insuffici ent si ze an d the rmal cap acity. T he moto r mounting cha racteristics should be in cluded if possi ble  in the spe cifications. 

As a consequence of full  power being applie d at e very step of a step per motor d uring its operation,  scheme s have been devel oped to reduce the ave rage appli ed power. A com mon ste pper driver featu re  that accom plishes this is the p ractice of restri cting t he “on-time ” of an ele ctrical step pulse to only a   fraction  of it s theo retical maximum duration. T his practice of o perating  with a redu ced or “partial  pulse  width” m ay redu ce ave rage motor power consump tion and internal motor heating, but it can also affect  motor steppi ng capability. If a pa rtial pul se width i s intended fo r operation, this n eeds to  be indicated early  on. Besid es the po ssibility of operating with a pa rtial pul se width, it is also com mon to have a current limit  applie d. Because the driver de sign can play a cri tical role in  the op eration  of a stepp er moto r, it is  importa nt to have the inten ded drive el ectronics and i ts feature s defined. 

Once the motor spe cificati ons have been de rived f rom system l evel re quire ments, d rive requi rements,  and trade -offs to a rrive at optimum rates and torque s, the issue of perfo rman ce margi n for th e me chanism  must also be addresse d. Perform ance margi n may be addr esse d as ma rgin on torque, voltag e, or losse s.  Perform ance margi n for space me chanisms can b e define d through use of several meth ods th at are   sometim es used tog ether as summa rized in Table 3 . It should b e noted that margin can b e applie d twice   or mo re through the  specification p rocess, sometimes unintention ally, resulting  in ha rdware that is ove rsized. 

Gene rally, th e hardware should  be designed  to me et seve ral tim es the actual perfo rmance requirement  (as dete rmined by the margi n), and then te sted to meet th e actu al req uirement ove r worst-ca se  conditions. If hard ware must be tested at seve ral times the actu al requireme nt, then the motor must be  sized with ad ditional fa ctors that may u nnecessa rily in crease moto r weig ht and v olume. Ta ble 3 assu mes  that margin is only applied one time for purp oses of meeting the specifie d req uirements de rived from the   actual m echanism dema nds. While t he Tabl e 3 method s are  commo nly kno wn, it is hoped th at  clarifi cation a nd po ssibly standa rdization  of their use will re sult. 

Table 3  provides guidelines that the system o r mechanism level designer can  use to  speci fy margin fo r  space stepper moto rs, so that when th e design i s created and analyzed  suffi cient de sign margin will be  present to assure actual torque s and spe eds are met over all the wo rst-case co ndition s at test. It is  importa nt tha t the sp ecifications be clear as to whether th e ma rgin appli es for design analysis purposes  only or if factors shall be a pplied at test as well. Some times it is d esired that th e requi rements for test be   set beyon d the actu al ne eded valu es by some ma rgin facto r. If this co urse i s chosen, the system o r  mech anism l evel engi neer mu st re alize that the st eppe r moto r will be sized acco rdingly with som e  addition al size and weight penalty.  

Torque loa d and inte rnal l oss margin fa ctors can va ry, becau se application s are different. Steppe r moto r  designers also vary in their con servatism. The actua l factors to apply sho uld rest in a mutu al agre ement  betwe en system, mechani sm, an d ste pper m otor design engineers for th eir specific proje ct but ba sed on  successful he ritage ma rgin values. In ge neral, it is  not uncom mon t o see lo ad an d loss torqu es increa sed  by factors of several times to assu re ste ppers will fu nction a s desi red (refere nce Table 3.a). T he sam e is  nearly t rue of taking  volta ge redu ction s, altho ugh these g enerally must b e applied more thoughtfully,  since voltage affects both speed a nd torque (referen ce Table 3.b ) 

Herita ge data (refe rence Table 3. c) may provide a referen ce of acceptabl e ma rgins from te st histo ry but  may not b e as clear regarding the  actual design margi n used if the h ardware i s from a very matu re  program. So,  heritage d ata sho uld be take n as reference if older or intend ed margi ns are unclear, an d  then com bined with on e of the other methods of  margin. It is of great i mporta nce that the margi n  methodol ogy agre es from specifi cation to  analysi s to test. 

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