Patent GB-599309-A (GB)
Title: Improvements in and relating to gun fire control systems and particularly to prediction apparatus therefor
IPC: F41G5/08
Filed: 1944-07-13
Abstract: 599,309. Gun sight predictors. SPERRY GYROSCOPE CO., Inc. July 13, 1944, No. 13461. Convention date, July 13, 1943. [Class 92 (ii)] Relates to predicting apparatus for computing a value Xp for a future co-ordinate of a moving target from present position data and their rates of change, of the kind in which the computation involves the determination of the time of flight tp to the target's future position and of the present co-ordinate X, of the target, and in which smoothing devices are employed to minimize errors in the computed position as the result of fluctuating errors of present position data set into the computer. According to the invention, the smoothing arrangements include servo means for computing from the quantity X0 and the time interval tp an auxiliary quantity X1 such that when there is added to it a quantity equal to the product of its rate of change X1 and the time interval tp, the result is substantially equal to the present co-ordinate X0, the computed value Xp of the future co-ordinate being obtained as a linear function 2 X0- X1 of the datum quantity and auxiliary quantity. Theory.-The solution for the predicted position Xp of the target in rectilinear coordinates is given by the equation where X represents any rectilinear co-ordinate, o and p represent present and predicted target positions and tp the time of flight of the projectile. With the target travelling at a uniform velocity, the value of X, is given by where a0 represents the X co-ordinate of the target at zero time and a, the target's speed in the X direction. These two formula give In actual practice errors are caused by the inability of the operators to track the target accurately, and by other factors such as backlash in the gearing, and it is assumed that these errors consist of sinusoidal oscillations of a magnitude # and a frequency #/2#. Thus, the present input data introduced into the predicting apparatus is represented by the real part of the expression If the predicting apparatus includes a pure mathematical differentiating device, means for multiplying the derivative by tp, and means for adding the product to the present position input data, the following value of Xp will be obtained. Thus, the error #e<j#t> introduced into the apparatus results in a magnified prediction error #(1+j#tp)e<j#t>, and with typical values of # and tp (for example #=3 and tp=20), the magnification or amplitude ratio is approximately 60. The apparatus according to the present invention is designed to reduce this amplitude ratio to the order of 2 under conditions ordinarily met in practice, and the Specification describes the connection between the theory and the apparatus described below. Apparatus.-Present target position X0 along one co-ordinate is fed to the prediction apparatus by a shaft 1 leading to a subtraction differential 2, the other input shaft 3 of which supplies data X1 referred to below. Output shaft 5 is thereby displaced by an amount X, - X1 and this is converted by a potentiometer unit 7 to produce a voltage signal corresponding to X0- X1 on a lead 12. The shaft 3 is driven by a servomotor 4 and serves to drive the shaft 17 of a permanent magnet generator 13, the output voltage of which is therefore proportional to X1. This voltage is fed to a potentiometer 20 whose contact arm 22 is rotated by a shaft 23 through an angle proportional to the time of flight tp, thus producing on a lead 24 connected to the arm a voltage proportional to X1tp. The two leads 12, 24 lead to a servo amplifier 11, details of which are given in Fig. 2 (not shown), this amplifier serving to subtract the voltage on lead 12 from that on lead 24 and to amplify the resultant difference voltage. This voltage, appearing on a lead 25, is introduced across the field windings of the servomotor 4, the armature of which is constantly energized by a constant source of direct voltage applied across terminals 27. Accordingly, the motor 4 actuates shaft 3 in a direction and at a rate determined by the magnitude and polarity of the difference in the voltage signals across leads 12 and 24. When a target moving at uniform velocity is being tracked and the apparatus reaches a state of equilibrium shaft 3 will be moving at the same speed as shaft 1 and X0=X1. Further, shaft 5 will be at rest, and the voltages in leads 12, 24 will, with a suitably designed amplifier 11, be approximately equal. From this it follows that the voltage in lead 12 is proportional to the prediction quantity Xotp and that the stationary shaft 5 is displaced by a corresponding amount. Further, this displacement X0tp equals the lag (Xo - X1) of shaft 3 behind shaft 1, so that if the displacement (X0- X1) of shaft 5 were added to that X1 of shaft 3 the resultant would equal that of shaft 1 X0 which corresponds to the target's present position, and if this resultant were added to the displacement X0tp of shaft 5, corresponding to the prediction quantity, the final result would correspond to the target's future position after time tp. Accordingly, shaft 5 is geared as shown through a 2 : 1 gearing 39 to a shaft 40 leading to a differential 28 to which also leads a shaft 33 geared as shown to the shaft 3, whereby the output shaft 41 is displaced through an angle corresponding to the predicted future position of the target. As can be seen from the drawing, this angle is proportional to the quantity 2X0-X1. Specifications 494,172 and 494,288 are referred to.