MCBIndustrialRobotFeatureArticleTheBarrettHandgrasperprogrammableflexibleparthandlingandassemblyAbstractThispaperdetailsthedesignandoperationoftheBarrettHandBH8-250,anintelligent,highlyflexibleeight-axisgripperthatreconfiguresitselfinrealtimetoconformsecurelytoawidevarietyofpartshapeswithouttool-changeinterruptions.Thegrasperbringsenormousvaluetofactoryautomationbecauseit:reducestherequirednumberandsizeofroboticworkcells(whichaverageUS$90,000eachnotincludingthehighcostoffootprint)whileboostingfactorythroughput；consolidatesthehodgepodgeproliferationofcustomizedgripper-jawshapesontoacommonprogrammableplatform；andenablesincrementalprocessimprovementandaccommodatesfrequentnew-productintroductions,capabilitiesdeployedinstantlyviasoftwareacrossinternationalnetworksoffactories.IntroductionThispaperintroducesanewapproachtomaterialhandling,partsorting,andcomponentassemblycalled“grasping”,inwhichasinglereconfigurablegrasperwithembeddedintelligencereplacesanentirebankofunique,fixed-shapegrippersandtoolchangers.ToappreciatethemotivationsthatguidedthedesignofBarrettsgrasper,wemustexplorewhatiswrongwithroboticstoday,theenormouspotentialforroboticsinthefuture,andthedead-endlegacyofgrippersolutions.Forthebenefitsofaroboticsolutiontoberealized,programmableflexibilityisrequiredalongtheentirelengthoftherobot,fromitsbase,allthewaytothetargetworkpiece.Arobotarmenablesprogrammableflexibilityfromthebaseonlyuptothetoolplate,afewcentimetersshortofthetargetworkpiece.Buttheselastfewcentimetersofarobotmustadapttothecomplexitiesofsecuringanewobjectoneachrobotcycle,capabilitieswhereembeddedintelligenceandsoftwareexcel.Liketheweakestlinkinaserialchain,aninflexiblegripperlimitstheproductivityoftheentirerobotworkcell.Grippershaveindividually-customized,butfixedjawshapes.Thetrial-and-errorcustomizationprocessisdesignintensive,generallydrivescostandschedule,andisdifficulttoscopeinadvance.Ingeneral,eachanticipatedvariationinshape,orientation,androbotapproachanglerequiresanothercustom-but-fixedgripper,aplacetostoretheadditionalgripper,andamechanismtoexchangegrippers.Anunanticipatedvariationorincrementalimprovementissimplynotallowable.Bycontrast,themechanicalstructureofBarrettspatentedgrasper,illustratedinFigure1,isautomaticallyreconfigurableandhighlyprogrammable,matchingthefunctionalityofvirtuallyanygrippershapeorfixturefunctioninlessthanasecondwithoutpausingtheworkcellthroughputtoexchangegrippers.Fortasksrequiringahighdegreeofflexibilitysuchashandlingvariablyshapedpayloadspresentedinmultipleorientations,agrasperismoresecure,quickertoinstall,andmorecosteffectivethananentirebankofcustom-machinedgripperswithtoolchangersandstorageracks.Foruninterruptedoperation,justoneortwosparegrasperscanserveasemergencybackupsforseveralworkcells,whereasoneortwosparegrippersarerequiredforeachgrippervariationpotentiallydozensperworkcell.And,itscatastrophicifbothgripperbackupsfailinagrippersystem,sinceitmaybedaysbeforereplacementscanbeidentified,customshapedfromscratch,shipped,andphysicallyreplacedtobringtheaffectedlinebackintooperation.Bycontrast,sincegraspersarephysicallyidentical,theyarealwaysavailableinunlimitedquantity,withallcustomizationprovidedinstantlyinsoftware.GripperlegacyMostoftodaysroboticparthandlingandassemblingisdonewithgrippers.Ifsurfaceconditionsallow,vacuumsuctionandelectromagnetscanalsobeused,forexampleinhandlingautomobilewindshieldsandbodypanels.Aspartsizesbegintoexceedtheorderof100gms,agrippersjawsarecustomshapedtoensureasecurehold.Asthedurablemainstayofhandlingandassembly,thesetoolshavechangedlittlesincethebeginningofroboticsthreedecadesago.Grippers,whichactassimplepincers,havetwoorthreeunarticulatedfingers,called“jaws”,whicheitherpivotorremainparallelduringopen/closemotionsasillustratedinFigure2.Wellorganizedcatalogsareavailablefrommanufacturersthatguidetheintegratororcustomerinmatchingvariousgrippercomponents(exceptnaturallyforthecustomjawshape)tothetaskandpartparameters.Payloadsizesrangefromgramsfortinypneumaticgrippersto100+kilogramsformassivehydraulicgrippers.Thepowersourceistypicallypneumaticorhydraulicwithsimpleon/offvalvecontrolswitchingbetweenfull-openandfull-closestates.Thejawstypicallymove1cmfromfull-opentofull-close.Thesehandshavetwoorthreefingers,called“jaws”.Thepartofthejawthatcontactsthetargetpartismadeofaremovableandmachineablysoftsteeloraluminum,calleda“softjaw”.Basedontheuniquecircumstances,anexperttooldesignerdeterminesthecustomshapestobemachinedintotherectangularsoft-jawpieces.Oncemachinedtoshape,thesoft-jawsetsareattachedtotheirrespectivegripperbodiesandtested.Thisprocesscantakeanynumberofiterationsandadjustmentsuntilthesystemworksproperly.Tooldesignersrepeattheentireprocesseachtimeanewshapeisintroduced.Asconsumersdemandawidervarietyofproductchoicesandevermorefrequentproductintroductions,theneedforflexibleautomationhasneverbeengreater.However,ratherthanmakegrippersmoreversatile,theroboticsindustryoverthepastfewyearshasfollowedtheexampleoftheautomatictoolexchangetechniqueusedtoexchangeCNC-millcuttingtools.Butapplyingthetool-changermodeltoserial-linkrobotsisprovingexpensiveandineffective.Unlikethestandardizedoff-the-shelfcuttingtoolsusedbymillingmachines,arobottooldesignermustcustomizetheshapeofeverysetofgripperjawsatime-consuming,expensive,anddifficult-to-scopetask.AlthoughgrippersmayseemcheapatonlyUS$500each,thelabor-intensiveefforttoshapethesoftjawsmaycostseveraltimesthat.Ifyoumultiplythatcosttimesadozengrippersasintheexampleaboveandthrowinatoolchangerandtool-storagerackforanadditionalUS$10,000,therealcostofthe“few-hundred-dollar”grippersolutionballoonstoUS$20,000toUS$60,000.Toaggravatematters,unknownsinthecustomizationprocessconfoundaccuratecostprojections.Sothecustomermustcommitapurchaseordertotheinitialinstallationfeeonatimeandmaterialsbasiswithoutguaranteeofsuccessoracostceiling.WhilepricedatUS$30,000,intelligentgraspersarenotcheap.However,onecan“customize”andvalidatetheprocessinsoftwareinamatterofhoursatthefactoryinasingleday.Ifthesystemdoesnotmeetperformancetargets,thenonlyadayslaboriswasted.Ifthesystemsucceeds,thentherearenotanyhiddenexpensesfollowingtheoriginalpurchaseorder.Beyondcost,thephysicalweightoftoolchangermechanisms,locatedattheextremeouterendofaserial-linkroboticarm,limitstheusefulpayloadanddynamicresponseoftheentiresystem.Theadditionallengthofthetoolchangerincreasesthecriticaldistancebetweenthewristcenterandpayloadcenter,degradingkinematicflexibility,dynamicresponse,andsafety.DescriptionoftheBarrettHandFlexibilityanddurabilityinacompactpackageTheflexibilityoftheBarrettHandisbasedonthearticulationoftheeightjointaxesidentifiedinFigure3.OnlyfourbrushlessDCservomotors,showninFigure4,areneededtocontrolalleightjoints,augmentedbyintelligentmechanicalcoupling.Theresulting1.18kggrasperiscompletelyself-containedwithonlyan8mmdiameterumbilicalcablesupplyingDCpowerandestablishingatwo-wayserialcommunicationlinktothemainrobotcontrolleroftheworkcell.Thegrasperscommunicationselectronics,fivemicroprocessors,sensors,signalprocessingelectronics,electroniccommutation,currentamplifiers,andbrushlessservomotorsareallpackedneatlyinsidethepalmbodyofthegrasper.TheBarrettHandhasthreearticulatedfingersandapalmasillustratedinFigure5whichactinconcerttotrapthetargetobjectfirmlyandsecurelywithinagraspconsistingofsevencoordinatedcontactvectorsonefromthepalmplateandonefromeachlinkofeachfinger.EachoftheBarrettHandsthreefingersisindependentlycontrolledbyoneofthreeservomotorsasshowninFigure6.ExceptforthespreadactionoffingersFlandF2,whichisdrivenbythefourthandlastservomotor,thethreefingers,Fl,F2,andF3,haveinnerandouterarticulatedlinkswithidenticalmechanicalstructure.Eachofthethreefingermotorsmustdrivetwojointaxes.Thetorqueischanneledtothesejointsthroughapatented,TorqueSwitchmechanism(Figure7),whosefunctionisoptimizedformaximumgraspsecurity.Whenafingertip,nottheinnerlink,makesfirstcontactwithanobjectasillustratedinFigure8,itsimplyreachesitsrequiredtorque,locksbothjoints,switchesoffmotorcurrents,andawaitsfurtherinstructionsfromthemicroprocessorsinsidethehandoracommandarrivingacrossthecommunicationslink.Butwhentheinnerlink,asillustratedinFigure9,makesfirstcontactwithanobjectforasecuregrasp,theTorqueSwitch,reachesapresetthresholdtorque,locksthatjointagainsttheobjectwithashallow-pitchworm,andredirectsalltorquetothefingertiptomakeasecond,enclosingcontactagainsttheobjectwithinmillisecondsofthefirstcontact.Thesequenceofcontactsissorapidthatyoucannotvisualizetheprocesswithouttheaidofhigh-speedphotography.Afterthegrasperreleasestheobject,itsetstheTorqueSwitchthresholdtorqueforeachfingerinanticipationofthenextgraspbyopeningeachfingeragainstitsmechanicalstopwithacontrolledtorque.Thehighertheopeningtorque,thehigherthesubsequentthresholdtorque.Inthisway,thegraspercanaccommodateawiderangeofobjectsfromdelicate,tocompliant,toheavy.Thefingerarticulations,notavailableonconventionalgrippers,alloweachdigittoconformuniquelyandsecurelytotheshapeoftheobjectsurfacewithtwoindependentcontactpointsperfinger.Theposition,velocity,acceleration,andeventorquecanallbeprocessorcontrolledoverthefullrangeof17,500encoderpositions.Atmaximumvelocityandaccelerationsettings,eachfingercantravelfullrangeineitherdirectioninlessthanonesecond.Themaximumforcethatcanbeactivelyproducedis2kg,measuredatthetipofeachfinger.Oncethegraspissecure,thelinksautomaticallylockinplaceallowingthemotorcurrentstobeswitchedofftoconservepoweruntilcommandedtoreadjustorreleasetheirgrasp.Whiletheinnerandouterfinger-linkmotionscurlanthropomorphically,thespreadmotionofFigure10isdistinctlynon-anthropomorphic.Thespreadmotionisclosestinfunctiontoaprimatesopposable(thumb)finger,butinsteadofoneopposablefinger,theBarrettHandhastwin,symmetricallyopposablefingerscenteredonparalleljointaxesrotating180degreesaroundtheentirepalmtoformalimitlessvarietyofgripper-shapesandfixturefunctions.Thespreadcanbecontrolledtoanyof3,000positionsoveritsfullrangeineitherdirectionwithin1/2second.Unlikethemechanicallylockablefinger-curlmotions,thespreadmotionisfullybackdrivable,allowingitsservostoprovideactivestiffnesscontrolinadditiontocontroloverposition,velocity,acceleration,andtorque.Byallowingthespreadmotiontobecompliantwhilethefingersclosearounda