ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Table2.DCMotorTerminology Description Symbol Definition MotorConstant Km Torque Sensitivity Constant BackEMFConstant Winding ResistanceKt KbKmdefines themotor's outputstalltorquecapability intermsofpower consumption. Generally, itisafactorallowing comparison among motors. Kmiscalculated as: Km=StallTorque /(Power Input) Or 0.5 Km=Kt/(R) whichareequivalent expressions.0.5 Ktdefines themotor's sensitivity toinputcurrent orabilitytoproduce torqueasafunction ofinputcurrent: Kt=Torque Output /Current Input Ktiscalculated duringthemotordesignanalysis basedonthemotor's number ofpoles,magnetic circuitflux,andtotalwinding conductors. Testing ofthemotorallowsconfirmation ofthecalculation. Kbdefines themotor's theoretical voltage requirement toachieve speed. Depending ontheunitsofanalysis used,KbisequaltoKtordiffersfrom Ktonlybyaconstant. Kb=VDC/unitofspeed. Rdefines theresistance ofthemotorwinding(s) acrosswhichoperating powerisapplied. Insomecases, phasewindings aretiedtogether for excitation, somotorRisthiscombined value. Thesebasicconstants helpdefinehowthemotorwillinterface frombothapowerinputandspeed-torque outputstandpoint. Theapplication ofthesefactors intoameaningful equation isshowninEquation (1): Vapplied-('_/Kt)oR +(cooKb)Eq.(1) where, Vappliedisthevoltage applied tothemotorwindings acrossR "cisthegrossmotortorqueinunitsconsistent withKt Ktisthetorquesensitivity constant forthemotor coisthemotorspeedinradians persecond Kbisthebackemfconstant ofthemotor Risthemotorwinding resistance inohms. Equation (1)expresses therequired motorvoltage intermsoftwocomponents, thevoltage duetotorque production andthatduetoachieving speed. Inthefirstterm,('_/Kt),isanexpression oftherequired current toproduce thetorque"_.Whenmultiplied byresistance, thevoltage required fortorqueproduction isexpressed. Thesecond term,co•Kb,isthevoltage required toachieve theoperating speed, co. Generally, thesumofthesetermsindicates theminimum current andvoltage required foroperating at torque'_ andspeedco.Equation (1)isusefulwhenstarting withaknownspeed, torque, andpowerto determine theapproximate motorwinding. Or,ifthemotorwinding isknown, thentheequation maybe usedtoestimate therequired voltage andcurrent toachieve acertain speed-torque. NotethatonceKt andRareknown, themotorconstant, Km,maybeobtained forcomparison ofmotors. Equation (1)maygenerally beusedinevaluation ofallDCmotortypes;however, therearemodifications required depending onthemotortypeanditsapplication. Thetorque"_showninfactmustinclude the combination ofoutputtorque, internal dragtorque, low-speed cogging torque, andspeedrelatedviscous torques. Ristakenasthewinding resistance without anytemperature effects. Butinpractice, ifthereis thermal rise,Rmustbeincreased (ordecreased ifcold)toaccount forwinding temperature changes. Equation (2)indicates howRchanges withtemperature: NASA/CP--2002-211506 92
BrushedDCmotorshavemagnetsplacedinastationaryringontheoutsideoftherotorassembly. Magnetstypicallyhavearadialorientationwithfluxgoingdirectlyintotheairgaptowardsthelaminated rotorstack.Itisnecessaryforthemagnets,windings,andcommutator tobe"clocked"withrespectto eachother.Hence,themagnetringmustbeadjustedbydesignorafterinstallationforoptimalposition. Thismayalsobeaccomplished byadjustingthebrushlocation,butthatisusuallyimpossiblewithhoused motors.Theadjustment fundamentally assuresthatthephasingbetweenthearmature,magnets,and commutator isoptimizedforperformance. Windingsfilltherotorslots,whichopenontherotorouterdiameterforinsertion.Thewindingstyleis typicallysimplexwave-wound, withidenticalcoilsuniformlyplacedinmotorslots.Inter-connections amongthecoilsaremadeatthecommutator bars.Theendresultisthatallcoilsareconnectedtogether butdifferences inexcitationpolarityoccurdependingonwherethebrushescontactthebars.Thespacing ofbrushes,windingcontacts,andcoilspansmustbecorrectforacceptable operation. So,forspaceapplications, thereareseveralover-riding concernsthatmustbeaddressed whenusing brushedmotors: (1)Isthemotordesignconservative enoughtoprovideacceptable brushwear? (2)Isthemotorbrushlifeexpectancy adequatetomeettheapplication liferequirement? (3)Willthepowerandperformance bemetwiththebrushmotorunderconsideration? Itisprudenttoverifythese(andother)concernsbothbyanalysisandbytestdemonstration. Chiefof theseconcernsforaspaceapplication iswhetherornotthebrushlifeexceedstheapplication requirement, sinceitwillbeimpossibletoreplacewornbrushesinspace. Brushless DCMotors Unlikethebrushed motors, brushless motorsaredesigned toeliminate thebrushes withtheirwear, speed,andlifeconcerns. Brushless DC(BDC)motors havebecome averyprominent choiceforspace applications, sincetheseconcerns areeliminated. However, brushless motorsrequire aconsiderable amount ofengineering toprovide thecommutation anddrivecircuitry external tothemotor.Instead of employing brushes andacoppercommutator, BDCmotors requiredevice(s) tofeedmotorrotorposition information tothedriveelectronics (seeFigure2.).Halleffects, encoders, resolvers, orothermeansmay beusedtosupplythisinformation, butthefeedback devices mustbeanintegral partoftherotating motor shaftassembly. L Figure2.Two-Phase, BDCMotorwithIntegral DualResolvers forCommutation Theissueofelectronic commutation isofprimary importance, sinceitistherotorposition information that isusedtocontrolwhichmotorphases arepowered atanygiventime.Notethatthisfeedback iscriticalto achieve smooth motion forclosedloopsystem control. Without awell-regulated position feedback and NASA/C P--2002-211506 94
possibleforthesizemotorchosen.Thisapproachisfairlystandardforalowspeedrange,highmotor constantandoutputtorqueBDCmotor. 960658 Figure3.Extremely Low0.2%Torque Ripple, Two-Phase, 24PoleBDC Torque MotorforHubble Telescope FinePointing Mechanism Abitmoredesignfinesse isrequired whenotherrequirements comeintoplay.Twocommon specialized requirements areforlow-loss, high-speed BDCmotors andlowtorquerippledisturbance BDCmotors (seeFigure3.)Toattainhighlevelsofspecialized performance, themotorconstant cannotbetheprimary requirement. Hence, whileenvelope andpowerconcerns arepresent, motordesign features must change tomeetthespecialized application. Themagnetic circuitmustchange topermituseoflowerloss materials atlowermagnetic fluxdensities. Theairgapwillbeincreased, andunique toothandpole features mayapplythatwillfurtherimpactthemotorconstant. Forhighspeed, low-loss designs, thesix-step method withHalleffectdevices isthebettercommutation choice. Halleffectsaresuitable forhigh-speed mechanisms of50,000rpmormoreifthepolecountistwo orfourpoles.Intense designeffortwillbefocused onminimizing lamination losses,whichgoupbothwith increases inspeedandthenumber ofpoles.Itisentirely possible thatextremely thinlaminations orno laminations atallwillbeused,depending ontheapplication. Circuitbackironmaysurround thewindings butrotatewiththemagnets tokeeplosseslow.Theobvious tradeoff lossisthedecrease inthemotor constant. Attheotherendofthespeed-torque spectrum, lowrpmBDCmotors withverylowtorquedisturbance characteristics (seeFigure3)arehardertooptimize thaneitherhighKmorhighspeedmotors. These applications require moreconsideration aboutthemethod ofcommutation andthedriveelectronics. Two andthreephaseBDCmotors havebeenusedsuccessfully. Oncethemethod ofcommutation hasbeen chosen, thenthemotordesign detailsdiverge. Withthree-phase Halleffectcommutation, magnet and winding design features aredifferent thanthosechosen fortheBDCtwo-phase, sinusoidal method, primarily duetoconcerns aboutthemotorbackEMFwaveform. Ingeneral, thecommutation method is likelytobedrivenbythesystem requirements andconcerns ratherthangivingthecommutation method consideration first.Therefore, themotordesignforalowtorquerippledisturbance application willrequire optimization basedon: (1)System levelconcerns andrequirements (system levelofacceptable torquerippleanddisturbances willbeestablished) (2)Commutation anddriveelectronics scheme chosen (twoorthreephases, sixsteporsinusoidal will usuallybedirected requirements fromthesystem level) (3)Trading offlowmagnetic cogging versus nocogging (should themotorhavelaminations orgo "ironless") NASA/CP--2002-211506 96
Figure4.Deployment Actuator forSpaceStation Photovoltaic Radiators SolarArrayDrives Pointing asolararrayisacomplex system dynamics problem. Factors suchasthelargeinertia,structural compliance, lowresonant frequencies, andeffectsofattitude control callforspecial measures. Accuracy isgenerally notveryimportant, afewdegrees isoftenadequate. Sincethesolararraymustrotateor cyclethroughout thelifeofthesatellite, longlifewithlowpowerconsumption isimportant. Almost universally, thestepper motoristhedriveofchoice. Itistypically two,three,orfourphase depending onthesystem electronics driveselected. Itcanbepowered inshortburststoadvance the array,andthenbedormant untilmovement isagainrequired. Itcanalsobepowered atveryslowstep ratesbuttheactualapplication ofpowercanbeontheorderof50milliseconds foreachstep.Inthisway, theaverage powerisminimal andthemotorisatrestmostofthetime,contributing tothelonglife needed. Italsohasahighun-powered holding (detent) torquewhichrestrains thearray. Thestepper motorneedsgearing tobeeffective. Backlash isaserious system problem soanti-backlash orzero-backlash gearing isused.Themostcommon gearing istheharmonic drivetype.Ithasclear advantages inseveral areas. Inaddition toitszero-backlash characteristics, ithasrelatively low compliance. Thestepper motorcan"windup"theflexible spline(cup)likeaspring,takingseveral steps beforethearraybeginstomove.Ifthiswerenotthecase,thestepper couldnotaccelerate themassin onlyonestepandmaintain synchronism withtheapplied pulsetrain.Stepper motorcharacteristics cause thesystem analysis tobecriticalasinstability isaconstant concern. Abrushmotorisnotunderconsideration duetotheshortlife.Brushless motorshaveotherlimitations and norealadvantages overastepper otherthansmoothness (desirable whereextreme spacecraft stability is needed). Detent canbedesigned intoabrushless motortoprovide un-powered holding torque. The controlelectronics ismorecomplex, however. Antenna Pointin.q Mechanisms Antenna drivesystems havecharacteristics similartosolararraydrives.Theymustbemoreaccurate, however, andusually require two-axis drives. Basiccomments onsolararraydrivesapplytoantenna pointing mechanisms also. Again,thestepper motorisdominant foritssimplicity andun-powered holding characteristics. Ascanbe seeninFigure5,twoharmonic driveactuators canbeattached toasuitable bracket forthetwo-axis drive capability. Advantages include theuseofsimilar(orthesame)actuators forbothsolararraydriveand antenna pointing mechanisms. Identical electronics canbeused.Thiscertainly canreducetotalcost. NASA/CP--2002-211506 99
Figure5.Stepper Motors inTwo-Axis Antenna Pointing Mechanism FansandPumps Mostfansandpumps operate atarelatively constant speedandtorque, usedtocirculate gasses or liquids. TheSpaceStation hasvarious applications inthelifesupport system aswellasgeneral cooling andrefrigeration needs.Continuous operation foryearsisoftenneeded alongwithgoodefficiency. Power consumption ismuchhigherthanneeded forpointing mechanisms duetotheusualsystem demands. Here,theBDCmotorisking.Typically commutated withHallcells,itisusually athree-phase designwith lowresistance forhighefficiency. Itcanberelatively quietbutgooddesignandprecise rotorbalancing is necessary. Electronic commutator capabilities caninclude speedortorquecontrol aswellascurrent limiting onstart-up. Theextremely lowefficiency ofstepper motors rulethemout,andbrushmotors donothavethelife capability. ThemotorshowninFigure6operates immersed inammonia forrefrigeration. Brushless motor designpermits therotortobehermetically sealed. Themotorandcommutator magnets areenclosed ina welded housing ofstainless steel.Similarly, thestatorandHallcommutator assembly isenclosed for protection against theammonia. $614-2-18/S,:[: _i Figure6.Three-phase BDCMotorforSpaceStation Ammonia Coolant Pump NASA/CP--2002-211506 100
MarsPathfinder LanderSidePetal . Figure1.Comparison ofNanorover totheMarsPathfinder Sojourner RoveronaMarsPathfinder Lander SidePetal(Demonstration Models} Nanorover Confiquration TheNanorover configuration withfourwheels andtheinternal components isshown inFigure2.The bodydimensions are140X140X85millimeters. PowerfortheNanorover isobtained fromsolarcells withaswitching powersupplytogenerate therequired regulation andadditional voltages forthecomplete system. Theelectronics subsystem consists ofacomputer; analog interface circuitry fortheinstruments, mobility, andengineering measurements; brushless dcmotordriverelectronics foroperation oftheten motordrivenfunctions ontheNanorover; andaradioforcommand anddatacommunications directly to theMUSES Cspacecraft. -". IRSpectrometer Gimballed Mirror ILoupe Mechanism Camera, Filters,Detector Wheel Figure2.Nanorover ConfigurationAlpha/X-Ray Spectrometer Shoulder Assembly NASA/CP--2002-211506 104
TheNanoroverinstruments arecomposedofanimagingcamerawiththreefocallengthsandninefilters withaclearposition,anInfraredspectrometer fordetermining themolecularcomposition oftheasteroid, andanAlpha/X-ray spectrometer fordetermining theelementalcomposition ofthesurface.Additional itemsintheNanoroveraresunsensors,alaserrangedetectorformeasuringdistances,asourceforinsitucalibrationoftheIRspectrometer, surfacecontactsensinginthewheelrims,andunidirectional treadsonthewheelsurface. Someofthemobilitycapabilities oftheNanoroverconsistoftheabilitytoturnitselfoverifiteverendsup onitsback,turninplacesteering,drivingvelocitycontrolfrom0.04to200mm/sec,raising,lowering,and positioning ofthebodyrelativetothesurfacebyclosingandopeningthestrutsthatsupportthewheels (seeFigure3),attitudedetermination usingstarscanningtechniques, andmicrogravity "hopping"totravel largedistancesquickly.Thehoppingmaneuverisaccomplished bytherapiddrivingofthestrutstogether (goingfromFigure3AtoFigure3B)toachieveaverticalvelocityoftheNanorover's centerofmass.The wheelsaredrivenforwardatthesametimeasthestrutsaredriventogether,producingacontrolled horizontalvelocity.Therelativemagnitudeofthetwovelocitiesdetermines howfartheroverwilltravel duringthehop. +Y (A) (B) Figure3.Twoviewsshowing theMobility System's Flexibility toPosition theNanorover Bodyin Different Poses Constraints ontheNanorover Design Several resource constraints wereimposed bythespacecraft system ontheNanorover, theusualones beingpower, mass,andvolume. Additional constraints werederived fromtheradiocommunication requirements tothespacecraft fromtheNanorover, thelaunchvibration environment, andthefree-fall andimpact method forgetting theNanorover ontothesurface oftheasteroid. Alongwiththesystem constraints, thethermal environment oftheasteroid posedasignificant challenge fortheNanorover hardware. Thethermal environment, coupled withthemassrequirements, werethemajordriving requirements fortheNanorover design. Thesetworequirement constraints, together forcedtheuseof someunusual solutions inthedesignoftheNanorover components. MassandVolume Duetothesignificant massconstraints ontheMUSECmission, theNanorover andallofitslaunch retention, deployment, communications, andprocessing support equipment onthespacecraft wasgiven amassrequirement ofnogreater than3.5kilograms. Thismasswasallocated as1.7kilograms forthe Nanorover and1.8kilograms fortheOrbiter Mounted Rover Equipment (OMRE). These mass requirements forthemission ledtotheoptimization ofeverycomponent oftheNanorover andOMREfor mass. NASA/C P--2002-211506 105
TheNanoroverpresentedasignificantchallengeinthethermalcontroldesign.Thevehiclehasverylittle externalsurfaceareaduetoitssmalldimensions anditissolarpowered.Sinceitispossibleforthe Nanorovertobeinanyorientationwithrespecttothesun(includingupsidedown),theNanoroverneeds solarcellsonallofitssidesthatcouldpossiblyfacethesuninordertomaintainpowertothecomputer andactuators.Thisresultedintheentireavailableareaonthetop,bottom,front,andbacktobe populatedwithsolarcells,ascanbeseenbythebluesurfacesinFigure1.Since;thesolarcellsused nearlyalloftheexternalareaavailable,therewasverylittlespaceleftoverthatcouldbeusedforheatdissipatingradiators.Anotherthermalcontrolproblemthataroverfacesistheintimatethermal"contact" withthesurfaceoftheplanet,comet,orasteroid.Thisconditionleadstoadditionalthermalcontrol systemdesignconstraints. ThesmallinternalvolumeoftheNanoroverandthetightmassconstraints ledtoacompletely passive thermaldesign.Therewasnospaceinthevehicletoplacebatteries,heaterunits,orthermalswitches andthefreevolumeneededforthermalblanketswasnotavailable.Theonlylocationthataradiatorcould beplacedonthevehiclewasthetwosidepanelsthatsupportthewheels,struts,andshoulder assemblies. Thecloseproximityoftheasteroidsurfacetothesesidepanelradiatorsresultedinaheat inputfromtheasteroidtotheNanoroverformostofthedaytimefunctionalperiod.Abalancewasstruck betweenimprovingtheheatrejectiontoslowthetemperature rise-timeduringthe;daytimeoperational periodandreducingtheheatrejectiontokeepthelowestnighttimetemperature withinareasonable bound. Thisbalancewasaccomplished withseveralinternalpassivefeaturesoftheroverdesign.Tominimize theheatinputfromthesolarpanelsduringthedaytime,thepanelsarethermallyisolatedfromtheinterior oftheNanorover. Theelectronics, mechanisms, andinstruments areallthermallytiedtotheoptical benchtomaintainalignmentandminimizethermalgradients.Tohandlethethermalloadofthe Nanorover's internalcomponents, theopticalbenchusesthewheelstrutsandthesidepanelsforheat rejectionradiators. Inordertoobtainanoperational periodthatwaslongenoughfortherequiredsciencereturn,the instrumentdetectorshadtobekeptcoolforalongerperiodoftimethantheopticalbench.Sincethe instrumentdetectorsaremounteddirectlytotheopticalbench,someformofadditionalheatcapacitywas necessarytoslowdownthetemperature riseofthedetectorsastheNanoroverheatedupduringtheday. ThecameraandIRspectrometer instrument detectorsaremountedontoanassemblythatusesa materialphasechangefromsolidtoliquidtoslowdownthetemperature riseofthedetector.Thesephase changeassemblies effectivelyincreasedtheheatcapacityofthedetectorbyahundredtimes.Thefinal thermaldesignoftheNanorover resultedinmostofthecomponents beingexposedtoatemperature cyclefrom-170°Cto+110°Cevery20hours.Adding+15°Cmargin fortesting, thedesigntemperature was-185°C to+125°C. Aplotofthetemperature ofvarious components oftheNanorover duringa complete day/night cycleoftheasteroid isshowninFigure4. NASA/CP--2002-211506 107
x,'v 8 Q. E t--400.O0 320.00 240.00 160.00 80.00 0.00Z 0 4 8 12 16 20 Time(hours) Figure4.Nanorover Temperature ProfileforOneDay/Night cycleof20Hours Mechanical Components Nearlyallofthemechanical components fortheNanorover aresmall,withthelargest piecehaving dimensions about20millimeters indiameter. Thecamera barrel,whichisathermalized overa200°C temperature change, isshown inFigure5withaquarter forscale.Thecamera hasafocussettosix meters inordertogetappropriate blurring ofstarstodetermine thelocation ofthestar'scentroid. To imagethesurface oftheasteroid, atwo-meter focusisrequired. Forclose-up science datataking,a70millimeter focusisneeded. Toobtainthesediverse focusrequirements, amechanism thatmovesapair oflensesintoandoutoftheopticalpathwasdeveloped. Themechanism, calledtheLoupeMechanism, hasthreepositions (lens1, lens2,nolens)andisshowninFigure6withaone-cent pieceforscale.The largestcomponents ontheLoupemechanism areabout10millimeters indiameter andthesmallest are lessthanonemillimeter indiameter. Figure!_7/b:_.<.>:_-.4.:/ 5.Camera Barrelwith"--OpticsInstalledFigure6.LoupeMechanism onVibration Fixture NASA/C P--2002-211506 108
TheNanorover System MassandDesign Maturity TheMUSES Cmission, likemostspacemissions, isverytightonmassmargin. Fromthebeginning ofthe Nanorover project, itwasunderstood thatmasswouldbeveryprecious. TheNanorover project utilized thestandard margin policies asoutlined inAIAAspecification number G-020-1992, titled"Guide for Estimating andBudgeting Weight andPowerContingencies forSpacecraft Systems". Thisspecification recognizes thatthesmaller themassofthefinished system, thehigherthemassmargins needtobeat various stagesoftheproject. AplotoftheAIAArecommendation formassmargin asafunction ofthe stageoftheprojectcompletion isshowninFigure7.Therecommendations aredivided intofourgroups basedonthesystem mass.ThefigureshowsthevaluesforClass1system designs only.2 O o O') ¢.,,m ¢- O u_ u_ =E co o O 13.50 50-500 kg 40 30 20I 10 0>2500kg Bid CoDR PDR CDR PRR Project Stage Figure7.MassContingency Recommendations versus Project Stage3 fromAIAAG-020-1992 forClass1System Designs Only EverydetailoftheNanorover design wasoptimized tominimize themassofeverycomponent. The motors haveamassof3gramsandthegearboxes haveamassof7grams.Thewheelassemblies with theirproximity sensing capability weighinat20grams. Thecamera barrelandopticshaveamassof137 grams. Thegimballed mirrorfortheopticalpathwasdesigned utilizing aHelmholtz coilarrangement, a beryllium mirror,andjeweled bearings tominimize itsmassof24grams[1].TheInfrared Spectrometer's4 massis90gramsandtheAlpha/X-Ray Spectrometer hasamassof95grams. Asthedesignmatured, themassgrewbuttherelative magnitude ofthegrowth wasmuchgreater than expected. Figure8showstheNanorover masshistory asthedesign progressed fromthePreliminary Design Review (PDR)totheCritical Design Review (CDR)forvarious components. Innearlyeverycase, theassembly masses wouldincrease dramatically inthebeginning ofthedesignasallofthefunctions wereincorporated intothedetailparts.Thenextphaseofthedesignwasamassreduction effortthat 2TheAIAAspecification defines aClass1system as"Anewdesign whichisone-of-a-kind orafirst generation device". TheAIAAspecification hasrecommendations forsystems ofdifferent levelsof maturity. Thispaperonlyaddresses theClass1category. 3Project stagedefinitions are:Bid=Proposal, CoDR=Conceptual Design Review, PDR=Preliminary Design Review, CDR=CriticalDesign Review, PRR=Preshipment Readiness Review. 4ThisisapartiallistingoftheNanorover components only. NASA/CP--2002-211506 110
maintained thefunctionality oftheassembly whilereducing themass.Typically, thefinalassembly mass wouldthenriseasmallamountasthecomponents weremanufactured andmeasured formass. 450 4O0 35O 300E ,-250 200 150 100 50_[Mobility__ComponentsE IElectronics IV Optic_ Componentsw J L TimefromPDRtoCDR Figure8.Selected Nanorover Component MassHistory Theelectronics assembly hadadifferent historythanmostoftheothercomponents. Thedesignmass dropped aftertheinitialestimates assomeofthecircuitry wasreduced andconsolidated intoagate array.Thenextphaseoftheelectronics designdetermined whatcomponents couldbeusedinthe thermal environment oftheNanorover. Thisresulted inseveralofthecomponents beingremoved and replaced, oftenwithmorecomponents oradditional circuitry. Onceallofthecomponents wereverifiedto operateatthethermalextremes, thefinalcircuitandpackaging designwascompleted. Thelargegrowth attheapproach toCDRisduetoseveralchip-on-board packaging detailsthatwerenotaccounted forin theoriginalpackaging plan.Figure9showsthemasshistoryoftheentireNanorover assembly fromPDR toCDR.Theeffectoftheelectronics assembly growthneartheCDRisevidentintheassembly history, eventhough othercomponent groups weregoing down inmass. NASA/CP--2002-211506 111