A New Stable Compensator Design for Exact and Approximate Loop Transfer Recovery

In this paper, a new compensator structure for loop transfer recovery (LTR) is proposed. The proposed compensator is (a) open-loop stable, (b) guarantees closed-loop stability and above all (c) requires much smaller values of gain than the conventional observer based controller for the same degree of loop-transfer recovery. The fact that the new compensator requires much smaller values of gain than the conventional controller results in several practical advantages, the most important among them being the reduction in controller band-width and freedom from the owes of control saturation. The trade-off between the value of gain and the degree of loop transfer recovery as well as the bounds on singular values of sensitivity and complementary sensitivity functions is shown clearly. Two numerical examples illustrate the advantages of the new compensator structure. The new compensator structure is inspired by a careful and clear understanding of how loop transfer recovery occurs when conventional observer based controllers are used. An elaborated version of the paper will appear in `Automatica´.