The advance of the multimedia system, importance of data storage systems is being increased. In the most recent study, high transfer rate and high density become inevitable necessities to improve the performance of the optical disk system.
When the optical disk system is going to increase the density and the transfer rate of the data, performance of optical disk actuator also has to be enhanced. In this paper, requirements of optical pickup actuator according to higher density and faster transfer rate are discussed. Maximum acceleration and stability for external disturbance of the actuator relate to the high speed, and minimum linearity and tilt angle of the actuator relate to the high density.
First, the actuators, which have been used in various optical disk systems, are evaluated in view of 4 new parameters. Using energy method, stiffness analysis of 4-wire actuator in 6-axis directions is performed, and magnetic circuit analysis and vibration analysis by FEM is executed. The analysis results match well with experiment results. In maximum acceleration, there is no difference between 4-wire actuator and rotary actuator. It means that new mechanism to improve the maximum acceleration is need. In stability for external disturbance, rotary actuator has superior to 4-wire actuator in tracking direction, but in focusing direction they have weak stability. Mass balanced actuator for focusing and tracking is needed. And minimum linearity relates to the positioning accuracy and servo ability. Rotary actuator has bad linearity because of surface roughness of hole and axis, spacing of hole and axis, and dynamic unbalance. 4-wire actuator has the guide of cantilever type so that it has no friction, less hysteresis, smooth movement. Finally, tilt angle of rotary actuator is easier to manage than 4-wire actuator, because it depends upon the mechanical tolerance of hole and axis. Tilt angle of 4-wire actuator is founded by magnetic field analysis and stiffness analysis, and the angle is measured over moving range. Consequently, rotary actuator has superior characteristics in stability while 4-wire actuator has superior characteristics in minimum linearity.
Next study is suggestion of new actuator with enhanced performance for 4 design parameters. Lever structure, which has been used in micro-positioning system, is introduced to achieve mass balance, and flexure hinge is used to improve the accuracy of the movement. This lever actuator has 4 levers, 4 focusing magnetic circuits, 4 tracking magnetic circuits. Tilt drive is added to the actuator, and the balance moving parts is made as same as the lens moving parts.
Considering flexibility of the lever arm, theoretical model of the lever actuator is derived by regular Newton method, and motion of equations for 3-axis direction is obtained. And in case of excitation of pickup base, the modeling of the lever actuator is found.
To design the lever system, sensitivity analysis for design variables is performed, and characteristics of the lever actuator is simulated. The effects due to the making error and driving condition are analyzed.
Prototype of the lever actuator is fabricated, each part is made precisely, and assembling jig is used. Evaluation for the lever prototype is performed for the static and dynamic characteristics. As the result of measurement of $T_A$, $1^st$ resonant of TA in lever actuator is changeable and its position depends upon the stiffness of lever arm. This lever actuator has the same minimum linearity as the 4-wire actuator, and it has stronger stability than other actuators. Moreover tilt drive solves the problem due to tilt, and maximum acceleration can be improved by dual actuation.
For the playability test of the lever actuator, evaluation apparatus is established. When the focusing and tracking servo is on, the actuator operates well.
Consequently, the lever actuator has excellent performance for a high speed and high density system, and its validity is proven by the experiment.