ABSTRACT
To improve anti-disturbance ability, disturbance observer based control (DOBC) strategies have been investigated and applied in many control areas [32, 85, 89, 108, 110, 121, 122, 133, 137, 197, 237]. DOBC has its roots in many mechanical applications in the last two decades, in particular for linear systems [108, 110, 121]. Attempts have been made to establish theoretical justification of these DOBC applications and extend DOBC from linear systems to nonlinear systems [32, 85], similar to nonlinear regulation theory [111, 234]. In [32], only single-input-single-output (SISO) nonlinear systems with well-defined disturbance relative degree were studied, and the disturbances were limited to be constant or harmonic signals. In [85], the DOBC approaches for a class of multiple-input-multiple-output (MIMO) nonlinear systems have been considered, where the nonlinear dynamics can be described by known and unknown nonlinear functions, respectively, and the disturbances were represented by a linear exogenous system. This extended the assumptions of the disturbances, which are restricted to being constant, harmonic or neutral stable [32, 111, 234]. However, it has been reported that when a disturbance has perturbations, the proposed approaches in DOBC (e.g., in [32, 85]) are unsatisfactory, which has been verified by the simulations in [85].
