Establishing Adequate Performance Margin for  Space Flight Stepper Motor Mechanisms 

David B. Mar ks* 

Abstract 

Adequate  Steppe r Moto r perfo rman ce margi n is critical for mission succe ss. Often, it is n ot clea r from   system specification s what the stepp er moto r pe rforma nce re quire ment s should b e, beca use the   concerns and method s for defini ng and ap plying  a stepp er motor may not be well  unde rstood.  “Establi shing Adequate Perform ance Margi n for S pace Flight Stepper Mot or Me chanisms” p resents  practical guidelines for the  corre ct specification an d appli cation of  stepp er mot ors used in space flight   mech anisms and provide s some b asic lesson s-learned from flight hard ware expe riences. 

Introduc tion 

System or mech anism specification s and the  associated stepper moto r specification s are typi cally  different in te rms of h ow the motor related cha racteristics are pre sented. It become s important then for  the stepp er motor spe cification s to in clude a comp rehen sive defi nition of motor pa rameters to cove r the  system  spec requirement s but also th ose requirements derive d and i ndirectly  required to  support th e  system m otion control. In the syst em specifi cation s, motor specification s, or within the mot or suppli er’s  sizing of th e design, the  perfo rman ce margi n over and a bove t he a ctual motor o r mechanism output  should be d etermin ed for st eppe r motors. 

Unlike brushless DC moto rs, step per motors cannot be relie d upon to prod uce ou tput torque as a functio n  of input curre nt. Motor spe ed, torque lo ading, an d system dynami cs affe ct the stepp er moto r’s ability to   respon d to input comm ands. Becau se step per mo tors g enerally op erate  witho ut feedba ck for  comm utation,  velocity, or p osition (an advantage that simplifie s the  system and reduces cost), care mu st  be taken to a ssure the stepper moto r supplie d can p erform ove r the ran ge of chara cteristics asso ciate d  with the p articular motio n cont rol system. Torque margi n or voltage ma rgin analyses must con sider  mech anism internal lo sses, external l oad and coupling variatio ns, ele ctrical  toleran ces, temperature   change effects, viscou s damping, and excitation  pulse charact eristics. The  specification  and  sizing  approa ch should eval uate minimum  torque conditions, minim um stability conditi ons, system  resona nce,  and minim um unpo wered holding to rque. Othe r consideratio ns may apply , too, depen ding on the   mechani cal configuration,  such as the maximum motor torque capability. 

So, to compensate and a ccount for in herent syste m variabilitie s and thei r influen ce on stepp er moto rbased me chanisms, adequate de sign margin must be built into th e motor sizing. Establi shed guid elines  exist among users and m anufa cturers of steppe r motors  for qua ntifying margi n requi rements, but every  appli cation will hold uniqu e con cerns. Perform ance anal ysis u sing motion si mulation tool s is usually   requi red to quantify the steppe r mechanism perform ance ove r many varying  conditions of ope ration.   Herita ge designs and th eir appli cation s serve as a good referen ce and should be con sulted alon g with   analysi s and simulatio n, but ultimately hard ware testing will demo nstrate  the success of the stepper motor  mech anism. 

The moto r specifi cation s, analysi s assumption s, an d hardware testing should agree in methodolo gy as  closely as possi ble. Otherwise, the design and test approach will be flaw ed. It is possibl e to provide a well  designed and  tested stepp er moto r or steppe r mechanism that d oes not me et the system req uirements if  the meth odology u sed does not relate well to how the  mechanism will a ctually be used. Or, specifi cation   requi rements that are  too severe can penali ze the  program with unnecessary weight, co sts and d elays. It                                                     * Moog, Inc., Durham, NC 

Proceedi ngs of the 37th Aerospa ce Me chani sms Sym posium, John son Spa ce Center, Ma y 19-21, 200 4  119