Disk storage system employing a discrete-time sliding mode controller for servo control

Abstract

A discrete-time sliding mode controller (SMC) controls the motion of a read head actuated over a disk storage medium during tracking operations. The discret-time SMC comprises a linear signal generator for generating a linear control signal, and a sliding mode generator for generating a sliding mode signal. These signals are combined and applied to a voice coil motor (VCM) for positioning the read head over a particular data track recorded on the disk. The linear signal generator and the gain (c&Ggr;) &Dgr; are designed such that once the sliding mode variable &sgr; crosses the sliding line the first time, it will cross the sliding line in every successive sample period resulting in a zigzag motion about the sliding line wherein &sgr; changes sign at every sample period. The switching action and resulting chatter of the sliding mode controller are minimized by adjusting the width of the boundary layer through appropriate selection of predetermined constants &Dgr; and &lgr;. The boundary layer can be made arbitrarily small so as to better approximate and thereby attain the benefits of continuous time SMC. In addition, the required control effort is minimized while still providing adequate immunity to parameter variations and external load disturbances.