Patent GB-1177641-A (GB)
Title: Improvements relating to Automatic Tracking Apparatus.
Inventor: BERRY PETER JOHN
IPC: F42C13/02
Filed: 1961-02-03
Abstract: 1,177,641. Optical direction-finding; transistor gating circuits. ELECTRIC & MUSICAL INDUSTRIES Ltd. 26 Jan., 1962 [3 Feb., 1961], No. 4254/61. Headings H3T and H4D. Apparatus for tracking a radiating bodymoving relative to the apparatus comprises a first lattice of elongated parallel radiationsensitive elements, a second lattice of elongated parallel radiation-sensitive elements at right angles to the first, means for focusing radiation on the two lattices from substantially the same point of view, first and second sampling means for sampling the elements of the two lattices respectively in predetermined manner to determine which, if any, of the elements is sensitized by radiation, and output means responsive to the sampling means to produce an output signal dependent on the direction of the radiating body relative to the tracking apparatus. Referring to Fig. 1, infra-red radiation emitted by a target is sampled by a mechanical chopper (not shown) and focused by a Schmitt optical system 1 on to two lattices 2 of infra-red sensitive cells carried by the missile. (Only one lattice 2 and its circuit is shown in Fig. 1). Each cell is scanned in turn by a series of switches or gates 3 controlled by a counter 4, and only that.cell on which radiation is focused will produce an output, the particular cell being determined by the angular position of the radiation source. This output is applied to a high-pass filter 5, amplifier 6 and blanking circuit 7 which cuts out components of the switching pulses; after passing through amplifier 9 the signal is detected at 10 to .reduce the A.C. component due to the mechanical chopper and applied to a Schmitt trigger circuit 11 which provides a fixed output whenever the signal rises above a predetermined value. The switches 3 are normally operated sequentially by a ring counter 4 which is driven by pulses from the counter control unit 8 via the counter driver 12. The counter control unit 8 produces driving pulses only when no signal is produced by unit 11. When a signal appears, e.g. from cell " x," the counter control unit 8, and hence the counter 4, stops and cell " x " remains connected to the output line as long as it is producing a signal. If the signal disappears, the unit 8 produces a drive pulse which stops the counter 4 in the direction in which it was last moving, for example to the (x + 1)<SP>th</SP> stage; if there is still no signal the unit 8 in conjunction with reversal unit 13 rapidly reverses the counter 4 two stages to the (x - 1)<SP>th</SP> stage and if there is still no signal the counter proceeds to count in this direction. The counter 4 is connected to a digital to analogue converter 14 whereby the count number at any instant is converted to a voltage proportional to the coordinate value. The signal from 14 is applied to a computer (not shown) and may, if desired, be gated into the computer by the " target present " signal from unit 11. A second input to the computer is derived from the second lattice to give the position of the target relative to the missile in two directions at right angles, and from these two values the computer calculates the angle of sight and its rate of change and hence the optimum moment of detonation. Blanking circuit.-The blanking circuit, Fig. 2, comprises a transistor T connected across a tuned circuit C 3 L 4  of which the centre tap of the coil L 4  is earthed to A.C. When T is cut off the coil L 4  behaves as an autotransformer and the signal is simply inverted in passing from the input amplifier 6 to the output amplifier 9. When T conducts the signal is fed to both halves of coil L 4  simultaneously and cancels out. Ring counter.-The ring counter shown in Fig. 3 comprises a number of stages each including a four-layer PNPN semi-conductor diode S connected to one of the lines ABC which are energized in sequence by means not shown. The potential applied to lines ABC is insufficient to cause conduction through a diode but sufficient to maintain conduction. When the circuit is switched on means are provided to ensure that one and only one diode will conduct (for example a non-linear impedance may be connected between the negative potential source and the junction of resistances RC1, RC2, RC3 ...). If line B is energized and diode S2 is conducting, the condensers C associated with diodes S1 and S3 are charged through diodes DB; this charge enables diode S1 or S3 to conduct if line B is de-energized and line A or C energized. Thus the stages can be energized in sequence in either direction according to the sequence of energization of lines AB and C.