Theory of Device Operation
4-8 C141-E192-01EN
The forces are compensated by adding the measured value to the specified current
value to the power amplifier. This makes the stable servo control.
To compensate torque varying by the cylinder, the disk is divided into 16 areas
from the innermost to the outermost circumference and the compensating value is
measured at the measuring cylinder on each area at factory calibration. The
measured values are stored in the SA cylinder. In the self-calibration, the
compensating value is updated using the value in the SA cylinder.
(2) Compensating open loop gain
Torque constant value of the VCM has a dispersion for each drive, and varies
depending on the cylinder that the head is positioned. To realize the high speed
seek operation, the value that compensates torque constant value change and loop
gain change of the whole servo system due to temperature change is measured
and stored.
For sensing, the firmware mixes the disturbance signal to the position signal at the
state that the head is positioned to any cylinder. The firmware calculates the loop
gain from the position signal and stores the compensation value against to the
target gain as ratio.
For compensating, the direction current value to the power amplifier is multiplied
by the compensation value. By this compensation, loop gain becomes constant
value and the stable servo control is realized.
To compensate torque constant value change depending on cylinder, whole
cylinders from most inner to most outer cylinder are divided into 14 partitions at
calibration in the factory, and the compensation data is measured for
representative cylinder of each partition. This measured value is stored in the SA
area. The compensation value at self-calibration is calculated using the value in
the SA area.
4.5.2 Execution timing of self-calibration
Self-calibration is performed once when power is turned on. After that, the disk
drive does not perform self-calibration until it detects an error.
That is, self-calibration is performed each time one of the following events occur:
• When it passes from the power on for ten seconds and the disk drive shifts to
Active Idle mode.
• The number of retries to write or seek data reaches the specified value.
• The error rate of data reading, writing, or seeking becomes lower than the
specified value.