ABSTRACT
In general, there are two kinds of methodologies focusing on the anti-disturbance control problem for nonlinear systems with unknown disturbances. The first one is the disturbance attenuation approach such as nonlinear H∞ control theory feasible for norm bounded disturbances [7, 80]. Another is the disturbance rejection method including nonlinear output regulation theory which has been shown to be good performance on the disturbance described by exogenous models (see, e.g., [47, 106, 126, 159, 180, 188]). A disturbance observer based control (DOBC) approach has been widely used in many practical engineering contexts, such as robot control, table drive systems, hard disks, active magnetic bearings, vibrational microelectronic mechanical system (MEMS) gyroscopes (see survey in [87] and references therein). In the first stage, only linear DOBC is involved in the frequency domain, where the nonlinearities and uncertainties are treated as a disturbance to be estimated and rejected [10]. In [85], DOBC was first generalized to uncertain systems with an exogenous disturbance model in the time-domain formulations. The nonlinear DOBC law was proposed for robots [32] with harmonic disturbances. Several valuable new related results have been given for nonlinear systems with special structures [31, 182, 221].
