ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
TheIRACShutter Mechanism: Residual Magnetism andtheRotary Solenoid ScottSchwinger*, ClaefHakun*, GaryBrown*, KenBlumenstock* Abstract TheIRACShutter mechanism wasoriginally presented inthepaper,"ALowPowerCryogenic Shutter Mechanism forUseonInfrared Imagers" atthe34thAerospace Mechanisms Symposium, May2000.At thattime,theshutter wasbelieved tobeperforming flawlessly andtherewaseveryindication itwould continue todoso.Inearlyspringof2001,thecalibration shutter, arotarysolenoid designed tobefail-safe open,remained inaclosedstatewithnopowertotheelectromagnetic coils.Theensuing investigation, subsequent testing, proposed remedy, andlessons learned arethefocusofthispaper. Introduction TheInfrared ArrayCamera (IRAC) isoneofthreeinstruments onboardtheSpaceInfrared Telescope Facility(SIRTF), thefourth"GreatObservatory". TheSIRTFinstruments willprovide imagery, photometry, andspectroscopy ofastronomical bodiesofinterest overaspectral rangeof3.6to160pm.IRACwilltake images infourbandscentered on3.6,4.6,5.8,and8.0pmwavelengths 1.Itisacryogenic instrument thermally coupled toasuperfluid Helium dewarwithanoperational temperature of1.4Kelvin(K). Shutter Backqround andDescription TheIRACshutter mechanism isrequired toblockincoming lightenergy, producing adarkinternal environment, andto allowviewing ofinternal calibration sources forIRACto calibrate itsdetectors. Thetwomostimportant requirements fortheshutter, asidefromtheleveloflightattenuation the shutter istoprovide, aretoexhibitlowpowerdissipation to limitcryogen depletion, andtobefail-safe opentoavoid lossofmission intheeventofafailureintheclosedstate. Thesetworequirements factored heavily intotheshutter design andwillbeaddressed further. Themechanism (Figure 1)isapproximately 0.15m(5.9in)longandhasa massof1.25Kg.Ittranslates amirrored panelthrough an arcof0.663rad(38deg).Theshutter isrequired to attenuate incoming radiation by1.0E+06. Ontheinside portion ofthepanel,mirrored surfaces arediamond turned toreflect anon-board calibration source through the instrument optics ontothefocalplanedetectors. This shutter isrequired tobefail-safe open,havealifetime of 20,000 actuations, operate at1.4K,anddissipate lessthan 5mWaverage power (0.5J/100s). Itisrequired tobe redundant, andtherefore, hastwoseparate actuators, Side AandSideB,located onopposite endsofthemechanism, driving acommon shaftasshown inFigure 2.Separate electronic cardsdrivetheSideAandSideBactuators independently. Onlyonecardcanbepowered atatimeand onlyoneactuator isrequired toclosetheshutter.Figure 1.IRACShutter Mechanism Thesuspension system oftheshutter reliesonatorsion flexure toprovide axialstiffness, torsional preload andrestoring torqueuponactuation (Figure 3).Thisflexure ischemically etchedfroma0.660NASAGoddard SpaceFlightCenter, Greenbelt, MD Proceedings ofthe36thAerospace Mechanisms Symposium, GlennResearch Center,May15-17,2002 NASA/C P--2002-211506 15
mm(0.026in)thicksheetofBerylliumCopper(BeCu)25AT.Theflexureisattachedtotheshaftofthe shutteratitscenterpointandisfixedateitherendbycomponents thatfixtheminrotation.Astheshutter closes,arestoringtorqueisproducedineachhalfoftheflexure.Thenominaltorsionalpreloadinthe flexureintheopenstateisapproximately 24.7mN°m(3.5in°oz).Closed,theflexureproduces49.4 mN°m(7in°oz).Bushingsateitherendoftheshaftprovideradialstability.Theylimitradialmotionofthe shaftduringvibrationandclosingoperations. Sinceradialforcesapproachzerowithnocurrentappliedto theactuators,frictionatthebushinginterfacesapproaches zeroastheshutteropens. RetainerElectromagnetic Coil H./ _ EndCap ousmg /Closeout /__Rotor._. _ / " ,........ \',\\,."\"\"\\ "- /"i Stator / Shaft Stator Counterweight ShaftHub Figure2.Shutter mechanism cross-section Theactuators arevariable reluctance rotarysolenoids designed atGSFCfortheIRACproject. All magnetic components aremadeofHiperco 50A,asoftmagnetic material withhighmagnetic saturation, whichwasheattreated tominimize coercive force.Withnopermanent magnets inthecircuit, the actuation ofthemotorisinsensitive tovoltage polarity acrossthecoil(i.e.thesemotors alwayspull,they neverpush).Thedesign ofthesemotors consists oftwostators andarotorhoused withinan electromagnetic coil.Acloseout cylinder thensurrounds thecoilandjoinsthetwostators ateitherend (Figure 4)thusclosing themagnetic circuit.Thecoilconsists of11500turnsof38gage99.99% pure copper wire.Coilresistance drops (SideCross-section) FlexureEndThreaded Piece r 1 --_ _/:___ _quare.... Flexure Flexure Hub Cross-section -_'_'/Clamp / MirrorMounting Surface(FrontView) Figure3.Shutter suspension systemfrom2700E_atroomtemperature to17_at1.4K.Lowpower dissipation isachieved byvirtueof thisresistance change andthe abilitytodropthecurrent input necessary toholdtheshutter closed. Thelowerholdcurrent isa product ofmagnetic tabsthat extend normal tothestatorfaces (Figure 5).These tabsare contacted bytherotorinthe closed stateandcomplete the magnetic circuitinthemotor.The closed magnetic circuitrequires dramatically lower current to achieve therequired fluxleveland resultant torque tomaintain a closedstateagainst therestoring torqueofthetorsion flexure. Sincepowertothecoilsis12R,lower current means lesspowerdissipation. Thedrivecircuitry provides approximately -60mAtoclosethe NASA/CP--2002-211506 16
shutter.Thecurrentisthenreducedto-2.5mAtomaintaintheclosedpositionwhiletheinstrumentruns calibrationprocedures. Currentisthenremovedtoallowtheshuttertoopen. Stator  Rotor  __ / Stator Latching Tab ____Closeout Electromagnetic Coil Figure4.Exploded viewofshutter actuator Figure5.Shutter rotorandstator Description ofFailure Duringground testingattheSIRTFsystem level,theshutterwasfoundtoremain closedwithnocurrent applied attheconclusion ofoneofthetestsequences, thusviolating thefail-open requirement. An investigative teamwasassembled todetermine iftherewere,infact,afailure,the;extent, repeatability, andcauseofthefailure,aswellashowtoresolve it. EventsLeadinq UpToTheFailure DuringareviewoftheIRACoperations, itwasdiscovered thattheshutter wasoperated inanillogical manner that,whilenotdamaging hardware, stressed certainelectrical components morethannecessary. Theoriginal shutter electronics andsoftware designengineers hadlongsinceleftNASA,sothedesign philosophy ofthecommand sequence isunclear. Theelectronics areprimarily aclosedloopcurrent drive withasolidstaterelayinserieswiththeshutter coiloutput. Whentherelayisopened, thefeedback electronics arezeroed outandheldinalowstate.Whentherelayisclosed, thesystem usesfeedback andaPIDcontroller totrackthecurrent levelcommand. Thecommand isa12-bitwordandisscaled suchthattheelectronics canoutputupto+/-80mA in40pAsteps.Theoriginal opening sequence in v3.13ofthesoftware was: Simultaneously command zerocurrent andopentherelay(with5mAofholding current flowing) -0.2second later,closetherelay -0.1second laterre-open therelay Thisopening andclosing ofarelaydrivingcurrent intoaninductive loadseemed inappropriate. Another problem withtheoriginal software wasthatthesensor wasnotbeingoperated properly. Tocorrect the sensor operation andreduce thestressonthisrelay,theshutter opening command sequence was changed inV3.2to: -Command thecurrenttozero(Waitatleast0.1second) -Opentherelay Soonaftertheimplementation ofV3.2,thefailureoccurred. Therefore, thefirstsuspect wasthesoftware. However, theinvestigation showed thesoftware sequence thatprovided smoother, gentler current profiles simplyuncovered thelowtorquemargins uponopening thatwereduetoresidual magnetism in theshutter. Investiqations onFliqhtHardware Thefailurewasfirstdiagnosed whileprocessing datatakenduringthelastfunctional testbeforebreaking configuration. Thefirstactionwastothoroughly examine alldataandtelemetry takenduringthattest. Therewasnoevidence ofimproper commands orcurrent anomalies, sothedetailed investigation began. NASA/CP--2002-211506 17 .J
placementoftheshimrevealedthatitwasnotplacedoptimallyandwassubjecttopotentialmagnetic shortingduetosmallvariationsinassemblyorshiftingofgapsduringshutteroperation. Considerable frustrationwasenduredduringinitialpostfailuretesting whichshowedverylittleeffectofmagnetics ontheabilityofthe shuttertoremainclosedwithzerocurrent.Uponexamination ofthe rotorandstatorparts,aslightburnishing oftherotorwhereit contactedthestatortabswasrevealed(Figure8).Clearlythemajority ofthetabsurfacewasnotinintimatecontactwiththerotor.Only whenthecontacting surfacesofasetofactuatorswerelapped together(Figure8)diditbecameapparentthatourinitialsuspicions weremostlikelycorrect.Approximately 12#m(0.0005in)ofmaterial wasremovedduringthelappingprocess.Whenthesecomponents wereassembled intoasparemechanism, theunitconsistently failed toopenwithzerocurrentapplied.Thiswasabreakthrough inthe testingprogram.Itshowedthatsmallvariationsinthenatureofthe contactbetweentherotorandthestatorhaveasignificantimpacton residualmagnetism and,therefore,theresidualtorquethatthe actuatorcouldexhibit.t i. Unlapped Burnishing Ma,qnetic Analysis andParameters Considerable magnetic analysis wasdoneontheactuator by Lapped constructing 2-Dand3-DBEMmodels. Unfortunately, thesemodels proved ofnouseinunderstanding "smallairgap"behavior. Itwas suspected thatlargefluxlevelscouldbedeveloped whenthegap Figure8.Rotorpreand between rotorandstatortabsapproaches zero.Sincelaboratory postlapping measurements onspareunitsfailedtoduplicate flightresidual magnetic torques necessary toovercome theflexuretorque(measurements werelessthan14mN.m(2 in.oz)compared withtheflexurereturntorqueof50mN.m(7in.oz))andresultina"stuck"condition, the question aroseastowhatthetheoretical maximum residual magnetic torquewas,andmoreover, how thisbehavior wasrelated toairgapdistance. Stator/rotor alignment isonefactoraffecting theminimum airgapandtabsurface qualityisanother. Oneanalytic solution wasutilized whichassumed allthematerial, Carpenter Hiperco 50A,was magnetized andthatallthereluctance wasintheairgap.Another casewasrunforthecaseofzeroair gapwherein thereluctance oftheHiperco wasusedtodetermine themaximum magnetic flux,Bmax. Hiperco 50A,analloyofnearly50/50ironandcobaltandafewotheringredients, wasselected duetoits abilitytocarryhighfluxlevelsandminimize actuator sizeandmass.Figures 9and10proved tobe interesting showing thattheB-fieldatthetabs,andthusthetorquegenerated bythetabs,risesrapidlyas theairgapreduces below5#m(0.0002 in). .............. .............. Magnetic FluxDensity vs.AirGap Torque versus AirGap 1.5 -o.5 _ 0 ' 0.0005 0.001 AirGap(inches)0.0015150 O | =100,m O '-L E5O 0 I--0 0.0005 0.001 AirGap(inches) ...........0.0015 Figure9.Analytic solution showing relationship offluxwithairgapFigure10.Analytic solution showing relationship oftorque withairgap NASA/CP--2002-211506 20
Non-Lapped Torque vs.Current _o__47_!!i:i_i¸i_'_¸¸i¸_¸_ii_!_:!I¸Ji_i,!i¸ O I-- -3 -2 -1 0 1 2 Release Current (mA) ........... ._oi I- -1 O I-Lapped Torque vs.Current -4 -2 0 Release Current (mA) ......................... Figure12.Torque vs.current fornon-lapped (left)andlapped (right)actuators demonstrated. Itisbelieved thatthenatureofthecontact ontheflightunitissomewhere inbetween the preandpostlappedcasesandthatthesetwocasesboundtheproblem. Therefore, theimportant concept tonoteisthatintheshutter mechanism assembly, theproblem isbounded: i.e.higher residual magnetism canstillbecancelled withslightreverse polarity currents. Themagnetic analysis confirmed thisbounding. Thereisnocondition inwhichthemechanism canremain closedaftersufficient reverse polarity current isapplied. Theefficiency oftheactuator intheclosedposition increases asthecontact between rotorandstator becomes moreintimate. Sincethegapissmaller withimproved contact, alargerpercentage ofthe possible residual fluxduetosaturation (-1.5tesla) canberetained. Atthesame time,since theactuator ismoreefficient, ittakesacomparably lessNiofreverse polarity tocancel theresidual fluxinthe magnetic circuit.Thus,theproblem isbounded. BackEMFMeasurements andDemonstration ofLikePolarity Release Currents Therestillremained thetaskofdevising atesttoprovethatthefailureoftheflightmechanism toopen withzerocurrent wasduetoresidual magnetism andnotfriction. Friction waseliminated asaplausible sourceofresistive torquethrough tolerance analysis andobservations. Multiple degauss schemes were devised toshowthattheresidual magnetism couldbeaffected byreverse polarity currents. Inanactuator withrelatively poorrotortostatoralignment (i.e.amechanism thatdoesnotstick),theeffectofreverse currents hasminimal effectonrelease currents sincetheamount ofresidual fluxissmall.Inaddition, sincearelatively smallvalueofcurrent saturates andre-gausses thetabs,itisdifficult todegauss the actuator withintheconstraints oftheflightmechanism. Infact,whenconstrained toaccess ofonecoilata Figure13.BackEMFmeasurements before(left)andafter(right) applying areverse polarity current NASA/C P--2002-211506 22
timeasintheflightset-up,itisvirtuallyimpossible. However,ifbothSideAandBareenergizedatthe sametimeitispossibletoshowthatresidualmagnetism existsinamechanism. Itisalsopossibleto showthatthisresidualcanberemovedbyaslightreversepolaritycurrent.Figure13showsthevoltage inducedontheshuttercoilduringopening,orBackEMF,beforeandafterasmallreversepolarity degausscurrentisapplied.Thefigureshowsthat-85%ofthemagnetism isremoved.Infacttestsat cryogenictemperature showedthatthetabscouldeasilybedegaussed iftheshutterwasheldclosed duringdegaussing. However,closingtheshutterwouldre-gaussthetabsandgeneratethesameresidual magnetism. Unfortunately, duetoSIRTFprojectmanagement concernsoverriskandschedulewewere notabletoduplicatethesetestsontheflightunit. Proposed Solutions andRecommendations fortheFlightSystem Oncetheproblem hadbeenassessed, weconsidered several solution remedies toguarantee recovery fromallplausible failuremodes. Opening thedewarandmodifying theelectronics werebothdeemed unacceptable sincetheflighthardware wasinitsfinaldelivered state.Thisimposed condition limited options, butasimpleharness modification solution wasdesigned andtested. This"softcross-strapping" concept simplyinvolved adding aresistor between thedriveoutputs oftheSideAandSideBboards (Figure 14).Thisresistor wouldbesignificantly largerthanthecoilresistance, butwouldallowasmall transient current toflowintheunused coil.Therefore, ifafailurecaused ashutdown andtheshutter remained closedonSideA,simplyactuating SideBwouldprovide enough current toSideAtoerasethe residual magnetism. Sincethecross-strapped current isafraction oftheprimary drivecurrent, itcouldbe controlled bycommand aswell.Models werecreated andtestsonanengineering unitwererunto determine anappropriate rangeofvaluesfortheresistor aswellaseffectsontheun-powered electronics board.Thesoftcross-strap approach successfully closedandopened theengineering unitshutterwhich hadbeenlapped tohavemaximum residuals onbothSideAandSideB.Effects ontheun-powered boardwerenegligible. Implementing thefixwithacombination ofseriesandparallel resistors, this solution promised afaulttolerant, robust, andsimplefixtotheFlightShutter problem. Theonlyfaultthat couldnotbecorrected wasiftheSideAcoilopened orshorted duringaclosing cycleusingtheSideA electronics andtheresidual magnetism washighenough tokeeptheshutter closed. Athorough studyof thematerials andfabrication techniques ofthecoilandharnessing indicated thisscenario wasextremely implausible, butnotabsolutely impossible. However, thefinaldecision ofSIRTFmanagement notwasto runfurthertestsorimplement therecommended fixontheFlightUnit. Voltage BufferCROSS-STRAP R CROSS-STRAP R CROSS-ST_RAP R CROSS_  x_,' ...iI m RHarness RHarness _Vv-- ,AAt Current SensinqVoltage Buffer  f_ i_1÷o__10__ .....Senn 10Rhm Inside Cold Assembly 10hm Figure14.Schematic showing softcross-strapping configuration NASA/C P--2002-211506 23
reality,anunfortunate byproductofimprovements incommandsequences, tightlytolerancedmachining, precisionalignment,andskilledassembly.Atfirstglance,thesearealldesirabletraitsforanymechanism. However,inconsistencies inanyoneoftheseareasduringthequalification phaseofadevelopment can fosterafalsesenseofsecurityandresultinsurprisesduringaphasewhenmorerigorouscontrolis applied.Fortunately, duetothecommitment ofatalentedgroupofindividuals, ahiddenproblemforthis mechanism wasfoundandapotentialmissionendingfailureavoided. References Schwinger, D.ScottandClaefF.Hakun. "ALowPowerCryogenic Shutter Mechanism forUseon Infrared Imagers." Goddard Space FlightCenter, Greenbelt, MD.34thAerospace Mechanisms Symposium, Greenbelt, MD,May,2000 Acknowledgments TedAckerson/GSFC -Quality assurance MartyBrown/OSC -I&T BillBurmeister/Ball Aerospace- BallSlRTFLeadI&T Giovanni Fazio/SAO -IRAC PI RainerFettig/Raytheon -Magnetics andmechanics JoseFlorez/GSFC- Electronics LeslieFrazier/SAO -Quality assurance SidJohnson/GSFC -Mechanical design, assembly, test RobertKichak/GSFC -Failureinvestigation teamchair EdLander/Raytheon -I&T NormLee/GSFC -Qualityassurance SteveMann/Hammers -Software Ahmean Mekhail/OSC -I&T Shirley PauI/OSC -Quality assurance LoisWorkman/GSFC -GSFCIRACprojectmanager NASA/CP--2002-211506 26
ACryogenic Half-Wave PlateModule toMeasure Polarization atMultiple FIRPassbands Timothy S.Rennick*, JohnE.Vaillancourt**, RogerH.Hildebrand +,andStephen J.Heimsath* Abstract Oneofthekeycomponents inafar-infrared polarimeter thatisbeingdesigned atTheUniversity of Chicago isalocally-powered half-wave platemodule. Thiscompact, lightweight, andreliable module will operate atcryogenic temperatures, rotating ahalf-wave plateaboutitsaxiswithintheopticalpath.By doingso,polarization measurements canbemade.Further, byutilizing multiple half-wave platemodules withinthepolarimeter, multiple wavelengths orpassbands canbestudied. Inthispaper,wedescribe the design andperformance ofarelatively inexpensive prototype module thatwasassembled andtested successfully, outlinethedifficulties thathadtobeovercome, andrecommend improvements tofuture modules. Thiseffortnowlayssomeofthegroundwork foranext-generation polarimeter forfar-infrared astronomy. Introduction Instruments designed forfar-infrared (FIR)andsubmillimeter astronomy generally operate withthe detectors andmostoftheopticalelements atcryogenic temperature. Whenthedesigncallsforinserting anyofseveral lensesorfiltersintotheoptical path,theremustbesuitable actuators, bearings, and sensors forthemoving parts.Theactuators haveusuallyconsisted ofexternal, roomtemperature, motors connected tothemoving partsbyinsulating shaftspassing through vacuum sealsandworking theirway through tightspaces attimesviarightanglegearing tothecoldparts.Thisapproach hasbeenworkable forrelatively simpleinstruments, buthasbecome increasingly difficult astheinstruments havebecome morecomplex. bearing_  /--worn,, shaf't, ,andbeorings HWPhoider----_ \ _,ir-_l/ ref . __ ,;y,_..,. plane#ary gearin9_ _'--_vorm g_ar gearholder cryogenic stepper rnotor_ Figure 1.Exploded viewofHWPmodule TheUniversity ofChicago, University ofChicago Engineering Center, Chicago, IL** TheUniversity ofChicago, Dept.ofAstronomy &Astrophysics, Chicago, IL;nowwiththeUniversity of Wisconsin, Dept.ofPhysics, Madison, WI +TheUniversity ofChicago, Dept.ofPhysics, Dept.ofAstronomy &Astrophysics, EnricoFermiInstitute, Chicago, IL Proceedings ofthe36thAerospace Mechanisms Symposium, GlennResearch Center,May15-17,2002 NASA/C P--2002-211506 27
Aluminum Housinq Withmanydissimilar materials usedinthemodule, therewereobvious concerns overinterference and binding duetoshrinkage asthemodule wascooled toitseventual operating temperature. After considering several designs, itwasdecided thatthebestapproach wastoaccommodate theshrinkage withintheassembly tolerances, particularly sincethedimensions involved werenotallthatgreatand shrinkage wasmostlyconcentric abouttheHWP.Takingthisapproach hadthedisadvantage ofcreating a"sloppier" mechanism thandesired atroomtemperature. However, bysurrounding thestainless steel HWPbearing withanelastomeric o-ringtocenteritandfillthepurposely-open gap,themodule wasable tooperate satisfactorily atbothroomtemperature andatcryogenic temperatures. Thesizeoftheo-ring (nominal 1.6-mm diameter) anddepthofthegroove inthehousing wereselected sothattheo-ringjust contacted theouterbearing raceatroomtemperature andthenexperienced roughly 10%diametral squeeze uponcooling. Alistingofallcomponents withintheassembled module isshowninTable1.Ascanbeseen,manyof thecomponents werecommercially available, generally reducing costassociated withtheprototype. The totalmassoftheassembled module was851g,anamount thatisprobably morethandesired forthe futureflightdesign. Component Stepper Motor Planetary Gearing HWPBearing Precision Worm Precision WormGear Magnetoresistive Sensor Housing ShaftTable1.Com Supplier Phytron, Inc. Phytron, Inc. Kaydon Corporation W.M.Berg,Inc. W.M.Berg,Inc.3onents ofHWPmodule PartNumber VSS19.200.0.6 VGPL22/25-UHVC-X Reali-Slim SA025XSO W32S-3F W32B30-F100Approx. Cost ($US) 4400 38O 66 50Material Stainless steelhousing Stainless steelhousing Stainless 440C Stainless 303 Bronze Alloy464 Infineon Technologies FP212L-100 b(580) Plastic housing Univ.ofChicago NA ~5000 Aluminum 2024 Univ.ofChicago NA c Stainless 303 ShaftBearings W.M.Berg,Inc. ShaftCoupling McMaster-Carr Flange Adapter Univ.ofChicago HWPRetainer Univ.ofChicago HWPHolder Univ.ofChicago GearRetainer Univ.ofChicago a-included incostofmotorB1-26-Q3 ABEC3 18 Stainless 440C 59925K88 90 Nickel NA c Stainless 303 NA c Aluminum 2024 NA c Stainless 440C NA c Stainless 440C b-sensor donated byInfineon Technologies, costinparenthesis associated withcircuitboardanddigitalcounter c-included incostofhousing Figure3showsafrontandbackviewoftheprototype module andsensoronceassembled andattached toabracket fortesting. NASA/CP--2002-211506 30
Figure3.Frontandbackviewofprototype module withmagnetoresistive sensor Inaddition, themodule wastestedfirstwithout andthenwithamodified tungsten disulfide drylubricant, Dicronite ®DL-5,applied tothewormgearing inhopesofreducing gearfriction andlessening themotor loadand,thus,heatproduced bythemodule. Thislubricant wasselected primarily because ofitslow cost,availability intheChicago area,anditsadvertised characteristics thatappeared tomatchourneeds. Because oftimeandcostconcerns, notesting orextensive investigation wasconducted toscreen various lubricants anddetermine themosteffective. Thiswouldbeworthwhile, though, inthefuture. Module Testing Testing oftheprototype HWPmodule wasconducted inacryostat thatwasoriginally designed asafarinfrared polarimeter foruseontheKuiperAirborne Observatory (Ref.3).Thecryostat hasadualstage He3refrigerator system capable ofreaching temperatures aslowas300mK.However, theHe3system wasnotusedinourtests,andonlycooling fromtheliquidhelium (LHe)reservoir wasusedtoobtain temperatures near4K.Thecryostat hastworadiation shields atLHetemperature, oneatliquidnitrogen (LN2)temperature (77K),andanoutervacuum shellatroomtemperature. Themagnetoresistive sensor waswiredusinglowthermal conductivity stainless-steel wireinsidethe cryostat. Thehighcurrent needsofthecryogenic motor(specified at0.6Aperphase), however, precluded theuseoflowconductivity wire.Therefore, copperwirewasusedinsidethecryostat tosupply powertothemotor.Byobserving liquidheliumboil-offrates,weestimated theextraheatconducted from roomtemperature tothe4Ksurface alongthese4copper motorwires(2perphase) nearlydoubled the heatreaching the4Ksurface. Thiswasaninitialconcern whentheideatorelocate themotorinsidethe cryostat wasconceived. Assuch,heatloadsonthe4Ksurface wereamajorfocusofthetesting. Inthe future,moreattention willbefocused onheat-sinking themotorleadstotheradiation shields andpossibly takingadvantage ofavapor-cooling effectbyroutingtheleadsinthevicinityofventing heliumgas. GoalsofHWPModule Testinq Thegoalsofthemodule testsweretwo-fold. First,andmostimportantly, wouldtheconcept ofalocallypowered half-wave platefunction atcryogenic temperatures andwoulditdosorepeatedly andreliably? Second, whatamount ofpower/heat wouldbedissipated atthe4Kstagewhilethemodule was operating? Ourastronomy application requires rotating theHWPby30degrees inabout5seconds, theequivalent of 1rpm.Giventhatonerevolution oftheHWPisapproximately 125,000 motorstepsthiswouldnecessitate NASA/CPm2002-211506 31