Hardware Reference
In-Depth Information
generations of HDD used to contain 4 or more disks. Current state of the
art in recording technology allows storage of approximately 100 Gbytes on
a single disk; for many applications a drive with one disk meets the storage
requirement. For a drive with 4 disks (8 surfaces), servo overhead is 12.5%, i.e.,
12.5% of the storage area is occupied by the servo patterns. Servo overhead is
increased if fewer disks are used. Both of these issues, thermal expansion and
increasing servo overhead, associated with HDDs with a dedicated servo surface
can be resolved using an alternative servo scheme where the servo patterns are
written on every track interleaved with the data blocks. With this scheme in
place, the servomechanism can control the position of any head using servo
information written on the corresponding surface as the feedback. Different
thermal expansion of different arms is no longer a problem, and servo overhead
is independent of the number of disks used. However, unlike in the scheme with
dedicated surface, feedback signal is available only at discrete sampling points.
The method used in the earlier generations of drive with position infor-
mation encoded on a dedicated surface is called the Dedicated Servo scheme,
whereas the other scheme having position information encoded on all surfaces
is called the Embedded Servo or Sectored Servo. The segment of the track
containing the servo information in an embedded servo drive is known as servo
sectors, and the section between two servo sectors is allocated for storing data
bits. Servo patterns, both in dedicated and embedded case, are created during
manufacturing of the drive and the
fi
rmware of the HDD takes care not to
overwrite them in any situation.
1.3.3 Track Seek and Track Following
The position of the read/write head is controlled by a closed loop servomech-
anism that uses the feedback signal generated by decoding the information
written on the disks. There are two modes of operation for this control loop -
(i) moving the head from one track to another in shortest possible time, and (ii)
regulate the position of the head such that the relative offset between the head
and the track-center is as small as possible. The
fi
rst of these modes is known
as Track Seek while the second mode is called the Track Following.Design
objectives of these two modes of operation are signi
fi
cantly different. Besides,
there must be a smooth transfer between the two modes. It is impossible to
meet the speci
fi
cations of both modes using a single control law. Two con-
trollers can be made to produce desired performances if each is designed and
tuned independent of the other. However, while designing and implementing
such controller, special attention must be paid to ensure that sudden change in
the amplitude of control signal does not occur at the time of switching between
modes. Sharp discontinuity in the control signal excites the lightly damped
resonances of the actuator. Occurrence of such jerk increases the time it takes
to settle and, therefore, must be avoided.
