Method and apparatus for controllably positioning a hydraulic actuator

A system for controllably positioning a hydraulic actuator operatively connected to a control surface subjected to external forces up to a predetermined maximum force, the system comprising:

• hydraulic supply means for supplying hydraulic pressure to the actuator to thereby position the actuator;
• main control valve means, in fluid communication with said hydraulic supply means, for creating a fixed restriction to a flow of hydraulic fluid to the actuator to thereby control the position of the actuator in response to a control signal indicative of a predetermined actuator position, wherein said main control valve means comprises at least one main control valve having a predetermined size which is selected to controllably provide sufficient hydraulic pressure to the actuator to position the control surface while said control surface is subjected to said predetermined maximum force;
• sensor means for monitoring the position of the actuator to generate actuator position signals indicative of the position of the actuator;
• a controller, responsive to the actuator position signals generated by said sensor means, for generating an error signal in response to a difference between the position of the actuator and the predetermined actuator position;
• and
• parallel control valve means, responsive to said controller and in fluid communication with said hydraulic supply means, and positioned in a parallel relationship to said main control valve means, for supplying additional hydraulic pressure to the actuator in response to the error signal to correct for differences between the position of the actuator as monitored by said sensor means and the predetermined actuator position specified by the control signal to thereby increase the stiffness of the hydraulic actuator, wherein said parallel control valve means comprises at least one parallel control valve for controllably providing sufficient hydraulic pressure to the actuator to provide the predetermined stiffness to the control surface while said control surface is subjected to the predetermined maximum force,
• wherein said main control valve means is independent of said sensor means and the error signal generated by said controller such that the fixed restriction created by said main control valve means in response to a control signal indicative of a predetermined actuator position is independent of the error signal generated by said controller in response to the difference between the position of the actuator and the predetermined actuator position.