  This paper proposes new practical design tools for the robust motion control
systems based on disturbance observer (DOB). Although DOB has long been used in
several motion control applications, it has insufficient analysis and design
tools. The paper proposes a new practical robustness constraint, which improve
the robustness at high frequencies, on the bandwidth of a DOB and nominal
inertia. Although increasing the bandwidth of a DOB and nominal inertia
improves the performance and stability, they are limited by the robustness
constraint. Besides, a novel stability analysis method is proposed for reaction
force observer (RFOB) based robust force control systems. It is shown that not
only the performance, but also the stability changes significantly by the
imperfect identification of inertia and torque coefficient. The robustness and
stability of a DOB based motion control system are improved by proposing new
design tools. The validity of the proposals are verified by experimental
results.
