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.

Pneumatic actuators are low-cost, safe, clean, and exhibit a high power to weight ratio. In this paper a new modeling approach and control law for pneumatic servo actuators are presented. The nonlinear system model is developed using a combination of mechanistic and empirical methods. The use of novel bipolynomial functions to model the valve flow rates is shown to produce a more accurate solution than prior approaches. A novel multiple-input single-output nonlinear position control law is designed using the backstepping methodology. The stability analysis includes the effects of friction modeling error and valve modeling error. Experiments are conducted with 9.5-mm bore and 6.4-mm bore pneumatic cylinders, and four low-cost two-way proportional valves. In experiments with the 9.5-mm bore cylinder and a 1.5-kg moving mass, maximum tracking errors of plusmn0.5 mm for a 1-Hz sine wave trajectory, and steady-state errors within plusmn0.05 mm for an S-curve trajectory were achieved.

Many researchers have investigated pneumatic servo positioning systems due to their numerous advantages: inexpensive, clean, safe, and high ratio of power to weight. However, the compressibility of the working medium, air, and the inherent nonlinearity of the system continue to make achieving accurate position control a challenging problem. In this paper, two control algorithms are designed for the position tracking problem and their experimental performance is compared for a pneumatic cylinder actuator. The first algorithm is sliding-mode control based on a linearized plant model (SMCL) and the second is sliding-mode control based on a nonlinear plant model (SMCN). Extensive experiments using different payloads (1.9, 5.8, and 10.8 kg), vertical and horizontal movements, and move sizes from 3 to 250 mm were conducted. Averaged over 70 experiments with various operating conditions, the tracking error for SMCN was 18% less than with SMCL. For a 5.8-kg payload and a 0.5-Hz 70-mm amplitude, sine wave reference trajectory, the root-mean-square error with SMCN was less than 0.4 mm for both vertical and horizontal motions. This tracking control performance is better than those previously reported for similar systems.