a. Each servo-control circuit sums a feedback signal from its respective actuator-position transducer
with a selected command signal. Until the feedback signal balances the command signal, a polarized
drive current is applied to the actuator control valve. This current drives the actuator toward command
position. As the actuator approaches command position, the level of the feedback signal increases and
valve-drive current decreases. At command position, the feedback signal balances the command signal.
At this point, the actuator stops.
b. Five command levels are provided for each servo-control circuit. These commands center or
extend or retract the actuators to 85 percent or hardover in either direction.
c. The feedback signal for each servo-control circuit is an ac voltage. This voltage increases from
zero to maximum amplitude as the actuator moves from center position to retracted or extended. The
signal is in-phase or 180° out-of-phase. When the actuator retracts, the feedback signal is in-phase.
When the actuator extends, the feedback signal is 180° out-of-phase. The feedback signal is demodu-
lated into its negative or positive dc equivalent before being summed with its command signal.
1-21. Square-Wave Generator.
Because the three servo-control circuits are identical except for input-
signal scaling, only the yaw circuit is described. A summary of input-
signal scaling for all three circuits is provided.
a. (See FO-4, FO-6, and FO-8.) The amplifier U2-14, 13, 12 operates as a non-inverting zero-cross de-
tector. Output voltage switches fully positive (+12-volt dc) when reference voltage passes through zero
going positive. It switches fully negative (-12-volt dc) when reference voltage passes through zero go-
ing negative. (See fig. 1-2.)
b. The square-wave output from the amplifier is inverted by U9-1, 2, 3. It is inverted again by U9-5,
6,4. This produces 180° out-of-phase 7-volt square wave at U9-3, and an in-phase 7-volt square wave at
U9-4 (fig. 1-2). The output signals are used for gating the electronic switches in the demodulator.
1-22. Feedback Signal Demodulator.
a. (See FO-4, FO-6, and FO-8.) The in-phase square-wave signal from U9-4 is applied to the gating
input of switch U13-13, 1, 2. The out-of-phase square-wave signal from U9-3 is applied to the gating
input of switch U13-5, 4, 3. These switches turn on during the positive halves of their gating signals.
They turn off during the negative halves.
b. When an in-phase retract signal is applied to resistor R5, positive half cycles are applied thru
U13-1, 2 contacts to the inverting input at U1-6. The negative half cycles are applied through U1 3-4,3
contacts to the non-inverting input at U1-5.
c. When an out-of-phase extend feedback signal is applied to resistor R5, the negative half cycles are
applied to the inverting input of the amplifier. The positive half cycles are applied to the non-inverting
input. (See fig. 1-2. ) The output of the demodulation amplifier is the positive dc equivalent value of the
feedback signal. The demodulated feedback signal is applied to resistor R6 in the input-summing net-
work for the valve drive amplifier.
1-23. Valve-Drive Amplifier.
a. (See FO-4, FO-6, and FO-8.) The valve-drive current amplifier applies drive current to the actuator
servo-valve. Current applied is in proportion to the difference between command and feedback signals,
Current polarity moves the actuator in the direction that causes the feedback signal to balance the com-
mand signal. Capacitor C 1 damps out oscillations.