Alignment insensitive optical position sensor

Abstract

An alignment insensitive optical position sensor produces a signal indicative of the position of an element in respect to a monitoring point. The position sensor (50, 100, 120, 180, 200) includes a light source (56, 126) that provides a test light and a reference light of different wavelengths. The test light and reference light are conveyed through an optical fiber (68, 128) and collimated by a lens (70, 110, 144) along a path toward an encoded track or surface (72, 102, 138, 152, 204, 206). The encoded track or surface includes a plurality of metallic dots (74, 104, 140, 154) that are of varying size and/or density to produce a variable reflectivity. The encoded surface also includes an interference filter (76, 106) that reflects the reference light, but transmits the test light, due to the differences in their wavelengths. The reflected test light and reference light are conveyed to a light detector (84, 148, 162) that determines the ratio of the intensity of reflected test light to the intensity of reflected reference light. This ratio is indicative of the relative position of the encoded surface. In several of the embodiments of the alignment insensitive position sensor, the test light and reference light are split along two paths and directed toward separate encoded surfaces that include metallic dots applied in symmetrically inverse relationship in respect to density and/or size. The sum of the ratios of the intensities of reflected test light and reference light for each of these two encoded surfaces should thus be proportional to a constant value, unless a fault has occurred in the position sensor. A processor (168) determines the ratio of the intensities of reflected test light and reference light from each of the encoded surfaces, and determines the sum of the two ratios in order to detect any failure of the position sensor.