thattaskwaschosentosettargetvaluesforaccuracyandrepeatability. ThereisatradeoffwithIEEM design;onethatcantoleratelargemisalignment wouldrequirelessaccuracyofthearmandvice-versa. Thusbothanaccurate wristandatolerant IEEMweredesigndrivers. Positioning aidscanmaketeleoperated tasksmucheasier. Oneexample isawrist-mounted video camera. Although theprimary videosource fortheRanger operator isthestereopairofcameras ona separate videoarm,several different camera angles aretypically required duringexecution ofatask. Auxiliary viewscouldreduce theneedtoreposition thevideoarm,simplifying andspeeding up operations. Also,someservicing tasksdesigned forNASA's SPDManditsend-effector, theORUTool Change-out Mechanism (OTCM), incorporate targeting crosshairs whichaidinvisually aligning theOTCM withthetaskequipment. Theseareoffset7.62cm(3in)fromthetaskcenterline tolineupwiththeOTCM wristcamera. Ifasimilarly-offset wristcamera wereprovided onRanger, theoperator couldtake advantage ofthosealignment aids. WristKinematics Thepurpose ofaroboticwrististofollowanarbitrary commanded trajectory intoolattitude, specified for instance byEulerangles((i),e,_)12withrespect totheforearm. Inpractice, wristsfailtofollowcommanded trajectories whentheyencounter jointlimitsorsingularities. Jointlimitsresultfromstructural interference orconstraints inherent intheactuator design. Singularities occurwhenthedegrees offreedom lose independence, e.g.whentwoaxesalignorthreeaxesfallintothesameplane.Singularities oftenconsist ofsinglepointsinjointspace, butaneighborhood around eachsingular configuration mustbeavoided duetotheexcessive jointvelocities generated. Atthesingularity itself,jointvelocities become infinitefor motionintheconstrained direction. Thissortoftrajectory failurecanbeveryfrustrating totheoperator ifitoccursfrequently inthecourseofa task.Violation oftheseconstraints ishardtopredict, sinceitdepends onoverallarmconfiguration. Even whencommanded toolattitude isheldconstant, awristjointlimitcanbeexceeded duetomotionofthe forearm. Lessrestrictive jointlimitsameliorate thisproblem, andhenceareamajordesigngoalforrobotic wrists.Sincesingularities mustalsobeavoided, theirlocation withintheworkspace requires attention as well. Wristgeometry canhaveastrongeffectonsuccessful trajectory following. Forexample, consider a trajectory ofincreasing (!)withconstant e,i.e.circular motionaround thestraight-out position. Witharollpitchwrist,therollangleis¢andthepitchangleise,soanylimitontherollanglewillbeexceeded at somepoint,haltingthetrajectory. Withapitch-yaw wrist,however, thistrajectory isjustanoscillation in bothpitchandyaw,andcanbemaintained indefinitely without exceeding jointlimits. Redundancy isanother meansofavoiding jointlimitsandsingularities. IfanextraDOFisadded,thereis nowarangeofjointanglecombinations foreachtoolorientation. Control algorithms cantakeadvantage ofthisbyfavoring combinations whichavoidtheboundaries (Ranger usesageneralized inverse method1°). Adding ajointtoawristcreates morepotential alignments between axes,butthesearenowof twotypes.Alignments inwhichthreeDOFsoftoolattitude aremaintained buttwojointslosetheir independence aremanageable. Someredundancy islost,buttrajectory following isnotimpaired. Singularities whereaDOFoftoolorientation islostmuststillbeavoided. Ranger wristtechnology evolved toitspresent statethrough aseriesofdesigns overseveral years.To illustrate theimprovements made,thecurrentsRPYR wristwillbecompared totwoearlierversions: arollpitch-roll (RPR)wristandanorthogonal RPYR(oRPYR) wrist.Eachdesignmeetstherequirement forall wristaxestointersect atacommon point,andincorporates twotooldrives. NASA/CP--2002-211506 122