Robust Control of a Pneumatic Servo System Using Disturbance Observer

Abstract

Recently, a pneumatic servo system is expected to be applied to various automation systems, because it has some advantages such as high power/weight ratio, functions of impact absorption and rigorous force control owing to air compressibility. However, the compressibility makes it a high order and nonlinear system, so its exact modelling and parameter estimation are not easy. To cope with this problem, the development of the control scheme with the robustness for disturbances and parameter changes are required.
We apply a disturbance observer, which is one of methods to attain the robust control, to the position and force control of a pneumatic servo system comprising a bellofram type pneumatic cylinder and a PCM digital control valve. The content of this paper is summarized as follows.
1) To compare the robustness of some types of control systems, we propose a estimation function being the ratio between open loop transfer functions included in the sensitivity function. Referring to this estimation function, control parameters can be rationally adjusted to improve the robustness of control system.
2) Comparing dynamic responses of the control system using a disturbance observer with the usual PI control system, it is proved that a disturbance observer is more effective to lower the sensitivity to both disturbances and plant parameter changes.
3) To avoid the trade-off problem between the low sensitivity and the high stability in the conventional control system using a disturbance observer, we add a compensator in the feedback loop of control input. The improved control system can provide the sufficient stability without sacrificing the low sensitivity to parameter changes.
4) The robust control system using a disturbance observer can achieve the satisfactory position and force control. Its availability to a pneumatic servo system is confirmed.