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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 fixed-structure disturbance observers, the disturbance observer design problem can be transformed into the synthesis problem of H/sub /spl infin// (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.