Process and apparatus for sensing magnitude and direction of lateral

Abstract

Process and apparatus therefor for sensing the magnitude and direction of lateral displacement of at least one line or slit element of an article which comprises producing two coherent light beams of the same intensity, one of the beams having a different frequency from that of the other beam, the frequency difference .DELTA.f being within the radio frequency range; converging the two beams to form an interference zone comprising, within the zone of convergence, a fringe pattern characterized by continuous lateral movement at a rate equal to .DELTA.f; adjusting the fringe period of the pattern to a width which is substantially wider than the element or the positive difference between the width of the element and an integer multiple of the fringe period; positioning at least the portion of the article bearing the element within the interference zone in such manner that the article lies in a plane defined by the x and y axes of the fringe pattern and the element is in a position, relative to the article, such that its longitudinal axis is substantially parallel to a fringe plane; laterally shifting the fringe zone and/or the article along the y-axis to produce relative lateral displacement between them while maintaining the element in substantially parallel fringe-plane alignment at least within the central portion of the fringe zone; continuously determining the phase of the RF signal component of the radiation scattered or transmitted by the element as the relative lateral displacement between the zone and article continues from a first to a second position of the element within the fringe zone; simultaneously and continuously comparing the phase of the scattered or transmitted radiation RF signal with the phase of a constant reference RF of .DELTA.f frequency; and sensing the magnitude and direction of phase shift between the RF signal component of the scattered or transmitted radiation and the RF reference between the first and second positions, whereby the magnitude indicates the extent of lateral displacement of the element between the two positions and the direction indicates the direction of relative lateral displacement. The element can comprise a single element, two essentially parallel elements, or a plurality of elements wherein at least each two adjacent elements are substantially parallel. The process and apparatus are particularly useful for accurate measurement of fractional distances between two elements with a high degree of fine resolution; the total magnitude and direction of displacement of elements of a measuring device, such as an encoder; and the distance from a reference element to a desired position on the article.