Optoelectronic pattern-recognition systems (optical encoders)for measuring positions of objects of interest at temperatures well below or well above room temperature are undergoing development.At present, the development effort is concentrated on absolute linear, rotary, and Cartesian encoders and Cartesian autocollimators for scientific instruments that operate in cryostats.

Like some prior pattern-recognition optical encoders, a system of the present type includes a backlit scale attached to the object of interest, a charge-coupled-device (CCD)camera, a lens to project a possibly magnified image of the scale onto the CCD, circuitry to digitize the image detected by CCD, and a computer to process the image data to determine positions of the optically projected scale marks in the reference frame embodied in the array of pixels of the CCD.Unlike in prior systems, neither the light source for illuminating the scale nor the CCD is located in the cold or hot environment that contains the object of interest and the attached scale.This arrangement makes it possible for the CCD to read the scale even though the CCD could not function properly if it were located in that environment.In the case of a cryogenic environment, this arrangement is particularly advantageous because it minimizes spurious heating by the light source and eliminates spurious heating that would otherwise be caused by dissipation of power in the CCD circuitry.

Fiber-Optic Bundles are used to feed light to and from a position-encoding scale in a cryostat.

A cryogenic implementation of a linear or rotary encoder of this type is conceptually straightforward (see figure). By use of a conventional illumination-type fiber-optic bundle, visible light from a source outside a cryostat is fed into the cryostat to back-illuminate the scale.In a typical case, an image of the scale can be acquired and fed to the CCD optics by use of a lens-tipped coherent fiber-optic bundle similar to a fiber-optic borescope or endoscope. A low-thermal-conductivity hermetic feed-through is installed at the point where the fiber-optic bundle passes through the cryostat wall. Alternately, an image of the scale can be projected out through a small window in the cryostat wall.Either way, the encoding function involves very little energy dissipation inside the cryostat.

This work was done by Douglas B. Leviton of Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category. This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

the Patent Counsel
Goddard Space Flight Center

Refer to GSC-14766-1.