GB1466471A - Optical digistising system - Google Patents

Abstract

1466471 Position finding CETEC SYSTEMS Ltd 25 Sept 1974 41620/74 Heading H4D The co-ordinates of a cursor movable on a platen are found by two radiation scanning devices which measure the time taken for a scanning beam to sweep from a fixed mirror to a mirror associated with the cursor; the times measured by the two devices represent the angular position of the cursor relative to the devices, and its co-ordinates are found from these angles by triangulation. In Fig. 1 visible, U.V., I.R. or microwave radiation from sources 17, 17A is directed to a cursor 10 by rotating mirrors 19, 19A. The cursor is a transparent body having a graticule 11, and is movable over platen 15; the body carries a reflective groove, or is externally reflective, or it can be a part sphere whose outer surface is 30% reflective and whose bulk is of refractive index 2. Detectors 20, 21 receive radiation from mirror 19 in device 16; as mirror 19 rotates, first detector 21 receives radiation, then detector 20 receives three beams in sequence, which are respectively the radiation from source 17 via, mirror 19, then via the cursor and mirror 19, then via fixed detector mirror 18 and mirror 19. These signals, and the similar signals from device 16A are processed at 115 for display at 116. Fig. 6 shows the processing circuit for device 16. When detector 21 receives radiation at the start of a measurement cycle, control unit 33 clears counters 36, 37. When detector 20 receives its first radiation the control unit opens both gates 34, 35 so that both counters receive clock pulses. Gate 34 is closed when detector 20 receives its second radiation input, and gate 35 closes when the detector receives its third input. The count at 36 thus represents the angle between source 17, mirror 19 and the cursor, while the count at 37 represents the angle between the source, mirror 19 and mirror 18. Since that latter angle is a known constant, the ratio of the two counts gives the angular position of the cursor assuming the mirror 19 rotates at a constant speed. The two counts are combined at 115 in Fig. 1, as are the similar ones from 16A, and the rectangular co-ordinates are found by standard triangulation. The unit 115 also receives signals from the control unit 33 of Fig. 6 to indicate the end of each measurement cycle and to sound an alarm if detector 20 receives more than three inputs per cycle, as it would if an extraneous reflective body were present.