Patent GB-946928-A (GB)
Title: Tracker for moving objects
Inventor: ASTHEIMER ROBERT WILLIAM
IPC: G01S3/787
Filed: 1960-05-20
Abstract: 946,928. Automatic photo-electric trackers. BARNES ENGINEERING CO. May 20, 1960, No. 17851/60. Heading H4D. Relates to a system for automatically tracking a small source of light or infra-red radiation comprising a pair of acquisition and fine tracking detectors for azimuth control and a corresponding pair of detectors for elevation control, the acquisition and fine tracking detectors producing respectively coarse and fine control signals. The four radiation detectors (two only, 13, 14, being shown in Fig. 1) are disposed quadrantally in a single plane around the axis of a tubular mounting 1 behind corresponding segmental annular apertures I, II, III, IV, Fig. 2, in a fixed mark 11, the small apertures I and III corresponding to the azimuth and elevation fine tracking detectors respectively and the larger apertures II and IV corresponding to the azimuth and elevation acquisition detectors. Each detector is on the optical axis of an associated objective lens 4 or equivalent mirror and a rotating disc 10 is disposed between the mask 11 and the detectors, the disc 10 having (1) an outer annular band 21 of alternate opaque and transparent radial bars of equal width which move horizontally and vertically respectively across the small fine tracking apertures I and III, (2) two contiguous annular bands 22 and 23 of similar bars which move horizontally and vertically respectively across the acquisition apertures IV and II, the bars in the inner band 23 being twice the width of the bars in the outer band 22 and the common boundary of the bands 22 and 23 intersecting the optical axis of the acquisition detectors. The speed of rotation of the disc 10 and the widths of the bars in the bands 21, 22, 23 are such that the chopping frequencies of the bands 21, 22, 23 are 1000 c/s., 1500 c/s. and 750 c/s. respectively. If the tube 1 is oriented so that an image of the source falls within the azimuth acquisition aperture II, then the frequency of the output from the corresponding acquisition detector will be 1500 c/s. or 750 c/s. according to whether the image is to the left or right of the optical axis; a corresponding output is obtained from the elevation acquisition detector depending on the vertical position of the image in the elevation acquisition aperture IV. These outputs are selected by filters and applied to frequency discriminators, Fig. 3 (not shown), to give corresponding error signals. When an image of the source falls within the azimuth fine tracking aperture I, any azimuth misalignment is given by phase comparison of the 1000 c/s. output from the azimuth fine tracking detector with a 1000 c/s. reference signal produced by focusing light from a lamp 16 through the annular band 21 of the disc 10 on to a photo-cell 18; the elevation misalignment is detected in a corresponding manner. The outputs from the fine tracking detectors are also applied to cut off the signals from the acquisition detectors; alternatively, this effect could be achieved by providing an opaque annular band between the bands 22 and 23. The error signals are applied to conventional servos controlling the orientation of the mounting 1 and the signals may also be displayed visually. In order to protect the detectors against intense radiation from the sun, a shield 6 is provided which is operated by a rotary solenoid 7 actuated by a radiation detector 8 receiving radiation on the central axis of the instrument through a small opening 9. When lead sulphide detectors are employed the square wave outputs therefrom may be distorted and such distortion may be removed by known frequency compensators and limiters. In order to avoid the production of a modulation component due to background radiation, the total area of transparent bars within each aperture must be constant for all positions of the disc 10 and this is achieved as shown in Fig. 2 and as disclosed in Specification 898,205 by serrating the radial edges of the aperture with a single or plurality of triangular sawteeth of depth 2n d, where d is the width of the bars and n is a small integer.