HDD dual-stage servo-controller design using a mu-analysis tool

A simple tool for assessment of the closed-loop performance of a linear standard dual-stage controller for track following in hard-disc drives (HDD) is presented. For this reason, a mu-control analysis problem is posed so that the uncertainty of the voice-coil-motor (VCM) actuator and of the secondary actuator can be considered while raising a performance constraint for the sensitivity magnitude. The algebraic formula for the resulting mu-control analysis problem is readily calculated and allows the computation of the amplitude of a worst case sensitivity frequency response for explicit evaluation of the prospective robust control performance. The validity of the approach is verified for a practical example using a HDD-system with piezoelectric secondary actuator.