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C141-E090-01EN4 - 18
(3) Preamble
This area is used to synchronize with the PLL, which is used to search the SSM by detecting
the ASM.
(4) Gray code (including index bit)
This area is used as cylinder address. The data in this area is converted into the binary data by
the gray code demodulation circuit.
(5) Pos A, Pos B, Pos C, Pos D
This area is used as position signals between tracks, and the device control at on-track so that
Pos A level equals to Pos B level.
(6) PAD
This area is used as a gap between servo and data.
4.7.4 Actuator motor control
The 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 track-following operation to position the head onto the target track.
(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 inner circumference shunt zone to the
normal servo data zone in the following sequence:
a) Micro current is fed to the VCM to press the head against the inner circumference.
b) A current is fed to the VCM to move the head toward the outer circumference.
c) When the servo mark is detected the head is moved slowly toward the outer circumference
at a constant speed.
d) If the head is stopped at the reference cylinder from there. Track following control starts.