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This chapter briefly describes the main results developed by the authors in the area of output feedback sliding mode control. For the sake of simplicity, the focus is maintained on uncertain single-input-single-output (SISO) nonlinear systems, although several results have been extended to the control of multi-input-multi-output (MIMO) systems. For the considered class of nonlinear systems, linear growth restriction on the unmeasured states is assumed, while less restrictive conditions are imposed to the growth of nonlinearities depending on the measured output. We present different tracking controllers for plants with arbitrary relative degree. We consider several approaches to overcome the relative degree obstacle: linear or variable structure lead filters, high-gain observers with constant or dynamic gain, global hybrid estimation schemes combining lead filters or observers with locally exact differentiators based on high-order sliding mode. Global or semi-global stability properties can be proved either for asymptotic exact tracking or for tracking within a small residual error. Some experimental results are presented to illustrate the applicability of the control schemes in real systems.

This chapter addresses the problem of model-reference sliding mode control of uncertain linear multi-input-multi-output systems by output-feedback. After a brief revision of the main approaches and results available in the literature, a new method to solve the problem is presented using unit vectors as switching functions to induce sliding modes. It is named Unit Vector Model-Reference Adaptive Control (UV-MRAC).