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StructuralanalysisincontrolsystemsdesignofhydraulicdrivesBennoStein,ElmarVierAbstractThedesignofhydrauliccontrolsystemsisacomplexandtimeconsumingtaskthat,atthemoment,cannotbeautomatedcompletely.Nevertheless,importantdesignsubtaskslikesimulationorcontrolconceptselectioncanbeefficientlysupportedbyacomputer.Prerequisiteforasuccessfulsupportisawellfoundedanalysisofahydraulicsystemsstructure.Thispaperprovidesasystematicsforanalyzingahydraulicsystematdifferentstructurallevelsandillustrateshowstructuralinformationcanbeusedwithinthedesignprocess.Anotherimportantpointofthispaperistheautomaticextractionofstructuralinformationfromacircuitdiagrambymeansofgraphtheoreticalinvestigations.KeywordsAlgorithmsandknowledgebasedmethodsforCACSDStructuralanalysisofhydraulicsystemsGraphtheory1.IntroductionHydrostaticdrivesprovideadvantageousdynamicpropertiesandthereforerepresentamajordrivingconceptforindustrialapplications.Largescalehydraulicsystemssuchasplantsinmarinetechnologyaswellasdrivesformachinetoolspossessalargenumberofactuators.Consequently,sophisticatedinterdependencesbetweensinglecomponentsorentiresubsystemsmayoccur,whichleadstoavarietyofchallenginganddemandingdesignandcontroltasks.Asarepresentativeexamplewithrespecttocomplexityanddimension,Fig.1showsthecircuitdiagramofacoldrollingplantWessling,1995EbertshaÈuser,1994.Here,morethan20actuatorsworkonthecoiledsteelstrips.Designingsuchlargehydrauliccontrolsystemsimpliesasystematicprocedure.Inpractice,thisisdoneratherimplicitly}basedontheintuitionandtheexperienceofthehumandesigner.Thispaperintroducesasystematicsofhydrostaticdriveswhichrevealtheirunderlyingstructures,aswellasrelationsanddependenciesamongsubstructures.Thisapproachallowsathoroughstructuralanalysisfromwhichfundamentaconclusionsfortheautomationofthedesignprocesscanbedrawn.Theconceptsofthispaperhavebeenrealizedandintegratedwithindeco,aknowledgebasedsystemforhydraulicdesignsupportStein,1995.Currently,decocombinesbasicCADfacilitiestailoredtouidics,checkingandstructureanalysisalgorithms,simulationmethods,andbasicdesignruleprocessing.Theoperationalizationofhydraulicdesignknowledgerequiresaformaldefinitionandautomaticextractionofstructuralinformationfromacircuitdiagram.Thepapercontributeswithintheserespectsitisorganizedasfollows.Section2describesbothconceptuallyandexemplarilythestructurallevelsatwhichahydraulicsystemcanbeinvestigated.Section3brieflydiscussesthebenefitsthatgoalongwithastructuralanalysis.Section4preciselydefinesdifferenttypesofcouplingsbetweenthefunctionalunitsofahydraulicsystem,henceestablishingabasisforacomputerbasedanalysis.Moreover,itisoutlinedhowastructuralanalysisisautomated.Section5outlinestheexploitationofstructuralinformationwithindeco.2.StructuralanalysisofhydraulicsystemsThemajorityofhydraulicsystemsisdesignedbyexploitingtheexperienceandintuitionofasingleengineer.Duetothelackofastructuralmethodology,athoroughanalysisofthesystemstructureisnotcarriedout.Instead,alimitedrepertoryofpossiblesolutionsisused,makingtheresulthighlydependentonthecapabilitiesoftheindividual.Suchanapproachissuitableonlyforrecurringdesigntaskswithlittlevariation.Inthefollowing,asystematicsofthestructuralsetupofhydraulicplantsisintroducedwhichleadstoaproblemorientedsystemanalysis.Itsapplicationtoahydrostaticdrivegivenasapreliminarydesignfacilitatesaconsequentandpurposivederivationofstructuralinformation,whichisnecessarytomakethesystemsbehaviormeetthecustomersdemands.2.1.StructurallevelsofhydraulicsystemsThesystematicsdevelopedhereisbasedonthreelevelsofabstraction.Thedifferentiationbetweenfunctionalstructure,componentstructure,andsystemtheoreticalstructurecorrespondstosystemdescriptionsofdifferentcharacteristicsFig.2.Fromthisdistinctionresultsanoverallviewofhowtoinfluencethesystemsbehavior.Toillustratetheconceptofstructurallevels,wewillconcentrateonasamplesubsystemofthecoldrollingplant,thefourrollstandissketchedinFig.3.Thefunctionalstructureshowsthefundamentalmodesofactionofahydrauliccircuitbyanalyzingthedifferenttasksfunctionstheplanthastofulfill.Itrepresentssomekindofqualitativesystemdescription.Akeyelementwithinthefunctionalstructureisthesocalledhydraulicaxis,whichisdefinedasfollows.AhydraulicaxisArepresentsandfulfillsasubfunctionfofanentirehydraulicplant.Adefinestheconnectionsandtheinterplayamongthoseworking,control,andsupplyelementsthatrealizef.Thehydraulicactuatorsofthefourrollstandperformtwotaskseachofwhichdefinedbyadirectionalloadandmotionalquantities.ArepresentationoftherollstandatthefunctionallevelisgiveninFig.4.Thedetectionofhydraulicaxesandtheirinterdependencesadmitsfarreachingconclusions,whicharestatedinSection3.Onthelevelofthecomponentstructurethechosenrealizationofafunctionisinvestigated.Thearrangementstructurecomprisesinformationonthehydraulicelementspumps,valves,cylinders,etc.aswellastheirgeometricandphysicalarrangement.BytheswitchingstatestructuretheentiretyofthepossiblecombinationsofswitchingpositionsischaracterizedAvalve,forinstance,canbeopenorclosed.Fig.6depictstherepresentationoftherollstandatthecomponentlevel.Thesystemtheoreticalstructurecontainsinformationonthedynamicbehaviorofboththehydraulicdriveasawholeanditssinglecomponents.CommonwaysofdescribingdynamicsaredifferentialanddifferenceequationsorthestatespaceformSchwarz,1991.Thesystemtheoreticalviewcomprisesinformationonthecontrolledquantities,aswellasthedynamicbehaviorofthecontrolledsystem.TheblockdiagraminFig.7revealsthesystemtheoreticalstructureoftherollstand.Bycomparinganalysisandsimulationresultswiththeperformancedemandsatthedrive,adecisioncanbemadeforeachhydraulicaxiswhetheropenorclosedloopcontrolconceptsareadequate.Inafurtherstep,anappropriatecontrolstrategylinear,nonlinear,etc.canbeassigned.Remarks.Whilethefunctionalstructureyieldsaqualitativerepresentation,thesystemdescriptionbecomesmorequantitativeatthecomponentandsystemtheoreticallevel,respectively.Moreover,theanalysisofthestructuralsetupshowsinwhichwaythebehaviorofahydraulicplantcanbeinfluencedcf.Fig.21atfirst,thefunctionalstructuremustbeconsideredasinvariant,becauseitresultsfromthecustomersdemands.Onlyifthegivenstructureprovestobeunsatisfactory,amodification}resultingfromaheuristicanalysisapproach}isadvisable2notethatatthecomponentlevel,acombinationofheuristicandanalyticmethodsisrequiredforthevariationorexchangeofhydraulicelements,whichformthecontrolledsystem3thesystemtheoreticallevelfacilitatestheinvestigationofthedynamicbehaviorcontroltheoryprovidesananalyticapproachfortheselectionofasuitablecontrolstrategy,parameterization,etc.2.2.HydraulicaxesandtheircouplingsFocusingontheinvestigationofthefunctionalstructureofhydraulicsystems,thedetectionandevaluationofhydraulicaxesisofcentralinterest.Theiranalysiscontributestoadeeperunderstandingoftheinnercorrelationsoftheplantandprovidesanoverviewoftheenergyflowswithrespecttothefunctionstobefulfilled.ThedefinitionofthehydraulicaxisgiveninSection2.1isbasedonthecriterionofelementsworkingtogetherinordertofulfillasinglefunction.Notethatseveralactuatorshydraulicmotors/cylindersmaycontributetothesamefunction,thusformingasinglehydraulicaxisFig.8.Thissituationisgivenforaidenticalsubcircuitsthatarecontrolledbyonesinglecontrolelement,bsynchronizedmovementsthatarecarriedoutbyopenorclosedloopcontrol,orc,dmechanicalcouplingssuchasguidesandgearunitsthatenforceauniquebehavior.Beyondtheconsiderationofisolatedhydraulicaxes,itisnecessarytoinvestigatetheirinterdependences.ThefollowingcouplingtypeshavebeenworkedoutLevel0Nocoupling.Hydraulicaxespossessnocoupling,ifthereisneitherapowernoraninformationalconnectionbetweenthem.Level1Informationalcoupling.Hydraulicaxeswhichareconnectedonlybycontrolconnectionsarecalledinformationallycoupled.Level2Parallelcoupling.Hydraulicaxeswhichpossesstheirownaccesstoacommonpowersupplyarecoupledinparallel.Level3Seriescoupling.Aseriescouplingconnectsthehydraulicaxeswhosepowersupplyordisposalisrealizedviatheprecedingorthefollowingaxis.Level4Sequentialcoupling.Asequentialcouplingisgiven,iftheperformanceofafollowingaxisdependsonthestatevariables,e.g.thepressureorthepositionoftheprecedingoneinordertoworkinasequence.ApplyingtheconceptoffunctionalstructuretothecoldrollingplantofFig.1,15hydraulicaxesalongwiththeircouplingscanbefound.ThelefthandsideofFig.9envisionsthemembershipofthecomponentsinthediagramtotheaxes,therighthandsideshowstheentirecouplingschemeintheformofatree.3.BenefitsofastructuralanalysisAstructuralanalysisofhydraulicsystemsrevealsbasicdesigndecisions.Especiallythefunctionalanalysis,whichisbasedonthedetectionofasystemshydraulicaxes,willsimplifythemodification,theextension,andtheadaptationofthesystemStein,1996.TheseparatetreatmentofhydraulicaxesremarkablyreducesthedesigneffortwithinthefollowingrespectsSmartsimulation.Smartsimulationisahumanstrategywhenanalyzingacomplexsystemsubsystemsareidentified,cutfree,andsimulatedontheirown.Thisstrategyreducesthesimulationcomplexityandsimplifiestheinterpretationofitsresults.Hydraulicaxesestablishsuitedsubsystemstobecutfree,sincetheyperformanindivisiblebutcompletesubtask.Staticdesign.Informationonthehydraulicaxesdrivingconceptopen/closedcenter,loadsensing,regenerativecircuit,etc.allowstheselectionofcomputationproceduresrelatingthestaticdesign.Moreover,theapplicationofmodificationknowledgehastoconsidertheaxescouplinglevels.Controlconceptselection.Theconsiderationofcouplingsbetweeninputandoutputvariablessuppliesanecessarydecisionbasisfortheselectionofcontrolconcepts.AnalyzingthedecouplabilitymatrixDSchwarz,1991yieldsacommonapproachhere.Notethatthesystemorderthatcanbetackledislimited.Thefunctionalstructureanalysisprovidesaseparationinto1SISOsystems,towhichstandardmethodsofcontrollerdesigncanbeapplied,and2coupledsubsystemsofareducedorder,forwhichdecouplabilitycanbeinvestigatedmoreefficientlyorevenbecomespossibleatall.Diagnosis.Havingahydrauliccircuitdecomposedintoitshydraulicaxes,thediagnosisprocesscanfocusontoasingleaxisaccordingtothefollowingworkinghypothesisifsymptomsareobservedmerelyatasinglehydraulicaxis,thenthedefectcomponentsmustbeamongstthecomponentsofthisaxis.Ifsymptomsareobservedatseveralaxes,theaxescouplingtypewillgivefurtheranswerswithrespecttodefectcomponents.HesseandStein1998describeasystemwherethisideahasbeensetintooperation.Notethatasmartclassificationofthecouplingsbetweenhydraulicaxesformstherationaleofwhetheradecompositionofahydraulicdesignproblemispermissible.Whilesubsystemswithlevel0orlevel1couplingscanalwaysbecutfree,additionalinformationisrequiredforparallel,series,andsequentialcouplings.
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