Summary. The efficient solution of several classes of controller approximation problems by using frequency-weighted balancing related model reduction approaches is considered. For certain categories of performance and stability enforcing frequencyweights, the computation of the frequency-weighted controllability and observability Gramians can be achieved by solving reduced order Lyapunov equations. All discussed approaches can be used in conjunction with square-root and balancing-free accuracy enhancing techniques. For a selected class of methods robust numerical software is available.
IntroductionThe design of low order controllers for high order plants is a challenging problem both theoretically as well as from a computational point of view. The advanced controller design methods like the LQG/LTR loop-shaping, H ∞ -synthesis, µ and linear matrix inequalities based synthesis methods produce typically controllers with orders comparable with the order of the plant. Therefore, the orders of these controllers tend often to be too high for practical use, where simple controllers are preferred over complex ones. To allow the practical applicability of advanced controller design methods for high order systems, the model reduction methods capable to address controller reduction problems are of primary importance. Comprehensive presentations of controller reduction methods and the reasons behind different approaches can be found in the textbook [ZDG96] and in the monograph [OA00].The goal of controller reduction is to determine a low order controller starting from a high order one to ensure that the closed loop system formed from the original (high order) plant and low order controller behaves like the original plant with the original high order controller. Thus a basic requirement for controller reduction is preserving the closed-loop stability and many controller