Positioning Control of Pneumatic Servo System with Pressure Control Loop Using Disturbance Observer

This paper describes one method to attain a robust positioning control of pneumatic driving system comprising a pneumatic cylinder and PCM digital control valves. A pneumatic driving system has become one of interests in the industrial fields, however, the control characteristics are inevitably affected by the friction force and parameter changes of plant due to air compressibility. The control characteristics of a pneumatic actuator depend on a pressure response. So if a pressure can follow the reference value which compensates the influences of the friction force and the parameter changes of plant, the friction force and the parameter changes of plant do not affect the control characteristics. A meter out speed control method, widely used in a pneumatic system, is employed. This method has a superior energy efficiency but gives a nonlinear pressure response. From these points of view, we proposed a positioning control system including an active controlled pressure loop. Two disturbance observers are employed in this control system. One is applied in a pressure control loop to improve the robustness against the influence of the piston speed which acts as a disturbance and to compensate the nonlinearity owing to the meter out method, the other is in kinetic part, from the pressure to the piston speed, to generate the compensating signal to counterbalance the friction force and the parameter changes of plant. Through some experiments about the robustness against the payload, the positioning accuracy and so on, the validity of this control method has been confirmed.