Introduction - Hard Disk Drive: Mechatronics and Control - page 18

track (in the direction along the track or down-track direction) is L bit = vT W ,
where v is the relative speed between the write head and medium. Dimension
of the write head is one of several factors that affect the dimension of a track
in the cross-track direction, i.e., along the radius of the disk. The head posi-
tioning error in the HDD servo loop is another factor that affects the decision
on track-to-track spacing. We should not create a new track that may erase
signi fi cant part of an adjacent track. Similarly, while data is read from a track,
the interference from the magnetic transitions recorded on an adjacent track
should be as low as possible. Maximum error in positioning the read/write
head by the closed loop servomechanism sets the limit on the allowable prox-
imity of two adjacent tracks. Let σ pes be the standard deviation of the tracking
error in the head positioning servomechanism, then 3σ pes is widely accepted
in the industry as a measure of the minimum track pitch. It should be noted
that actual track pitch is larger than the 3σ pes achieved by the head position-
ing servo. Different factors such as spindle eccentricity, mechanical vibration,
environmental disturbances etc contribute to the tracking error. If the track
width is W trk and minimum separation between two transitions is L bit ,then
A bit = L bit W trk is the area occupied by a single bit of information. The stor-
age density or bit density of a magnetic recording system is the inverse of A bit .
Inverse of L bit and inverse of W trk are known as the linear density and track
density, respectively. Imperial units are widely used in the HDD industry and
bits per inch 2 , bits per inch and tracks per inch are the units of areal density,
linear density, and track density, respectively.
1.3.2 Locating Data
When the host system sends data for recording, it is recorded in chunks of
512 bytes. Each of these chunks is called a data block . Bits of a data block
are recorded sequentially along the track. In order to locate a data block on
the surface, they are tagged with an identi fi cation number. From system level
point of view, each data block is assigned with a Logical Block Address or LBA,
starting at 0 and ending at a number appropriate for the capacity of the entire
drive. These block addresses, however, are not suitable for low level access
to the data. Access at the low level uses head number , cylinder number,and
sector number assigned to each LBA. An HDD may contain one or more disks
with data recorded on both surfaces of a disk. Data on each surface is accessed
(forreadingaswellaswriting)usingaseparate head for that surface. If the
number of disk surfaces used is S in an HDD, there will be S head sliders in
it. Each used surface of the disk stack is identi fi ed by the corresponding head
number; for an HDD with 8 usable surfaces, heads are numbered 0 to 7. There
are tens of thousands of tracks on each surface, numbered 0 on the outermost
track and increasing inward. If we consider a stack of disks, then track 0 of all
disks form a cylinder and is identi fi ed as cyl 0. One of the cylinders is shown
in Figure 1.8. Each cylinder is assigned with a unique identi fi cation number.