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Self-Healing Actuator Servo for Hard Disk Drives

IP.com Disclosure Number: IPCOM000013575D
Original Publication Date: 2000-Apr-01
Included in the Prior Art Database: 2003-Jun-18
Document File: 9 page(s) / 96K

Publishing Venue

IBM

Abstract

Disclosed is a systematic onboard tuning-method that allows lowering the bandwidth, while increasing the gain-margin (GM) and keeping the phase-margin (PM) fixed at its maximum. A method and apparatus are shown for improving the servo performance of a deteriorating hard disk drive (HDD). This allows for a constant servo stability for the deteriorating HDD, while sacrificing (lowering) its open-loop servo bandwidth.

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This is the abbreviated version, containing approximately 19% of the total text.

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Self-Healing Actuator Servo for Hard Disk Drives

   Disclosed is a systematic onboard tuning-method that allows lowering the
bandwidth, while increasing the gain-margin (GM) and keeping the phase-margin
(PM) fixed at its maximum. A method and apparatus are shown for improving the
servo performance of a deteriorating hard disk drive (HDD). This allows for a
constant servo stability for the deteriorating HDD, while sacrificing (lowering)
its open-loop servo bandwidth.

This invention deals with the graceful performance
degradation of an HDD (hard disk drive) over time.
Just like the performance (acceleration, braking,
control) of an electro-mechanical automobile
deteriorates with time, the performance of an
electro- mechanical HDD will be reduced. While the
new automobile responds quickly to commands, the
older automobile does not. However, with good care
and maintenance (healing) the automobile may last
with acceptable performance for many years.

The servo performance of an HDD control system
refers to both the control systems frequency-domain
and time-domain characteristics. In the
frequency-domain, measurement terms like
phase-margin (PM) and gain-margin (GM) is commonly
used. An acceptable servo performance is often
associated with PM > 30 degrees and GM > 10 dB.
The response to a step change is used in the
time-domain to measure performance, for example,
step- overshoot (%), step rise-time (10% - 90%),
and step settle- time. Time-domain performance
goals for HDDs may be, overshoot <20%, fast
rise-times and fast settle-times.

It is commonly known that the servo performance in
an HDD shows individual variations at manufacturing
time between each sensor and
head/suspension/actuator structure. Over time, the
servo performance will usually deteriorate due to
unavoidable mechanical changes in the
head/suspension/actuator structure and changes in
the position sensors. Some factors causing the
deteriorating servo performance are:

     Actuator voice-coil-motor (VCM) magnetic
field changes due to temperature or
magnet-degaussing with age.

     Reduction in VCM coil-current due to
increased VCM coil- resistance with temperature.

     Changes in damping and frequency of arm and
suspension resonances due to mechanical wear, like
loose swage joints, dust, temperature, etc.

While changes occur in the individual sensor and
head/suspension/actuator structure, the servo
compensator, defined as the estimator/controller
complex, resides in microcode and does not change
with time or temperature. This microcode is stored

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in the microprocessor and consists of several
stored algorithms with an individual set of tuned
coefficients for each sensor and
head/suspension/actuator structure. At the time of
manufacture, tuning individually the compensator
for each head/suspension/actuator structure is
always possible. This means that each sensor and
head/suspension/actuator structure has a unique set
of coefficients. However, in the customers work
area, to recompute another unique set of
coefficients is difficult....