Discus sion 

The HCM qu alification te sting verified that the me chanism i s robust in desi gn and con struction and ha s  sufficie nt torque m argin, on-orbit adjustability, and  lifetime ma rgin to complete its t wo-year nomi nal  missi on. However, the d egradatio n of th e repeatabilit y performan ce over th e mechanism’s lif etime was  shown to be a limitation of our de sign. A full expl anation of th e HCM p erformance and limitations to   perfo rman ce would requi re analy sis an d testing  that are beyo nd the scope  of this develo pment p rogram.  However, sev eral gen eral observed  phenomen a may explain th e perform ance of the HCM th rough out its  developm ent and testin g. 

The de crease in the repe atability of th e HCM ove r its life can be  traced to sel ection of Bra ycote 600 EF   grease as a lubricant. While this de cision was conservative, base d its herita ge and compatib ility with the   designs of th e COR1 an d COR2 telescop es, typi cal small  chan ges in its p roperties over life greatly  affected th e perfo rman ce of the HCM design. It wa s expected that incre ases or d ecreases in bearing  friction could be counte racted by in creasing or decreasing th e HCM  delay setting . However, it wa s not the   total cha nge i n bearing friction, but vari ation of be aring friction at low spee ds and sm all an gular distances  that decrea sed the me chanism repeatability over the course of its life.  It has be en de monstrate d that   beari ng to rque noise in creases as the grease lubricant begi ns to  deterio rate. This effe ct is esp ecially  stron g at low speeds where slight torq ue variation s affect the rotation sp eed rela tively more [3].  

The effect of  increa sing t orque noise is strongly linked to the repeatability of the mech anism. After  cessation of comm utation,  the rotor coasts, with m oderate b raking, to a stop in a detent. Ch anges in the  friction to rque over th e 2° braking phase prior to stop t hat cau se 1%  change  in averag e rotor speed  could  affect the fin al stop ping position of the rotor by as mu ch as 70 arcseco nds. Since it app ears that the noise  is ran dom, a nd based in certain lo catio ns in the bea ring, the torq ue noise is di fferent for ea ch movem ent  to a particul ar location a nd causes the ro tor to stop in an ever-wi dening ra nge of positio ns.  

Slight wea r in greases do not typically affect the average runnin g torqu e of the be aring at high spee ds.  However, variation in the averag e stea dy-state spin speed of the HCM, typicall y about 5 rad /s (280 °/ s),  showed som e effects of torque n oise. Sampling th e revolution time of the mech anism over seve ral  thousand revolution s at the me chanism end -of-life  showed that the spe ed varied a s much as 10%  betwe en consecutive rota tions and typically vari ed about  5%. T his compares to be ginnin g-of-life spin  speeds  that were typic ally 1% diffe rent from rotation to rotation.  

The total speed de pendent bearin g friction in creased a total of 71 % durin g the approximatel y 3.4 million   testing an d lifetime simula tion ope rations, manife sting itself in a n app roximat ely 20% decrease in spi n  speed. Be cause this data  was derived  from measures of th e spin speed over a revolution, th e increase is  likely due to i ncreasing to rque noi se. 

It is un known what effect t emperature will h ave on repeata bility a t the en d of t he HCM  life. A de crease in  repeatability wa s seen with increa sing tempe rature d uring th e the rmal functio nal test pe rform ed ea rly in   the life of the mecha nism. It is likely th at this trend will be exa cerbated with mech anism li fe due to the  greate r stress to the grea se lubricant. 

The d rastic decline seen over the la st 600,000 operations of the HCM  test sequen ce (shown in Figure  15a) i s not easily expl ained. Cata stroph ic degradatio n of perfo rmance of m echanism s with  Teflon to roid  separated bearing s lubricated with Bray cote grea ses was seen pre viously in sev eral LMSAL mech anisms  constructed for NASA’s Solar B F ocal Plane Pack age. T hese mechanisms, however, operated at  constant velo city in a single dire ction, instead of intermittently as in the SECCHI HCM. The ir failure wa s  characterized by a sha rp increa se in bearin g friction at app roximately 20 mi llion revol utions and was  caused by si gnifica nt wea r to the Teflo n separators. While thi s failure mo de is quite differe nt than se en in  the HCM, the re is evide nce that torque noise also in creased in Sol ar B motors p rior to their fai lure. 

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