Elsevier

Mechatronics

Volume 13, Issue 3, April 2003, Pages 259-272
Mechatronics

Long-stroke tracking actuator for both coarse and fine motion in optical disk drives

https://doi.org/10.1016/S0957-4158(01)00025-3Get rights and content

Abstract

In an optical disk system, the tracking system consists of a coarse actuator and a fine tracking actuator. This 2-stage actuator requires many devices in order to obtain both a large stroke and precise movement. The moving parts also have a heavy mass because of their complicated configuration. As a result, the inertia and friction forces caused by the large mass interfere with fast response and precise movement. This paper describes the design and construction of a one-dimensional tracking actuator that combines a fine tracking actuator with a coarse actuator. The system is designed in the following way: first, a voice coil motor (VCM) is used as the basic drive mechanism for high precision. The permeance method is used to obtain a large driving force for the actuator. The results of this analysis are verified by simulation and experiment. Second, a focus spring is designed to enhance the stability of the tracking actuator and the sensitivity of the focusing actuator. Third, since friction is the dominant error source, the friction force is reduced by a plastic coating, and friction identification is carried out. After the actuator is constructed based on the above design schemes, PD control is applied. The actuator has a resolution under ±0.1μm and a stroke over 33 mm.

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 0.1μm, 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 ±0.1μm 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 actuatormpẍf(t)+cẋf(t)+kfxf(t)=F(t)=Bgli(t),where 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.

References (7)

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