ProceedingsoftheSeventhInternationalConferenceonMachineLearningandCybernetics,Kunming,12-15July2008978-1-4244-2096-4/08/$25.002008IEEE3800ANOVELSPHERICALWHEELDRIVENBYOMNIWHEELSCHIA-WENWU1,CHI-KUANGHWANG21InstituteofEngineeringandScience,ChungHuaUniversity,Hsing-Chu,Taiwan,R.O.C.2Dept.ofElectricalEngineering,ChungHuaUniversity,Hsing-Chu,Taiwan,R.O.C.E-MAIL:.tw,.twAbstract:AnovelcombinationofOmniwheelandsphericalwheelunit(CWWU)ispresentedinthispaper,anditsoperatingprincipleisbasicallyasphericalwheeldrivenbytwoperpendicularpairsofOmniwheels.Thus,aCWWUbaserobotcanperformthestaticanddynamicbalancingfunctionthatasinglewheelwithinversemouse-balldrivedevelopedbyCarnegieMellonUniversity(CMU)canachieve.Thepreviousrolling/balancingmachinescannotimmediatelydriveinagivendirectionwithoutfirstre-orientingthedrivemechanism.ThemajorfeatureofthepresentedCWWUbaserobotandtheCMUrobotisthatbothofthemcanmovedirectlyinanydirection.AscomparedtheOmniwheelbaserobotwiththeproposedone,themaindrawbackoftheOmniwheelrobotisthecancellationofoppositeportionofthedrivingforces.Moreover,theCWWUbaserobothasseveralexclusivefeatureswhichcanresolvesomeproblemsinheritedintheCMUrobot.Especially,thesimultaneousdemandofbothahigh-frictionandlow-frictionmaterialforthesphericalballisacompromiseneededtomakefortheCMUrobot,butnotfortheproposedCWWUbaserobot.Keywords:CWWU；Omniwheel；Sphericalwheel1.IntroductionIn1994,atwo-wheeledrobotusingtheinversependulumcontroltoeliminatethethirdcastorwheelhasbeendevelopedinJapan1.Theyalsopresentedaone-wheeledbalancerobotinwhichtheellipsoidshapewheelisadoptedanddrivenwithanaxlealongthemajoraxis2.Recently,thetwo-wheelRMProboticplatformsbasedontheSegwayaretheinterestingresearchtopicinroboticlocomotion3.Thereafter,asinglewheelwithinversemouse-balldrivecanachievethestaticanddynamicstabilityhasbeendevelopedbyCarnegieMellonUniversity(CMU)4-5.Theoveralldesignofthesystem,suchasactuatormechanismandcontrolsystemispresented.Thensomeinitialresultsincludingdynamicbalancing,stationkeeping,andpoint-to-pointmotionarealsodiscussed.Mostofall,theirpaperspointedoutthatunlikebalancing2-wheelplatformswhichmustturnbeforedrivinginanydirection,andthesingle-wheelcanmovedirectlyinanydirection.Therefore,theyarethefirstgrouptoproposeabalancingrollingmachinewhosebodyissupportedbyasingleOmni-directionalsphericalwheel.However,fortheCMUrobot,theconflictdemandofbothahigh-frictionandlow-frictionmaterialatthesametimeforthesphericalballbecomesthemajorconcerntobecompromised.TheworkingprincipleofthepresentedCWWUinthepapercanbevirtuallyexpressedasasphericalwheeldrivenbytwoperpendicularpairsofOmniwheels.EachpairhastoparallelOmniwheelslinkedtothesameactuator.ItindicatesthattheCWWUneedstwoactuators.Themotivationofthepresentedoneisinitiallyinspiredbythesinglewheelwithinversemouse-balldrivedevelopedbyCMU,sotheirstaticanddynamicbalancingabilityisalsooneofimportantcharacteristicsofthepresentedone.However,forthepresentedone,theweightorloadofrobotcanbedirectlytransferredtothecontactsurfacewhichisthecrucialpointrelatedtothenormalcontactforcebetweenthedrivingpartandthesphericalwheelinthesimilarscheme.Thereby,theapplicabilityandrobustnessoftheinventedonecanbegreatlyenrichedbythisdistinctandparticulartrait.Thepresentedoneisalsoinsensitivetothedust,andthesphericalwheelcanbemadebytherubberwhichissamematerialasthecartire.Theabovetwoimportantfeaturessuggestthatthepresentedoneassembledwitharubbersphericalwheelcanbepromisinglyadoptedintotheautomobileindustry.Moreover,thehighefficiencyandbetterbalanceofdrivingforceassociatedwiththepresentedonecanimproveitsquality.2.SystemDescriptionSingleOmniwheel,asshowninFigure1,hasoneseriousnobrakingability.Therefore,multi-Omni-wheeledrobotsarepopularlyadoptedanddeveloped.Usually,therobotformedbythemulti-Omni-wheelhasonemajordisadvantage,thatis,thelowefficiencyofdrivingforce.Ifastraightlinepathisplannedforathree-Omni-wheelbaseProceedingsoftheSeventhInternationalConferenceonMachineLearningandCybernetics,Kunming,12-15July20083801robottofollow,thenonlyonehalfofthedrivingforcecanbeusedowingtoanequiangulartrianglearrangementofthesethreeOmniwheels.Thiskindofarrangementwillcreateundesiredcancellationofthedrivingforce,sothelowefficiencyofthedrivingforceisincurredbythecancellation.Furthermore,forthecaseoffour-Omni-wheelbaserobot,itislikelyunsuitablefornon-smoothfloororobstacles.BecausethesuspensionofoneofthesefourOmniwheelsincidentallyoccursduetounevenfloor,itcanresultinmalfunctionoftherobot.Figure1.Omniwheel(http:/www.phxhs.k12.az.us/education/club/club.php?sectiondetailid=27681and/photo/39/)DrivemechanismoftheCMUisbasicallytheinverseofamouse-balldrive4-5,thatis,rollersdrivetheballtoproducemotion,asshowninFigure2.Theballisactuatedbyapairsmoothstainlesssteelrollersplacedorthogonallyatthespheresequator.TheserollersarelinkedtotwohightorqueDCservomotorsthroughtimingbelts.Therearetwospring-loadedpassiveidlerrollerswhichareoppositethedriverollers,andidlerrollerscanapplyforceattheballsequatortomaintaincontactbetweenthedriverollersandtheball.Thisarrangementcanreducetheundersideeffectcausedbyun-avoidedslippage.Forexample,ifonerollerisbeingdriven,theorthogonalrollermustbeslipping.Thisunwantedsimultaneousdemandofbothahigh-frictionandlow-frictionmaterialfortheballbecomesacompromisetobemade.Becauseitisalwaysdesirabletohavehighfrictionbetweentheballandthefloor,thehighfrictioncanavoidtheseriousslippageonthefloortocausethedifficultytobalancetherobot,orevenworsethemalfunctionoftherobot.Therefore,theypointedoutthatthedriveworkswellbuttheinitialballeventuallyworeout.Threecommerciallowfriction,omni-directionalballtransferdevicesareadoptedtosupporttheentireBallbotbodyrestsontopoftheball.Thisdesignissmoothlytotransfertheweightorloadofrobotthroughtheballintothefloor.Figure2.CMU_BallbotEricAndAnish(/projects/ballbot/)TheCWWUistheabbreviationofthecombinationofOmniwheelandsphericalwheelunit.BasicallytheCWWUisanovelsphericalwheeldrivenbyOmniwheels,soitcombinestheadvantagesofbothtypesofwheels.Figure3depictsthedrivemechanismoftheCWWUbaserobot.TherearetwopairsofOmniwheels(AandB,CandD),andeachpairofOmniwheelsisdesignedtobedrivenbyanactuator,suchasahightorqueDCmotororStepmotorwithanencoder.ThesetwopairsofOmniwheelsarearrangedperpendicularlyeachother.ThesphericalwheelisactuatedbythesetwopairofOmniwheels.ItcanbemadesimilartotheCMUrobotinwhichtheballisa200mmdiameterhydro-formedsteelshellcoveredwitha3.2mmthickurethaneouterlayer.However,thepreferredoneismadebyrubberlikesthetireofcars.Thus,theproposedCWWUbaserobotcanbeeasilyappliedintheroboticlocomotion,anditcanworkwellintheautomobileenvironmentinthefuture.Apre-determineddistancebetweentwoOmniwheelsinthesamepaircanaffectthenormalforceexertedonthesphericalwheelbythepairofOmniwheels.Inotherwords,itreliesonwhetherthefrictionforcebetweenthesphericalwheelandthepairofOmniwheelsislargeenoughtoavoidtheundesiredslippage.TheadvantagesoftheCWWUbaserobotaresummarizedasfollows.zTherearenoidlerollersworkingintheoppositemovingdirectionlikestheCMUrobot,sotheproposedonedoesnotencountertheseriouswore-outproblemoftheball.zDemandsofbothahigh-frictionandlow-frictionmaterialfortheballisnotrequiredfortheproposedone.zFortheCMUrobot,threecommerciallowfriction,omni-directionalballtransferdevicesareadoptedtosupporttheentireBallbotbodyrestsontopoftheball.TheadditionalthelowfrictionmaysometimesbecomeProceedingsoftheSeventhInternationalConferenceonMachineLearningandCybernetics,Kunming,12-15July20083802seriouswhentheweightorloadoftherobotisincreasingsignificantly.zBothOmniwheelsinthesamepairaredesignedtodrivethesphericalballsimultaneouslyunliketheCMUrobotwhichonlyonedriveroller.SotheefficiencyandbalanceofdrivingforceoftheCWWUrobotisbetterthanthatoftheCMUone.zThedrivingforcealwaysexertsonthesphericalwheelalongthetangentdirection,sothemaximumtorquecanbeattainedbytheproposedone.zThecontactsurfacebetweensphericalwheelanddrivingpairofOmniwheelsislargerthanthatoftheCMUrobot.Sotheundesiredslippagecanbemigratedeasilybytheproposedone.zThesmallidlewheelsoftheOmniwheeltaketurntocontactthesphericalwheel,sotheproposedoneiscapableofreducingtheunwantedeffectofdustascomparedwiththeCMUrobot.Inotherwords,theproposedonecanbeappliedtodustyenvironments.zTheweightofrobotcanbeeffectivelytransferredintothenormalcontactforcebetweenthesphericalwheelandthedrivingOmniwheelpairs.Thiscontactforceisessentialtoreducethepossibilityofundesiredslippage.zDifferentloadingscanbeeasilyadoptedbytheproposedone.Iftheloadingisincreasing,thenthecontactforceisalsoproportionallyincreasing.zAsmentionedabove,apre-determineddistancebetweentwoOmniwheelsinthesamepaircanaffectthenormalforceexertedonthesphericalwheelbythepairofOmniwheels.Itnotesthattheshorterpre-determineddistanceis,thehigherthenormalforceis.Therefore,differentapplicationscanbedesignedflexiblybasedonvariousloadingoftherobotwithasuitabledistance.zThesphericalcanbemadebyrubberlikesthematerialoftireusingbycars.Itimpliesthattheproposedonecannotonlybeeasilyappliedintheroboticlocomotion,butalsoworkwellintheautomobileenvironmentinthenearfuture.However,theCMUrobotissosensitivetothedeformationofitsballwhichmayshiftthetightcontactsurfacebetweenrollerandtheballtobenon-tightone.Moreover,intheworstcase,aseriousdeformationcancausethemalfunctionoftherobot.zTheproposedonehasgoodbrakingability,anditabilitycanbeenhancedbythepre-determineddistance.Incaseofincreasingtheweightorloadofrobot,therequirementbrakingabilityisalsoincreasing.Fortunately,thebrakingabilityisalsoupgradedproportionallytoweightorloadfortheproposedone.Figure3.ThemechanismofaCWWU3.ConclusionsAnovelcombinationofOmniwheelandsphericalwheelunit(CWWU)ispresentedinthispaper,anditsworkingprinciplebasicallycanbedescribedasasphericalwheeldrivenbytwopairsofOmniwheels.Themotivationoftheinventionisinspiredbythesinglewheelwithinversemouse-balldrivedevelopedbyCMU,sotheproposedonealsopossessesthestaticanddynamicbalancingability.Previousrolling/balancingmachinescannotimmediatelydriveinagivendirectionwithoutfirstre-orientingthedrivemechanism.ThemajorfeatureofthepresentedCWWUbaserobotandtheCMUrobotisthatbothofthemcanmovedirectlyinanydirection.However,fortheproposedone,theweightorloadofrobotcanbedirectlyutilizedastheimportantnormalcontactforcebetweenthedrivingpartandthesphericalwheel.Thisfeaturecangreatlyenrichtheapplicabilityandrobustnessoftheinventedone.Forexample,thesphericalwheelcanbemadebythesamematerialasthecartire.Additionally,itisalsoinsensitivetothedust,sotheCWWUcanbepromisinglyappliedtotheautomobileindustryinnearfuture.Therearenoidlewheelsworkingintheoppositemovingdirection,sotheconflictfrictionrequirementscanbediminishedbytheproposedone.Th