ACTUATOR PISTON
CENTERING SPRING DETENT PISTONS (2)
ACTUATOR PISTON
OUTPUT (SURFACE) POSITION LVDT
INTERNAL OUTPUT CRANKCRANK CONNECTED TO PCA LINKAGE
Autopilot Pitch Control Servo Figure 4
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(b) Electrohydraulic Servovalve (EHSV)
1) One electrohydraulic servovalve is installed on each APCS. The EHSV contains a sealed torque motor, a feedback spring, a projector jet and a piston. Hydraulic pressure through the valve can be applied to either of two output ports. When no error signal is applied to the torque motor, a small amount of hydraulic fluid flows through a flexpipe attached to the torque motor and out the projector jet. From the jet, equal pressure is applied to opposite ends of the piston holding it at center and closing both output ports.
2) When an error signal is applied, the motor armature rotates in proportion to the magnitude and direction of the input signal and moves the projector jet accordingly. The jet directs more hydraulic pressure to one end of the piston than the other causing it to move and opens the corresponding output port to complete hydraulic fluid flow through the APCS. When the error signal is nulled, the motor armature and jet return to center. This equalizes the pressures on the piston and, with the aid of the feedback spring, causes the piston to recenter and close both output ports.
3) The EHSV is installed with 4 bolts and sealed with a gasket plate. Electrical pins are mated when the EHSV is bolted in position.
4) The EHSV is controlled by the elevator error signal (ELEV SERVO CMD) from the FCC. In response to this command, the EHSV ports hydraulic pressure to the right or left side of the actuator piston. Feedback from the actuator piston LVDT nulls the error signal at the FCC and piston movement stops.
(3) Linear Variable Differential Transducers (LVDT)
(a) The APCS has two LVDTs. The actuator piston (servo position) LVDT functions as a linear follow-up transmitter for closing the loop around the EHSV. It is operated by the actuator piston. The output position (surface position) is operated by the internal crank which is connected to the output crank and elevator control linkage. Both LVDTs are variable reluctance transformers with an output that varies directly with linear motion. The LVDT uses 26 vac excitation from the associated FCC. The LVDTs are not considered line replaceable units since they require nulling adjustments to be completed on the bench to match actuator piston with crank position.
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(4)
Pressure Regulator and Relief Valve
(a) The pressure regulator-relief valve regulates and limits hydraulic pressure applied to the actuator piston and detent piston. The pressure relief function allows manual inputs from the control linkage to override autopilot control.
(5)
Actuator Piston Assembly
(a)
The actuator piston translates autopilot input command through the EHSV into mechanical positioning of the elevators. The actuator piston is normally centered by two springs and is moved right or left by hydraulic pressure from the EHSV when the autopilot is armed or engaged. The springs center the actuator piston when pressure is released.
(b)
Two detent pistons inside the actuator piston are normally retracted by disengage springs. When the autopilot is engaged, SV2 opens. Hydraulic pressure through SV2 overrides spring tension and locks the detent pistons against the roller of the internal crank. Detent pistons may be forced back if sufficient force to overcome hydraulic pressure is applied manually through the control linkage (camout).
(c)
The actuator piston LVDT provides an electrical signal proportional to actuator piston position. This signal nulls the autopilot elevator position error signal from the FCC to stop movement of the actuator piston.
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