Abstract: An encoder for providing positional information about a moveable member comprising code member means couplable to the moveable member for movement with the moveable member, reference means positioned to be stationary relative to the code member, an index sequence positioned on the code member, a mask sequence positioned on the reference member means, and means for detecting the degree of registration between the index sequence and the mask sequence, wherein the index sequence and the mask sequence include a plurality of building block patterns that are separated by predetermined spacings.

Abstract: An improved apparatus for generating an index signal in incremental encoders, wherein an index sequence of code elements is provided on the code disc, and the degree of registration between the index sequence and a mask sequence is detected, and further wherein the mask sequence and index sequence are configured so that there is maximum registration of the elements between the index sequence and mask sequence for the index position of the code disc and the degree of registration for all other angular positions of the code disc is at or below a predetermined background registration level, and so that the degree of registration for angular positions adjacent to the index or zero reference position is greater than zero and preferably no greater than the background registration level.