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Single Track Step

IP.com Disclosure Number: IPCOM000044866D
Original Publication Date: 1984-Dec-01
Included in the Prior Art Database: 2005-Feb-06
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Arter, NK: AUTHOR [+3]

Abstract

In an optical or magnetic disk drive there is a track following servo which keeps the transducer (laser scanning spot or magnetic recording head) positioned over a single track. This servo must also be able to step the transducer to an adjacent track quickly and reliably. It is not feasible to do this under closed loop control because of the unstable region of the tracking error signal between tracks and because of imperfections in the disk or optics such as varying track widths, defects, and lens off axis performance which make the tracking error signal quite variable.

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Single Track Step

In an optical or magnetic disk drive there is a track following servo which keeps the transducer (laser scanning spot or magnetic recording head) positioned over a single track. This servo must also be able to step the transducer to an adjacent track quickly and reliably. It is not feasible to do this under closed loop control because of the unstable region of the tracking error signal between tracks and because of imperfections in the disk or optics such as varying track widths, defects, and lens off axis performance which make the tracking error signal quite variable.

Typical open loop stepping is very unreliable because of the relatively large run out of the disk compared to the track pitch. In an optical disk drive, the lens has to follow disk run out of up to 200 tracks so there are large variations in acceleration, velocity, and position conditions over which the single track step must work. If the step is not completed properly, the loop can be closed in an unstable region causing the lens to be driven across many tracks and even causing the drive to lose focus.

Fig. 1 shows a block diagram of apparatus for stepping a laser spot that is simple and reliable. Fig. 2 shows the tracking error signal 1 vs. track position. Fig. 3 shows the expected signals during a step.

In Fig. 1, during normal track following both sample and holds 10 and 12 are in sample mode and the system uses conventional servo techniques to keep the spot centered either over the land or over the groove. The remainder of this discussion will be for land tracking but it applies equally to groove tracking. A system set up for land tracking is stable while over lands but is unstable over the groove area because of the sign change of tracking error vs. position. (See Fig.
2.)

The tracking error signal is proportional to the amount of current in the tracking coil which determines the position of the lens. In order to step, sample and hold 10 is put into hold mode by logic 11 and a slight accelerating voltage of the proper polarity is applied to amplifier 14 by generator 15. The effect of this is to provide an acceleration of the lens toward the desired track, while holding the servo loop at its previous value. By putting the loop on hold, the initial conditions at the start of step are held in effect during the step. The step is accomplished in about one millesecond so there is very little difference between the initial servo conditions for the track that is being stepped to and the conditions at the start of the step.

As the lens moves over the groove, comparator 16 senses the middle of the groove by detecting a zero crossing of the tracking error signal 1. At this point, the accelerating voltage 8 is turned off, sample and hold 12 is put into hold mode and sample and hold 10 is put into sample...