OPTO-ELECTRONIC SCALE-READING APPARATUS

摘要

A scale (10) is read by a readhead (13) to determine relative displacement between them. The readhead contains no gratings, as would be conventional. Instead, it has reflective surfaces (17, 18) so that light from a source (14) is reflected and has interactions with the scale at A, D, and G. At G, a fringe field FF is developed, which has the same pitch as the scale. Thus, a detector (20) receives light modulations during relative movement, as the fringe field interacts with the scale at G. These modulations can be counted to determine the distance moved.