confirmed the accuracy and reliab ility of automated testing method.   It was imm ediately ob served th at the total a ngular dista nce of the m ove gre atly impacte d the   repeatability-t o-position of  the me chanism. Short moves of less th an 30° (12 steps) showed poor  repeatability with stan dard deviation s above 10 arcm inute s. As the mov e distan ce increased, th e  repeatability was improved. Move di stances of 60° showed standard deviations of less than 60  arcse conds and move dist ances of gre ater than 90 ° were repeatable to less than 20 a rcseconds. It was  also observe d that the re peatabilit y of each of the mech anism’s 144 ta rget d etents was different for a   given del ay setting. As a corollary, t he best repeatability of each target detent occurred at different delay  setting. 

As a re sult, the rep eatabi lity requirem ent on the HCM was refined to refle ct the nominal  operationa l  mode s of the HCM. Th e nominal move d istance an d direction of 12 0° clo ckwise (CW) that will  be use d on  orbit wa s defi ned a s a ba seline movem ent for all rep eatability testing.   

Figure 5 i s a histo gram of the stoppin g position of t he HCM  rotor when  driven 12 0° to  target  position  no. 1 0, 200 times at  a del ay settin g of 80. T he distribution has a sin gle standard  deviation (1 -σ) of 20 arcseco nds where the averag e sto pping  positio n ha s been normalized to zero. This measureme nt  was made early in the  life of the mech anism. Similar  measurement s sho wed th at each of the 144 target po sition s  had uniq ue distrib utions and that  they would ch ange over  time and with  modification s to the delay setting. 

Figure 6  shows the results of an auto mated test seque nce.  Each of the  points recorded on the  plot rep resent the   stand ard deviation of the stoppi ng p osition for e ach of the  144 moto r detents after a 120° rotation.  The a ggrega te delay  for ea ch delay setting h as been superimpo sed on  the plot  and is represented by the gray soli d line. The minim um point  of this agg regate  measure sho ws that a  delay setting  of 80 and   a re peata bility of 13  arcse conds are th e  nominal mech anism  delay an d the  mech anism  repeatability at this  delay.  

The e ncode r system   proved  instrumental i n  developin g a bette r  unde rstanding of the   dynamics of the moto r,  especi ally in its  stoppi ng beh avior and   how the d elay setting controls the fin al positio n. With  a delay setting of zero, th e ine rtia of th e roto r is not  enou gh to carry the m otor over to th e next detent, so it returns t o the d etent before  the ta rget p osition.  With the m aximum del ay setting of 255,  the delay ti me provide s sufficie nt commutation ti me to d rive the   motor throug h the targ et detent and into  the next dete nt. There are unsta ble del ay settings th at can result  in the moto r slidin g forwa rds to the next detent, ba ckwa rds to the previou s detent, or even coming to rest  at the top  of the dete nt potentia l between steps. Th e extrem es of the autom ated run results shown in  Figure 6, wh ere the ag gregate rep eatability is poor e nough n ot to be sho wn on the scale, a re the re sult of 010203040 50 60 70 80 90 100 110 Delay SettingAverage Standard Deviation  of Final Position (arcseconds)

Figure 6.  Re sults of a n Automa ted Ex ternal Enco der Run Used  for Delay   Setting Selection and Repeatab ility Specification Verification 05101520 -30 -20 -10 0 10 20 30 Final Posi tion Rel ative to  Average (arcs econds)Frequency

Figure 5.  Histogr am of HCM Rotor  Stopping Po sitions of 1 20° Rotations 19