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C141-E090-01EN4 - 8
4.5 Self-calibration
The disk drive occasionally performs self-calibration in order to sense and calibrate
mechanical external forces on the actuator, and VCM torque. This enables precise seek and
read/write operations.
4.5.1 Self-calibration contents
(1) Sensing and compensating for external forces
The actuator suffers from torque due to the FPC forces and winds accompanying disk
revolution. The torque vary with the disk drive and the cylinder where the head is positioned.
To execute stable fast seek operations, external forces are occasionally sensed.
The firmware of the drive measures and stores the force (value of the actuator motor drive
current) that balances the torque for stopping head stably. This includes the current offset in
the power amplifier circuit and DAC system.
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 14 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.