Patent GB-707642-A (GB)
Title: Improvements in or relating to servo-control systems
Inventor: ROBERTS DEREK AUBREY
IPC: F02D1/02
Filed: 1950-09-29
Abstract: 707,642. Gas-turbine plant ROLLS ROYCE, Ltd. Dec. 24, 1951 [Sept. 29, 1950], No. 23952/50. Class 110(2) [Also in Group XXIX] A servo-control system suitable for controlling the fuel supply to a jet-propulsion gas turbine engine comprises a pressure-operated device such as the piston and cylinder device 21 adapted to actuate a controlled element and loaded differentially by two fluid pressures the first being derived directly from a fluid pressure source and the second from a point between a flow metering means 40 and a restricted vent or relief-flow-restricting means connected in series to the fluid pressure source to convey liquid therefrom. According to the invention, means are provided for controlling the flow of servo-fluid through the flow-metering means so that it is substantially proportional to the square. root of the pressure drop across the relief-flowrestricting means. This flow may be controlled so as to be exactly proportional to the square root of the pressure drop or where the difference between the " first " and " second " fluid pressures is small in relation to their absolute values, the flow of servo-fluid is arranged to be approximately proportional to the square root of the pressure drop across the relief-flow-restricting means. In Fig. 1, the fuel burners 10 of the turbine are supplied through a pipe 11 from an engine driven swashplate pump 13, the swash-plate 17 of which is adjusted by the piston and cylinder device 21 under the pressures in the spaces 22, 23, and the load of a spring 24. The space 22 is supplied from the pump delivery. The space 23 is connected to a point between the flow-metering means 40 and a' relief-flow-restricting means in the form of a vent valve 28, or 34. The vent valve 28 controls the outlet from a pipe 29 having a branch 25 to the space 23 and forms part of a barometric pressure control unit 26. The valve 28 is carried on a lever 27 loaded by (1) an evacuated capsule 30 exposed to ambient atmospheric pressure, (2) by a plunger 31 exposed to pump delivery pressure and (3) by a spring 32. The vent valve 34 forms part of an over-speed control 33 and is actuated by a diaphragm 35 loaded by a tension spring 36 against the fluid pressure in a chamber 38. This pressure is generated centrifugally by radial passages 37 in the pump rotor 14 and is therefore related to engine speed. In a third form of control, the reliefflow-restricting means are controlled in accordance with the instantaneous flow of fuel to the engine and the instantaneous compressor delivery pressure as described in Specification 651,599. The flow metering means 40 comprises a restricted orifice 41 connected to the space 22 by a pipe 42 and to the pipe 29 through a valve 43 operated by a pair of diaphragms 45, 46. The diaphragm 45 is operated in the sense to close the valve 43 by the pressure drop across the orifice 41. The diaphragm 46 is operated in the opposite sense by the pressure drop across the relief-now-restricting means, the pressure downstream of such means being conveyed by a pipe 19a communicating with the pump suction 19. In a modification of the flow-metering means 40, Fig. 2, the valve 43 controlling the pressure drop across the restricted orifice 41 is actuated by a single diaphragm 51 subject to the pressure between the orifice 41 and the valve 43 and to the pressure between a pair of restrictors 49. 50 in a pipe 48 connecting the pipe 42 upstream of the orifice 41 with the pump suction 19. Specifications 577,016, [Group XXIX], also is referred to.