Patent GB-865418-A (GB)
Title: Improvements in or relating to servo-mechanisms
IPC: F15B15/26
Filed: 1956-04-19
Abstract: 865,418. Fluid-pressure servomotor control systems; hydrodynamic couplings. ROBINSON & CO. Ltd., A., and HEARN, J. July 18, 1957 [April 19, 19561. No. 11891/56. Classes 69 (2) and 135. [Also in Groups XXVII, XXVIII and XXXVIII] A servo-mechanism 11, Fig. 1, for moving a control-member, e.g. the speed governor control of an internal-combustion engine and/or the control scoop of an hydraulic coupling in its transmission to any desired position between two terminal positions comprises a piston 10 connected to the control member 18, the opposite sides of the piston normally being both connected individually through pipes 19, 20 containing valves 21, 22, to the pressure fluid supply pipe 27, one or other of the valves 21, 22, being operated to exhaust the space on one side of the piston 10 to permit the piston to be moved by the pressure on the other side. Dash-pot members 16, 17 are provided to clamp the piston movement. The piston 10 comprises two piston members 34, 35 capable of slight movement relative to the piston-rod 12 between end-stops 36, 38 and a central stop 37, the members being urged apart by springs 42 and having situated therebetween a friction member 44. When pressure-fluid, e.g. compressed air, acts on both sides of the piston, the members 34, 35, are urged together to force the friction member 44 against the cylinder wall to hold the piston stationary. On release of the pressure on one side of the piston, the members 34, 35 are moved slightly further apart by the springs 42 to release the friction member 44 and permit free movement of the piston. In Fig. 3 a control circuit for a marine internal-combustion engine propulsion unit comprises two servo-mechanisms 11b, 11a, of the construction shown in Fig. 1, controlling respectively the scoop of an hydraulic coupling in the transmission and a speed governor, the servo-mechanisms being arranged to operate in sequence so that when it is desired to increase speed the mechanism 11b moves the scoop control member 13b to its extreme right position before the mechanism 1 la is operated to move the governor control and vice versa when the speed is decreased. The compressed air supply pipes to the right-hand sides of both servo-pistons contain control-valves 51a, 51b controlled by a change-over valve 54 and the supply pipes to the left-hand sides of the pistons contain valves 52a, 52b, controlled by a changeover valve 64, the valves 51a, 51b, 52a, 52b, normally being in a position permitting supply of pressure fluid to both sides of the pistons. To increase speed a control button 62 is depressed to supply -pressure fluid from pipe 27 through valve 54 to move valve 51b to a position in which the right-hand side of cylinder 11b is exhausted, whereupon the servo-piston is actuated to move the scoop control-member 13b to the right. At the end of its stroke an abutment 56 operates valve 54 to direct pressurefluid to pipe 51a which is operated to connect the right-hand side of cylinder 11a to exhaust, whereupon the servo-piston is moved to move the governor control-member 13a in the increased speed direction. On release of the button 62 the valves 51a, 51b, return to their normal positions in which pressure fluid is supplied to both sides of the servo pistons so that they are held locked in the positions attained at the time of release of button 62. To decrease speed, button 65 is depressed which first operates valve 52a to cause servo-mechanism to close the throttle valve, an abutment 66 on member 13a then operating changeover valve 64 to cause valve 52b to set servomotor 11b to return the scoop control member. The pressure supply lines to both sides of the servo pistons contain valves 19a, 19b, 20a, 20b, which enable rapid exhaust of the pressure fluid. In a modification, Fig. 4 (not shown), the pressureoperated control-valves 51a, 51b, 52a, 52b and change-over valves 54, 64, are replaced by electro-pneumatically operated control valves and limit switches. To provide an indication of the position of a servo piston in its cylinder a rack 101, Fig. 5, connected to the plate 13 attached to the servo piston actuates a pressure-reducing valve 103 in a pressure fluid line 27 leading to a pressure gauge 104 having a bleed 106 to atmosphere, the reading on the gauge giving an indication of the piston position. To avoid the reading on the gauge lagging behind the piston position when the piston moves to the left and reduces the pressure supplied to the gauge, a subsidiary bleed passage 108 containing a non- return valve 109 is connected between the gauge line and the pressure fluid supply line 110 to the left-hand side of cylinder 11 to ensure a sufficiently rapid drop in pressure in the gauge line when the line 110 is connected to exhaust. In Fig. 6 the servo-mechanism 11 is controlled by a telegraph handle 112 and actuates a controlmember 111 which, through rack-and-pinion gears 132 ... 135, controls a throttle valve or hydraulic coupling scoop. A cam 126 on the member 111 also actuates valves 127 ... 129 to set a gear change lever 121 into the ahead, neutral or astern positions. Clockwise movement of the handle 112 causes temporary closing of contacts 115 by a follow-up cam 113, whereby relay 117 is energized to operate an electropneumatically-operated control valve 68 which connects the right-hand side of cylinder 11 to exhaust producing movement of the frame 111 to the right, whereby valve 127 is actuated to set gear lever 121 to " ahead " and the rack 132 engages pinion 134 to increase the throttle speed. Simultaneously a contact 118 is moved along a centre-tapped resistor associated with a repeater circuit 120 whereby the cam 113 is made to follow up the movement of handle 112 until the contacts 115 are broken, when the relay 117 is de-energized and the fluid supply to both sides of the servo piston is resumed. Similarly on movement of handle 112 in the anticlockwise direction contacts 114 are closed to energize relay 116 which causes the servo piston to move frame 111 to the left until the contacts 114 are reopened, the speed being reduced. When the handle 112 is in " neutral " position, neither of racks 132, 133, is in engagement with the pinions 134, 135, but movement of the lever 112 in either direction from neutral causes one or other of the racks 132, 133 to rotate the governor or scoop controls in a direction to increase speed, the cam 126 simultaneously moving to actuate either valve 127 or 129 to move the gear lever from " neutral " to " ahead " or " astern." The gear lever 121 is connected to a piston in a cylinder 123 which is connected to a second piston 124 in a stationary cylinder 125. Actuation of the " astern " valve 129 moves both pistons to the left of their cylinders 123, 125 to hold the gear lever in position 121. Actuation of the " neutral " valve 128 moves piston 122 to the right of cylinder 123 to move the gear lever to position 121a and actuation of the " ahead " valve 127 moves both pistons to the right of their cylinders 125, 123, to move the gear lever to position 121b.