GB1470293A - Digitising system - Google Patents

Abstract

1470293 Optical measurement of position CETEC SYSTEMS Ltd 25 Sept 1974 [18 July 1973] 34144/73 Heading G1A The digital co-ordinates of a point on an object, e.g. a map, on a platen are found by two interferometers co-operating with two mirrors; either the interferometers are fixed to the platen and the mirrors are mounted on a cursor 10 placed over the point, as in Fig. 1, or the interferometers are mounted on the cursor and the mirrors are fixed to the platen as in Fig. 8. The Fig. 8 embodiment allows direct measurement of the co-ordinates by moving the cursor from a reference point and counting the fringes in each interferometer 16, 16A. These interferometers are similar to those used in Fig. 1, but this latter requires triangulation to determine the co-ordinates. In Fig. 1 light from laser 20 passes to interferometers 17, 17A and then via steerable mirrors 18, 18A to reflectors mounted on cursor 10. The reflected light passes back to the interferometers where it interferes with the outgoing light to cause fringes. These fringes are detected by two photo-cells in each interferometer, arranged to have a phase difference of about 90 degrees to give directional information, Fig. 3 (not shown). The photo-cell signals are counted to give a digital measure of the path length from the interferometers to the cursor, and the co-ordinates are found by triangulation in an associated computer, Fig. 7 (not shown). The computer can give cartesian or polar information, or may show how far the cursor has been moved from a given point. The mirrors 18, 18A in Fig. 1 are controlled by photo-detectors 67, 67A which respond to the reflected light; the mirrors thus track the cursor. If a reflected beam is blocked by an extraneous object this is also detected by the detectors and the computer is disabled; the mirrors are then rotated until the beam is unblocked and tracking resumes.