1. Field of the Invention
The present invention relates to an actuator for use in a pickup device which moves a lens holder holding an objective lens.
2. Description of Related Art
A disk drive apparatus for recording/reproducing data on/from an optical disk such as a CD (Compact Disk), DVD (Digital Video Disk), or the like is constituted by including an optical pickup device.
The pickup device guides a laser beam emitted from a light source such as a laser diode or the like to an objective lens by a predetermined optical system component, and irradiates the laser beam on a data recording surface of the optical disk.
Normally, the objective lens is arranged to face the optical disk. The laser beam from the light source is guided firstly to a position immediately below the objective lens along an optical path parallel to the optical disk by the optical system component. The laser beam is then oriented upward vertically by an uprise mirror arranged immediately below the objective lens and reaches the objective lens.
In order to record/reproduce data correctly on/from the optical disk, the pickup device performs focus control for controlling the distance between the data recording surface of the disk and the objective lens, and also performs tracking control for controlling the objective lens to follow data tracks of the optical disk.
To achieve the focus control and tracking control, the objective lens is fixed by adhesion to a lens holder throughout the entire circumference of the substantially ring-like outer peripheral portion of the objective lens. The pickup device is provided with an actuator which moves the lens holder in the focusing direction and in the tracking direction by means of an electromagnetic drive.
Considered as an electromagnetic drive of this kind will be a mechanism which has a coil board attached to the lens holder and a magnet provided near the coil board. A drive current is fed to the coil board in a magnetic field formed by the magnet, to move the lens holder in the directions described above relative to the optical disk.
A drive current is thus fed to the coil board of the electromagnetic drive, and the frequency of an electromagnetic wave generated by the coil board reaches a predetermined servo band (e.g., 0 to 5 KHz). Then, the electromagnetic drive is activated so that the lens holder moves together with the coil board.
If the lens holder resonates in the servo band, there may be a drawback that the actuator stops. It is hence necessary to set the resonance frequencies of movable components such as the lens holder, coil, and the like higher than the servo band.
Any optical disk has very small concaves and convexes on its surface due to molding precision. Therefore, if focus servo control is performed, not only the surface rocking of the optical disk occurs but also focus error signals are generated following the concaves and convexes, so a high frequency is incurred. If a part of the frequency components of the focus error signals coincides with the resonance frequency of the lens holder, the lens holder resonates. As a result, the lens holder is deformed and the stress of deformation is transmitted to the objective lens, causing an optical aberration.
That is, the objective lens is securely fixed to the lens holder by a thin adhesion layer throughout the entire circumference of the substantially ring-like outer peripheral portion of the objective lens. Therefore, the deformation of the lens holder caused by the resonance is directly transmitted to the objective lens, so that the lens distortion may be incurred to degrade the quality of the pickup device.
In this case, the servo band is the frequency band that the device should respond to when reading data from an optical disk. Sufficient reading is possible even when the pickup device does not respond to a higher band.
It is possible to design a low-pass filter which extracts the desired servo band of 0 to 5 KHz as an error signal. However, there are difficulties in designing a filter having sharp pass-band characteristics. The frequency of the electromagnetic drive therefore has characteristics that the frequency of 0 to 5 KHz is allowed to pass and any higher frequency gently and gradually attenuates. There is however a case that some frequency beyond the servo band coincides with the resonance frequency of the lens holder. In this case, the deformation of the lens holder as described above is incurred.