The present invention relates to an objective lens driving apparatus for an optical pick-up suitable for use in an optical disk apparatus such as a CD player, a CD-ROM or the like.
An objective lens driving apparatus for an optical pick-up has a lens holder and a base, and the objective lens is retained by the lens holder. The lens holder is mounted on the base so that it can be moved in the vertical direction (or focusing direction) with respect to an optical disk for focusing purposes, and in the radial direction (or tracking direction) of the optical disk for tracking purposes. Typical known arrangements for mounting lens holders on bases include a supporting arrangement utilizing wire means, such as disclosed in Japanese Patent Unexamined Publication No. 61-115249, and a supporting arrangement utilizing a supporting shaft, such as disclosed in U.S. Pat. No. 4,752,117.
In the former known arrangement, an objective lens is positioned at a center of a lens holder, and the lens holder is fixed to intermediate portions of a plurality of wires of which first ends are fixed to the base and opposite ends are inserted in respective bores formed in the base. In other words, the supporting arrangement utilizing wire means is characterized by the structure in which the lens holder is retained in a suspended state. The lens holder is provided with focusing coils and tracking coils, and the base is provided with a plurality of permanent magnets for forming a magnetic field.
When the focusing coils or the tracking coils are energized, the lens holder is made to move in the focusing direction or the tracking direction by elastically deforming the wires by an electromagnetic action produced between the energized coils and the permanent magnets provided on the base, thereby making adjustment of focus or tracking. When the focusing coil or the tracking coil is de-energized, the wires are recovered from elastic deformation and the lens holder is restored to its original position.
The above arrangement has a number of disadvantages. For example, since the lens holder is, as described above, retained in a suspended state by the wires, it is subject to rolling or pitching due to dynamic external forces acting on the objective lens driving apparatus. Further, the flexure and twist characteristics of the wires are difficult to equalize due to subtle differences between the fixed state of the wires. These disadvantages have adverse influences on a servo control system for the lens holder and, for example, the objective lens may deviate from the predetermined direction to impair the accuracy of position control.
In the latter arrangement, an objective lens is mounted on the extension of a lens holder, and the lens holder is rotatably and slidably fitted onto a supporting shaft mounted on a base at a position of the center of balance of the lens holder. A plurality of rubber members for restoration to an original position are connected between the lens holder and the base. The points of the respective rubber members connected to the lens holder are displaced from one another in the direction of the height of the supporting shaft because of the space in the objective lens driving apparatus. In addition, the lens holder is provided with the focusing coils and the tracking coils, and lead wires connected to the respective coils are suspended from the lens holder.
When the focusing coils are energized, the lens holder slides along the supporting shaft by an electromagnetic action produced between the focusing coils and focusing magnets provided on the base, thereby enabling adjustment of the focus of the objective lens. When the tracking coils are energized, the lens holder rotates about the supporting shaft by an electromagnetic action produced between the tracking coils and tracking magnets provided on the base, thereby enabling adjustment of tracking.
In the above-described arrangement, since the lens holder is fitted onto the supporting shaft, it is possible to suppress severe rolling and pitching due to dynamic external forces which may be experienced with the supporting arrangement utilizing wire means. However, since the points of the respective rubber members connected to the lens holder differ from one another in the direction of the height of the supporting shaft, a moment acts to incline the lens holder. As a result, the lens holder may be inclined and an edge of the fitting portions of the lens holder is engaged with the supporting shaft, thereby impairing the smooth rotary or sliding motion of the lens holder. In addition, since the material of the elastic members for restoration to an original position is a rubber, the variation of the characteristics thereof is large, thereby making it difficult to realize accurate position control. Furthermore, since the lens holder instantaneously rotates or slides, the inertia of lead wires suspended from the lens holder has an adverse influence on the control of the lens holder and this may make it more difficult to realize accurate position control. Also, since the lens holder repeatedly rotates or slides, repetitive bending stresses act on the lead wires suspended from the lens holder. As a result, the lead wires may be broken by being fatigued with the repetitive bending stresses.