Nonlinear loop shaping for suppressing sensitivity hump in hard disk drive servo systems

In hard disk drive (HDD) servo system that uses voice coil motor (VCM) as the actuator, the sensitivity hump is unavoidable according to Bode´s integral theorem (BIT). That results in amplification of disturbances with frequencies beyond bandwidth. This paper proposes a nonlinear loop shaping method to suppress sensitivity hump of the conventional servo. The proposed nonlinear algorithm consists of two parts. One is the conventional track-following control law, the other is a nonlinear PD type control law designed based on the conventional one to shape its high frequency response. The simulated describing function shows that sensitivity peak can be suppressed with the proposed algorithm without degrading low frequency disturbance rejection, which could lead to increase the track-following accuracy. The efficacy is verified through vibration test with disk flutter. Although sensitivity is improved, the effect of high frequency measurement noise on the position error signal (PES) which is governed by complementary sensitivity function is not degraded. The robustness is also verified with high frequency plant uncertainties.