TRACKING SERVO
The two side-beams generated by the grating in Fig. 20.2 are used for guiding the objective lens along the pit-spiral. To achieve this the entire slide (carrying all the components in Fig. 20.2) must be slowly moved from near the centre of the disc to the edge. The slide is motor driven, but its inertia (and that of the motor and drive system) is too great to enable it to follow the possible sideways ‘track-wobble’ which may typically be 100 microns – the light spot must remain centred on the disc-track to within 0.1 micron. The resolution of the track- ing servo, then, must be better than 1 part in 1000.
To obtain this degree of accuracy the objective lens in Fig. 20.2 is used to steer the light beam on the disc surface. It is fitted with permanent magnets and a coil in which the strength and direction of the current determine the position of the lens. The coil is part of a closed-loop servo whose feedback signals are derived as follows.
The two side-beams (tracking beams) generated at the grating early in the outgoing light path are displaced on either side of the main (scanning) beam by half the track width at the disc surface, so that they straddle the edges of the pit-row. Their reflections from the disc surface are conveyed to separate pick-up photodiodes (E and F, Fig. 20.3) sitting at either side of the main diode quadrant already described. The amount of reflected light received by each of these diodes depends on the tracking beams’ view of the pit-row. If the triple-beam should wander to the left, the left-hand beam will see flat disc-surface and a great deal of light will be reflected back into the ‘left-hand’ photodiode as a result; simultaneously the right-hand tracking beam will be continuously viewing pits whose reflectance is lower, causing the ‘right-hand’ photodiode surface to go dark. The converse is also true. By amplifying and inverting the diode currents for passage through the lens positioning (tracking) coil a feedback loop is set up whereby the main beam is kept centred on the pit track by continual maintenance of a balance in the light falling on the tracking photodiodes, hence a balance in the pit/disc views taken by the equi-spaced tracking light spots. The concept has much in common with the ATF track-following systems described for videotape in Chapters 13 and 15.
The objective lens assembly is very small and light, enabling it to respond quickly to tracking errors. Its range of movement is limited, however, and it needs to be kept in the centre of its operational range as the slide assembly gradually tracks outwards from the disc centre. The current in the tracking coil is monitored in a long time-constant circuit which only produces an output when a sustained deflection is taking place. This output controls the tracking motor which drives the slide assembly, and the overall action is to keep the average volt- age across the tracking coil at zero.