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amplifier current to the actuator voice coil is
differentiated to provide an indication of actuator
velocity.
3-83. Slope Selector.
The slope selector circuit,
programmed by the Even In signal from the mic-
roprocessor, selects a PES slope of the proper
polarity for the tachometer to provide a negative
feedback.
3-84. Current Inverter.
The current inverter,
with a gain of
±
1 and programmed with the Odd
In signal from the microprocessor, selects a current
of the proper polarity from the power amplifier
for input to the tachometer.
3-85. On-Peak Detector.
The on -peak detector
operates at approximately 3. 5 volts to initiate the
switch between the integrated PES signal and the
differentiated motor current
in the tachometer.
The output of the on -peak detector is also coupled
to the microprocessor to signal track crossings.
3-86. Power Amplifier. The power amplifier is a
transconductance amplifier which supplies current
to the actuator voice coil in response to an error
voltage input.
The amplifier output lines are
labeled VCMA, VCMB. The amplifier is enabled
by the Servo Enable line from the microprocessor.
3-87. Inverter.
The inverter has a gain of
±
1
and is programmed with signal Odd In from the
microprocessor. The purpose of the inverter is to
select an error voltage of the correct polarity for
input to the power amplifier. The input to the in-
verter is from the tachometer (seek mode) or signal
PES (track follow mode), as selected by the mode
select switch.
3-88. TRACK -FOLLOW MODE.
The track-
follow mode of operation is selected by the micro-
processor at the end of a seek operation when the
addressed track has been reached. Signal PES is ap-
plied via the track -follow position of the mode
select switch through the inverter to the input of
the power amplifier. Signal Odd In from the mic-
roprocessor programs the inverter to select a PES
voltage of the proper polarity to provide a negative
feedback signal to the amplifier.
Theory of Operation
7942 and 7946
3-89. OFF-TRACK DETECTOR. The off -track
detector monitors the amplitude of the PES signal
and provides an Off Track output to the micro-
processor whenever the PES voltage exceeds plus or
minus 1.5 volts.
3-90. SEEK OPERATION. The disc drive must
have Ready RDY-L and Seek Complete SKCMP-L
true before a seek operation can begin. With the
disc drive in this state, the microprocessor is in its
basic loop, monitoring Motor Speed, Off Track, and
looking for either a Step STEP-L pulse or Write
Gate WGATE-L, both of which are gated by Drive
Select DSO-L. When a Step pulse is received, the
Step signal forces Seek Complete SKCMP-L false.
If
Write Gate is true and Seek Complete goes false,
a fault condition will be issued, automatically
deactivating the write circuitry. When a Step pulse
is received and Write Gate in not true, then the
microprocessor will initiate a seek operation. The
microprocessor will set the direction of the seek
from the information on the Direction DIR-L line.
The microprocessor will also set the seek mode, at
which time the actuator servo control circuit will
cause the actuator to move. The speed at which the
actuator moves is dependent on the rate at which
the Step pulses are received.
Maximum perfor-
mance of the actuator is attained if the time inter-
val between Step pulses is less than 39 micro-
seconds.
The microprocessor counts the track crossings until
there are no more Step pulses and the actuator is
crossing the last track. At this time the micro-
processor switches the actuator servo control from
the seek mode to the track follow mode. The mic-
roprocessor now starts a settling timer and looks
for an off -track condition. If an off -track condi-
tion exists, the microprocessor restarts the settling
timer. This is done until the off -track condition is
cleared plus the settling time. Once this is ac-
complished, the microprocessor sets Seek Complete
line SKCMP-L.
3-91. RESTORE TO TRACK 0 OPERATION.
To perform a restore to track 0 operation, the mic-
roprocessor enables the actuator servo control cir-
cuit and moves the actuator over the data area and
out of the landing zone into the outer guard band
area. Once the microprocessor detects that the ac-
tuator is in the outer guard band area, the micro-
processor will cause the actuator servo control
circuit to settle the actuator on a data track. The
3-19
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