Sensorless Tilt Compensation for a Three-Axis Optical Pickup Using a Sliding-Mode Controller Equipped With a Sliding-Mode Observer

A sliding-mode controller equipped with a sliding-mode observer is synthesized and applied to a novel three-axis four-wire optical pickup for the purpose of sensorless tilt compensation. The three-axis pickup owns the capability to move the lens holder in three directions of focusing, tracking and tilting, which is required particularly for higher data-density optical disks and precision measuring instruments to annihilate nonzero lens tiltings. To achieve the sensorless compensation, Lagrange´s equations are first employed to derive equations of motion for the lens holder. A sliding-mode controller is then designed to perform dynamic decoupling and forge control efforts toward the goals of precision tracking, focusing, and zero tilting. Along with the controller, a sliding-mode observer is designed to perform the online tilt estimation of the lens holder. This estimated tilt allows the previously designed sliding-mode controller to be implemented in most existing commercial pickups without additional photodiodes to detect the tilting motion of the lens holder. A full-order high-gain observer is next forged to estimate the moving velocities of the lens holder in order to provide low-noised feedback velocity signals for the designed sliding-mode controller. Simulations are carried out to choose appropriate controller and observer gains. Finally, experiments are conducted to validate the effectiveness of the controller for annihilating lens tilting and the capability of the tilt observer for performing sensorless tilt compensation.