Patent GB-515567-A (GB)
Title: Improvements in or relating to the detection of the position of objects by electro-magnetic rays
IPC: G01S17/06
Filed: 1938-05-06
Abstract: 515,567. Photo-electric apparatus for direction finding. MOBSBY, E. G. H., and SLAZENGER, R. C. G. May 6, 1938, No. 13639. [Class 40 (iii)] For detecting the position of an object, such as an aeroplane, waves reflected from the object in a projected beam of electro-magnetic rays within the infra-red, visible, and ultraviolet wave band are directed on to an electrically responsive element and a shutter is arranged to move across the reflected rays so that an indication is obtained of the movement of the shutter across the reflected rays relatively to the total movement across the image field. A parallel beam of infra-red or visible light rays A<3>, Fig. 1, is projected by a projector A with which is coupled a receiver C by which rays reflected from an object D in the beam are focussed in front of the sensitive surface of a caesium or other photo-electric cell C<1>. The receiver is provided with a shutter E rotated in front of the cell C<1> by a motor J. As shown in Fig. 2, the reflected rays from the object D are focussed to form an image D<2> on the cell C<1>. The shutter E is shown in the position in which its leading edge E<1> is just entering the image field, which coincides with the cell C<1>. During rotation to the position F no electrical change is caused in the cell by the shutter, since no reflected rays fall on this part of the cell. From the position F to the position F<1> the pencil of reflected rays is interrupted by the shutter, causing a variation in the output of the cell. Further movement to the position F<2>, in which - the shutter completely covers the cell, produces no change in the output of the cell. The output of the cell in relation to the movement of the shutter across the cell is indicated by a cathode ray tube H and thereby indicates the position and size of the image D<2> in the field. Two separate shutters may be arranged so that their leading edges move across the cell at right angles to one another so as to give the complete position of the image. Alternatively a single shutter shaped as shown in Fig. 3 may be employed. Pairs of slots E<3>, E<4> are arranged alternately with the two slots in a pair at right angles so that the edges of the slots E cross the cell at right angles to the direction of movement of the edges of the slots E<4> across the cell. Each slot is the same width as the cell C', so that an impulse is obtained in one direction from the cell as the leading edge exposes the cell and an impulse in the other direction as the trailing edge screens the cell. the two impulses forming a flat-topped wave with a length depending on the size of the object. An alternative form of shutter is shown in Fig. 5, in which slots E<6> are formed in a cylinder E so that their axes are at right angles. The cell C<1> is connected to the control electrode H<1> of the cathode ray tube H through an amplifier G so arranged that normally the intensity of the electron beam is insufficient to produce a visible spot, which is only produced when an impulse is received from the Cell C<1>. In order to produce scanning of the electron beam, the motor J driving the shutter E also drives an inductor generator K comprising a sawtoothed laminated iron rotor K<1>, Fig. 6, and a stator K<2> having twice as many poles K<3> and coils K<4> as there are rotor teeth. Alternate stator coils K<4> are connected in series through an amplifier G<1> to one pair of deflecting plates H<2>, the remaining stator coils being connected in series through an amplifier G<2> to a second pair of plates H<3> at. right angles to the plates H<2>. Saw-toothed scanning impulses are thus applied alternately to the two sets of deflector plates, so that the electron beam moves horizontally across the screen as the slot in the shutter moves horizontally across the cell C<1> and vertically as the shutter slot moves vertically across the cell. Traces L, L<1>, Fig. 8, are thus produced on the screen of the cathode ray tube and give the co-ordinatesof the position and size of the object in the beam. By applying suitable biassing voltages to the deflector plates, the traces L, L<1> may be shifted into positions L<2>, L<3> in which they intersect and give. the position and size of the object directly. This may be effected by applying to one set of deflecting plates, during the scan by the other set, a biassing voltage dependent on the impulse from the cell C<1> during the scan immediately preceding the one in question. For biassing the deflecting plates H<5>, H<6>, Fig. 9, a condenser R is charged for the time from the beginning of the preceding scan of the other pair of plates h <5>, h<6>, until the production of the signal impulse by the cell C<1>. A gas or vapour filled discharge tube M having a cathode M<1> and anode M<3> is connected through the secondary, winding N of a transformer to an adjustable tapping p<1> on a resistance P connected across a D.C. source. This tapping is adjusted so that the voltage applied to the anode M<3> is insufficient to cause ignition of the tube M but sufficient to maintain the discharge when initiated. The condenser R to be charged is connected between an auxiliary electrode M<2> and the cathode M<1> in series with an adjustable resistance R<1>. A gas-filled tube S with an ignition control electrode S<1> is connected across the condenser R. A primary winding N<1> is controlled by the sudden Impulse in the scanning voltage at the end of the preceding scan. This causes sufficient increase of anode voltage to start the discharge of the tube M. A second primary winding N<2> has a voltage applied to it by the signal impulse, which causes sufficient decrease in the anode voltage to stop the discharge of the tube M. The condenser R is thus charged to a voltage proportional to the time between the beginning of the scan and the instant at which the impulse is received from the photoelectric cell. To avoid distortion due to the application of the condenser voltage to the deflecting plates during charging, a second condenser R<2> is charged from a second auxiliary electrode M<4>, the voltage of this condenser being applied to deflector plates H<7>, H<8> in the opposite direction to the voltage applied to the plates H<5>, H<6>. In order that the plates H<5>, H<6> may have the necessary biassing action during the succeeding scan, the plates H<7>, H<8> are rendered ineffective by discharging the condenser R<2> at the end of the scan during which it is charged by means of a discharge tube S<2> having its ignition control electrode S<3> connected to the primary winding n<1> of a transformer in the anode circuit of a discharge tube m which charges a condenser r for biassing the second pair of deflecting plates h<5>, h<6>. In order to avoid potential discharge of the biassing condenser R through the circuit supplying the scanning voltage to the deflector plates, the plates H<5>, H<8>, h<5>.. h<8>, are separate from the plates H<2>, H<3> employed for scanning. Alternatively the scanning may be effected magnetically by coils, deflector plates being used for positioning or biassing the scan axis. Instead of the automatic biassing arrangement the electron beam may be arranged to produce two visible bands the widths of which correspond to the lengths of the lines L. L<1>. The bright square produced where these bands cross indicate the position and size of the object. To produce these bands a highfrequency voltage is applied to the deflector plates at right angles to those producing the main scan. The high-frequency voltage may be obtained from a time base circuit or from an inductor generator of the type shown in Fig. 6 and having a rotor with an appropriate number of teeth of sine-wave form. These teeth may be superimposed on the main teeth shown in Fig. 6 for producing the saw-toothed scanning impulses. Instead of a cathode-ray tube, the indicating apparatus may comprise an oscillograph of the rotary mirror type provided with two mirror drums at right angles, or a single mirror drum and stationary optical systems, the arrangement being such that two intersecting bands are produced on a screen from a source of light controlled by the receiver responsive element. This source of light may be a neon tube which is rendered luminous by the impulses derived from the cell. Instead of a light source whose brilliance is directly controlled by the photo-electric cell, a source of constant brilliance may be used in conjunction with a polarizing device and an adjustable slot or other means controlled by the cell for modulating the light reaching the mirror drums. Alternatively a discharge tube normally illuminated may be so modulated by the signal impulse that the brilliance is reduced, producing a dark trace. The distance of the object from the projector may be determined by using two similar sets of apparatus spaced apart so that the range can be calculated from the bearing obtained, or range finding apparatus as described in Specification 493,704 may be employed, either separately or combined with the apparatus described above. Alternatively the cathode ray tube M may be provided with further circuit arrangements so that it can be used to measure the phase displacement between fluctuations produced in the projected beam and fluctuations in the reflected rays, for example a voltage may be applied to deflector plates, this voltage being obtained from a condenser which is arranged so that its charge is initiated by a fluctuation in the projected beam and stopped by the fluctuation in the received reflected rays. The effect of background reflected rays may be neutralized by applying to the circuit of the receiver responsive element a voltage equal and opposite to that produced by the background rays.