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2.3.6
Actuator Positioning Circuit
While reading the following, refer to the schematics in SECTION 3.
To
control the actuator, the microprocessor calculates the desired actua-
tor current, places 8 bits corresponding to the current on the MAD
bus, strobes the bits into actuator DAC U3 with -DAC CS and -WR, and
asserts +ACT DAC.
The output of op-amp U21 is 3.7 volts to 7.4 volts
for 0 to 255 (decimal) DAC input.
U5 is another voltage amplifier; its output swings around the refer-
ence voltage--the full range of positive and negative actuator cur-
rents is obtained even though a negative supply is not available.
A
DAC output of 128 (decimal) corresponds to zero (nominal) actuator
current.
While seeking, the gain of U5 is four times its gain when
settling and following.
The gain is switched by +GAIN LOW, which
turns on Q11 and connects R72 into the feedback path.
U4 (A) is an error amplifier, a part of the actuator driver chip U4.
Driver stage U4 (B) has a differential output.
Power transistors Q12
through Q15 limit the dissipation in U4 (B), and force through the
actuator coil a differential current that is proportional to the out-
put of the error amplifier.
U5 senses the current as the difference
voltage across R42 and R43, amplifies it by four, and closes the loop,
giving accurate positioning.
The R-C network from output to input of U4 (A) compensates for the
actuator coil time constant; the R-C network from U4 (A) output to
ground plus C62 and C63 compensate U4 (B).
CR13 - CR16 suppress
inductive transients across the actuator coil.
2.3.7
Read/Write Circuit
Follow this circuit description on Figure 2-4, a block diagram of the
drive, and the schematics in SECTION 3.
The waveforms are shown in
Figure 2-13.
The alternating magnetic fields on the disk generate a differential
voltage across the coil in the head selected by +BUF HSO - +BUF HS2.
This voltage is preamplified by the chip in the flex circuit (giving
RDX and RDY) , and ac coupled to the AGC amplifier, U2 (A), part of the
8464 pulse detector U2.
The AGC amplifier output is filtered by a
balanced five pole filter that slims the pulse to reduce shouldering
errors.
Q1, Q9, Q2, and Q16 drive the filter, which is terminated in
R57 and R63.
Q23 and Q26 are emitter followers that couple the filter
output to the U2 (B) pulse detector inputs.
2-27
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