US Patent 5073882 - Servo-controlled actuator with two-peak flux density distribution

The present invention relates generally to servo-controlled actuators, and more specifically to a lens actuator which is servo-controlled to keep the optical axis of a tracking following device on the right track of a recording disk such as optical disks.

Conventional track following devices include a permanent magnet mounted on a base, and at least one pair of tracking coils and a focusing coil which are mounted on a resiliently movable support. These coils cooperate with the magnet to produce force components in a direction transverse to the direction of tracks to keep the optical axis of a lens system on a desired track and in an orthogonal direction to focus a light beam on the track. Magnetic flux density distribution generated by the permanent magnet has only one peak. As the tracking coils are moved in a focusing direction, flux densities at the upper and lower portions of the tracking coils tend to differ from each other, causing a rotary moment to occur in the movable support. The optical axis of the lens is thus tilted with respect to the vertical.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an actuator which eliminates the tilting of a major axis of a movable support by the provision of a magnetic flux density distribution having two peaks.

According to a broader aspect of the present invention, a magnetic flux density distribution having two peaks of equal magnitudes are generated, the two peaks being located symmetrically on opposite sides of a center axis of the distribution and arranged in a first direction perpendicular to the center axis. A pair of first coils and a second coil are mounted on a resiliently movable support. The first coils have first coil sections which cooperate with the magnetic flux to generate force components contributing to movement of the support in a second direction perpendicular to the first direction and second coil sections which cooperate with the magnetic flux to generate force components not contributing to the movement of the support in the second direction. The second coil sections of the first coils are located respectively at the peaks of the magnetic flux density distribution. The second coil cooperates with the magnetic flux to produce a force component that moves the support in the first direction. Because of the two peaks, flux densities do not substantially differ between upper and lower sections of the first coils when they are moved in a focusing direction. Specifically, the magnetic flux density distribution is generated by a permanent magnetic having opposite pole faces on major surfaces thereof and a groove formed on one of the pole faces, the groove extending in the second direction.

According to a second aspect, the actuator of the present invention is provided for optical disk and comprises a magnetic flux generator mounted on a base to generate a magnetic flux density distribution having two peaks of equal magnitudes, the two peaks being located symmetrically on opposite sides of a center axis of the distribution and arranged in a first direction perpendicular to the center axis. A pair of tracking coils and a focusing coil are mounted on a support resiliently movable with respect to the base. The tracking coils have first coil sections which cooperate with the magnetic flux to generate force components contributing to movement of the support in a second direction perpendicular to the first direction and second coil sections which cooperate with the magnetic flux to generate force components not contributing to the movement of the support in the second direction. The second coil sections of the tracking coils are located respectively at the peaks of the magnetic flux density distribution. The focusing coil cooperates with the magnetic flux to produce a force component that moves the support in the first direction. An optical lens is mounted on the movable support so that an optical axis thereof extends in the first direction to form a light spot on the recording disk