Patent GB-458176-A (GB)
Title: Improvements in or relating to the automatic control of variable physical characteristics
IPC: G05D16/18
Filed: 1935-03-08
Abstract: 458,176. Automatic control systems. CALLENDER, A., and STEVENSON, A. B., Winnington Hall, Northwich, Cheshire, and IMPERIAL CHEMICAL INDUSTRIES, Ltd., Millbank, London. March 8, 1935, No. 7369. [Class 38 (iv)] Temperature, &c.-A system for the automatic control of a physical characteristic, wherein a reversible servomotor of any kind actuates correcting or compensating means as determined by a relay subject to the differential operation of means responsive to deviations of the characteristic from normal and means responsive to the action of the compensating means, is characterized by a coupling between the compensating means and the relay which comprises fluid within two or more containers and means displacing a wall of one of the containers in proportion to changes in the compensating effect, together with means tending gradually to restore the resulting pressure within another container to a standard value. The system is designed to produce a control law in which the compensating effect V is related to the deviation # according to the two equations k1, k2, k3, C, alpha and # being constants chosen according to the circumstances. It is however, adaptable to a simpler law in which #=Z or k3 = O. Fig. 1 shows such an arrangement. Tilting mercury switches in a relay 42 control a reversible electric motor operating a valve in a pipe supplying heating steam and are subject to the differential action of members 19 and 36 operated respectively in accordance with changes in the compensating effect, e.g. rate of steam flow, and in the controlled quantity, such as temperature. The coupling between the member 19 and the valve 2 comprises a cam T designed to produce displacements of a member R proportional to changes in the compensating effect. Fluid, such as oil, is enclosed in containers A, B and C. Container A has a movable wall constituted by the piston 4 actuated by the member R. It is connected directly to the container C, while container C is connected through a partly closed throttle valve 16 to a container B in which a standard pressure is maintained by a weighted piston 11. Throttle valves 17, 18 may be introduced between containers A and C for the purpose of introducing the third term in the equation defining the law of control. If the container A is not elastic (having a spring controlled wall 8) a compression tension spring is inserted in the couplings described. Back lash in the means actuating the steam valve is compensated by lost motion in the cam T with or without a friction device U. Changes in the controlled characteristic may be transmitted by fluid pressure proportional thereto to chamber 31 whereby a suitable pressure is applied to a piston in a container F which is directly connected to a container G. This is connected through a partly closed throttle valve to a container K having a spring-controlled wall 142. The members 19, 36 are respectively operated according to the pressures existing in containers C and G or these fluid pressures may themselves operate differentially upon a piston actuating the relay switches. The indicator of the controlled characteristic may directly operate upon the relay, either mechanically or by fluid pressure. The relay may be constituted according to Specification 440,115 or, as stated in the Provisional Specification, according to Specification 453,602. In order to anticipate changes which may occur in the controlled characteristic due to changes in a causal' quantity, such as steam pressure, a fluid pressure varying with the causal quantity may be applied to container A or a wall of this container is displaced in accordance with the magnitude of the causal property. The system of containers A, B, C may be double acting as shown in Fig. 4. Fig. 5 shows an arrangement in which two spring bellows AA, AB are secured centrally to a support and have their adjacent ends secured together ; their outer ends are jointly actuated by the member R. The containers AA, AB, are connected together through a throttle valve 161 and respectively to two further spring bellows CA, CB. Throttle valves 17A, 17B, 18A, 18B, are also arranged as shown. The movements of the common wall of containers CA, CB are communicated to the relay 42, which is coupled differentially to the member 361 of a system similar to that in Fig. 1, but also constructed of spring bellows. In a further modification, Fig. 6, the fluid pressure output from a double-acting system of containers is introduced through a throttle valve 88 to a spring bellows chamber 91 also subject to the fluid pressure output from a system of containers F, G, K; this spring bellows controls a four-point mercury contact system 92 according to Specification 440,115. The differential action of the members 19 and 36 or the differential fluid pressures may act upon a pilot valve system controlling a hydraulic servomotor. According to the Provisional Specification the compensating means may act upon the coupling by means of fluid pressure variations fed into the container A. A dash-pot was associated with the relay and the average size of each regulating step was adjustable. An arrangement was also suggested to obtain a law of control including the next derivative of the deviation #.