Analysis of Optical Data Storage Systems—Tracking Performance With Eccentricity

The tracking control performance of optical data storage systems is analyzed in the presence of the disc eccentricity. The eccentricity is the maximum deviation from the geometric center of the circular tracks and the rotating center of the disc, which is inevitable in practice. This is mainly caused by the inaccuracy of the tracking forming devices. In the paper, tracking control performance is discussed in terms of pull-in capability and steady track-following ability. Firstly, it will be explicitly shown that the pull-in capability will degrade in proportion to the multiplication of the eccentricity and rotational frequency. This analysis provides a guide for tracking controller design with reliable pull-in actions. Secondly, in order to improve the steady track-following performance, a feedforward compensation method is introduced, which is simple enough to be implemented in practice. Furthermore, an interesting connection to the disturbance observer approach in the time domain is examined.

I. Introduction

The optical data storage systems (ODSSs) such as compact disc read-only memory (CD-ROM) or digital versatile disc (DVD) drives for personal computer peripherals, and CD or DVD players for consumer electronic devices are representative of the optomechatronics systems. Reading or recording data on a disc relies on laser optics and, at the same time, the optical pickup unit is controlled and manipulated by the mechanical components such as electromagnetic linear motors (for fine actuators) and dc motors (for a coarse actuator and spindle rotation). For example, the optical pickup unit for DVD playback consists of an objective lens, a laser diode with 650-nm wavelength, photodetectors, components for optical paths, and focusing and tracking actuators. In order to restore data from a disc, the objective lens should be controlled so as to follow a sequence of data pits in circular tracks. To this end, the robust precision control of focusing and tracking actuators is necessary in the presence of optical disturbances such as disc eccentricity, defects, fingerprints, and variation of discs substrate thickness. Once the objective lens is placed at a focal distance, the signals from the photodetectors are managed to obtain the error signals for the feedback control, and RF signal for the data decoding. Although there are technical difficulties in precision control, it is noted that the optomechatronic structure of ODSSs enables easy distribution of a large amount of information at low cost, which has led the information technology for a couple of decades. In particular, cost effectiveness and portability are the advantages of the ODSSs and optical discs over the hard disc drives. For example, 4.7 GB of MPEG2 data can be easily distributed via a single layered DVD disc (i.e., a DVD title).