Patent GB-756176-A (GB)
Title: Electronic spotting device, applicable, in particular, for the guiding of rockets and other high speed appliances
IPC: H01J31/26
Filed: 1953-12-11
Abstract: 756,176. Target finding apparatus. HARDY, R. J. Dec. 11, 1953 [Dec. 17, 1952], No. 34529/53. Class 40 (3). [Also in Group XL (a)] In electronically - operated target - finding apparatus such as a self-homing projectile an optical image of the field of investigation is converted into an electronic image, the electronic image being traversed relatively to a thin discharging means so as to locate the position of the image. As shown (Fig. 1) an optical image i of the target is converted into an electronic image e by known means and the image as a whole is traversed relatively to a fixed electrode 10 in the form of a vertical strip which is arranged to lie in the position of the image when at rest. The crossing of the electrode by a target 16<SP>1</SP> located in the image produces an output pulse from the electrode and the position in the scanning cycle of this pulse, is an indication of the location of the target in the image. A tuned circuit 17 assists in the separation of the pulse produced by the traverse of the target across the electrode. In a modification (Fig. 3, not shown), the image is traversed relative to a slit in a screen so that only electrons traversing the slit are able to reach a collector electrode behind the slit. Various secondary electrodes may be so arranged behind the slit as to form an electron multiplier. In the arrangement of Fig. 4, not shown, an iconoscope is used, the image of the target area being represented as charges on the elemental photo-cells and the area being scanned by a thin rectangular electron beam. In order to derive both co-ordinates of the target in this arrangement, the image may be rotated through 90 degrees by a known optical arrangement so that two images appear on the iconoscope screen and scanning in a single direction gives first one and then the other co-ordinate of the target position. However, both co-ordinates may be derived in arrangement of the type in which the entire image of the target area is traversed relative to the first electrode by using a cruciform electrode, as shown in Fig. 7, the centre part of the scanning area being traversed from 73 to 74, quickly from 74 to 75 so as not to produce any appreciable pulse if the target crosses an electrode, from 75 to 77, and finally returned quickly to position 73. This movement of the scanning area is produced by means of waveforms such as those shown in Fig. 8a, and b, applied respectively to the " X " and8 " Y " plates. Arrangements for deriving these waveforms will later be described with reference to Fig. 16. General operation of system (Fig. 12).-The image of the target area is traversed with respect to the cruciform electrode 146 by means of a time base generator 161 (to be described), generating waveforms 200, 201, so that an output signal may appear at any two of four output terminals 181 to 184 representing the distance to one side or the other of the two arms of the cruciform electrode. The output from the electrode 146 is amplified in a stabilized amplifier 158 feeding a device 160 which produces an invariable pulse 160. An output from the device 160 may be used to alter the dimensions of the electronic image and will be later described. The output pulses from circuit 160 are fed to four circuits 168, 169, 170, 171 comprising negatively biased valves. However, only two of these circuits 170, 171, are operable during the horizontal scanning of the target and the other two during the vertical. This is achieved by means of a bi-stable oscillator 162 synchronised with the time base 161 to render two of the circuits operable in turn. Also fed to circuits 168 to 171 are the output and the inverted outputs of a time base generator 165. The biasing of valve 168 to 171 is such that the valves are conductive only for half the sloping part of the sawtooth waveform (Fig. 11) and during this period the invariable input pulse 143 is additive to the instantaneous value of the waveform to produce a voltage 145 which is representative in amplitude of the distance of the target from the electrode. As soon as the target is located the output pulse from electronshaping circuit 160 is arranged to apply a voltage to alter the dimension of the image and at the same time reduce the extent of the scanning sweep. Confusion due to the location of a second target is thus avoided. In a modification (Fig. 13) guard electrodes placed near the outer ends of the cruciform prevent electrons reaching the cruciform when a potential is applied to them, thus limiting the target area to a central portion 212. Circuits. Amplifier and limiter 158, 160, Fig. 14.-This amplifier is concerned with producing a pulse of constant amplitude from the traverse of the target relatively to the electrode. The electrode is connected to terminal 146 of a first tuned stage 301 followed by an aperiodic stage 302, and a valve 303 which is so biased that the crests only of the signal 20 are able to pass. This action is obtained by negatively biasing the grids of valves 202 and 203 by means of large grid resistors so that the amplifier operates near cut-off. The signal at the plate of valve 302 is amplified at 306 and feeds a rectifier 307, integrator 307a and filter 309. The rectifier and integrator produce a signal corresponding to background noise which is re-injected to the valve 302 to eliminate background while filter 309 prevents passage of the frequency of successive lines of the image scan. In each scan when the signal appears condesner 310 discharges through thyratron 311. The signal thus produced is nearly rectangular and is fed to a phase splitter 312 to obtain one positive and one negative pulse. The oscillator (Figs. 15 and 16) (165 of Fig. 12) comprises a thyratron 315 associated with a condenser 316. The sawtooth voltage derived is phase split and fed to terminals 321, 312. The vertical parts of the teeth are fed from the anode of the phase splitter and control a flipflop circuit-comprising thyratron 325, 326 (which may be as described in Mass. Inst. of Technology, Vol. 19, page 47). This bi-stable oscillator corresponds to oscillator 162 of Fig. 13 and its output is combined with the saw teeth from oscillator 165 at the grids of antiphase valves 329, 330 so as to produce the deflecting waveforms 333, 334. These complex waveforms are then applied to the grids of valve 337 (Fig. 16), again combined with the castellated waveform and used to produce the deflecting waveform 200 which is phase split at 343 and used to control the deflection of the image. The output pulses from the amplifier of Fig. 15 are also integrated and used to block a valve 366, the anode potential of which so changes that in conjunction with rectifiers 371, 372 it limits the sweep of the scanning circuits by clipping its crests. The voltage at point 170 may be applied to the image enlarging system. Producing output voltages (network 168 of Fig. 13) (Fig. 17).-As shown in Fig. 13, network 168 which is to derive one of the four voltages defining the position of the target must be fed with the sawtooth voltage derived from circuit 165 (Fig. 16),.the castellated voltage derived from circuit 162 (Fig. 16), and the output pulse of amplifier 158 (Fig. 15).. As shown the constant amplitude output pulse from the amplifier is applied to terminal 350 while the sawtooth waveforms are applied to lines 321, 322. These two signals are combined and the grid potential is such that the signal occurring during the first half of the sawtooth is repressed while that occurring during the second is additive so that at the anode of valve 353 appear a series of pulses which represent the position of the target and which are integrated in a circuit 354, and applied to the grid of a valve 355. The anode potential of this valve is connected to the deflecting plates of the electron image and tends to keep the image in the centre of the cruciform. This in conjunction with the steering of the missile has the effect of aiming the missile at a point ahead of the target. The output signals of valve 353 also feed valve 402 connected to an integrator circuit 403 which supplies at 181 one of the four output voltages which control the missile through servomotors.