On the invariance of dynamic output feedback VSCS

This note has studied the invariance of dynamic output feedback variable structure control systems (VSCS) for square, invertible plants. Based on a result of Diong and Medonic (1992), we show that such systems can be made invariant to disturbance and uncertainty, with stability, only if the plant being controlled is minimum-phase. Thus if dynamic output feedback VSC is to be applied to this class of plants, then it is almost always necessary to robustify the sliding mode dynamics against the residual disturbance and uncertainty. One approach was given by Diong and Medonic, and other methods are being studied. The invariance of dynamic output feedback VSCS for other classes of plants is also under investigation