Hardware Reference
In-Depth Information
2.3 Feedback of Position Signal
The feedback signal for the head positioning servo loop is obtained by decod-
ing spatially coded magnetic patterns written on the disks. These patterns,
known as servo pattern, are created at the time of manufacturing HDD after
the spindle, disks, actuator and heads have been assembled inside the drive en-
closure. The process of writing servo patterns is known as Servo Track Writing
(STW) and is carried out using a very high precision equipment that controls
the position of the actuator of HDD and writes the servo patterns on the disks.
For all HDDs manufactured these days, the enclosure is covered and sealed af-
ter servowriting and, as a result, the same head-suspension-actuator assembly
and disk-spindle assembly are used for normal operation of HDD. In the past,
however, a different scheme was in use where only one surface of one disk was
pre-written with servo patterns. Writing of servo patterns were performed in
bulk, several tens of disks at a time. One servo-written disk and several other
blank disks were then assembled inside the drive enclosure. In an HDD assem-
bled in such fashion, there is one surface of a disk containing servo pattern.
This scheme of creating servo information is known as dedicated servo,where
only one side of one disk in a multi-disk HDD contain the spatial servo patterns.
In such drive, the signal necessary for position feedback came from the head
accessing the servo surface. Since a single VCM actuator controls the motion
of sliders on all surfaces simultaneously, moving the slider on the servo surface
to a desired track is equivalent to moving any other head to the same track.
The main assumption for proper functioning of this scheme is that sliders on
all surfaces are displaced by precisely the same amount which, in reality, is im-
possible. The suspension arms get elongated due to thermal expansion if the
drive is kept ON for long time and the amount of elongation may be different
for different arms. For older generation drives, the acceptable tracking error
(20 ยต-inches or more) was much larger compared to the differences between
the thermal expansions of arms. The dedicated servo scheme worked well for
drives of the past, but its limitations started to surface with the trend of in-
creasing track density when the disparity between thermal expansions became
comparable to error tolerance. The need to overcome the thermal expansion
related problems to pave the way for higher track density gave birth to a new
scheme of servo encoding - embedded servo or sectored servo,inwhichservo
patterns are created on all surfaces and same head is used for accessing both
servo and data. Instead of one dedicated servo surface, the embedded scheme
puts servo patterns on all surfaces interleaved with the data blocks. The servo
sector, a small segment of the track containing servo patterns, are created at
regular intervals, and the space between two servo sectors is designated for
storing data. The embedded servo scheme with interleaved data blocks and
servo sectors is illustrated in Figure 2.12.
Besides the problem of thermal expansion, large servo overhead is another
drawback of dedicated servo scheme when only a few disks are used in an
HDD. Servo overhead is the percentage of available area that is consumed by
