Sliding-mode disturbance observer with switching-gain adaptation and its application to optical disk drives

This paper deals with the design of a sliding-mode disturbance observer (SDOB) with switching-gain adaptation for a class of nonlinear systems subject to exogenous signals. The proposed SDOB, referred to as IMP-SDOB, is based on the internal-model principle and thus achieves robust asymptotic disturbance estimation, whereas the conventional SDOB leads to nonzero estimation error to a time-varying disturbance. Moreover, while the conventional SDOB needs a switching gain greater than the magnitude of an unknown disturbance, the proposed IMP-SDOB design releases this constraint and requires only a small switching gain as compared with a conventional design, further alleviating the chattering problem. The stability of the combined controller-observer system is derived on the basis of a Lyapunov analysis. Moreover, in contrast with the previous switching-gain adaptation law that inevitably yields a nondecreasing switching gain and asymptotic convergence of the switching function, a novel adaptation law is proposed to reduce the switching gain whenever appropriate and to guarantee finite-time convergence of the switching function. The effectiveness of the proposed scheme is demonstrated through experimental studies on the track-following system of an optical disk drive suffering from significant disk runout.