QFT design of a PI controller with dynamic pressure feedback for positioning a pneumatic actuator

Quantitative feedback theory (QFT) is applied towards the design of a simple and an effective position controller for a typical low-cost industrial pneumatic actuator with a 5-port three-way control valve, that is subject to disturbing forces. A simple fixed-gain proportional-integral control law with dynamic pressure feedback is synthesized to guarantee the satisfaction of a priori specified closed-loop performance requirements, including robust stability, tracking performance and disturbance attenuation, despite the presence of nonlinearities and parametric uncertainty in the pneumatic functions. A novel outer-inner design approach is proposed to avoid the synthesis of an unnecessarily complex outer loop controller. The merits of the inner loop feedback are examined from the perspective of system responses to step changes in the reference position and step changes in the disturbing force. Simulation results show clearly that the inner loop feedback improves the closed-loop disturbance response by eliminating oscillation and reducing the overshoot. The main contribution of this paper is the presentation of a systematic approach to the design of position controllers for pneumatic servos with dynamic pressure feedback, within the framework of QFT.