|ENGINEERING TRIPOS PART II A and EIST I|
|EIET LABORATORY||ENGINEERING AREA ACTIVITY|
Areas 5,6,7 and 8
|INVESTIGATION OF THE DESIGN OF CD PLAYERS|
SUBTOPIC: MOTOR AND DRIVES
The CD player and CD-ROM require several electromechanical motors and actuators to provide some of the primary functions of the unit. The most important functions are controlling the rotation of the disc, the radial tracking of the optical head and the focusing of the laser. The precision required is astounding and these systems must accomodate various problems with the discs, such as the error in the location of the disc in the central clamping mechanism and warpage of the disc.
There is a wide choice of devices suitable for the three main electromechanical actuators. Each requires an interface to the microprocessor and a drive circuit. Feedback control is also essential. For example, in the Philips CD player, the signal is read from the disk by an electro-optical front end, and then after amplification and filtering by front-end electronics, the resulting analogue electronic signal is fed into the SAA7310 decoder IC. This powerful IC demodulates the signal for audio use, in the process extracting bit-timing information used to control the disc rotation speed.
The main aim of this exercise is to understand the operation of the drive circuits, investigate the performance of the actuators and to attempt to understand and explain the factors that influenced the design.
To do this, you will need to draw on a number of sources of information. However, the majority of the work is electronics. You will also have to communicate with the other groups in your session to identify the control signal paths and to understand their importance. You will be able to access some relevant signals inside the CD player.
The main sources of useful information are likely to be:
* Available to you when you start doing the activity.
To make efficient use of your time, you need to plan your investigation. The following is a suggested outline plan, but you are free to vary it:
A modified Cd player is available for you to examine in detail the drive circuits for the spindle drive, tracking arm and focusing actuator. The drive sircuits themsleves have been duplicated onto and external circuit board, so that you have access to all of the signals. There is also an extra mechanism for you to drive directly.
Find out about the physical characteristics of the actuators.
Consider the track spacing, rotation/linear speeds required.
Investigate the CD930 technical manual, locate the relevant signals from the front-end amplifiers to the SAA7310, and the relevant output signals.
Read the descriptions of the custom electronics, and find out what they are designed to let you do.
Using the test points provided, examine the dynamic rage of the signals applied to the actuators.
Test the actuators using external supplies.
Assess the design decisions that led to the CD electromenchanical system being designed in the way it is.
Is it over- or under-specified? What are the consumer requirements? What alternative methods can you find?
Put together your presentation, aiming to explain briefly what the motors and actuator systems were designed to acheive (in terms, for example, of the disc speed and track spacing), what technical solutions have been chosen, what their overall performance characteristics are.
Full details will be provided when you start the activity.
Dr P.R. Palmer, October 2003