A study on multirate sampled-data control for hard disk drives

This paper proposes a head-positioning controller design for hard disk drives using multirate sampled-data H^∞ control theory. Two types of upsampling-by-2 controllers have been derived by solving a mixed sensitivity problem, one is with exogenous inputs at the plant input side (Type A) and the other is with those at the plant output side (Type B). The controlled plant used here has a mechanical resonance at a higher frequency than the Nyquist frequency, which is the half of a sampling frequency of controllers. As a result, it has been confirmed that Type A controllers are robust against aliasing of the mechanical resonance and that Type B controllers, which have the same features as conventional digital controllers using analog notch (band-eliminated) filters, attains a wider control bandwidth than that of Type A. Although the control system is affected by the resonance aliasing, a Type B controller has achieved higher positioning accuracy than that of Type A in a benchmark simulation of a hard disk drive with dual-stage servo mechanism.