Browse Prior Art Database

Improved Settle Algorithm

IP.com Disclosure Number: IPCOM000102397D
Original Publication Date: 1990-Nov-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 3 page(s) / 96K

Publishing Venue

IBM

Related People

Baker, P: AUTHOR [+2]

Abstract

This article describes an improved calculation of when it is safe to allow the data channel to write on the track of a DASD disk file.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Improved Settle Algorithm

       This article describes an improved calculation of when it
is safe to allow the data channel to write on the track of a DASD
disk file.

      When a disk file actuator head is moved to a new track it is
necessary to decide when it is close enough to the track center to
allow the data heads to write and whether it is likely to stay within
that required distance.  The following factors complicate the
process:  the actuator will be arriving with some velocity and some
position error;  the loop dynamics dictate how these will decay.
Also resonances in the system will have been excited together with
noise on the position error signal (PES) and imbalance in the spindle
motor that will cause the tracks to be eccentric.

      Prior art algorithms generally use some combination of delay
and a threshold, i.e., the PES is within the threshold for more than
five consecutive samples then consider the actuator settled, an
approach with  problems of eccentricity and overshoot.  In the
situation where the disk has an eccentricity of say 5% of a track
(trk) and noise on the PES of, say, 3% trk, then setting a threshold
of 10% trk will cause the settle times in some cases to be
significantly longer than necessary.  With overshoot, the delay will
have to be large enough to cope with the situation where the actuator
enters within the threshold, crosses track center and then overshoots
the other threshold.

      The new algorithm stems from the notion that although the
eccentricity is a large percentage of the threshold, the servo
requires only a small current to follow it.  Also it is sensible to
factor the velocity into the decision. Clearly, being 5% from track
center with a small velocity toward the track center is better than
5% off with the same velocity away from the center.  The demand is a
mix of the position and velocity.  A moving average is formed from
the demand, and when it falls below a set value, the actuator is
considered settled.  However, the input to the averager must be a
unipolar signal to avoid cancellation.  As a consequence the
algorithm squares the demand and adds it to the moving average which
is now a measu...