Patent GB-743354-A (GB)
Title: Electronic control device
IPC: G05B11/01
Filed: 1953-05-29
Abstract: 743,354. Automatic control systems. BENDIX AVIATION CORPORATION. May 29, 1953 [Aug. 18, 1952], No. 15043/53. Class 38 (4). [Also in Group XXXVIII] A signal of constant amplitude having a frequency proportional to one variable is applied to a thermionic device the resonant frequency and gain of which are modified by a second variable, the output of the thermionic device serving to modify a reference voltage. The circuit arrangement is used for regulating the fuel supply to a gas-turbine during acceleration. Control circuit.-As shown, Fig. 1, a voltage proportional in magnitude and frequency to a variable, e.g. speed, is fed at 12 through a limiter 10, giving a constant voltage output, a portion of which is fed by a potentiometer 18 to the grid of an amplifier tube 16. The tube anode is fed through a parallel resonant network 28, the resonance frequency of which is varied by a second variable, e.g. temperature, sensed by a device 35, which also adjusts the portion of the limiter output fed to the tube 16 to control the gain thereof. The amplifier output is fed through a cathode follower 38 and rectifiers 44, 62 to a point P on a potentiometer 52 fed with a backing-off voltage. The amplifier output (Y axis), Fig, 5, approaches a peak value as the input frequency at 12 (X axis) approaches the resonance frequency of the network 28, variations of the resonant frequency (Z axis) altering the position of the peak along the X axis. The backing-off voltage is pre-adjusted so that the rectifiers 44, 62 conduct only as peak conditions of the amplifier output are approached, whereby a reference voltage taken from a point 60 on the potentiometer 52 rises only as peak conditions are approached, i.e. in dependence on the two variables. Application to gas turbine control, Fig. 2. For controlling the fuel valve 86 of a gas turbine 78 so as to avoid stall conditions during acceleration, an engine driven alternator 76 feeds the first variable frequency signal to the limiter 10, and the sensor 35 responds to the turbine air inlet temperature. The potentiometer 52 and a further potentiometer 102-104 form the arms of a bridge which compares the reference voltage at point 60 with the output of a thermocouple responsive to engine tailpipe temperature, Fig. 6 (not shown). The bridge output is connected to a servo amplifier 98 controlling through a motor 88 the fuel valve 86. Fig. 3 shows fuel-weight used plotted against speed for constant temperature acceleration K, L, M. Stalling tends to occur when acceleration is effected at high temperature, e.g. 1650‹ F., i.e. at the shaded area CF along the curve K corresponding to this temperature. During acceleration, e.g. from the point A on the steady state curve M to the point H thereon it is desirable to do so along A, B, C-F, G, H. The selection of the new-speed by the pilot connects the servo amplifier 98 into the circuit of Fig. 2, and by means, not shown, the valve 86 is opened to increase the fuel supply to point B on the curve K which is traversed from B to C with an increase in engine speed to N1. At this stage the output from amplifier 16 approaches its peak value to increase the reference voltage at point 60 whereby the bridge is unbalanced and through the servo amplifier 98 the engine is caused to operate along the lower curved portion C, F of line K until at F the normal curve K is followed. The curves, Fig. 3, are derived empirically and the time constants of the signal modifying circuit designed so that the operating curve just skirts the shaded stall area.