Patent ID: 6771377
Filing Date: 2004-08-03
Classification: G01D

Abstract:
A device for measuring the relative displacement of a scale along a measuring axis, the scale having a grating formed along the measuring axis, the device comprising:a split light beam input portion; two or more light beam directing elements; two or more retroreflector elements; and an optical detector; wherein: the split light beam input portion is positioned to input two split light beams along respective light paths; the two or more light beam directing elements are positioned to receive the two split light beams along their respective light paths and direct the two split light beams along respective converging light paths toward a first zone on the scale grating, the two split light beams giving rise to two diffracted light beams from the first zone along respective light paths which diverge to enter the two or more retroreflector elements; the two or more retroreflector elements are positioned to receive the two diffracted light beams from the first zone and retroreflect two retroreflected light beams along respective converging light paths toward a second zone on the scale grating; and the two retroreflected light beams give rise to two later-diffracted light beams along respective light paths from the second zone that diverge to be received by respective light beam directing elements and are directed to enter a shared zone; and the optical detector detects at least one illumination characteristic arising from the shared zone, the detected at least one illumination characteristic usable to determine the relative displacement of the scale.