Design optimization and control of pickup actuator for near field recording system

Abstract

As the high capacity multimedia contents become popular among consumers, the demand for the high capacity storage device is increasing. Especially for the optical storage device, the near field recording (NFR) system becomes a key candidate for high-density recording method with the introduction of solid immersion lens (SIL). Using the solid immersion lens and its optical property, a gap between disc and lens could be calculated and controlled under nano-scale by the precise actuating motion of wire type actuator. Generally, a 3-axis wire type actuator is widely used for the conventional ODD system, and it guarantees a precise servo not only in the focusing-tracking direction, but also in the low frequency radial tilt direction. With this good performance of 3-axis actuator, the NFR system also adopted a 3-axis actuator from the initial stage of development. However, when the linear and rotational motion coupled together, unwanted motion was observed, and it produced unstable servo performance. Moreover, since previous research was focused on angle compensation of disc for low frequency range, it is not necessary to understand high frequency characteristics of actuator in the rotational direction. Therefore, this paper analyzed the coupled motion of 3-axis actuator and tried to introduce the design guide of 3-axis actuator by constructing the generalized equation of motion. Coupled behavior was analyzed by singular value decomposition of transfer function matrix, and it showed that rotational motion affected to the linear motion in the high frequency range. Therefore, to decouple linear and rotational motion, off-diagonal term of transfer function matrix should be minimized, and to do that, two approaches were considered. At first, by adding decoupling matrix in front of actuator plant model, system optimization was examined. By synthesizing decoupler and original plant model, pseudo-plant could be built and by simple arithmetic operation, off-diagonal te...