Hardware Reference
In-Depth Information
Figure 2.15: Two consecutive transitions (top) and corresponding readback
signal (bottom).
When the disk spins, the entire track containing all data block and servo
sectors is scanned by the head. The read head positioned at the point marked
0 in Figure 2.14 senses the burst patterns scanning them along the dashed line
and produces a series of voltage pulses with alternating polarities. For this
case, the read head senses maximum flux from burst C and no flux from burst
D. For bursts A and B,theflux linkage is smaller than that for burst C.Asa
result, the amplitude will be maximum for the burst C, zero for burst D,and
non-zero but less than maximum for bursts A and B. Corresponding waveform
is shown in Figure 2.16.
Followingthesamemethod,onecaneasilyfind the waveforms generated
by read head scanning the servo patterns along different off-track positions,
marked 1, 2, 3 and 4 in Figure 2.17 .
It is revealed from the observation of waveforms shown in Figure 2.16 and
Figure 2.17 that the amplitudes of different burst waveforms vary with the
cross-track location of the read head. The information contained in the am-
plitudes of these bursts is used to measure the displacement of the read head
with respect to the burst patterns. A signal proportional to the off-track error,
called the Position Error Sensing (PES) signal, is obtained by demodulating
the burst waveforms. A down-track line (that is line along the track) is des-
ignated as the center of a track if the amplitudes of burst A and burst B are
equal when the head scans the burst along this line. The difference between
the amplitudes of these two bursts is called the in-phase PES signal,
PES in−phase = A A −A B .
A A and A B are the amplitudes of the waveforms of burst A and burst B,
respectively. When the read head moves away from the center of the track,
one of these amplitudes become larger than the other and the PES in−phase
becomes non-zero. Further th e head is from the track-center larger is the
Thelinesshowninthis fi gure are fi ctitious; in reality there is no such physical line along
the track.
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