812,711. Fluid-pressure servomotor control systems. AUTOMOTIVE PRODUCTS CO. Ltd. May 17, 1957 [May 17, 1956], No. 15441/56. Class 135. A servomotor system with follow-up gear for providing correspondence between movement of input and output members has a main control valve member 10 for the piston and cylinder power device 2, 3 and a rotary pilot valve member 12 mounted in an extension 91 of the main valve member and controlling the movement of the latter. As shown in Fig. 1, the piston 2 is fixed at one end 4 and the cylinder 3 is movable to actuate the controlled member which may be a flying control surface of an aircraft, through a transmission member 9. The member 9 is connected to the moving cylinder by a pin 7 on which is also mounted a manual control member 15 having a link 14 connected to an arm 13 for rotating the pilot valve member 12. Liquid under pressure is supplied to a port 73 from which it has access through ports 68, 84, 41, 38 to one side of the piston which is of smaller area than the other side. The larger area of the piston is subject to the pressure in a chamber 37 which is connected by passages 30, 29 with a reduced part of the control valve member 10. The arrangement is such that movement of the member 10 to the left places the chamber 37 in communication with the pressure liquid supply whilst movement of the member 10 to the right connects the chamber 37 with an exhaust 70. The valve member 10 is slidable at its left-hand end over a fixed plug 85 and at its right-hand end over fixed plug 95 of larger area than the plug 85. The pilot valve member 12 has a neutral position from which it may be turned in either direction to place a chamber 94 in communication either with the pressure liquid supply through a passage 81 in the member 10 or with exhaust through passages 112, 50, 49, 70, the member 12 being provided with a diametrical bore and diametrically opposite flats (not shown) and a surrounding, relatively fixed, sleeve 102 having axially spaced sets of ports, of which one set 107 is shown, for this purpose. Each set of ports in the sleeve 102 communicates with an annular groove of which the one, 104, communicating with the passage 81 is shown. The other two connect respectively with the chamber 94 and the passages 112. Movement of the lever 15 in an anticlockwise direction causes anticlockwise movement of the valve member 12 to connect the chamber 94 to the pressure liquid supply whereby the valve member 10 is caused to move to the left to pressurize the chamber 37 to cause the cylinder 3 to move to the right. Such movement causes rightward movement of the link 14 to return the valve member 12 to its neutral position and thereafter to move beyond its neutral position in the opposite direction to vent the chamber 94 whereby the valve member 10 is moved back to its neutral position. This latter movement of the valve member 10 also causes the valve member 12 to be returned to its neutral position. Fig. 6 illustrates a modification in which pressure fluid is supplied through a passage 141 and the rotary pilot valve member 12 is of simplified form, having an arcuate groove whereby a passage 109 leading to the chamber 94 may be connected either with the pressure supply through a passage 81 or with exhaust through a passage 111 depending upon its direction of movement. The input to the valve member 12 may be through an electrical motor device 143 connected to an automatic pilot or from a manual input member 15 as in Fig. 1, the manual input being connected to the casing of the device 143. The device 143 is of a kind having a rotor which is held in a predetermined position by an electric current of a certain magnitude and moved angularly in one direction by an increase of current and in the other direction by a reduction of current. Alternatively it may be of a kind which is resiliently biased to a central position and moved in either direction by electric currents of opposite senses. The device 143 is pivotally mounted about the axis of the valve member 12 and its rotor is coupled to the member 12. In a further modification, electrical inputs only are catered for, the servomotor being of the kind in which the cylinder is fixed and the piston is movable and the followup obtained by rheostats actuated by the main valve member and by the piston of the servomotor and feeding back into means which control the device 143. Modifications are referred. to consisting of hydraulic or pneumatic actuation of the pilot valve member, the provision of a pilot valve member of " open centre " type in which both ends of the main valve member are normally connected to the exhaust and movement of the pilot valve member from its mean position closes the connection between the exhaust and one or other end, and the employment of other than differential pistons for actuating the main valve member and the servomotor.