“…The disturbance observer technique was first presented by Ohishi et al [2] for a position servo system in the late 1980s. During the last three decades, DOBC schemes for linear and nonlinear systems have been proposed and applied successfully in various practical areas, e.g., servo control systems [3,4], robotic systems [5,6,7], hard disk drive systems [8,9], flight control systems [10,11], and process control systems [12,13,14].…”