Hardware Reference
In-Depth Information
ID region of the disk can be servo-written later using self servowriting
technique. This technique is discussed later in section 5.4 of this chapter.
These methods can help to reduce RRO in the head positioning servomech-
anism of the assembled HDD, but RRO can not be completely eliminated if
disks with pre-written patterns are assembled in an HDA. The RRO that re-
mains after all these precautionary and corrective measures must be corrected
by the head positioning servomechanism of HDD.
Application of air bearing spindle motors reduces the nonrepeatable radial
error, which can be in the range of about 0.5 µin when carrying a load of
12 disks of 95 mm diameter
. At rotational speed of above 10,000 RPM,
which is desired for fast servo writing, the vibration due to disk NRRO, sus-
pension, as well as slider-disk interaction is no longer negligible. Such radial
movements displace the center of a servo-written track, while the vibrations
from slider and suspension displaces the write head causing distortions in the
servo pattern and, therefore, increases the written-in RRO and track encroach-
ment. Incremental reduction of vibration from various sources via improved
electromagnetic design of the spindle motor and using bearings with higher
stiffness and other components with low vibration is still necessary to improve
the STW quality with increasing TPIs. If a self servowriting loop with capa-
bility of writing servo patterns using the mechanics and head/medium of the
HDD is added, then it is possible to suppress vibrations via active control.
This measure improves the quality of servowriting.
†
5.4 Self Servowriting
Conventional methods of servo track writing need external devices for posi-
tioning of the write heads of the HDD and writes the servo patterns using
these heads. They face increasingly difficult challenges as the move towards
ever increasing TPI continues in the industry. These challenges come from
different factors. Firstly, conventional servo writing requires few revolutions
of the spindle to create one servo track and, therefore, the total time required
to servo write an HDD increases proportionately with increasing number of
tracks per surface. Because of this decrease in the throughput of individual
STW more servo-writers are required to meet the production target. This,
in turn, requires more
fl
oor space in clean room since these STWs are oper-
ated in such environment. Both MSTW and HDA level STW suffers from this
problem. Secondly, for drives of smaller form factors, the jigs and
fi
xtures of
the STW becomes smaller. It is challenging to design the pushpin mechanism
that is small and yet sufficiently stiff. Finally, if HDA servo-writing is to be
used then the drives must be assembled with expensive components of better
quality as this servo-writing makes use of product level components, i.e., the
†
see for example http://www.seagullsolutions.net/
