Vibration control with thin-film-coil actuator for head-positioning system in hard disk drives

In a head-positioning system of hard disk drives, various mechanical resonances may degrade performance or stability of the control system. In these mechanical resonances, a torsional mode of a head-stack assembly (HSA), in which a resonant frequency is between 3 kHz and 4 kHz, has a large negative impact for the performance of hard disk drives because it can be easily excited by operational vibrations of other hard disk drives in a datastorage server. To overcome this issue, we have developed a vibration control method with a thin-film-coil actuator (called "film actuator") for the head-positioning control system of hard disk drives. The film actuator is attached to a coil of a Voice Coil Motor (VCM). This proposed control system is a triple-stage-actuator system: the first stage is a VCM actuator for moving the HSA, the second stage is a PZT actuator for moving a suspension in the HSA, and the third stage is the film actuator to control the torsional mode of the HSA. The proposed triple-stage actuator system can compensate for the vibrations of the torsional mode by using the film-actuator. Comparison of sensitivity functions showed that the triple-stage-actuator system can improve a sensitivity function at the torsional mode's frequency without decreasing a servo bandwidth of the control system.