The control of uncertain nonlinear systems by output feedback is addressed. A model-reference tracking sliding mode controller is designed for uncertain plants with arbitrary relative degree. Nonlinearities of a given class are incorporated as state dependent and possibly unmatched disturbances of a linear plant. Such class encompasses nonlinear systems which are triangular in the unmeasured states. In contrast with previous works, exact tracking is achieved by means of a switching strategy based on a locally exact differentiator, and a monitoring function is used to cope with the lack of knowledge of the control direction. Global or semi-global stability properties of the closed-loop system are proved.

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In this paper, we propose an adaptive output-feedback controller for uncertain linear systems without a priori knowledge of the plant high-frequency-gain sign. To deal with parametric uncertainties and unmodeled dynamics, we consider a robust adaptive strategy named binary model reference adaptive control, which has the good transient properties and robustness of sliding mode control with the important advantage of having a continuous control signal free of chattering. The effective way of tackling unknown high-frequencygain sign is employing monitoring functions. The developed adaptive control guarantees global exponential stability of the closed-loop error system with respect to a compact residual set. Moreover, in the absence of unmodeled dynamics, exact tracking of a reference signal can be achieved. Numerical simulations illustrate the efficacy of the proposed approach.

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