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Load-independentcontrolofahydraulicexcavatorEugeniuszBudny*,MiroslawChlosta,WitoldGutkowskiInstituteofMechanizedConstructionandRockMining,ul.Racjonalizacji6/8,02-673Warsaw,PolandAccepted23August2002AbstractTheprimaryfocusofthisstudyistoinvestigatethecontrolofexcavationprocessesbyapplyingload-independenthydraulicvalves.Thisapproachallowsavoidingclosedloopcontrolsystemwithsensorsandtransducersmountedontheexcavatorattachment.Thereare,then,nosensorcellsmountedonthemachineattachment.Theconsideredsystemiscomposedoftwosubsystems:amicrocomputerandahydraulicunit(apumpandload-independentvalves).Inthemicrocomputerunit,thebucketvelocityvectorisrelatedtotheoilflowintothreecylindersthroughtheapplicationofinversekinematics.Then,flowsaretransferredintotheelectricsignalsactuatingtheload-independentvalves.Theirmotionispresentedbyapplyingtransferfunction.Theperformanceofthesystemisverifiedbyassuminganabruptchangeoftheoilflowintocylinders.Thelastpartofthepaperisdevotedtotheobtainedexperimentalresults.Thefirstresultdealswithverticaldrilling.Thesecondresultdealswithanexcavationalongahorizontaltrajectory.D2002ElsevierScienceB.V.Allrightsreserved.Keywords:Excavator；Hydraulicsystems；Control；Trajectoryexecution1.IntroductionDuetoencouragingresultsofrecentresearch,thereareincreasingpossibilitiesforenhancementofalargespectrumhumaneffortsinexcavationpro-cesses.Thismayoccurmainlythroughcontrolofrepetitiveworktasks,suchastrenchinganddrilling,requiringconstantattentionofmachineoperatorsduringtheperformanceofeachtask.Particularattention,inresearch,ispaidtoexcavationalongprescribedtrajectoriessubjectedtovaryingsoilenvi-ronment.FundamentalsdealingwithcontrolledexcavationprocessesarediscussedbyVahaandSkibniewski1,Hemami2,andHillerandSchnider3.Aninter-estingapproachtopilingprocessesbyadirectangularsensingmethodisproposedbyKeskinenetal.8.BudnyandGutkowski4,6proposedasystem,applyingkinematicallyinducedmotionofanexcavatorbucket.Inthisapproach,influenceofasmallvariationofhydraulicoilflowintocylinders,applyingsensitivityanalysis,isdiscussedbyGut-kowskiandChlgosta5.Huangetal.7presentedanimpedancecontrolstudyforaroboticexcavator.Theyappliedtwoneuralnetworks:first,asafeed-forwardcontrollerandthesecondasafeedbacktargetimpedance.Anotherimpedancesystem,apply-ingahybridposition/forcecontrol,isproposedbyHaetal.9.Thefirstgenerationofrobotswasconceivedasopenlooppositioningdevices.Thisimpliedthatallpartshadtobemanufacturedwithaveryhigh0926-5805/02/$-seefrontmatterD2002ElsevierScienceB.V.Allrightsreserved.PII:S0926-5805(02)00088-2*Correspondingauthor.E-mailaddress:.pl(E.Budny).URL:/locate/autconAutomationinConstruction12(2003)245254andcostlyaccuracy.Next,positioningrobots,withsensors,reducedthisaccuracyrequirementconsider-ably.Herewereseveralapproaches,mentionedinabovereferences,toextendtheindustrialrobotscapabilitiestoroboticexcavator.Systemsofforcecells,longitudinalandangularsensorshavebeenapplied.However,twomaindifferencesbetweenrequirementsformanufacturingrobotsandroboticexcavatorsshouldbenoted.Thefirstdifferenceisthatmanufacturingrobotsareworkinginalmostperfectconditions,freeofvibrations,protectedagainstshocks,humidity,andotherpossibledamag-ingconditions.Theseconddifferenceistherequire-mentsforhighaccuracyofmanufacturingrobots,oftenwithinmicrons.Onthecontrary,roboticexca-vatorsareworkinginverydifficultconstructionsiteconditions,andrequiredaccuracyoftheexecutedtrajectories,comparingwithindustrialrobots,islimited,saywithincentimetres.Withdifficultcon-ditionsofexcavationsworks,allsensorsattachedtotheboom,arm,andbuckethavetobeverywellprotected.Bearinginmindtheabovedifferences,itwouldbeofinteresttoinvestigatethepossibilitiesofcontrollingexcavationtrajectorybyahydraulicmodulecom-posedofapumpandload-independentvalves.Inotherwords,toinvestigateasystemfreeofsensorcellsmountedattheexcavatorattachment,combinedwithafeedbackcontroller,includedinthehydraulicunitofthemachine.Themainobjectiveofthepresentpaperistoextendthediscussion,initiatedbytheauthors10,onthepossibilitiesofapplyingload-independentvalvesinstalledinsideofoperatorcabinonly.Underthisassumption,thesystemisfreeofsensorslocatedontheexcavatorattachment.Afterdiscussingmathematicalmodelofthesystem,pre-liminaryexperimentalresultsarepresentedattheendofthepaper.2.StatementoftheproblemThepaperdealswithacontrolled,stablemotionofanexcavatorbucketalongaprescribedpath.Theproblemisbasedonpreviousauthorstheoreticalinvestigations4ofquasi-static,kinematicallyin-ducedexcavationprocessesforassumedtrajectories.Inthisstudy,thefollowingassumptionsaremade.Theexcavatorattachmentisaplanarmechanism,composedofaboom,anarm,andabucket.Three,independentlydriven,hydrauliccylindersoperatethesystem.Theyareassuringauniquerepresentationofthethreedegreesoftheplanarbucketmotion,twodisplacementsandarotation.Theexcavationprocess,intheexperimentsper-formed,isassumedtobeslowenoughtoconsideritasaquasi-staticone.Inertiatermsinmotionequationsofattachmentcanbethenneglected.Onlyspooloftheservomechanismisassumedtomovewithaccelera-tions,whichcannotbeneglected.Theforce(pressure)disturbancesareassumedtohavesinusoidalform.Theacceptableparametersofthesinusoidaredefinedfromstabilityconditionsofthesystem.Thesoilisassumedhomogeneous.Somesmallinclusionsintheformofstonesareacceptable.Theproposedcontrolsystemofexcavationisoperator-assisted.Itmeansthatinacaseofalargerobstacle,theoperatorhastointervene.Ifsuccessful,theproposedcontrolsetupcouldapplytostandardexcavatorswiththeaimofenhance-mentofalargespectrumofhumaneffortsinrepetitiveprocessessuchastrenchinganddrilling.Theexperimentisconsideredasasystemcom-posedofthreesubsystems,namely:microcomputerwithPLC；hydraulicarrangement(apump,valves,cylinders)；andthemechanismwiththreedegreesoffreedomofthebucket.Next,thesubsystemsareconsideredassetsofcomponents.Inthefirstsub-system,thefollowingcomponentsarerecognised:personalcomputerwithappropriatesoftware,trans-formingintroducedequationsandinequalitiesofmotionandtrajectoryplanersintoelectricsignal.ThelatterissendtoaPLCunit,whichinturncausesanelectricalactuationofsolenoidvalves.Pressuresfromthesolenoidvalvesarecausingchangesinspoolpositions,assuringassumedflowofthehydraulicoilintocylinders.Thespoolposition,inturn,iscon-vertedthroughatransducertoanelectricfeedbacksignalsenttothesolenoidvalves.Openedspoolsarelettingthehydraulicoiltoflowintothethirdsub-system,namelycylindersoftheexcavatormechanism.Finally,thelastsubsystemiscomposedofthreecomponents:thehydrauliccylinders,theboom,thearm,andthebucket.Withthemotionoftheexcavator,armsandthebucketitself,thepressuresincylindersE.Budnyetal./AutomationinConstruction12(2003)245254246arechanging.Informationaboutthesechangesissenttothesecond,hydraulicsubsystem,wherethefeed-backsignalcorrectspositionofspoolsassuringtheoilflowaccordingtothedesignedtrajectory.Inthepaper,transferfunctionsofallsystemcomponentsareinvestigatedfromthepointofviewofstability.Thefunctionsaredefinedtheoretically,ornumericallyfromdiagramspresentedincataloguesofhydraulicequipment.Joiningalltransferfunctionofparticularcomponent,thetransferfunctionofthewholesystemisdiscussedfromthepointofviewofperformanceunderabruptunitsignal.Severalexperimentswereperformed,showingthatitispossibletoassurestable,assumedmotionofthebucket.Amongexperiments,onewasdevotedtodrill-ing.Inotherwords,thekinematicallyinducedtrajec-torywasastraight,verticalline.ExperimentallyobtainedlineispresentedinRefs.6and10.Itisinterestingtonotethatthevariationofexperimentallinedoesnotexceed10cm.3.ThreesubsystemsoftheexperimentalsetupThediscussedsystemisdividedinthreesub-systems,namely:microcomputer,hydraulicvalves,andexcavatorarmswithabucket.Below,theyarediscussedseparatelyandthenajointcontrolprob-lemisdefined.3.1.MicrocomputerasasubsystemWestartwithdefiningamodeloftheend-effector(bucket,drill,hammer)motion.Theend-effector,initsplanemotion,hasthreedegreesoffreedomaj(j=1,2,3)(Fig.1).Theyarerotationsoftheboom,ofthearm,andoftheeffector.Denotingbyx1p,x2ppositionoftheend-effectortip,andbyx3itsrotation,thekinematicsoftheconsideredmechanismisrepresentedbyvectorrelation:x1px2px3p266664377775c1c2c30s1s2s30000a3266664377775C1l1l2l3266664377775；1wherecjandsjdenotecosajandsinaj,respec-tively.Infurtherconsiderations,thesubindexpisomittedasthepositionofonlyonepointiscon-sidered.VelocityofthepointP,v=v1,v2,v3T=x1,x2,x3TisobtainedbytakingtimederivativeofEq.(1),andbyreducing3C24matrixtoa3C23matrix:xvAaAw；2whereAC0l1s1C0l2s2l3s3l1c1l2c2l3c3001266664377775:3TakinginverseofAmatrixequalto:AC01l2c2l1c10C0l2s2C0l1s10l2l3f23l1l3f13l1l2f12266664377775C11l1l2c1