An electromagnetic objective lens actuator for optical disc drives has been developed in order to reduce higherorder resonance amplitude in frequency response characteristics of the actuator. The actuator has a configuration in which each set of three adjacent magnets is located to oppose each of the two sides of the lens holder and each polarization of the center magnet is set in reverse to those of the two outside magnets. The driving force in the positive focusing direction is generated at the focusing coil portions opposing the two outside magnets and the driving force in the negative focusing direction is generated at the focusing coil portions opposing the center magnet. This driving force distribution cancels the bending deformation of the vibration mode at the higher-order resonance. The resulting amplitude of the higher-order resonance in the focusing direction of the developed actuator with this three-polar magnetic structure for 9.5 mm-height Blu-ray Disc drives was successfully reduced to 2 dB or under.

Objective lens actuators are mechanical devices for positioning the objective lens in optical disc drives. The moving part in the objective lens actuator, which is comprised of the objective lens, a holder holding the objective lens, and coils attached to the holder, is supported movably relative to the fixed part by suspension wires and moved electromagnetically in the focusing and tracking directions. The resonance of the supporting system with suspension wires occurs as a main resonance in the frequency response characteristics of an objective lens actuator. It influences the driving sensitivity and settling time of the servo system. Therefore, it is necessary to design the frequency and damping of the main resonance appropriately. The frequency response characteristic of a supporting system with suspension wires is different from the general response of a spring-mass-dashpot system due to the viscoelasticity of the silicone gel added to the suspension wires for damping. In this paper, silicone gel is considered as a spring, the elastic modulus of which is expressed in a function of the frequency so that the vibration model of the supporting system of suspension wires is described easily. Simulated results with this model showed good agreement with experimental results, and it was confirmed that this model is effective for analyzing the frequency response characteristics of objective lens actuators.

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