1. Field of the Invention
The present invention relates generally to an objective lens driving apparatus with an actuator movable unit which is supported on an actuator frame displaceably and elastically, the unit being configured such that a driving coil is mounted on a lens holder including an objective lens, and, more particularly, to an objective lens driving apparatus that cancels rolling generated when an actuator movable unit is driven in a tracking direction.
2. Description of the Related Art
In an objective lens driving apparatus in an optical head that optically reads or writes signals from or to an optical disc, as is well known, an actuator movable unit is mounted with a focus coil and a tracking coil, or a tilt coil as needed, on a lens holder with an objective lens attached therein and is displaceably supported by an actuator frame; an effective area of each driving coil of the actuator movable unit is located within a predetermined magnetic field formed by a magnetic circuit; and therefore, the objective lens is driven in a focus direction and a tracking direction or a tilt direction as needed, in accordance with a drive signal supplied to each driving coil.
In the objective lens driving apparatus, since the objective lens is mounted on the top surface of the lens holder such that the objective lens is located closer to a disc, and due to the arrangement of each driving coil and the configuration of the magnetic circuit, the center of gravity of the actuator movable unit generally deviates in the vertical direction, i.e., the light axis direction of the objective lens, from the point of action of the driving force generated by driving the actuator movable unit in the tracking direction, and when the actuator movable unit is driven in the tracking direction, rolling is generated which is lateral swinging of rotational motion of the actuator movable unit in a radial direction of the disc.
To prevent the rolling, in a known objective lens driving apparatus, the placement location of the tracking coil and the configuration of the magnetic circuit generating a magnetic flux applied to the tracking coil are devised to supply the drive signal to the tracking coil, such that a driving force is generated in a direction preventing the rolling, along with the driving force in the tracking direction (see patent document 1).
In the objective lens driving apparatus using the driving force of the electromagnetic force to prevent the rolling, since no counter weight is added to the under side of the lens holder to keep a weight balance of the actuator movable unit, deterioration of sensitivity due to the increase in the weight of the actuator movable unit is prevented (see, e.g., Japanese Patent Application Laid-Open Publication No. 2001-222830).
In the objective lens driving apparatus shown in patent document 1, since a tracking coil is needed at each side of a side surface of the lens holder facing to a magnet effective surface and a magnetic pole having reverse polarity must be magnetized obliquely with respect to the focus direction on the magnet effective surface, it is problematic that the magnetic circuit has many restrictions.
Since the boundary of the magnetic pole is oblique on the magnet effective surface, if the actuator movable unit is displaced in the tracking and focusing directions, a positional relationship between the magnetic pole of the magnet effective surface and an effective side of each tracking coil becomes unbalanced in each tracking coil. That is, if the actuator movable unit is displaced in the tracking direction, a positional relation of each tracking coil relative to the magnetic pole boundary is changed depending on this displacement. Therefore, the driving force generated by each tracking coil for preventing the rolling becomes unbalanced, and moment is generated around the center of gravity of the actuator movable unit.
Therefore, since an electromagnetic force is generated by the drive signal supplied to each tracking coil and the moment generated around the center of gravity of the actuator movable unit is changed, the rolling cannot sufficiently prevented and a fundamental solution cannot be achieved.