Robust output regulation by observer-based feedforward control

In this paper, a new method for the design of disturbance and reference feedforward controllers with disturbance observers is presented for systems, where the disturbance input location is unknown. The resulting controller achieves asymptotic tracking in the presence of the disturbances despite plant parameter variations and changing disturbance input locations, and is therefore robust. It is shown that the new controller design method is dual to Davison's approach for the application of the internal model principle. However, different from the latter one, the new design procedure allows a simple and complete prevention of controller windup by driving the disturbance observer with the saturated input. Furthermore, a straightforward separate reference channel design is possible in order to improve the tracking behaviour if the types of the reference and the disturbance signals differ. A simple example demonstrates the advantages of the new design method.