4.7 Servo Control
4.7.4 Actuator motor controlThe voice coil motor (VCM) is controlled by feeding back the servo data recorded on the data surface. The MPU fetches the position sense data on the servo frame at a constant interval of sampling time, executes calculation, and updates the VCM drive current.
The servo control of the actuator includes the operation to move the head to the reference cylinder, the seek operation to move the head to the target cylinder to read or write data, and the
(1) Operation to move the head to the reference cylinder
The MPU moves the head to the reference cylinder when the power is turned. The reference cylinder is in the data area.
When power is applied the heads are moved from the outside of media to the normal servo data zone in the following sequence:
a)Micro current is fed to the VCM to press the head against the outer direction.
b)The head is loaded on the disk.
c)When the servo mark is detected the head is moved slowly toward the inner circumference at a constant speed.
d)If the head is stopped at the reference cylinder from there. Track following control starts.
(2) Seek operation
Upon a data read/write request from the host, the MPU confirms the necessity of access to the disk. If a read/write instruction is issued, the MPU seeks the desired track.
The MPU feeds the VCM current via the D/A converter and power amplifier to move the head. The MPU calculates the difference (speed error) between the specified target position and the current position for each sampling timing during head moving. The MPU then feeds the VCM drive current by setting the calculated result into the D/A converter. The calculation is digitally executed by the firmware. When the head arrives at the target cylinder, the track is followed.
(3) Track following operation
Except during head movement to the reference cylinder and seek operation under the spindle rotates in steady speed, the MPU does track following control. To position the head at the center of a track, the DSP drives the VCM by feeding micro current. For each sampling time, the VCM drive current is determined by filtering the position difference between the target position and the position clarified by the detected position sense data. The filtering includes servo compensation. These are digitally controlled by the firmware.