Functional Requirements Alljointaxesofthewristarerequired tointersect atasinglepoint.Thisenables theuseofacontrol scheme whichpartitions thekinematics intotwoindependent groups: thearm,fourDOF(oneis redundant Iusedtoposition thewristcenterandthewrist,atleastthreeDOFwhichdetermine theattitude ofthetool0.Ifwristaxesdonotintersect atacommon point,toolposition andattitude areinterrelated andcannotbesolvedforseparately. Inaddition toatleastthreeindependent DOFforattitude, thewristmustinclude twotooldrives:alowspeed, high-torque driveproviding atleast33N°m(25ft°lbf)at45deg/s,andahigh-speed, low-torque driveproviding atleast0.66N°m(0.5ft°lbf)at7200deg/s.Additional gearing canbeprovided withineach toolasneeded. Oneexample ofatoolusingbothdrivesisagripper withacentralsocketdrive(required byseveral existing ORUs). AsixDOFforce/torque sensorisrequired atthewristinordertoimplement anadmittance controller. This allowsthearm'slocalelectronics tocontroltheeffective compliance ofthetool-tip, andreactimmediately tolimitcontact forcesratherthandepending onthe(remote) human operator. Foraccurate readings, all forcesandtorques between thearmandwristmustbestructurally routedthrough thissensor. Sincemostspaceservicing taskstodatehavebeendesigned withastronauts inmind,human capabilities arereflected inthetaskspecifications. TheRanger arms(atthetooltip)arethusrequired toequalor exceed the110N(25Ibf)and33N°m(25ft°lbf)capabilities considered standard forahuman inEVA11. Actuator choicewasinfluenced bypackaging requirements forthedriveelectronics (notdetailed here), whichplacedalimitof10Aonactuator current. Thetool-exchange mechanism mustcouple thewrist'sfinalDOFtothestructure ofthetool,and simultaneously engage thewrist's twotooldrives. Sinceitmustoperate intheneutral-buoyancy environment, theIEEMcannotuseanyelectrical connections oradditional powered actuators; itmustbe apassive device. Forreliability andsimplicity, noadditional actuators onthewristmaybeaddedfortoolexchange. Alltool-exchange actuation mustbeprovided bymotionofthearmendpoint. Thedesign ofthetoolstorage facilitymustnotrestrictthesizeoftools.Thiswasadrawback ofthe previous design4,whichrequired everytooltopassthruaholeoffixeddiameter. Desi.qnDrivers Theseparameters havebeenidentified ashavingastrongeffectonproductivity andeaseofoperation of telerobots. Thewristkinematics shouldprovide alargesingularity-free workspace. Everytypeofwristgeometry has singularities, buttheangular workspace thatcanbereached without crossing onedepends strongly on theangular limitsofeachDOFandtheirarrangement. Thecombined wrist/IEEM shouldbecompact inalldimensions toallowoperation inconfined spaces. For instance, thedistance fromeachtooltothebackofthewristshouldpreferably belessthanthatofa human (inEVAglove)holding acomparable tool.Thetool-tip towrist-center distance isaparticularly important parameter tominimize. Iftheoperator adjusts tooltipattitude, anarcofthatradiusbackfrom thetooltipmustbeclearofobstacles. Wristangular speedcapabilities weretargeted at45deg/sat33N°m(25ft°lbf)torqueoutputinany direction throughout theworkspace. Nearasingularity, thismayrequire muchhigherspeeds from individual actuators. Theprimary modeofRanger operation iswithahuman directly incontrol. Accuracy andrepeatability are notascriticalinateleoperated modeofrobotcontrolasinafullyautonomous one.However, productivity wouldbenefit fromtheprovision ofsomesimpleautonomous routines suchastoolchangeout. Hence, NASA/C P--2002-211506 121