ROBUST OUTPUT TRACKING CONTROL OF NONLINEAR MIMO SYSTEMS VIA SLIDING MODE TECHNIQUE

The robust output tracking control problem of general nonlinear multi-input multi-output (MIMO) systems is discussed. The robustness against parameter uncertainties and unknown disturbances is considered. A second order sliding mode control (SMC) technique is used to establish the desired tracking. Input/output (I/O) linearization, relative degree, minimum phase and matching condition concepts are reviewed. Some earlier SMC strategies which are restricted to the systems in canonical form are extended to a much broader class of nonlinear dynamics. It is also shown that for unperturbed dynamics, the sliding phase of the SMC applications have a direct correspondence to the I/O linearization operations. Interesting parametric flexibilities emanate within the formation of the second order SMC, designating the "s dynamics" and the "error dynamics" segments as frequency domain filters. However, a critical impasse is posed in the off-line selections of the design parameters. A set of example cases is presented for a spacecraft attitude control problem. These examples manifest that the proposed control strategy is tunable to a desired response despite the disturbances and uncertainties.