据库的瞬态部分。现在我们列出有一个直接内置数据库系统的PLC的优点。(1)数据建模技术：输入和输出缓冲器是一个对真实世界的活动比力散乱的暂态模型，因此，卡尔建模法是传统数据库建模技术中比力好的方式。(2)历史数据库：可编轨范控制器一段时间内例行处理分歧版本的数据。因此，历史数据建模技术以及历史数据把持技术可以取代专案方式把持的历史数据的可编轨范控制器。(3)用户界面友爱：目前在市场上买到的该PLC的软件。能有限的展示信息和可变数据信息存储。例如，接触直方图显示功能作为开机/关机史上的一个特定的主记忆体。(4)处理大量数据：由于一个数据库和查询语言的增加，PLC能够分析大容量数据。(5)在PLC水平上的数据压缩和压实：目前，由简单系统或是简单处理的的信息，收集的数据由PLC的传送并储存到主机的内存中。由于目前的可编轨范控制器不能分析出令人满意的大部分的数据，而只是将数据传输到主机。在一些科学实验和应用中，需要收集到的数据是如此之大。现在还没有PLC的数据库和实时数据库的书面文献报道。然而，就在比来，惠普颁布颁布了1989年获得惠普实时数据库[惠普88]所用的内部结构。显然，结构图3的主机通信线路胜于结构。然而，个人电脑可能超载太大都据。现在我们开始讨论可编程数据库的性能。(1)实时数据库：相当短的“实时”一般从微秒到秒，数据的输入缓冲器必需经过扫描。因此,对查询必需包管其将低于必然的“实时”。有时限的，几乎总是低于5至10秒。(2)内存数据库:查询响应限制必需的主内存只有数据库。(3)分级数据库：一旦PLC的环境和要求的相关应用轨范被决意，相当长的时期就留在一个固定的可能的查询类型的数据库中。由于响应时间的问题是至关重要的，该数据库应该加码,所需要常规本能机能(例如，检索方式，数据结构等)都可以纳入傍边。接下来，我们讨论可编轨范控制器的特点，并简要地介绍了梯形图语言。就第三节讨论的特点,提出了可编程控制器的数据库系统。2PLC和梯形图语言的一般特征在一般情况下，PLC的硬件大多是量身订做的。偶尔现成的硬件，则由一个CPU(或多个中央处理器)，内存，“产业终端”而组成高、中速数据通信硬件组成复杂的内存容量，16k字节(5至10年前)8兆字节(当前时间)。虽然CPU有一个指令集类似CPU的16位和32位机。它分外配备了快速位操作指令。工业终端是一个特殊键盘使编程的PLC容易融合应用程式。该PLC的软件包孕操作系统(从一个非常简单的监控器(