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Independent Sense Coil for Disk Drive Actuator Velocity Control

IP.com Disclosure Number: IPCOM000116215D
Original Publication Date: 1995-Aug-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 2 page(s) / 74K

Publishing Venue

IBM

Related People

Albrecht, TR: AUTHOR

Abstract

Disk drives which use ramp load/unload must control actuator speed during loading to avoid disk damage. Actuator speed may be measured by monitoring the back-emf generated in a voice coil driver, the speed of actuator rotation being proportional to the back-emf voltage. In systems which monitor the back-emf of the voice coil while simultaneously driving the voice coil with an applied current, the back-emf appears as an offset voltage added to the resistive and inductive voltages developed across the voice coil. The accuracy with which the back-emf (and therefore the actuator speed) may be measured is limited by the accuracy to which the resistive and inductive voltages are determined.

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Independent Sense Coil for Disk Drive Actuator Velocity Control

      Disk drives which use ramp load/unload must control actuator
speed during loading to avoid disk damage.  Actuator speed may be
measured by monitoring the back-emf generated in a voice coil driver,
the speed of actuator rotation being proportional to the back-emf
voltage.  In systems which monitor the back-emf of the voice coil
while simultaneously driving the voice coil with an applied current,
the back-emf appears as an offset voltage added to the resistive and
inductive voltages developed across the voice coil.  The accuracy
with which the back-emf (and therefore the actuator speed) may be
measured is limited by the accuracy to which the resistive and
inductive voltages are determined.  While voltages due to inductance
become negligibly small if measurements are taken sufficiently long
after a current change, voltages due to resistance remain large.
Resistance can be measured to arbitrary accuracy prior to initiating
a load cycle, but temperature changes due to current in the voice
coil can change the coil resistance significantly on the time scale
needed to complete a load cycle.  Furthermore, all voltage
measurements, including those used to determine coil resistance,
require the use of a relatively expensive analog-to-digital converter
(A/D) with large dynamic range, linearity, and resolution.  For a
typical 2.5-inch drive with an actuator torque constant of 0.03
Newton-meter per amp, a resistance of 10 ohms, and a target
rotational speed of 1 radian per second during loading, a target
back-emf of 30 millivolts will be generated as an offset on a
resistive voltage component which may be as high as a few volts when
high actuator torque is required (for example, when the actuator is
on the load/unload ramp).  For such a system, conventional
measurement of back-emf as a...