A Low  Mass Translation Mech anism for Planetar y FTIR Spectrometry  using an  Ultrasonic Piezo Linear Motor 

Matthew Heverly*, Sean Dough erty*, Geoffrey T oon**, Alejandro S oto**, Jea n-Francois Blavier** 

Abstract 

One of the key comp onen ts of a Fouri er Tra nsform Infr ared Spe ctromete r (FTI R) is the line ar tran slatio n  stage used t o vary the o ptical path l ength between the two a rms of the interf erom eter. Thi s translatio n  mech anism must p roduce extremely constant velo city motion a cross its entire range of travel  to allow the  instru ment to  attain high signal-to -noise ratio and spectral resolvin g power. A new spectrometer i s being  developed at the Jet Propul sion Laboratory under NASA’s Planetar y Instrum ent Definition and  Develo pment  Progra m (PIDDP). Th e goal of this project is to build upo n existing sp aceborne FTIR  spectrometer techn ology t o produ ce a new i nstrume nt prototype that has drastically superior spe ctral  resolution an d sub stantiall y lower ma ss, making it feasib le for pla netary exploration. In orde r to achieve   these goals, Alliance S pacesystem s, Inc. (ASI) has develop ed a  linear tran slation me chanism using a  novel ultraso nic piezo line ar moto r in conjun ction with a fully kin ematic,  fault tolerant li near rail system.   The piezo motor provid es extremely  smooth m otion, is inherently  redundant, a nd is capable of producing  unlimited trav el. The kin ematic rail u ses sphe rical Vespel rollers and b ushings, which e liminates the  need  for wet l ubrication,  while p rovidin g a fault tolera nt platform  for smooth  linear motion th at will n ot bind  unde r misalignment o r structural  defo rmation. Thi s system can produce velo cities from 1 0 – 100 m m/s  with less tha n 1% velocity erro r over th e entire 100-mm length of  travel for a total mech anism ma ss of  less than  850 grams. T his system h as performed over h alf a million strokes un der vacuum without   exce ssive wear or degrada tion in pe rformance. Thi s pape r covers the design, d evelopme nt, and te sting of   this lin ear tra nslatio n me chani sm a s part of the  Planetary Atmo sphere Occultat ion Spe ctrometer (PAOS)  instru ment prototype devel opment p rogram.  Introduc tion and Backgr ound 

The Plan etary Atmosphe re Occultation  Spect romete r uses the solar occultatio n techniqu e to search f or  trace gases in a planeta ry atmosp here. The inst rument, a Fouri er Tran sform S pectromete r (FTS), views  the sun th rough a plan et’s atmosp here at sunrise a nd sunset. As th e sunlight passes thro ugh the pla net’s  limb, the ga sses in th e atmosp here selectively abs orb certai n wavelen gths of light. The instrument   measures th e flux of the  incident light at different  wavelength s and is able to quantify the atmosp heric  comp ositio n as a fun ction of altitude f rom the depth s of the vari ous absorption  lines. Since the sun i s a  very bright ra diation so urce, these occu ltation spe ctra can  be of very high spectral resolution  and signal - to-noi se ratio, providing ult ra-hig h sensitivity to trace g ases in the pl anetary atmo sphere under study. 

At the heart of any FTS is a Mich elson interferome ter, which spl its the incomi ng radi ation and send s it  down two a rms with different path l engths. When t he two beams recombin e interfe rence occu rs. By   varying the o ptical p ath length differe nce between th e two arms of the interfe rometer, the in coming lig ht is  interferometri cally mo dulated. The different wavel engths presen t in the radiation a re modulate d at  different freq uencies. So applying a F ourie r transform to the inte rferogram produ ces a spe ctrum of lig ht  absorptio n levels vs. wave length and reveals th e compositio n of t he observed  atmosp here. The spectral  resolution  of an FTS i s a functio n of th e maximum  optical path differen ce (OPD), a nd the  quality of th e  spectra is cri tically de pendant u pon the unifo rmity of the movin g cube  corn er’s motio n. An exampl e  spectrum for the atmosphe re of Saturn’s moon Titan i s shown in Fig ure 1.                                                   

* Alliance Spa cesystem s, Inc., Pasad ena, CA  ** Jet Propul sion Lab orato ry, California In stitute of Te chnology, Pasad ena, CA 

Procee dings of the 37th Aeros pace Mec hanisms Sy mpos ium, Joh nson S pace Ce nter, May 19-2 1, 200 4  251