Design of Robustly Stable Disturbance Observers Based on Closed Loop Consideration Using H-Infinity Optimization and Its Applications to Motion Control Systems

In disturbance-observer-based control, the closed loop system consists of a main feedback control loop and an inner disturbance observer loop. This paper presents a design method for a disturbance observer to satisfy closed loop performance specifications, provided that the main feedback controller is known. Taking advantage of certain fixedstructure disturbance observers, the disturbance observer design problem can be transformed into the synthesis problem of H∞ (sub-)optimal static output feedback gain for an extended plant. The static output feedback gain, and therefore the disturbance observer, can be obtained by solving a series of convex optimization problems. Since the disturbance observer is designed based on closed loop consideration, the robust stability of the closed loop system is guaranteed.