Loop shaping issues in hard disk drive servo system design

In this paper, the design of hard disk drive (HDD) servo systems based on state-space techniques is considered. In particular, we analyze the performances of discrete-time controllers obtained by means of LQG optimal control theory. The key control design issue we address is related to the need of achieving sufficiently high close-loop bandwidth while granting adequate disturbance rejection in the loop gain crossover frequency region. It turns out that the shaping of the sensitivity function is a critical issue, in particular when run-outs compensation schemes are introduced. The effectiveness of discrete-time loop transfer recovery techniques in achieving a satisfactory loop shaping design starting from an LQG regulator is discussed. The proposed control algorithms are then evaluated on a test platform based on a commercial HDD. The experimental results thus obtained are reported and discussed.