A QFT design of disturbance observer for the track-following control system of an optical disk drive

For track-following control in an optical disk drive, a compensator should be designed to satisfy disturbance attenuation property, overall system stability, and so on. In order to ease the compensator design procedure by reducing trial and error, a disturbance observer is added on to a conventional single loop track-following control system, which may cause control input saturation due to excessive loop gain. Because the track-following control system is conditionally stable, control input saturation leads to actual reduction of loop gain whereby the system becomes unstable. To preserve the overall stability during transient response, where control input saturation possibly occurs, the circle criterion based on the pull-in condition is utilized to check the stability under control input saturation. The constraints from the circle criterion and robust stability condition are converted into frequency domain bounds, which are represented on a Nichols chart. Then a design method using quantitative feedback theory (QFT) is applied to satisfy the constraints shown on the Nichols chart. To show the feasibility, some experiments are conducted, and the results are presented.