Robust output feedback control for unknown non-linear systems with external disturbance

The authors propose a robust output feedback control for unknown non-linear systems with external disturbance. The main contribution of this study is that the proposed method guarantees the global uniform ultimate boundedness of the output tracking error using only output feedback for unknown non-linear systems with external disturbance. The key idea of the proposed method is that the plant non-linearities and external disturbance, and their derivatives are lumped into augmented state variables. The robust high order augmented observer (RHOAOB) is developed to estimate the lumped augmented state variables and full state. The benefit to using the RHOAOB is that the estimation error can be reduced without increasing the bandwidth of the RHOAOB. The backstepping controller is designed to guarantee the global uniform ultimate boundedness of the output tracking error occurred by the disturbance estimation error. To the best of the authors' knowledge, it might be the first attempt of the analysis of the estimation and output tracking performance of the RHOAOB-based controller with the measurement noise in both time and frequency domains. The analysis shows that very small estimation error is not necessarily required to obtain the precise output tracking using an input-to-state stability property. This merit results in avoiding the amplification of the measurement noise.This work was supported by the Dong-A University research fund