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Cosine Error Detection Apparatus

IP.com Disclosure Number: IPCOM000077886D
Original Publication Date: 1972-Oct-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 3 page(s) / 31K

Publishing Venue

IBM

Related People

Harrison, RW: AUTHOR

Abstract

This device allows the detection of laser interferometer beam misalignment (which causes reading errors) to be made without disturbing the system setup. It has twice the detection sensitivity of prior methods and is self-calibrating.

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Cosine Error Detection Apparatus

This device allows the detection of laser interferometer beam misalignment (which causes reading errors) to be made without disturbing the system setup. It has twice the detection sensitivity of prior methods and is self-calibrating.

Application of laser interferometers to linear distance measurement, requires that the measuring beam be precisely aligned parallel to the geometric path of travel of the corner cube target prism located on the moving object, else an error be introduced. This so-called "cosine error" is on the order of 1 ppm for each 3.5 arc min of beam misalignment. Present art makes use of a centering detector mounted in place of the target prism, to detect motion of the impinging beam as the subject part undergoes excursion; beam alignment is then adjusted to reduce this motion to a negligible level. (For the above-quoted 1 ppm accuracy, 0.001 inch of motion per inch of excursion must be detected).

The device described allows for detecting the beam misalignment without disturbing the system setup - that is. without replacing the target prism with a centering detector. Also, for a given beam misalignment the motion to be sensed by the centering detector is twice that above, thus halving the necessary sensitivity. (For 1 ppm accuracy, 0.002 inch of motion per inch of excursion must be detected). The device is also self-calibrating.

The device utilizes the fact that a beam entering a corner cube target prism distance x from the apex axis of the prism emerges parallel to itself at a distance x from the apex axis, but diametrically opposite the point of entrance. Hence, if the prism moves laterally to the entrance beam the distance Delta the return beam moves the same direction, laterally, the distance 2 Delta. serving the lateral motion of the beam returning from the prism; namely, if the return beam shifts th...