Browse Prior Art Database

Hybrid Position Error Signal Generation for Disk File

IP.com Disclosure Number: IPCOM000050556D
Original Publication Date: 1982-Nov-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 4 page(s) / 64K

Publishing Venue

IBM

Related People

Case, WJP: AUTHOR [+2]

Abstract

A head-positioning servo system for a multi-disk file derives a hybrid position error signal (PES) from continuous position information on a dedicated servo surface and from sampled intermittent position information on the data surfaces. The hybrid PES is essentially the dedicated PES adjusted, after every sample, to the value of the data PES. However, since both component position error signals represent displacement from the nearest track center and the data PES is linear over less than one track width, offsets of more than half a track between servo and data heads could result in the incorrect data track being followed. To avoid this, Gray-coded track-type information is included in the servo samples and compared with track-type information derivable from the dedicated PES to determine any necessary correction.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 33% of the total text.

Page 1 of 4

Hybrid Position Error Signal Generation for Disk File

A head-positioning servo system for a multi-disk file derives a hybrid position error signal (PES) from continuous position information on a dedicated servo surface and from sampled intermittent position information on the data surfaces. The hybrid PES is essentially the dedicated PES adjusted, after every sample, to the value of the data PES. However, since both component position error signals represent displacement from the nearest track center and the data PES is linear over less than one track width, offsets of more than half a track between servo and data heads could result in the incorrect data track being followed. To avoid this, Gray-coded track-type information is included in the servo samples and compared with track-type information derivable from the dedicated PES to determine any necessary correction. To give an adequate extended linear range on either side of the target data track center, a step function, corresponding to the difference between dedicated track type and the target track address supplied to the positioning system, is added to the hybrid PES.

The principles of the hybrid PES generation will be illustrated by reference to the various waveforms shown and to a schematic circuit for combining the dedicated and sampled information. The dedicated PES is derived from a quadrature servo pattern as described in (1). This produces a normal signal, N, and a quadrature signal, Q, the variation of which with position, as the servo head crosses the tracks, is shown in idealized form in Fig. 1. In this example, the servo head is twice as wide as the data head, the sampled or data PES is derived from a so-called "null" servo pattern written in servo sectors on each data surface as described in (2). The idealized form of this signal, as the data head crosses the tracks, is shown in Fig. 2. It will be realized that, in practice, sampling operations will produce a discontinuous data PES which will alter abruptly between points lying on the waveform of Fig. 2. The two idealized PES are shown as being in registry with each other, corresponding to no offset between servo and data heads. The boundaries between tracks are shown by vertical lines below the waveform of Fig. 2. The tracks are identifiable as one of four possible types indicated by the numbers in Fig. 2.

The data track type is identified by means of two track type identifying bits which precede the analog servo pattern in each servo sample. These bits are encoded in a Gray code so that only one of them changes from track to track. The format of a servo sample is shown in Fig 5. The start of a servo sector is identified by a first Mark which is distinguishable from recorded data. Following the mark is the two-bit Gray code. A second Mark identifies the start of the analog servo data as described in (2). The basic data PES is converted into the switched data position error signal, SWDAT, shown in Fig. 4, by inver...