Abstract:A global discrete-time sliding mode controller is presented for the systems with parameter uncertaintiesand external disturbances.In order to reduce chattering,the boundary layer method using variable structure concept is applied.By choosing an optimal boundary layer width,the systems states reach sliding mode surface exactly in the shortest time and thereafter stay on without chattering on the condition that all the uncertainties are known.A disturbance compensator is constructed to ensure favorable robust stability to the close-loop system in the sense that unknown uncertainties are varying slowly,when the optimal design parameter is suggested.Simulation results have shown the effectiveness of the proposed algorithms.