Experimental comparison of two pneumatic servo position control algorithms

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 non-linearity of the system continue to make achieving accurate position control a challenging problem. In this paper two control algorithms are designed for the pneumatic servo problem and their experimental performance is compared. The first algorithm uses position plus velocity plus acceleration feedback combined with feedfoward and deadzone compensation (PVA+FF+DZC). The second algorithm is a form of sliding-mode control (SMC). 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 SMC was 59% less than with PVA+FF+DZC. For a 5.8 kg payload and a 0.5 Hz, 70 mm amplitude, sine wave reference trajectory the root mean square error with SMC 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.