Disturbance-observer-based repetitive control with sliding modes

This paper proposes a sliding repetitive control (SRC) scheme based on sliding disturbance observers (SDOB) for repetitive tracking control tasks. The three control strategies which are synthesized to yield excellent tracking performance are: 1) the pole-placement feedback control to specify desired error dynamics; 2) SDOB-based feedback compensation as the robust part; and 3) a learning feedforward component that gradually takes the place of the SDOB-based feedback compensation, and refines the control to improve system performance through repetitive trials. The SDOB-based feedback compensation ensures acceptably small tracking error during initial learning trials and enhances system insensitivity to exceptional and aperiodic disturbances. The learning compensation is updated according to a switching signal that is equivalent to the error of the feedforward compensation, yielding fast convergence of the learning process from trial to trial. Because the switching actions to ensure global existence of a sliding mode take place in the controller instead of the plant, the proposed scheme thus alleviates the chatter problem often encountered in conventional sliding-mode controls.