Patent GB-774745-A (GB)
Title: Improvements in or relating to servo systems
IPC: B64C11/42
Filed: 1955-04-25
Abstract: 774,745. Fluid-pressure servomotor-control systems. UNITED AIRCRAFT CORPORATION. April 25, 1955 [April 28, 1954], No. 11859/55. Class 135. In a servomotor system comprising an hydraulically-actuated power device continuously urged in one direction and having a working chamber 42, the device being connected to an element, such as the shaft 39 of an aircraft propeller pitch-changing mechanism, adapted to be maintained in a given position, means, for example a flow-control valve 46, connected with the chamber 42 for varying the pressure in the latter to operate the device, a first pressure regulating means, such as a valve 43, connected with the valve 46 and utilizing the pressure in the chamber 42 to establish a first fluid pressure a given amount above the pressure in the chamber 42, a second pressureregulating means, for example a valve 44, connected with valve 43 and utilizing the pressure in the chamber 42 to establish a second fluid pressure a given amount below the pressure in the chamber 42, means such as an electronic governor, not shown, and a solenoid 77 are provided for actuating valve 46 to selectively connect the valve 43 or 44 with the chamber 42 in response to variations of the shaft 39 from the given position. (a) Pump.-A pump unit 10 comprising a pair of gear pumps 14, 16 in tandem is actuated from the aircraft propeller driving gear 20 one pump 14 draining fluid from an atmospheric tank 12 and discharging it into a pressurized tank 15 from which it is sucked by the other pump 16 and discharged into a line 18; the latter is also connected to a similar pump unit 21 that is operated, when the propeller is stationary, by an electric motor 25 which may be automatically controlled by means, not shown. (b) Power device.-Downstream of a check valve 27 in the line 18 the fluid passes into two conduits 28 and 29, the former being connected to both a relief valve 30, the outlet from which opens into the tank 15, and to one chamber 41 of an axially movable cylinder 32 of the power device, the piston 40 of which may be stationary, as shown, or capable of reciprocation. The conduit 29 leads into a normally open shut-off valve 33 from which a line 34 conducts fluid to a solenoid-operated reversing valve 35. The power cylinder 32 which embodies a rack 37 that is engaged by a pinion 38 on the shaft 39 connected to the pitch-changing mechanism, not shown, has a greater effective area, double for example, in chamber 42 to the right of the piston 40 than in the other chamber 41. (c) Pressure-regulating valves.-The highpressure valve 43 comprises a valve member 47 with a through passage 52 and a loading spring 48 in a lower space 49 connected by a channel 50 with the chamber 42 so that the pressure therein and the spring force move the member 47 upwardly until passage 52 is aligned with a conduit 53 branching from line 34 whereupon pump pressure, which is always higher than the pressure in chamber 42 is admitted into an upper space 51 and thrusts the member 47 down into the position shown until the passage 52 is no longer in communication with conduit 53 and the pressure in space 51 will equal that in space 49 together with the pressure set up by the spring 48 so that space 51 provides a source of fluid at a pressure which is a fixed amount, for example 30 p.s.i., greater than that in the chamber 42; the fixed amount is equal to the pressure exerted by the spring. The space 51 is connected by a passage 54 to the top port of a centrifugal flow-control valve 45. The low-pressure valve 44, which is similar to valve 43 but inversely disposed, comprises a valve member 55, and upper and lower spaces 57 and 59, the former housing a spring 56 and being connected to the lowermost ports of valves 45 and 46 by lines 58 and 58a, and space 59 being open to the chamber 42 via channel 50; a conduit 60 opening into the valve leads to the tank 15. The arrangement is such that the pressure in space 57 will be a fixed amount, for example 30 p.s.i., less than that in the chamber 42, which amount is equal to the pressure exerted by the spring 56. (d) Flow-control valves.-The passage 54 leads through a normally open port in valve 45 and a channel 61 to the uppermost port of valve 46 whilst the intermediate ports of both valves are interconnected by a line 62 which opens by a conduit 63 with the chamber 42. The valve member 65 of the governor valve 45 is attached to a centrifugally-operated governormechanism 67 driven from the gear 20 and at normal propeller speeds the member is in the position shown whereby line 62 is closed and channel 61 is open. Should over-speeding occur the member 65 is moved upwardly and closes channel 61 and opens line 62 to enable fluid at the same pressure as that in chamber 42 to flow into line 58 to move the valve member 55 down and uncover drain conduit 60 so that pressure in the chamber 42 of the power device is reduced causing leftward movement of the cylinder 32 to increase the propeller pitch. The valve member 75 of valve 46 is fixed to the armature of a double solenoid 77 energized by signals from an electronic governor, not shown. Should the propeller rotate at a lower speed than that set the member 75 is forced down thereby interconnecting channel 61 and line 62 to increase the pressure in chamber 42 and hence cause rightward movement of cylinder 32 to decrease propeller pitch. Conversely if the propeller rotates at higher speed than that set the member 75 is lifted to interconnect lines 62 and 58a thereby lowering valve member 55 and opening line 58 to drain conduit 60 thereby lowering the pressure in chamber 42 and causing movement of the cylinder to increase the propeller pitch. (e) Initial operation.-With the engine inoperative the hydraulic system is supplied with fluid by pump unit 21 upon energizing the electric motor 25 whereupon fluid enters chamber 41 and since the other chamber 42 is empty the cylinder 32 will travel to the left to the maximum or some lesser pitch position, as determined by a stop 85. Fluid will also flow to the reversing valve 35 and high pressure valve 43 which is in the uppermost position due to the action of spring 48 alone so that fluid is admitted to passage 54 and across the governor valve 45 to the control valve 46 and since no signal is transmitted to the solenoid 77 one is imposed thereon so that the valve member 75 is urged down to admit fluid to the chamber 42 via channel 61 and line 62 to decrease the propeller pitch; to prevent a protrusion 87 on the cylinder 32 engaging an adjustable and retractable stop 88 another signal is imposed on the solenoid 77 to raise the valve member 75 to permit drain of the fluid from chamber 42 through the valves 46 and 44 and the conduit 60. The system may be bled and filled with fluid by supplying the solenoid with alternating signals. When the engine is operative the system is supplied by pump unit 10 and the other unit 21 is closed down whereupon the apparatus functions as described above. (f) Feathering.-Upon depressing a plunger 90 the valve member 91 of the shut-off valve 33 is thrust downwards closing the line 34 and opening the chamber 42 to the tank 15 via the line 62, and conduits 95 and 96. This flow to the tank 15 may be supplemented by transmitting a signal to the solenoid 77 and permitting drain from the line 62 through the valve 44 and conduit 60 so that the cylinder 32 will move to the maximum pitch position causing the propeller to be feathered. (g) Reversing pitch.-When the solenoid of the reversing valve 35 is energized fluid from line 34 is admitted to a pipe 104 and moves a sleeve 103 in a direction opposite to the power cylinder 32 and hence by means of a spring 105 and a lever 106 pivots the stop 88 to permit the protrusion 87 to pass thereby as the cylinder 32 is moved through the minimum pitch position to reverse the propeller pitch angle; the limit of such movement is defined by an adjustable stop 107. Pipe 104 is also connected to a cylinder 112 in which is a piston 113 that actuates a lever 114 to prevent actuation of the governor valve 45 whilst the propeller blades are being set at reverse pitch. Specification 692,119, (Group XXXIII], is referred to.