(typically)50teeth,causingthemotortofunctionasifithad100poles.Thisapparentincreaseinthe numberofpolesisakeyfeatureinobtainingthesmall1.8degreestepforeachpoweredinputpulse. Othernon-conventional methodsexistforprovidingsmallanglesteppersinenvelopesof50mmorless. Athoroughdiscussionofsteppermotordesignandapplication issuesiscomplex.However,thereare severalkeydesignandapplication issuesthatneedtobeaddressed: (1)Simulatingthesteppermotorinthesystem;algebraicanalysisisnotadequate (2)Understanding thetypesofmarginusedandhowmuchmarginisneededfortheapplication (3)Powersupplyoptionsandrestrictions includingcurrentlimiting,pulsewidthlimiting,voltage,and power (4)Operatingspeedandtorquerangeswithattentiontoneedforastart-upramp (5)Specialsystemrequirements suchasunpowered holdingtorqueorratchettorquelimitationsof harmonicdrivegearing (6)Stepsizesmallenoughtoassuremechanism resolutionismet (7)Damping,magneticcoggingrange,dragtorque,bearingtorquevariationsovertemperature. DCMotors inSpaceApplications Athorough reviewofspacecraft mechanisms andtheirspecial requirements isbeyond thescopeofthis paper.Instead, thecharacteristics ofthemostcommon systems arepresented. Similar systems use similardrivemotorsforverygoodreasons. Themostcommon applications ofthevarious motortypesare presented alongwithcautions andlessons learned. SolarArrayandBoomDeployment Surprisingly, theselargeandrelatively heavysystems consume verylittlepowerduringdeployment. For various reasons, deployment rateisslow.Thisresultsinamechanical powerrequirement assmallasone watt,evenonlargesystems. Akeyrequirement istohavesubstantial torquemargin inthedrivesystem. Unexpected events suchasdirtybearings orapanelthatsnagspartofasolarblanket canprevent deployment unlessthereissufficient powertoovercome theseanomalies. Somesystems usespring deployment withdamper control. Ifamotorisselected, itisgenerally abrushorBDCmotor.Thebrush motorisoftenacceptable duetotheshortliferequirements. Having specialcontrolcapabilities, brushless motors aresuccessfully usedbymanysystems manufacturers. Also,brushmotors wereasignificant problem intheearlydaysofspaceoperation. Advances overtheyearsmakethemviablechoices aslong asthepotential problems areunderstood andpastlessons learned arenotrepeated. Lifeinavacuum canbeveryshort,typically from10to400hours.Thisisduetoaccelerated brushwear. Whynotuseastepper motor?Thisiscertainly possible buttherecanbeunexpected problems. Whena stepper motordriveisloaded withahighinertia, thereisthepossibility ofinstability causing erratic operation. Also,astepper motorcan"runaway"withaidingtorqueloads,typically atspeeds several timesfasterthanthecommanded rate. Inmostapplications, adrivemotorrunsatamoderate speedandisgeared downtoproduce hightorque ataslowspeed.Figure4showsaBDCmotor/brake drivingthrough planetary gearing. Notethatthesize isdominated bythegearing andastronaut over-ride mechanism. Evenso,thetotalpackage ismuch smaller thanwouldberequired foradirectdrivetorquemotor.Powerrequirements wouldbemanytimes higherusingdirectdrive. Veryoften,asolararrayorboomwillhaveself-deployment characteristics. Ifsomeself-deployment occurs, themotormustactually actasagenerator undertheseconditions torestrain andcontroltherate. Allowances mustbemadeinthecontrolelectronics topermitreverse current flow.Otherwise themotor couldreachahighrateofspeed.Sometimes aZenerdiodeisusedtoprovide acurrent path.Whenthe motoractsasagenerator, itcontrols deployment speedmuchbetterthanadamper basedsystem. NASA/CP--2002-211506 98