ABSTRACT
One simple and effective robust control technique is disturbance observation-cancellation. It is necessary to have modes of disturbance in the controller for proper regulation. The low-order disturbance observer estimates an equivalent disturbance only several steps ahead. The estimated disturbance added to input cancels out the disturbance. This function is the same as feed-forward compensation and improves the transient performance to the disturbance, as well as, the steady-state operation of the plant. To regard disturbance as a state variable, the disturbance should have certain dynamics. The disturbance observation and cancellation technique is widely used in industrial controllers including position servo controller, video head controller, industrial robots, pneumatic servo controller and automobile active controller. The main reason is that higher performance is achieved by a simple and inexpensive controller. Since the equivalent disturbance includes parameters variation, the entire controller is expected to be robust against not only the disturbance but also the parameters variation.
