Patent GB-1470063-A (GB)
Title: Video tracking system
IPC: G08B17/00
Filed: 1974-08-28
Abstract: 1470063 Optical tracking THOMSON-CSF 28 Aug 1974 [31 Aug 1973] 37675/74 Heading H4D An automatic target tracking system comprises a pick-up camera 1, Fig. 1, which supplies a video signal V for an observed field, as well as synchronizing pulses SH, SV for line and frame scanning, respectively, circuitry 6 for generating an electronic window, a TV receiver 3 for displaying said field and said window, a circuit 11 for adjusting the size of the window to that of the target, acquisition means 8, 9 to position the window on the target, and automatic tracking means 10 to slave the window to the target. The tracking means 10 comprises a circuit for measuring the mean level exhibited line by line by the video signal away from but close to the target, a comparison circuit to adjust the target video signal to said measured mean level, and a circuit for the full wave rectification of said adjusted video signal. The rectified video signal is applied to two channels which relate, respectively, to orthogonal X (line-scan) and Y axes, and which each consists of circuits for selecting the video signal contained in the window, two circuits for measuring cyclically at the frame-scan frequency, or at a multiple thereof, the integral of this latter signal in the first and second halves of the window respectively along the axis in question, and a comparison circuit which supplies an error signal which is equivalent to the difference between the two measurements, and which represents substantially the divergence along said axis between the centre of the window and the centre of the target. As described, the optical axis AV of the camera 1 corresponds with the centre of the image formed on the screen 4 of the TV receiver 3. A synchronizing circuit 5 extracts the synchronizing pulses SH, SV for application where required in the system, including ancillary exploitation circuits 12, which also receive from the circuitry 6 signals representative of the position of the centre of the electronic window. The circuits 12 are thereby enabled to measure the aiming error of the camera and to provide signals for the automatic correction thereof. The circuitry 6 comprises in respect of each of the axes (X and Y) a series arrangement of a sawtooth scan signal generator 13, a summator circuit 14, which is also fed by relevant channels (i.e., X or Y) of the acquisition and tracking circuits 9, 10, and a circuit 15 which supplies window and half-window signals (Fig. 3, not shown). The circuit 15 also receives control signals from the circuit 11. Brightness boosting signals are produced by a circuit 16 from the window signals, and are mixed in a circuit 7 with the video signal from the camera, so as to display an electronic window F on the receiver screen along with the field of observation. When a target to be tracked appears in said field it is initially acquired by placing the end of a photoelectric pen 8 thereupon and operating an associated press-button switch. A signal SD is given to the circuit 9, which also receives signals from the sawtooth scan signal generator 13 and from the window signals generator 15, and which comprises logical circuitry for developing signals to the summator circuit 14 for positioning the window to the target (Fig. 5, not shown). After a short period the auto tracking circuits 14 take over. As is shown in Fig. 4 for the Y channel of the generator 15, the output of the summator 14Y is supplied to three threshold comparators 20, 21, 22 which are connected as triggers. Threshold values V1Y, V2Y, V3Y corresponding to the sides and centre of the window (Fig. 3, not shown) are provided by the circuit 11. A logic assembly which combines an exclusive OR circuit 23 and three AND circuits 24, 25, 26 enables the window signal FY and the two halfwindow signals F1Y and F2Y to be obtained. The circuit 16 may be formed by a gate circuit 27 which is controlled by the signal FY and which receives the FX signal of the X channel at a second input, said FX signal only being transmitted to a circuit 28 for producing the brightness boosting signals SF when the signal F Y is applied, so that a rectangular window is displayed on the screen 4 (Fig. 2, not shown). In the auto tracking circuit 10 the video signal from the camera is filtered and applied directly to one input of a comparator (such as a differential amplifier) and through three serially connected circuits to the other input thereof (Fig. 8, not shown). The first of said circuits is an analogue gate circuit controlled by a signal which provides a narrow window on each side of the window F (Fig. 7, not shown). The second of said circuits integrates the video samples from the first circuit, and provides a mean level, which is transferred to the third circuit (a memory) at the end of each line scan. The output of the third circuit, which is applied to the second input of the comparator, is thus one line out of step but it is stated that this is of no practical disadvantage. The output of the comparator is applied to a full wave rectifier, which provides an output signal S13. This is applied to two gate circuits 47, 48, Fig. 9, which are associated, respectively, with a Y and an X channel, and which are controlled respectively, by the window signals FX and FY. Considering the X channel, output from the gate circuit 48 is fed in parallel to two gates 50X, 51X, which are controlled, respectively, by the signals FIX and F2X for the two successive halves of the electronic window along the X axis. The outputs of the gates 50X, 51X are integrated in circuits 52X, 53X, which feed memory circuits 54X, 55X. The two memory circuits supply continuous voltages E1X, E2X to a comparator circuit 57X of the differential amplifier type, the output of which, i.e. (E1-E2)X, represents an error signal. In order to provide automatic gain control this is normalized by division by (E1+E2)X, using an adding circuit 56X and a divider circuit 58X. Finally, the signal ((E1-E2)/(E1+E2))X is processed to make allowance for the speed of the target. It is applied to an adjustable divider circuit 60X on the one hand directly, and on the other via an integrator circuit 59X (controlled by a synchronizing signal 58Y) to obtain a position error signal S4X which consists of two parts, i.e., the directly applied signal multiplied by k, and the integrated signal multiplied by(1-k). The factor k is less than one, and is determined experimentally. The error signal S4X is applied to the summator circuit 14, Fig. 1.