Patent GB-1050888-A (GB)
IPC: G01S1/02
Abstract: 1,050,888. Photo-electric position control. A. C. COSSOR Ltd. Aug. 13, 1964 [Aug. 16, 1963], No. 32532/63. Heading H4D. The invention relates generally to photoelectric means for generating signals indicative of orthogonal position co-ordinates of a light source relative to an arrangement of three or more photo-sensitive cells, and more particularly to arrangements for positioning marker symbols on a radar cathode-ray tube screen. In a first embodiment, a probe, Fig. 2A (not shown), comprises four photo-voltaic cells a, b, c and d, arranged in a square in a transparent annulus 11, connected to a handle 12, which handle also carries the leads 13 from the photocells. The photo-cells a, b, c and d are connected to the circuit of Fig. 3 (not shown), wherein opposing cells a and b are connected to a first differential amplifier 14, to give a signal e representing the difference of their outputs, and opposing cells c and d are connected to a second differential amplifier 15, to give a signal f representing the difference of their outputs. The signals e and f pass through gates 16 and 17, opened during interscan periods, to the Y and X deflection systems of the cathode-ray tube. In use, the four photocells are placed over the image of the marker symbol in its initial position, and are then moved to the desired position of the marker symbol. As the photo-cells are moved such that the marker symbol is no longer in the middle of the photo-cell arrangement, the illumination thereof by the symbol is no longer equal for the four photo-cells, and signals e and f are produced, which are fed to the deflection systems to cause the marker symbol to move back into the middle of the photo-cell arrangement. By including stores 18 and 19 in the arrangement, the marker may be retained in its new position after the probe has been removed. The stores may each comprise a pulse generator, having its pulse recurrence frequency controlled by signals e and f, and adding or subtracting its pulses from the contents of digital registers. The outputs of the registers are applied through digital-to-analogue converters, to the deflection system. Alternatively, analogue stores such as capacitors or motor driven synchros or potentiometers may be used. A press-button switch may be provided on the probe so that the output of the photo-cells are provided only when needed. In the foregoing description the arrangement was used to correct to position of an initially roughly positioned marker. It may also be used however to bring about the rough positioning, when the marker is to be in register with a target blip on a P.P.I. screen, in the following way. The probe is positioned on the target blip and the four photo-cells emit a signal when the blip is next painted. The phase of the signal within a radial scan interval defines the range polar-co-ordinate of the desired marker position, whilst the angular polarco-ordinate of said position is given by the angle moved through by the rotating trace from some datum, said angle being given by a simple " clock " when the trace rotates with constant angular velocity, or otherwise by a digitizer fixed to the radal aerial shaft. The polar co-ordinates can be converted into cartesian co-ordinates to position the member. Alternatively the signal from the four photocells may be used to cause an almost instantaneous sample of the X and Y time base voltages, and so directly derive the required cartesian co-ordinates. After the rough positioning of the marker, the probe may be used as earlier described, to correct its position. As an alternative to the arrangement of Fig. 2A, the probe may comprise a pencil-like member, with the photo-cells positioned at one end perpendicular to its axis (see Fig. 1, not shown). In a second embodiment, Fig. 4 (not shown), and Fig. 5 the four photo-cells a, b, c, d (cell a being hidden behind cell b), are positioned on the bottom of a box 20, carrying in its roof, by means of a universal ball-and-socket joint 22, a joystick arrangement 21, having a light source 23 fixed to its lower end. The photocells a, b, c, d are connected to the circuit of Fig. 3 (not shown), such that movement of the joystick and light source 23 causes corresponding movement of the marker symbol on the cathode-ray tube screen. The scale of the signals e and f can be varied by changing the brightness of the light source 23, or by lowering or raising the light sources, or by changing the amplification of amplifiers 14 and 15. Switches on knob 24 may provide for switching the photo-cells in and out of circuit, changing the ratio between movement of the joystick and the signals e and f, or to convert from translational to velocity control, wherein the marker symbol moves with a velocity dependent on the position of the joystick. If a large movement of the joystick causes a small movement of the marker symbol, the marker symbol can be moved in steps, the photo-cells being switched off after each movement and then switched on again after the joystick has been returned to its initial position. Three photo-cells may be used instead of four, and three dimensional coordinates may be defined by using the photocells in two planes. The photo-voltaic cells described may be replaced with photo-conductive cells and suitable potential sources.