1. Field of the Invention
The present invention relates to an actuator for use in an optical pickup, and more particularly, to improvements in retaining the neutral point of tracking in an actuator of the type in which focusing and tracking are performed by sliding the lens in the axial direction and by rotating the line about a shaft, respectively.
2. Description of the Prior Art
In an actuator in which focusing and tracking are conducted by sliding a lens in the axial direction and rotating the lens around a shaft, respectively, it is common to use a rubber spring to retain the neutral point of tracking. FIG. 2 shows an example of such a conventional actuator. In the Figure, one end of a rubber spring 21 is fixed to a pin 23 set up in a fixed or stationary portion such as a yoke, and the other end thereof is fixed through a pin 22 to an objective lens retaining tube 1 constituting a movable or rotary portion and having a central portion and an annular peripheral portion. When the objective lens moves in the tracking direction indicated by the double-headed arrow A, its neutral point is retained by the resilient force of the rubber spring 21.
This conventional method of retention by a rubber spring, however, requires the provision of space for the rubber spring, and is therefore very inconvenient from the view point of the desirability of reducing the size of the pickup, which has been requested more and more in recent years.
If the rubber spring is inaccurately mounted in such a way that it is distorted, nonlinearity may be generated in the spring action, or the neutral position of the objective lens may be deflected from the outset. This makes the process of mounting the rubber spring very troublesome.
In addition, the rubber spring does not have good temperature characteristics owing to the physical properties of its material, and is subjected to deterioration as its resonance frequency or sensitivity changes.
When the objective lens retaining tube 1 is raised to its operating point by passing a bias current through a focusing servomechanism, the rubber spring generates a force which pulls it downward, thereby necessitating to flow a larger bias current. In this way, the rubber spring affects the movement of the objective lens in the focusing direction.