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5-68. Missing PWM Clock Detector.
The two
inputs to this circuit are a square wave from the
100-kHz generator in the pulse-width modulator
(PWM-L)
and a clock from the position detector
circuit
(GOCLK-H).
If the 100 kHz signal quits, the
detector output will go high after a maximum of
one sector (approximately 125 microseconds). The
detector output is labeled No 100 KHz Clock
(NO
CLK-H).
Failure of the 100-kHz oscillator will cause the
servo power amplifier to self-oscillate at about 15
kHz. This causes a high power dissipation in the
output filter. As an added precaution, there is a
fuse in the output filter.
5-69. Undervoltage
Detector.
The
circuit
monitors the +45V and -45V supplies. If either
supply should fail, (possibly because of a fuse
opening), then the servo power amplifier will be
turned off. Trip points for the circuit to indicate a
failure are approximately +24V for the +45V sup-
ply, and -24V for the -45V supply. The detector
output is labeled Undervoltage
(UNDER VLT-L).
5-70. Overvelocity
Detector.
The
circuit
monitors Actuator Current
(ACUR)
and is used to
indicate if the actuator is moving too fast. The
circuit contains a low-pass filter which basically
integrates
ACUR.
The output of the filter gives a
relative indication of the velocity of the actuator.
The filter output goes to a window comparator
which trips if the voltage gets above +9.2V or
-9.2V. The output from the comparator section is
called Overvelocity
(OVER VEL-H).
The filter is
reset if Fine Servo Enable
(FSEN-L)
stays low for
over 1.5 milliseconds.
It
is necessary to reset the fil-
ter after every seek since the low-pass filter must
start at zero volts in order to properly indicate an
overvelocity condition.
5-71.
SERVO SYSTEM ERRORS
Servo system ,errors can be divided into three
categories: track follower-PLL errors, seek errors,
and power amplifier errors. Some of the errors are
hard-wire errors, that is, they immediately shut
down a data write, whereas others are detected in
the servo controller and are communicated only to
controller PCA·-A6.
Functional Description
7936 and 7937
The occurrence of most of the errors will cause
the servo controller to jump to an abort routine. In
this routine, various servo protect routines will be
taken, depending on the severity of the error. The
severity of the error is indicated by the immediate
status message reported to the controller.
An
immediate status of 0 indicates no error. An
immediate status of 1 indicates a soft error for
which the servo controller commands a coast with
the position loop open (no current to the actuator)
and reports the immediate status to the controller.
An immediate status of 02H indicates a hard error
for which· the servo controller does all of the
above plus turn off the servo amplifier.
Details of the errors in each category are provided
in the following paragraphs. Refer to the flow-
charts in figures 5-6 through 5-11 for the control
paths taken to derive the errors. The errors appear
as "status
=
xx" blocks in the flowcharts. Drive er-
rors (DERRORs) and test errors (TERRORs) are
discussed in detail in chapter 8.
5-72. TRACK FOLLOWER - PLL ERRORS.
The errors in this category are: servo timing, AGC
error (sampled), off track
(OFFTRK),
and spindle
speed error.
5-73. Servo Timing. There are servo timing er-
rors (STEs) and servo timing faults. Servo timing
error is asserted when the fixed amplitude pulse of
the sampled servo field on a data surface does not
occur within the proper time windows inside the
sampled servo gate (generated from the dedicated
servo surface). Servo timing error is expected to
be true any time the drive is off track.
Either Timing Error
1 (TERR1-L)
or Timing Error
2
(TERR2-L)
will trigger Servo Timing Fault
(SRTF-H),
which in turn will cause the servo con-
troller to abort any write operation.
In the sampled servo timing circuit,
TERR1·L
is as-
serted when
GAP-L
and
SMDT -H
do not occur at
the proper time, relative to one another.
TERR2-L
is asserted when three consecutive
sampled servo sync pulses are missing, or do not
occur at the proper time.
5-25
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