Patent GB-1041191-A (GB)
Title: Improvements relating to light-beam displacement sensing apparatus
Inventor: FORD BRIAN ARTHUR
IPC: G01S3/787
Filed: 1961-10-18
Abstract: 1,041,191. Optical position finding. BRITISH AIRCRAFT CORPORATION (A.T.) Ltd. Jan. 16, 1963 [Oct. 18, 1961], No. 37423/61. Heading H4D. [Also in Division F3] Apparatus, detecting the displacement of a radiation beam from a central axis within a predetermined area comprises radiation sensing means detecting the beam in all positions within the area, and a rotary shutter continuously rotating about the axis and interposed in the path of the beam to the sensing means, the rotary shutter being so formed that the fraction of each revolution in which it allows the beam to pass varies with the distance from the axis of the point of impingement of the beam on the shutter. The invention is particularly described as applied to gunnery or missile guidance practice wherein the beam of light simulates the path of the missile. A beam of light 15, Fig. 1, produced by arc lamp 11 and lenses 12 and 14 is directed towards a target 20 via movable mirrors 16 and 17, adjustable to position the beam. The target comprises a transparent disc 22 rotating in ball bearings and having an opaque portion with a radial leading edge 23, Fig. 2 (not shown), and a trailing edge 24 in the form of an Archimedean spiral, the centre of the disc being the centre of the target. Behind the disc a photo-electric cell 21, of the same receiving area as the disc, detects the light beam when it passes through the disc. The shape of the opaque portion of the disc is such that the fraction of each revolution of the disc during which cell 21 receives the beam, varies linearly with the distance of the point of impingement 34 on the disc. So that the apparatus shall not be influenced by ambient radiation, the beam 15 is chopped by a rapidly rotating disc 70, Fig. 4 (not shown), having a series of transparent circular portions 72. One face of the disc is provided with a conductive coating, except in areas 73 bounded by the radial tangents to the respective portion 72. The pulsed output from the cell 21 is passed to an amplifier 40, Fig. 5 (not shown), tuned to the frequency of the pulses, and then to a demodulating circuit (also in 40) wherein, by means of contacts 74 and 75, Fig. 4, co-operating with the conductive coating on the disc 70, every other disc pulse is shorted out. The resulting demodulated output is passed to a saturable amplifier 50, Fig. 6 (not shown), to produce a steep-sided pulse output with a mark/space ratio proportional to the distance of the spot 34 from the disc centre. This output can be displayed on a C.R.T. 61 having a time-base controlled by the A.C. voltage rotating the disc 22, or may be passed to an average reading voltmeter. To enable the angular position of the spot 34 to be found a contact 31 connected to the disc, and in line with edge 23 makes contact with a fixed contact 32 once per revolution, to send a pulse to a bi-stable flip/flop 80, Fig. 7 (not shown), to trigger it into a positive state. The flip/flop is then returned to its initial state by the cutting off of the light beam by the leading edge 23, using the amplifier output as a triggering voltage. The flip/flop thus gives a pulsed output which can be displayed on a C.R.T. or voltmeter to represent the angular displacement of the direction of the spot. The radial and angular outputs can be combined on a single C.R.T. to give a displayed spot on the C.R.T. screen corresponding to the position of spot 34.