“…One is the disturbance observer (DOB) technique originally presented by Ohnishi [18]. Following this direction, many DOB-based control methods have been reported in different applications, e.g., robotic systems [19], hard disk drive systems [28], general motion control systems [20][21][22][23][24], inverted pendulum systems [25], PMSM systems [26,2,27,7], etc. Among these results, different kinds of DOB have been developed, including linear DOBs [26,2,22,28], fuzzy DOBs [25], neural network based DOBs [27], and other nonlinear DOBs [19][20][21]23,7,24], etc.…”