Figure 9.  PAOS Mechani sm Rear Vie w Ceramic Driv e  Strip  Linea r Piezo  Motor  Linea r Optica l  Encoder  The pi ezo ele ments can be stacked side by side  and operate d in parallel to  give a propo rtional  increa se in  driving fo rce. This al so introdu ces redunda ncy such that if on e eleme nt fail s, it sim ply result s in a  decrea se in available fo rce rath er th an cata strophic failu re (Unif ormity of vel ocity in thi s configuration i s  yet to be quantified). The curre nt protot ype use s a va cuum-rated,  four-fing ered piezo moto r with a mass  of 73 g rams. It is able to provide approximately 4 poun ds of force in th e driving directio n. This allow s  ample m argin for d riving the me chanism in any orientation (in cluding vertical ly again st gravity), which  allows for versatility in earth testing.          A factor i n the decision of wheth er or not to use pi ezo moto rs for this develop ment was th eir lack of flight  heritag e. It was de cided that although these moto rs have never flown on a spac e flight missi on, they  posse ssed all the ne cessary qualities for a flight me chanism and offered  such d ramatic benefits that they   could not be overloo ked. Given the  research and development nature of th e PIDDP progra m, the pie zo  motors were i ncorporated into the d esign. Piezo electric mate rials have bee n flown in the p ast on proje cts  such as the Large Angl e and Sp ectrometric Coro nagraph (LAS CO) tele scop e ab oard the  Solar and   Heliospheri c Observatory (SOHO) spacecraft, but have prim arily been used  as a stack of piezo wafers  with an  applied DC volta ge to give  highly accu rate, but extrem ely small  deflection s, in t he µm range  (Burg er, 1995). The ultra sonic pie zo mot or uses the same materi als, but employ s an AC volta ge to excite   the natural freque ncy of the piezoele ctric elem ent to cre ate motion. These motors h ave been used for  several years in vacuum e nvironm ents f or appli cations such as scannin g elect ron micro scopes and are   available  in vacuum -rated  version s rated d own to  10-10 torr. Na nomotion  Ltd. has conducted the rmal  testing, provi ng a su rvivability temperature a s low a s –120 deg C and an op erational ran ge of –55 deg C  to +85 deg C. A 4.6 million-cy cle vacuum life test has also been run showing no degradation in mot or  perfo rman ce over the life of the eighty-d ay test. Ou tgassing te sts perform ed per ASTM E 595 -93 sta ndard  have sho wn a total ma ss loss (TML ) of 0.60% an d less th an 0.018% collected volatile  condensable  material s (CVCM) un der vacuum conditions, whi ch exceeds the re commen ded T ML < 1.0% and CVCM <  0.1%. ASI is curre ntly wo rking to  fully q ualify these  motors for space flig ht app lication s. The develop ment  of this motor as an alte rnative to traditional motors lo oks very pro mising, a s the most sub stantial issue s  have alre ady been a ddressed in the co nstruction of th ese moto rs for terrestri al vacuum a pplications.   

Rail Desi gn  Several rail system s were tested in t he ea rly stag es of the p rogram in cluding recirculatin g linea r ball   beari ngs, cro ssed roller bearin gs, as well a s flexu re and linkage systems. With all systems th at used  recirculatin g balls, th e imp act of th e ball  bearings striking the g uiding rod was evident in the  velocity erro r  (whose m easurem ent is describ ed later). The  prototy pes that u sed crosse d roller bearings all  perfo rmed  well, but the crossed rollers have i ssues that ma ke them le ss de sirable fo r space mechani sms. Line ar  crossed roller bea rings require a caging system  to separate  and equally spa ce the cylind rical rolle rs. Over  time the cag e can mig rate to on e en d of travel. Th is will l ead to the cage striking one end of the rail  system an d gradually redu cing the r ange of travel, which will even tually lead to a mech anism failure. This  is not gen erally a concern i n terre strial a pplication s because the cage ca n easily be reset by hand, but for  255