Table 3.  Specif ying Marg in in Stepper  Motor Perfo rmance 

Specif ying Margin in  Steppe r Motor  Performance     Specifica tion   Analy sis   Test 

(3.a) Apply F actors to  Load s and Losses  durin g analy sis to  assure actu al  requi rements are met in  test     States sp ecific load a nd  loss torques  with fac tors  that shoul d be applie d  durin g analy sis but not  in hard ware test     Use torque values  that  include the m argin  factors and simulate to  show complia nce   Test at the re quire d  torque valu es under  worst case conditions  but with no fa ctors  applie d 

(3.b) Red uce Applied  Voltage du ring  analyses; but  test with  norm al voltag e      Specify a red uced  applie d voltage that  reflect s desired margin  for analysi s purposes  only     Use red uced voltage for  simulatin g operation  and spe cific  perfo rman ce points   Test u sing th e norm al  voltage to meet  specified p erforma nce 

(3.c) Use Heritage  Mechanism Data to  Qualify Similar New  Designs     Referen ce the Herita ge  specifications   Referen ce the Herita ge  analyses a nd make  minor mo difications a s  need ed   Test similarly to the  heritag e me chani sm 

The ap plication of ma rgin factors to loa d and l oss to rques may take o n another form besides that sho wn  in Tabl e 3, th e torq ue m argin bei ng derived from an equatio n rela ting mea sured to requi red value s. For  example, a m easure d maxi mum outp ut torqu e value under worst ca se conditions would be div ided by an   expecte d torq ue lo ss valu e (sometime s measured also) to  arrive at a m argin factor a bove i nternal l osses.  Additional torque loadi ng may be adde d to repre sent an output load or may be  factored into  the margin   equatio n. 

This approach is not u ncommon b ut tends to comp ound th e ma rgin withi n a stepper moto r appli cation.  The re ason f or this i s that when m argin factors with in a sp ecification must be demon strated  at test by a   calculatio n based on measured to rques, the analy sis comp onent of the p rocess must contain “addition al  paddi ng” so that margin over the  test ed am ounts is present. A  step per’s response is not linear o r  predi ctabl e like th at of a  brushless design. In  order for a stepp er to produce, for example,  thre e time s  more torque output than the req uirement, the sizing  of the motor must  go so me factor b eyond that in   order to a ctually test out a margi n factor of thre e. Hence, the mo tor de sign might need to i ncorporate a  factor of five over expe cted torque s, for example, in  orde r to test out a factor of three time s the torqu e  requi rement. So, the spe cification of torq ues and application of ma rgin mu st be  thoug ht thro ugh very well  and correlate d with the an alysis a nd test approa ch to achieve a rea sonable o utcome. 

Evaluation, Simulation, and Tes ting of the Stepp er Motor 

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 data. Thi s involves lo oking at the available torq ue pro duction capa bility, and re alizing that the rated  value of to rque will need to  be de-rate d in accordance with th e ma rgin fa ctors chosen. Spee d maximum s  should also be revie wed with a close lo ok at th e moto r step  angl e and number of motor pha ses. The  peak  and continu ous outp ut data (if available ) must al ways be ju dged relative to the con ditions asso ciated with  the ratin g. Typically, space enviro nments offe r the  least favorable  condition s from a th ermal stan dpoint.  Toleran ces on the moto r winding constants, mag netic detent, volta ge, cu rrent, tempe rature e xtremes, etc.  125