Thesegoalswereaccomplished withamechanism consistingofa7.3-cm(2.875-in)diametercylinder4.8 cm(1.875in)inlength,withthreeconcentricmountingsurfacesonthedistalface(Figure2).Theouter ringsupportsthestructureoftheinstalledtool,themiddleringcouplestothemanipulator's high-speed, low-torquetooldrive,andtheinnerringcouplestothelow-speed, high-torque tooldrive.Theopposite faceconnectsrigidlytothewristoutputflangewithamechanism inspiredbyacameralensbayonet mount.Whennotinstalledonthewrist,eachIEEM(andtool)isrigidlyattachedtooneofseveral"tool posts"usedfortoolstorage.Theendofthetoolpostresemblesamushroom, andiscapturedbya rotatingsleeveontheIEEMasthemanipulator disconnects fromthetool. Thefollowingsectionspresentthefunctionalandenvironmental requirements fortheRanger wristand IEEM,followed bydetailed discussion ofthedesigns. Requirements andDesign Drivers Design objectives fallintotwocategories. Intheterminology usedhere,a"requirement" specifies astrict threshold valueforsomeparameter (e.g.,temperature range)whichthedesignmustmeet,butnobenefit accrues fromimprovement beyond thatvalue.A"design driver"specifies afigureofmerit;aparameter to bemaximized orminimized subject toabalance ofalltheotherdesignconstraints. Anexample ofthe latterwouldbetominimize thedistance between thetooltipandthewristcenter, withasmaller value allowing workintighterspaces. Onlytherequirements anddesign drivers withamajorimpact on configuration willbediscussed here.Manyotherssuchaslaunchandlanding vibration loads,corrosion resistance, etc.wereaccounted forbuthadminoreffect. Environmental Requirements TheRanger armsandtoolsmustbeabletofunction inthelaboratory, invacuum, andneutral-buoyancy simulation (underwater, maximum depth12.2m(40fl)).Forunderwater operation thearmsare pressurized withairtoaround 20kPa(afewpsi)abovetheambient waterpressure. Analysis oflikelyorbitscenarios setanoperating temperature rangeof-40°Cto+125°Cformostofthe mechanism, withvalues upto155°Cpossible adjacent tomotorwindings. Inneutral buoyancy simulation, thetypicalambient watertemperature is32°C. SafetyRequirements Thedecision wasmadein1996todesignRanger tocomply withtheNASAsafetystandards applicable to Shuttle payload-bay experiments 6.In1999theprojectpassed itsPhase2Payload SafetyReview. This effortinvolved identifying andclassifying potential hazards associated withoperating thetelerobot inthe Shuttle payload bay.Eachlevelofhazard (critical, catastrophic, etc.)thenrequires appropriate safeguards. TheRangier projectwasthefirsttoconform tothenewstandards for"Computer Control of Hazardous Payloads". DetailsoftheoverallRanger safetyapproach aredescribed inreference (8). Theadopted safetyplanrequires eachDOFtohavetwoindependent meansofmeasuring relative joint angle,toensure thatactuator runaway conditions canbedetected. Inaddition, eachjoint(excluding tool drives) musthaveameans ofinitializing therelative anglemeasurements toknownpositions. Thiswas essential fortheboundary-management scheme, designed toprevent therobotfromperforming hazardous actions suchasreleasing anuntethered orbitalreplacement unit(ORU)intofreespace, or drivingthetoolintoanon-contact zoneoftheshuttle. Forthetool-exchange mechanism, lossofatoolintospaceisapotential catastrophic hazard. Thus securetransfer fromtoolstorage tothemanipulator armandbackisrequired, andthemechanism must satisfyprovisions oftheappropriate NASAsafetystandard 9. Electrical safetyconsiderations alsoinfluenced thechoice ofactuators. Themaximum voltage levels considered safeforneutral-buoyancy operations wassetat32V,and48Vforlaboratory operations. NASA/CP--2002-211506 120