Patent ID: 6441908
Filing Date: 2002-08-27
Classification: G01B

Abstract:
A system for high-precision determination of the position of a rotatable component of an optical scanning device, comprising:a source of light that emits light; a light-sensitive sensor; a lens used to focus an image onto said sensor; and a rotatable polygon mirror system that re-directs the light emitted from said source of light to a plurality of locations of an external surface of a component, the rotatable polygon mirror system comprising: a shaft; a plurality of primary mirrors positioned substantially concentrically surrounding the shaft; a first turning mirror positioned to intercept the light emitted by said source of light subsequent reflecting off each of the primary mirrors at an angle representing a start-of-scan; a second turning mirror positioned to intercept the light emitted by said source of light subsequent reflecting off each of the primary mirrors at an angle representing an end-of-scan; a first bi-cell detector positioned to receive the light reflected off the first turning mirror, said first bi-cell detector comprising two photo-detectors, wherein a first voltage difference signal is determined from the output of the two photo-detectors of the first bi-cell detector, and wherein a first trigger pulse is generated upon occurrence of the first voltage difference signal equaling zero; a second bi-cell detector positioned to receive the light reflected off the second turning mirror, said second bi-cell detector comprising two photo-detectors, wherein a second voltage difference signal is determined from the output of the two photo-detectors of the second bi-cell detector, and wherein a second trigger pulse is generated upon occurrence of the second voltage difference signal equaling zero; and a timing mechanism that measures the elapsed time between the first and second trigger pulses; wherein the average rotational velocity of the shaft is determined using said elapsed time, and wherein the average rotational velocity is used to establish the position of the rotatable polygon mirror system during the rotation of the rotatable polygon mirror system.