Douglas B. Leviton is an optical physicist at NASA's Goddard Space Flight Center (GSFC) in Greenbelt, MD. He has worked on a variety of optical technologies for ultraviolet through infrared wavelengths for NASA science missions including the Hubble Space Telescope (HST), COBE, SOHO, POLAR, and EOS. His absolute optical encoder technology was honored as NASA's Government Invention of the Year for 1999.

Leviton graduated from Emory University in 1981 with a B.S. in physics; he also holds an M.S. in applied physics from Georgia Institute of Technology. His research areas have included ultra-high sensitivity optical encoders, interferometry, radiometric instrument calibration, automation and motion control, diffraction grating, optical filter, and optical component evaluation, scatter measurements, optical metrology, and development of various far ultraviolet measurement techniques including high-resolution image evaluation, flat fielding, and efficiency measurement in nitrogen purge environments.

NASA Tech Briefs: What is an absolute optical encoder?

Douglas B. Leviton: Optical encoders measure mechanical position and report it to a computer as a number. For example, a rotary encoder mounted on a large telescope's support bearings tells you the telescope's pointing angle. In a milling machine or a lathe, linear encoders mounted on the machine's slides tell you the position of the cutting tool with respect to the part being machined. Some encoders are incremental, which means that if you turn the power off and turn it back on, the encoder gets lost because its reference is lost. With an absolute encoder, you can turn power off and back on, and when the encoder wakes up, it still knows exactly where it is. This is especially advantageous in space mechanisms where conserving power is important.

NTB: What advantages does your invention provide?

Leviton: Conventional absolute encoders sense a bit pattern on glass that is unique at each position. The limitation on resolution is, "How small can you make the finest little bit?" Instead of reading out individual bits, my absolute encoder takes digital pictures of a different kind of pattern and interprets them through software to get position with much higher resolution. The pattern resembles coarsely spaced pickets in a fence. Each picket has a little bar code that uniquely identifies it. The camera sees two or three pickets at a time. Since only one position is being measured, each one of those pickets gives the same answer. This means that a pretty vast area on the scale can be damaged and the encoder still works fine.

NTB: What was NASA's first application for the new encoder?

Leviton: The very first application was in 1998 for a system we built to calibrate flight prisms for the HST Advanced Camera for Surveys (ACS). That system was essentially a vacuum UV version of the classical, prism refractometer for measuring refractive index of transmissive optical materials. We needed a cheap rotary encoder in vacuum with sub-arcsecond accuracy in a hurry, so we engineered a special version of the new encoder in just a few weeks for less than $1,000. Meanwhile, we ended up enhancing the world's database of far UV optical material properties with data of unprecedented accuracy and spectral coverage. The materials we measured are crucial to other NASA UV science missions and are being used increasingly in optical designs of wafer steppers for advanced semiconductor production.

NTB: What kind of software does the encoder use, and who developed it?

Leviton: I developed the software -- conceived the way it works, conceived the pattern. I've gone through a number of patterns over time to arrive at one that lends itself to the most efficient image processing. Then I wrote the code that examines the images; they are basically taken as fast as the computer can take them. It takes a picture, does some computation, decides where the position is, then discards that picture and gets another one -- and does that repeatedly.

NTB: What future NASA applications might there be for the new encoder?

There are other applications, possibly, for the phasing of the adjustment of the mirror segment for the Next Generation Space Telescope (NGST), where they have to adjust the mirrors very precisely, one to the next, with a linear encoder. This encoder can also work in cryostatic environments -- down near absolute zero temperature, which for NGST or any of these other future far-infrared space probes will be important because things have to be kept extremely cold.

NTB: What are some potential commercial applications for your invention?

Leviton: Any mechanism requiring absolute encoders more sensitive and compact than existing encoders can take advantage of this new technology. One example would be linear positioning platforms for microlithography steppers. Other potential applications include inspection equipment, robotics, machine vision, coordinate-measuring equipment, aviation, surveying, profilometers, and disk-drive manufacture.

Mr. Leviton can be reached at doug.leviton@gsfc.nasa.gov.

 

Related Tech Brief
"Absolute Position Encoders Using Pattern Recognition" can be accessed at www.nasatech.com/Briefs/GSC13703.html

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