This paper presents a multirate digital controller design for HDD servo systems via direct parameter optimization approach. In order to find a satisfactory multirate controller with good transient and steady state performance, a linear quadratic performance index with stability constraints is introduced. The controller parameters are selected based on the HDD servo system transient response data instead of a detailed plant model. The proposed method can improve the performance of LQG multirate servo designs without knowing the exact plant parameters or noise model. The effect of controller order on system performance is also studied. For a 4th order system studied, it is found that a 4th order controller is sufficient to achieve the optimal performance.
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