a flight m echanism,  additional design element s must be introdu ced to mitig ate this proble m (Rob erts,  1994 ). A cro ssed roll er bearin g is al so inherently  over-co nstrained an d dep ends upo n tight machi ning  toleran ces to prevent the system from bindin g. A sli ght misalig nment or sm all chang e in preloa d (du e to  thermal expa nsion for exa mple) can p roduce a drastic increa se in the force required to move the sled,   which could cause bindin g.   

After investigating the p ros a nd co ns of variou s  designs, a rail system  was develop ed that uses the   prelo ad of the piezo moto r in conju nction with three   spherical roll ers to creat e a fully ki nematic rail  system  that guide s the moving co rner cu be sled .   Figure 10 sh ows a two -dimensi onal schematic of th e  rail configu ration. A sphe rical roller (furthest ri ght in  the figure ) is attached to  the base structu re. Th e  sled has an integral V -groove t hat ri des on  the  spherical roller, re sultin g in two conta ct point s. A  separate spherical roller (furthest left in the figure) is  also atta ched to the ba se structure  and cre ates  anothe r point conta ct on a flat surfa ce on the moving   sled. Th e piezo m otor is preloa ded again st the   moving sled pro ducing a force th at re sults in a  moment a round the V-gro ove rolle r that is re acted  by the singl e roller, resulting in a fully kinemati c  design. Piezo  Motor Single  Roller  V Groove Roller  Figure 10.  2-D Kin ematic  Rail Schematic 

The rail d esign can  be expanded to three  dimen sions by adding a second V -groove roller  (shown in Fi gure 1 1). If the sin gle roll er and pi ezo  motor preloa d force a ct betwee n the two v-g roove   rollers, the rail system becomes kinematic in three  dimen sions, leaving the t ranslatio n of th e sle d as the  only re maini ng d egree of freed om. S ince the rail  system i s kinematic, the force requi red to move th e  sled should not ch ange if  small mi salignment s are  introdu ced due to thermal distortio n of the structu re.  The de sign also hel ps to redu ce the ne ed for tight   machining tolerances in order to align th e system  properly. Figure 11.  3-D Kin ematic  Rail Schematic 

All three of the sphe rical rollers are m ade from Ve spel SP3. Ini tially, a pair of prel oaded radi al bal l  beari ngs was used in side each of the  spherical rollers, yiel ding good vel ocity uniformity p erform ance.  Tests we re then ru n with out the ball b earin gs using only the Vespel rolle rs as bushi ngs, sliding on steel  pins coated with Dicro nite dry film lubrica nt as sh own in Figu re 12. These tests yielded sim ilar re sults to  the test s using bea rings, but without th e requireme nt for wet lub rication. T his is advantage ous becau se  the lub ricatio n un der va cuum conditions could outgas and contaminate t he optical  surfa ces. With the  rollers acting as bu shings the force necessary to mo ve the mech anism i s proportional to the prelo ad of  the pie zo motor and the coefficient of fri ction b etween the sli ding Vespel/ Dicronite interfa ce. Und er norma l  ambient con ditions the mechanism req uires app roxim ately 4 N of force to move  the sled. Un der va cuum  conditions, the coeffici ent of friction for Vespel SP3  redu ces by a factor of 10 due to the  removal of   moistu re on  the co ntacti ng su rfaces.  This further reduces the power required by the s ystem an d  increases the motor’s force margin ov er stall co nditions. 

The V-gro ove and  flat tra ck of the sled are ha rd anodized alum inum with a surface fini sh of 8, giving a   hard, smooth  surfa ce for t he rolle rs to contact. And e ven if the rollers were to bi nd on the ste el pins, they  are still abl e to slide o n the anodi zed alu minum tra cks. This makes the system single fault tole rant. Test s  256