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reference voltage that represents the maximum allowed
gyro/manual control, and rate gyro. In normal operation,
system error. If it is exceeded the system will go from
only gyro signals are used. In stab-lock mode, the LVDT
ready to not ready and turn out the GSI light. System
signal is the input with manual control being used for
errors existing during turn-on would trigger a false not
testing.
ready light. To prevent this, a delay is included in the
The zero adjustment on the two amplifiers are used
error circuit.
to reduce any offsets in the amplifiers to zero. The
A schematic of the error circuit is shown in fig-
compensator circuit (R3-C1) is used to reduce the sys-
ure 3-31. Errors may be of either polarity; thus the
tem gain at higher frequencies. At high frequencies, the
op-amp A3 accept signals on both its inverting and
capacitor will act as a short circuit and the op-amp gain
noninverting inputs. If the servo error is positive and on
will be cut in half. Amplifier A2 is used as a volt-
the roll axis, it will pass through diode CR7 and resistor
age-to-current driver. This is necessary because the
R12. This voltage will be added to the reference voltage
servo valve is current controlled. The current driver
(A2) circuit is similar to the voltage amplifier previously
set on pot R8. If the reference is greater than the error,
described.
the output of op-amp A3 will be positive. Its amplitude
will be 3.25 times the difference of the error voltage
When a voltage is applied to the amplifier input, a
minus one half the reference.
current flows in the servo valve coil and through resistor
R12. The current in R23 causes a voltage drop across
Servo error gain is 7.5. Capacitor C5 averages out
itself. This voltage is provided as feedback to the input
the varying error signals so short-term errors (spikes) do
not trigger errors. The output of op-amp A3 drives
through R11. If 1 volts dc is applied to TP-B, a current
will flow through R12 to generate 1 volt across it since
transistor switch Q1, which turns on the error relay. In
its energized state, the error relay turns on the remote
R11/R8 = 1.R12 is 105 ohms, so the current in R12
for 1-volt input is approximately 10 ma (fig. 3-30). The
panel READY lamp and actuates the GSI lamp control
other input to A2 is from the dither oscillator and is
relay.
attenuated by a voltage divider.
The delayed start circuit is charged when the system
first goes into ready. Capacitor C7 keeps the system in
Dither Oscillator
ready (error relay energized) for about 6 seconds to
allow the system errors to settle out.
The dither oscillator provides a high-frequency
(compared to system response) signal to the servo valves
Gyro Alarm Circuits
to keep them in constant motion to prevent sticking at
null.
The SGSI system incorporates an independent fail-
The dither oscillator is a phase shift oscillator. It
ure detection circuit that detects any failure that will
depends on the phase shifts inherent in RC networks to
result in a loss of stabilization. It does this by comparing
shift the phase of the amplifier feedback 180. This will
an input from the ship's gyro with the output of the
cause a sustained oscillation if the amplifier gain is high
platform LVDT. When the system is operating correctly
enough. The gain also determines the quality of the sine
in the internal gyro mode, the output of the LVDTs is
wave.
directly proportional to the ship's motion. If the ship's
Resistor R5 (amplifier gain control) is adjusted
motion from the LVDTs is out of phase (reverse polarity)
until the amplifier starts oscillation and has a clean sine
to the ship motion from the ship's gyro, the two will
wave with no flattening of the tops. The zener diode acts
cancel. Any voltage left over from the summation will
as an upper limit for the amplitude. The relay on the
be the error between the ship gyro and the platform. The
dither oscillator is parallel to the stab-lock relay, which
error is compared against a preset limit, and if it exceeds
controls the rate gyro information.
this limit the platform error relay is tripped. The ship
gyro input is required for the gyro alarm and is also used
Error Circuit Card
for ship gyro stabilization and for the rate lead. The rate
lead circuits are used to reduce velocity lag of the
The error circuit card is used to monitor the pitch
platform and increase system dynamic accuracy. In the
and roll servo errors. It allows monitoring of the gyro's
ship gyro stabilization mode, the system operates at a
internal pendulum reference for test purposes. Since the
reduced accuracy due to null errors and LVDT linearity
system is not perfect, servo errors are present. Voltages
error. Therefore, the ship gyro mode is to be used as a
representing system errors are compared with a
backup mode only.
3-21

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