826,215. Fluid-pressure servomotor-control systems. GENERAL ELECTRIC CO. Aug. 28, 1957 [Aug. 30, 1956], No. 27098/57. Class 135. [Also in Group XXXV] A combined manual and automatic servocontrol system comprises a power drive, power drive control servomotor, manual input, and automatic signal input to control the servomotor, the arrangement being such that when the system is operating automatically the manual input member is locked with respect to the power drive output member and the power drive control is placed under the control of the servomotor as energized by the automatic signal input, while when the system is under manual control the servomotor may be either locked to place the power drive control under the exclusive control of the manual input member, or may be operative to superimpose an automatic control input on the manual input. In the aircraft control system shown, an elevator 10, or other control surface, is operated through a link 12 by a power drive 16 comprising a fixed piston 20 and moving cylinder 18, with a control valve 32 operable under automatic control conditions by an hydraulic servomotor or amplifier 42, and under non-automatic conditions by the pilot's control lever 24. The amplifier 42 includes a pair of valves 61, 54 operated by solenoids 63, 59 energized by signals from an autopilot 28 and " transient " damper 26. Valve 61 controls the admission of pressure liquid to valve 54, which in turn controls the operation of a power piston 48 in a cylinder 50, the end 44 of the piston-rod being connected to the piston valve 32. A pipe 95 leads from the passage between valves 61, 54 to a cylinder 87, Fig. 2, incorporating a spring-loaded tapered plunger 85 by which piston 48 may be centred and locked with respect to cylinder 50. With pressure established in pipe 95 plunger 85 is held retracted to allow free relative movement of the piston and cylinder, but should the pressure fail plunger 85 engages rod 48 and locks it to cylinder 50. Cylinder 50 may also be locked to, or permitted a degree of lost motion in relation to, the housing 52 of amplifier 42, which is secured to power cylinder 18, the housing carrying opposed pistons 99, 101, Fig. 4, engageable with a lug 105 attached to cylinder 50. Admission of pressure liquid to the pistons is controlled by a valve 109 operated by a solenoid 111. The system may operate in the following modes:- (a) Direct manual.-In the event of hydraulic power failure control surface 10 may be operated directly from the pilot's control lever 24 through link 82, cylinder 18 and link 12, a bypass 117 between the supply passages of cylinder 18, normally obstructed by a spring-loaded obturator 119, being opened to permit free transfer of liquid between the cylinder ends. (b) Power-assisted manual.-Under these conditions the contacts of a switch 69 are moved to position " 1," solenoids 63, 111 being inoperative so that cylinder 50 is locked to rod 48 by plunger 85, but is free to move relatively to housing 52. Movement of pilot's lever 24 is thus communicated directly from member 82 to piston valve 32 to operate power drive 16. The system will function in this manner in the event of electrical power failure. (c) Damped power-assisted manual.-Switch 69 is moved to position " 2," energizing damper 26, input amplifier 53 and solenoid 63, thus retracting plunger 85 and releasing piston 48 from cylinder 50. Movements of pilot's lever 24 are, in effect, transmitted directly to valve 32, but damper 26 may superimpose signals on valve 54 to effect corrective movements of piston 48 to compensate for wind gusts, or other transient disturbances. Under these conditions a potentiometer 71 furnishes a feedback signal indicating the position of piston 48. (d) Fully automatic.-Switch 69 is moved to position " 3 " energizing damper 26, autopilot 28, input amplifier 53 and solenoids 63, 111. Amplifier 42 operates as in condition (c) but in addition responds to signals from autopilot 28. Cylinder 50 being locked to housing 42, all movements of control surface 10 and power drive 16 are directly transmitted to pilot's lever 24, which moves in correspondence. Under all conditions of operation the pilot may exercise over-riding control by exerting sufficient force on the control lever to overcome the limited force exerted by pistons 99, 101.