Impedance Control of a Pneumatic Servo System with Adaptive Control Method

Abstract

One of the typical features of a pneumatic servo is a relatively high compliance due to air compressibility. This feature may be useful for constrained tasks such as deburring, polishing, and assisting humans, in which the relationship between position and force is important. If this relationship of a pneumatic servo becomes actively controllable, it can be effectively applied to these tasks. In order to control this relationship, an impedance control concept has recently been proposed. The impedance of the overall control system depends not only on the manipulator but also on the manipulated object of which the characteristics are usually unknown. Therefore, to attain the desired impedance over extensive operating conditions, an adaptive control strategy is required. This paper proposes an impedance control method of a pneumatic servo, using a position based approach, where an adaptive position control system is constructed inside the force feedback loop. The proposed method is applied to an experimental pneumatic servo system comprised of a pneumatic cylinder, electro-pneumatic proportional control valves, and a spring object. From the experiments, the following has been verified: 1) both static stiffness and dynamic impedance of the pneumatic servo system can be independently regulated by setting a desired reference model; 2) the impedance can be held constant with changes in system parameter such as object stiffness; and 3) the instability problem for the low stiffness setting can be overcome by setting high damping in the reference model. The proposed impedance control method may prove to be effective for both improving a pneumatic servo and developing its new applications.