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