An optoelectronic system discussed herein measures the state of alignment (SOA) of an object under test in four of its six degrees of freedom (DOFs). This system was originally intended for use in determining the SOA of an optical bench in a vacuum chamber. The basic design and mode of operation of this system are also applicable to similar measurement problems that arise in a variety of endeavors, including general alignment of optics, alignment of structures, studies of deformations of structures, geodetic surveying, photogrammetry, and construction. Heretofore, in order to obtain the alignment information provided by this system, it would have been necessary to use at least two expensive theodolites or else multiple expensive low-distortion camera heads aimed along different lines of sight. The present system contains simpler, less-expensive optics, and its basic design is amenable to tradeoffs among range, sensitivity, and configuration.

From the Measured Positions of the Laser Spots on the position-sensing photodiodes, the parametersz, qx, qy, and qz are calculated.

The prototype of the system (see figure) includes (1) a reflective diffraction grating mounted on the object under test; (2) a collimated laser diode fixed in a known position and orientation on, and relative to, a dimensionally stable rail; and (3) at least two position-sensing photodetectors mounted at known positions and orientations on the rail. In an alternative version, the laser diode could be affixed to the object under test.

The laser diode is aimed toward the expected location of the reflective diffraction grating on the object under test, thereby generating a fanlike array of diffracted, collimated laser beams. The position-sensing photodetectors are positioned and oriented so that at least one of the diffracted beams would be i tercepted by each such detector if the object under test were in approximately the expected alignment. The outputs of the detectors are digitized, processed by image-data-processing software, and further processed by software that implements the geometric relationships among (1) the locations of the centroids of the laser-beam spots on the detectors, (2) the laser wavelength, (3) the spatial period of the grating, (4) the distance between detectors, (5) the diffraction order, and (6) the position and orientation of the diffraction grating (and thus of the object under test). The final data product is a set of parameters that specify the position and orientation of the object in four degrees of freedom: range (distance along the laser beam between the laser diode and the center of the grating), the angle of rotation (qz) of the grating about the laser-beam axis, and the angles of rotation (qx and qy) of the grating about two coordinate axes perpendicular to the beam axis.

This work was done by Eric B. Hochberg of Caltech for NASA's Jet Propulsion Laboratory.


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System for Measuring Three Tilts and Distance of an Object

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This article first appeared in the November, 2001 issue of Photonics Tech Briefs Magazine.

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