Hardware Reference
In-Depth Information
the total moving mass is increased. Besides, the contact between the two is not
perfectly rigid and it causes signi fi cant resonances in the frequency response
of the overall system. This resonance at around 800 Hz limits the achievable
servo bandwidth to around 300 Hz [198]. Lower servo bandwidth means re-
duction in positioning accuracy and therefore increased written-in runout. It
also results in longer seek-settle time and thus reduces the throughput of the
servo track writing process.
This positioning system is external to the HDD enclosure, and is usually
located at the bottom of the nest tray of STW which rigidly clamps the HDD
enclosure. Insertion of the mechanical push-pin inside the enclosure to make
it in contact with the HDA's actuator makes it necessary to leave an openning
in the device enclosure. Thus, it is essential to perform the servo track writing
using the above mentioned method inside a clean room so as to minimize any
contamination to the disk platters.
The HDD and the external positioner should be securely and rigidly clamped
in a nest or tray, typically made of marble, to minimize the vibrations between
them. Furthermore, the nest or tray is placed on a vibration isolator which
absorbs external vibration or shock and prevents them from being transmit-
ted to the HDA and pusher during the servo writing process. However, using
vibration isolator or using heavy mass as the platform can not eliminate the
vibration contributed by the rotation of spindle motor and disks, vibration
caused by the movement of the arm, and vibration of the suspension and
slider excited by the air circulating at very high speed. These vibrations are
still present at the time of servo track writing and they induce written-in errors.
5.2.3 Control of Radial Position using Optical Push-pin
The arm of the external VCM used as the mechanical push-pin contributes
large share to the total moving mass during the servo track writing. Such
heavy mass is not desirable as it increases response time and therefore reduces
throughput. Various approaches have been tried to get rid of the external and
heavy mechanical actuator. One of these methods uses a laser beam shining
from the top on the E-block of the VCM actuator which carries a fi ne grating
sheet scale [198]. The position of the actuator arm can be precisely determined
using the laser beam and the grating sheet scale, and therefore, be controlled
using closed loop feedback system. In this case, the plant to be controlled
by the servomechanism is only the VCM actuator of the HDD and not any
external mechanical structure. The bandwidth achievable is quite high and
similar to that of the HDD servomechanism.
As reported in [198], the sheet scale pasted on the VCM actuator can be
of printed diffraction grating of 1 µm pitch which produces an optical source
signal of 0.25 µm. When using a grating of 10 µm pitch with an optical source
of 5 µm pitch, a sensing resolution of 0.5 nm can be achieved. Moreover, this
system is less sensitive to vertical vibration of the actuator arm compared to
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