signifi cant vib ration. Me chanical noi se in these systems i s minim ized by controlling be aring cleanli ness  and p reload. Close attention is al so paid to h armonic drive inte rface s. Prope r run-i n and cleaning are  critical for sm ooth tran smission o peratio n. 

System Ope ration 

Operation of the micro stepping system  is different fro m operation of a st andard stepp er motor. Parti cular  attention mu st be p aid to construction of the out put motion profile. The p rofile should b e smooth and  avoid un necessary a ccelerations. T his is primarily de pend ent on how speed change i s commande d. As  step frequ ency is typically controlled  by discrete  values, ea ch speed chan ge results in a n instantan eous  change in step frequ ency. The magnit ude of this freque ncy cha nge dete rmines the mag nitude of the  resultant di sturbance to rque. Typi cally, the a ccepta ble ma gnitud e for any discrete  frequency chan ge is  determi ned th rough syste m simulation s and later verifi ed by test. 

Additionally, the system m ust be operated consistent with the  inherent limitatio ns of the mi croste pping  system. T he limitations that must b e considered when o perating th e microste pping syste m are  summ arized belo w.  • During operation the m otor m ust be contin uously energized to  maintain  the sub-i ncrem ental   positio n.  If p ower is remo ved, the motor will settle to the nearest st able dete nt position  • As the unpo wered hol ding torque of the system is redu ced to minimize dist urbance torq ues,  precautio ns must be take n whe n operating in a 1- g environm ent. Typically this requi res power to  be maintain ed so lon g as the mechani sm is maintain ed outsi de of a gravity neutral po sition.  • As the m otor will settle to an inherent detent pos ition when power i s remo ved, the motor must be  comm anded in increm ents of the i nherent step a ngle of the motor. Thi s allows po wer to be  remove d with out disturbing  motor po sition.  • As the abov e con straints requi re the motor to be energized for longe r periods relative to a   stand ard ste pper sy stem , powe r consum ption and he at dissipatio n are  elevated i n a  microste pping system. Thi s can limit t he achieva ble operatio nal dut y cycle. 

Test Design  

The di sturb ance level s achievable with a micro stepping sy stem are extre mely low, on the orde r of 10 -  100 mi cro-g’s. Mea suring torques and forces at the se level s is a challenge.  The key re quirements for th e  test fixture desig n are se nsor sensitivi ty and stiffne ss. Sen sor sensitivity must be sufficien t to measure   torque s in th e 0.1 i n-oz range  across a typical  frequency range  of 0.1 Hz to 500 Hz. Also  the fixture   stiffness mu st be sufficie nt to achieve a natural  frequ ency out side the freque ncy band s of interest. 

To sati sfy these de sign require ment s an instrum ented hexap od is empl oyed. Each strut of  the hexapo d  contai ns a quartz fo rce sen sor.  The dynamic range of  the force sensors is 1 to 300 Hz. While the sen sor  range does not meet th e overall m easurement goals, it has be en previously demonstrated  that the data i s  sufficie nt to chara cterize system perfo rmance. The data colle cted from the hexapod struts does requi re  post processing to t ranslate the measured strut fo rces back to  torque s and fo rces along six degrees of  freedo m in a n ortho gonal coordinate sy stem. A Matl ab sc ript is u sed to perfo rm this op eration a s well as  the requi red FFT to gene rate the frequ ency dom ain data req uired for compli ance verification . 

During testin g, the system  is operate d on a vibration isolatio n table  in an acou stically quiet ro om. At these   measurement  levels both  seismic a nd acoustic noise are significant. The  measured noise thre shold for  this system was 10 micro-g’ s. Given these con straints, it is not possibl e to verify disturb ance levels i n  either th ermal or va cuum conditions. The data  colle cted unde r ambient con ditions is used to generally  confirm overa ll system ope ration. 

Test Resul ts 

The hig hest fidelity testing  perfo rmed  to date was on the Deep I mpact anten na poi nting g imbal. In thi s  test, the disturbance to rques from  the mechanism were m easured while  the gimbal  was p erformi ng the  151