Single/dual-rate digital controller design for dual stage track following in hard disk drives

This paper presents the design of single rate and dual rate digital controllers for dual stage servo systems. The dual stage actuator system consists of a coarse actuator (voice coil motor) and a fine actuator (piezoelectric transducer, PZT). The design is divided into two steps. In the first step, the stability of the coarse actuator loop is an important consideration. In the second step, the fine actuator loop is designed by loop shaping for superior performance of the overall system. An important consideration in the second stage is to ensure that the phase difference is away from /spl plusmn/180 degrees between the coarse and fine actuator paths when the magnitudes of the two are close to each other. By loop shaping of fine actuator loop, appropriate contribution of each actuator to the final output is adjusted differently in different frequency range. In the dual rate control, the coarse actuator works at slow sampling frequency and the fine actuator works at fast sampling frequency. Simulation and experimental results justify the proposed design.