A novel fixed-time sliding mode disturbance observer for a class of nonlinear systems with unmatched disturbances

This paper develops a sliding mode control (SMC) approach for a class of uncertain nonlinear systems with unmatched uncertainties/disturbances. The newly proposed method is based on a novel fixed-time convergent sliding mode disturbance observer which can estimate the unmatched disturbance in a fixed time (time whose upper estimate is independent of the initial conditions). By designing a novel sliding surface based on the disturbance estimation, a disturbance-observer based SMC method can counteract the unmatched disturbance. Simulations verify the validity of the proposed approach.