CD-ROM DRIVE

Case Study

Term II, 2000

EE 352 – Network Theory II & Circuit Lab

Cece Thompson

Drew Fultz

Table of Contents:

1) Introduction

2) CD ROM Mode of Operation

3) Components and their function

4) DC Motors in the CD-ROM

5) DC Motors Mode of Operation

6) Power Calculations

7) Conclusion

INTRODUCTION:

This case study describes the basic functioning components of the CD-ROM drive. For this purpose we chose to study the Creative Labs model CD420E 2X Internal CD ROM. In this report we will first give a brief description of how a CD-ROM works. Next we will identify the major components of the device. Then we will discuss in detail the use and operation of DC motors in this CD-ROM drive. Finally we will discuss the power consumption of the motors in this circuit. Lets begin with a brief description of what a CD-ROM is.
CD ROM OVERVIEW

Fig 1

“Compact Disc Player”, Scientific America “Working Knowledge Series”

CD-ROM, in computer science, is an acronym for compact disc read-only memory, a rigid plastic disk that stores a large amount of data through the use of laser optics technology. Data is read from a CD-ROM with a low power laser contained in the drive that bounces light – usually infrared – off of the reflective surface of the disk and back to a photo detector. Electronics and software interpret this data and accurately access the information contained on the CD-ROM. A CD-ROM holds approximately 630 megabytes. By comparison, a regular floppy disk holds approximately 1.44 megabytes. CD-ROM players are used almost exclusively as computer components or peripherals. They may be either internal (indicating they fit into a computer’s housing or external (having their own housing and are connected to the computer via an external port).

["CD-ROM," Microsoft® Encarta® Encyclopedia 2000]

MODE OF OPERATION

The CDROM drive receives instructions from the computer telling it to retrieve information from a CD and deliver that information back to the computer (Fig 2). The CD drive responds by performing several tasks. The tray motor pushes the CD tray out, so that a CD can be loaded onto it, then pulls it back in. The Lens sled motor, which is ideally a linear stepper motor, moves the laser send/receive unit into position to read the start of the CD data track. The spindle motor then engages, spinning the disk so that the information can be read from the tracks.

	Fig 2

DC MOTORS

The majority of work in a CDROM drive is handled by 3 DC motors: The tray motor; the lens sled motor: and the spindle motor (Fig 3). These motors are supplied with 5 to 12volts from the Driver board. When the control board needs these motors to engage, it sends a 5volt signal to the driver board. The driver board circuitry then supplies a ground to one of the motor terminals, which drops creates a closed circuit and causes the motor to turn. Drew will now give you some more information on how these DC motors function.

	Fig 3

DC MOTORS OPERATION

Fig 4

In direct-current (DC) motors, a split-ring commutator switches the direction of the current each half rotation to maintain the shaft's direction of motion (Fig 4). In a brushless DC motor, the rotating portion contains a permanent magnet and the conducting coil of wire is stationary. In any motor, the stationary parts constitute the stator, and the assembly that turns is called the rotor, or armature. This creates a torque ripple that causes a normal dc motor unable to be used for the purposes of rotating the cd-rom disk since the reader needs a nearly constant speed in order to reliably read data from the disk. The spindle motor compensates for this problem by having multiple magnets and stator, these allow for high starting torque with minimum torque ripple.

Fig 5

POWER CALCULATIONS:

A CD-ROM drive uses DC power and components. This simplifies power calculations greatly. There are many power consuming components in the standard CD-ROM drive however we will concentrate on the three motors since they use the greatest amount of power. The schematic below (Fig 6) is a representation of the circuitry for the 3 motors. The tray and sled motors use the same 12volt power source; the spindle motor uses the 5volt source. Only one of the 12volt motors will run at a time, but the sled motor and the spindle motor will run simultaneously. We measured the internal resistance of the three motors and used that amount for our power calculations (Fig 7). The largest power draw will occur when the sled motor and spindle motor are running. The power used in this situation would be 10.21W + 3.7 W or about 14W.

		Fig 6

CONCLUSION:

This case study shows one application of program driven electromechanical controls or “Mechatronics”. The components used to run a CD ROM: The Control board, Driver Board and Electromechanical elements are typical of what are found in all modern appliances. Control boards replace the old method of using switches, gears and cams for timing control. The driver board replaces relays and other electrical switches. The individual servomotors replace a whole series of belts, chains and clutches that were used to deliver mechanical power throughout older machines. The technology in this application has allowed us to make modern appliances smaller, lighter and cheaper than in the past. As this technology improves, CDs and the newer DVDs will become faster and more versatile.

SOURCES

Archibald, Roger, Extending storage beyond the enterprise. (Technology Information). Vol. 11, HP Professional, 11-01-1997, pp 38(2).

"CD-ROM," Microsoft® Encarta® Encyclopedia 2000. © 1993-1999 Microsoft Corporation. All rights reserved. "CD-ROM," Microsoft® Encarta® Encyclopedia 2000. © 1993-1999 Microsoft Corporation. All rights reserved.

Whelan, Carolyn, DVD-Rewritable Donnybrook.(Philips, Panasonic battling over standards) (Company Business and Marketing). Vol. 44, Electronic News, 06-29-1998, pp 40(1).

Bigelow, Stephen J “Troubleshooting & Repairing PC Drives & Memory Systems”, 2nd ed., McGraw Hill, New York. 1998

Pohlmann, Ken C. “Compact Disc Player”, Scientific America “Working Knowledge Series” http://www.sciam.com/1998/0998issue/0998working.html