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OCTOBER 2003 1 Inside a DVD Player: Laser Lens Focus and Tracking John Y. Hung, Senior Member, IEEE Auburn University Electrical & Computer Engineering Department 200 Broun Hall, Auburn, AL, 36849-5201, U.S.A. E-mail: [email protected] URL: www.eng.auburn.edu/hungjoh Abstract The laser lens focus and tracking mechanism described here is from a Philips DVD/VideoCD/CD player (model # DVD 711, manufactured December 2000). A brief overview of the overall mechanism is presented, followed by a more detailed description of the focus and fine tracking system. I. THE OVERALL MECHANISM The laser system platform in a DVD/CD player is mounted two rails, which can be seen in Fig. 1. The disk clamping mechanism and spindle drive motor can also be seen in the upper right hand corner of the figure. During normal use, disks are clamped to the spindle mount between a permanent magnet and a thin steel washer. The entire platform rests on a set of soft rubber mounts that serve two functions: 1) Isolate the laser system from external vibration, and 2) Allow the far end of the platform to pivot up and down. Tilting the platform downward sufficiently far causes the thin steel washer to peel away from the spindle mount, thus releasing the disk for removal. On the left hand side of the picture, one can make out the rack and pinion gear mechanism use for coarse positioning of the laser system. Another view of the coarse positioning system and the laser lens actuator is shown in Fig. 2. II. THE LENS ACTUATOR A closer view of the laser lens actuator is shown in Fig. 3. The lens actuator in this DVD player generates two degrees of motion: tracking and focus. Tracking is a fine motion control in the direction parallel to the two main rails. Motion control error must be well below 0.5 micron, since the distance between adjacent tracks is 0.74 microns on a DVD (1.6 microns on CD). Typical diameter of a pit on a DVD track is about 0.28 micron [1]. Laser spot ocus is adjusted by moving the lens vertically. Note that the round crystal-like object shrouded by the black cover is the lens; it is not the laser, which is actually located roughly 0.5 cm below the lens. A. Cover removed Removing the black plastic cover of the focus and tracking actuator reveals the key elements of the actuator, as seen in Fig. 4. The movable lens carrier at the center rides on a vertical pin, and two degrees of motion are possible. Slight rotation of the carrier around the pin translates into fine linear motion (tracking). Sliding the carrier up and down along the pin results in focus adjustment. The tops of four magnets surround the lens carrier, and the outer case functions as the actuator stator.

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Page 1: DVD Lens Actuator

OCTOBER 2003 1

Inside a DVD Player:Laser Lens Focus and Tracking

John Y. Hung,Senior Member, IEEE

Auburn UniversityElectrical & Computer Engineering Department

200 Broun Hall, Auburn, AL, 36849-5201, U.S.A.E-mail: [email protected]

URL: www.eng.auburn.edu/ ∼hungjoh

Abstract

The laser lens focus and tracking mechanism described here is from a Philips DVD/VideoCD/CD player (model# DVD 711, manufactured December 2000). A brief overview of the overall mechanism is presented, followed bya more detailed description of the focus and fine tracking system.

I. THE OVERALL MECHANISM

The laser system platform in a DVD/CD player is mounted two rails, which can be seen in Fig. 1. Thedisk clamping mechanism and spindle drive motor can also be seen in the upper right hand corner of thefigure. During normal use, disks are clamped to the spindle mount between a permanent magnet and athin steel washer. The entire platform rests on a set of soft rubber mounts that serve two functions:

1) Isolate the laser system from external vibration, and2) Allow the far end of the platform to pivot up and down. Tilting the platform downward sufficiently

far causes the thin steel washer to peel away from the spindle mount, thus releasing the disk forremoval.

On the left hand side of the picture, one can make out the rack and pinion gear mechanism use forcoarse positioning of the laser system. Another view of the coarse positioning system and the laser lensactuator is shown in Fig. 2.

II. T HE LENS ACTUATOR

A closer view of the laser lens actuator is shown in Fig. 3. The lens actuator in this DVD playergenerates two degrees of motion: tracking and focus. Tracking is a fine motion control in the directionparallel to the two main rails. Motion control error must be well below 0.5 micron, since the distancebetween adjacent tracks is 0.74 microns on a DVD (1.6 microns on CD). Typical diameter of a pit on aDVD track is about 0.28 micron [1]. Laser spot ocus is adjusted by moving the lens vertically. Note thatthe round crystal-like object shrouded by the black cover is the lens; it is not the laser, which is actuallylocated roughly 0.5 cm below the lens.

A. Cover removed

Removing the black plastic cover of the focus and tracking actuator reveals the key elements of theactuator, as seen in Fig. 4. The movable lens carrier at the center rides on a vertical pin, and two degreesof motion are possible. Slight rotation of the carrier around the pin translates into fine linear motion(tracking). Sliding the carrier up and down along the pin results in focus adjustment. The tops of fourmagnets surround the lens carrier, and the outer case functions as the actuator stator.

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OCTOBER 2003 2

B. Actuator motion

The combination of tracking and focus motions is illustrated by the two image sequence shown inFig. 5. A regular wooden pencil is pointing at the lens at the start of the sequence (left hand image). Inthe right hand image, the pencil tip has moved the lens to the left and downward. Evidence of downwardmotion is revealed by increased exposure of the vertical pin tip.

III. T HE ACTUATOR DISASSEMBLED

In this actuator, the lens carrier is suspended at the pin midpoint by permanent magnets, and motionis achieved by adjusting electrical currents in tiny coils mounted on the carrier. Four conductors in a thinflexible ribbon can be seen in the foregrounds of Fig. 4 and Fig. 5. The coils are revealed by lifting thecarrier off the pin, as shown in Fig. 6. Two pairs of coils are mounted on the outer circumference of thecarrier. One pair is used for tracking adjustment, and the other pair for focus adjustment.

Changing coil currents induces motion in the same way that audio speakers produce motion: electriccharge moving in a magnetic field experience force (Lorentz force phenomenon). Actuators of the typefound in this mechanism are known as “moving coil” actuators. A moving coil actuator has the char-acteristics of low mechanical inertia and low electrical time constant, both desirable properties in highperformance electromechanical motion control systems.

IV. D EEPERINSIDE

Two pairs of magnets are revealed in Fig. 7, which shows the lens carrier completely off the verticalpin, and flipped over to one side. The thin ribbon of four conductors is also visible. The magnets are therectangular shaped elements cemented to the inside of the actuator wall, which constitutes the actuatorstator. The laser and pickup head unit are clearly visible to the right of the pin (the glassy blue element).In operation, the laser beam is actually reddish in color (650 nm wavelength).

V. FINAL COMMENTS

The lens focus and tracking actuator system in a DVD/CD player is a beautiful example of “mechatron-ics’, the engineering field that develops advanced electronically controlled mechanical systems. Controlelectronics are tightly integrated and “embedded” within the mechanical structure. The lens focus andtracking controllers, as well as laser power controller, are still implemented in analog electronics toachieve high performance in an extremely small volume. The truly astounding fact is that a completeDVD player can be purchased today for US$40!

In fall 2002, Philips Semiconductors announced the OPU66.20 optical pickup unit [2], featuring 3-Dlens motion control: focus, tracking, and tilt. Tilt control is used to reduce astigmatism in the laser beamspot. The actuator is supported by wire springs, and entire pickup unit weighs a mere 45 g! Analogelectronics remain at the heart of the control system.

REFERENCES

[1] Pioneer Corp, “DVD Technical Guide,” 2003. Available in electronic form at http://www.pioneer.co.jp/crdl/tech/dvd/1-e.html[2] Philips Semiconductors, “OPU66.20 Optical Pickup Unit for DVD+R/RW and CD-R/RW,” document order # 9397 750 10613, October

2002.

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Fig. 1. A DVD/CD disk handling system - the laser system rides on two rails

Fig. 2. Coarse positioning system (rack and pinion gear, motor) in the foreground

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Fig. 3. Laser lens focus and fine motion tracking unit

Fig. 4. Focus and tracking unit with cover removed

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Fig. 5. Actuator motion: initial state (left picture), lens pushed to the side and down (right picture)

Fig. 6. “Moving coils” on the lens carrier

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Fig. 7. Actuator magnets exposed; laser and pickup unit is also visible