ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Theconstantslopeover thetemperature range indicatesthatthegear trainefficiencyremains highatalltemperatures; however,theintercept (i.e.,startingcurrentat zeroforce)increasesat coldtemperatures indicatingatemperaturedependentdrag.This behaviorisreasonable becausethefriction coefficientofthedryfilm lubricant(Vitro-Lube 1220C)onthegearsis nottemperature sensitivebutthegrease lubricant(Braycote601) inthebearingsisknownMT2779 17 [] =,s •Ambient Data [] OColdData Q_'13 16 []HotData _- OO gLinear(Combined) _ vO 15n- [] _: °uu__o° °w14 13 I I l _ I I I I I -200 -150 -100 -50 050100 150 AidingLoad(N) LDUDriveWheelEffective Radius Versus Load. toincrease itsviscosity significantly attemperatures below -50°C. Inanyevent,thecuretothisproblem wasanincrease tothetorquelimitparameters inthe controlsoftware.2OO DriveWheelLossofFriction ThelifetestingoftheLDUwasperformed inambient airovera21/2monthperiod.TheLDUdrivewheel performance wasnominal atthestartandoperated atasustained forcelevelof423N(445Npeak) withoutslipping(notethatthesevaluesarenotthelimitoftheLDUcapability butjustthemaximum tested value).Afterrunningfor32kmoftravel(1/2lifeofuse)underapreloadof623Nandlateralloads varyingupto375N,thedrivewheelwasabletooperateatasustained forcelevelof400N,butshowed indications ofslipping. Bytheendof64kmoftravel(onefulllifeofoperation), thedrivewheelcouldonly sustainloadsupto285Nwithoutslipping, indicating thatthefrictioncoefficient haddropped to0.46. Thelowdriveforceexperienced duringqualification lifetestinghasbeentracedtosiliconecontamination ofthetestrailbytheelastomer tire(notethatlifetestingconsisted ofrepeated operations overthesame, verylimitedsections oftestrails).Chemical testingoftherailrevealed siliconedeposits, estimated tobe oftheorderof7nmthick,onlyintheusedportionofthetestrail.Furtherfrictiontestsestablished that thelife-tested wheelcouldsustainadequate driveforceonunusedrailswhiletheuntested redundant wheelexhibited poorperformance onthecontaminated railsection. Thesourceofthecontamination is believed tobethefreesiliconeoilthatwasusedtodisperse thecatalystduringwheelmanufacturing. Working oftheelastomer tire,asinthelifetest,apparently releases thefreesiliconeoil.Overthe 18daysofthermal-vacuum testing,including threehot-cold cycles,therewasnoindication thateither vacuum ortemperature extremes accelerate thereleaseofsiliconecontaminants. Although theeffectof longtermexposure isyettobedetermined, thereareindications thatagradualreleaseofasilicone contamination willnotcompromise MToperation on-orbit. Thespacestationfliesinalowearthorbit (400km)whichexposes surfaces facinginthedirection offlighttoastrongfluxofatomicoxygen(AO). TheLDUdrivesurfaceisnormally exposed tothisfluxandtestingonsilicones indicatethattheatomic oxygenwillbreakupsiliconecontaminants intoaresidueofsilicondioxideandvolatilecomponents which rapidlyevaporate. Testshaveestablished thatexposure ofthecontaminated testrailtoatomicoxygen doesmaintain thefrictioncoefficient forthedrivewheel. 129
Concluslons TestingoftheLDUtranslation driveassembly demonstrates thatafrictiondriveisapractical wayto translate theMobileTransporter alongtherailsoftheInternational SpaceStation.Observations made duringtheMTprogram indicatethatspecialcaremustbepaidtothesystemdesigntomaintain adequate driveforceforcrossing majorgapsandoffsetsintherailsegments. Swingarm-mounted drivewheel designswithintegralgearpassesallowstationary attachment ofdrivemotorsandcanprovideapreload relatively independent ofdriveload.Theeffectofexternal loadsontraveldistance andpositioning accuracy mustbeconsidered forflexible-tire drivesystems. Drivewheelandidlerwheelmaterial selection programs yieldedacceptable selections fortheLDU translation drive.Silicone elastomers aregoodchoicesashigh-friction drivetirematerial; however, compounding shouldminimize theuseoffreesiliconeoilstopreventlossoffrictionoveroperating life. General ElectricSE6660 siliconeelastomer displayed acceptable performance characteristics fortheLDU application, butotherrubbermaterials alsoshowed promise. Polyetheretherketone (PEEK)plastic,used fortheidlerwheelsoftheLDU,isrecommended asatough,nonabrasive material foruseinathermalvacuum environment. Translation drivetestingsubstantiated lubricant selections. Forthehighlyloadedinterfaces andgear teethapplications oftheLDUgearboxes andswingarms, Vitro-Lube 1220Cdry-filmlubricant performed successfully throughout thermalvacuum andlifetesting.Braycote 601greaseperformed adequately in bearings although itsviscosity doessignificantly increase atlowtemperatures. Overall,theLDUdevelopment, production, andtestprogram validated thechoiceofagear-driven, swingarm-based, frictiondrivesystem. TRWAstroAerospace andtheBoeingCompany lookforwardto theopportunity tovalidatetheLDUtranslation drivedesignthroughon-orbitoperation oftheMobile Transporter. 130
Development ofaCryogenic Nanometer-Class Repeatability LinearActuator RubenNalbandian* andAlsonE.Hatheway'" Abstract ThegoaloftheNanometer Resolution LinearActuator development projectwastodemonstrate an extremely lightweight, veryhighprecision andhighstiffness actuator capable ofoperating uniformly welloverthetemperature rangeof20Kto300K.Thedevelopment wasajointeffortbetween Alson E.Hatheway, Inc.(AEH)andMoog,Schaeffer Magnetics Division (SMD),foruseinspacecraft optical instruments, notably theNextGeneration SpaceTelescope (NGST). Thispaperdescribes thedesign challenge ofdeveloping alightweight, compact (35mmdiameter by100mmlength), highstiffness, lowpower,thermally stablelinearpositioning mechanism. Thekeytoachieving highresolution, low power,andstability istoeliminate theclosed-loop controlsystem thatisnormally applied to overcome non-linearities andhysteresis inherent insometechnologies, suchaspiezoelectric and magnetostrictive transducers. Thiswasaccomplished byusingAEH'spatented elasticRubicon TM transducer techniques thatareinherently linearandhysteresis free. Introduction Thelinearactuator presented inthispaperiscomprised oftwostages, afinestageandacoarse stage.Bothstagesuseahighreliability space-rated stepper motordesigned andmanufactured by Moog,SMD.Theuniquedesignfeatures ofthisactuator thatcontribute toitslightweightand extremely precise motioncapability include astepper motordrivingaleadscrew through areducing gearbox. Thesamemotor,gearbox andleadscrew combination isusedinthefinestage,butthefinal outputmotionisreduced _'afactorofmorethan200bytheelasticactionoftheRubicon TM transducer. TheRubicon transducer consists oftwoelasticelements; asoftspringandastiff flexural member. Thesoftspringexertsaforceinputonthestiffflexure. Tomaintain linearity, the displacements, andhencethestresses, developed inbothelasticmembers arekeptwellbelowthe elasticyieldstrength ofthematerial. Thefinestageisdesigned toproduce anominal axialmotionof about3nanometers foreachstepinthefinemotor.Thematerials oftheactuator partswerecarefully selected toprovide anathermal designatthemid-stroke position andtominimize theeffectsof thermal expansion mismatch ontheoutputstage. Requirements andConstraints Driving theDesign Theprincipal characteristics forNGSTthatthelinearactuator mustbedesigned tomeet: Resolution Lifecycles Stroke Operating Temperature Calibration Mode(CM)HeatDissipation Observatory Mode(OM)HeatDissipation Mass Outside Diameter Creep,OM Thermal Stability, OM AxialForce,Set&Hold,OM PowerConsumption, CM PowerConsumption, OM AxialStiffness Stowed AxialLength<10nanometers 1,000,000 cycles 10millimeters 20to300K 0.5milliwatts 0.0milliwatts <140grams <50millimeter <0.01nanometers/day <50nanometers/K >1.0newtons <1watts 0.0watts >1,000,000 newtons/meter <10.0centimeters ,,Moog, Inc.,Schaeffer Magnetics Division, Chatsworth, CA A;sonE.Hatheway, Inc.,Pasadena, CA Proceedings ofthe34_Aerospace Mechanisms Symposium, Goddard SpaceFlightCenter,May10-12,2000 131
Design andDevelopment TheRubicon actuator, asectional viewofwhichisdepicted inFigure1,hasbeenjointlydeveloped by AEHandMoog,SMD.Thedesignconsists oftwostages, afinestageandacoarsestage.Both stagesaredrivenbyhighreliability space-rated stepper motorsdesigned andconstructed byMoog, SMD.Thestepper motorsare15degreetwophasemotors, producing twentyfourfullstepsper revolution. Sincemicro-stepping isnotusedinthedesign, andunpowered holding torqueisexhibited ateverycommanded step,theRubicon actuator iscapable ofholding position withthepoweroff. Themotorforthecoarseaxisdrivesa#4-40standard threadleadscrew nutthrough areducing gearbox witharatioof(1:45.2). Withthisoverallrating,approximately 1085motorstepsarerequired toprovide onerevolution ofthenut.Theleadscrew threadpitchof0.635millimeter resultsina resolution of0.585micrometer ofaxialmovement permotorstep.Toachieve the10-mmstrokeon thecoarsestage,thecoarseactuator usessixteen fullturnsofitsleadscrew nut. ThefinestageoftheRubicon actuator usesthesamemotorandgearbox,butinstead ofarotating nutdrivingtheleadscrew,itrotatesthescrew,andthenutisallowed totranslate. Theplunger nut exercises theRubicon elastictransducer (seeFigure2),toreducetheoutputmotionbyafactorof 0.00462. Thefinestageistherefore capable ofproducing anominal axialmotionof2.7nanometers foreachstepofthefinemotor. Eleventurnsofthefinestagejackscrewareusedtoproduce atotal strokeofabout30nanometers. Thedesignissuchthattheoutputofthefineandcoarsestagesaresummed attheactuator output shaft.Thisfeature allowsbothfineandcoarseposition adjustment oftheoutput. Tominimize weightandachieve thegearreduction ratio,acustom four-pass spurgeartransmission wasdesigned. Thegearsweremachined from15-5PH stainless steelmaterial. Table1summarizes thegearreduction characteristics. Forcryogenic temperature operation, itwasdeemed necessary to dry-lubricate themoving parts,i.e.,bearings, lead-screws, nutsandgears. Tungsten disulfide (Dicronite) drylubrication waschosen, basedontheheritage ofthislubrication oncryogenic mechanisms usedintheCOBE, UARSandAIRSprograms. Dicronite offerssuperior performance at ambient conditions, notexhibiting thehumidity absorption problems inherent withMoS2.Thegears weremanually lappedandelectro-polished toaccommodate thedrylubricant build-up thickness. The bearing racesandballsweredrylubricated bythevendorandassembled priortoshipment toMoog, SMD.Table2summarizes thelubrication trade-off studies. Table1.GearReduction Characteristics Designation Diametral Pitch Ratio FirstStage ........... Second Stage ThirdStage FourthStage Overall GearReduction Ratio1.800 120 96 2.929 96 2.929 80 2.929 45.211 Table2.Summary ofLubrication Trade-offs Parameter Coefficient ofFriction @ 300K,760torr 20K,101°torr Thickness, micrometer WearRate,m/m.MPa Application Method NotesMoS2 0.15 0.05 0.5to10 1.2x101° Impingement Poiymer Coating Sputtering Wells/Reservoirs OutgassingWS2(Dicronite) 0.05 0.05 0.5 1.2x10-_u Impingement LowRTFriction Molecular Bonding 132
Thematerialsofthepartsintheloadpathwerecarefullyselectedtoprovidefor1)anathermaldesign atmid-strokepositionand2)minimizefrictionalcharacteristics oftheleadscrew/nut combination. Table3summarizes thematerialchoicesoftheactuatorparts.TheRubicontransduceriscomprised ofa410stainlesssteelhelicalspringanda416stainlesssteelmachinedflexure.Thetransducer wassizedtoachievearelativelyhighstiffness(2.61x10sN/m)andlowoperating stresses (120MPa incylindrical shellflexureand171MPashearstressinhelicalcoilspring). Table3.Material Selection PartDescription General Construction Ballbearings Rubicon Transducer cylindrical shell Rubicon Transducer coilspring GearsMaterial Titanium Alloy6AI-4V Stainless Steel440C Stainless Steel416 Stainless Steel410 Stainless Steel15-5PH Lead-screw Stainless SteelA286 Nuts Stainless SteelNitronic 60 Athermalization portionofoutputshaft Invar Lessons Learned Thelineardisplacement inputtothetransducer's softcoilspringwasprovided byaplunger nutriding ontheactuator outputleadscrew. Theplunger assembly washeldagainst rotation byapininthe housing andanenlarged holeintheplunger. Thecoilspringwasacustom springmanufactured at SMD,withclosedandground ends.Duringinitialperformance characterization testingitwasfound thatthefinestageactuator hadarelatively largeandunpredictable deadband. Acloserexamination revealed thefollowing contributors: Largediametric clearance between theanti-rotating pinandtheplunger hole.Aclosertolerance radialslotwasmachined intheplunger toreducethisproblem. Thecoilspringdidn'thaveperfectground closedendsperpendicular tothecoilaxis.Asaresultthe plunger wasloadedeccentrically causing ittoskewandbind.Abettermachined springwasinstalled. TheCoulomb frictionintheinterface ofthespringtoplunger wasunpredictable, causing theplunger torotatebackandforthastheactuator wasbeingexercised. Thelargeclearance intheplunger antirotation holewasamplifying thiseffect. Thenextgeneration actuator incorporates amachined springwithintegral internal threaded nutanda mounting flangetoeliminate alloftheabovecontributors totherelatively largedeadband problem. Evaluation Testing TheRubicon actuator completed characterization evaluation testingatbothroomtemperature and cryogenic temperature. Roomtemperature andcryogenic temperature testingwasconducted atJet Propulsion Laboratory (JPL),androomtemperature measurement testingwasconducted atMoog, SMD.Thecryogenic testingatJPLusedavertical stainless steeldewarasdepicted inFigure4.The testusedaPolytech Series3000lnterferometer operating atawavelength of630nanometers, witha resolution of7nanometers. Thedatawererecorded onacomputer workstation. Fordetailed analysis, thedatafileswereconverted toASCIItextandreadintoanExcelspreadsheet. Theroom temperature testingatSMD(Figure 3)usedaprecision capacitance gauging metrology technique developed byAEH.Thecapacitance wasmeasured withanAndeen-Hagerling 2500precision capacitance bridgethathasaresolution ofabouttwonanometers. Theactuator isdesigned 133
specifically forperformance characteristics independent ofambient temperatures overtherangeof 20Kto300K.Thisgoalwasachieved, Thetestingindicated thattheperformance oftheactuator is insensitive totemperature. Figures 5and6showtypicalroomtemperature behavior intheMoog SMDlaboratory. Thetestswereconducted at200pulsespersecond witha0.350ampere regulated current. Following aretheoverallperformance characteristics oftheactuator: Coarse Stage: Available stroke: Smallest increment:10.3millimeters 0.58micrometer FineStage: Available stroke: Smallest increment:31.9micrometer 7.9nanometer (twomotorsteps) Thesmallest finestageincrement wasmeasured atbothcryogenic androomtemperature atJPL. Theresultswerenotrepeatable duetotestequipment andtestareanoiselevels. Theyweremore accurately observed atMoog,SMDundermorecontrolled testconditions atroomtemperature. The smallest detectable increment forthefinestagewasdetermined tobetwomotorsteps.Forfurther detailed information onthetestresultsrefertoReference 3. References -Hatheway, AlsonE.andHammond, JamesC."Passive Cryogenic Structural Actuators for theNextGeneration SpaceTelescope", Actuator Technology andApplications, ed.AlsonE. Hatheway (Bellingham, WA,SPIE,1998)Volume 3429B-48 . Hatheway, AlsonE.,"Metrology forCharacterizing Actuators withSub-atomic Repeatability," Proceedings oftheAnnual Meeting, (Raleigh, NC,American Society ofPrecision Engineers, 1994),pp.246-9 .Hatheway, AlsonE.andOwens, DanielE."Cryogenic andRoomTemperature Testing ofa Nanometer-Class Structural Actuator fortheNextgeneration SpaceTelescope", Optomechanical Design andEngineering, (Bellingham, WA,SPIE,1999)Volume 3786 _Hatheway, AlsonE.,"Results ofTesting aNanometer-Class Structural Actuator forNGST," NGSTScience andTechnology Exposition, )Provo, UT:Astronomical Society ofthePacific, September, 1999). _ "Rubicon" isatrademark ofAlsonE.Hatheway Inc. 134
Figure3.SMD'sRoomTemperature TestFacility Figure4.JPL'scryogenictestfacility 9OTen2-stepBursts BO L-:  4,0870 u.tu 60 average =7,86nm. =+a°_=_t _ J10.74 5O 401 ,,_lU.b4 ,_8.01 30i L._11.49 20, ;24 101 F_ TM OIL...... 50 1O0 150 2i -10] #41secondsIOTen10-step bursts 45(] 4o(] 35O 300 20<1 _150 101) 50 0 -50! , 50 100 150 seconds Figure5.Finestage,Twostepincrements Figure 6.Finestage,Tenstepincrements 136
Application ofMagnetic SmartMaterials toAerospace Motion Control ChadH,Joshi"andBruceR.Bent Abstract Energen, Inc.hasdeveloped linearactuators andlinearstepper motorsbasedonmagnetostrictive materials. Thesedevices aredesigned tooperate efficiently inharshvacuum andcryogenic environments. Theyprovide submicron positioning resolution overarangeofmotionof20mmormore. Thelinearactuators havehighbandwidth andashortstrokemaking themidealforactivevibration damping highspeedopticalpositioning. Thestepper motorshavealongstrokeandwillholdposition whenpowered off. Magnetostrictive Materials Magnetostriction isachange inanydimension ofamagnetic material caused byare-orientation ofthe atomicmagnetic moments. Inferromagnetic materials, anapplied magnetic fieldcauses rotation ofthe magnetization towards thefielddirection withindomains and/ormotionofthedomain wallstoincrease thesizeofthedomains withmagnetization vectors closeindirection totheapplied field.Whenthe magnetic moments arecompletely aligned, saturation occursandnofurthermagnetostriction canbe produced byincreasing theapplied magnetic field.Theamount ofmagnetostriction atsaturation isthe mostfundamental measure ofamagnetostrictive material. Themodern eraofmagnetostriction beganin 1963whenstrainsapproaching 1%werediscovered intherareearthmaterials, terbium (Tb)and dysprosium (Dy),atcryogenic temperatures. Sincethen,manymaterials (seeTable1)havebeenshown toexhibitmagnetostrictive behavior including several materials atroomtemperature, butthehighest magnetostriction isfoundinalloyswithCurietemperatures belowroomtemperatures making themideal forcryogenic deviceapplications. Table1-Saturation strainandCurietemperatures ofselected materials. 1 Material Saturation Strain CurieTemperature Ni Fe SmFe2 Fe304 DyFe2 TbFe2 Tb0.3Dy0.TFe1.9 (TerfenoI-D) Tbo.sDy0.4 @77K Tb0.5Zn0.5 Tbl__DyxZn(x10-6) (K) -50 630 -14 1040 -2340 690 60 860 650 630 2630 700 1600-2400 650 6300 210 5500 180 5000 200 Measurements ofmagnetostriction onTbDyZn alloysweremadeat77Kandat4.2K.Thedatafor77K areveryreproducible andagreewithdatafromtheliterature. 2Figure1showsthemeasurements onone sample oftheTbDyZn alloyat4.2K.Thesedataarebelieved tobethefirstdirectmeasurement of magnetostriction inthismaterial system at4.2Kandindicate thatthehighsaturation strainremains at thatlowtemperature. •Energen, Inc.,Billerica, MA 1Magnetostrictive Materials, K.B.Hathaway andA.E.Clark,MRSBulletin, Vol.XVIII,No.4,April1993. 2A.E.Clark,HighPowerMagnetostricfive Materials fromCryogenic Temperatures to250C,Materials Research SocietyFallMeeting, Boston,MA,November 28-30,1994. Proceedings ofthe34_Aerospace Mechanisms Symposium, Goddard SpaceFlightCenter,May10-12,2000 137
100 8OA 60 i,o N20  i ! ! 010 20 30! 40 50 Current (A) Figure 1-Magnetostriction ofaTbDyZn alloyat4.2Kshows highsaturation strain. Actuators Alinearactuator consists ofarodofmagnetic material surrounded byanelectrical coil.Energizing the coilwithelectrical current causes themagnetic material toelongate inrelation tothecurrent amplitude. Precision positioning canbeachieved byprecisely controlling thecurrent. Suchanactuator canbeused forprecise mechanical positioning, vibration control, orswitchandvalveoperation. Energen, Inc.has builtseveral sizesoflinearactuators thatcanprovide veryhighforcesinacompact device. Figure2 showsseveral actuators manufactured byEnergen, Inc.(thelargestofwhichweighs only40g)having strokes thatrangefrom5to300microns andforcecapabilities upto1200N(270Ib). Largeractuators canbebuiltbyincreasing thesizeofthemagnetostrictor rod.Energen, Inc.hasbuiltan actuator witha22.2kN(5000Ib)forcecapability foruseinsuperconducting particle accelerators. Typical response ofoursmallactuators isshowninFigure3. Figure2-Energen's linearactuators 138
150 A E :L "-"100 c E -_5O ffl 0J 0 2 4 6 8 10 Current (A) Flgure3-Typical response ofanactuator SRFTuners Particle accelerators andfreeelectron lasersarebeingusedtogenerate monochomatic lightsources in theinfrared, ultraviolet anddeepultraviolet regions oftheelectromagnetic spectrum. Whileparticle accelerators havetraditionally beenthetools-of-the-trade ofhighenergy physicists, freeelectron lasers arefindingincreased applications inindustrial materials processing suchaspolymer andmetalsurface processing, metalandceramic surface micromachining, andsemiconductor surface preparation. Figure4-Prototype cavityfinetuner. Energen, Inc.hasdesigned, builtanddemonstrated afinetuningmechanism (shown inFigure4)for superconducting radiofrequency (SRF)cavities usedinparticle accelerators. Thistunerwasspecifically designed tomeettherequirement oftheContinuous Electron BeamAccelerator Facility attheThomas Jefferson National Accelerator Facility inNewport News,VA.Thetunerisinstalled inthedeadlegofthe existing mechanical cavitytuningsystem (asshowninFigure5)whichconsists ofawormwheel andball screwassembly. Themechanical tunerisusedforcoarsetuning. Asthemagnetostrictive SRFtuneris energized, thedeadlegexpands providing precision finetuningcapability. Because ofitshighforce capability, thefinetunercanovercome the5644N/m(50,000 Ib/in)stiffness oftheSRFcavity. The magnetostrictive tunerprovides 2000Hzoftuningbandwidth andcanbeactively controlled duringthe 139
operationoftheSRFcavity.Thelargestrokeandhighforcecapabilityofouractuatorsmakesitpossible toreplacethemechanical tunerwithamagnetostrictive tunertherebyincreasingthereliabilityofthe cavitytuningsystemandreducingthethermalloadontherefrigerator. 1497.0025 _.1497.0020 I _v1497.0015 _1497.0010 o" _1497.0005 E 1497.0000 1496.9995 1496.9990 0 10 20 30 40 50 DriveCu_ent(A) Figure 5-ThefinetunerwastestedatJefferson Laboratory anddemonstrated excellent tuning capability6O LinearStepper Motor Energen, Inc.hasusedthebasicconcept ofalinearactuator tocreatealinearstepper motor. Thelinear stepper motorusesasetofthreemagnetostrictive actuators thatenableittomovearodforward or backward inastepwise fashion. Thismotorcanprovide alargestrokeofseveral millimeters. Variations onthisdesigncanyieldanactuator withastrokelimitedonlybythelengthofthetranslating rod. Figure6showsaphotograph ofalinearstepper motordesigned foraerospace applications alongwithits operating sequence. Thetranslating rodiscomprised ofthreesegments assembled endtoend.The centersection isamagnetostrictive rodsurrounded byasuperconducting coil.Clamps oneachsideof theactuator grabontoconnecting rods.Theseclamps contain amagnetostrictor actuator thatwhen energized willcausetheclamptorelease itsholdontherod. Thelinearstepper motorcanbeoperated inoneoftwomodes-stepper modeorfinetunemode. In stepper mode,theclamps andtranslating actuator areturnedonandoffinthepropersequence tocause thetranslating assembly toindexforward orbackward. Finetunemodeoperation provides highpositioning resolution. Inthismode,theforward actuator is energized torelease andthecurrent inthecenteractuator ismodulated thereby moving theforward end oftheshaftproportionally. Underthisoperating mode,thepositioning resolution islimitedbythecurrent regulation. Thus,thismotoriscapable ofproviding alongstrokewithhighpositioning resolution. Itis capable ofholding position withzeropowerdissipation sincetheclamps holdatzerocurrent. 140