Prototy pe Mechanism 

The PAOS instru ment wil l incorporate new technol ogy in several  different are as. It will use new la ser  diode s for the refere nce lase r and ne w elect ronics fo r on-bo ard data pro cessing an d compre ssion. T he  subsystem with the lowest  technolo gy readin ess level at the time  of the PIDD P propo sal, h oweve r, wa s  the transl ation mech anism and thus it has be en th e focus of th e PIDDP effort up to the time of this  publi cation. T he me chanism is required to move a  125-gram cube corn er retro-reflecto r through a 100  mm stro ke at a con stant v elocity of 30 mm/s with le ss than 1% e rror in velo city (± 0.15 mm/s rms) over a  life of one million stro kes. The goal of  the develo pment wa s to make a m echanism that cou ld accompli sh  all the se requirements wh ile pa ckag ed as small as possibl e, con sumin g less than 5 watts of po wer on  averag e and having a ma ss less tha n 4 kg. Becau se this is an interferomete r, it is se nsitive to motions as  small a s a few nan omete rs and can sense disturban ces into the megah ertz range. 

In modern F TSs, the effects of velo city erro rs are remo ved, to first o rder, by mean s of a referen ce laser  that traverse s the same  optical p ath as the infrared radiation of inte rest. T he laser is used to trigge r the  sampli ng of the infrared in terfero grams, ensuring th at they are  at equal  increm ents of opti cal delay. But   due to th e fact that the la ser beam hit s different pa rts of the opti cs from th e IR beam, and  the f act that the   laser is at a different freq uency, there still remain  significa nt second-order  effects that arise from  velo city  non-unifo rmities. Sin ce the se second and third order disturban ces are ve ry ha rd to model analytically, ou r  developm ent approa ch wa s to build sev eral prototyp es an d test them as e arly as po ssible. Once tested,  the perfo rmance of the me chanism could be characterized and the causes of vel ocity jitter co uld be b etter  unde rstood. Several diffe rent con cepts includin g voice coils, l ead screws, linkag es, and belt d rive  systems we re tested. Off the shelf  mech anisms a s well a s quick, custom p rototypes were built an d  evaluated. T his helped t o reduce t he time involved  in the development proc ess while still providing  valuable data  about the ro ot cau ses of velocity error.  The lesson s learn ed du ring each of these test s were  then folded back into the next iter ation of the desig n cycle t o continu ally improve the  mecha nism  perfo rman ce and ro bustness.   

The final p rototype mecha nism, sho wn in Figure 3, u ses an ult rasonic pi ezo m otor to move the sled th at  carrie s the moving h ollow cube-corner retro-refle ctor (HCCRR). The sled is guided by a kinematic rail  system that u ses three sph erical Ve spel rollers along with the p reload of the pi ezo motor to full y con strain  the cube  corner sl ed’s motion. Th e sled is moved thro ugh the requi red 100 mm stroke and foll ows a  trape zoidal motion p rofile spending 85% of its duty cy cle at th e targ et spee d of 30  mm/ s. The  second   cube corner, requi red for t he interfe rometer, is h eld stationary an d is attached to  a comm on base plate. A  top view of the mechani sm is sho wn in Figures 4 a nd 5 with the moving sle d at its left and right end s of  travel positio ns re spectivel y.   

  Cube Corner Sled  21cm Moving Cu be  Corner  Kinematic Ra il  Rollers Base Plate Stationary  Cube Corner

Figure 3.  PAOS Prototy pe Mecha nism  253