Browse Prior Art Database

File Flyheight Monitor With No Added Hardware

IP.com Disclosure Number: IPCOM000100493D
Original Publication Date: 1990-Apr-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 4 page(s) / 157K

Publishing Venue

IBM

Related People

Cunningham, EA: AUTHOR

Abstract

A method to monitor changes in flyheight of heads in a Direct Access Storage Device (DASD) without any additional hardware requirements in the device uses special patterns recorded during the manufacture of the system and the normal data detection hardware in the device.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 42% of the total text.

File Flyheight Monitor With No Added Hardware

       A method to monitor changes in flyheight of heads in a
Direct Access Storage Device (DASD) without any additional hardware
requirements in the device uses special patterns recorded during the
manufacture of the system and the normal data detection hardware in
the device.

      During the manufacture of the DASD using flying heads over
magnetic media, one or more special pattern sectors are written with
each head in a protected area.  The written pattern is composed of
high and low frequency content.  The high frequency content output
signal experiences a more rapid change in the well known separation
loss than does the low frequency content signal.  Changes in fly
height over time, which may indicate problems, may be detected by
comparing the ratio of the high to low frequency signal to the value
tested at the time of manufacture.

      A commonly used peak-detection system is assumed for
illustration purposes, having a preamplifier for the head output
signal, and an Automatic Gain Control (AGC) amplifier that adjusts
the gain to maintain the peak amplitudes equal to a set value.  The
detection of the signal includes a differentiator and zero crossing
detector to determine the time position of the peaks.  However, the
system also includes an amplitude qualification circuit, to verify
that a peak indicated by the zero derivative, is an actual pulse from
the recording, and not just a noise bump.  One of the simplest
qualifiers is a threshold detector, typically set to about 1/2 of the
nominal peak amplitude.  Peaks over the 1/2 level are qualified as
legitimate peaks, and those less are disqualified.  The indicated
zero crossing is then eliminated from the data.  Other qualifier
circuits also work for the disclosed system.

      The special pattern recorded depends on the recording code, but
for illustration a commonly used code is assumed. The lowest
frequency obtained with a simple pattern for this code is one with 7
zeros between ones.  This represents a magnetization having 8 clock
cell times between transitions. Consecutive transitions are of
alternate polarity, so the period of the waveform is 16 clock cell
times.  This is the base pattern of low frequency content to be used.

      The high frequency portion of the pattern is generated by a
special circuit, since it is not of the form of a legal data pattern.
 The circuit causes two additional transitions spaced close together
to be inserted into the recording data stream midway between the
transitions that are 8 clock cells apart.  The spacing of the added
transitions is varied from a separation of about one clock cell time
to about two clock cell times.  At the beginning of a sector, the
transitions are added at the close spacing, and the spacing is
uniformly increased until the separation at the end of the sector is
about two cell times.

      The addition of the transitions causes a high...