Loop-Shaping Method with Unstable Poles for Magnetic-Head Positioning Control in Hard Disk Drive

In this paper, we present a loop-shaping method that incorporates unstable poles for the magnetic-head positioning system in a hard disk drive (HDD). The proposed method involves introducing unstable poles instead of the conventional stable poles for the resonance filter used in loop shaping. In our approach, the resonant filter is designed to realize a circle that includes the coordinate $[-1,0 j]$ on the Nyquist plot of the open-loop characteristics in the control system. Validation results using the HDD benchmark problem demonstrate that our proposed methods effectively reject disturbances in the high-frequency range while maintaining robust performance.