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HybridHydraulicElectricPowerUnitforFieldandServiceRobotsKurtAmundson,JustinRaade,NathanHarding,andH.KazerooniDepartmentofMechanicalEngineeringUniversityofCalifornia,Berkeley,California,94720,USAexoberkeley.edu,http//bleex.me.berkeley.eduAbstractEnergeticautonomyofahydraulicbasedmobilefieldrobotrequiresapowersourcecapableofbothelectricalandhydraulicpowergeneration.Whilethehydraulicpowerisusedforlocomotion,theelectricpowerisusedforthecomputer,sensorsandotherperipherals.Aninternalcombustionenginewasusedastheprimemoverduetothehighenergydensityofgasoline.TheprimaryspecificationforthishybridHydraulicElectricPowerUnitHEPUisthatitmustoutputconstantpressurehydraulicpowerandconstantvoltageelectricpower.Anonboardcomputerusesapressuresensorandaspeedsensortoregulatethepressureandvoltagebymodulatingahydraulicsolenoidvalveandanenginethrottle.Thespeedregulationalsoresultsinasystemnoisewithpredictablefrequencybandwhichallowsforoptimalmufflerdesign.Anovelcharacteristicofthispowersourceisitscoolingsysteminwhichhydraulicfluidisusedtocooltheenginecylinders.SeveralhydraulicelectricpowerunitswerebuiltandsuccessfullydemonstratedontheBerkeleyLowerExtremityExoskeletonBLEEXshownonbleex.me.berkeley.edu/bleex.htm.Aprototypepowerunitweighs27Kg,outputs2.3kW3.0hphydraulicpowerat6.9MPa1000psi,and220Wofelectricpowerat15VDC.IndexTermsmobilepowersources,hydraulicpower,powergeneration,fieldandservicerobotics,BLEEXI.INTRODUCTIONCurrentlymosthumanscaleandfieldroboticsystemsarepoweredbytethersorheavybatterysystems.Inorderforaroboticdevicetoobtainenergeticautonomyfreefromtethersandheavybatteries,acompact,portablepowerunitprovidingbothmechanicalpowerforactuationandelectricalpowerforcomputationandcontrolisessential.Batteriesareacommonpowersourceformobilerobots.TheNiMHbatterypackinASIMO,Hondashumanoidwalkingrobot1,isonesuchexample.However,batterieshavealowspecificenergyenergypermass0.5MJ/kgforahighperformancelithiumionbattery2.Duetothislowspecificenergy,batteriesbecomelargeandheavyunlesstheoperationtimeisshortortheroboticsystemrequireslittlepower.Afuelwithahigherspecificenergythanbatteriesisdesirableinamobileroboticsystem.PreviousworkattheUniversityofCalifornia,Berkeleyfocusedontheuseofamonopropellantpoweredfreepistonhydraulicpump2,3.Thissystemgenerateshydraulicpowerthroughdecompositionof90concentratedhydrogenperoxide.Monopropellantsaremoreenergeticthanbatteriesbuttheirspecificenergy1.2MJ/kgfor90concentratedhydrogenperoxideissignificantlylowerthanthatofafuelsuchasgasoline44MJ/kg.Simplicityisakeyadvantageofmonopropellants.Thesystemrequiresnopremixing,aircompression,ignition,orcoolingsystem.Alloneneedsistocontroltheamountofmonopropellantfuelthroughasolenoidvalveviaacomputertocreateaproperpressuredifferentialinthetworeactors.Howevertherelativelylowspecificenergy,thesubstantialrequiredsafetyfeatures,andthefuelcostpreventedusfromfurtherpursuingmonopropellant–basedpowerunitsforroboticapplications.See4,5foranothernovelutilizationofmonopropellantinwhichfuelisdirectlyconvertedtomechanicalpower.InternalcombustionICenginesutilizethehighspecificenergyofgasoline.ThepowerunitdescribedhereutilizesatwostrokeopposedtwincylinderICenginetoproduceacompact,lightweightpowersource.ThisisprimarilymotivatedbythefactthatICengineshavebeentheprimarysourceofpowerforautomobiles,earthmovingmachinery,motorcycles,andotherwheeledvehicles.Weenvisionmobilefieldrobotsasanotherclassofthesefieldvehiclesthatoperateoutdoorsforperiodsofhours.InfactseveralfieldandserviceroboticsystemshavealreadyexperimentedwithICenginesastheirprimemover68.ICengines,unfortunately,areloud.HoweveritisourbeliefthatcurrentlowvolumemarketandsmalldemandforsmallICengineshavepreventedthedevelopmentofthetechnologiesthatleadtoefficientandquietsmallenginesforfieldroboticsystems.LargevolumefieldandmobileroboticsystemswillleadtodevelopmentofquietandefficientICenginebasedpowerunits.Infact,bothHondaandYamahahavealreadydevelopedsmall,efficient,andquietICenginebasedportableelectricpowerunitsfornonroboticoutdoorapplicationswithanoptimizedstructureandmufflerthatproduceameasured75dbnoiseat5ft.ThispaperdescribesthebasicdesignchallengesofagenerichydraulicelectricpowerunitHEPUforroboticapplications.AlthoughthedesignspecificationsforthispowerunitwerederivedfromtheoperationalrequirementsofBLEEX911,thedesignrulesapplytootherfieldroboticsystems.Thearchitecture,hydraulicandelectricpowergeneration,coolingsystemandcontrolaredescribedindetail.Experimentaldataarepresentedtoshowthesystemperformance.ThisworkispartiallyfundedbyDARPAgrantDAAD190110509.II.HEPUSPECIFICATIONSThedesignrequirementsforamobilefieldableroboticsystemarefunctionsoftherobotsize,itsmaneuveringspeed,anditspayloadcapability.ThedesignofthehybridpowerunitdescribedherewasmotivatedbytherequirementsoftheBLEEXproject911.Afterdesigningseveralpowerunits,wehavecometorealizethatmobileroboticsystemswithsimilarweightandsizetoBLEEXwillrequirepowersourceswiththesamecharacteristicswhichdifferonlynominally.ThemainfeatureofBLEEXandmanyotherfieldroboticsystemsthateffectsthedesignoftheirpowerunitsistheloadcarryingcapabilityinthefield.Whilemanywalkingsystems12,13aredesignedtocarryonlytheirownweight,BLEEXwasdesignedtocarryexternalloads.Whilehighpressurehydraulicsoftenleadstolesspowerloss,wechose6.9MPa1000psiasthesystempressure.Thisleadstomorereasonablehydrauliccomponentsformobilesystemsthatneedtoworkinthefieldandperhapsinproximityofhumans.Werecommendhigherworkingpressuree.g.20.7MPaor3000psiifsafeandappropriatehydraulicdeliverycomponentscanbeincorporatedinthesystem.Thehydraulicflowrequirementsareusuallycalculatedusingthespeedcharacteristicsoftherobot.Highspeedmovementsleadtolargehydraulicflowrequirements.InthecaseoftheBLEEXproject,thewalkingspeedfromCGAclinicalgaitanalysisdata9resultedin20LPM5.2GPMofhydraulicflow.Ourexperienceinbuildingvariousexoskeletonsystemssuggestthatonerequiresapproximately220Wofelectricpowerforonboardrobotcomputersandsensorsinadditiontothepowerunitsensorsandcontroller.ThemasstargetoftheHEPUis23kg50lbstoallowforasignificantpayloadcapacity.Table1summarizesthepowerunitspecifications.TABLE1HYDRAULICELECTRICPOWERUNITHEPUSPECIFICATIONSFORBLEEXHydraulicFluidPower2.3kW3.0hpElectricalPower220WHydraulicFlow20LPM5.2GPMWorkingPressure6.9MPa1000psiMassTargetLessthan23kg50lbsMaximumNoiseLevel78dBAat1.5m5ftIII.OVERALLHEPUARCHITECTURETheHEPUisdesignedtoprovideelectricandhydraulicpower.ItusesacompacttwostrokeopposedtwincylinderICenginecapableofallangleoperation.Fig.1andFig.2showhowtheengine1drivesasingleshaft2topoweranalternator3forelectricpowergeneration,acoolingfan4foraircirculation,andagearpump5forhydraulicpowergeneration.Thissingleshaftdesignelegantlyavoidsnoisyandheavybeltdrivemechanismscommoninsystemscomprisingmanyrotatingshafts.Ahydraulicsolenoidvalve7regulatesthehydraulicfluidpressurebydirectingthehydraulicflowfromthegearpumptoeitheranaccumulator10ortothehydraulicreservoir13.Theaccumulatorconsistsofanaluminumcylinderinwhichafreepistonseparatesthehydraulicfluidfromthepressurizednitrogengas.Acarbonfibertank11isattachedtothegassideoftheaccumulatorasreservoirforthenitrogengas.Ingeneralthelargerthevolumeofthisgasreservoiris,thesmallerthepressurefluctuationwillbeinthepresenceofhydraulicflowfluctuations.Apressuretransducer9measuresthepressureofthehydraulicfluidforthecontroller.Amanifold6isdesignedtohouseboththesolenoidvalve7andfilter8.Anovelliquidcoolingschemeutilizesthereturninghydraulicfluiditselftocooltheengine.Thehydraulicfluidfromtherobotactuatorsisdividedintotwopaths.Approximately38ofthehydraulicfluidisdivertedtocooltheenginecylinders.Aheatexchanger12removestheheatfromthishydraulicfluidbeforeitreachesthehydraulicreservoir13andismixedwiththeremaining62ofthefluid.Accumulator10GearPump5TwoStrokeEngine1HeatExchanger12HydraulicReservoir13ServovalvesandActuatorsHeatExchangerBypass12.5LPM3.2GPM7.5LPM2.0GPM20LPM5.2GPMNitrogenTank11Shaft2ABSolenoidValve7Alternator3CoolingFan4ShaftHydraulicNitrogenFilter8PressureTransducer9Manifold6Fig.1HEPUschematiclayout.ComponentslabeledwithnumbersinparenthesesalsocorrespondtoFig.2.IV.MECHANICALPOWERPRODUCTIONThetwostrokeopposedtwincylinderICenginemodel80B2RV,manufacturedbyZDZModelMotorcapableofproducing6kW8.1hpofshaftpowerat8200rpmisusedastheprimemoverofthispowerunit.Thisenginehasan80cm3displacementandweighsonly2kg4.4lbs.Sincethegearpumpwaslimitedtoturnatmaximumspeedof6300rpmandsinceweintendednottoutilizeanytransmissionspeedreducerinthispowerunit,wewereforcedtodrivetheengineatspeedslowerthanthemaximumpowerspeedoftheengine.Theenginecanproduceapproximately3.06kW4.0hpat6300rpmwhichisgreaterthantherequiredpower2.5kWor3.4hp.Ingeneral,usingalargerengineatlowerspeedsresultsinlessnoisethanusingasmallerengineathigherspeeds.Theengineiscontrolledwithaservomotormountedtoitsthrottle.Theenginedirectlydrivesanalternator,acoolingfanandagearpump.ThepumpmodelWP03B1B032L20MA12,manufacturedbyHaldexhasa3.2cm3displacementvolumeperrevolutionandthereforeintheoryitcantransfer20.2LPM5.3GPMofflowatitsmaximumspeedof6300rpm.3148561311101279214151617Fig.2HEPUphysicallayout.Engine1shaft2,notvisiblealternator3coolingfan4gearpump5manifold6solenoidvalve7filter8,notvisiblepressuretransducer9,notvisibleaccumulator10nitrogentank11heatexchanger12hydraulicreservoir13muffler14batteries15carburetorandthrottle16heatexchangerfans17.Internalbafflingaroundengineisnotshownforclarity.V.CONTROLARCHITECTUREAuniquecontrolschemewasneededtomaintainconstantoperatingpressurewithafixeddisplacementpumprunningataconstantspeed.Anaccumulatorattheoutletofthepumpsuppliesthefluidtotheactuatorsandfunctionslikeacapacitortocompensatefortransientpeakflows.Thehydraulicpressureisreadbythepressuresensor.Thecomputercontrolsthesolenoidvalvetomaintainthepressure.Whenthepressurereachesthedesiredvalue6.9MPainthiscase,thecomputerdivertsthehydraulicflowtothereservoirbymovingthevalvetopositionAasshowninFig.1.Topreventpressuredropintheaccumulatorwhenthehydraulicfluidintheaccumulatorisconsumedbytheservovalvesandtheactuators,thecomputerdivertstheflowtotheaccumulatorbymovingthevalvetopositionB.Themodulationofthisvalvebasedonthemeasuredpressureallowsthesystemtooutputhydraulicpoweratnearconstantpressure.Theoperatingpressureintheaccumulatorismaintainedinabandof6.9/0.2MPa1000/30psi.Whenthesolenoidvalvedivertsthehydraulicfluidtothereservoir,theenginespeedincreasesrapidly.Theoppositeisalsotruewhenthevalvedivertsthehydraulicfluidtotheaccumulator,theenginespeeddecreasesrapidlyandtheenginemightevenstall.Thevariationofenginespeedcausesexhaustsoundwithvaryingfrequenciesthatisundesirableforoptimalnoisereduction.Furthermore,theenginespeedvariationleadstoalargevoltagevariation.Additionallythehighenginespeedsmightdamagethepump.Fortheabovereasons,itisdesirabletocontroltheenginespeedtoaconstantvalue.Itwasdecidedtomaintainthespeedat6300rpmmaximumallowablepumpspeed.Insummary,anonboardcomputerusesapressuresensorandaHalleffectsensortoregulatethepressureat1000psiandenginespeedat6300rpmbymodulatingahydraulicsolenoidvalveandanenginethrottle.VI.COOLINGSincetheenginewasdesignedforhighperformancemodelaircrafts,itrequiresalargeamountofairforcoolingitscylindersairisgenerouslyavailablewhentheengineisinstalledonaircraftmodels.Fortheapplicationoffieldrobotics,itisnecessarytopackagetheenginetightlyinasounddeadeningshieldthereforeliquidcoolingwasrequired.Anovelliquidcoolingschemewasdevisedthatusesthehydraulicfluiditselftocooltheengine.TheenginecylinderheadsweremodifiedtoallowhydraulicfluidtopassthroughthemandabsorbheatFig.3.Thismakestheadditionofawaterbasedcoolingsystemunnecessaryandresultsinasimplifiedsystemwithfewercomponents.Usingthehydraulicfluidasthecoolingmediumincreasestheloadontheheatexchangersincetheheatfromtheenginemustberemovedtopreventthehydraulicfluidfromexceedingtheoperatingtemperatureofanyhydrauliccomponents.Themaximumtemperatureallowablewasdeterminedbythepumpwhichhadthelowesttemperaturetoleranceofanycomponentinthesystemthegearpumprequiredhydraulicfluidtemperaturecoolerthan65°Cor149°F.Thefluidreturningfromtheactuatorsissplitintotwoseparatepaths,asshowninFig.4.Approximately62ofthehydraulicfluidreturnsdirectlytothereservoir.Theremaining38passesfirstthroughthecylinderheadswhereexcessheatisextractedfromtheengine,thenthroughaheatexchangerwheretheheatinthefluidisdissipated,andfinallyreturnstothereservoir.AsshowninFig.4,theheatexchangermustremovetheheatgeneratedfromthedissipativeeffectoftheservovalvesontheactuatorsinadditiontotheheatgeneratedintheenginecylinderheads.Increasingfluidvolumeinthereservoirincreasesconvectiveheattransfercoolingtoambientairandallowslongeroperationtimes.Thisisatypicalsolutioninindustrialhydraulics,butisnotfeasibleinthisapplicationwherealargereservoirisundesirable.Therefore,carefulsizingoftheheatexchangerwascriticaltoensureadequatecoolingataminimumweight.Athermalmodelwascreatedusingmeasureddatafromtheteststandwheneverpossibletoestimatethebehaviorofthehydraulicsystemandevaluatethehydraulicfluidtemperatureatthemostsensitivecomponent,thepump.Datawastakenfromanexperimentalrunwiththeengineproducing3.06kWofshaftpower.Adutycycleof50wasusedtosimulateouroperatingconditionsi.e.,1.53kWcontinuousshaftpower.Thereservoirwasmodeledasaperfectmixerwithzeroheattransfertoambient.Thepumpexhibitedaminimumof80efficiencyshaftpowertofluidpowerhence20oftheengineshaftpower3.06kW0.500.200.306kWor0.41hpisconvertedtoheatintothehydraulicfluid.Theheattransfertoambientairinthehydrauliclineswasestimatedat0.373kW0.50hp.Theactuatorsandservovalveswereassumedtoconvertallthehydraulicpowerflowingthroughthemtoheatintothehydraulicfluid3.06kW0.500.801.22kWor1.64hp.Thesumoftheheattransferratesfromthereservoir,pump,lines,andvalvesis12203060373115OtherQ....null−kW1.54hp.Theheattransferratefromtheenginecylinders,EngineQnull,wasmeasuredat2.85kW3.82hp.TheperformanceoftheheatexchangerischaracterizedbyathermalparameterKthwhichistheheattransferrateatagivenflowrateoffluiddividedbytheinitialtemperaturedifferencebetweenthehotfluidenteringtheheatexchangerandtheenvironmentatTambient.2ExchangerthambientQKTTnull−−1ThetemperatureT4inFig.4isequaltothepumpinlettemperaturesincethereisnoheattransferinthereservoir.Atsteadystatetheheattransferfromeachcomponentcanbeexpressedbythefollowingequations.14OthertotalPQmcTTnullnull−221EnginecoolPQmTT−nullnull332ExchangercoolPQmTT−nullnull4wheretotalmnullisthetotalhydraulicmassflowrate,coolmnullisthecoolingflowrate,andcPisthespecificheatofthefluid.Sinceatsteadystate0ExchangerEngineOthersQQQnullnullnull5equations1–5canbesolvedexplicitlyforthesteadystatepumpinlettemperature,T4.4ExchangerEngineOtherambientthcoolPtotalPQQQTTKmcmcnullnullnullnullnull−−−6Variousheatexchangerspecificationswereinsertedin6toestimatethesteadystatehydraulicfluidtemperatureandevaluatetheperformanceofagivenheatexchanger.Atsteadystatetheselectedheatexchangerremoves4.00kWandthecalculatedpumpinlettemperatureis61°C141°F,underthemaximumallowablepumptemperature,65°C.2022181921192220213118151116Fig.3Detailoftheenginedepictingthecoolingjacketsonthecylinders.Engine1alternator3nitrogentank11batteries15carburetorandthrottle16hydrauliclines18exhaustpipe19coolingjacket20sparkplug21cylinderhead22.CylinderHeadHeatExchangerActuatorsValvesPumpLinesReservoirQEngine2.85kWQValves1.22kWQLines0.373kWQPump0.306kWQExchanger4.00kWT164.3°CT278.2°CT357.6°CT461.3°Cmcool7.5LPMmtot20LPMT164.3°CT164.3°CFig.4CoolingsystemschematicoftheHEPU.VII.ELECTRICALPOWERGENERATIONTheHEPUgenerateselectricalpowerforthesensors,coolingfans,andthecontrolcomputer.TheelectricalpowergenerationandregulationdesignisdepictedinFig.5.Thetotalelectricalsystempowerbudgetis220W,with100Wforcoolingfansand65Wforthecontrolcomputerandsensors.Theremaining55Wareexpectedtobeconsumedinlossesandotherperipheralcomponents.Athreephase,12poleframeless,brushlessDCmotormodelRBE1812,manufacturedbyKollmorgenisusedasanelectricpowergenerator3inFig.2.Thethreephaseswereconvertedtosinglephase,240VDCbyabridgerectifierthebackEMFconstantofthemotoris26.9V/krpmsothatattheoperationalspeedof6300rpmtherectifiedvoltageis240VDC.TwoDCDCconvertersareusedtocreatetwo15VDCbusvoltagestobeusedfortwosetsofcomponents.One15VDClineisusedtopowertheelectricallynoisycomponentssuchassolenoidvalves,coolingfans,andtheignitionfortheengine.Thesecond15VDClineisusedtochargeasetofbatteries,powerthecontrolcomputer,HEPUcontroller,andthethrottleservo.TheexternalpowershowninFig.5isusedtopowerthesystemwhentheengineisoff.ThebatteryshowninFig.5powersthecontrolcomputer,HEPUcontroller,throttleservo,
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