ABSOLUTE POSITION MEASUREMENT

摘要

An absolute measurement scale (18) comprises an incremental scale (10) with light reflecting lines (12) alternating with non-light reflecting lines (14) in which absolute data, in the form of discrete codewords, is embedded in the incremental scale by removing light reflecting lines (22). The codewords are arranged such that the scale is palindromic and cyclic. A look-up table is used to determine a coarse absolute position by comparing absolute position data extracted from the scale with absolute codewords in the look-up table. The absolute position is determined by combining the coarse absolute position with the position of the start of the first codeword in the data extracted. The absolute position may be determined to within a scale pitch by combining the absolute position with the incremental position.