Figure 5.4: Block diagram for solving the control design problem.
where Φ is a weighting factor that can be chosen to be less than 1.
Instead of using the track pro fi le f(k−K) of the previous track for control-
ling the error through the use of F(z), part of the f(k −K) information can
be used in its place for the same purpose . For example, some repeatable
components of f(k −K) which represents the written-in runout signal can be
used and gradually reduced in the writing process.
We can use the frequency domain properties of the PES signal to eval-
uate the performance of the self servowriting process. Figure 5.5 shows the
frequency spectrum of PES in adjacent tracks for the case of self-servowriting
without proper error containment algorithm used. There is a steady growth of
the frequency component of PES at a frequency approximately 15 times the
spindle frequency. It grows continuously until the process reaches a situation
where the non-circularity written in the track becomes too large and the loop
When the error containment algorithm is used, the error does not grow
steadily as shown in the Figure 5.5. The use of the compensator F(z)can
effectively stop the error amplitude from growing and allows more tracks to be
propagated using the self-servowriting process.
5.4.3 Clock Propagation
Accurate reference of tangential displacement is essential for writing the mag-
netic transitions of the new track aligned with the transitions of the previous
track, i.e., the reference track. This is achieved with the help of a phase lock
loop (PLL) that generates a clock signal precisely locked to the timing marks
sensed from the reference track. The performance of the PLL is adversely af-
fected by the noise entering the loop, and the generated clock does not remain
perfectly in phase with the readback signal sensed from the reference track.