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OntheMechanicalDesignoftheBerkeleyLowerExtremityExoskeletonBLEEXAdamZoss,H.Kazerooni,AndrewChuDepartmentofMechanicalEngineeringUniversityofCalifornia,Berkeley,CA,94720,USAexoberkeley.edu,http//bleex.me.berkeley.eduAbstractThefirstenergeticallyautonomouslowerextremityexoskeletoncapableofcarryingapayloadhasbeendemonstratedatU.C.Berkeley.ThispapersummarizesthemechanicaldesignoftheBerkeleyLowerExtremityExoskeletonBLEEX.TheanthropomorphicallybasedBLEEXhassevendegreesoffreedomperleg,fourofwhicharepoweredbylinearhydraulicactuators.Theselectionofthedegreesoffreedomandtheirrangesofmotionaredescribed.Additionally,thesignificantdesignaspectsofthemajorBLEEXcomponentsarecovered.IndexTerms–BLEEX,exoskeleton,wearablerobotics,mechanicaldesign,leggedlocomotionI.INTRODUCTIONHeavyobjectsaretypicallytransportedusingwheeledvehicles.However,manyenvironments,suchasrockyterrainsandstaircases,posesignificantchallengestowheeledvehicles.Thusleggedlocomotionbecomesanattractivemethodoftransportationwithinthesesettings,sincelegscanadapttoawiderangeofextremeterrains.TheBerkeleyLowerExtremityExoskeletoncommonlyreferredtoasBLEEXisthefirstfieldoperationalroboticsystemwhichiswornbyitsoperatorandprovidesitswearertheabilitytocarrysignificantloadsonhis/herbackwithminimaleffortoveranytypeofterrain.BLEEXiscomprisedoftwopoweredanthropomorphiclegs,apowersupply,andabackpacklikeframeonwhichavarietyofheavypayloadscanbemountedFig.1.BLEEXprovidesloadcarryingcapabilitythroughleggedlocomotionguidedbyhumaninteraction,butinsteadofactivelydrivingthevehicle,BLEEXshadowstheoperatorsmovementashe/shewearsitlikeapairofartificiallegs.Bycombiningthestrengthcapabilitiesofroboticswiththenavigationintelligenceandadaptabilityofhumans,BLEEXallowsheavyloadstobecarriedoverrough,unstructured,anduncertainterrains.Exoskeletonsareusuallyconceivedassystemsincludingupperextremities,lowerextremities,orboththeBLEEXprojectfocusessolelyonlowerextremityexoskeletons.Upperextremityexoskeletonsaregenerallyformanipulatingheavyobjectsandareusuallyusedinwarehouses,manufacturingfacilities,anddistributioncenterse.g.1–4.Lowerextremityexoskeletonsaregenerallyforcarryingheavyobjectslongdistancesusuallyoutdoorsandonpathsthatarenotpassablebywheeledvehicles.ThereisnoThisworkispartiallyfundedbyDARPAgrantDAAD190110509.practicalreasonforusingalowerextremityexoskeletoninasettingwherewheeledvehiclescanbeusedorwheredistancesaresoshortthatoverheadcranesarepractical.Lowerextremityexoskeletonsaremostsuitedforcarryingheavyobjectsalongunstructured,outdoorpaths.PayloadKneeAnkleHipExoskeletonFootExoskeletonShankExoskeletonThighHarnessPowerSupplyComputerFig.1Conceptualsketchofalowerextremityexoskeleton.Properactuationoftheroboticlegsremovesthepayloadweightfromthewearer,whileallowingthewearertoeffortlesslycontrolandbalancethemachine.BLEEXhasnumerousapplicationsitcanprovidesoldiers,disasterreliefworkers,wildfirefighters,andotheremergencypersonneltheabilitytocarrymajorloads,suchasfood,rescueequipment,firstaidsupplies,communicationsgear,andweaponry,withoutthestraintypicallyassociatedwithdemandinglabor.ItisourvisionthatBLEEXwillprovideaversatiletransportplatformformissioncriticalequipment.II.BACKGROUNDThefirstactiveexoskeletonsappearedinthelate1960sandearly70satGeneralElectricGEandtheMihajloPupinInstituteinBelgrade.TheHardimanprojectatGE5wasalarge,fullbodyexoskeletonweighing680kgandcontrolledusingamasterslavesystem.Safetyconcernsandcomplexityissuespreventeditfromeverwalking,orevenstablymovingthelegs.TheBelgradeexoskeletonwasahumansizedlowerextremityrobotdesignedtohelprehabilitateparaplegics6.SimilartotheHardimanproject,itcouldnevercarryitsownpowersource.TheBelgradeexoskeletononlyfollowedpreprogrammedwalkingmotions,whichgreatlylimiteditsusefulness.However,thisprojectdidproduceZeroMomentPointcontrol,whichisstillusedinhumanoidrobots.Followingthe1970attempts,relativelyfewpeopleinvestigatedlowerextremityexoskeletons.Oneprojectin1993wastheElectricPowerExtenderattheUniversityofCaliforniaatBerkeley1.ThisfullbodyexoskeletonusedelectricactuationtoamplifyhumancapabilitiessimilartotheHardiman.TheBerkeleyprojectusedforcesensorstodetectandamplifythehumansforces,butstillhadlimitedsuccessinwalking.The21stcenturyhasseenresurgenceinexoskeletoninvestigation.InJapan,theKanagawaInstituteofTechnologyhasdevelopedafullbodywearablepowersuit,poweredbyuniquepneumaticactuators7.Theforcesattheirthreeactuatorsknee,waist,andelbowarecontrolledbymeasuringthehardnessofthecorrespondinghumanmuscles.Limitedactuationandlackofaportablepowersupplyrestrictsthisexoskeletonsapplications.TsukubaUniversityinJapandevelopedthelightweightpowerassistdevice,HAL8.UsingEMGsensorsonthehumanslegmusclesandgroundreactionforcesensors,HALcontrolsitselectricactuatorsatthekneeandhip.Thisexoskeletonhasaportablepowersupply,butonlyassiststheoperatorslegmusclesitcannotcarryanexternalload.Stillindevelopmentareseveralotherlowerextremityexoskeletonsdesignedtoaiddisabledpersons9–11.Besidesexoskeletons,someotheractivelowerextremitydevicesareworthmentioning.Amodernrehabilitationdeviceisthelowerlimblocomotiontrainer,Lokomat12and13.Insteadofcarryingaload,therobotstorsoismountedtoastandandmovestheoperatorsfeetinapredeterminedpath.Whilenotanexoskeleton,theLokomatisasuccessfulproductthatfacessimilarchallenges.TheRoboKnee,developedbyYobotics,isapoweredkneeorthoticdesignedtoenhancetheoperatorsstrengthandenduranceduringwalking14.RoboKneeusesgroundreactionforcestoestimatethedesiredkneetorques.Alternatively,researchersatHokkaidoUniversityinJapanarecreatingapowerassistdeviceforthelowerback15.Attachedatthethighandtorso,thedeviceusesEMGsensorstocontrolitselectricmotors.Avarietyofotheractiveorthosesarealsobeingdeveloped,suchasapneumaticmusclepoweredankleorthotic16.TheBerkeleyLowerExtremityExoskeletonBLEEXprojecthasdevelopedanenergeticallyautonomousexoskeletoncapableofcarryingitsownweightplusanexternalpayload.Allpreviousexoskeletonsareeithertetheredtoafixedpowersupplyornotstrongenoughtocarryanexternalload.Also,BLEEXtransfersthepayloadforcestothegroundinsteadofwearer,unlikeorthosesandbraces.Tocombatthecomplexitiesinherentwithcreatingawalkingexoskeleton,theBLEEXprojectdevelopedanovelcontrolscheme,whicheliminatesmeasurementsofthehumanorhumaninteractionwiththerobot.III.EXOSKELETONCONTROLTheBLEEXcontrolalgorithmensuresthattheexoskeletonshadowstheoperatorwithminimalinteractionforcesbetweenthetwo.Additionally,thecontrolschemeneedsnodirectmeasurementsfromtheoperatororwheretheoperatorcontactstheexoskeletone.g.noforcesensorsbetweenthetwoinstead,thecontrollerestimates,basedonmeasurementsfromtheexoskeletononly,howtomovesuchthatthepilotfeelsverylittleforce17.Thecontrolmethodeliminatestheproblemsassociatedwithmeasuringinteractionforcesorhumanmuscleactivity.ThebasicprincipleforthecontrolofBLEEXrestsonthenotionthattheexoskeletonneedstoshadowthewearersvoluntaryandinvoluntarymovementsquickly,andwithoutdelay.Thisrequiresahighlevelofsensitivityinresponsetoallforcesandtorquesthepilotimposesontheexoskeleton.TheBLEEXcontrolincreasestheclosedloopsystemsensitivitytotheoperatorsforcesandtorquesbymeasuringvariablesonlyfromBLEEX17.TheBLEEXcontrolschemedoeshavetworealisticconcerns.First,anexoskeletonwithhighsensitivitytoexternalforcesrespondstoexternalforceswhetherornottheyarefromtheoperator.Forexample,ifsomeonepushedagainstanexoskeletonthathadhighsensitivity,itwouldmoveasiftheforceswerefromitsoperator.Thekeytostabilizingtheexoskeletonandpreventingitfromfallinginresponsetoexternalforcesdependsontheoperatorsabilitytomovequicklye.g.stepbackorsidewaystocreateastablesituationforherself/himselfandtheexoskeleton.Forthis,averywidecontrolbandwidthisneededsotheexoskeletoncanrespondtoboththeoperatorsvoluntaryandinvoluntarymovementsi.e.reflexes.Thesecondconcernisthatthiscontrolmethodhaslittlerobustnesstoparametervariationsandthereforerequiresarelativelygooddynamicmodelofthesystem17.IV.DESIGNARCHITECTUREFundamentaltodesigningalowerextremityexoskeletonisselectingtheoverallstructuralarchitectureofthelegs.Manydifferentlayoutsofjointsandlimbscancombinetoformafunctioningleg,butanyarchitecturegenerallyfallsintooneofafewcategoriesA.AnthropomorphicArchitectureAnthropomorphicarchitecturesattempttoexactlymatchthehumanlegFig.2.Bykinematicallymatchingthehumandegreesoffreedomandlimblengths,theexoskeletonslegpositionexactlyfollowsthehumanlegsposition.Thisgreatlysimplifiesmanydesignissues.Forexample,onedoesnothavetobeconcernedwithhuman/exoskeletoncollisions.However,onemajordifficultyisthatthejointsinhumanlegscannotbeduplicatedusingthecommonstateoftechnologyindesigningjoints.Forinstance,thehumankneedoesnotexhibitapurerotationandduplicatingallitskinematicswillresultinacomplicatedandperhapsnonrobustmechanicalsystem.Anothermajorpointofconcerninthisarchitectureisthattheexoskeletonlimblengthsmustbeequaltothehumanlimblengths.Thismeansthatfordifferentoperatorstoweartheexoskeleton,almostalltheexoskeletonlimbsmustbehighlyadjustable.Ingeneral,theanthropomorphicarchitectureiserroneouslyregardedtobethepreferredchoicebecauseitallowstheexoskeletontoattachtotheoperatorwhereverdesired.Fig.2ExamplesofAnthropomorphicArchitectureLeftandNonAnthropomorphicArchitectureRightB.NonanthropomorphicArchitectureWhilenotascommoninexoskeletondesigns,manynonanthropomorphicdevicesarehighlysuccessful,suchasbicycles.NonanthropomorphicarchitecturesopenupawiderangeofpossibilitiesforthelegdesignaslongastheexoskeletonneverinterferesorlimitstheoperatorFig.2.Oftenitisdifficulttodeveloparchitecturesignificantlydifferentfromahumanlegthatcanstillmovethefootthroughallthenecessarymaneuverse.g.turningtightcornersanddeepsquats.Safetyissuesbecomemoreprominentwithnonanthropomorphicdesignssincetheexoskeletonmustbepreventedfromforcingtheoperatorintoaconfigurationtheycannotreach.Anotherproblemwiththisarchitectureisthattheexoskeletonlegsmaycollidewiththehumanlegsorexternalobjectsmoreoftenbecausetheexoskeletonjointsarenotlocatedinthesameplaceasthehumanjoints.C.PseudoanthropomorphicFormaximumsafetyandminimumcollisionswiththeenvironment,theBLEEXprojectchoseanarchitecturethatisalmostanthropomorphic.ThismeanstheBLEEXlegiskinematicallysimilartoahumans,butdoesnotincludeallofthedegreesoffreedomofhumanlegs.Additionally,theBLEEXdegreesoffreedomareallpurelyrotaryjoints.Sincethehumanandexoskeletonlegkinematicsarenotexactlythesamemerelysimilar,thehumanandexoskeletonareonlyrigidlyconnectedattheextremitiesfeetandtorso.Anyotherrigidconnectionswouldleadtolargeforcesimposedontheoperatorduetothekinematicdifferences.However,compliantconnections,allowingrelativemotionbetweenthehumanandexoskeleton,aretolerable.AnotherbenefitofnotexactlymatchingthehumankinematicsisthatBLEEXiseasiertosizeforvariousoperators.V.DEGREESOFFREEDOMSinceBLEEXispseudoanthropomorphic,ithaship,knee,andanklejointslikeahuman,butthedetailsofthesejointsdifferfromahuman.Overall,BLEEXhassevendistinctdegreesoffreedomperleg3degreesoffreedomatthehip1degreeoffreedomatthekneepurerotationinthesagittalplane3degreesoffreedomattheankleThehumanhipisaballandsocketjointwiththreedegreesoffreedom18.Itisnaturaltodesignathreedegreeoffreedomexoskeletonhipjointsuchthatallthreeaxesofrotationpassthroughthehumanballandsockethipjoint.However,throughthedesignofseveralmockupsandexperiments,welearnedthatthesedesignshavelimitedrangesofmotionandresultinsingularitiesatsomehumanhippostures.Therefore,thehiprotationjointforbothlegswaschosentobeasingleaxisofrotationbehindtheperson,asshowninFig.3thusitnolongerpassesthroughthehumanshipjoint.Additionally,analternativerotationjointwasaddeddirectlyaboveeachexoskeletonlegfortestingpurposes.Bothhipabduction/adductionandflexion/extensionaxespassthroughthehumanhipjoint.Fig.3BLEEXHipDegreesofFreedomviewedfromback.Althoughboththeabduction/adductionandflexion/extensionaxespassthroughthecenterofthehumanhipjoint,therotationaxisdoesnot.Theadjustmentbracket,betweenthetwoabduction/adductionaxes,isreplaceabletoaccommodatewearersofvariouswidths.Thehumankneejointisacomplexcombinationofrollingandslidingbetweenthefemurandtibiawhichallowsthejointscenterofrotationtomoveasthekneeflexes18.ChoosingapurerotaryjointfortheBLEEXkneeleadstosimplicityandrobustness,inadditiontomorestraightforwarddynamicmodeling,butcausestheexoskeletonkneetovaryfromthehumansknee.Also,theBLEEXkneelacksthehumankneesabilitytolockoutthelegbecauseitdoesnothavethemovingcenterofrotation.Likethehumansankle,theBLEEXanklehasthreedegreesoffreedom.Theflexion/extensionaxiscoincideswiththehumanankleflexion/extensionaxis.Fordesignsimplification,theabduction/adductionandrotationaxesontheBLEEXankledonotpassthroughthehumansfootandformaplaneoutsideofthehumansfootFig.4.AnadditionaldegreeoffreedomisaddedtotheBLEEXfoot.Thefrontoftheexoskeletonfoot,undertheoperatorstoes,iscomplianttoallowtheexoskeletonfoottoflexwiththehumansfootseeSectionIX.B.Fig.4BLEEXAnkleDegreesofFreedom.Onlytheflexion/extensionaxispassesthroughthehumansanklejoint.Abduction/adductionandrotationaxesarenotpowered,butareequippedwithappropriateimpedances.VI.RANGEOFMOTIONTheBLEEXkinematicsareclosetohumankinematics,sotheBLEEXjointrangesofmotionaredeterminedbyexamininghumanjointrangesofmotion.Attheveryleast,theBLEEXjointrangeofmotionshouldbeequaltothehumanrangeofmotionduringwalkingshownincolumn1inTable1,whichcanbefoundbyexaminingClinicalGaitAnalysisCGAdata19–21.SafetydictatesthattheBLEEXrangeofmotionshouldnotbemorethantheoperatorsrangeofmotionshowninColumn3ofTable122.Foreachdegreeoffreedom,thesecondcolumnofTable1liststheBLEEXrangeofmotionwhichis,ingeneral,largerthanthehumanrangeofmotionduringwalkingandlessthanthemaximumrangeofhumanmotion.TABLE1BLEEXJOINTRANGESOFMOTIONHumanWalkingMaximumBLEEXMaximumAverageMilitaryMaleMaximumAnkleFlexion14.1°45°35°AnkleExtension20.6°45°38°AnkleAbductionnotavailable20°23°AnkleAdductionnotavailable20°24°KneeFlexion73.5°121°159°HipFlexion32.2°121°125°HipExtension22.5°10°notavailableHipAbduction7.9°16°53°HipAdduction6.4°16°31°TotalRotationExternal13.2°35°73°TotalRotationInternal1.6°35°66°Ideally,toarriveatthemostmaneuverableexoskeleton,onedesirestohaveasystemwithrangesofmotionslightlylessthanthehumansmaximumrangeofmotion.However,BLEEXuseslinearactuatorsseeSectionVIII,sosomeofthejointrangesofmotionarereducedtopreventtheactuatorsaxesofmotionfrompassingthroughthejointcenter.Ifthiswasnotprevented,thejointcouldreachaconfigurationweretheactuatorwouldbeunabletoproduceatorqueaboutitsjoint.Additionally,allthejointrangesofmotionweretestedandrevisedduringprototypetestingFig.5.Forexample,mockuptestingdeterminedthattheBLEEXankleflexion/extensionrangeofmotionneedstobegreaterthanthehumananklerangeofmotiontoaccommodatethehumanfootssmallerdegreesoffreedomnotmodeledintheBLEEXfoot.Fig.5BLEEXMockupusedtotestandrevisetheBLEEXdegreesoffreedom,rangesofmotion,andergonomics.TheseprototypeswerebuiltonaFusedDepositionModelingFDMmachine.VII.WHICHJOINTSTOACTUATEEachBLEEXleghassevendegreesoffreedomeightcountingthetoeflexibility,butactuatingthemallleadstounnecessarilyhighpowerconsumptionandcontrolcomplexity.Instead,onlyjointsthatrequiresubstantialpowershouldbeactuated.Asafirststep,theactuationwasdesignedprimarilyforwalking,soCGAdatawasusedtodeterminewhichdegreesoffreedomconsumepowerwhilewalking.Asexpected,thehighestamountofpowerisconsumedforflexion/extensionattheankle,knee,andhip18,19–21,Fig.6.Theankleandhipbothrequiresignificantpositivepowerandthusneedtobeactuated.Thekneerequiresmainlynegativepoweritabsorbspowerwhilewalkinghowever,whenclimbingstepsandslopes,orsquatting,thekneebecomescriticalforaddingpositivepowertothesystem23Fig.7.Therefore,thekneejointisalsoactuated.Besidestheflexion/extensionjoints,hipabduction/adductionrequiresthemostpowerduringwalkingsinceitprovidesthelateralbalancingforcesthus,theBLEEXhipabduction/adductionjointisactuated.AccordingtoCGAdata,theotherdegreesoffreedomhiprotation,anklerotation,andankleabduction/adductionallhaveverysmallpowerconsumptionswhilewalkingandthusremainunactuatedFig.6.Fig.8summarizesallofthedegreesoffreedomchosenfortheBLEEXandindicateswhichofthesejointsarepowered.Theunactuatedjointsstillmayhavesprings,orotherimpedances,toreducetheloadonhumanmusclesandincreasecomfort.VIII.ACTUATORSELECTIONBLEEXiscompletelyautonomous,carryingitsownpowersource,sopowerconservationiscriticalformissionduration.Everyeffortwasmadetokeeptheexoskeletonlegs,andactuators,compactandlightweighttoreducetheexoskeletonspowerconsumption.Additionally,thepowerefficiencyoftheactuationsystemiscrucial.