Dither Oscillator

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