This paper presents a novel disturbance observer (DOB) for overcoming the limitation of uniformly bounded stability in existing DOB-based control systems and to attain asymptotic stability. The DOB is derived by considering the residual disturbance remaining after disturbance decoupling is performed. It focuses on the elimination of the residual disturbance as well as minimization of the estimation error. By considering the system state and the residual disturbance simultaneously in a Lyapunov stability function, the proposed DOB cancels the residual disturbance term and attains asymptotic stability. In contrast, existing DOBs consider these factors separately in different Lyapunov functions. A basic novel DOB is initially derived for achieving asymptotic stability and is later combined with the existing DOB. This forms a novel extended DOB that achieves a better estimation rate and better performance with regard to DOB-based control than those of existing DOBs.
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