Anti-shock control for optical storage drives

Abstract

Maturity in the industry of optical storage drives has drawn dramatic competition in recent years between drive performance and prices. One of the main technical challenges of the optical storage drive industry is how to achieve reliable data read-out in the presence of all kinds of disturbances. Disturbances are either periodic or non-periodic. Periodic disturbances can effectively be handled by various linear control methods like repetitive control and feedforward compensation. The amount of research published on rejection of non-periodic disturbances like shocks is limited. Yet, it is this so-called anti-shock performance that is often the most limiting in high capacity / high speed storage drives in consumer electronics. This thesis addresses several anti-shock control methods to reduce the influence of external shock disturbances to optical storage drive systems. The optical storage drive shock environment and specification for consumer electronics is studied systematically. A dynamic model of the optical storage drive under shock disturbances is established. Quantitative relations between design parameters of the drive suspension and control system and the dynamic shock behavior are studied to provide practical design guidelines for better shock performance. These studies indicate the limitations of obtaining high shock immunity that exist in present linear servo control systems. A cost-effective way towards better anti-shock performance by designing a more robust servo control system is therefore proposed. More specifically, the estimator-based discrete-time sliding mode control with variable control gains is developed to replace the original linear servo control system. This is done not only to improve the drives robustness for external shock disturbances but also to maintain drive per-formance characteristics under linear control that are needed for playback and recording performance over disc defects. The proposed technique guarantees asymptotic stability and requires certain model knowledge. Stability is maintained in case of bounded uncertainties. Special attention is given to solving the chattering problem in the application of discrete-time sliding mode control. A practical implementation method for estimator-based discrete-time sliding mode control with two switching control gains is presented as a main outcome of the thesis. Additionally, a new shock detection method to detect the occurrence of shocks and disk defects is illuminated. The feasibility and effectiveness of the control techniques addressed in the thesis are verified both by computer simulation and experimental results.