Suppressing sensitivity hump in HDD dual-stage servo systems

In Hard disk drive (HDD) single-stage servo system that uses voice coil motor as the sole actuator, the sensitivity hump appears unavoidable due to the Bode’s integral theorem, resulting in amplification of disturbances at frequencies higher than the open-loop gain crossover frequency. This paper studies the sensitivity transfer function limitation in HDD dual-stage servo system. A compensation method for the micro-actuator dynamics, termed as “near-perfect modeling” (NPM), is proposed to generate an effective compensated plant model with zero relative degree in order to suppress the sensitivity hump of the dual-stage servo loop. Therefore, high-frequency disturbances would not be amplified by the servo loop. Experimental results show that the sensitivity hump of dual-stage servo loop can be reduced to very close to 0 dB without amplification of the corresponding measurement noise.