ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Operation oftheSMAdrivemotormechanism SMAwireactuators aremounted onbackside oftheoutermost solarpanelandtheironeendis connected totheleverthrough flexible wireropelinkguidedonpulleys(Figure 7).Tostep-drive the motorintherequired direction, thecorresponding SMAwireactuator isheatedbypassing electric current. Thewirecontracts initslengthandpullsthelevertotheright,turning itby16degrees untilthe leverisstopped bythestopper pinontheotherend.Atthesametime,thepawlmounted andhingedon theleverridesoverthepawlguideandgetsengaged withtheratchetwheelteethandturnsitbyone toothpitch.Thefirsthalfofleverstrokeisusedforpawlengagement andtherestforratchetwheel rotation. Theratchetwheel,whichisrigidlyconnected tothemainshaft,rotatestheshaftby~8-degree step,equaltotheratchetwheeltoothpitch.Thestrokeoftheleverinitsoperation islimitedbytwo stepper pins,whichalsoactaspawlguides. OncethecurrenttotheSMAactuator isremoved, itcools down,becomes soft,andelongates toitsoriginal longerlengthbyitstwo-way memory. Atthesametime, thebiastorsionspringmounted ononesideofleverwillpulltheleverbacktoitsoriginal placeuntilitis stopped bythestopper pinatthebottomend.Thepawlmounted ontheleverisalsopulledback.When thepawlispulledback,itgetsdisengaged fromtheratchetwheel. Disengaging thepawlisnecessary for thenextstepmotionoftheflap.Inthenextactuation, thepawlengages tothenextteethoftheratchet wheelforanother stepmovement. Heating theotherSMAwireactuator willgiveanoutputstep movement intheopposite direction astheratchetwheelteethorientation; leverandpawlareinreverse direction totheearlierasshowninFigure5.Thespurgearreducer stepsdownthemainshaftrotation. Thegearreducer outputshaftisconnected totheflapdrivehingeshaftthrough aflexible shaftcoupling todrivetheflap. Design details Drivetorqueanddetenttoreue Thenetdrivetorqueofthemechanism attheoutputshaftisgivenby: Drivetorque =[(FsxL-Td)xN]-"If Fs=SMAwirepullforce L=Leverlength Td=Detenttorque N=Gearratio "If=System frictiontorque Thedetenttorqueisattheflaps,afterthegearreducer wasspecified foron-orbit condition. Thedetent mechanism wasdesigned towithstand on-orbit disturbances withamarginofabout8underlowestonorbitassumed frictioncoefficient of0.05.Thedrivetorquecalculations assume aworst-case friction coefficient of0.3.WithanSMApullforceofabout9N,themechanism provides 0.4N°mdrivetorqueat theoutputshaftoverandabovethemaximum detenttorqueontheshaft. DesignofShapeMemory AlloyActuator SMAwireactuator waschosen withanoperating strainlevelof~3.5%. Actuator wirelengthwasselected basedonthestrokerequirement fortheratchetwheelandpawlmechanism asunder: 100 SMAlength=--(St+Sp+Sm),where,3..5 St=Strokerequired foronestepofratchetwheelrotation Sp=Strokerequired forpawlengagement withRatchet wheel Sm=Marginformemory lossandadditional strokemargin. Theactuator wirediameter depends uponthetorquerequirement ofthemechanism. SMAwireof0.25 mmdiameter andpullforceof9Nmetalltherequirements andprovided sufficient designmargin. 245
_Design ofloadlimitin_ sprin_ Thespringisdesigned toprovide 8.5-Npreload withitspre-compression inassembly andlimittheloadin theSMAwireto12Nwhenthemechanism isprevented fromrotation. Liferequirement Liferequirement forthemechanism was10,000operation inbothdirections. Withafactorof10,theSMA wireactuator wastestedfor105cyclesusingaspecial PC-based testsetuptounderstand memory fatigue/permanent lossofSMAwirestroke. Electrical Heating Actuation SMAwirecanbeassumed aselectrical resistance wirehavingadefined resistance valueperunitlength. ttcanbeactuated usingDirectCurrent (DC)heating orPWMheating. DCheating wasnotpreferred because of(i)uneven heating ofSMA,and(ii)overheating resulting inhotspots,whichcancause memory loss.PWMheating isprovided foruniform heating andforincreased lifecycletimeofthewires. Eachactuation ofSMAwireforonestepmovement involves aheating periodof"1"1seconds foractuation andafteractuation thewireshouldbeallowed aminimum cooling periodofT2seconds, sothatitreturns toamartensitic stateandisreadyforthenextoperation. T1andT2periods varydepending onthe thermal environment andelectrical power. If1"1ismorethantherequired, thenover-heating ofthe actuator mayresult,andifless,itmaynotheatthewiretoitstransformation temperature. Theminimum cooling timeT2couldbe1second to180seconds depending onthesurrounding environment. Cooling timeiskeptlargeenough totakecareofallenvironmental conditions. Aprovision tochange theheating timeforon-orbit operation isprovided byprogrammable on-time command tocovervariation inpanel temperature andpowergenerated. Toensurepositive cooling oftheSMAwire,aminimum of5minutes isspecified between twoactuation commands. Instead ofdrawing powerfromspacecraft bus,anewconcept isusedinpowering theSMAwire:a separate solarcellarrayismounted onthefirstsolarpanelexclusively forflapSMApowering. Thisarea onthepanelwastobeleftblankotherwise tocleartheshadow ofspacecraft antennae. Thepower switching signals aregivenbysatellite attitude controlelectronics asshowninFigure8. Spacecraft AOCE Ontme a'ac°mma 0 TTTForwarddriveenable-- Reversedriveenable FlapdisablePWMSignalI I 1Powerswitching I Electronics I !; I+ FlapSolararray ] IswII J Drivemechanism Solararray_.J Figure 8.Flapdriveelectronics blockdiagram 246
TestsConducted onSubsystem Elaborate testplansweredrawnatthecomponent, subassembly andassembly levelstoqualifythe mechanism. Oneoftheimportant testsisthelifetestontheSMAwireactuator tounderstand the memory/stroke loss.Aspecial PC-based system wasdesigned forconduct ofthelifetest.Thesystem setup,showninFigure9,consisted ofaloadsimulation fixture,LVDTforstrokemeasurement, power supply, powerswitching box,andaPCforselection ofnumber ofcycles, heating time,cooling time,and number ofcyclesofoperation. I I_[ PowerSwitching Box I L°TSupply Amplifier]_ PowerSupply 3 E I 7777"77_ Figure9.SMAactuator lifetestsetup Important testsconducted onthemechanism included: •Loadelongation tests,determination oftransformation temperature,swage jointstrength test, andlifetestsontheSMAlinearactuator amounting tomorethanI(Tcyclesofoperations. Life testingwasdoneontheSMAdrivemotormechanisms underthermal-vac. •Transfer orbitcondition simulation onSMAlinearactuator andmechanism priortolifetests. •Heating andcooling timeevaluation atambient andthermal-vacuum condition fortheSMAwire actuator toarriveatoptimum heating andcooling timesforon-orbit operations. •Drivetestingofthemechanism withtorqueloadsimulation, withflap,andwithspacecraft before andaftervibration andacoustic tests. 247
On-Orbit Performance TheINSAT-2E spacecraft waslaunched successfully onApril2,1999onboard theArianeVehicle from Kourou, French Guyana. Thesolararraydeployment wasdonesuccessfully onApril11,1999.Asthere wasnoneedtomovetheflapsmounted ontheoutboard endofthearrayatthattime,theywerenot movedimmediately. Subsequently, theflapmovement testwasdoneon14July1999.Asanfurther precaution toavoidover-heating theSMAwires,thedrivewasattempted withalowerheating timeof0.9 second. Theheating timewasincreased to2.3seconds instepsof0.2second, untiltheflapsstartedto move.Theflapsstartedtomovewithaheating timeof2.3seconds, whichcloselymatched withthe ground thermal-vac testdata.Boththeflaps(Flap-1 andFlap-2)forward andreverse movements were testedandallthemovements occurred withsame2.3-sec heating timeindicating theconsistency ofthe mechanism. Inaddition topotentiometers indicating thechanging anglewitheachstepcommand, the flapmovement wasalsoindicated bythechanges seeninsatellite gyro-rate asshowninFigure10. Sincethen,boththeflapdriveshavebeenoperated manytimesandareoperating consistently. 0.05 o,olo++++++ g0,  4,015++++-ti++++"_++++ / +++++ ++++ ++++++++++++++++++++++ t,,LJ 19202122 567'8'91113l 15t61718'"' ' Timeinminutes Figure10.Satellite gyro-rates during flapdeployment L Thesatellite attitude controlratesdisturbances wereconsidered toarriveatthedetenttorquevalue(to holdtheflapsinthedrivenposition) withamarginofmorethaneightwhiledesigning themechanism. Its adequacy wasalsoprovedwithflapsholding totheirposition duringorbitcorrections. Butwhenthesouth side(two)22-Nthrusters werefiredincontinuos modefornorth-south stationkeeping operation, flap-l, whichwasopened tothefull180°position, hittheendstopandbounced back.Fromthedwelldataofthe flapmovement, velocity oftheflapatthetimeofhittingtheendstopwascomputed. Usingthevelocity andbounce-back angletheon-orbit detenttorquewasestimated andfoundtobeasperthedesign. A checkonthethruster plumeimpingement torqueontheflaps,whichare9metersawayfromthe thrusters, indicated veryhighplumetorque. Thishightorquewhichwasinexcess ofdetenttorque provided inthemechanism andcaused theinadvertent movement oftheflap. 248
Presently,north-south stationkeepingoperationsarebeingcarriedoutafterkeepingtheflapsinthefully closedpositionorinthefullyopenpositiontoreduceprojectedareaofflapstothethrusterplume.For futuremodels,itisplannedtoincreasethedetenttorque. . , , . . o ,Lessons Learned Duringrepeated cycling, afterabout50,000cyclesofoperations, alossinstrokeoramemory fatigue ofaround 12%wasobserved, andremained thesameupto105cyclesofoperations. Electrical actuation parameters areveryimportant forlonglifeoperations ofSMA,asoverheating/creation ofhotspotscouldresultinlossofmemory. Memory/stroke lossduetocycling is smallandcanbeaccommodated inthedesignandmemory lossduetoover-heating resultsintotal lossofstroke. Directheating oftheSMAactuator utilizing higherresistance ofNITINOL wireswithpulsewidth modulated electric heating wasfoundtobemoresuitable foruniform heating compared todirect electrical heating. Thisfurtherensures longlifeoftheSMAactuator. Provision foronboard programmable datacommand withadjustable ontimeforSMAelectrical powering andactuation isnecessary. Thisensures thattheSMAwiredoesnotgetover-heated and canactuate atdifferent temperatures ofon-orbit environment. Foron-orbit operation, aminimum coolingtimeof5minutes isprovided toensure complete cooling of SMA.Thermal-vac testsconducted onthewireactuator andonmechanism todetermine SMAwire heating timeandcooling timesinvacuum provedtobeveryusefulforon-orbit operation planning. Heating timecooling timesseeninvacuum weretotallydifferent fromambient. Chances foroverheating werefoundtobemoreinvacuum thaninambient. APC-based testsystem wasusedforallground tests.Software protection hadtobeincluded to avoidover-heating ofSMAwiresduetomanual settingerrorsduringthermal-vac tests,whereasin ambient tests,withairpresent, over-heating problem wasnotseen. Accurate teethalignment between thetworatchetwheels isnecessary infixingthewheels ontothe mainshaftformatching offorward andreverse steppositions. Conclusion Anovelmechanism usingashapememory alloylinearactuator todrivesolarflapshasbeendeveloped andissuccessfully operating ontheINSAT-2E satellite. Themechanism iscapable ofdrivingtheflapin 2°steps,andcansustain morethan105cyclesofrepeated operations. Extensive testingofthe mechanism atcomponent, subsystems andsystem levelhasindicated thatthemechanism isreliable for repeated operations. Themechanism ismagnetically cleanandhasthecapability toreplace conventional electric stepper motorforslow-speed operations. Various lessons learned inthe development; testingandon-orbit operations havebeenhighlighted. 249
Evolution ofanActuator Family PaulStigell', PekkaKyrenius*, TomiYlikorpi*, KaiViherkanto* Abstract V-I-I"Automation gotinvolved inspaceflight engineering inthemid-1980's andhelpedintheformation ofthe Finnish spaceindustry. Thefirstdesignwasawormgearmechanism thatopened anauxiliary solarpanel fortheASPERA instrument onboardtheSovietPhobos spacecraft. NextcametheSolarWindANisotropy (SWAN), including a2-axisscanning mechanism, onboardESA'sSolarandHeliospheric Observatory (SOHO). TwoSWANsensorunitscoverafieldofviewofnearly4_VTTAutomation developed scanning mechanisms fortwoinstruments onboardNASA's Cassini spacecraft, namely theACTactuator forthe CAPSinstrument andtheTTturntable fortheMIMI/LEMMS instrument. Theactuator concept wasfurther developed inESA'sMPRAproject. Aversion ofACTactuator iscurrently beingmanufactured forNASA's TwoWideAngleImaging Neutral AtomSpectrometers (TWINS) mission. Introduction ASPERA Inthemid-1980's, theFinnish PrimeMinister wasaskedonavisittoMoscow aboutFinland's interest to haveaFinnish AirForceofficerflyasacosmonaut intheSovietmanned spaceprogram. Thiswas politically undesirable forFinland sincemostnon-Russian cosmonauts camefromtheSovietUnion's satellite countries. TheideaofaFinnish cosmonaut wasconverted intotheco-operation between Finland andRussiainthefieldofscientific spacecraft andtheirinstruments. ThisledFinnstoparticipate inthe Phobos, Mars-96, Interball, Spectrum-RSntgen-Gamma andRadioastron missions. ASPERA (meaning "Difficulty") wasaSwedish plasma instrument forthePhobos mission. Itcontained a "singleshot"smallsolarpanelopening mechanism. Itusedasmallelectric motorandawormgear.The factthatthiswasthebeginning ofFinnish spacecraft engineering ismanifested bythefactthathardlyany projectdocumentation canbefoundwhichiscontrary toalllaterspacecraft projects. TheFinnish-built hardware functioned welluntilthetwospacecraft vanished nearMarsin1988. SWAN VTTstartedthedesign, manufacturing andtestingofSWANinstrument's structures andmechanisms in 1989foraFrench-Finnish science team.Themechanism designstartedfromscratch sinceSOHOwas Finland's firstspacecraft assignment forESA.SWAN's scanmechanism isshowninFigure1. Theinstrument's conceptual designfreezing driftedafterendofPhaseB.Various mechanism concepts wereinvestigated including theuseofwormandbevelgearsandeventhepossibility tomanufacture the motorsin-house wasconsidered sinceknowledge ofmagnet technology wasavailable. Finally,the mechanism designevolved aroundtheFrenchSAGEM toroidal hybridstepper motors(35PP810101 WW)andinternally preloaded MoS2solidlubricant-coated thin4-pointbearings witharobotics textbook sort ofconfiguration. Themotorhasamaximum outputtorqueof0.4N-mandapower-off detenttorqueof0.040N-mthatis enough tokeepthemechanism stationary eveniftheunitisunpowered andthespacecraft maneuvers. Because ofambitious scientific goals,whichintheendwerenotsoughtafter,atoowideopticalpath(DIA *VTTAutomation, Finland Proceedings ofthe34_Aerospace Mechanisms Symposium, Goddard SpaceFlightCenter,May10-12,2000 251
40mm)wasbuilttothesensorunit.Thisledtoaheavierandmoreexpensivemotorbeingselected.The designdidbecomeadvantageous whenVTTAutomation startedtobuildrotatingactuatorsforCassini. Thebearingsarefour-pointinternallypreloadedonesbyKaydon[1].Oillubrication usingFomblin Z25, ceramic composite bearings andMoS2orleadplatedbearings wereconsidered. MoS2wasselected because thebearings wereclosetoopticalelements andalsobecause therewaslimitedvolume forseals. Thefrictionofthebearings isnominally 15-25mN-m(peak35mN-m). TheMoS2-coated bearings were procured bytheEuropean SpaceTribology Laboratory (ESTL) [2]andthatrelationship hasworked wellever since.Forprototype testing, liquidlubricants havebeenusedbutmoving fromsolidtoliquidlubricants has neverbeenconsidered forflightusebecause ofthelearning curveeffect.'_Whether tobuyordevelop components" question wassolvedtherightway.ThisprojectledtoatriptoCannes forthe4thEuropean SpaceMechanisms andTribology Symposium [3]tolearnaboutvacuum tribology. MuchlaterMIL-A-83577 [4]wasread.Bothconfirmed thatourthinking isrightforspacemechanism design. Inthosedays,basic training (e.g.,[5]and[6])andEuropean guidelines [7]werenotavailable. Resistive position sensors byPenny&Giles[8]wereusedbecause theyfitmechanically theconfiguration best.Thedrivingofthestepper motorisdoneinthemicrostepping modethatmakesthedisturbance torquesmall.Feedback controlisusedbutincasetheencoder (beingoneofrelatively manynonredundant itemsinthedesign) information islost,themotorcanbedrivenincommand modecounting the step. BaffleMirror Figure1.SWANSensor Unit'speriscope mechanism. SWAN's "periscope mechanism" waslatched duringthelaunchwithpin-in-the-hole-mechanism thatwas released in-orbitbyStarsys Research Corporation's [9]pinpullermechanism thatisbasedonaHigh OutputParaffin (HOP)actuator. Thisusageofaresettable non-pyrotechnic actuator isbelieved tobethe firstofitskindinESAflightprojects. SOHOhasstringent cleanliness requirements andtherefore theissue oftheparaffin waxwasdiscussed atlengthuntiltheactuator wasshowntobehermetic. Thelockandthe actuator pindidtendtocoldweld(metal-metal pair)orrelease debris(metal-plastic pair).Aftertestingfive different designs, aNitronic steelpinwithaVespel liningwaschosen. 252
Duringthisproject,VTTAutomation gotridofdrawingboardsandstartedtouseAutoCAD. Structural analysis wasperformed usingI-DEASandNASTRAN onaCray-supercomputer. Theengineering staffmore thandoubled because theSpaceTechnology Groupdeveloped simultaneously threeotherspace instruments, participated infeasibility studyofanationalsatellite, etc.I-DEAS3D-modelling was introduced firstforfaster(thanESATAN) thermalanalysis (usingTMGsoftware) oftheinstruments andalso theSWANmechanism. Aspacecraft projectrequiresworkplanning andcostfollow-up. Typically 80%ofthetotalcostislabor cost.Thustheextension ofproject,asinthecaseofSWAN,meansaddedcostjusttokeepthe knowledge withintheteam.VTT(aresearch centerwithpersonnel of3000)hasacomputerized cost accounting systemthatallowsmulti-level costbreakdown (downtoworkpackages). Actually, despiteof thislargesystemtheprojectmanager musthandlethecostfigurestogainrealinsight.AgainKelly Johnson's "SkunkWorks"rules[10]arehandy:theprojectmanager mustkeepthecoreteamsosmall thatheknowswhodoes/did whatandwhen.Assembly, integration andtestingmaybecome easierand fasterwithmorepeople,butdesignengineedng doesnotunlesstheteamislackingsomespecial knowledge. SWANmechanisms [11]areassumed tohavegonewaybelowtheircoldthermallimittemperature during thesummer 1998incidentwhereflightcontrollostSOHOforseveralweeksandthespacecraft wasin survivalmode.Sincerecovery ofSOHO,SWANhasbeenworkingwellandstartedits5thyearofoperation inDecember 1999. Rotating Actuator Requirements andProperties Theactuatorfamilydescribed hererotatesapayloadthatismounted ontopoftherotatingactuator witha transition platethatallowsadjustment ofthepayloadcenterofgravity(COG)totheaxisofrotation. Typical requirements andproperties ofthedifferent actuator modelsaresummarized inTable1.CoGoffsets, mass,andlimitloaddetermine structural issues,especially fortheMarman clamp.Therestofthe parameter valuesdetermine themotionaspects, i.e.customer requirements. Table1.Theactuator family-ACT,TT,MPRAandTWA-allhaveheritage fromSWAN. Mass,kg Power,W Rotated mass,kg CoGelevation from Marman clamp centerline, mm Designload,g Molaboutaxisof rotation, kg-m2 Scanrange,degrees Scanspeed,deg/s Pointing accuracy, degrees Life_cycles Limittemperatures, °C, operationalSWAN CAPS/ACT LEMMS/TT MPRA TWINS/TWA 2 4 3.2 3-4 3.6-4.0 <1 <10 25 small +90peraxis 0-5 _+0.13 18 218 32 0.4 ±110 0.53 6 150 32 0.3 continuous 2-12 0.5 25000 250000 500000 -45to+55 -35to+50 -40to+55 Radiation tolerance, 100 100 1002-3 5-20 100-250 <4O 0.2-0.5 continuous3 17 150 28 0.5 +90 1-120 3 0.01 ±0.05 3M 700000 -45to+55 -45to+55 vanous 100 253
krad Magnetic disturbance, >2.5 2.5 2.5 various 10 nTat1m Feedthroughs optical 20wires,cable slipring various 20wires,cable ... wrap wrap CAPS/ACT andLEMMS/TT Backaround Originally theCAPSinstrument forNASA's Cassini wasdesigned tobelocated onarotating platform (delivered byNASA)attheendofaboom.In1992,theCassini mission wasde-scoped andtheso-called platform instruments weremounted directlyonthespacecraft. Inordertorestorethescientific capabilities ofCAPS,theprojectteamdecided toaddanactuator (ACT)thatrotatestheinstrument inawindshieldwiperfashion [I9].VTTAutomation didthedevelopment, design, fabrication andtestingoftheACT including itsdriveelectronics. Another instrument ontheCassini FieldsandParticles Platform, namely LEMMS, alsosuffered fromthe lossoftheboom,andVTTAutomation builtanACT-like actuator (calledturntable, I-F)fortheLEMMS too. Thesetwowere"similar" andthedesignwastohavelotsofsynergy. However, LEMMS required aslipring anddifferent motioncontrolelectronics. Theheights, thepayload masses, andlaunchloadsweredifferent. Alsocustomer requirements weredifferent. Twodifferent designs evolved. Motion ACT,showninFigure2,rotatesCAPSinawindshield-wiper fashion +110degrees atanaverage scan speedof1deg/s.LEMMS/TT rotatescontinuously atvariable speedintherangeof0to5deg/s. Pot_ Stot_rPurge EIc,ctron[c,s IT_e I;_',,_rd_ ThinTIQ,ncja Spr'tttgholder Plasticlin_r Cablewrap Corr_r_ssion spring Figure 2.Crosssection ofCAPS/ACT. Thechosen stepper motoristheSameasintheSWAN. instead ofastepper motor,aDC-motor witha wormorbevelgearswasconsidered, butthesesystems wereabandoned because ofhighmassandi L-; 254
r_l --IJI Ii I! i! Ir__ Bearing j,.J Mounting bolts  Flexible flange _ Housing II  Threaded holeformounting  Adjusting ring  Motorstator  Motorrotor  Shatt  Magnetic shields ---------- Housing I _-I_ Encoder "I/////////,¢ Figure3.CrosssectionofCAPS/ACT motorbearing assembly. Thedrylubricant forthebearings ision-plated leadassuggested byESTLwhoalsoprocessed thecoating. Thelubricant androllingsurfaces easilysufferseveredamage iftheballsdonotrollbutskidalongtheball track.ESTLusedtheirspecialsoftware thatgivescalculated valuesforminimum andmaximum preload undergiventemperature difference between thehousingandtheshaft,andforgivenbearingseataxial stiffness. Thesoftware givesalsoanestimate forsteadymeantorquevs.preload. Withtheaidofthis information therequired bearingpreloadandhousingstiffness weredetermined. Anadjusting ringthatwasplacedbetween thebearing's innerringstunedthepreload. Thickness ofthering definesdistance between thebearinginnerrings,whilethehousingpressestheouterringstowardseach other.Bearingpreloadisdetermined bymeasuring theheightofthemotorhousingwithandwithoutthe bearings. With1N/_mbearinghousingaxialstiffness thenominal68-Naxialbearingpreloadstaysbetween 45Nand 92Nwhenthethermalgradient overthebearings is-beloW 10°Cineitherdirection. Thebearingaxialstatic limitloadis1030Nandaxialdynamic limitloadis290N.Themotorbearingassembly hasbeenoperated overthetemperature rangefrom-50°Cto+60°C.Duringtests,theirfrictiontorquehasbeenmeasured with aKistlertorquemeter.Thefrictionwasdem6nst_rated tovaryonlyalittlewithrespecttotemperature. ,Structural designoftheMarman clamD Thebearings cannotcarryanylaunchloadsfromtheinstrument massloadsduringthelaunch.Theloads aretransferred between theupperandloweractuator partsthroughaMarman clamp.Thefirstapproach wastoseparate theACTrotorandstatorinthelatchedstatewithaW-shaped latchring.Thefriction between thelatchprofileandACTflangeswasnotenoughtopreventpayloadrotationduringvibration testing. Itwasdifficult-to addstructural rotationlockssincetheymightobstruct opening ofthelatchring. Thedesignwaschangedto presstheACTrotorandstator againsteachotherwithaV-shaped latchring. Tooth-shaped PEEK-p|astic ringplacedbetween thebodiesprevents rotationwhenlatched. Operating thelatchre-quires a2-mmaxialseparation between therotorandstator,whichismadepossible byaflexiblem0UntTngbetween themotorshaftandtheACTstator.Thesteppermotorstator,thepartwith windings, isrigidlymounted totheACTrotor.Whenlatched,therotormoves2mmdown(Figure1)andthe motorassembly moveswithit.TheupperbearingmounIed tothethinbearingflangepressesthehollow shaftdownwards. Theshaftismounted tothecompression springsupported bytheflowerthatalsomoves downagainstthecompression springs. Theflowerpartismounted totheACTstatoralsowithathinplate springthatallowsaxialmovement butnolateralmovement orrotation. Thecompression springskeeptheupperbearing=pre!oaded andactagainstvibration loadsoftheshaftand flowerduringspacecraft lift-off.Thelowerbearingisunloaded duringlaunch.Thisispermissible sinceit doesnotcarryanyexternal loads.Whenunlatched, therotor,theshaft,andtheflowermovebacktoupper 256