Hardware Reference
In-Depth Information
disk by controlling the position of the actuator arm with the help of an
external mechanical push-pin or optical push-pin. Any disturbance and
eccentricity present during this process will appear as writen-in RRO
for the head positioning servomechanism of HDD. This written-in RRO
increases TMR that must be compensated for by the servomechanism
of an operational HDD. Naturally, the requirements on the accuracy in
positioning the write heads is more stringent in the STW than in the
HDD servo system. The magnitude of the written-in RRO is a yardstick
used to measure the performance of the STW in creating servo patterns.
2. The servo sectors of any track must be precisely aligned with the servo
sectors of adjacent tracks. In an STW, this is done with the help of a
clock track written prior to the writing of servo tracks. A separate head
is used to sense the transitions in the clock track, and these transitions
are used as the reference timing marks for writing the transitions in
the servo patterns. The misalignment between the servo patterns and
timinginformationontheclocktrackcanbemeasuredbytiming jitter .
Excessive timing jitter causes distortion in read back waveform that the
recording channel must compensate.
Figure 5.1 shows the functional block diagram of a servo track writer. The loop
consisting of the optical position sensor and/or PES demodulation controls the
position of the write head so that concentric tracks can be created, one track at
a time. Unevenness of the track is exaggerated in this
fi
gure to illustrate that
the proximity with which the tracks can be created depends on the vibrations in
head-disk assembly. The clock signal generated from the transitions in a clock
track controls the tangential position (along the track) of the transitions in
servo patterns. A servo sector consists of a series of magnetic transitions. Since
these patterns in one track must be aligned with those of the adjacent tracks,
controlling their tangential positions precisely is an important requirement for
servo writing. The quality of the servo-writtentracksisassessedbymeasuring
both the written-in RRO and the timing jitter.
The mechatronics and control system used in STW must be of ultra-high
precision which makes the equipment expensive. As explained later in this
chapter, the conventional servo writing process needs openings in the drive
enclosure to make the heads, media and actuator of the HDD accessible to
the mechanics of STW. It makes it necessary to carry out the process of servo
writing in a very clean environment so that the space inside the HDD does not
get contaminated with particles that can damage the head and media. Need
for good clean room facility further increases the cost of manufacturing. The
throughput of this process is an important factor to be considered.
Let
• ω be the RPM of the disks during the STW process,
• T be the desired track density (TPI) of the drive to be servo written,
