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.
This paper addresses the visual servoing control problem for robot manipulators without using velocity measurements, and considering a fixed but uncalibrated camera with an optical axis perpendicular to the robot workspace. A novel visual servoing strategy via sliding mode control (SMC) and a monitoring function based switching scheme is presented to deal with the uncertainties in the camera calibration parameters and to remove any restriction on the unknown camera orientation angle. The developed method provides global stability, disturbance rejection properties, and exact output tracking with better transient performance than adaptive controllers. Experimental results illustrate the robustness and practical feasibility of the proposed scheme.
SUMMARYA novel output-feedback sliding mode control strategy is proposed for a class of single-input single-output (SISO) uncertain time-varying nonlinear systems for which a norm state estimator can be implemented. Such a class encompasses minimum-phase systems with nonlinearities affinely norm bounded by unmeasured states with growth rate depending nonlinearly on the measured system output and on the internal states related with the zero-dynamics. The sliding surface is generated by using the state of a high gain observer (HGO) whereas a peaking free control amplitude is obtained via a norm observer. In contrast to the existing semi-global sliding mode control solutions available in the literature for the class of plants considered here, the proposed scheme is free of peaking and achieves global tracking with respect to a small residual set. The key idea is to design a time-varying HGO gain implementable from measurable signals.
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