3. The step size should be calculated in the tangential direction also based
on periodic calibration of geometrical error. Using wrong step size should
be avoided, otherwise one track may erase other track's space.
4. In SSTW, we can use higher PES sampling frequency to achieve a more
effective servo design. The write to read switching time, limited by
switching off the inductive writing head, must be minimized because
such delay reduces the sampling frequency.
5. Similar to HDD servo, missing trigger patterns must be detected. When
the trigger pattern detector either triggers before the desired written trig-
ger due to noise, or fails to trigger due to a defective or missing trigger
pattern, the STW process must continue to robustly propagate and write
servo and timing patterns. To achieve this, the detector can be enabled
for only a short time (150 ns) before the trigger pattern is expected. This
greatly limits noise and defect induced “extra” triggers. Alternatively, if
a trigger pattern does not occur within a speci fi ed time after the expected
time, a fake trigger is generated by the system hardware. Moreover, all
trigger times can be compared in software against the expected time.
If the trigger falls outside a speci fi ed duration the trigger is ignored for
propagation purposes and all measurements and writing are referenced
to the last valid trigger. The process has a speci fi c threshold for consec-
utive invalid triggers which causes process termination. In practice such
termination is very rare as the invalid trigger rate is typically less than
To place narrower servo tracks closer without being limited by the spin-
dle motor runout and head/disk assembly vibration, a hybrid STW (HSTW)
system can be used which is a combination of the conventional STW and the
5.5 A Laboratory-scale Example
The equipment used for servo track writing is a high precision mechatronics
system with speci fi cation for error tolerance in positioning of the write-head in
nanometer scale. An experimental setup, with the capability of servo-writing at
a precision of 2.37 nm RMS is described in the following subsection. The setup
is designed and developed in the Data Storage Institute, Singapore (DSI) .
This system is also capable of simultaneous servo-writing of multiple disks. It
uses an active control loop for suppression of vibrations.
5.5.1 Con fi guration of the System
The hybrid servo track writing (HSTW) system is shown in Figure 5.8.