Hardware Reference
In-Depth Information
parallel to disk surface is effectuated by the torque generated by VCM. This
is the motion a sliders go through during repositioning of the head over a new
track, as well as during the track following, i.e., when the head is regulated
over a track. Both of these operations, track seek and track following, uses
the same actuator to create the motion parallel to disk surface. Error toler-
ance during the track-following is in the scales of nanometers, and it must be
achieved in presence of various disturbances acting on the slider, suspension
and actuator arm. On the other hand, the transfer of head from one track to
another is expected to be performed in few milliseconds.
The HDD servomechanism is a unique example of practical applications
that demonstrate the degree of precision achieved in a mechatronics system.
Current state of the art in HDD industry enables laying out of data tracks on
disk surfaces at density greater than 100, 000 tracks per inch (TPI), that is, the
centers of two adjacent data tracks are separated by 10 µ-inch or 0.25 µ-m. The
read/write head is expected to fly above the center of data track as precisely
as possible while writing binary information on the data track or retrieving it
from the track. Deviation of the head from this desired position increases the
probability of occurrence of erroneous bits by either accidental overwriting on
adjacent track or unwanted interference from the adjacent track, and hampers
the reliability of the disk drive.
∗
Head-positioning error tolerated in an HDD is
typically less than 10% of track pitch which is equivalent to 1 µ-inch or 25 nm
in a 100, 000 TPI drive. Projections suggest that track density will reach
400, 000 TPI in laboratory demonstration by the year 2009 and in production
by 2013, particularly for small form factor drives, i.e., the drives using disks of
diameter 2.5-inch or smaller. Desired error tolerance of the head positioning
servomechanism for such drives will be 0.25 µ-inch or 6.3 nm.
The HDD market is now dominated by 3
2
inch form factor drives; but the
smaller form factors have shown a growth potential comparable to those of the
3
2
inch drives in the early years of 1990s. Starting in 2003, the growths of 2
2
inch, 1.8 inch, and 1 inch drives are 36%, 380% and 55%, respectively. Global
shipment of small form factor drives (2
2
inch and below) was 50 million units
in 2003 and is expected to reach 100 million units in 2006. Insatiable demand
of notebook PC and application in consumer electronics, e.g., MP3, video
camera etc are the main driving force behind this growth in the small form
factor drives. As HDDs are being used in new applications, they are expected
to meet more stringent performance specifications. For example, drives to be
used in PDAs, camcorders, or automobiles must be able to withstand much
larger vibration than those experienced by drives used in PC.
A comprehensive illustration of the closed loop head positioning servomech-
anism of HDD is shown in Figure 2.2. The VCM actuator moves the read-write
head between tracks (track seek mode) and regulates the position of the head
(track following mode). During track following, the head must follow the ref-
Error probability greater than 10
−
10
is not acceptable in a properly functioning HDD;
this means only one bit in error is permitted out of 10
,
000
,
000
,
000 bits read from the disk.
∗
