Patent US 3,967,533

A fluid servo network having a servo fluid pressure responsive movable output member controlled by a variable area orifice the effective area of which is established by a movable servo valve which is actuated by lever means in response to a variable force input applied thereto. The lever means includes a main input lever responsive to the variable force input and an opposing variable feedback force which varies in response to the reaction of the output member to establish a force balance on the main input lever. The main input lever is attached to a roller loaded by a constant reference force and movable along a secondary lever mounted for pivotal movement about a fixed axis to vary the effective lever arm of the secondary lever which is attached to the servo valve. The input torque tending to close the servo valve is opposed by the torque derived from a characteristic orifice force unbalance acting through the secondary lever. The forces acting on the secondary lever are isolated from the main input lever thereby making the size and rate of the variable area orifice independent of the force balance relationship of the main input lever.