drive ratcheti ng to rque rating. Othe rwise, dama ge to the ha rmonic drive co uld result. This addition al  aspect an d others li ke it associ ated wit h uniqu e system desi gn co ncerns must be add ressed as well as  the obviou s perform ance margi n concerns. 

The Elect ronic Driver   The elect ronic driver for the stepp er motor ca n have as signif icant an effect on  performance a s the  system dyna mics. Excl uding the o bvious facto rs of a vailable volta ge an d curre nt, other d river pa rameters  if overloo ked can im pair th e resulting system pe rform ance. Proba bly the most si gnifica nt driv er pa rameter  to be con cerned about is the use of a partial pul se widt h. As already explai ned, this i s implemente d to  help re duce the avera ge power consum ption of the motor. But, if the par tial pulse wi dth feature is ove rlooked in th e motor design analy sis, then the performance can  suffer at lower speed  ranges where i t  takes effe ct. Specification s must be  clear about this  featu re, an d perform ance sim ulation s must ta ke its  use into a ccount. A similar and mo re obv ious featu re is the cu rrent limit.  

In some systems, a ste pper may be re quire d to start  at a rate that exceed s its capability. So, an ele ctronic  rate ra mp ca n be pa rt of the drive el ectronics to start the motor at a wo rkable speed a nd qui ckly adva nce  to the highe r rate. Thi s type schem e is t ypically not p roposed in a driver b ut is the re sult of the motor and  system pe rformance analyses, which may sho w that  such a feature is required. If the perform ance  analyses fail to uncover thi s concern, th en it will surel y be uncove red duri ng test ing.  

A more com mon pro blem that occu rs is the resul t of using commercial d rive electro nics for moto r  comp onents testing th at usually differs in  design from the  space flight d rive ele ctronics. Thi s can  introdu ce minor perfo rmance differe nces or majo r ones, depending on the electronics. In m any ca ses the  flight drive el ectro nics are  not availabl e for use with the moto r compone nts, b ut a re asona ble comp romise  is to build a commercial g rade drive b ox bas ed on the  flight electro nics schem atic. 

Summar y and Conclusio ns 

Adequate steppe r moto r perfo rmance margin i s critical for m ission su ccess. System  or mechanism  specification s and th e associate d stepper moto r specifications are typically dif ferent in te rms of h ow the  motor related  characteristics a re presented. Unlike br ushless DC motors, stepp er moto rs cannot be relied   upon to prod uce o utput torque a s a fu nction of inp ut current. M otor sp eed, torqu e loadi ng, and syste m  dynamics affect the stepp er motor’ s ability to resp ond to input command s. To comp ensate and acco unt  for inh erent motor a nd system varia bilities an d the ir influen ce on step per motor-ba sed mechanisms,  adeq uate design margin must be built into the  motor si zing. The motor specifications, analy sis  assumptio ns, and hardware testing sho uld agr ee in methodol ogy as cl osely as possibl e. 

It is ne cessary in applying  a step per motor to care fully define rang es (minim ums or maximu ms) on m any  parameters, such as the  motor speed (pulse rate) to a ssure th at the m otor and  load rotation remai n  synchro nous over all con ditions of voltage variation , temperature  and  internal frictio nal drag chan ges,  load variatio ns, etc. The proce ss of deri ving motor require ment s from system o r mecha nism perfo rman ce  requi rements and of impo sing sufficie nt margin fo r the particul ar mech anism a pplication is essential t o  proper stepper moto r sizing. A comp rehensive stepper moto r specificatio n mu st includ e an approach that  correl ates th e sp ecifications, sim ulation and a nalysis, a nd ha rdwa re te sting. Table 1 i s applicatio ns  oriente d, dire cted towards the system s or me chani sm designer desiring to utili ze a stepp er for a sp ace  flight mecha nism, and p rovides gui delines o n specifying and correlating ste pper moto r req uirements fo r  motor spe cification, an alyses, and te sting. 

Prelimina ry evaluation of the step per m otor pe rform ance comes through a revie w of its catal og or hi storic  use d ata. Wh ile most of the relevant pa ramete rs are readily availa ble from the system de scription, motor  catalo g spe cification s, and  histori cal ref erences, it is essential that the performance analy ses inclu de  these over th e full ra nge of ope rating conditions. Failure to consider the worst-ca se extre mes of these   parameters will re sult in  improp er perform ance asse ssment and op eratin g anomali es. Performa nce  simulatio n is best don e usi ng method ologies that hav e been succe ssful in the p ast in co rrelating with test   results. Correlation mu st also b e present between performan ce modelin g and lab test s. Testing at the   motor component level  looks at the  parameters th at determi ne to rque, speed, p ower, angular moveme nt,  129