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Disk File Proximate Time

IP.com Disclosure Number: IPCOM000111812D
Original Publication Date: 1994-Apr-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 4 page(s) / 166K

Publishing Venue

IBM

Related People

Pham, IV: AUTHOR

Abstract

A new seeking algorithm can be used in the high performance disk file to improve the worst case average access time about 10%. The algorithm is similar to the prior art digital servo, except it uses variable velocity clip and feed forward voltage to ensure servo robustness with respect to actuator parameter variations. Velocity clip is to limit the velocity trajectory output and the feed forward signal is to be added into the control signal before output to the D/A converter. They are adaptively adjusted based on estimated actuator parameters.

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Disk File Proximate Time

      A new seeking algorithm can be used in the high performance
disk file to improve the worst case average access time about 10%.
The algorithm is similar to the prior art digital servo, except it
uses variable velocity clip and feed forward voltage to ensure servo
robustness with respect to actuator parameter variations.  Velocity
clip is to limit the velocity trajectory output and the feed forward
signal is to be added into the control signal before output to the
D/A converter.  They are adaptively adjusted based on estimated
actuator parameters.

      Prior art digital servo uses velocity trajectory to generate
the velocity command during seeking.  Trajectory is generated by
integrating system equations backward in time.  It must be designed
to work with both short and long seeks.  If it is only optimized for
long seek then it will overshoot very badly for a short seek.  This
is due to Voice Coil Motor (VCM) current rise time, switching delay
and velocity error when servo switches from acceleration to
deceleration.  Prior art servo overcomes the problem by reducing the
trajectory slope for the last part of the seek.  In addition, the
access time is further degraded due to digital servo implementation.
Velocity trajectory must be slightly conservative to allow digital
estimator settling and to overcome the uncertainty in discreet
switching time.  One sample switching delay can cause several tracks
over shoot in position signal.

      Prior art digital servo generates the velocity trajectory using
the worst case parameters.  The advantage of this design philosophy
is that the servo will be robust and work for the full range of
actuator parameters.  The disadvantage is that is causes the good
machine to perform like a worst case machine.  One technique to
overcome "part" of the problem is to adaptively adjust the trajectory
based on estimated power supply and forward gain.  The draw back for
this method is that both power supply and forward gain can note be
used to modify the trajectory slope for the last part of the seek.
In addition, it does not include the effect of coil resistance which
has little initial tolerance but can vary with temperature.

      This disclosure solves all problems which was described in the
last tow paragraphs.  it provides a proximate time optimal control
for all seek lengths and for any machine with any combination of
actuator parameters.  It computes the velocity trajectory using the
nominal actuator parameters and uses velocity clip and feed forward
signal to ensure robustness.

      Velocity clip is calculated at the beginning of the seek as a
function of seek length, estimated motor force factor over mass
ratio, estimated maximum control voltage and estimated temperature
rise.  Temperature rise is used to compensate for the effect of
temperature on coil resistance and motor force factor.  Expression
for velocity clip is as follows:

     ...