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Browse Prior Art Database

Use of Dither in Digital Servo Control for DASD

IP.com Disclosure Number: IPCOM000102139D
Original Publication Date: 1990-Oct-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 3 page(s) / 77K

Publishing Venue

IBM

Related People

Daede, RJ: AUTHOR [+3]

Abstract

A zero-mean, low amplitude square wave or dither signal, with a period equal to an integer multiple of the period of the controller zero-order-hold, is added to the output control signal of DASD actuator controller. The dither signal causes continual motion of the actuator so that nonlinear effects do not degrade the controller performance. The controller is designed assuming that the dynamics of the actuator are linear. The dither frequency is chosen to be at a zero of the controller zero-order-hold. Dither amplitude is chosen to minimize the cost function defined as the sum of the squares of the position error signal for a fixed time period. Dither amplitude and frequency could also be made adaptive by minimizing on-line the previously given cost function.

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Use of Dither in Digital Servo Control for DASD

       A zero-mean, low amplitude square wave or dither signal,
with a period equal to an integer multiple of the period of the
controller zero-order-hold, is added to the output control signal of
DASD actuator controller.  The dither signal causes continual motion
of the actuator so that nonlinear effects do not degrade the
controller performance. The controller is designed assuming that the
dynamics of the actuator are linear.  The dither frequency is chosen
to be at a zero of the controller zero-order-hold.  Dither amplitude
is chosen to minimize the cost function defined as the sum of the
squares of the position error signal for a fixed time period.  Dither
amplitude and frequency could also be made adaptive by minimizing
on-line the previously given cost function.

      Actuator positioning controllers are typically designed
assuming the actuator dynamics are linear.  For small motions or
tracking, frictional effects in the bearings can be highly nonlinear
(e.g., stiction).  As a result, the controller may have poor
performance for small motions or tracking.  The idea we propose is to
add a zero-mean, low amplitude square wave or dither signal, with a
period equal to an integer multiple of the period of the controller
zero-order-hold, to the position control signal.  This idea is
outlined in Fig. 1.

      Note that the dither signal constantly keeps the actuator in
motion to eliminate the stiction e...