Servo Track Writer - Hard Disk Drive: Mechatronics and Control - page 313

Writing of Servo Pattern
The patterns of transitions de fi ning different fi elds of the servo sectors are
stored in a shift register of N bytes, from where they are shifted out serially
to the write circuit. The clock signal generated by the PLL locked to the clock
track is used to shift the bits out of the shift register. The output of the shift
register, being 0 or 1, is written as the servo pattern at the desirable tangential
position determined by the clock count. The INDEX mark in the clock track
is also used to synchronize the pattern generator. If the patterns for all servo
sectors are the same then the required size of the shift register is shortened as
thesamepatterncanberepeated.

5.2.2 Control of Radial Position using Mechanical Push-
pin

Another very important task in the servo writing process is to ensure accurate
and precise control of the radial position of the write head in compliance to
the requirements of track density or TPI.
The traditional method of servo writing uses an external mechanical posi-
tioner whose position is controlled with ultra-high precision. The VCM of the
HDA is biased with a DC current forcing the actuator arm to remain fi rmly in
contact with a part of the mechanical positioner known as the push-pin.Since
the actuator arm of the VCM is in contact with the mechanical positioner, con-
trolling the position of the mechanical positioner is equivalent to controlling
the position of the VCM actuator's arm. The external mechanical positioner
provides an accurate radial reference position for servo writing process. The
external positioner and the pushpin pushes the VCM and therefore the write
head to the desired radial positions. Once the head is positioned at the desired
location, the servo pattern shift register, triggered by the clock signal, sends
out pattern to be written.
The push-pin, being part of a positioning system external to the HDD,
must be inserted into the enclosure of the HDD so that the HDA's actuator
canbebiasedtoforcethearm fi rmly against the push-pin. The insertion of
the push-pin can be either from the side of the HDD enclosure or from the top
of the enclosure.
A rotary actuator with a grating as position sensor can be used as the
external positioner. A feedback controller is used to control with ultra-precision
the position of the actuator of external positioner. One such device available
in the market is MirocE micro positioner PA 4046 G whose resolution can be
as high as 4.68 nanoradians with an accuracy of 35 nanoradians rms ∗ .
It is possible to achieve a servo bandwidth greater than 1 kHz for the posi-
tion control loop using the external positioner, which is a VCM actuator with
optical feedback. However, when the push-pin pushes the actuator of the HDD,

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See webpage at http://www.microesys.com/