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: FEEDBACK CONTROL LOOPS

1. INTRODUCTION

CD players employ a number of control loops which are vital to their function.

  1. The size and spacing of the pits on the disc is constant over the whole disc. As a result, the rotational speed of the disc decreases from about 10 to 3 Hz as the disc is read (from inside to outside). The bits read from the disc are written into a FIFO (first in first out) buffer. Using a clock, the bits are read out at a constant rate from the FIFO buffer. A control loop regulates the disc rotation speed with the aim of keeping the buffer half full. The loop has a bandwidth of about 10 Hz.
  2. The light intensity from the laser diode is regulated by means of a photo sensor on the rear end of the laser diode. This control loop has a bandwidth of about 150 Hz.
  3. The radial control loop governs the position of the laser spot in the direction orthogonal to the track. To maintain readability, the radial tracking error has to be kept below about 0.2mm whereas track eccentricities may be of the order of 0.2 mm. The actuator consists of the radial arm and a linear DC motor (coil and magnet). This loop has a bandwidth of about 500 Hz.
  4. Regulation of the focus depth is necessary due to irregularities in the disc height, undulations of the surface and un-orthogonal clamping of the disc. The maximum height variation is 2 mm, while the desired positioning accuracy is 2mm. The actuator consists of a lens, leafsprings and a coil-magnet linear DC motor. The focus control loop has a bandwidth of about 800 Hz.

    This activity will explore the design and operation of the above control loops. The study of the radial and focusing loops will form the main part of the activity. These loops will be investigated with the aid of measurements performed on a control rig.

    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: