Like all ste pper motors used in spa cecraft applicatio ns, the sele ction betwe en curre nt mode  and voltage  mode control is criti cal. Voltage mode co ntrol is the si mplest an d require s the lea st electroni cs. Howeve r,  when operating in volta ge mode th e motor p erformance is subject to signifi cant variation resulting f rom  bus voltag e fluctuatio n and  temperatu re sensitivity of the win ding re sistance. As motor torque is ultimately  controlled by curre nt, cha nges in volta ge an d resistance will di rectly affect motor curre nt and outp ut  torque. 

Curre nt mod e control  is typically p referred in preci sion systems as the effect of external  variable s is  reduced. Thi s does come at the exp ense of mo re co mplex e lectro nics. In  stand ard st eppe r moto r  appli cation s, simple  curre nt mode  con trol can be implemented using pe ak current control . Using thi s  scheme the motor current  is allowe d to ramp up to a pre set thre shold and th en modul ated3 to maintain  the pea k current level. 

Unfortu nately , peak cu rrent control do es not lend it self well to high perfo rman ce microste pping system s.  This i s an artifact of the relatively poor dynamic range offered by peak cu rrent c ontrol. Dyna mic rang e is a  measure the ability to maintain c ontrol accuracy over a broad range. Contro lling peak current is  compli cated by the switch ing n oise a ssociate d with t he current feedba ck signal. At small er signal level s  the noi se becomes mo re significa nt than  the un derlying feed back signal a nd it i s impossibl e to extract th e  actual p eak current level. This limits th e achi evabl e dynamic range. This is n ot desired in m icrostepping  appli cation s as the entire p rofile of the sinusoidal excitation sig nals sho uld be a ccurately gene rated. 

As su ch, av erage cu rrent control i s prefe rred for micro stepping appli cations. Using av erage cu rrent  control, the t ransients in t he current fe edba ck signa l are heavily attenuated  with a multi-pol e low pa ss  filter. While t he switchi ng noise still corrupts the fee dback signal, the effect is minimized as the desi red  signal level is increa sed. This i s due t o the fact  that the entire  motor cu rrent is being measured a s  oppo sed to just the pea k. 

Mecha nism Design 

The me chanism de sign for the low to rque distu rbance appli cation s is al so critical. As the m icrostepping  system is in herently ope n loop, the me chanism m ust operate qui etly and smo othly. The micro stepping  control sche me doe s not comp ensate for output mot ion error an d cannot atten uate external  source s of  mech anical noise. The  key aspe cts t o mechanism de sign for these a pplication s are stiffness, zero   backla sh, an d smooth ness. 

Determinin g the app ropriate mechani sm stiffness is critical for proper system o peratio n. The  mech anism  stiffness m ust be  mat ched to  the system in ertia to control resonate  frequen cies. The resultant   amplificatio n from re sonant modes can significantl y increa se resulta nt torq ue distu rbances. In most  system s de signed to date , harmoni c drive gea ring has been u sed to achi eve the desi red stiffness.   Attention also has to b e paid to the structu ral elements within  the mech anism to insure  the desi red  system stiffness i s not degraded by any comp onent in the stiffness path. 

The ha rmonic drive al so provide s zero ba cklash, anothe r key mecha nism desi gn re quire ment fo r  microste pping application s. Backl ash effects sy stem performan ce by creatin g a region of  relatively low  stiffness and damping. Any  perturbation s imparted to t he system  can result  in oscillatory  motion withi n the  backla sh region. Furthe rmore, this m otion ca n produ ce torque im pulses e ach time the ba cklash regio n is  traversed. 

Smoothne ss, often asso ciated with servo m echanism design, is also importa nt in microste pping  appli cation s. Smooth mecha nism o peration is characterized by  low friction  and relativ ely con stant  resisting to rques. Me chanism sm oothn ess can b e the limiting variable when d esigning a mi crostepping  system.  Me chanical noi se ca n exci te stru ctural m odes that ca n allo w mino r noi se level s to produ ce                                                    3 In motor dri vers, mo dulation refe rs to  the pulse  width modul ation or th e proce ss of swi tching th e  excitation on and off at a high frequ ency to cont rol the effective voltage appli ed to the motor.  150