ENCODER WITH REFERENCE MARKS

摘要

An encoder comprises a scale (10) with a main scale track (12), along which a readhead (22) is movable. The scale has reference marks (18, 20), spaced laterally on each side of the main scale track (12). A common reference mark detector (30) produces a combined output from detection of both the reference marks (18, 20). This averages the Abbe errors of the reference marks, reducing the effect of any yaw misalignment of the readhead. Instead of a common reference mark detector, there may be a respective detector for each reference mark, outputs of which are combined electronically.