Virtual absolute position encoder

摘要

A rotary stage (26) is positionable in precise angular increments that are determined by a virtual absolute position encoder disk (106). The angular increments are determined to a 2,400 arc-second absolute resolution by a bar code scale (110) and to a 0.125 arc-second relative resolution by interpolating interference patterns generated by a diffraction grating-based incremental encoder scale (108). The position encoder may be considered a virtual absolute encoder because as little as 0.66 degrees of rotation is required to know the absolute angular position of the rotary stage. The bar code scale eliminates the need for stop bits and quiet zones, which results in increased bar code resolution. The preferred bar code format includes a continuously abutting series of 15-bar slot position bar code patterns positioned as a single track encircling the encoder disk. The first four bar slots of each pattern contain a fixed start code (132), and the remaining 11 bits contain a unique sector-identifying code (134) that does not match the start code.