ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
camsinunknown positions. Adouble engagement operation is performed tosteadythe MTduringEVAreplacementofafailedLTU. Duringunlatching froma worksite, theboosterfoot assembly initiallyprovidesaverticallifting force,bypushing onthe railwithaspringload, andthenliftsclearofthe railafteradrivewheelis engaged. EVAcamsare usedtoplacethedrive wheelsintheirlaunch positions toallowengagement ofthe swingarm launchlocks. Theyalsoallowan astronaut todisengage bothtranslation drive wheelsandreinstalltheDrive IdlerWheel Worksite MotionSensorMT2773 ement/ Disengagement DriveIMCAs Assembly Preload Spring Assembly Swingarm Struc!ure /I_CamAss_emb.ly.. tIDeletedforCladh o 0 05m o0 Scale Figure3.LDURearIsometric View. launchlocksforEVAremoval andreplacement oftheLDU.Allcomponents oftheLDUarehousedina common structure assembly tofacilitate packaging asanORU.TheLDUORUisattached totheMT structure withfourEVAORUbolts. Translation Drive Eachtranslation drivesystemcontains anIntegrated MotorController Assembly (IMCA),ageartrain including aplanetary gearbox, apreloadspringassembly, abrakeassembly, anda30.5cm(12inch)- diameter drivewheelwithanelastomer tire.Thedrivecomponents aretiedtogether byacommon swingarm structure whichhousespartofthegeartrain,acceptstorqueinputfromthegearbox/IMCA combination, supports thedrivewheelandbrakeassembly, andacceptsattachment ofthepreloadspring assembly. Thepivotingendoftheswingarm assembly ismounted tothemainstructure oftheLDU.The gearbox/IMCA combination ismounted ontheopposite sideoftheLDUstructure wallthatsupports the swingarm. Thepreloadspringassembly loadsthedrivewheelandtheidlerbogiewheelsagainstthe trussrailtogenerate thetractionrequired fortranslation. Figure4displays across-section ofthe swingarm drivecomponents coupledtothegearbox assembly andIMCAdrivemotor. Powerandcommunication isprovided totheIMCAsineachofthetwodrivesthroughdedicated electrical strings. Duringnormaltranslation, onlyonetranslation driveisengaged tothetrussrailandtheother remains initsdisengaged positionasabackup. Eithertranslation drivecanbetheactivedriveorthe backupdrive.Thebrakeassembly oneachswingarm provides abrakingtorquetotheddvewheelwhen poweristurnedoffinordertocontrolMTstopping uponunexpected lossofpower.Normalstopping is accommodated throughcontrolled deceleration oftheIMCA. Engagement/D sengagement Drive Theengagement/disengagement driveassembly consists ofaprimeandaredundant system,eachwith itsownIMCA,gearbox, spurgearpass,switches andcam.Athirdcam,referred toasthecentercam,is 119
alsoincorporated betweenthetwomain camsintheE/D assembly.Thecams canbeseeninFigure 2.Undernormal engagement/disengagementoperation, theIMCAofthe activeE/Ddrive powersthegearbox, whichrotatesthe spurgearthatdrives thecam.Thecam liftsorlowersthe associated translation drivethroughacam followerattachedto thetranslationdrive swingarm. Thecentercamisa contingency mechanismthatenables aDrive-/_ Wh{_el Whe_;I. Gear Drive Gear InLe Gear Input Swingarm Structure Planet Swingarm PivotPlanetGear RingGearMT2776 SunGear ®©© GearBoxLflMCA backupsystemto Figure 4.Translation DriveTrainCross-Section. disengage afailed translation drive.Thecentercamcarriesapinthatengages withslotsineachofthemainE/Dcams.If theprimarytranslation orengagement/disengagement systemfails,powerisremoved fromthefailed systemandthebackupsystemispowered. ThebackupE/Dcam,rotatingintheopposite direction from normalwheelengagement, firstengages thebackuptranslation drivewiththerailandthendrivesthe centercam,viaitspin,toraisethedisabled translation drive.Translation canthenbecontinued withthe backuptranslation drivesystem. Motionofthecamsiscontrolled throughtheuseofhermetically-sealed, magnetic reedswitches positionedontheLDUstructure andmagnets embedded inthecamstoindicatethetranslation drivedisengagement positions forallthreecams.Switches arealsousedtoindicatetheswingarm engaged positions. BogieAssembly Thebogieassembly consists oftwo7.6-cmidlerwheels,abogiehousing, motionsensors, andworksite sensors. Theidlerwheelsengagethetrussrailfromtheunderside, andreactthepreloadappliedbythe drivewheelduringtranslation. Whencrossing gapsintherails,oneidlerwheelisalwaysincontactwitha railsegment. Thewheelsareattached throughredundant bearingsetstothebogiehousing, whichisa rigidlyattached component oftheLDUstructure. Magnets ineachidlerwheelareusedtotriggermagneticreedswitches toformtwomotionsensors. TheMTtranslation direction andacoarseindication of speedcanbederivedfromthesequence andrateofswitchtransitions toprovideanindependent check oftheprimarysensors intheIMCA.Additional magnets andreedswitches areconfigured intotwo worksite positionsensorswhichcandetectferromagnetic strips,embedded intherails,tosignalthe locations ofworksites alongthestationtruss. 120
DriveTrainDesign Theprimaryrequirements forthetranslation driveare toprovideapeakdriveforceof374Nandacontinuousdriveforceof156Natcontrolled speedsbetween 0.13cm/sand2.5cm/s.Driveperformance hastobe maintained invacuum, overtemperatures ranging between -43°Cand+60°C,throughout theservicelife oftheMT.Thedesignaccommodates a11°Cqualificationmarginathotandcold. Fordriverequirements, theLDUusesaBoeing/Allied Signal-provided Integrated MotorController Assembly (IMCA)consisting ofabrushless DCmotor,amotor controller, aninternal15:1reduction gearbox, andan outputclutch.TorqueoutputfromtheIMCAiscoupled toa7.14:1reduction, single-pass, planetary gearbox assembly viaasplined-shaft connection. Thisconnectiondrivesthesungearofthegearbox assembly subsequently rotatingthethreeplanetgearswithina ringgear.Outputfromthegearbox assembly is delivered fromasplinedshaftintegraltotheplanet carrier.Withintheswingarm assembly, torqueis transmitted throughaspur-gear passtoapairof coaxialgearswhich,inturn,driveasecondspur-gear passthatpowersthedrivewheel.Figure5depictsthe swingarm assembly geartrain.Thebrakeassembly is connected tothedrivewheelthroughaseparate spur-MT2775 Wheel DriveGearWheel Gear Inl:Swingarm Pivot arm Structure Figure 5.Swingarm GearTrain. gearpass.Collectively, thetranslation driveassembly possesses a51.5:1gearratiotoprovideuptoa 534Ndriveforcefromthe1.6N.mmaximum outputtorqueoftheIMCA. Toavoiddrivetrainefficiency lossesduetomechanical interference fromthermalexpansion, fitsbetween matingpartsofdissimilar materials wereestablished toprovideatleast0.0013cm(0.0005inch)clearanceatworst-case temperature conditions. Axialfitswerealsoestablished toguarantee minimum bearingpreloads wherenecessary andtoavoidthermal-induced interference. Unfortunately, thisapproachcausescertaininterfaces toberelatively looseunderparticular temperature conditions; however, qualification testingdispelled concerns ofexcessive wearandfrictionpotentially resulting fromthis situation. Swingarm Configuration TheLDUimplements aswingarm designtoallowverticalmotionofthewheelwhencrossing stepsinthe railsandduringMTlatchingataworksite. Anadvantage ofaswingarm designisthatthedrivemotor, alongwithitsplanetary gearbox, canbefixedattheswingarm pivotandcommunicate thedrivetorqueto thedrivewheelthroughageartrainattached totheswingarm. Anissuewiththisapproach isthatthe introduction ofadrivetorqueatthepivotaxiswillproduce atorqueontheswingarm thatcanincrease or decrease thepreloadofthewheeltotheraildepending onthedirection oftravelandthegeometry ofthe swingarm. Variations inwheelpreloadneedtobeminimized sincedecreased preloadcanleadtowheelslippingand increased preloadcausesincreased driveforcerequirements forcrossing railsteps.However, thedrive wheelpreloadcanbemadeindependent ofdrivetorquethrougha"torqueneutral"designwhichmatches thegearpassesontheswingarm withtheoperating angleoftheswingarm toensurethatdriveforces resultinnonettorqueontheswingarm. Anaddedbenefitoftorquebalancing theswingarm isthat t21
eliminating torque-induced preloadminimizesrotationofthedrivewheelduetorotationoftheswingarm. ThisallowstheMTtoaccuratelyholditspositionalongtherailsasthedrivewheelrisesduringalatching operationataworksite. Thefollowinganalysisdisplaysthedependency ofwheelpreloadonswingarmorientationandgeartrain geometry.Thenormaldrivewheelpreloadgeneratedbythespringpackdoesnotaffectthetorque balanceoftheswingarm;therefore,inthefollowinganalysiswewillignorethespringpreloadandonly address theresulting preloadduetotheinputdrivetorqueandtheoutputdriveforce.Asindicated previously,theLDUswingarm hastwopassesofspurgearstoincrease thedrivetorqueatthewheel.When thegearsareunderload,reaction forcesaregenerated atthegearpivotpoints,whichcontribute tothe nettorqueexertedontheswingarm. Theindividual forcesontheswingarm (neglecting thespring preload) arerepresented inFigure6whereitisassumed thatthegearsmeshattheirstandard pitchradii. Theswingarm structure willonlyexperience forcesatthegearandswingarm pivotpointsasindicated in theresultant forcediagram ofFigure7.Staticbalanceoftheswingarm requires thatthenettorqueabout theswingarm pivotbezerowhichresultsinthefollowing condition. (rl+r2_1+r2/i=(rl+r2+r3+r4)[r2_+c°s(°_)Fn- sin(°')r2r4--_1r3jrl -Lr3r1 r3r5rlJ(I) Solvingforthetorque-induced preload, F°,intermsoftheinputtorqueresultsinthefollowing expression. -"_ [rlr3-r2r4+sin((z)r2r4] Fncos((_)r, r3(r,+r2+r3+r,) r3rS(2) Torquebalance isachieved whenFequalszero,whichisachieved when: r,(r2r,-r,,3) =r2r,(r'+r2+r3+r,)(3) FortheLDU,where(rt,r2,r3,r4,rs)=(3.387,4.180,2.328,13.600,15.240)cm,torquebalance is achieved fortheswingarm angle(_=34°. wasnotfullyachieved withtheas designed nominal 30°swingarm angle. However, theresulting preload fluctuations duetoamaximum drive torqueequivalent toa374Ndriveforce arepredicted byEquation 2tobeonly+25 Naboutthenominal 623Npreload. Duringqualification andacceptance testingoftheLDUnodifference in performance wasnotedforchanges in driveforceordirection. Material Selections Theselection ofsuitablematerials was drivenbystrength, stiffness, contact stress,weight,fracturecontrol,corrosion resistance, outgassing, atomicoxygen resistance andfrictionrequirements.Duetolatechanges ingearratios,complete torquebalance  Sr5'WheelTire MT2778 // Swingarm_ r4,wheelGear II {_Ft /III_--r2,1ntermediate Gear // i_._.. _.._////-_-r3, WheelDriveGear _k, _/_-- rl'InputGear  ,np°tTorqoe  _RailSurface Translation Force,Ft=t_(r21r3)(r,Zr5) Figure6.Torque Induced Forces onLDU Swingarm Assembly. 122
Structuralcomponents, suchastheswingarm structureandgearboxhousing,areprimarily constructed of7075-T7351 aluminumalloywhile CRESCustom455TMwaschosenforgearsand shafts.Thestructural portionofthedrivewheel anditssupporting spindleareTi-6AI-4V. Bearingsareofangularcontact, radialball,thrust,and needlerollerdesign,fabricated fromthetraditionalspace-bearing material CRES440C.The idlerwheelsaremadeofpolyetheretherketone (PEEK)toavoidcontactstressdamage tothe trussrail.Asiliconematerial wasselected forits superior frictionproperties anddurability touse asthedrivetire.Selection processes fortheidlerMT2777 c_,Swingarm _.7__r_,'_ Angle _wingarm J_-.._-'_  Swingarm Pivot--" Figure7.Moments AboutSwingarm Pivot. wheelsanddrivewheelmaterials aredetailed insubsequent sections. Tetrafluoroethylene (TFE)-Iined bushings areusedtoreduceslidingfrictionattheswingarm pivot.Alsointended tominimize sliding friction,VespelTMwashers areusedtolineinteriorsurfaces ofthegearbox assembly. Animportant selection criterion forlubricants istheabilitytoperformundervacuum overawidetemperaturerange.Allgearsandsplinesinthegearbox assembly andswingarm assembly aredry-film-lubricated withVitro-Lube 1220CTMtoreducefrictionatslidinginterfaces. Additionally, bearings withrollingelementsarelubricated withBraycote 601TMgrease. Allgeartrainsunderwent operational conditioning cyclestobreak-in thedry-film-lubricated components andverifybasicperformance. Materials Development Testing IdlerWheelMaterial Amaterials development program wasundertaken toidentifysuitablematerials fortheLDUdriveand idlerwheels. Addressing firsttheidler(i.e.,bogie)wheels,sevencandidate materials werefirstselected basedonareviewofmaterial properties. Fullsize(7.6cmdiameter) testwheels,consisting ofanaluminumhubwitha0.55cmthickplastictire,weremanufactured forallcandidate tirematerials. Inwhat turnedouttobeapoordesignfeature,whichwascorrected inthefinaldesign,thetiresonthetest wheelswerekeyedtothehubthroughnotchesonthetirerimandlugsonthehub.Duringtesting,the stressconcentration atthetirenotches didinitiatecracking forsomematerials. Thetestwheelswere subjected toaseriesoftestsassummarized inTable1. Table1.Candidate IdlerWheelTireMaterials Material ImpactTest LifeTest Thermal Vacuum =_thylene-chlorotrifluoroethylene (E-CTFE) Passed Distorted No Oolyap/Isulfone (PAS) Passed Cracked No Oolysulfone (PSO) Cracked No No Oolyetherimide (PEI),with30%glassfiber Abrasive No No Ooly(amide-imide) (PAl),3%TiO2,0.5%fluorocarbon Passed Cracked No Oolyetheretherketone (PEEK) Passed Passed Passed Oolyetheretherketone (PEEK), Abrasive No No Nith30%glassfiber ThevarioustestslistedinTable1wereperformed byrunningthetestwheelswitha490Npreload againstarotating30.5-cmodiameter anodized aluminum drum.Thetestwheelswerefirstsubjected tothe coldtemperature ImpactTest,whichsimulated theMTcrossing gapsandoffsetsinrailsegments by 123
Table2liststhevarious wheelcandidate materials alongwiththeirhardnesses andsomecomments on theirperformance. Underslidingloadstheelastomers exhibited twofrictioncoefficients: thestaticfriction coefficient determined bytheforcerequired toinitiateslidingandthedynamic frictioncoefficient determinedbytheforcerequired tosustain sliding. Elastomers havetheunusual property thatthesliding frictioncoefficient isgenerally greater thanstaticfrictioncoefficient. FortheLDUdrivewheelapplication, thedynamic frictioncoefficient wasusedbecause theMThastoproduce highdriveforcesonlyforavery shortduration andalimitedamount ofdrivewheelslipcanbetolerated inthissituation. Ingeneral, thesoftermaterials havethebestfrictionproperties, butdonotstandupaswelltohigh loading conditions asdothehardermaterials. TheverysoftCV-2287 material wasdropped earlyonasa candidate because itstartedtotearduringjointcrossing tests.TheveryhardM-950-A-41 material was eliminated laterbecause itsslidingfrictioncoefficient inairhaddropped fromtheinitialvalueof0.94toa valueof0.59attheendofextended testing, whichisbelowtheminimum requirement of0.6.Asa general comment, alloftheremaining candidates showed promise aswheelmaterial. However, thelast fivecandidates inTable2wereeliminated fromfurthertestingbecause processing problems ledtofullor partialdebonding ofthetiresfromthewheelhubsforallthewheels withtheAramid fiberbeltandforall thewheels withtheSRS185andSRS105materials. Unfortunately, thetightdevelopment schedule did notallowustoremanufacture testwheels forthedebonded materials sowewereunabletofurther investigate SRS105orSRS185materials ortoresolve ifreinforcing, suchasembedded fabricbeltsor random fibers,provides anyadvantage. Table2.Candidate DriveW1_eelTireMaterial Testing inAir Material ShoreAMinimum Comments Hardness Friction Wacker Silicone, M-950-A-41 85 0.59 hardest material, eliminated forlowfriction McGhan Nusil,CV-2287 30 - softestmaterial, tearsunderload DowComin9,STILT50 60 1.04 bestallround General Electric, SE6660 60 0.71 goodallround,secondchoice RhonePoulenc Rhodorsil, 65 0.71 goodallround,thirdcl_oi'ce..... MM771/70 RhonePoulenc Rhodorsil, 65 debonding oftirefromhub MM771/70 withAramidbelt ArlonSRS185 48 0.86 verygoodfriction, possible debonding ArlonSRS185withAramid 48 debonding oftirefromhub belt ArlonSRS105 57 debonding oftirefromhub ArlonSRS105withAramid 57 0.80 goodallrouncl,I_o.ssible debonding belt Twocandidate materials, theDowComing STILT50andtheGESE6660, wereselected foradditional thermal-vacuum lifeandfrictiontesting. Previously unused testwheelsforthetwomaterials werefirst vacuum bakedat+71°Candthenwereruninvacuum against therotating testdrumoverthetemperature rangeof66°Cto-54°C,withfrictionmeasurements madeperiodically throughout thetest.Overthe 5.3daysofvacuum testing, thewheels accumulated atotaltravelequivalent togreater thantwolivesof operation ofthefullsize30.5-cm LDUdrivewheel. 125
LDUMechanism Testing Development Testing Priortocompletion ofthedetaileddesign,theLDUpreliminary designwentthroughalimited"proofof concept" breadboard testprogram wherefullsize,butnotflightlike,swingarms, drivewheels,engage/ disengage camsandabogieassembly werefabricated andtestedasaunit.Thisprocess contributed to thematurity ofthedesignbuthadonlylimitedvaluebecause itlackedmuchofthedetailofthefinal design.TheLDUdesignwasalsomovedforwardand,moreimportantly, themanufacturing processes wereestablished throughtheconstruction ofanLDUmasssimulator forMTsystemlevelmodalsurvey andvibro-acoustic testing. Because theLDUwasbigenoughtocontribute significant localmodestothe MTmodalresponse, theLDUmasssimulator structure andORUattachment hardware wasbuilttoearly flightdrawings inordertoproviderepresentative stiffness properties. Qualification andAcceptance TestProgram Atthecompletion ofthedetaileddesignprocess, aflight-like LDUqualification unitwasbuiltandtestedin ordertovalidatethedesign. Duetoschedule pressures andtheextensive qualification testprogram, the actualflightLDUwasbuiltandacceptance testedbeforethequalification testsfinished. Sincethe purpose oftheacceptance testsoftheflightunitaretoverifyworkmanship ratherthantovalidatethe design,thetestswereonlyasubsetofthequalification tests.Thetestsperformed onthetwoflight-like unitsaresummarized inTable3. Table3.LDUQualification andFlightTesting Unit/Test Function Vibration Thermal Thermal Thermal Stiffness Strength Vacuum Balance FuII No 'i.0limit Qual. ,T,n, Flight Full180sec. qualification 60sec. screening3cycles -54to71°CHeater function 8cycles No No No No -43to60°CLife 1.1to1.52x10 limit},ears No Allfunctional testing,including thermalandthermal-vacuum, wasperformed onaspecialLDUtestfixture whichusedapowered andinstrumented railcarriage tosimulate LDUoperating conditions, suchas offsetsandgapsinrails,aidingorretarding loads,andMTlatchingoperations. Vibration testingand stiffness/strength testingusedspecialtestfixtureswithflight-like attachment hardware. Acceptance testingoftheflightLDUprogressed relatively smoothly; however, thequalification test program didsurfaceanumberofproblems thatwereresolved withfixesbeingincorporated intotheflight hardware aswell.Therecovery fromavibration overtest, duetoashakerrunaway, became anopportunitytofixsomeswitchproblems andcorrectaswingarm clearance issue.Asisoftenthecase,thetest fixture,whichhadtoworkoverthesameextremes andwasalmostascomplicated astheLDUitself,had numerous problems, whichsignificantly impacted thetestschedule. Onelessonlearnedwasthat, overall,thetestprogram wouldhavebeencompleted soonerandmorecosteffectively ifmoredesign efforthadbeeninvested inthetestequipment. Threemajorissuesregarding theLDUdrivesystemsurfaced duringqualification testing: •Adependence oftraveldistance andvelocityonload •Anincrease inmotortorquerequirements atcoldtemperatures •Alossofwheelfrictionduringlifetesting 127
DriveWheelEffective Radius Oneofthemoreinteresting discoveries fromtheLDUtestingwasthestrongeffectthedriveforcehadon thedistance traveled foreachrotationofthewheel.Anominally 15.2-cm-radius wheelcouldbehaveasif itsradiuswerebetween 13.5to16.5cmdepending onwhether loadswereaidingorretarding. Wheel slippingwasquicklyeliminated asapossible causeandtheeffectwastracedtoadistortion ofthesilicone elastomer attheinterface withtherail.Radialcompression ofthewheelcannotaccount forthemagnitudeofthiseffect. Asillustrated inFigure8foradrivewheelunderapreload (i.e.,anormalforce)andworkingagainstaload(i.e.,a lateralforce),theelastomer tirenearthecontactpointon therailwillbestretched ononesideofthewheeland compressed ontheothersideofthewheel. Whenthe wheelturns,aspotonthesurfaceoftheelastomer tire travelsfasterthannormalintheregionofstretching and slowerthannormalintheregionofcompression; butany spotonthetirestillcompletes onecomplete rotationfor eachrotationofthewheel.Forthesamewheelrotation rate,theLDUwilltravelmoreslowlyifthetirerotatesonto theregionofslowermovingtire(i.e.,rotatesagainstthe lateralload)andwilltravelmorequicklyifitrotatesinto theregionoffastermovingtire(i.e.,rotateswiththelateral load).Tire AluminurMT2578 Theendresultisthatforanelastomer tire,thedistance Figure 8.TireDistortion forLoaded traveled alongtherailforeachturnofthewheelisgreater DriveWheel. foraidingloads(i.e.,loadsinthedirection ofmotion)and islessforretarding loads(i.e.,loadsagainstthedirection ofmotion). Notethattherubberisinintimatecontactwiththerailatalltimesandthereisnotireslipping orscuffing, juststretching andcompressing oftherubber. The"effective radius"ofthewheel,definedasthedistance traveled dividedbythewheelrevolutions in radians, isausefulwaytocharacterize thewheelbehavior. Thermal-vacuum testdataforambient, hot andcoldconditions arepresented inFigure9asanLDUdrivewheeleffective radiusversusdriveforce plot,whereretarding forcesarerepresented asnegative aidingforces.AsevidentinFigure9,theessentiallylineardependence ofeffective radiusondriveforceisverysignificant butanyvariation asafunction oftemperature iswithinthescatterofthemeasured data.Forpredicting MTperformance thefollowing formulawasderivedfromalinearleastsquares fitofthecombined data. effective radius(cm)=15.52+O.O0708x(aiding loadinN) (4) Thisbehavior isnotdetrimental totheLDUperformance because itispredictable. Theresulting alteration oftraveldistance isaccommodated bytailoring thetranslation software stopping parameters. TorqueLimitsatTemperature TheIMCAdrivemotorshaveconsiderable excesstorquecapability butarenormally operated inamode whereoutputtorqueislimitedbylimitingmotorcurrentbasedondownloaded software parameters. At coldtemperature extremes, thetorque-limited motorscouldnotcomplete somefunctions. Inorderto resolvethisproblem, theLDUoutputforceversusmotordrivecurrentwasmeasured overthefullthermalvacuum rangewiththetorque/current limitoff.Theresultsindicatealineardependence ofoutputforce onmotorcurrentwithaslopewhichisrelatively independent oftemperature. 128