Hardware Reference
In-Depth Information
(can be assumed 1 for simplicity) of the optical sensing device, and n
1
be
the noise in the sensing loop. The PZT actuator loop in the MicroE arm is
modeled as P
2
(s), with its controller is represented as C
2
(s). Position of the
R/W head is y
3
.LetT
n−1
be the n−1
th
track written on the disk, v
1
be the
suspension and slider vibrations, v
2
be the vibrations in spindle and disk, y
ref
be the track center of the n−1track,y
4
be the measured difference between
the track center of the n−1 track and the track to be written, and n
2
be the
measurement noise contributed mainly by the R/W process. Two feed-forward
signals, u
3
and u
4
,areusedwhereu
3
is the RRO measured from the previous
track (i.e., T
n−1
)withC
2
(s) cut open and u
4
is the
fi
ltered y
1
.
Figure 5.12: HSTW servo loop block diagram.
C
2
(s) is designed such that the PZT loop compensates for the vibrations
between head and media, i.e., the head position Y
2
(s) follows V
2
(s) − V
1
(s),
to minimize the written-in error. The reading element is used as the sensor to
measure the vibrations. We note that taking out T
n−1
from the loop (by inject-
ing u
3
) relaxes the error propagation problem and reduces the written-in error
similar to the self-servo track writing method. Referring to the Figure 5.12,
we have
t
n
= h−v
2
= T
s
(s)(r
2
−n
d
)+S
2
(s)(v
1
−v
2
)
+S
2
(s)T
1
(s)(r
1
−n
0
)+S
2
(s)S
1
(s)P
1
(s)d
1
,
(5.3)
where T
1
(s),T
2
(s),S
1
(s)andS
2
(s) are the complementary sensitivity functions
and sensitivity functions for the optical servo loop and the second servo loop
respectively. In this con
fi
guration, optical servo loop provides coarse movement
oftheheadtothedesiredtrackandenablesthesecondlooptocontrolprecisely
the head with reference to the previous ideal track center by decoupling the
previous written-in error T
n−1
from PES for writing the current servo track.
The PZT actuator can attenuate the PES induced by head and disk vibrations
by reducing their effect via the feedback loop using the R/W head as the sensor
as opposed to using the external optical sensor. Furthermore, the selection of
