for eve ry application. The best fi rst approach is to identify stepp er motors and  manufa cturer’ s data that   are comp arable with the new a pplication, either  through knowledge of step pers that h ave been u sed  before or th rough m anufa cturer’s catal og data. Th en, narro w the sel ection of motor mod els based on  further trade s and analy ses. 

Figure 2.  Single Axis Stepper Harm onic Driv e Unit 

Load torque is esse ntial informatio n for developi ng initial steppe r motor sp ecification s, but it must be  unde rstood that sele cting a stepp er fro m catalog data base d on load torq ue requireme nts will not yield  satisfa ctory result s. Typically, catalog data is p rovided for o peration a room te mperature and has little if  any margin applied to assure that  performance goals will be m et in a space application. It become s  importa nt, then to  also know the f ull range of the  mechanism’s loading over tem perature and othe r  extreme s and the amount of margin th at is deeme d sufficient. Fro m the stepp er motor’ s perspective, the  internal  drag asso ciate d with its be arings un der co ld start-up con ditions can be as sign ificant as the  external lo ad refle cted th rough th e ge ar redu ction.  Hence, step motor specification s must attempt to  address the range of external loa ds, int ernal l oads, and the extent of torqu e margi n required befo re a  motor can be sized. 

Another torq ue characteristic typi cally nee ded with stepper motor appli cations is called  “unpowered  holdin g torqu e.” Thi s property is th e result of m agnetic detent wi thin the m otor that causes it to hold  its  positio n whe n no power i s applied. A certai n am ount of detent is necessary to assure that w hen step  excitation has ceased the motor rotor positio n rest s at the expe cted l ocation.  This may not be t rue if  external dyna mic effects are sufficie nt to overco me the unpowered hol ding torque. Unpowered holdi ng  torque  is commonly u sed within space mech anisms to hold the  mechanism po sition a gainst rotation once  power is removed from the  motor. 

If a minimum  amou nt of u npowered holding to rque is present within the  moto r design, th en a mini mum   amount of m echanism hol ding can be expected. So, it is comm on for the me chanism a nd he nce moto r to  have an u npowered holdi ng torq ue specification. It sh ould also be noted th at too much motor d etent  torque ca n interfere with meeting dynamic torque re quire ment s, so it may be n ecessary to have both a  minimum an d maximum ho lding torq ue spec (c ommon ly defined through the mot or’s detent). 

Unlike bru shless dc servomotors, which have  varyi ng p ower in puts depending o n the changi ng load  requi rements,  stepp er m otors operate on a constant level of power in put. This at first  may se em  surpri sing, but once it i s realized that th e ste pper actuates ba sed on series of e lectrical pulses, the power  situation becomes better unde rstood. The full level of power is applied at eve ry motor step regardless of  the loadin g situation. Since the po wer i nput is e ssentially at maximu m for eve ry step, an asse ssment of   the duty cycl e, averag e power input, and moto r thermal ca pability  become s very important. A motor may   be cho sen that meet s step re solution and the speed – to rque requireme nts but b e found to have  123