Considerations on disturbance suppressions of sliding mode control with disturbance observer and two-degree-of-freedom control

In the field of robot manipulator control, one of the important issues is robustness of the servo-system. The robustness of a servo-system applied to each joint of a manipulator enables non-interferences among other joints. Several papers have been published concerning this issue(1)_??_(6).Particularly, the authors have proposed such a robust-servo-system, based on a combination of disturbance observer and sliding mode control(1)_??_(3). This control law has an advantage in the robustness with the less chattering even at the relatively low sampling frequency. The introduction of the disturbance observer improves the robustness of conventional sliding mode control.On the other hand, two-degree-of freedom control is derived from stability and achievable transfer characteristics of a linear system(7). As a result, it includes equivalently various linear control systems such as a disturbance observer(4) (8). The disturbance observer is very robust servo-system against a disturbance and a fluctuation of plant parameters. And the effectiveness has confirmed in some applications.In this paper, from the view point of disturbance reduction, a sliding mode control with disturbance observer is compared with two-degree-of-freedom control which includes an equivalent system to the disturbance observer. The comparison of these control schemes makes clear that the introduction of non-linear control (sliding mode) improves the disturbance suppression characteristics of a linear system with a disturbance observer or two-degree-of-freedom control system.In the simulations and experiments, both control methods were applied to DC servo motor positioning control, and it was proved that the proposed non-linear control has an advantage over the two-degree-of-freedom control in the feature of disturbance reduction.