October 15, 2010, 1100 Webb

Hyungbo Shim received his B.S., M.S. and PhD. degrees from Seoul National University, Korea in 1993, 1995 and 2000, respectively all in electrical engineering. From 2000 to 2002 he was a post-doctoral researcher at the Center for Control Engineering and Computation, University of California, Santa Barbara. In 2002 he joined the faculty of Hanyang University, Seoul, Korea. From 2003 he has been with the School of Electrical Engineering at Seoul National University, Korea, where he is currently an Associate Professor. His research interests include analysis of control nonlinear systems with emphasis on observer design and output feedback stabilization. He is an Associate Editor for European Journal of Control and International Journal of Robust and Nonlinear Control, and has served as Vice IPC Chair for IFAC World Congress in 2008.

Linear disturbance observer (DaB) design has often been used in practice as a heuristic method for robust output-feedback control and disturbance rejection. Its benefits include that as an inner-loop controller it can also be added on the existing closed-loop system working in harmony with pre-designed (possibly non·robust) outer-loop controller, and that it robustifies the closed-loop system in a way that that uncertain plant under external disturbance behaves like a disturbance-free nominal plant. However, it has not been analyzed in a rigorous manner in its most general setting. Motivated by this, we utilize singular perturbation theory and derive necessary and sufficient condition for robust stability. This analysis also answers several long-standing questions from the practice and provide an extension for nonlinear systems. Finally it is shown that the DOB approach does not guarantee robust transient performance and then, a modification is presented to achieve it.