Accurate position control of a pneumatic actuator using on/off solenoid valves

The development of a fast, accurate, and inexpensive position-controlled pneumatic actuator that may be applied to a variety of practical positioning applications is described. A novel pulse width modulation (PWM) valve pulsing algorithm allows on/off solenoid valves to be used in place of costly servo valves. The open-loop characteristic is shown both theoretically and experimentally to be near symmetrical. A comparison of the open- and closed-loop responses of standard PWM techniques and that of the novel PWM technique shows that there has been a significant improvement in the control. A linear process model is obtained from experimental data using system identification. A proportional integral derivative controller with added friction compensation and position feedforward has been successfully implemented. A worst case steady-state accuracy of 0.21 mm was achieved with a rise time of 180 ms for step inputs from 0.11 to 64 mm. Following errors to 64-mm S-curve profiles were less than 2.0 mm. The controller is robust to a sixfold increase in the system mass. The actuator's overall performance is comparable to that achieved by other researchers using servo valves.