Modeling and compensation of low fly-height vibrations in high density Hard Disk Drive servo systems

The areal density of the Hard Disk Drive (HDD) is expected to reach 10 Tb/in² by 2014. At such high density, the fly height of the HDD heads have to be lowered, even to the point of contact with the media in order to have good signal to noise ratio. As such, contact recording, where the slider and HDD surface are in contact through lubricant surface, is the one of several possible technologies that is being tried today. This leads to the new vibrations in the HDD servo loop, which are named as low fly-height vibrations (LFHVs) caused due to the continuous contact of the read/write head with the media. Therefore, it is desirable that the servo system can work against these new vibration source. This paper aims to model the LFHV in the HDD servo loop through experiments, characterize its effect and propose a compensation method. The LFHVs were modeled as output disturbances to HDD servo and the vibration parameters were identified from the spectrum of vibrations due to continuous contact of the slider and the media measured in the off-track direction. In what follows, based on the state-space model of the HDD servo and LFHV dynamics, an observer-based state feedback (OBSF) control scheme is adopted for actively suppressing the LFHV. To the end, numerical simulations are made to evaluate the performance of the OBSF method for LFHV compensation.