可编程控制器的数据库系统中英文翻译资料 大学论文.docx

可编程控制器的数据库系统中英文翻译资料 大学论文.docx

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可编程控制器的数据库系统中英文翻译资料大学论文

DatabaseSystemsforProgrammableLogicControllers

Inthispaper,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.Introduction

Aprogrammablelogiccontroller（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,datacollectedbyPLCsgettransmittedandstoredintoahostcomputerfileusinganarchitectureshown..

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.Architecture

Weproposeatwo-level,single-userdatabasesystemarchitectureasshowninfigure6.WehaveomittedfromourarchitecturetheexternalmodelofthetraditionaldatabasearchitecturenotbecausePLCsarenotpowerful,butbecauseconcurrentlyrunningapplicationprogramsusingdifferentviewscreateproblemsinaccuratelyestimatingtheapplicationprogramscantimes.Thatis,inamultitaskingenvironmentwheretaskscompetefortheresourcessuchasdatabaserelationsandcommunicationlines,decidingasingletop-tobottomexecutiontimeofataskinactualtimeisratherdifficult（ifatallpossible）.Asfarasthehardwarecomputingpowerisconcerned,thepresentdayPLCsareaspowerfulaspersonalcomputers（and,indeed,insomerecentproducts,PLCsarepersonalcomputers）,andcancertainlysupportmultipleindustrialterminalsanddatasharingamongtheapplicationprograms.

3.DataModelingIssues

ThetraditionaldatamodelingtechniquesdirectlyapplytoPLCdatabases.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.DBMSIssues

Thereareanumberofissuesthatneedtoberesolvedinatime-constrained,singleuserDBMSenvironment.Theseare

（a）DataArchival.

（b）DatabaseBackup.

（c）DatabaseIntegrityenforcement.

5.QueryProcessingIssues

Theconventionaldatabasemanagementsystems（DBMS）donothavethecapabilityofdynamicallymeetingthetimeconstraintswhentheamountofdataistoolargetoprocesswithinagiventimequota.Ourapproachis,foragiventimequota,totakeanappropriateamountofsampledatasuchthatthesetofsampleddataisguaranteedtobeprocessiblebytheDBMSwithinthegiven.

SomebasicPLCinstructionshavealsobeenextendedtoincreasetheirfunctionality.Forexample,wehaveextendedthe"examinelogicswi