Method and apparatus for controllably positioning a hydraulic actuator

A system and method for controllably positioning a hydraulic actuator which is operably connected to a control surface includes at least one main control valve for controlling the hydraulic pressure supplied to the actuator in response to a control signal indicative of a predetermined actuator position and at least one parallel control valve for supplying additional hydraulic pressure to the actuator in response to an error signal indicative of the difference between the position of the actuator and the predetermined actuator position to thereby correct for any differences indicated by the error signal. By correcting for any differences indicated by the error signal, the errors in positioning the control surface can be decreased and the stiffness of the control surface can be increased. The control system also includes a sensor for monitoring the position of the actuator and a controller for generating the error signal in response to the difference between the position of the actuator and the predetermined actuator position. However, the at least one main control valve is independent of the sensor and the error signal such that the hydraulic pressure controllably supplied to the actuator by the at least one main control valve in response to the control signal is independent of the error signal. Thus, the stiffness of the control surface can by enhanced by the control system even though the size and weight of the at least one main control valve and the flow requirements of the hydraulic system are significantly reduced.