High-bandwidth servo control designs for magnetic disk drives

In order to attain high track density, a head must accurately follow position information recorded on a disk. To achieve high accuracy head positioning, disk drives require increased servo bandwidth. However, servo bandwidth is restricted by the main resonance frequency of an actuator and by the sampling frequency. In this paper, we discuss several servo technologies. First, we present the limit of the servo bandwidth of digital loop-shaping servo methods to the mechanical resonance at which the main resonance frequency of a voice coil motor (VCM) actuator is 5 kHz, and the sampling time is 30 μs. We show that the 1.5 kHz servo bandwidth can be attained by using a phase stabilized compensation method. It is also shown that by using the main resonance pole zero compensation method drawn from H-infinity theory, the servo bandwidth increases 1.1 times. Next, we discuss a vibration-sensing feedback method which uses a sensor to measure and carry out analog feedback of the main resonance characteristic of the VCM actuator. The resonance mode can be suppressed by the vibration-sensing feedback servo, and it is shown that the servo bandwidth increases 1.2 times. Finally, we present the phase stabilized compensation design for a dual-stage actuator. The 2-kHz servo bandwidth is achieved when the main resonance frequency of the fine PZT actuator is set to 7 kHz. The impulse response characteristic of each control system, from the torque disturbance to the head, is also discussed