Anotherrequirement wasthatthemotorheat,atthenormaldutycycle,shouldnotincreasetheboil-off rateoftheliquidheliumreservoirsignificantly andtherebyimpactobservation time. Cryogenic Motor Priceandrecommendations weremajorconsiderations intheselection ofthecryogenic motor. In addition, theparticular motorselected hadtobesmall,lightweight, runonrelatively lowpower, butyet havesufficient torque, andbeeasilycontrollable. Ageneral design envelope defined theextentofthe motorsize.Simple computations wereperformed toassess thetorquerequirements baseduponHWP inertia. Thesecomputations accounted foraccelerating thesmallrotating massoftheHWPandits holder. Theresultsindicated verylowvaluesofmotortorquewereneeded. Itwasanticipated, though, thatfriction inthegearing andbearing, whichcouldnotbereliably predicted, wouldbethemajormotor load.Toboosttheoutputofthemotor,anintegral planetary gearunit,witharatioof25:1,wasselected. Boththemotorandplanetary gearing cameasoneunit,fullywired,internally lubricated, andreadytorun. HWPBearinq Alowprofilebearing waschosen tosupport theHWPinthemodule because ofspacelimitations. Further, theeventual implementation ofthisHWPmodule wouldbeonboardanaircraft, exposed toa flightenvironment. Tominimize jitteroftheHWPandpossible microphonic effects pickedupinthe instrument's detectors, aprecision X-bearing (i.e.fourpointcontact) wasselected. Withabearing ofthis typeinacryogenic application, itwascriticalthatthematerial oftheracesmatchthatoftheballbearings. Stainless steelalloy440Cwasselected forboth. Gearinq SincetheHWPwasatthecenterofthemodule andrequired aclearaperture, themotorthatdroveithad tobeoffset.Thisrequired gearing, besides theplanetary gearunit,totransfer themotion. Awormgear arrangement waschosen, eventhough thistypeofgearing suffers fromlargeamounts offriction, because itwasconducive tothespaceavailable andhadarelatively highgearratiotofurtherboostthe outputtorqueandstepcount.Forexample, thestepper motoris200steps/rev; theplanetary gearratiois 25:1;thewormgearratiois25:1.Thus,thetheoretical stepcountforacomplete revolution is200x25x 25=125,000 steps.Therotational accuracy desired forthepolarization studies was+1degree orbetter, andthiswasthought tobeeasilyachievable. Ma.qnetoresistive Position Sensor Toindependently sense theHWPposition, smallcryogenic sensors wereinvestigated. A magnetoresistive sensorfromInfineon Technologies wasrecommended andsamples wereprovided. The sensor wasadifferential magnetoresistive sensor (typeFP212L100)andconsisted oftwoseries coupled InSb/NiSb semiconductor magnetoresistors, whoseresistance couldbemagnetically controlled, mounted onaninsulated ferritesubstrate (Ref.2).Apermanent magnet, whichsupplied abiasing magnetic field,wasfixedtothebaseofthesensor. Theentiresensorwascontained inaplastichousing andhadthreeconnecting terminals thatweresubsequently soldered toasmallcircuitboardformounting andwiring.Thebasicroomtemperature resistance ofthesensorwas2x100Q.Theseparticular sensors wereselected andhadbeentestedsuccessfully onarelatedprojecttoatemperature of12K,whichgave ussomeconfidence thatthesensors wouldoperate atthetemperature thatwedesired. Notonlycould thesesensors detectposition, butbecause thereweretwomagnetoresistors, thedeviceasawholecould sensedirection; although, wedidnotchoose tousethiscapability inourprototype testing. Thesensor wasquitesmallatabout6mmx3mmand0.35g,anditwasrelatively easytoimplement. A5Vpotential wascontinuously applied, andoutputsignals wereontheorderoftenstohundreds ofmillivolts. Asmall electronic circuitwascreated toamplify theoutputsignal,andacommercial off-the-shelf digitalcounter wasusedtorecord movement oftheHWP.Sincetheprecise HWPlocation information wasnot necessary fortheprototype testing, sampling oftheHWPposition wasconducted onlyonceper revolution byattaching asmallferromagnetic chipontherotating HWPholdertoactuate thestationary sensor(byaltering themagnetic fieldofthepermanent biasing magnet andeliciting aresponse fromthe magnetoresistors) eachtimeitpassed. Futureimplementation ofthissensorwilllikelyincorporate more exactinformation onHWPposition, including anyincremental amount ofrotation andpossibly asenseof direction. NASA/CP--2002-211506 29