Design and robustness analysis of dual stage servo system

This paper presents the design of a digital dual stage servo and the analysis of its robust stability and performance. The dual stage actuator system consists of a coarse actuator and a fine actuator. The design consists of two steps: 1) the coarse actuator loop is designed for basic performance and stability; and 2) the fine actuator loop is designed by loop shaping for superior performance of the overall system. Appropriate contribution of each actuator to the final output is adjusted differently in different frequency range. An important consideration in the second stage is to ensure that the phase difference is within ±120 degrees between the coarse and fine actuator paths when the magnitudes of the two are close to each other. Robustness is studied by the μ-analysis after the closed loop system is augmented by various weightings. Simulation and experimental results justify the proposed design