perfo rman ce margi n in a stepp er me chani sm. Lessons learned from stepp er motor usag e on sp ace  mech anisms can b e cate gorized into fo ur differe nt areas:  • Torque Pro duction Assum ptions  • Temperature Effects  • Over / Und er Margi n Use  • Electri cal Driver F eatures / Differences. 

Incorre ct Torque Pro duction Assumptio ns  Incorre ct torq ue pro duction assumption s can be ma de during the m otor sel ection and si zing p hase due to   the ca sual usage of the motor Kt. In a step per m otor, the Kt is a reference figure used  in helpin g to  determi ne the windi ng scaling and in compari ng m otor capabilities, alon g wit h the motor constant, Km .  But, if the Kt is multiplied by the curren t to obtai n a “torque estim ate” witho ut reco gnizing th e open -loop  operation characteristics of  the ste pper and without a ny margi n consideratio ns, then the m otor sizing will  be very la cking in to rque capability. Th is con cept is at the he art of establi shing adequate perform ance  margi n, because Kt an d Kb by themsel ves cann ot be used to p redict what the  speed-to rque output will  be.  

Unanticipated Temperature Effe cts  Unanticipated temperature effects can have a g reat impact. The firs t concern is  attempting to size a motor  workin g only with the a mbient tempe rature resistance. The h ot ambient temp erature can b e used to easily  calculate the  cha nge in windi ng resi stance from n ominal to ho t condition s. But of equal  or greate r  importa nce, i s the pote ntial for th ermal rise d ue to  internal  moto r heatin g. Thi s is much more difficult t o  address th an the effects of a hot am bient  temperature. The m ountin g condition s and the rmal t ransfer f rom  the motor into its flight mounting may n ot be well def ined; this in combinatio n with an unce rtain duty cycle  can create uncertainties in  the wi nding temperature.  Elevated win ding temperatures, which may be within  the ratin g of the moto r material s, can exceed th at expe cted i n the p erformance m odel. And, so  the  expecte d perf ormance ca n suffer. 

Inadeq uate o r Inapp ropriate Uses of Ma rgin  Inadeq uate o r inapprop riate uses of m argin eithe r by the torq ue or volta ge method i s sure to cause  perfo rman ce deficie ncies. Becau se step per moto rs are different in design an d their appli cations al so vary   considerably, there is some und ersta nding that to rque m argin factors may vary within a  reasonable  range. As th ese fa ctors are applied  over a histo ry of pro grams, a ba sis for establi shing the ne cessary  ranges of factor valu es is develo ped. To the ext ent that perf ormance sim ulation tool s increa se in  accuracy of predi ction, th e facto rs may be re duced a bit. Fund amentally, the  stepp er m otor’s outp ut is  very dep endent upo n the  dynamics of th e system. An d, so to eval uate a stepper motor ind epende ntly of  the sy stem in  which it will be used is not accepta ble. Hen ce, the  need i s present to in corporate to rque  margi n factors from a well-kno wn rang e to help assu re that performance goal s will be met. 

It should also be unde rstood that the inclu sion of such margin factors does not gu arante e that  perfo rman ce will be sati sfactory unde r every condit ion, because other i nfluential co nditio ns may exist   such as syst em re sonances. In fact, so me con ditions of the worst -case system  analysi s are not corre ctly  simulate d if t he to rque margin is a pplied to th e loads and losses. These simulation s that evaluate th e  motor an d system sta bility are bette r served if the margi n facto rs are omitted  and losse s are redu ced  closer to the  minimum va lues. Cl osely related to  this type of concern i s speed de pendent dampin g  asso ciated with the motor magneti c properties and th e electroni c drive circuitry.  

In a b roader sense, to cover th e full perspe ctive of performance margin, it is necessa ry to evaluate  not  just the torque requi rements and as sociated margi n factors but al so the dynami c stability of t he motor in  its syst em. While thi s is certainly un derstood  by system and mech anism desi gners, for the sa ke o f  presentin g a complete p erspective of steppe r mot or perform ance it is ment ioned. In ad dition to the  conditions of operation where facto rs are applied  and those conditions wh ere they are not, other unique  conditions should al so b e con sidered. For examp le, if there is the possib ility of the mech anism  encounte ring hard-stops, then the  requireme nts o utlined in  harmonic drive catalogs fo r ratch eting torque   must b e observed. Restriction s may be required to  prevent the over-pro duction of m otor torque, that i s,  the maximum  motor torq ue possible should not reach or exce ed a percenta ge of the maximu m harmoni c  128