Minimizing Torque Disturba nce of Stepper Motors 

Jim S prunck* 

Abstract 

As the resol ution of space-borne instruments improves, the stability of the satellite platform becomes  more critical. While the atti tude co ntrol system is p aramount in providing this stability it doe s have finite   ability to reject external torques and forces im parted to the spacecraft. Thi s has translated to dem andin g  requi rements for low disturban ce m echanisms. Starsys has successfully employed mi crostepping  techni ques in moving m echanisms to re duce di sturban ce levels. Thi s allows smooth  motion to b e  achieved u sing ste pper motors and avoids th e ne ed for hig h cost bru shless D.C. servo systems. Thi s  pape r discu sses the key a spects of sy stem, comp onent and test de sign for mi crosteppi ng sy stems. 

Introduc tion 

Histo rically, many spacecraft me chani sms have been de signed using ste pper moto rs. Ste pper motor s  provide a si mple m eans of controlling both positi on and rate without complex closed loop control .  However, as platform stabi lity has become more crit ical, the t orque impul ses resultin g fro m increm ental  motion of a stepp er moto r can be un accepta ble in some a pplications. Thi s results in a need fo r an  alternate control sche me fo r stepp er mo tors that can li mit torque re action s. 

The solution is to em ploy comm ercial microste pping technology. Mic rostepping techniques have long  been  used in comme rcial appli cation s where low cost motio n control i s required. Mi crostepping  allows  relatively coa rse stepp er motors to be operated  with very fine  step resolution. A secondary benefit in  the  comm ercial system i s the sm oothn ess of o peratio n resulting  from t he reduced step size. It is this  characteristic that has bee n exploited in  spacecraft m echanisms to  redu ce torqu e disturban ce. 

When designing a s ystem us ing micr ostepping it is important to note  the differe nces betwe en thi s  approa ch an d standard steppe r moto r system de sign. This techn ology ha s unique characteristics and  inherent limit ations that m ust be considered in the design p rocess. T he motivation for thi s paper is to  summ arize these characteristics and limi tations a s a referen ce for microste pping system d esign. 

Theor y of Operatio n 

The ba sis for microstep ping is ba sed in the fundam ental theory of  steppe r mot or operatio n. A modern  stepp er moto r em ploys both rotating  and statio nary magneti c fields.1 The rotating field i s generate d by  perm anent magnet s installed on a rotor assem bly. The statio nary field is generated by a wound stato r  assembly. Th e stato r is wound i n multi ple ph ases with each winding ph ase corresponding to a specific  electrome chanical phase angle. By sel ectively exciti ng one or m ore stator ph ases, an ele ctromagnetic  field is generated at a spe cific angle. Unre strained, t he ro tor rotat es to align  its field with  that of the stator.  Furthe r rotati on is pro duced by sequ entially exciting the individu al motor pha ses. In each case the roto r  rotates to maintain align ment with the ge nerate d field so lo ng as applied torque s and comma nded spee ds  are withi n the capability of the motor. 

The typical e xcitation scheme u sed with a stepp er motor advan ces the moto r in increm ental step s from  one detent p osition  to the  next. Using  microste pping it is  possible to p osition  the rotor to some  finite                                                     * Starsys Research Corporation, Boulde r, CO  1 The term m odern is u sed here to differentiate bot h perm anent magnet and hy brid moto rs from variabl e  reluctance m otors that g enerate  no i nternal  field. A s variable reluctance moto rs are extrem ely size a nd  weight ineffici ent, they are rarely and pe rhaps neve r used in space appli cation s.  Procee dings of the 37th Aeros pace Mec hanisms Sy mpos ium, Joh nson S pace Ce nter, May 19-2 1, 200 4  147