(4)Upperlimitofmotortorqueripplepermissible (depending onpercentage thedesignwillundergo furtherchanges...typically motortorqueripplelessthan1.0%requiresanextraordinary design.) (5)Manufacturing concerns(themorespecialized thedesign,themorecriticalthemanufacturing procedures willbedrivingyieldandcost) Itisimportanttounderstand thatbeingoverlyconservative inspecifyingverylowlevelsoftorqueripple anddisturbance willresultinsignificanttechnicalchallengesandmuchhighermanufacturing complexities andcost.Thetemptationtoapplysignificantmargintotorqueripplerequirements shouldberesisted. Systemoptionsshouldbeconsidered carefully,sincetheytendtoprovidethemostdirectionforthemotor design. Stepper Motors Whiletherearemanydesign similarities between stepper motors andBDCmotors, therearesome profound differences. Stepper motorsreallyfallintoacategory bythemselves, atleastfromanapplication standpoint. Unlike BDCorbrushed motors, steppers aredesigned without apositive feedback commutation method. Whilethemotorwindings arecommutated bythedriveelectronics, thereisnorotor position feedback toassure thatthesteppers arefollowing thecommands tostepandrotate.Tothe extentthatthestepper motoranditsloadfollowthecommands, thesystem issaidtobe"synchronous." If themotororloadbreakoutofthecommand sequence, thenthestepping is"non-synchronous." Amajor portion ofthestepper design should focusondetermining howlikelythesystem istoremain synchronous. Stepper motordesignmustinclude consideration forthesystem mechanical dynamics and theelectronic driver.Tothisend,selection ofastepper motorforanapplication needstoinclude a system levelanalysis. Stepper motors aredesigned toprovide arepeatable angular rotation foreveryinputpowerpulse command. Whilethetwo,three,orfourphasestepper motorexcitation mustfollowacertain sequence, noexternal rotorposition sensing isrequired toinstruct theelectronic driverwhentogotothenext sequence. Thisfeature simplifies thesystem byeliminating rotorposition feedback andrelated electronics. However, thecontrol electronics mustbedesigned tocommand thestepper withinapredetermined rangeofspeeds sothatthestepper doesnot"losestep"withthecommands. Theprocess of analyzing thesystem dynamics toestablish thepermissible command limitscanbecomplex. Ifthemotor andsystem aredesigned andanalyzed correctly, simplycounting thenumber ofmotorstepcommands givenandmultiplying itbythemotorstepanglewillprovide theoutputposition. Conventional stepper motors aredesigned andconstructed muchlikeBDCmotors withwound stators andmagnetic rotors.Because stepper motors runopenloop,thepresence ofmagnetic cogging and internal motordamping areimportant instabilizing thestepping actionforeachstep.So,whileBDC motors trytominimize cogging anddamping, stepper designs willusesmaller airgapstoassure magnetic cogging ispresent andintentionally permitrunning lossestocreatedamping effects. Other design concerns peculiar tosteppers existsuchasassuring thepositioning during powered steps remains thesamewhenpowerisremoved. Stepsizeusually indicates arangeinthestatordiameter, because asmaller stepanglerequires larger diameter motors. Stepangleisaninverse function ofthenumber ofphases andthepolecount.So,to obtainasmallstepangle,atypicaltwo-phase stepper motorwillrequire alargerpolecountandhencea largerdiameter. Forexample, amotorwithfifteendegrees perstepcanrequire a75-mm outerdiameter. Conversely, largeanglesteppers (45or90degrees perstep)arefairlysmallwith25-mm diameters. Because oftheneedforachieving smallstepangles withinareasonable diameter, alternate design approaches havebeendeveloped, suchasthehybridstepper motor. Thehybridstepper motorcanprovide a1.8-degree stepanglewithina50-mm diameter. Itishybridinthe sensethatithasfeatures ofbothpermanent magnet andvariable reluctance motortypes.Somehybrids arelargerandsomesmaller depending onthetorqueneeds. Hybrids havebeenusedsuccessfully in spacemechanisms. Theirdesignapproach relieson"tricking" themotorintobelieving ithasmanymore polesthanitactually does.Atwo-pole discmagnet isoriented axiallyandsandwiched between twosteel polefacesdirecting fluxradially towards manystatorteeth.Eachrotorpolefaceismachined with NASA/CP--2002-211506 97