Improvement of tracking performances in hard disk drive servo via tracking-differentiator and composite nonlinear control

This paper deals with the design of hard disk drive (HDD) servo systems using a tracking differentiator (TD) and a composite nonlinear feedback (CNF) control to improve its tracking performances. The CNF law consists of linear and nonlinear feedback parts. The linear part is designed for fast rising time while the nonlinear part is designed to increase the damping ratio as the system output approaches the target value to minimize the overshoot during track following in the presence of actuator saturation. In order to reduce the impact of the control gains of both the above linear and nonlinear parts on the tracking performance, a tracking differentiator is used. This tracking differentiator generates a smooth approximation of the reference step input, so as to produce a smooth change of tracking error as the systems approach the final value. Thus the problems of the large or small gain control theory does not arise with this method. The stability of the CNF control law along with the TD is proved through appropriate Lyapunov function. The implementation results show that the proposed control design out-performs the conventional proximate time-optimal servomechanism.