Hardware Reference
In-Depth Information
that the reader can sense the clock information reliably. The preampli fi er is
carried on a fl ex cable which provides write/read signal path. Signal tracks for
differential read back signal are shielded with ground plane to reduce noise.
Two MR data heads are connected to the on board SR1581 preamp with
programmable write current and MR bias current as well as head selection.
There are four other terminals for connecting the piezoelectric micro actuators.
The signal tracks for connecting PZT are separated from the R/W signals by
a ground track.
Two on board SA5209 variable gain low noise radio frequency ampli fi ers
serve as buffers for read back servo pattern and clock signals. The differential
signals are transmitted to the R/W board via shielded coaxial cables. It also
routes bi-directional serial con fi guration data to or from preampli fi ers. On the
R/W circuitry, two 32P4752 read channel IC's are used to recover clock pulse
and servo burst. The servo bursts are digitized by AD7482 and then send to
DSP module for servo control.
The pattern generation circuitry uses three Altera Flex 10KA CPLD de-
vices. It has a PLL circuit to generate the write clock which is driven by
the clock track read back signal. A clock track closure measurement circuit
is also developed so that a desirable clock can be written after compensating
the timing error in an initial trial clock track. The 4 MHz clock track signal is
also stepped down to lower frequency for spindle motor speed control via the
CPLD.
The pattern generator adaptor bridges the pattern generation board and
the R/W electronics. It serves to extend the capability of the original pattern
generation board. The Altera EPM9320 CPLD on the adaptor is programmed
to receive con fi guration data from the pattern generation board and then sent
them to preampli fi ers, read channels on R/W board, spindle motor control unit
and a delay generator accordingly. Registers for preampli fi ers, read channels
and motor control units can also be read by sending command via the pattern
generator board.
5.5.2 Measurement and Reduction of Disk-Spindle Pack
Imbalance
Since the imbalance of the disk-spindle pack generates radial force and thus
excites vibrations in the spindle motor and disk, its value is typically controlled
to be below certain amount in high speed HDDs and servo track writers.
To measure the disk-spindle pack imbalance, Figure 5.11 shows a disk spin-
dle pack mounted on a fl exure [86]. The disk-spindle pack eccentricity force
drives the disk spindle pack to vibrate on the fl exure bearing. A laser Doppler
Vibrometer (LDV) is used to measure the vibration velocity signal whose am-
plitude is proportional to the eccentricity force, and hence the eccentricity. A
bandpass fi lter whose center frequency is at the disk rotating speed can be
 
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