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the head so the AMC gives equal amplitudes for these two bursts.
The
difference between the burst amplitudes is a measure of how closely
the head is centered; if the difference becomes too large (about one-
fifth of a head width), the drive stops reading or writing.
Adaptive Gain Parameters
To improve servo operation, three adaptive gain parameters are mea-
sured during recalibration and stored in RAM.
In addition, two of
these values are updated while the drive is operating.
See the Power-
Up Sequence, Figure 2-15.
o
KAG, or servo gain.
This is essentially a measure of the
physical head width, though it depends to a lesser extent
upon other factors.
Since the head width doesn't vary with
temperature, KAG is measured for each head, but only once--
at the middle track, during recalibration.
This is a feed-
forward value, used by the microprocessor to predict and
compensate for burst amplitude variations when switching
heads.
o
NULL I, or force bias.
This is the dc actuator current
required to overcome forces on the actuator from bearings,
windage, the flex circuit, gravity (in some orientations),
and to compensate for any offset in the actuator circuitry.
NULL I is measured by changing the value in small steps
until the head accurately centers over the track.
It is
measured at the inner cylinder and outer cylinder, and is
the same for all heads.
Linear extrapolation between the
two values accurately predicts the parameter for all
cylinders.
Since NULL I changes with temperature, it is
constantly measured and the value in RAM is updated.
o
Servo AGC, the gain of a head when reading head position
data from bursts.
It is measured for each head, at the
inner and outer cylinders.
Linear extrapolation accurately
predicts the parameter at other cylinders.
To correct for
changes with temperature, a fixed offset from the predicted
value is constantly measured, and the updated offset is
stored in RAM.
When reading servo data in a wedge, Servo
AGC overrides the CAGC signal from U2 (B), and controls the
gain of the U2 (A) AGC amplifier.
AMC Circuit Description
The AMC measures servo-burst amplitude by full-wave rectifying the
signal, integrating it, and then converting it from analog to digital.
2-24
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