StructuralanalysisincontrolsystemsdesignofhydraulicdrivesBennoStein,ElmarVierAbstract：Thedesignofhydrauliccontrolsystemsisacomplexandtime-consumingtaskthat,atthemoment,cannotbeautomatedcompletely.Nevertheless,importantdesignsubtaskslikesimulationorcontrolconceptselectioncanbeefficientlysupportedbyacomputer.Prerequisiteforasuccessfulsupportisawell-foundedanalysisofahydraulicsystemsstructure.Thispaperprovidesasystematicsforanalyzingahydraulicsystematdifferentstructurallevelsandillustrateshowstructuralinformationcanbeusedwithinthedesignprocess.Anotherimportantpointofthispaperistheautomaticextractionofstructuralinformationfromacircuitdiagrambymeansofgraph-theoreticalinvestigations.Keywords:Algorithmsandknowledge-basedmethodsforCACSD；Structuralanalysisofhydraulicsystems；Graphtheory1.IntroductionHydrostaticdrivesprovideadvantageousdynamicpropertiesandthereforerepresentamajordrivingconceptforindustrialapplications.Large-scalehydraulicsystemssuchasplantsinmarinetechnologyaswellasdrivesformachinetoolspossessalargenumberofactuators.Consequently,sophisticatedinter-dependencesbetweensinglecomponentsorentiresubsystemsmayoccur,whichleadstoavarietyofchallenginganddemandingdesignandcontroltasks.Asarepresentativeexamplewithrespecttocomplexityanddimension,Fig.1showsthecircuitdiagramofacold-rollingplant(Wessling,1995；Ebertshauser,1994).Here,morethan20actuatorsworkonthecoiledsteelstrips.Designingsuchlargehydrauliccontrolsystemsimpliesasystematicprocedure.Inpractice,thisisdoneratherimplicitlybasedontheintuitionandtheexperienceofthehumandesigner.Thispaperintroducesasystematicsofhydrostaticdriveswhichrevealtheirunderlyingstructures,aswellasrelationsanddepen-denciesamongsubstructures.Thisapproachallowsathoroughstructuralanalysisfromwhichfundamentaconclusionsfortheautomationofthedesignprocesscanbedrawn.Theconceptsofthispaperhavebeenrealizedandintegratedwithindeco,aknowledge-basedsystemforhydraulicdesignsupport(Stein,1995).Currently,decocombinesbasicCADfacilitiestailoredtouidics,checkingandstructureanalysisalgorithms,simulationmethods,andbasicdesignruleprocessing.Theoperationalizationofhydraulicdesignknowledgerequiresaformaldefinitionandautomaticextractionofstructuralinformationfromacircuitdiagram.Thepapercontributeswithintheserespects；itisorganizedasfollows.Section2describesbothconceptuallyandexemplarilythestructurallevelsatwhichahydraulicsystemcanbeinvestigated.Section3brieflydiscussesthebenefitsthatgoalongwithastructuralanalysis.Section4preciselydefinesdifferenttypesofcouplingsbetweenthefunctionalunitsofahydraulicsystem,henceestablishingabasisforacomputer-basedanalysis.Moreover,itisoutlinedhowastructuralanalysisisautomated.Section5outlinestheexploitationofstructuralinformationwithindeco.2.StructuralanalysisofhydraulicsystemsThemajorityofhydraulicsystemsisdesignedbyexploitingtheexperienceandintuitionofasingleengineer.Duetothelackofastructuralmethodology,athoroughanalysisofthesystemstructureisnotcarriedout.Instead,alimitedrepertoryofpossiblesolutionsisused,makingtheresulthighlydependentonthecapabilitiesoftheindividual.Suchanapproachissuitableonlyforrecurringdesigntaskswithlittlevariation.Inthefollowing,asystematicsofthestructuralset-upofhydraulicplantsisintroducedwhichleadstoaproblem-orientedsystemanalysis.Itsapplicationtoahydrostaticdrivegivenasapreliminarydesignfacilitatesaconsequentandpurposivederivationofstructuralinformation,whichisnecessarytomakethesystemsbehaviormeetthecustomersdemands.2.1.StructurallevelsofhydraulicsystemsThesystematicsdevelopedhereisbasedonthreelevelsofabstraction.Thedifferentiationbetweenfunctionalstructure,componentstructure,andsystem-theoreticalstructurecorrespondstosystemdescriptionsofdifferentcharacteristics(Fig.2).Fromthisdistinctionresultsanoverallviewofhowtoinfluencethesystemsbehavior.Toillustratetheconceptofstructurallevels,wewillconcentrateonasamplesubsystemofthecold-rollingplant,thefour-rollstandissketchedinFig.3.Thefunctionalstructureshowsthefundamentalmodesofactionofahydrauliccircuitbyanalyzingthedifferenttasks(functions)theplanthastofulfill.Itrepresentssomekindofqualitativesystemdescription.Akeyelementwithinthefunctionalstructureistheso-calledhydraulicaxis,whichisdefinedasfollows.AhydraulicaxisArepresentsandfulfillsasubfunctionfofanentirehydraulicplant.Adefinestheconnectionsandtheinterplayamongthoseworking,control,andsupplyelementsthatrealizef.Thehydraulicactuatorsofthefour-rollstandperformtwotaskseachofwhichdefinedbyadirectionalloadandmotionalquantities.ArepresentationoftherollstandatthefunctionallevelisgiveninFig.4.Thedetectionofhydraulicaxesandtheirinterdependencesadmitsfar-reachingconclusions,whicharestatedinSection3.Onthelevelofthecomponentstructurethechosenrealizationofafunctionisinvestigated.Thearrangementstructurecomprisesinformationonthehydraulicelements(pumps,valves,cylinders,etc.)aswellastheirgeometricandphysicalarrangement.Bytheswitching-statestructuretheentiretyofthepossiblecombinationsofswitchingpositionsischaracterized:Avalve,forinstance,canbeopenorclosed.Fig.6depictstherepresentationoftherollstandatthecomponentlevel.Thesystem-theoreticalstructurecontainsinformationonthedynamicbehaviorofboththehydraulicdriveasawholeanditssinglecomponents.Commonwaysofdescribingdynamicsaredifferentialanddifferenceequationsorthestate-spaceform(Schwarz,1991).Thesystem-theoreticalviewcomprisesinformationonthecontrolledquantities,aswellasthedynamicbehaviorofthecontrolledsystem.TheblockdiagraminFig.7revealsthesystem-theoreticalstructureoftherollstand.Bycomparinganalysisandsimulationresultswiththeperformancedemandsatthedrive,adecisioncanbemadeforeachhydraulicaxiswhetheropen-orclosed-loopcontrolconceptsareadequate.Inafurtherstep,anappropriatecontrolstrategy(linear,nonlinear,etc.)canbeassigned.Remarks.Whilethefunctionalstructureyieldsaqualitativerepresentation,thesystemdescriptionbecomesmorequantitativeatthecomponentandsystem-theoreticallevel,respectively.Moreover,theanalysisofthestructuralset-upshowsinwhichwaythebehaviorofahydraulicplantcanbeinfluenced(cf.Fig.2):(1)atfirst,thefunctionalstructuremustbeconsideredasinvariant,becauseitresultsfromthecustomersdemands.Onlyifthegivenstructureprovestobeunsatisfactory,amodificationresultingfromaheuristicanalysisapproachisadvisable；(2)notethatatthecomponentlevel,acombinationofheuristicandanalyticmethodsisrequiredforthevariationorexchangeofhydraulicelements,whichformthecontrolledsystem；(3)thesystem-theoreticallevelfacilitatestheinvestigationofthedynamicbehavior:controltheoryprovidesananalyticapproachfortheselectionofasuitablecontrolstrategy,parameterization,etc.2.2.HydraulicaxesandtheircouplingsFocusingontheinvestigationofthefunctionalstructureofhydraulicsystems,thedetectionandevaluationofhydraulicaxesisofcentralinterest.Theiranalysiscontributestoadeeperunderstandingoftheinnercorrelationsoftheplantandprovidesanoverviewoftheenergyflowswithrespecttothefunctionstobefulfilled.ThedefinitionofthehydraulicaxisgiveninSection2.1isbasedonthecriterionofelementsworkingtogetherinordertofulfillasinglefunction.Notethatseveralactuators(hydraulicmotors/cylinders)maycontributetothesamefunction,thusformingasinglehydraulicaxis(Fig.8).Thissituationisgivenfor(a)identicalsub-circuitsthatarecontrolledbyonesinglecontrolelement,(b)synchronizedmovementsthatarecarriedoutbyopenorclosedloopcontrol,or(c,d)mechanicalcouplingssuchasguidesandgearunitsthatenforceauniquebehavior.Beyondtheconsiderationofisolatedhydraulicaxes,itisnecessarytoinvestigatetheirinterdependences.ThefollowingcouplingtypeshavebeenworkedoutLevel0(Nocoupling.)Hydraulicaxespossessnocoupling,ifthereisneitherapowernoraninformationalconnectionbetweenthem.Level1(Informationalcoupling.)Hydraulicaxeswhichareconnectedonlybycontrolconnectionsarecalledinformationallycoupled.Level2(Parallelcoupling.)Hydraulicaxeswhichpossesstheirownaccesstoacommonpowersupplyarecoupledinparallel.Level3(Seriescoupling.)Aseriescouplingconnectsthehydraulicaxeswhosepowersupply(ordisposal)isrealizedviatheprecedingorthefollowingaxis.Level4(Sequentialcoupling.)Asequentialcouplingisgiven,iftheperformanceofafollowingaxisdependsonthestatevariables,e.g.thepressureorthepositionoftheprecedingoneinordertoworkinasequence.Applyingtheconceptoffunctionalstructuretothecold-rollingplantofFig.1,15hydraulicaxesalongwiththeircouplingscanbefound.Theleft-handsideofFig.9envisionsthemembershipofthecomponentsinthediagramtotheaxes,theright-handsideshowstheentirecouplingschemeintheformofatree.3.BenefitsofastructuralanalysisAstructuralanalysisofhydraulicsystemsrevealsbasicdesigndecisions.Especiallythefunctionalanalysis,whichisbasedonthedetectionofasystemshydraulicaxes,willsimplifythemodification,theextension,andtheadaptationofthesystem(Stein,1996).Theseparatetreatmentofhydraulicaxesremarkablyreducesthedesigneffortwithinthefollowingrespects:Smartsimulation.Smartsimulationisahumanstrategywhenanalyzingacomplexsystem:subsystemsareidentified,cutfree,andsimulatedontheirown.Thisstrategyreducesthesimulationcomplexityandsimplifiestheinterpretationofitsresults.Hydraulicaxesestablishsuitedsubsystemstobecutfree,sincetheyperformanindivisiblebutcompletesubtask.Staticdesign.Informationonthehydraulicaxesdrivingconcept(open/closedcenter,loadsensing,regenerativecircuit,etc.)allowstheselectionofcomputationproceduresrelatingthestaticdesign.Moreover,theapplicationofmodificationknowledgehastoconsidertheaxescouplinglevels.Controlconceptselection.Theconsiderationofcouplingsbetweeninputandoutputvariablessuppliesanecessarydecisionbasisfortheselectionofcontrolconcepts.Analyzingthedecouplabi