mech anism delay setting  for be st op eration given  the actual on-orbit op erating tempe rature of the   mech anism. This te sting  was limite d to temperatures  above 0°C because of limitations on the operation of  the external e ncoder. 

As assum ed in the desi gn of the mecha nism, ch anges in bea ring f riction – a nd the co rrespon ding lo ss of  repeatability – could be mitigated by modifying the mechanism delay setting. At higher temperatures  where total f riction i s less, the del ay setting can be decreased to maintai n acceptable repeatability.   Incre asing the del ay setting comp ensates for higher than n ominal  friction. Fi gures 11a and 11b illu strates  the corre sponden ce bet ween the dela y setting at  best re peata bility and temperature fo r the HCM .  Modifying the  delay setting  –1.75  units (112 µ sec) per degree Celsius fo r tem peratures chan ges from  the  nominal m echani sm test  temperatu re of 20°C provided ro ughly the best  mecha nism repeata bility  perfo rman ce. The minimu m and maximum acce ptable delay  settings b racket the ran ge of delay setting s  where the aggregate re peatability of th e mechani sm meet s the 30 arcsec ond requirement. 

HCM Li fetim e Vacuum O peratio n 

The key compone nt of the  HCM q ualification effort was a four-times life ope ration sequ ence while und er  vacuum and at elevated  temperatu re. As in the thermal fun ctional te st portion of the effort, the   mech anism p erform ed a serie s of ope rations simila r to those exp ected o n-orbit with perio dic autom ated  function al tests to asse s the health of the mech anism and trend ch ange s in mechani sm perfo rmance. The   mech anism li fetime simula tion wa s performe d at the uppe r limit of the mech anism op eratin g temperatu re  of 40° C in order to  provide the greatest stress to  the b earing grease lubricant. The exte rnal e ncode r  system was not empl oyed  during thi s section of the qualification effort  due to  the compl exity of adaptin g  the system fo r use in va cuum. 

In the first life-test effort , the proto-qualificat ion HCM was placed in the  vacuum chambe r after  approximatel y 850,000 break-in an d other testing operation s had been completed. The  mecha nism  perfo rmed  as expe cted while u nder vacuum a nd the fr iction-with-time profile  was as expected for a  Braycote 600 lubri cated bearing wi th Tef lon toroids. However, after 1.8 million operatio ns under vacuum  (or a bout 2.6  million total operation s), the mechani sm stalled du ring a stand ard move seq uence. To that  point, the mecha nism h ad shown no adverse signs of wear o r incipient failure.  

An anomaly investigatio n wa s initiated to isolate the cause of the stall prio r to brea king va cuum. The lifetest sup port e quipme nt was tested an d che cked. An attempt was made to com mand the mot or to move in  the sam e direction the mot or had bee n moving du ring the test. Th e motor di d not move and drew full stall  curre nt. Next, the motor was comma nded to run i n the opp osite di rection. Th e motor stutte red at first, and   then b egan to operate  normally. Figure 12 is the mechanism d rive curre nt trace durin g the  stall recove ry.  Evidence such as this in dicated po ssible particle contamination. 

Figure 12.  Current and Encoder Tr ace of Failure Recov ery Encoder Signal C Encoder Signal B Stalled HCM Initial Star t-up Transient (Move Direction Opposite to the Move Direction at the Time of Stall )HCM Curre nt Dra w (Nomin al Ste adyState Current i s   108 mA) Encoder Signal A 

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