SMADriveMotorMechanism Themechanism requirements/specifications arelistedbelow: OutputDrivetorque Drivestepsize No.ofoperations Direction ofrotation Driveangle Operating temperature Massofflap Inertiaaboutrotation axis Drivemechanism Mass:0.2N.m(minimum) :2degree :10,000stepsofoperations inbothdirections :Bothclockwise andanticlockwise :0to180° :-20°Cto70°C •0.440kgeach ;0.15kg.mz :0.190kgeach TheSMAdrivemotormechanism mainlyconsists of:(a)SMAwireactuator elements, (b)Levers, Pawls andRatchet wheels, (c)Detentmechanism, (d)Spurgearreducer, (e)Loadlimiting springmechanism andotherparts.Figure5showsthedisassembled viewofthemechanism. Twobrackets, bracket 1and bracket 2,givethenecessary structural support tothemechanism. Onealuminum alloysingle-piece thermal coverisprovided overthemechanism. _;MAwireactuators TwoSMAlinearwireactuators ofdiameter 0.25mmareusedtogenerate theforcerequired todrivethe mechanism. Onewireisusedfortheopening direction driveandtheotherfortheclosing direction drive. Linearwireactuators werepreferred overspring-type SMAactuators astheyaremoreefficient, require lesselectrical power,havefasterheating andcooling rates,higherlifeforrepeated operation, andlow mass.Thesetmemory inthewireactuator isshorter length. TheSMAwirelengthshortens whenheated abovetransformation temperature. Heating oftheSMAwireisachieved bypassing PulseWidth Modulated (PWM)electric current directly through thewire.PWMelectric current heating wasselected overDCheating fortheimproved lifecycleoftheSMAwireactuator. TheSMAwiretransformation temperatures wereselected suchthattheyarewellabovetheestimated on-orbit temperature ofthewire. Theshaperecovery temperature oftheselected SMAactuator wirewas-90°C. Thelengthofactuator wirewaschosen toprovide therequired strokewithin3.5%recovery strain.TheSMAwirealsohadtwowaymemory andreturned toitslongerlengthoncooling belowitslowertransformation temperature. Hencebiasforcewasnotrequired tostretchtheSMAtoitslongerlength(coldshape). Thisalsohadthe advantage ofhighernetoutputforce. Levers. PawlsandRatchet wheels Theyareusedtoconvert SMAactuator's linearstroketorotarystepmotion. Twosetsoflever,pawland ratchetwheelassemblies (onesetforopening andothersetforclosing direction) aremounted onthe mainshaftasshowninFigure5.Theratchetwheelsarefixedtothemainshaftwiththeirteethin opposite direction toeachother.Thelevers,fittedwithbushing bearings arefreetooscillate ontheshaft. Eachpawlisfittedwithabushing bearing andismounted ontoapinfixedononeendoflever.Apawl torsionspringisalsomounted onthepintokeepthepawlpressed. Thepawlsareprovided withaguide contour toguidethemfortheirengagement/disengagement withtheirratchetwheels. Theotherendof theleverisconnected tothewireropeterminal, whichinturnlinkstheSMAactuator. Pawlandratchet wheelteetharelubricated withbonded filmmolybdenum disulfide. Detentmechanism Adetentwheelandapairofspring-loaded leversprovide thedetenttorquethatisnecessary forthe ratchetwheels toholdthemintheirdiscrete toothpositions. Thedetentwheelmounted directly onthe mainshaftasshowninFigure5,holdsthemainshaftinposition whentheleverandpawlarepulledback bythebiasspring. Thedetentwheelisnecessary, asaftereachdrivesteptheshaftwouldhaverotated backtotheoriginal position andmightnotproduce anynetrotation. 242