4.5 Self-calibration
Figure 4.3 Power-on operation sequence
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 tarque. 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.
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Theory of Device Operation
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 varing by the cylinder, the disk is divided into 8 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 8 partitions at calibration in the factory, and the compensation data is measured for representive 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 executed when:
• The power is turned on.
• The disk drive receives the RECALIBRATE command from the host.
• The self-calibration execution timechart of the disk drive specifies selfcalibration.
The disk drive performs self-calibration according to the timechart based on the time elapsed from power-on. The timechart is shown in Table 4.1. After poweron, self-calibration is performed about every five or ten or fifteen minutes for the first 60 minutes or six RECALIBRATE command executions, and about every 30 minutes after that.
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