Patent GB-771057-A (GB)
Title: Improvements in or relating to electrical control systems
IPC: G05D9/12
Filed: 1953-08-14
Abstract: 771,057. Automatic fluid level control systems. SWARTWOUT CO. Aug. 14, 1953 [Sept. 6, 1952], No. 22467/53. Class 38 (4). The level of liquid in a tank T, Fig. 1, is maintained constant by control of a valve V in conduit C leading to the tank. Variations of level are detected by a float 10 moving the core 28 of a differential transformer 11 which is compensated for ambient temperature and phase changes. A. similar manually adjustable transformer 12 is provided to set the desired level and the difference between the alternating voltage outputs of transformers 11 and 12, representing the error in level, is applied to an A.C. amplifier 14 including triode valves 42, 47, 48, the sensitivity of which is adjustable by a potentiometer 59 controlling negative feedback. The output of amplifier 14 is applied to a phase-sensitive rectifier 16 which produces a D.C. signal corresponding in polarity and magnitude to the sense and magnitude of the error in level. This D.C. signal is applied to an amplifier triode 77 provided with negative feedback through an adjustable resistance 81, 84 and a shunt capacitor 85, so that a component dependent on rate of change of error is introduced into the D.C. signal which s then applied to a two valve amplifier 18 including valves 100, 101 provided with negative feedback through a series capacitor 104 and adjustable shunt resistor 105, and positive feedback through resistances 108, 109, so that a component dependent on the integral of error is introduced into the D.C. error signal which, through leads 163 and 119, energizes a moving coil 115. Moving coil 115 acts, through a lever pivoted at 118, on a diaphragm 125 so that a valve 132 or a valve 127 is opened, according to the direction of movement, until the air pressure in a chamber 124 is representative of the error by increase from an air supply 129 or reduction to atmosphere through vent 126. The pressure in chamber 124 is communicated through conduit 136 to a larger diaphragm 137, movement of which controls larger valves 142, 144 in a manner similar to valves 127, 132 so that a pressure corresponding to the error is produced in chamber 139 to control valve V through conduit 148. Power supply for the system is provided through a constant voltage transformer 20. Rectifier 22 provides the H.T. supply while rectifiers 87, 103 provide backing-off voltages in the feedback circuits. The rate component may be eliminated by appropriate setting of resistance 84 and the integral component by operation of switch 107. Automatic control of transformer 12 on a time basis is referred to. There is provision for sense reversal of the phase-sensitive rectifier and moving coil 115 to suit different types of system. A system for switching from automatic to manual control and vice versa is described with reference to Fig. 2 (not shown). During manual control moving coil 115 is disconnected from amplifier 18 and controlled from a D.C. source by a variable series resistance, an equivalent dummy load being connected in place of the moving coil. An intermediate switching position is arranged in which the dummy load is connected to the manual control circuit so that the series resistance can be adjusted, with the aid of a voltmeter connected between the automatic and manual control circuits, to a position at which smooth changeover to manual control can be effected. A similar intermediate switch position is provided to ensure smooth changeover from manual to automatic control. In this position, a relay 110<SP>1</SP> is energized also to give faster integral control. The voltmeter serves as an indicator of level during automatic and manual control. Plug-in manual or automatic control units facilitating replacement or changeover to automatic control without rate and/or integral components is referred to.