Servo Track Writer - Hard Disk Drive: Mechatronics and Control - page 330

controller C 2 (s) for vibration V 2 (s)−V 1 (s) rejection and noise n 2 attenuation
can follow the same way as in HDD servo design.
5.5.4 Test Results
In the experiment set up only two 1.27 mm thick glass disks were installed
and they were spun at 5,400 RPM. After the servo writing process, the NRRO
is measured with the microactuator feedback loop disabled, i.e., with only
the MicroE optical loop. The measured NRRO is found to be at the level of
3σ =17.9 nm. This has been an improvement from the initial 21.5nmwhen
no air shroud around the disk was employed.
With an open loop cross-over frequency of 885 Hz when we close the servo
loop in the servo writing process, vibrations below 800 Hz can be attenuated.
Figure 5.13 shows the PES spectrum with a normal peak fi lter and with a phase
lead peak fi lter. As shown in the fi gure, the PZT loop effectively rejected the
vibrations at 650 Hz and below using the phase lead peak fi lter. The PES 3σ
achieved is 6.4 nm. Assuming that a 10% track width is required of the 3σ,
this STW con fi guration can easily support writing of 395 kTPI on the disks
spun by fl uid bearing motor. With better control design, actuator and sensor
technologies, we can expect the low frequency vibration and also the peak at
3.8 kHz (contributing roughly 0.3 nm rms error to the PES) better attenuated,
and achieve a 425 kTPI on such a platform.
Figure 5.13: PES spectrum with normal peak fi lter and phase lead peak fi lter.
The example given above shows the feasibility of servo track writing with
ultra high TPI. In this hybrid STW example, the optical position feedback
loop determines the average track center whereas the previously written servo
information determines where the next servo burst is laid. The control design
philosophy for the PZT actuator loop follows that of the HDD servo control