A study on tracking position control of pneumatic actuators

A tracking position control method is proposed and experimentally evaluated for a linear positioning system. The positioning system is composed of a pneumatic actuator and a 5-port proportional valve. The proposed controller has an inner pressure control loop and an outer position control loop. A PID controller with feedback linearization is used in the pressure control loop to nullify the nonlinearity arising from the compressibility of air. The position controller is also a PID controller augmented with friction compensation using either neural network or nonlinear observer. In the former case, the nonlinearity due to friction is coded on the neural network in the training mode. During the operation, the neural network calculates the proper input voltage that counterbalances the inherent friction. This input voltage is used to cancel out the friction and to linearize the plant dynamics. In the latter case, the compensation method uses an observer to estimate the friction which is assumed to be proportional to the sign of the velocity. The positioning system can then be regarded as a linear one, provided that both nonlinearities in the inner loop and in the outer loop are completely compensated for. Experimental results indicate that the proposed controller significantly improves the tracking performance.