In this note, a new H-infinity performance preserving controller order reduction method is proposed, Here performance preservation indicates that the H-infinity norm bound of the closed loop transfer function with reduced-order controller is not greater than the H-infinity norm bound of the closed loop transfer function with full order controller, We assume additive perturbations to the closed-loop transfer function and obtain a sufficient condition for performance preservation. Two kinds of useful weightings are derived, and the controller reduction problem is solved via a frequency weighted model reduction problem.
Wang, G., Sreeram, V., & Liu, W. Q. (2001). Performance Preserving Controller Reduction via Additive Perturbation of the Closed-Loop Transfer Function. IEEE Transactions on Automatic Control, 46(5), 771-775. https://doi.org/10.1109/9.920799