Substrate advance measurement system using cross-correlation of light

Abstract

Method and apparatus for determining the distance a sheet of paper, or other reflective or transmissive substrate material with a suitable, approximately planar surface, has moved in a given direction. The substrate is illuminated by reflection or transmission of light at non-normal incidence, and a linear array of N uniformly spaced light sensors is provide to receive and sense light issuing from the illuminated substrate. This produces a first or reference array of light signal strengths s.sub.1 (k) (k=1,2, . . . , N). The substrate is then moved in the given direction, and a second array of signal strengths s.sub.2 (k) (k=1, 2, . . . , N) is produced. A cross-correlation function C(K), formed from consecutive portions of the first and second light signal strength arrays, is then examined to determine the distance the substrate has moved in the given direction. A maximum in the cross-correlation function C(K) at K=K.sub.0 corresponds to displacement of the substrate in the given direction by a distance approximately equal to (MF)(K.sub.0 -i)d, where i is selected positive integer determined by the initial position of the substrate, the distance d is determined from the known spacing of the light sensors, and MF is the system optical magnification factor. The cross-correlation function can be computed with uniform or non-uniform weights in the sum. This one-dimensional approach is extended to determining the vector of two-dimensional movement of the substrate, including translation, rotation and scaling, in a two-dimensional plane.