A sufficient condition for robust internal stability of closed-loop systems with disturbance observer

The question of robust internal stability of closed loop system with disturbance observer (DOB) is studied in this paper. A sufficient condition, which can assure the closed-loop system to be robust internally stable, is presented based on NYQUIST criterion. The controller is designed based on the synthesis technique in the frequency field. By means of giving a sufficient condition, a conclusion can be drawn as follows: the controller can be designed with respect to the plant nominal model and the DOB can be easily designed, and at the same time robust internal stability of the system is guaranteed.