Patent GB-901582-A (GB)
Title: Improvements in or relating to servo-control mechanisms
IPC: F16K31/42
Filed: 1957-07-25
Abstract: 901,582. Hydraulic apparatus; fluid-pressure servomotor-control systems. ROBINSON & CO., A., and BELL, G. Oct. 24, 1958 [July 25, 1957], No. 23533/57. Classes 69 (2) and 135. A servo-control system for a member, such as a valve member 12, Fig. 1, controlling the flow of steam in a marine installation or a variable-pitch propeller, comprises a primary hydraulically - operated servo - mechanism 10 connected to, and located at a position local to, the member for actuation thereof, and a secondary air-operated servo-mechanism 14 responsive to a controlling-member 15 and itself controlling the primary servo-mechanism. The member 12 is connected to one end of a rod 11, on the other end of which is a piston 53 in a cylinder 21 of the primary servo-mechanism. The ends of the cylinder are connected by conduits 26, 31 to non-return valves 25, 30, in turn connected by conduits 24, 29, 38, 40 to a piston-and-cylinder device 27, 43. Both ends of the cylinder 27 are so connected to each valve 25, 30 that whenever the piston 43 moves in the upward direction, liquid flows through conduit 24, valve 25 and conduit 26 to the upper end of cylinder 21 to move the member 12 downwards. At the same time, pressure in conduit 40 opens valve 30 to permit the liquid from the lower end of cylinder 21 to pass to the lower end of cylinder 27. The piston 43 is connected to a rod 13 having on its other end a piston 42 in a cylinder 41 of the secondary airoperated servo-mechanism. The ends of the cylinder 41 are connected by conduits 47, 49 and valves 46, 48, 51, 52 to an air supply 56. The valves 46, 48 are operated by the controlling member 15 which, on movement in one direction or the other, connects one or other of the valves 46, 48 to exhaust; the air supplied by the other valve being supplied to the cylinder 41 to move the piston 42, and hence the piston 43 of the primary servo-mechanism, in one direction or the other. The valves 51, 52 provide means local to the cylinder 41 for controlling the movement of the piston. Adjustable needle valves 54, 55 may be provided in the conduits 24, 29 to control the rate of movement of the piston 53, and adjustable throttle devices may be provided in the air conduits 47, 49. In an alternative embodiment, the ends of the cylinder 41 of the air-operated servo-mechanism are connected by conduits 62, 63, Fig. 2, to chambers 73, 74 of a valve 64 having a third chamber 75 connected to an air supply 76. Valves 70 normally connect the chamber 75 with chambers 73, 74 so that both ends of the cylinder 41 are connected to the air supply and the piston 42 remains stationary. Each valve 70 is connected to a valve 67 which co-operates with a seating 66 on a hollow plunger 65 through which air can pass to exhaust 103. The plungers 65 are movable by a lever 79 pivotable about one end 80 by varying the flow of air from a supply 97 by a regulator 98 to a diaphragm 89 and cylinder 90, the diaphragm being held in a neutral position by a spring 94 connected to the diaphragm by links 85, 88; one or more additional diaphragm devices may be provided to effect a firm control. Movement of the diaphragm moves links 82, 84 to rock the lever 79, say, clockwise, to close valve 67 in chamber 73, to open valve 67 in chamber 72, and to close the left-hand valve 70 in chamber 75 so as to supply air to the right-hand end of cylinder 41, the opposite end of the cylinder being connected by the open valve 67 in chamber 72 to the exhaust 103 in the plunger associated with chamber 72. The movement of the piston 42 imparts movement to the piston 53 of the primary hydraulic servo-mechanism as before. The piston in the second embodiment, however, is connected by links 100-102 to the lever 79 to impart a feedback characteristic to the operation of the valve 64. The lever 79 is provided with a handle 106 for manual operation of the system local to the member to be controlled. The piston-andcylinder devices in both the primary and secondary servo-mechanism may be replaced with diaphragm devices. The servo-control systems may be employed for controlling a screw-down valve in which a member 214, Fig. 3, is movable to control the flow through the valve by a hand wheel 213 or a piston 215, a sleeve carrying the piston being internally threaded to receive the valve spindle 212 which has the member 214 at one end and the hand wheel on the other and is releasably secured to the sleeve by a cotter pin 218; a scale 217 and collar 216 give an indication of the position of the piston relative to its cylinder 210. The piston-and-cylinder 215, 210, which correspond to the piston-and-cylinder 53, 21 in the servo-control systems described, are connected by bores 219, 220 to a piston-and-cylinder 225, 230, corresponding to piston-and-cylinder 27, 43, the piston 230 being connected by a rod 231 to a piston 229, corresponding to piston 42. Bores 232, 233 along one side of the cylinder 225 connect to a throttle valve and a non-return valve, corresponding to valve 25, respectively, and lead to bore 219 or 220 in cylinder 210, and bores diametrically opposite the bores 232, 233 lead to a non-return valve, corresponding to valve 30, and a check-valve, respectively leading to the other of bores 219 or 220. Additional bores 250, 251, which may be provided with oil nipples, are connected to cylinder 210 to permit the removal of air from the system and the making-up of any leakage of oil that may occur; similarly, the cylinder 225 is provided with additional bores for the same purpose. Expansion valves in the form of spring-loaded plungers may be connected to the conduits to lift if the pressure in the system rises above a predetermined value because of temperature increases expanding the oil. When used in a high-temperature system, the piston-and-cylinder 210, 215 may be displaced from the valve to reduce heat transmission therefrom.