Patent GB-641358-A (GB)
Title: Improvements in or relating to control systems
IPC: G05B11/32
Filed: 1947-08-08
Abstract: 641,358. Automatic control systems for physical variables. ASKANIA REGULATOR CO. Aug. 8, 1947, No. 21998. Convention date, May 18, 1946. [Class 38 (iv)] , A stabilized control system for controlling a variable comprises a controller, e.g. valve, responsive to the combined action of a first signal generator responsive Co the departure of the variable from a predetermined value, and of a second signal generator responsive to the adjustment of the controller, the second signal generator being arranged to reduce the rate at which the controller is adjusted as it approaches the end of its initial adjustment by introducing an opposing control, which is removed at an exponentially decreasing rate after the initial adjustment. As shown in Fig. 1, a valve 11 under the control of a motor 12 is adapted to control the pressure in a conduit 10. The motor 12 is controlled by three signal generators 17, 18, 19, the first, 17, producing a signal corresponding to the predetermined value at which the variable is to be maintained, the second, 18, producing a signal equal in magnitude but opposite in character to that of the generator 17, when the variable is at the predetermined value, and increasing or decreasing in magnitude if the variable varies in one direction or the other, so that the resultant signal applied through an amplifier 25 to the motor 12 due to the generators 17 and 18 is of a magnitude corresponding to the degree of departure from the predetermined value of the variable, and of a character corresponding to the direction of departure. The third signal generator, shown as a potentiometer 20, fed by a fixed source of potential 21, is arranged to counteract or offset progressively the resultant signal produced by the generators 17 and 18. The potentiometer 20 has an arm 22 driven by a gear 34 through a planetary gear system 31 having a gear 32, driven by the motor 12 and gear 33, driven by a resetting device 30, so that movement of the arm 22 depends on the difference in the movements of the gears 32, 33. The resetting device 30 is such that it tends to restore the arm 22 to its neutral position at an exponentially decreasing rate and comprises a motor 38 driving a disc 37 on which is mounted at right angles a further disc 39 connected to the gear 34 driving the potentiometer arm 22 so that on rotation of the gear 34 the disc 39 is offset axially with respect to the disc 37 causing a roller 40 driving the gear 33 to rotate. Linear movement of the disc 39 causes an exponential rate of increase or decrease in the speed of the gear 33 and therefore in the rate of adjustment of the arm 22. In operation a deviation in the pressure in the conduit 10 causes a signal to be applied to the motor 12, thereby adjusting the valve 11 in a sense to correct the deviation and also rotating the gear 32 and, therefore, the arm 22 causing the potentiometer 20 to generate a signal which eventually balances that produced by the generators 17, 18 and bring the motor to rest. With the first movement of the arm 22, however, the resetting device 30 is rendered operative tending to restore the arm 22 to the neutral position at a rate which decreases exponentially as the arm 22 approaches this position. In a modification (Fig. 2, not shown), in place of the fixed source of potential 21 a source of variable potential, itself operated by a second device similar to the device 30, is em. ployed so that the potential applied to the potentiometer 20 is decreased as the arm 22 moves from its' neutral position. Thus, for large deviations of the variable only a small opposing voltage is initially generated. In a further modification (Fig. 3, not shown), the opposing voltage is derived from two devices operating both the potentiometer 20 and resetting device 30 and is such that an aiding signal may initially be obtained in the case of a large deviation of the variable.