GB779664A - Improvements in or relating to fluid pressure control mechanisms - Google Patents

Abstract

779,664. Fluid-pressure servomotor-control systems. ROLLS-ROYCE, Ltd. Sept. 2, 1955 [Sept. 9, 1954], No. 26207/54. Class 135. In a servomotor-control mechanism comprising a differential servo-piston 51 wherein a pressure fluid is supplied directly to the smallerarea end of the piston and through a restriction 59 to the larger-area end, the latter being connected via a duct 64 to a bleed valve 66, a diaphragm assembly 72, 73 subjected to a controlling pressure load and operating the bleed valve 66 so that the piston position is determined by. the controlling pressure load, and means, such as fluid admitted to a space 89b, temporarily loading the diaphragm assembly on movement of the piston 51 to an extent dependent on the displacement and rate of displacement of the piston, the load is arranged to decrease gradually and to oppose the load which disturbs the equilibrium of the diaphragm assembly to cause the piston displacement and hence restore the equilibrium conditions of the diaphragm assembly. A conduit 49, which conveys compressed air from the compressor of a gas-turbine engine to an air turbine driving a re-heat fuel-pump, not shown, embodies a throttle valve 50 actuated through a rack and pinion by the piston 51 working against a spring 54. The valve 66 is mounted on a, carrier 67 connected at one end to an evacuated capsule 70 and at the other end to diaphragms 72, 73 ; another bleed port 60 controlled by a solenoid-operated valve 63 is also provided. Chamber 75 between the diaphragms is open to the conduit 49 by way of a duct 76, whilst a chamber 77 above the upper diaphragm 73 is connected via a pipe 78 to the engine exhaust ; the pipe 78 contains a restrictor 79 and is connected with the duct 76 by a conduit 80 incorporating a restrictor 81 so that the pressure in chamber 77 is proportional to but greater than that in the pipe 78. One or both of the restrictors 79, 81 may be adjustable. The control will operate to maintain the ratio of the pressures in the chambers 75 and 77 at a value which is predetermined by the effective diaphragm areas. The capsule 70 has an effective area equal to that of diaphragm 72 and its resilient load is balanced by a spring 83. An annular structure 90 in the chamber 77 supports a pair of diaphragms 91, 92, two spaces 89a, 89b being formed therebetween by a fixed wall 93 with a central aperture ; the diaphragm centres are joined by a frame 94 having a portion 94a passing through the aperture and forming therewith a viscous restrictor joining the spaces 89a, 89b and carrying a nib 95 which bears on the diaphragm assembly 72, 73. The frame 94 carries an evacuated capsule 96. The space 89b is connected by a duct 98 to the interior of a bellows 99 loaded in compression by a main spring 100 and tied by a tension spring 102 to a lever 103 connected to the piston 51. The bellows 99 and spaces 89a, 89b are filled with liquid. Upon a pressure decrease in chamber 77, the diaphragm assembly 72, 73 moves to close the valve 66, which movement is opposed by a reduction in the volume of the space 89b, whereupon liquid flows through the viscous restrictor into the space 89a and compresses the capsule 96 so that in a short time the opposing force disappears. The valve 66 then closes and the piston 51 moves to open the throttle 50 and concomitantly rocks the lever 103 to reduce the load on the spring 102 and allows the main spring 100 to compress the bellows 99 and force liquid through duct 98 into the space 89b so applying a load to the diaphragm 92 and hence to the diaphragm assembly in the opposite sense to the unbalanced load due to the pressure decrease in the chamber 77. The liquid in space 89b leaks into the space 89a in a short time to equalize the pressure in these spaces and removes the resulting load from the diaphragm assembly 72, 73, the change of volume of the bellows being accommodated by the capsule 96. An increase in pressure in chamber 77 causes a reverse operation. Specification 694,322 is referred to.