Hardware Reference
In-Depth Information
controller's operation. This is a disturbance signal on top of the NRRO present
in the head-disk assembly and spindle-disk assembly.
Figure 3.43: A schematic diagram of disk runout.
The PES signal extracted from the servo bursts written on the tracks con-
tains both RRO and NRRO components. The RRO is synchronized with the
disk's rotation but the NRRO component is not. Synchronous averaging of the
PES signal can separate the repeatable components from the PES signal.
If there are N servo sectors in an HDD, we get N samples per revolution
when PES is measured. For the sake of averaging, we need to measure PES for
several revolutions. If measurement is performed for M revolutions then the
measured PES y is a M × N matrix. Let the fundamental frequency of RRO
signal be ω. The repeatable component R
y
(n) of PES and the nonrepeatable
component N
y
(n) can then be obtained according to the following:
X
M
R
y
(n)=
1
M
y(n, m), n=1···N,
(3.83)
m=1
N
y
(n, m)=y(n, m)−R
y
(n), n=1···N,m=1···M.
(3.84)
Figure 3.44 shows a sample of PES obtained from a track of an HDD with 62
sectors and rotational speed of 3325.65 RPM. The power spectral density of
the PES is shown in Figure 3.45.
Since the RRO and NRRO are added to head position to form the measured
position error signal (PES), only the frequency components that fall within
the servo bandwidth can be attenuated by the basic servo loop that we have
discussed previously.
When the servo system is to follow the track center that contains large
amount of runout, the actuator has to move more frequently in both direction
and therefore uses higher servo power. Reducing the amount of RRO compo-
