The dynamical behavior of a variable structure system (VSS) in sliding mode is determined by the sliding surface. On the surface, the system trajectory is immune from all uncertainties and external disturbances which are in the range of the control space, i.e., disturbances and uncertainties satisfy the so called matching condition. This property of invariance towards matched uncertainty makes sliding mode an attractive methodology for designing robust controllers for uncertain systems. However, the attribute of disturbances rejection for the variable structure system is lost if the sliding surface function contains uncertainties due to the presence of disturbances. In this paper, we present a simple way to construct discontinuous sliding surface to control an electro-mechanical systems. We show that disturbance rejection still can be attained even when matching condition in the original system is not satisfied.
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