ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
actuatorstepstoachievethatmotion.Characterization oftheactuatormotionwasnotdoneuntillaterin theprogram.Thiswasamistake,ascharacterization turnedupasignificantdesignissue. TheIW-700inchwormconsistsofamovingshaft,twoPZTclamps,andathirdPZTelementthatchanges thedistancebetweenthetwoclamps.Figure5showshowtheseelementsworktogethertomovethe shaft.First,PZTElement1clampstheshaftwhilePZTElement2extends.Thiscausestheshafttomove totheleft.Thedistancemovedisdetermined bythevoltageappliedtoElement2.A1Vincrement causesasinglestepof2-4nm.After665steps,Element2becomesfullyextended.ThenElement3 clampstheshaftfollowedbythereleaseofElement1.Inthisconfiguration, theshaftwillcontinueto movetotheleftasElement2contracts.Thisprocessisrepeatedtomovetheshaftthroughtheactuator's entirerangeofmotion. 2 1 • 3 • Off Unclamp 1 [ [ ] I I:_•F::Z_ Clamp1 Contract 2 ] I Extend 2 Clamp 1 I I I Clamp3 Unclamp 3 Il I [_ I [_ • IIi•, Figure 5.Inchworm Actuator Motion Characterization testsoftheactuators revealed thatamotion discontinuity occurs whentheclamps exchange. Imperfections inthewaytheclamps griptheshaftcausetheshafttomoveforward or backward byanuncontrolled amount. Testsshowed thisdiscontinuity tobeaslargeas215rim.Since thousands ofclampchanges occurthroughout theactuator's range,thisprevents theactuator frombeing accurately commanded inanopen-loop fashion. Furthermore, itwasfearedthattheactuator couldhave "dead-zones", positions withinthelengthofadiscontinuity thattheactuator couldneverreach. Figure6showsagraphofcommanded actuator position plottedagainst measured position determined usingaZygolaser-ranging interferometer. Theactuator hasbeenrunbackandforthrepeatedly through thesameclamping cycle.Thatis,theactuator wasrunthrough 1330steps,causing element 2togo through fullexpansion andcontraction, andcausing bothclamps toopenandcloseontheshaft.Thenthe actuator wasrunbackwards toitsstarting position, andtheprocess wasrepeated. Thediscontinuities canbeclearlyseenevery665steps.Itisimportant tonotethatdiscontinuities occurin boththeforward andreverse directions, butwithdifferent magnitudes. Discussions withthemanufacturer suggest thatthemagnitudes varydepending ontheactuator's loading condition. Thisresultsuggested thatbymoving backandforthacross adiscontinuity, theactuator couldachieve anyposition withinits 152
Figure6.Actuator MotionDiscontinuities range.Thus,thediscontinuities donotcauseany"dead-zones". Inordertooperate theactuator inthis fashion, however, high-resolution positionfeedback mustbeprovided tothecontrolsystem. TheDCATT controls teammadeeffortstoaddress thisproblem, butthatworkisbeyondthescopeofthispaper. Anyone considering theuseofhigh-resolution inchworm actuators shouldbeawareofthesepotential discontinuities, anddesigntheirsystemaccordingly. Discuss thisissuewithvendors beforeselecting actuators foradesign.Earlyactuator characterization isalsostronglyrecommended. 6-DoFMotionTesting The6-DoFTestFacility Inthehexapod mechanism, allsixactuators moveincombination toachieve motionalongonedegreeof freedom. Therequired motionfromeachactuator iscalculated usingthehexapod kinematics model. Deviations fromthegeometry usedinthatmodel,including machining andassembly errors,willintroduce errorsintothatcalculation. Earlyinthedesignitbecame clearthatcharacterizing andcalibrating the motionofeachassembled hexapod wouldbecritical.Inordertoaccomplish this,atestwasrequired that couldmeasure thehexapod's motioninallsixdegrees offreedom simultaneously. Devising thistestwith therequired resolution wasnottrivial. Laser-ranging interferometers wereselected tomakemostofthemeasurements. Byreflecting offa mirrored cubeattached toahexapod, threebeamscouldbeusedtomeasure linearmotionofthe hexapod. Parallel beamshittingthesamecubefacecouldbeusedtomeasure rotation overasmall range.Aslongasrotations remained small,alltheinterferometers couldmakemeasurements simultaneously. Anautocollimator reflecting offanother mirrorprovided 2-axisrotational measurements withgreaterrangethantheinterferometers butlessresolution. Figure7showsaprototype hexapod thatwasusedtohelpdevelop the6-DoFtestingfacility.Figure8 showstheprototype inthefacility.Aflat"dummy segment" hasbeenboltedtotheprototype toprovidea mounting surfaceforthemirrors. Inthispicture,twolasershavebeensplittofeedthreeinterferometers. 153
Figure 7.Prototype Hexapod Figure8,6-DoFTesting Facility Theseinterferometers aremeasuring thethreelineardegrees offreedom aswellasrotation aboutthe pistonaxis.Anautocollimator isbeingusedtomeasure tipandtiltrotation. 6-DoFTestin,q Results The6-DoFfacilitywasneverusedtofullycharacterize ahexapod. Duringtestingwiththeprototype, the designteamdiscovered significant dimensional stability problems. Partofthisinstability wasthermal expansion inthehexapod, whichisdiscussed inthefollowing section. Therestoftheproblem was attributed tothermal expansion ofthetestsetup.Itbecame clearthatdimensional stability ofthetest setupwasimperative formeasuring thehigh-resolution motion thatwasdesired fromthehexapod. Machined aluminum partstraditionally usedtomountopticalelements wereunacceptable forthistest. Plansweremadetoreplace thealuminum partsinthetestsetupwithIow-CTE metalssuchasInvaror SuperInvar.Thiseffortwasnotcompleted beforetheprojectwascancelled. Although 6-DoFtestswereperformed ontheprototype hexapod, thatdatawillnotbepresented here.In addition todimensional stability errors,thesetestsoccurred beforetheinchworm motion discontinuities werediscovered andaddressed. Withmultiple sources oferrorinthemeasurements, thedatafromthese testsisconsidered unreliable. Despite errorsinthedata,the6-DoFtesting effortisconsidered apartialsuccess. Thefacilitywas sensitive enough todetectbothdimensional stability problems andsystematic errorsthatresulted from inchworm discontinuities. Itisbelieved thatDCATT's 6-DoFtestingfacilityisagoodmodelforsimilar testing endeavors. Aswillbedescribed inthefollowing section, designers ofsimilar facilities should carefully examine thedimensional stability oftheirpotential testsetup.Whenattempting tomeasure movement onthescaleofnanometers, thermal expansion ofthetestsetupcanbeaslargeorlargerthan themotions thatarebeingmeasured. Dimensional Stability andNewRequirements Discovery oftheDimensional Stability Problem Inkeeping withtheearlygoalofaself-compensating telescope, thedimensional instability ofaluminum wasatfirstseenasabonus, Asthetemperature changed, itwasdesirable tohavethetelescope structure expand orcontract atthesameratethatthemirror's figurewaschanging. Experiments withthe prototype hexapod inthe6-DoFtesting facilitybrought tolightserious flawswiththisthinking. Initial discussions ofthermal expansion assumed thatthestructure wouldexpand orcontract asaunit.Testing, 154
however,showedhowthehexapodscouldbehaveasindividualstructuresgrowingatdifferentrates. Worsestill,individualhexapodlegsmightgrowatdifferentrates,causingthemechanism's positionto driftinall6degreesoffreedom. Prototypetestsshowedthatthehexapod'spositionwasdrifting.Inanattempttoisolatethismotion,tests wererunwiththehexapodinastaticposition.Thetestfacilitymonitoredchangesinthehexapod's positionovertime.Figure9isanexampleofthisdatatakenbytheautocollimator. Ascanbeseen,the measurement driftsintiltbyabout1arc-second andthenbackduringthecourseofthetest.Thebottom axisofFigure9isgiveninthenumberofmeasurements taken.Thetotaltimeofthetestisabout1hour. TiltDriftOver1hour 0 2000 4000 6000 8000 10000 12000 DataPoint Figure9.Position DriftinPrototype Hexapod Afterensuring thattherewasnodatadriftcaused bythemeasuring tools,itwashypothesized thatthis driftwastheresultofthermal expansion withinthehexapod and/orthetestsetup.Thenextstepwasto analyze thehexapod's dimensional stability inmoredetail. Hexapod Dimensional Stability Analysis Asimplethermal-expansion modelwascreated toanalyze thedimensional stability ofthehexapod. The analysis focused onlyonthehexapod legs,notthebaseormounting platform. Thiswasdoneforseveral reasons. First,thelegscomprise mostoftheheightofthehexapod, sotheircontribution togrowth is muchlargerthaneitherthebaseortheplatform. Thepartsinthelegsarealsolessmassive thanthe baseorplatform, sotheyarelikelytochange temperature faster.Lastly,theactuators themselves area sourceofheat,andfluctuations inthatheatwilleffectthelegsmorethanthebaseorplatform. Sinceall legsareidentical, onlyonehexapod legwasmodeled. Inthemodel,eachpartwasrepresented byaneffective length(L),acoefficient ofthermal expansion (_), andatemperature differential (AT).Onlypartsthataddedtothegrowth ofthelegwereincluded inthe model,andLrepresents onlytheportion ofthatpartwhichcontributes toleggrowthwhenitexpands. The valuez_Trepresents astatic,bulktemperature increase intheleg.Thegrowth (_L)ofeachpartwas calculated bymultiplying thesethreecharacteristics together: _L=L*(z_AT Thetotalgrowth ofthelegwascalculated byadding thegrowths ofalltheparts.Thermal expansion coefficients werebasedonpartmaterial, withtheexception oftheactuator itself.Fortheactuator, the 155
valueofL_o_wasdetermined experimentally. Inthemodel,thegrowthcontribution oftheactuatoris determined bymultiplyingthisL_e_byAT. Twoextremecaseswereusedtoequateleggrowthtohexapodpositiondrift.Inthefirstcase,alllegson onehexapodareassumedtogrowthesamerelativetoanotherhexapodinthesystem.Thus,2_Tisa differenceintemperature betweentwohexapods. Theresultingleggrowthsproduceapistonerror betweenthetwosegments.Thesizeofthispistonerrorwascalculatedusingthehexapodkinematics model.Thesecondcaseassumesthattwoadjacentlegsonthesamehexapod growrelative totheother legsonthathexapod. Thisproduces atilterror.Thesizeofthiserrorwasalsocalculated usingthe hexapod kinematics. Itshouldbenotedthatbothofthesecasesareconservative. Inreality,differential leggrowthwillberandom, resulting inlesssegment driftthanpredicted formostcases. Table4showsthespreadsheet thermal modelforasinglehexapod legwitha1°CATapplied. The hexapod driftuncovered bythisanalysis waslargeenough toprompt acloserlookatthedimensional stability requirements forthehexapods. Part Number PerLeg Rodend 2 Flexcouple 2 !Uppershaft 1 1 Upper !housing Actuator Lower housin9 Flexpivot !LowershaftTable4.Thermal Expansion Analysis ofHexapod Leg Length (mm) 12.07 6.02 9.14 107.44 1NA 1 30.48 1 7.62 1 11.94Material o_ AT at20C(IC)(degC) 303Stainless Steel 1.55E-05 1 303Stainless Steel 1.55E-05 1 303Stainless Steel 1.55E-05 1 AI6061-T6 2.29E-05 1 SteelandAI 2.38E-07 mPC li AI6061-T6 2.29E-05 1 420Stainless Steel 9.90E-06 303Stainless Steel 1.55E-05Growth TotalGrowth (nm) (nm) 186.77 373.53 93.19 186.37 141.55 141.55 2456.12 2456.12 238.37 238.37 696.77 696.77 75.44 75.44 84.801 184.80 1 4352.96 Hexapod Height change ifallactuators grow: 5005.90 nm Hexapod tiltif2actuators grow: 8.22arc-sec NewDimensional Stability Requirements DCATT's largest concern withposition stability wasthatoncetheprimary mirrorwasaligned, itshould stayaligned fortheremainder oftheexperiment. Thelengthofanexperiment wasunclear, butwas expected tobebetween 1hourand8hours.Duringthattime,theDCATT scientists defined twonew requirements thatthehexapods hadtomeet. First,nohexapod couldgrowinpistonmorethan10nmrelative toanyotherhexapod. Allthehexapods couldmovetogether bymorethanthisamount, buttheirrelative pistonpositions hadtostaywithin10 nm.Thus,significant thermal growthinthehexapods wasallowed. Differential thermal growth, however, wouldviolatethenewrequirement. Second, nohexapod couldtiltmorethan0.025arc-second fromitscommanded position. Tipandtiltdrift wouldresultfromdifferential growthofindividual hexapod legs.Again,allthelegsinahexapod could growsignificantly without causing enough drifttoviolate therequirement. Onlydifferential leggrowth posedproblems. Thehexapod-leg thermal modelwasusedtocalculate thetemperature difference between legsthat wouldexceed thesenewrequirements. Thatvalueis0.002degC. 156
Characterizinq theThermal Environment Withthethermal modelinhand,thenextstepwastodetermine theactualtemperature variation thatwas likelytoexistbetween hexapod legs.Thermocouples wereattached toeachlegofaprototype hexapod, andthathexapod wasplaced ontheDCATT testbed. Additional thermocouples wereplaced onthe hexapod's base,adummy segment mounted tothehexapod platform, andthetelescope baseplate. After thelegscametoequilibrium intheenvironment, theactuators wereturnedonandtemperature datawas takenforaperiodof24hours.Thisdataincluded aonetotwohourperiodduringwhichthelegsrosetoa newequilibrium temperature duetoactuator heating. Theremainder ofthedatashowed howthe temperature ofthelegsvariedwithtime. Figure10isagraphoftherawdatafromthistest.Notethatleg#2appears tobesignificantly colderthan theotherhexapod legs.Examination ofthetestsetuprevealed thatleg#2'sthermocoupte hadbecome partially unattached duringtheexperiment. Forthisreason, leg#2'sdatawasnotincluded intheanalysis. 21.900 21.800 21.700 21.600 21.500 21.400 -- 21.300 21.200 21.100 21.000 _"20.900 "20.800E 20.700 20.600 20.500 20.400 20.300 20.200 20.100 20.000 19.900 19.800 19.700 10:0022HourHexapod Temperature Measurements -PowerOncafibrated TCsfrom7/14_cewater DataTakenonTestbed Nocover Actualors activated I I datatakenevery30seconds  J ,,, 7119-7/20 ...... _ _ll,_ 1 " n..... ,h,ng ii l_. _"IW""""tl_p" '' -........ Dummy Segment ...........Base Actuator 1 Actuator 2 --Actuator 3 --Actuator 4 .....Actuator 5 Actuator 6 .....Baseplate L 12:00 14:00 16:00 18:00 20:00 22:00 ClockTime0:00 2:00 4:00 6:00 8:00 10:00 Figure 10.Hexapod Temperature Measurements Analyzin,q theHexapod intheDCATT Thermal Environment Because thedimensional stability requirements areeffected byrelative leggrowth, itisnecessary to compare thetemperatures ofonelegwithanother. Figure11showsthreecurvesderived bysubtracting oneleg'stestresultsfromanother, afterthoseresultsweresmoothed witharunning average. Ideally, it wouldbedesirable forthesecurvestobeconstant atzero.Thiswouldproduce nosegment drift.Ifthe curveswereconstant butnon-zero, thesegments wouldmoveinpiston,butwouldhavenorelative drift orsignificant tip/tiltdrift.Thiswouldstillmeetthelong-term stability requirements. Ascanbeseenin Figure11,however, thedifference curves arenotconstant. Therefore, undesirable driftwilloccur.The 157
worstvariation inthetestoccursinthedifference between leg#5andleg#4.Thiscurvewasusedforthe restoftheanalysis, andwasreferred toasthe5-4curve. Temperature Differences Between Legs 0150 0140 0.130 0120 I-....6-1 5-4 ..........4-1 0110 0t00 0090 12:00! 14:00 16:00 18:00 20:00 22:00 0:00 2:00 4:00 6:00 ClockTimeI 8:00 Figure11.Temperature Difference Curves forSelected PairsofHexapod Legs Toexamine thehexapod's response, the5-4curvewastreated asalegtemperature vs.timecurveand variations initwereconsidered tobeATvalueswhichcouldbeinputintothehexapod-leg thermal model. Thelargestvariations inthecurvefor1,2,3,4,and8-hourintervals weredetermined. Thesevaluesare giveninTable5.Recallthattherequirements willbeexceeded ifthedifferential change intemperature between hexapod legsislargerthan0.002degC.Table5showsthattherequirement willbeexceeded duringanyofthetimeintervals considered. Table5.Maximum Variation in5-4CurveforSelected TimeIntervals Time Interval 1hr. 2hr. 3hr. 4hr. 8+hr.TimeRangefor AT Largest AT (degC) 18:30-19:30 0.026 21:52-23:52 0.032 16:55-19:55 0.035 21:52-01:52 0.043 14:00-22:00 0.05 Table6shows howthethermal growth modelresponds totheATvalues inTable5.Again, the requirements areexceeded inallofthetimeintervals byatleastanorderofmagnitude. Controlling the temperature ofthehexapod legsto0.002degCwasconsidered unreasonable, soitwasdecided thatthe aluminum hexapod designwouldnotmeetthenewrequirements. 158
Table6.Predicted DriftofHexapod inDCATT Environment Time AT Piston Tip/Tilt Interval (degC)Error(nm)Error(arc-sec) 1hr. 0.026 130 0.214 2hr. 0.032 160 0.263 3hr. 0.035 175 0.288 4hr. 0.043 215 0.353 8+hr. 0.05 250 0.411 Requirements: 10 0.025 Dimensionally StableDesign foraHexapod Leg Proposed Desiqn Thegoalofthenewdesignwastomaximize thedimensional stability ofthehexapods legsbyusing materials withlowornegative CTEs.Thefirstiteration usedonlyIow-CTE metals: InvarandSuperInvar. Thefinatdesignincorporated graphite-epoxy composites withanegative CTEalongtheaxisoftheleg. Figure12showsthefinallegdesign. Housing (Formerly UpperandLowerHousings) (Graphite-Epoxy composite tube bonded toSuperInvarend-fittings) -... Flex-Pivot _'_ (Unchanged) Flex-Couple k(Invar) x_' _Clamps  _ (Unchanged)  LowerShaft \RodEnd (Super Invar) (Invar)RodEnd \Actuator (Unchanged)l\Flex-Couple (Invar) UpperShaft (SuperInvar) UpperEnd-Cap (NewPart) (Super Invar) LowerEnd-Cap (NewPart) (SuperInvar) Figure12,Dimensionally StableHexapod Leg 159
Composite Desiqn Effort Thedesign ofthecomposite tuberequired itsowneffort,withassistance fromamaterials engineer specializing incomposites. Analysis suggested thatwiththeproposed legconcept, acomposite tube couldbedesigned thatwouldprovide thelegassembly withanear-zero CTEintheaxialdirection. The goalforthetubewastohaveaCTEintheaxialdirection of-4.4E-6 perdegC.Specific analysis wasalso required toestimate theeffective CTEoftheoverlap between thetubeandtheSuperlnvarend-fittings thatwerebonded toit. Thecomposite material designed uses9layersofAmoco's T50graphite fiberwithCytecFiberite's 9542Acyanate esterresin.Alayupof30130/-30/-30/0/-30/-30/30/30 waspredicted toprovided thedesired CTEof-4.4E-6 perdegC.Thislayuptakesadvantage ofthelargePoisson's ratioofthematerial to amplify thenegative thermal expansion ofthetube.Asthetemperature increases, thetubeexpands in diameter, andthiscauses thetubetoshrinkintheaxialdirection. Thisisinaddition toaxialshrinking caused bythenegative CTEofthecomposite fibersthemselves. Thislayupdesignhasreduced strength andstiffness compared tootherlayupoptions. Inthisapplication, however, thestrength andstiffness properties weredetermined tobeadequate. Thiswasanacceptable trade. Sincethehexapod wouldbeinaterrestrial environment, dimensional changes caused bymoisture absorption inthecomposite wereaconcern. Thecomposite selected hadalargecoefficient ofmoisture expansion (CME). Thiseffecthadtobeminimized, orthenewhexapod wouldbejustasdimensionally unstable astheoldone.Thisproblem wassolvedbyapplying amoisture barrier. Parylene-C, apolymer commonly usedforconformal coating electronics, wasselected. Thisvapor-deposited coating iseasyand inexpensive toapplyandhasaverylowrateofmoisture transmission. Thecoating itselfisthinand compliant, soitwasnotexpected tochange thethermal expansion properties ofthepart. FinalDesiqn Table7showsthepredicted expansion ofthedimensionally stablehexapod designwitha1degCAT applied. Table8shows howthemodelbehaves whenthetemperature variations fromTable5are applied. Theanalysis predicted thisdesignwouldmeettherequirements withsignificant margin. A1.53 degCtemperature change between legsisnecessary tocausethismodel toexceed thedrift requirements. Part Number PerLeg Rodend 2 Flexcouple 2 !Uppershaft 1 Uppercap 1 Upperbond Housing Actuator* 1 LowerCap 1 Lowerbondi1 Flexpivot 1 Lowershaft 1Table7.Expansion ofDimensionally StableHexapod Design Length Material (z t_TGrowth (mm) at20C(IC) (degC)(nm) 12.065 6.0198 9.144 6.48 15.875 70.74i NA 28.94 15.875 7.62 11.938Invar 1.25E-06 Invar 1.25E-06 1 SuperInvar 3.00E-07 1 SuperInvar 3.00E-07 1, Invar-to-Comp. Joint Graphite-Epoxy SteelandAI !SuperInvar Invar-to-Comp. Joint-1.60E-06 -3.83E-06 2.38E-07 3.00E-07 -1.60E-06 1 420Stainless Steel 9.90E-06 1 SuperInvar 3.00E-07 115.08 7.52 2.74 1.94 -25.35i -270.93 238.37 8.68 -25.35 75.44 3.58 Hexapod Height change ifallactuators grow: 62.48nmTotalGrowth (nm) 30.16 15.05 2.74 1.94 -25.35 -270.93 238.37 8.68 -25.3_ 75.44 3.58 54.33 Hexapod tiltif2actuators grow: 0.10258 arc-sec 160
Table8.Predicted Response ofNewHexapod Design toMeasured Tern Time AT Piston Tip/Tilt Interval (degC)Error(nm)Error(arc-sec) 1hr. 0.026 0.17 0.00028 2hr. 0.032 0.22 0.00034 3hr. 0.035 0.23 0.00038 4hr. 0.043 0.28 0.00046 8+hr. 0.05 0.33 0.00054 Requirements: 10 0.025}erature Environment InvarConsiderations Beforeincorporating InvarorSuperInvarintothemechanism, thedesignteamdidconsiderable research tofullyunderstand thesecomplex metals. Invar's CTEvariesnon-linearly withtemperature, sothe operating thermal environment mustbewellunderstood. Achieving thelistedproperties forInvarrequires specific heattreatment, andmachining Invaraftertheheattreatment willaltertheseproperties. Therefore, heattreatment isrequired againforfinished parts.Lastly,Invarissubject todimensional creep overlongperiods. Consider alltheseproperties carefully beforedeciding touseInvarorSuperlnvar.The primary sourceofInvarinformation usedbytheDCATT teamwas"TheInvarEffect''2. FinalStatusofDCATT Hexapods Fabrication andAssembly Status TheDCATT projectwascancelled shortlyafterthedimensional stability designeffort.Limited fundswere madeavailable tofabricate partsforonehexapod intheinterest ofverifying designelements thatmight beusefultofutureprojects. Atthistime,allpartshavebeenfabricated, butthehexapod itselfhasnot beenassembled. Thermal expansion testshavebeenperformed onsomeofthecomponents toverify designpredictions oftheirCTE.Finalassembly andtestingofahexapod isnotcurrently beingpursued duetofunding andmanpower limitations. Itispossible, however, thatrenewed interest inthefuturemay revivethisdevelopment effort. Component-Level CTETestinq CTEtestshavebeenperformed onseveral ofthecomposite tubes.Theresultsshowanaverage CTEof -3.83E-6 perdegCoveratemperature rangeof7-31degC.Thisiswithin13%ofthedesignvalue.One measurement wasmadeoneachoftwosub-assemblies consisting ofcomposite tubesbonded totheir SuperInvarend-fittings. BothCTEmeasurements were-1.9E-6 perdegC.Moremeasurements onthe sub-assemblies werenotpossible ontherestricted budget, sothesub-assembly CTEscarryless confidence thanthetubeCTEs. Dimensional Stability Predictions withMeasured CTEs Ifthemeasured CTEsareincluded inthehexapod thermal models, thedesignperformance degrades by nearlyafactorof10.Thisstillmeetsthedriftrequirements, however. Worstcase8-hourdriftsare6nmin pistonand0.01arc-second intip/tUt.Driftrequirements areexceeded bya0.09degCdifferential temperature change. Table9furtherdetailstheseresults. 2"TheInvarEffect:ACentennial Symposium." International Symposium ontheInvarEffect(1996: Cincinnati, Ohio).ed.JerryWittenauer. Warrendale, PA:Minerals, Metals&Materials Society, 1996 161
Table9.Hexapod DriftPredictions withMeasured CTEs ModelDescription Analytical modelwithnomeasured CTEs Modelwithmeasured CTEofcomposite tubes Modelwithmeasured CTEoftubesub-assemblies (lowconfidence inmeasurement)Predicted 8-Hour Drift Piston(nmITip/Tilt IdealCI 0.33 0.00054 3.1 0.0051 5.9 0.0096ATtoExceed Req.(degC) 1.53 0.17 0.09 Conclusions TheDCATT hexapod isaresponse totheneedforahigh-resolution, 6-DoFpositioning mechanism for useinalarge,active-optics system. Although thefinaldesignhasnotbeenassembled ortested, the development brought tolightseveral challenging obstacles whichwereanalyzed andovercome. With activeopticsbeingproposed forincreasing numbers spaceflightmissions, theexperiences ofthe hexapod designteammayproverelevant tofutureaerospace mechanisms. Themostimportant lessons learned fromthehexapod designeffortareasfollows: High-resolution inchworm actuators mayhaveunacceptable motiondiscontinuities caused bythe clamping andunclamping oftheinchworm mechanism. High-resolution, 6-DoFmotiontestsrequiresignificant designeffort.Analyze dimensional stability in thetestsetup. Relative dimensional stability maybemorerestrictive thanabsolute dimensional stability inactiveopticssystems. Largenegative CTEscanbeachieved withgraphite epoxy.Tubestructures canamplify thisbytaking advantage ofthePoisson's ratioofthematerial. Composites withalarge,negative CTEmayhavereduced strength andalargeCME. Ifacomposite's CMEposesaproblem, Parylene-C canbeasimple, cost-effective moisture barrier. 162