Browse Prior Art Database

Track Seek Using Continuous Servo

IP.com Disclosure Number: IPCOM000045136D
Original Publication Date: 1983-Feb-01
Included in the Prior Art Database: 2005-Feb-06
Document File: 4 page(s) / 59K

Publishing Venue

IBM

Related People

Hansen, NH: AUTHOR [+3]

Abstract

Track seeking is the process of moving a recording head over several tracks on magnetic disk to the desired track. Current seek methods move the head either open loop or with velocity feedback to the desired track, then switch to position feedback to hold the head on track. The timing of that switch in feedback modes is critical and position error transients often occur during the transition time.

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Track Seek Using Continuous Servo

Track seeking is the process of moving a recording head over several tracks on magnetic disk to the desired track. Current seek methods move the head either open loop or with velocity feedback to the desired track, then switch to position feedback to hold the head on track. The timing of that switch in feedback modes is critical and position error transients often occur during the transition time.

A method is presented here where the seek and track hold can be done entirely with position feedback, eliminating switching between feedback modes. This technique is best used when the servo information from the disk is continuous rather than sampled.

The head position information or PES (Position Error Signal) is derived from the servo signals. For many servo patterns (i.e., tri-bit, di-bit, two frequency, etc.) the PES is triangular in shape, as shown in Fig. 1. The track centers coincide with the zero crossings of the PES. As the head moves across the servo tracks, a triangularly shaped PES is generated. The period between zero crossings is inversely proportional to the head velocity. The PES will have long periods between zero crossings at the start of a seek, will gradually decrease the period until the maximum head velocity is achieved, and will then increase the track crossing period until the final track is reached.

To use the PES during the seek operation, it is necessary to generate a PES reference with the desired number of zero crossings and the correct period between the zero crossings. The actual PES is then subtracted from the reference signal to develop the feedback signal.

The PES, as shown in Fig. 1, changes slope every track. This causes a polarity reversal in the position feedback control loop that must be accounted for in the servo design. A simple sign changer that has one polarity for all even tracks and has the opposite polarity for odd tracks will correct for the polarity reversals in the PES.

Fig. 2 shows a functional diagram for such a proposed seek method. In this figure, the velocity profile generator determines the velocity reference used for the seek.

This generator develops the velocity reference signal from the track count. Similar velocity reference generators are used in current track seek systems. For such a method, the velocity reference signal is not used for feedback control but is used to drive the position reference generator. The output of the position reference generator is the PES reference. It has a triangular shape similar to that expected from the PES during seek.

The use of the position reference generator and the technique used to generate the position reference are new to this seek method. The position reference generator integrates the velocity reference signal. When the integrator output reaches a preassigned limit, the polarity of the integrator input changes and the integrator output moves toward the negative limit. When it reaches the

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