HybridHydraulic-ElectricPowerUnitforFieldandServiceRobotsKurtAmundson,JustinRaade,NathanHarding,andH.KazerooniDepartmentofMechanicalEngineeringUniversityofCalifornia,Berkeley,California,94720,USA,Abstract-Energeticautonomyofahydraulic-basedmobilefieldrobotrequiresapowersourcecapableofbothelectricalandhydraulicpowergeneration.Whilethehydraulicpowerisusedforlocomotion,theelectricpowerisusedforthecomputer,sensorsandotherperipherals.Aninternalcombustionenginewasusedastheprimemoverduetothehighenergydensityofgasoline.TheprimaryspecificationforthishybridHydraulic-ElectricPowerUnit(HEPU)isthatitmustoutputconstantpressurehydraulicpowerandconstantvoltageelectricpower.Anon-boardcomputerusesapressuresensorandaspeedsensortoregulatethepressureandvoltagebymodulatingahydraulicsolenoidvalveandanenginethrottle.Thespeedregulationalsoresultsinasystemnoisewithpredictablefrequencybandwhichallowsforoptimalmufflerdesign.Anovelcharacteristicofthispowersourceisitscoolingsysteminwhichhydraulicfluidisusedtocooltheenginecylinders.Severalhydraulic-electricpowerunitswerebuiltandsuccessfullydemonstratedontheBerkeleyLowerExtremityExoskeleton(BLEEX)/bleex.htm.Aprototypepowerunitweighs27Kg,outputs2.3kW(3.0hp)hydraulicpowerat6.9MPa(1000psi),and220Wofelectricpowerat15VDC.IndexTerms-mobilepowersources,hydraulicpower,powergeneration,fieldandservicerobotics,BLEEXI.INTRODUCTIONCurrentlymosthumanscaleandfieldroboticsystemsarepoweredbytethersorheavybatterysystems.Inorderforaroboticdevicetoobtainenergeticautonomyfreefromtethersandheavybatteries,acompact,portablepowerunitprovidingbothmechanicalpowerforactuationandelectricalpowerforcomputationandcontrolisessential.Batteriesareacommonpowersourceformobilerobots.TheNiMHbatterypackinASIMO,Hondashumanoidwalkingrobot1,isonesuchexample.However,batterieshavealowspecificenergy(energypermass):0.5MJ/kgforahighperformancelithiumionbattery2.Duetothislowspecificenergy,batteriesbecomelargeandheavyunlesstheoperationtimeisshortortheroboticsystemrequireslittlepower.Afuelwithahigherspecificenergythanbatteriesisdesirableinamobileroboticsystem.PreviousworkattheUniversityofCalifornia,Berkeleyfocusedontheuseofamonopropellant-poweredfreepistonhydraulicpump2,3.Thissystemgenerateshydraulicpowerthroughdecompositionof90%-concentratedhydrogenperoxide.Monopropellantsaremoreenergeticthanbatteriesbuttheirspecificenergy(1.2MJ/kgfor90%-concentratedhydrogenperoxide)issignificantlylowerthanthatofafuelsuchasgasoline(44MJ/kg).Simplicityisakeyadvantageofmonopropellants.Thesystemrequiresnopremixing,aircompression,ignition,orcoolingsystem.Alloneneedsistocontroltheamountofmonopropellantfuelthroughasolenoidvalveviaacomputertocreateaproperpressuredifferentialinthetworeactors.Howevertherelativelylowspecificenergy,thesubstantialrequiredsafetyfeatures,andthefuelcostpreventedusfromfurtherpursuingmonopropellantbasedpowerunitsforroboticapplications.See4,5foranothernovelutilizationofmonopropellantinwhichfuelisdirectlyconvertedtomechanicalpower.Internalcombustion(IC)enginesutilizethehighspecificenergyofgasoline.Thepowerunitdescribedhereutilizesatwo-strokeopposedtwincylinderICenginetoproduceacompact,lightweightpowersource.ThisisprimarilymotivatedbythefactthatICengineshavebeentheprimarysourceofpowerforautomobiles,earthmovingmachinery,motorcycles,andotherwheeledvehicles.Weenvisionmobilefieldrobotsasanotherclassofthesefieldvehiclesthatoperateoutdoorsforperiodsofhours.InfactseveralfieldandserviceroboticsystemshavealreadyexperimentedwithICenginesastheirprimemover6-8.ICengines,unfortunately,areloud.HoweveritisourbeliefthatcurrentlowvolumemarketandsmalldemandforsmallICengineshavepreventedthedevelopmentofthetechnologiesthatleadtoefficientandquietsmallenginesforfieldroboticsystems.LargevolumefieldandmobileroboticsystemswillleadtodevelopmentofquietandefficientIC-engine-basedpowerunits.Infact,bothHondaandYamahahavealreadydevelopedsmall,efficient,andquietIC-engine-basedportableelectricpowerunitsfornon-roboticoutdoorapplicationswithanoptimizedstructureandmufflerthatproduceameasured75dbnoiseat5ft.Thispaperdescribesthebasicdesignchallengesofagenerichydraulic-electricpowerunit(HEPU)forroboticapplications.AlthoughthedesignspecificationsforthispowerunitwerederivedfromtheoperationalrequirementsofBLEEX9-11,thedesignrulesapplytootherfieldroboticsystems.Thearchitecture,hydraulicandelectricpowergeneration,coolingsystemandcontrolaredescribedindetail.Experimentaldataarepresentedtoshowthesystemperformance.ThisworkispartiallyfundedbyDARPAgrantDAAD19-01-1-0509.II.HEPUSPECIFICATIONSThedesignrequirementsforamobilefieldableroboticsystemarefunctionsoftherobotsize,itsmaneuveringspeed,anditspayloadcapability.ThedesignofthehybridpowerunitdescribedherewasmotivatedbytherequirementsoftheBLEEXproject9-11.Afterdesigningseveralpowerunits,wehavecometorealizethatmobileroboticsystemswithsimilarweightandsizetoBLEEXwillrequirepowersourceswiththesamecharacteristicswhichdifferonlynominally.ThemainfeatureofBLEEXandmanyotherfieldroboticsystemsthateffectsthedesignoftheirpowerunitsistheloadcarryingcapabilityinthefield.Whilemanywalkingsystems12,13aredesignedtocarryonlytheirownweight,BLEEXwasdesignedtocarryexternalloads.Whilehighpressurehydraulicsoftenleadstolesspowerloss,wechose6.9MPa(1000psi)asthesystempressure.Thisleadstomorereasonablehydrauliccomponentsformobilesystemsthatneedtoworkinthefieldandperhapsinproximityofhumans.Werecommendhigherworkingpressure(e.g.20.7MPaor3000psi)ifsafeandappropriatehydraulicdeliverycomponentscanbeincorporatedinthesystem.Thehydraulicflowrequirementsareusuallycalculatedusingthespeedcharacteristicsoftherobot.Highspeedmovementsleadtolargehydraulicflowrequirements.InthecaseoftheBLEEXproject,thewalkingspeedfromCGA(clinicalgaitanalysis)data9resultedin20LPM(5.2GPM)ofhydraulicflow.Ourexperienceinbuildingvariousexoskeletonsystemssuggestthatonerequiresapproximately220Wofelectricpowerforon-boardrobotcomputersandsensorsinadditiontothepowerunitsensorsandcontroller.ThemasstargetoftheHEPUis23kg(50lbs)toallowforasignificantpayloadcapacity.Table1summarizesthepowerunitspecifications.TABLE1HYDRAULICELECTRICPOWERUNIT(HEPU)SPECIFICATIONSFORBLEEXHydraulicFluidPower2.3kW(3.0hp)ElectricalPower220WHydraulicFlow20LPM(5.2GPM)WorkingPressure6.9MPa(1000psi)MassTargetLessthan23kg(50lbs)MaximumNoiseLevel78dBAat1.5m(5ft)III.OVERALLHEPUARCHITECTURETheHEPUisdesignedtoprovideelectricandhydraulicpower.Itusesacompacttwo-strokeopposedtwincylinderICenginecapableofall-angleoperation.Fig.1andFig.2showhowtheengine(1)drivesasingleshaft(2)topoweranalternator(3)forelectricpowergeneration,acoolingfan(4)foraircirculation,andagearpump(5)forhydraulicpowergeneration.Thissingleshaftdesignelegantlyavoidsnoisyandheavybeltdrivemechanismscommoninsystemscomprisingmanyrotatingshafts.Ahydraulicsolenoidvalve(7)regulatesthehydraulicfluidpressurebydirectingthehydraulicflowfromthegearpumptoeitheranaccumulator(10)ortothehydraulicreservoir(13).Theaccumulatorconsistsofanaluminumcylinderinwhichafreepistonseparatesthehydraulicfluidfromthepressurizednitrogengas.Acarbonfibertank(11)isattachedtothegassideoftheaccumulatorasreservoirforthenitrogengas.Ingeneralthelargerthevolumeofthisgasreservoiris,thesmallerthepressurefluctuationwillbeinthepresenceofhydraulicflowfluctuations.Apressuretransducer(9)measuresthepressureofthehydraulicfluidforthecontroller.Amanifold(6)isdesignedtohouseboththesolenoidvalve(7)andfilter(8).Anovelliquidcoolingschemeutilizesthereturninghydraulicfluiditselftocooltheengine.Thehydraulicfluidfromtherobotactuatorsisdividedintotwopaths.Approximately38%ofthehydraulicfluidisdivertedtocooltheenginecylinders.Aheatexchanger(12)removestheheatfromthishydraulicfluidbeforeitreachesthehydraulicreservoir(13)andismixedwiththeremaining62%ofthefluid.Accumulator(10)GearPump(5)Two-StrokeEngine(1)HeatExchanger(12)HydraulicReservoir(13)ServovalvesandActuatorsHeatExchangerBypass12.5LPM(3.2GPM)7.5LPM(2.0GPM)20LPM(5.2GPM)NitrogenTank(11)Shaft(2)ABSolenoidValve(7)Alternator(3)CoolingFan(4)ShaftHydraulicNitrogenFilter(8)PressureTransducer(9)Manifold(6)Fig.1HEPUschematiclayout.ComponentslabeledwithnumbersinparenthesesalsocorrespondtoFig.2.IV.MECHANICALPOWERPRODUCTIONThetwo-strokeopposedtwincylinderICengine(model80B2RV,manufacturedbyZDZModelMotor)capableofproducing6kW(8.1hp)ofshaftpowerat8200rpmisusedastheprimemoverofthispowerunit.Thisenginehasan80cm3displacementandweighsonly2kg(4.4lbs).Sincethegearpumpwaslimitedtoturnatmaximumspeedof6300rpmandsinceweintendednottoutilizeanytransmissionspeedreducerinthispowerunit,wewereforcedtodrivetheengineatspeedslowerthanthemaximum-powerspeedoftheengine.Theenginecanproduceapproximately3.06kW(4.0hp)at6300rpmwhichisgreaterthantherequiredpower(2.5kWor3.4hp).Ingeneral,usingalargerengineatlowerspeedsresultsinlessnoisethanusingasmallerengineathigherspeeds.Theengineiscontrolledwithaservomotormountedtoitsthrottle.Theenginedirectlydrivesanalternator,acoolingfanandagearpump.Thepump(modelWP03-B1B-032L-20MA12,manufacturedbyHaldex)hasa3.2cm3displacementvolumeperrevolutionandthereforeintheoryitcantransfer20.2LPM(5.3GPM)offlowatitsmaximumspeedof6300rpm.3148561311101279214151617Fig.2HEPUphysicallayout.Engine(1)；shaft(2,notvisible)；alternator(3)；coolingfan(4)；gearpump(5)；manifold(6)；solenoidvalve(7)；filter(8,notvisible)；pressuretransducer(9,notvisible)；accumulator(10)；nitrogentank(11)；heatexchanger(12)；hydraulicreservoir(13)；muffler(14)；batteries(15)；carburetorandthrottle(16)；heatexchangerfans(17).Internalbafflingaroundengineisnotshownforclarity.V.CONTROLARCHITECTUREAuniquecontrolschemewasneededtomaintainconstantoperatingpressurewithafixeddisplacementpumprunningataconstantspeed.Anaccumulatorattheoutletofthepumpsuppliesthefluidtotheactuatorsandfunctionslikeacapacitortocompensatefortransientpeakflows.Thehydraulicpressureisreadbythepressuresensor.Thecomputercontrolsthesolenoidvalvetomaintainthepressure.Whenthepressurereachesthedesiredvalue(6.9MPainthiscase),thecomputerdivertsthehydraulicflowtothereservoirbymovingthevalvetopositionAasshowninFig.1.Topreventpressuredropintheaccumulatorwhenthehydraulicfluidintheaccumulatorisconsumedbytheservovalvesandtheactuators,thecomputerdivertstheflowtotheaccumulatorbymovingthevalvetopositionB.Themodulationofthisvalvebasedonthemeasuredpressureallowsthesystemtooutputhydraulicpoweratnearconstantpressure.Theoperatingpressureintheaccumulatorismaintainedinabandof6.9+/-0.2MPa(1000+/-30psi).Whenthesolenoidvalvedivertsthehydraulicfluidtothereservoir,theenginespeedincreasesrapidly.Theoppositeisalsotrue:whenthevalvedivertsthehydraulicfluidtotheaccumulator,theenginespeeddecreasesrapidlyandtheenginemightevenstall.Thevariationofenginespeedcausesexhaustsoundwithvaryingfrequenciesthatisundesirableforoptimalnoisereduction.Furthermore,theenginespeedvariationleadstoalargevoltagevariation.Additionallythehighenginespeedsmightdamagethepump.Fortheabovereasons,itisdesirabletocontroltheenginespeedtoaconstantvalue.Itwasdecidedtomaintainthespeedat6300rpm(maximumallowablepumpspeed).Insummary,anon-boardcomputerusesapressuresensorandaHalleffectsensortoregulatethepressure(at1000psi)andenginespeed(at6300rpm)bymodulatingahydraulicsolenoidvalveandanenginethrottle.VI.COOLINGSincetheenginewasdesignedforhighperformancemodelaircrafts,itrequiresalargeamountofairforcoolingitscylinders(airisgenerouslyavailablewhentheengineisinstalledonaircraftmodels.)Fortheapplicationoffieldrobotics,itisnecessarytopackagetheenginetightlyinasound-deadeningshield；thereforeliquidcoolingwasrequired.Anovelliquidcoolingschemewasdevisedthatusesthehydraulicfluiditselftocooltheengine.Theenginecylinderheadsweremodifiedtoallowhydraulicfluidtopassthroughthemandabsorbheat(Fig.3).Thismakestheadditionofawater-basedcoolingsystemunnecessaryandresultsinasimplifiedsystemwithfewercomponents.Usingthehydraulicfluidasthecoolingmediumincreasestheloadontheheatexchangersincetheheatfromtheenginemustberemovedtopreventthehydraulicfluidfromexceedingtheoperatingtemperatureofanyhydrauliccomponents.Themaximumtemperatureallowablewasdeterminedbythepumpwhichhadthelowesttemperaturetoleranceofanycomponentinthesystem(thegearpumprequiredhydraulicfluidtemperaturecoolerthan65Cor149F).Thefluidreturningfromtheactuatorsissplitintotwoseparatepaths,asshowninFig.4.Approximately62%ofthehydraulicfluidreturnsdirectlytothereservoir.Theremaining38%passesfirstthroughthecylinderheadswhereexcessheatisextractedfromtheengine,thenthroughaheatexchangerwheretheheatinthefluidisdissipated,andfinallyreturnstothereservoir.AsshowninFig.4,theheatexchangermustremovetheheatgeneratedfromthedissipativeeffectoftheservovalvesontheactuatorsinadditiontotheheatgeneratedintheenginecylinderheads.Increasingfluidvolumeinthereservoirincreasesconvectiveheattransfer(cooling)toambientairandallowslongeroperationtimes.Thisisatypicalsolutioninindustrialhydraulics,butisnotfeasibleinthisapplicationwherealargereservoirisundesirable.Therefore,carefulsizingoftheheatexchangerwascriticaltoensureadequatecoolingataminimumweight.Athermalmodelwascreated(usingmeasureddatafromtheteststandwheneverpossible)toestimatethebehaviorofthehydraulicsystemandevaluatethehydraulicfluidtemperatureatthemostsensitivecomponent,thepump.Datawastakenfromanexperimentalrunwiththeengineproducing3.06kWofshaftpower.Adutycycleof50%wasusedtosimulateouroperatingconditions(i.e.,1.53kWcontinuousshaftpower).Thereservoirwasmodeledasaperfectmixerwithzeroheattransfertoambient.Thepumpexhibitedaminimumof80%efficiency(shaftpowertofluidpower)；hence20%oftheengineshaftpower(3.06kW*0.50*0.20=0.306kWor0.41hp)isconvertedtoheatintothehydraulicfluid.Theheattransfertoambientairinthehydrauliclineswasestimatedat-0.373kW(-0.50hp).Theactuatorsandservovalveswereassumedtoconvertallthehydraulicpowerflowingthroughthemtoheatintothehydraulicfluid(3.06kW*0.50*0.80=1.22kWor1.64hp).Thesumoftheheattransferratesfromthereservoir,pump,lines,andvalvesis12203060373115OtherQ(.).null=+=kW(1.54hp).Theheattransferratefromtheenginecylinders,EngineQnull,wasmeasuredat2.85kW(3.82hp).TheperformanceoftheheatexchangerischaracterizedbyathermalparameterKthwhichistheheattransferrateatagivenflowrateoffluiddividedbytheinitialtemperaturedifferencebetweenthehotfluidenteringtheheatexchangerandtheenvironmentatTambient.2ExchangerthambientQKTT()null=(1)ThetemperatureT4inFig.4isequaltothepumpinlettemperaturesincethereisnoheattransferinthereservoir.Atsteadystatetheheattransferfromeachcomponentcanbeexpressedbythefollowingequations.14OthertotalPQmcTT()nullnull=(2)21()EnginecoolPQmTT=nullnull(3)32()ExchangercoolPQmTT=nullnull(4)wheretotalmnullisthetotalhydraulicmassflowrate,coolmnullisthecoolingflowrate,andcPisthespecificheatofthefluid.Sinceatsteadystate:0ExchangerEngineOthersQQQnullnullnull+=(5)equations(1)(5)canbesolvedexplicitlyforthesteadystatepumpinlettemperature,T4.4ExchangerEngineOtherambientthcoolPtotalPQQQTTKmcmcnullnullnullnullnull=(6)Variousheatexchangerspecificationswereinsertedin(6)toestimatethesteadystatehydraulicfluidtemperatureandevaluatetheperformanceofagivenheatexchanger.Atsteadystatetheselectedheatexchangerremoves4.00kWandthecalculatedpumpinlett