Fluid-pressure servoactuator

942,123. Fluid-pressure servomotor control systems. MOOG SERVOCONTROLS Inc. Sept. 17, 1962, No. 35292/62. Heading G3P. A hydraulic servopiston 12 is controlled by a spool valve 21 which is centred by springs 63, 64 and displaced by variations of pressure in chambers 42, 44 occasioned by movement of a flapper 50 relative to nozzles 35, 36. The flapper is carried by a flexure tube 54 and has a portion passing between permanent magnetic pole pieces 60 so that it is caused to be deflected by energization of a coil 62. The flapper is subject to negative feedback through a cam 65 movable by the servopiston and a follower 66 engaging a cage 68 containing springs 74, 75 and biased by a spring 85. In a modification, the cage 68 is omitted and the flapper is engaged on its upper side by a single, adjustable spring and on its lower side by a spring which is engaged by a cam follower carried by a leaf spring. Further modifications referred to include a flapper arrangement as in Specification 845, 109, the control of gaseous instead of hydraulic fluid by the flapper, and arrangements in which a mechanical feedback between the spool valve and flapper are provided, and in which the valve spool is subject to load reaction feedback which may act through a high frequency pass arrangement. A three-stage control valve arrangement is also referred to.