Hardware Reference
In-Depth Information
an HDD due to cost constraints. Special techniques such as actuator arm
with low vibration [154], air shroud around the disks [42], very well balanced
disk-spindle pack by centering the disks, biasing the disks, or active control
methods (see [87]-[86] and the references therein) can be used in the MSTW to
enhance its performance by controlling the mechanical vibration, timing error,
and position sensing error to a relatively lower level compared with those of
HDA servo writing.
Once the patterns are written on disks using MSTW, these disks can be
assembled in an HDD. An alternative method suggests assembling the HDD
with one disk servowritten by MSTW and few other virgin disks with no servo
patterns. Then the servo patterns from the master disk are replicated on other
disk surfaces. In both cases, the patterns written on the master disk can be
either the complete servo pattern or intermitently written servo patterns. For
the case of complete servo pattern, all the servo tracks are written by the
MSTW. On the other hand, the intermittent servo patterns can be of different
types such as spiral pattern [190] or partial servo track writing [22]. In the
fi
rst of these two cases, the write head is moved precisely from the outer edge
of the disk towards the inner edge while the sector patterns are being written.
This creates a pattern of sector marks spiralling from outer diameter to the
inner diameter. For partial writing, few servo tracks are written at different
radii of the disk leaving the reamining sections blank.
The processes of creating the intermitent servo patterns or servo tracks can
be carried out in the HDA, instead of media level writing. During the second
stage, the complete servo pattern is written on all disk surfaces by referring
to the initial patterns and bank writing the remaining disks [94] or using self
servowriting. Which process can be used in the second stage is determined by
the type of initial pattern used.
When a disk with pre-written servo patterns is assembled in the HDD,
the eccentricity of the disk and therefore of the patterns with reference to
the centre of rotation of the spindle must be tightly controlled. Misalignment
between the center of disk and the center of rotation introduces RRO. Even the
force with which the disks are clamped causes deformation in the disk which
increases the RRO further. These problems can be alleviated using one of the
following methods:
1. Mark the disks during the MSTW process so that these marks can be
used later during the assembly of the disks in the HDA to make them
properly aligned,
2. Write the
fi
nal servo pattern or user data by not following the eccentric
master servo track centers but a virtual data track centers such as the
zero acceleration path (ZAP) reported in [33],
3. Leave the ID region unwritten in the process of MSTW. Deformation
of disk due to clamping is more severe in the inner region. The blank
