Long-stroke tracking actuator for both coarse and fine motion in optical disk drives
Introduction
Optical disks are an essential component for data storage. High rotation speed has been the development target for high data read/write speeds in optical disks. However, there is a limit to performance improvement by high rotation speed alone because vibration at high speeds makes the system unstable. Consequently, a multi-beam access method has been suggested as an alternative to increase data read/write speeds. When multiple beams are used, a dove actuator for beam rotation and additional devices are required. Thus multi-beam systems result in both high costs and a large size.
An optical disk arm drive mechanism consists of a tracking actuator and a focusing actuator. Since a tracking actuator is required to have both a large range, over 33 mm, and high resolution, under , it is usually divided into a coarse actuator for gross movement and a fine tracking actuator for precise movement. However, this 2-stage tracking actuator has a slow access time since fine control is carried out after coarse access. A 2-stage tracking actuator also requires a number of devices [1].
In this paper, a one-dimensional tracking actuator is proposed to reduce the slow access time and the amount of devices required.
Many studies have been done on combining a coarse actuator with a fine tracking actuator. But they were limited to reducing the mass of the actuator because of the complexity of the optical system construction. The large friction and inertia force created by the large mass of the moving parts make fine movement difficult. One of the conventional studies had used a separate optical system in order to reduce the moving mass. However, since this design needs many optical devices, the optical system becomes very complex and has a limited practical use [2].
Currently, the optical pickup has a small size and mass because of a simple optical system. In general, it is difficult to obtain the required performance in the optical disk system with a single sensor, but the tracking actuator in the optical disk drive can use the track as the displacement reference. Coarse motion is detected by counting the number of tracks that the pickup has passed over, and fine motion is sensed by the deviation of the beam from the track. This multi-sensing method enables the realization of an actuator with a large range and high resolution.
When the coarse actuator and fine tracking actuator are combined, various problems develop, such as stability, sensitivity, and friction. In this paper, we present approaches to solve these problems. The actuator is manufactured and tested for fine motion within a long stroke. The developed actuator has stable motion and a resolution of under ± resolution over a range of several millimeters.
Section snippets
System design
The actuator is driven by a voice coil motor (VCM) since this driver can easily achieve highly precise movement and a large stroke. The electromagnetic actuator, shown in Fig. 1, has a dual path, which ensures minimal flux leakage and good linearity. During system design, three considerations are important: a large driving force, the leaf springs, and friction force.
First, the permeance method is used to design and analyze a VCM with a large driving force. To increase the driving force, many
Dynamic characteristic experiment
A current feedback driver is used to eliminate phase drop and provide an adequate current supply. Eq. (15) is the motion equation for the focusing actuatorwhere mp is the pickup mass, c is the damping coefficient, kf is the spring constant, xf is the displacement of the pickup focus, i is the input current and l is the effective length of the coil. A dynamic analyzer (HP35670A) was used in order to obtain the transfer function of the actuator. Fig. 7 shows
Conclusion
This paper describes the design and control of a one-dimensional precise tracking actuator in an optical disk drive with the performance of 2-stage tracking. Three design categories were needed as follows. First, the driving force had to be large enough to overcome the friction force and the inertia force. The VCM was designed using the permeance method. The result was verified by an electromagnetic analysis program and by experiment. Second, when the leaf spring for focusing is designed, the
Acknowledgments
This work was supported in part by the Brain Korea 21 Project.
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