Servo Track Writer - Hard Disk Drive: Mechatronics and Control - page 311

requires precise reference along the disk's circumference, i.e., the reference for
tangential position. This reference is created by fi rst writing a clock track,
which contains pre-de fi ned number of regularly spaced transitions. Writing
of the clock track and reading its transitions later are done with a separate
head other than the heads of the HDA. This head, known as the clock head,
is external to the HDA and is inserted inside at the time of STW. Creation of
the clock track involves few steps:

1. decide the number of clock cycles required per revolution,

2. write the clock track with precisely the desired number of transitions in
it, and

3.verifythequalityoftheclocktrack.

Since a separate head is used for the clock track, the clock head signal is
available throughout the entire process of writing servo patterns. A phase
lock loop (PLL) circuit takes the signal from the clock head as its reference
and the VCO (Voltage Controlled Oscillator) of the PLL produces a clock
signal synchronized to the clock track. The clock track also include an INDEX
pattern that de fi nes the beginning of the track. Using the INDEX mark from
the clock track and the synchronized clock signal from the PLL, a timing
generator circuit produces reference for every sector and the fi elds within a
sector. These references and the clock signal are then used to trigger the
process of writing servo patterns on different tracks.

Calculation of the Number of Clock Cycles per Revolution
Care must be taken in deciding the number of cycles to be written on the
clock track. If the disk rotates at N rpm RPM and if the frequency of the
patterns to be written is f 0 , then a nominal, integer number of clock cycles
(N)canbedeterminedbyN = f 0 × N rpm

60 =60f 0 /N rpm .
The rotational speed varies in any practical system. To accommodate such
variations in speed (e.g.: σ =0.01%), the clock track is written in few attempts.
Every time after writing it, quality of the clock track is veri fi ed. If the written
clock track does not meet the speci fi cations, the process is repeated.
Following the notation of [21], let ∆ second be the interval between sam-
pling the servo sectors in HDD, N rpm be the spindle speed in RPM, f clock be
the clock track frequency, and the integer m sector be the number of sectors per
revolution. Then we have,

∆ = m sector N rpm

1

,

60

which determines how fast the PES will be sampled by the HDD head posi-
tioning servomechanism. This parameter affects the performance of the head
positioning servo controller.