Development oftheMkIIActuator Obiectives Theglovedesigners atILCsetthereduction ofjointtorqueastheirprimary goalforthesecondgeneration glove.Thiswouldhaveanimmediate benefitinreducing thepower(andheatdissipation) requirements oftheactuator, andpossibly allowareduction insize.Atargetwassettoreducemaximum cableforcefromaround 70N(16Ibf)to45N(10Ibf). TheSSLgoalsforthesecond-generation actuator weretoreducesize,reduceheatdissipation, improve "smoothness" ofperformance, permitoperation inNBSandvacuum environments (whichrequired bringing themaximum voltage backunder30VDC)andimprove maintainability ofthedesign, particularly thecablechangeout procedure. Actuator performance parameters ofmaximum forcecapability, powerdissipation, effective inertia, friction, andsizealldepend strongly onthemotorandtransmission ratioselected. FromtheMkItesting wehaddeveloped agoodfeelfortherelative importance ofeachofthese,andcarefully examined the tradeoffs between themfortheMkIIdesign. Acentralissueisthechoiceoftransmission ratio,whichincludes anygearing usedandthediameter of thecapstan orspoolthatthecableiswoundon.Toillustrate thetradeoffs, definetheeffective spool radiusRastheratiobetween motortorqueandcableforceR=T/F.Givenamotorwithmaximum torque capability TM,andgiventhatpeakforceFMmustbeapplied tothecable,themotormustdriveaspool withradiusR=RM=TM/FM. Inotherwords,anupperlimitforRissetbythetorquelimitofthemotor. WhatthensetsalowerlimitforR? Possibility 1:Spoolsize.IfRistoosmall,itbecomes difficult toattachthecableandthebendradiusis tootightforcablelongevity. Experience showed thislimitisaround 2.5mm(0.1in)diameter. Thiscan berelieved somewhat byusinggearing toincrease thephysical spoolradiuswithout changing the effective R.TheMkIactuator, forexample, utilizeda2.5:1gearratiotoincrease thecapstan diameter to amanageable size.Ultimately, thesizeandefficiency lossesofahigh-ratio geartrainwouldbecome prohibitive, butasinglestagewithratioupto5:1isfeasible inthisapplication. Thislimitsetsaminimum R=0.5mm(0.02in).Thetargetof45N(10Ibf)maximum cabletension thensetaminimum torque capability of22.5mN.m(3.2oz-in)andconsequently aminimum sizeforthemotorsconsidered. Possibility 2:Powerdissipation P.P=12r=T2rlKT2=R2F2rlKT 2whereIiscurrent, risresistance, andKT isthemotor's torqueconstant. Therefore decreasing Ractually improves (decreases) powerdissipation. Thisconsideration drivesthedesigntowardlowerRvalues. Possibility 3:Motorvelocity limit.AsRdecreases, therequired motorvelocity increases ininverse proportion. Thehighest velocity attainable occurswhenthebackemfplustheresistive dropareequalto thesupplyvoltage. Highervelocities canbeachieved byincreasing thevoltage, uptothe30VDClimit setbyNBSsafetyconsiderations. Ifthatisnothighenough, different motorwindings cantradevoltage requirements forcurrent requirements. Asaresult,motorvelocity wasnotacriticallimitation inthe regimeconsidered. Possibility 4:Friction. Staticfrictionandstiction atthecableoutputareproportional toIlR,viscous frictionisproportional toIlR2.Therefore toreducetheeffectsofinternal friction, Rmustbeincreased. Thisconsideration drivesthedesigntowardhigherRvalues. Possibility 5:Effective inertia. Theeffective inertiaoftheactuator rotoratthecableoutputisJ/R2,where Jisthemotorrotorinertia(kg.m2). Iftheactuator inertiabecomes toohigh,theactuator can'tkeepup withtheglove.Theresultisslackinthecableandlossofforcecompensation (gloveopening), or 96