Patent GB-479836-A (GB)
Title: Improvements in or relating to the automatic control of temperature
IPC: G05B11/01
Filed: 1936-06-05
Abstract: 479,836. Automatic control systems. CALLENDER, A., STEVENSON, A. B., ALCOCK, T. C., and IMPERIAL CHEMICAL INDUSTRIES. Ltd. June 5, 1936, Nos. 15844/36 and 12300/37. [Class 38 (iv)] Temperature.-In a system comprising a resistance thermometer 1, Fig. 1, in a Wheatstone bridge and a reversible servomotor 13 governed by a relay 7 responsive to unbalance of the bridge and correcting temperature variations a property of a resistance 3 in the bridge is controlled by a coupling from the means 16 by which a temperature change is compensated so as to be temporarily changed in proportion to the change in compensating effect, the property then tending to a predetermined value corresponding to the desired temperature ; this coupling may also change the property of the resistance in proportion to the rate of change of temperature. The property may also be changed independently by variation of a quantity, such as steam pressure in a heating system, which would cause a change in temperature. The resistance may also be varied in accordance with any desired behaviour of the temperature. As shown in Fig. 1, a cam 51, providing the proportionality, varies the level of liquid in a container 54 according to the change in compensating effect ; a float 55 adjusts the resistance 3 and a capillary connection 57 to a large-capacity container gradually restores the level and resistance to normal. Specification 440,115 is referred to. Fig. 2 shows a hydraulic system comprising spring bellows 61, 62, 66, a constant pressure container 67 and throttling valves 63, 64, 65 as in Specification 468,176 adapted to alter the resistance 3 in the manner set forth. A mechanical method is shown in Fig. 3. The cam 51 acts upon a lever 74 directly, or through a spring 71 against a dashpot 73 if the rate of change effect is required ; the lever 74 turns on a pivot supported by an adjustable dashpot 75 and the other end adjusts the resistance 3 and is constrained by one or two springs 78, 79 to return to its normal position after displacement. Fig. 4 includes an electrical method of restoring the setting of resistance 3. The cam 51, an induction disc 88, and a shaft 90 are interconnected through a differential 81. The shaft controls the resistance 3 and also the energization of the disc 88, by displacing a contact on a potentiometer 83 ; the potentiometer is bodily adjustable to determine the temperature setting. Fig. 5 shows means whereby an auxiliary current in the resistance 3 is varied in accordance with the change in temperature compensation. The voltage of the grid P of a valve 4 wording on the straight part .of its characteristic is normally determined by a battery 25 and is temporarily changed, according to the change in compensating effect caused when the relay 7 adjusts the compensating means 16, by movement of a potentiometer contact X connected to the grid through a, condenser 22 ; means 17 provide an equivalent backlash to annul the backash in the compensating means and the potentiometer 21 is graded to ensure changes proportional to those in the compensating effect as in Specification 453,602. Resistances may be inserted between the resistance 23 and the grid P and condenser 22 respectively together with a condenser between the grid and cathode to add a component in accordance with the rate of change of compensation. Fig. 7 shows a relay arrangement in which two pentodes normally produce no current in opposed relay windings 35, 36, their grid circuits being reversely connected respectively across the conjugate points a, b of the bridge and suitably biassed. If the arrangements of Figs. 5 and 7 are combined a condenser 45 connected as shown will provide the rate of change factor. D.C. for the bridge may be obtained by winding 41 and rectifier 42. Specification 479,836 also is referred to.