Amplifier Error Detection - HP 7936 Support Manual

Functional Description
7936 and 7937
5-65. Amplifier Control and Fault Indication.
This circuit monitors certain functions of servo
power amplifier operation and shuts off the
amplifier if a fault condition is detected.
Inputs to the circuit are Power On Reset (POR-L),
Servo Processor Reset (SPOR-L), Under Voltage
(UNDER VLT-L), Over Velocity (OVER VEL-L),
Missing 100-kHz Clock (NO ClK-H), and Amplifier
Enable (AMPEN-H).
There are three output signals from the circuit.
Two of these signals are digital signals. One digital
signal, Amplifier Fault (AMPFlT-L), is latched low
if any fault condition occurs (any input active ex-
cept AMPEN-H). The fault latch is cleared on the
rising edge of AMPEN-H, as long as there is not an
existing fault.
The other digital signal is the NAND combination
of AMPEN-l and AMPFl T -l, and is called Switch
On
(SWON-l).
Signal
SWON-l
is gated with Pulse
Width Modulation (PWM-l), and controls the in-
put signals to the two transistor driver stages.
When this signal is low, both FET driver circuits
are commanded to turn off their respective output
transistor.
Since the SWON-L signal operates from +5V, it can
only turn off the FETs as long as the +5V supply is
functioning. Because of this, an analog signal is
also used to turn the power FETs off. This signal is
called Transistor Enable
(QEN-H).
If there are no
fault conditions, QEN-H is connected to +5V
through a transistor. Signal QEN-H is connected to
the optical isolator of the N-channel transistor
driver, the actuator crowbar circuit, and to some
circuitry connected to the P-channel transistor
driver. When QEN-H is equal to +5V, the optical
isolator for the N-channel is operational, the ac-
tuator crowbar is disabled, and the P-channel
driver operates normally.
When a fault condition exists, QEN-H does not
source any current. This causes the N-channel
transistor driver to turn off the N-channel FET,
the actuator crowbar circuit is activated, and the
P-channel circuit is forced to turn off the
P-channel FET. All of these functions operate even
when any of the +5V, -5.2V, +12V, or -12V supplies
are at zero volts. The actuator crowbar circuit
5-24
requires that the +45V supply to be about lOY in
order to operate, and the output FETs will remain
turned off for any value of either the +45V or
-45V supplies.
5-66. Actuator Crowbar. The actuator crowbar
circuit is activated if a fault is detected in the ser-
vo power amplifier. The purpose of the circuit is
to prevent the actuator from hitting the crash
stops at a high speed. The circuit employs a triac
which is activated when a fault is detected in the
servo power amplifier. When the triac gate con-
trol circuit is triggered by Transistor Ena ble
(QEN-H) from the amplifier control and fault in-
dication circuit, the actuator is effectively "short-
ed" and the output from the servo power amplifier
is also shorted. This prevents the amplifier from
putting high current into the actuator, in the event
that an output transistor shorts to the power
supply.
The triac also provided some braking action to the
actuator, since the triac looks like a low impedance
load, with the actuator acting like a generator. This
reduces the velocity of the actuator during a fault
condition.
The triac is triggered by its gate control circuit as
long as the fault condition continues to exist. Once
the fault condition stops, the gate drive to the triac
is removed. However, the triac will continue to
conduct as long as there is current flowing through
it. Once the current flow through the triac ceases,
it will turn off.
5-67. AMPLIFIER
ERROR DETECTION.
There are three error detection circuits in the ser-
vo power amplifier. These circuits are required in
order to prevent a failure in the electronics from
causing damage to the head-disc assembly or caus-
ing a safety hazard. The three errors that are
detected are: failure of the 100 kHz triangle
waveform wave generator (NO ClK-H), either the
+45V or -45V supply is too low (UNDER VL T-L),
or the actuator velocity is too high (OVER VEL-H).
The errors are reported to the amplifier control
and fault indication circuit previously discussed.