Patent GB-453602-A (GB)
Title: Improvements in and relating to automatic control of variable physical characteristics
IPC: G05B11/01
Filed: 1935-02-13
Abstract: 453,602. Automatic control systems. CALLENDER, A., and STEVENSON, A. B., Winnington Hall, Northwich, Cheshire, and IMPERIAL CHEMICAL INDUSTRIES, Ltd., Millbank, London. Feb. 13, 1935, No. 4692. [Class 38 (iv)] Temperature &c.-A relay system operating a reversible motor to compensate changes in a variable physical characteristic is rendered operative when primary and secondary devices, responsive respectively to the changes in the characteristic and to changes in the compensating effect obtained from the compensating means, are not in correspondence. The secondary device, e.g. M2, Fig. 1 (Prov.), essentially comprises means 5 whereby an electrical potential proportional to the compensating effect is applied at a point X to an electrical system comprising a point P, to the potential of which the secondary device is responsive, which is connected on the one hand through a condenser 7 to the point X, and on the other hand through a resistance 9 to a point Q, the potential of which is determined by the desired value of the physical characteristic. The condenser 7 and the points P, Q, may each be connected by resistances 8, 12 to a common point, the point P being connected through a condenser 11 to a point Y maintained at constant, e.g. earth, potential. The system as a whole is adapted to provide a law of control expressed as -V = k1 ## dt + k2# + k3d#/dt, where V is a measure of the compensating effect, # a measure of the deviation of the characteristic and k1, k2, k3 are constants. Substantially the condenser 7, the resistance 12, and the condenser 11 provide the terms respectively proportional to #, ##dt and d#/dt, and the values of resistance and capacity determine the constants. The system may be modified to omit certain terms from this law of control. An exponential factor providing temporarily exaggerated compensation to deal with sudden changes in the characteristic may be introduced by adding a second electrical system, Fig. 3 (Prov.), comprising a point L, to the potential of which the primary device M1 is responsive, connected on the one hand through a resistance 84 and condenser 85 to a point of constant potential, and on the other hand through a second condenser 83 to a point 81 at which the potential changes in correspondence with changes in the characteristic. The characteristic may be controlled according to any desired behaviour thereof by varying the potential at the point Q in the first electrical system in the manner desired. The potential at the point X may be varied additionally according to changes in the magnitude of any further causal characteristic or characteristics, variations in which may be expected to lead to variations in the value of the main characteristic, e.g. steam pressure in a steam heating-system. Where large time lags are involved in the operation of a control system and large condensers would be required, these may be replaced by an electrochemical polarization cell, such as a Daniell's Cell, fed with a constant current in a direction and for a time according with the actuation of the compensating means. The invention is described as applied to the control of temperature, the compensating means being a valve through which heating medium is supplied. As shown in Fig. 2 (Prov.) the primary device M1 is responsive to amplified temperature variations the thermometer operating an arm 20 controlling a reversible electric motor 32 actuating follow-up contacts and gearing 37 giving the arm M1 a movement of say 60‹ for the permissible range of temperature. The secondary device is a milliammeter 43 actuated in accordance with the adjustments of the valve motor VM through the electrical system referred to, which is added a pentode thermionic valve 44 whereby the changes of potential at the point P are converted into a current actuating the needle M2. Contacts 38, 39 and 42 cooperate to control selectively thermionic valves 48, 51 with time delayed grid circuits and these actuate a reversing switch 50 for the motor VM. This motor drives also a potentiometer resistance 5 calibrated to accord with the variations in compensating effect produced by the valve and the steam potential of the point X is applied through condenser 7, and resistances 8 to the junction between resistances 9, 12 which is normally maintained at a potential determined by a point 9 on a potentiometer 13. The potential of this junction is applied through resistance 9 to the grid of the pentode between which and the cathode, which is maintained at a fixed potential, is connected the condenser 11. Resistances 59, 60 may be variable by causal characteristics. An adjustable resistance 47 is in parallel with the milliammeter 43 in the anode circuit of the pentode. The contact device actuated by the primary and secondary devices M1, M2, may be of the four-point mercury-surface type described in Specification 440,115. Fig. 1 (Comp.) shows a modification wherein the primary and secondary devices are thermionic valves 140, 130 the anode potentials of which normally balance a polarized relay 150 adapted selectively to control the valves 160, 170 through time-delayed grid circuits and so operate a reversing switch for the valve motor VM.. The connection between the valve and the potentiometer S mav include a lost-motion device 4 to compensate any back-lash in the valve operating mechanism. The potentiometer 122 adjusted in accordance with temperature changes by the arm 120 may be replaced by a liquid resistance 124 .. 126. In a modification a thermocouple is connected in the grid circuit of a pentode which forms the first stage in a three-stage resistance-condenser coupled amplifier replacing the valve 140. Fig. 3 (Comp.) shows a polarization cell 191 adapted to replace the condenser 7 and the resistance 12. Two electrodes 192, 193 lie in a cuprous salt solution about a porous diaphragm and are associated with a steep-slope pentode 190 whereby amplified potentials derived from the cell are applied to the valve 130. This arrangement is adapted for control systems having large time lags owing to the higher leakage resistance represented by the gridcathode resistance of the pentode but may be adapted to shorter time-lags by a resistance in parallel therewith.