Adaptive robust control of a class of uncertain nonlinear systems with unknown sinusoidal disturbances

In this paper, nonlinear observers are incorporated into the discontinuous projection based adaptive robust control (ARC) to synthesize performance oriented controllers for a class of uncertain nonlinear systems with unknown sinusoidal disturbances. In addition to magnitudes and phases, frequencies of the sinusoidal disturbances need not to be known as well, so long as the overall order is known. A nonlinear observer is constructed to eliminate the effect of unknown sinusoidal disturbances to improve the steady-state output tracking performance - asymptotic output tracking is achieved when the system is subjected to unknown sinusoidal disturbance only. The discontinuous projection based adaptation law is used to obtain robust estimate of all unknown parameters. In addition, a dynamic normalization signal is introduced to construct adaptive robust control laws to effectively deal with various uncertainties for a guaranteed robust performance in general. Compared with the existing internal model principle based robust adaptive designs for unknown sinusoidal disturbances, the model uncertainties considered in the paper can be of unmatched. Furthermore, in the presence of other disturbances and uncertainties in addition to the sinusoidal disturbances, the proposed approach achieves a guaranteed output tracking robust performance in terms of both the transient and the steady-state, as opposed to the robust stability results of the existing internal model principle based designs.