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Use of Elliptic Filters With Disk Drive Servo

IP.com Disclosure Number: IPCOM000101404D
Original Publication Date: 1990-Aug-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 2 page(s) / 69K

Publishing Venue

IBM

Related People

Ottesen, HH: AUTHOR

Abstract

The unique application of using an elliptic notch (bandstop) filter in cascade with the voice coil motor (VCM) actuator to effectively suppress mechanical resonances in a disk drive servo minimizes the degradation in servo performance by maintaining good phase margin.

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Use of Elliptic Filters With Disk Drive Servo

       The unique application of using an elliptic notch
(bandstop) filter in cascade with the voice coil motor (VCM) actuator
to effectively suppress mechanical resonances in a disk drive servo
minimizes the degradation in servo performance by maintaining good
phase margin.

      The use of filters to reduce the effects of mechanical
resonances in the casting/actuator/suspension structure of a disk
drive has been commonly used for the last 20 years. Early on, all
filters were analog, but today both analog and digital filters are
being cascaded with the actuator plant.

      The problem with such filters, analog or digital, is that they
reduce the servo stability by reducing the available phase margin.
The use of low-pass filters is in general ruled out since they cause
very large phase shifts at the servo O dB crossover frequency (ZCF).
Notch (bandstop) filters exhibit much less phase shift at ZCF and are
therefore much more attractive.

      The most frequently used type of notch filter in disk drive
servo control has been the Butterworth notch filter, where one or
more second-order sections are cascaded with the actuator plant.  The
attenuation characteristics of a typical Butterworth filter versus
frequency is a very sharp and narrow notch.

      The sharp Butterworth notch has a problem.  A Butterworth
filter with a notch at fO (Hertz) may attenuate a mechanical
resonance at fO (Hertz) very well; however, the resonance frequency
can vary as much as, plus or minus, 20 percent about its nominal
value over the entire file population.  This is quite common in disk
drives, and the attenuation at the extremes...