  The goal of this paper is to clarify the robustness and performance
constraints in the design of control systems based on disturbance observer
(DOB). Although the bandwidth constraints of a DOB have long been very
well-known by experiences and observations, they have not been formulated and
clearly reported yet. In this regard, the Bode and Poisson integral formulas
are utilized in the robustness analysis so that the bandwidth constraints of a
DOB are derived analytically. In this paper, it is shown that the bandwidth of
a DOB has upper and lower bounds to obtain good robustness if the plant has
non-minimum phase zero(s) and pole(s), respectively. Besides that the
performance of a system can be improved by using a higher-order disturbance
observer (HODOB); however, the robustness may deteriorate, and the bandwidth
constraints become more severe. New analysis and design methods, which provide
good robustness and predefined performance criteria, are proposed for the DOB
based robust control systems. The validity of the proposals are verified by
simulation results.
