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MCB–IndustrialRobotFeatureArticleTheBarrettHandgrasper–programmableflexibleparthandlingandassemblyAbstractThispaperdetailsthedesignandoperationoftheBarrettHandBH8250,anintelligent,highlyflexibleeightaxisgripperthatreconfiguresitselfinrealtimetoconformsecurelytoawidevarietyofpartshapeswithouttoolchangeinterruptions.ThegrasperbringsenormousvaluetofactoryautomationbecauseitreducestherequirednumberandsizeofroboticworkcellswhichaverageUS90,000each–notincludingthehighcostoffootprintwhileboostingfactorythroughputconsolidatesthehodgepodgeproliferationofcustomizedgripperjawshapesontoacommonprogrammableplatformandenablesincrementalprocessimprovementandaccommodatesfrequentnewproductintroductions,capabilitiesdeployedinstantlyviasoftwareacrossinternationalnetworksoffactories.IntroductionThispaperintroducesanewapproachtomaterialhandling,partsorting,andcomponentassemblycalledgrasping,inwhichasinglereconfigurablegrasperwithembeddedintelligencereplacesanentirebankofunique,fixedshapegrippersandtoolchangers.ToappreciatethemotivationsthatguidedthedesignofBarrettsgrasper,wemustexplorewhatiswrongwithroboticstoday,theenormouspotentialforroboticsinthefuture,andthedeadendlegacyofgrippersolutions.Forthebenefitsofaroboticsolutiontoberealized,programmableflexibilityisrequiredalongtheentirelengthoftherobot,fromitsbase,allthewaytothetargetworkpiece.Arobotarmenablesprogrammableflexibilityfromthebaseonlyuptothetoolplate,afewcentimetersshortofthetargetworkpiece.Buttheselastfewcentimetersofarobotmustadapttothecomplexitiesofsecuringanewobjectoneachrobotcycle,capabilitieswhereembeddedintelligenceandsoftwareexcel.Liketheweakestlinkinaserialchain,aninflexiblegripperlimitstheproductivityoftheentirerobotworkcell.Grippershaveindividuallycustomized,butfixedjawshapes.Thetrialanderrorcustomizationprocessisdesignintensive,generallydrivescostandschedule,andisdifficulttoscopeinadvance.Ingeneral,eachanticipatedvariationinshape,orientation,androbotapproachanglerequiresanothercustombutfixedgripper,aplacetostoretheadditionalgripper,andamechanismtoexchangegrippers.Anunanticipatedvariationorincrementalimprovementissimplynotallowable.Bycontrast,themechanicalstructureofBarrettspatentedgrasper,illustratedinFigure1,isautomaticallyreconfigurableandhighlyprogrammable,matchingthefunctionalityofvirtuallyanygrippershapeorfixturefunctioninlessthanasecondwithoutpausingtheworkcellthroughputtoexchangegrippers.Fortasksrequiringahighdegreeofflexibilitysuchashandlingvariablyshapedpayloadspresentedinmultipleorientations,agrasperismoresecure,quickertoinstall,andmorecosteffectivethananentirebankofcustommachinedgripperswithtoolchangersandstorageracks.Foruninterruptedoperation,justoneortwosparegrasperscanserveasemergencybackupsforseveralworkcells,whereasoneortwosparegrippersarerequiredforeachgrippervariation–potentiallydozensperworkcell.And,itscatastrophicifbothgripperbackupsfailinagrippersystem,sinceitmaybedaysbeforereplacementscanbeidentified,customshapedfromscratch,shipped,andphysicallyreplacedtobringtheaffectedlinebackintooperation.Bycontrast,sincegraspersarephysicallyidentical,theyarealwaysavailableinunlimitedquantity,withallcustomizationprovidedinstantlyinsoftware.GripperlegacyMostoftodaysroboticparthandlingandassemblingisdonewithgrippers.Ifsurfaceconditionsallow,vacuumsuctionandelectromagnetscanalsobeused,forexampleinhandlingautomobilewindshieldsandbodypanels.Aspartsizesbegintoexceedtheorderof100gms,agrippersjawsarecustomshapedtoensureasecurehold.Asthedurablemainstayofhandlingandassembly,thesetoolshavechangedlittlesincethebeginningofroboticsthreedecadesago.Grippers,whichactassimplepincers,havetwoorthreeunarticulatedfingers,calledjaws,whicheitherpivotorremainparallelduringopen/closemotionsasillustratedinFigure2.Wellorganizedcatalogsareavailablefrommanufacturersthatguidetheintegratororcustomerinmatchingvariousgrippercomponentsexceptnaturallyforthecustomjawshapetothetaskandpartparameters.Payloadsizesrangefromgramsfortinypneumaticgrippersto100kilogramsformassivehydraulicgrippers.Thepowersourceistypicallypneumaticorhydraulicwithsimpleon/offvalvecontrolswitchingbetweenfullopenandfullclosestates.Thejawstypicallymove1cmfromfullopentofullclose.Thesehandshavetwoorthreefingers,calledjaws.Thepartofthejawthatcontactsthetargetpartismadeofaremovableandmachineablysoftsteeloraluminum,calledasoftjaw.Basedontheuniquecircumstances,anexperttooldesignerdeterminesthecustomshapestobemachinedintotherectangularsoftjawpieces.Oncemachinedtoshape,thesoftjawsetsareattachedtotheirrespectivegripperbodiesandtested.Thisprocesscantakeanynumberofiterationsandadjustmentsuntilthesystemworksproperly.Tooldesignersrepeattheentireprocesseachtimeanewshapeisintroduced.Asconsumersdemandawidervarietyofproductchoicesandevermorefrequentproductintroductions,theneedforflexibleautomationhasneverbeengreater.However,ratherthanmakegrippersmoreversatile,theroboticsindustryoverthepastfewyearshasfollowedtheexampleoftheautomatictoolexchangetechniqueusedtoexchangeCNCmillcuttingtools.Butapplyingthetoolchangermodeltoseriallinkrobotsisprovingexpensiveandineffective.Unlikethestandardizedofftheshelfcuttingtoolsusedbymillingmachines,arobottooldesignermustcustomizetheshapeofeverysetofgripperjawsatimeconsuming,expensive,anddifficulttoscopetask.AlthoughgrippersmayseemcheapatonlyUS500each,thelaborintensiveefforttoshapethesoftjawsmaycostseveraltimesthat.IfyoumultiplythatcosttimesadozengrippersasintheexampleaboveandthrowinatoolchangerandtoolstoragerackforanadditionalUS10,000,therealcostofthefewhundreddollargrippersolutionballoonstoUS20,000toUS60,000.Toaggravatematters,unknownsinthecustomizationprocessconfoundaccuratecostprojections.Sothecustomermustcommitapurchaseordertotheinitialinstallationfeeonatimeandmaterialsbasiswithoutguaranteeofsuccessoracostceiling.WhilepricedatUS30,000,intelligentgraspersarenotcheap.However,onecancustomizeandvalidatetheprocessinsoftwareinamatterofhoursatthefactoryinasingleday.Ifthesystemdoesnotmeetperformancetargets,thenonlyadayslaboriswasted.Ifthesystemsucceeds,thentherearenotanyhiddenexpensesfollowingtheoriginalpurchaseorder.Beyondcost,thephysicalweightoftoolchangermechanisms,locatedattheextremeouterendofaseriallinkroboticarm,limitstheusefulpayloadanddynamicresponseoftheentiresystem.Theadditionallengthofthetoolchangerincreasesthecriticaldistancebetweenthewristcenterandpayloadcenter,degradingkinematicflexibility,dynamicresponse,andsafety.DescriptionoftheBarrettHandFlexibilityanddurabilityinacompactpackageTheflexibilityoftheBarrettHandisbasedonthearticulationoftheeightjointaxesidentifiedinFigure3.OnlyfourbrushlessDCservomotors,showninFigure4,areneededtocontrolalleightjoints,augmentedbyintelligentmechanicalcoupling.Theresulting1.18kggrasperiscompletelyselfcontainedwithonlyan8mmdiameterumbilicalcablesupplyingDCpowerandestablishingatwowayserialcommunicationlinktothemainrobotcontrolleroftheworkcell.Thegrasperscommunicationselectronics,fivemicroprocessors,sensors,signalprocessingelectronics,electroniccommutation,currentamplifiers,andbrushlessservomotorsareallpackedneatlyinsidethepalmbodyofthegrasper.TheBarrettHandhasthreearticulatedfingersandapalmasillustratedinFigure5whichactinconcerttotrapthetargetobjectfirmlyandsecurelywithinagraspconsistingofsevencoordinatedcontactvectorsonefromthepalmplateandonefromeachlinkofeachfinger.EachoftheBarrettHandsthreefingersisindependentlycontrolledbyoneofthreeservomotorsasshowninFigure6.ExceptforthespreadactionoffingersFlandF2,whichisdrivenbythefourthandlastservomotor,thethreefingers,Fl,F2,andF3,haveinnerandouterarticulatedlinkswithidenticalmechanicalstructure.Eachofthethreefingermotorsmustdrivetwojointaxes.Thetorqueischanneledtothesejointsthroughapatented,TorqueSwitchmechanismFigure7,whosefunctionisoptimizedformaximumgraspsecurity.Whenafingertip,nottheinnerlink,makesfirstcontactwithanobjectasillustratedinFigure8,itsimplyreachesitsrequiredtorque,locksbothjoints,switchesoffmotorcurrents,andawaitsfurtherinstructionsfromthemicroprocessorsinsidethehandoracommandarrivingacrossthecommunicationslink.Butwhentheinnerlink,asillustratedinFigure9,makesfirstcontactwithanobjectforasecuregrasp,theTorqueSwitch,reachesapresetthresholdtorque,locksthatjointagainsttheobjectwithashallowpitchworm,andredirectsalltorquetothefingertiptomakeasecond,enclosingcontactagainsttheobjectwithinmillisecondsofthefirstcontact.Thesequenceofcontactsissorapidthatyoucannotvisualizetheprocesswithouttheaidofhighspeedphotography.Afterthegrasperreleasestheobject,itsetstheTorqueSwitchthresholdtorqueforeachfingerinanticipationofthenextgraspbyopeningeachfingeragainstitsmechanicalstopwithacontrolledtorque.Thehighertheopeningtorque,thehigherthesubsequentthresholdtorque.Inthisway,thegraspercanaccommodateawiderangeofobjectsfromdelicate,tocompliant,toheavy.Thefingerarticulations,notavailableonconventionalgrippers,alloweachdigittoconformuniquelyandsecurelytotheshapeoftheobjectsurfacewithtwoindependentcontactpointsperfinger.Theposition,velocity,acceleration,andeventorquecanallbeprocessorcontrolledoverthefullrangeof17,500encoderpositions.Atmaximumvelocityandaccelerationsettings,eachfingercantravelfullrangeineitherdirectioninlessthanonesecond.Themaximumforcethatcanbeactivelyproducedis2kg,measuredatthetipofeachfinger.Oncethegraspissecure,thelinksautomaticallylockinplaceallowingthemotorcurrentstobeswitchedofftoconservepoweruntilcommandedtoreadjustorreleasetheirgrasp.Whiletheinnerandouterfingerlinkmotionscurlanthropomorphically,thespreadmotionofFigure10isdistinctlynonanthropomorphic.Thespreadmotionisclosestinfunctiontoaprimatesopposablethumbfinger,butinsteadofoneopposablefinger,theBarrettHandhastwin,symmetricallyopposablefingerscenteredonparalleljointaxesrotating180degreesaroundtheentirepalmtoformalimitlessvarietyofgrippershapesandfixturefunctions.Thespreadcanbecontrolledtoanyof3,000positionsoveritsfullrangeineitherdirectionwithin1/2second.Unlikethemechanicallylockablefingercurlmotions,thespreadmotionisfullybackdrivable,allowingitsservostoprovideactivestiffnesscontrolinadditiontocontroloverposition,velocity,acceleration,andtorque.Byallowingthespreadmotiontobecompliantwhilethefingersclosearoundanobject,thegrasperseeksmaximumgraspstabilityasthespreadaccommodatesitsposition,permittingthefingerstofindtheirlowestenergystatesinthemostconcavesurfacefeatures.ElectronicandmechanicaloptimizationIntelligent,dexterouscontroliskeytothesuccessofanyprogrammablerobot,whetheritisanarm,automaticallyguidedvehicle,ordexteroushand.Whileroboticintelligenceisusuallyassociatedwithprocessordrivenmotorcontrol,manybiologicalsystems,includinghumanhands,integratesomedegreeofspecializedreflexcontrolindependentofexplicitmotorcontrolsignalsfromthebrain.Infact,theBarrettHandcombinesreflexivemechanicalintelligenceandprogrammablemicroprocessorintelligenceforahighdegreeofpracticaldexterityinrealworldapplications.Bystrictmathematicaldefinition,dexterityrequiresindependent,intelligentmotorcontrolovereachandeveryarticulatedjointaxis.Forarobottobedexterous,atleastnindependentservomotors,andsometimesasmanyasn1or2n,arerequiredtodrivenjointaxes.Unfortunately,servomotorsconstitutethebulkiest,costliest,andmostcomplexcomponentsofanydexterousrobotichand.So,whilethestrictdefinitionofdexteritymaybemathematicallyelegant,itleadstoimpractical
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