High-Accuracy Position Control of a Rotary Pneumatic Actuator

Rotary pneumatic actuators provide several advantages over electromechanical actuators (e.g., higher power-to-weight ratio, lower cost, and inherent safety) but are generally inferior in terms of accuracy and robustness when closed-loop position controlled. This paper presents the modeling, controller design, and experimental verification of a high-accuracy position-controlled rotary pneumatic actuator. A novel inverse valve model enables the development of a fast and precise inner-loop pressure control law. The outer-loop position control law combines feedback with model-based feedforward terms, including an adaptive friction compensator. The robust stability of the inner and outer subsystems is analyzed. Experimental results are included for rotating an arm in the vertical plane. The hardware features low-cost ON/OFF solenoid valves and low-cost pressure sensors. For a multiple cycloidal reference trajectory covering a 90 degrees range, the root-mean-square error (RMSE) averaged over five tests was 0.156 degrees. Steady-state errors less than or equal to 0.0045 degrees were achieved for these moves and for moves as small as 0.045 degrees. Robustness to unknown payloads was achieved using an improved payload estimator. For example, for a payload 53% less than nominal, the RMSE with the estimator was 75% less than without it. The experimental results are superior to those reported in the prior literature.