HCM F unctional Testin g 

Functio nal testing of the HCM em phasized the ch aracteristics and perfo rmance of the mecha nism a s a  whole, rathe r than the p ropertie s of in dividual co mpone nts such as the bearing or encoder ele ctronic  parts.  All te sting was completed with the  mechanism fully asse mble d and yield ed bulk measurements of   the mech anism prop erties and direct inf ormatio n abo ut how the m echanism wo uld ope rate o n-orbit. 

Specific test s were u sed to verify th e HCM’ s two key re quirements of 1 00% torque margi n and  repeatability-t o-position of  less th an 30 arcse conds standard deviation. The total torque m argin  requi rement specified th at the moto r be capable of operating normally give n a 10 0% in crease in total   friction. Th e repeata bility requireme nt specifie d that t he distri butio n of the stop ping po sition of the roto r  must have a stand ard d eviation of less than 30 a rcseconds. 

A spe cial met ric was devised to test the  total HCM to rque m argin. The moto r drive voltage was in creased  increme ntally from a sm all value – typically 4V – as  the motor was comm ande d to move fro m each of its   144 po sition s. The minimu m voltage, and thus th e mini mum current for the stalled motor, at which th e  motor was able commutate  normally fro m all p ositions was record ed as the me chanism mini mum start-up   voltage. Minimum sta rt-up voltages le ss than 7.5V indicate d a torq ue margin of greate r than 100%.   

This te st refle cted the typical ope rational  mode of the mech anism a nd provided an aggregat e measure of   beari ng perfo rman ce. It was extremely  effective in mappi ng the effect of b earing life o n torque margin  durin g both the functio nal testing an d the vacuum o peration p ortion of the qualification series. Chan ges in  the mea surement over th e lifetime of the me chanism could then be attribute d to cha nging bearin g brea kaway torque and lo w-sp eed friction. T esting the moto r with d ecreased drive volt ages not o nly verified that   the mech anism could o perate with de creased spa cecraft bus voltage, but wa s also u sed as an anal og for  increa sed b earing fri ction. 

Early in the functional testing of the HCM,  it wa s obvious that the repeat ability requirement of 30  arcse conds standard devi ation (al so denoted a s 1-σ) could not be efficiently  verified with  our current  measurement  method s. In the case of MDI’s Mi chelson Tuni ng Mot ors, a theo dolite wa s em ployed to sp ot  check the motor’s stopping position fo r a few m otor detents. This method wa s extremely time consumin g  and was not easily a dapte d to statistica l analysi s of the repe atabili ty of all the HCM  moto r positio ns. In   addition, it was also ne cessary to te st the me chanism at a l arge numbe r of potential d elay setting s to  select the on e that woul d result in  its m ost robust an d rep eatabl e operat ion. A total of more t han 5 0,000  measurement s were requi red to ve rify the mech anism repeata bility and effectivel y tune its perf ormance. 

The solutio n was to atta ch a small b ut highly a ccurate  external e ncoder (Figure 4) to the HCM and p erform  an autom ated test se quen ce that  rotated th e  mech anism to all of the  motor targets many times a t a  range of d elay setting s. An optical increme ntal  encode r m anufa ctured by Gurley Preci sion  Instrum ents was chosen because its ine rtia and friction  were small and it was a ccurate enough to obtain  data  with a rcsecond resolution . The extern al en coder was  attache d to a shaft mou nted in an aluminu m puck  designed to simulate th e mass an d inertia of the   polari zing opt ic. The motor was comma nded to ma ke  moves to e ach positio n a statistically re levant numb er  of times (typi cally 40 ). This wa s done f or 8 to 10 d elay  settings.  

Figure 4.  External Enco der Con figura tion  Manual  spot  checks of the accuracy of the extern al  encode r system with a  theodolite  sho wed good  agreement betwe en the  two measurem ents a nd  18