(4)
Linear Variable Differential Transducer (LVDT)
(a) Each servo has two identical 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) LVDT is operated by the intermediate crank, which is connected to the surface control linkage. Both LVDTs are variable reluctance transformers with an output that varies directly with linear motion. The LVDT uses 26vac excitation from the associated FCC. The LVDTs are not considered line replaceable units. They require nulling adjustments to be completed which match actuator piston with piston position when the servo is on the bench.
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(5)
Pressure Regulator and Relief Valves
(a) The pressure regulator and relief valve regulates and limits hydraulic pressure applied to the actuator piston and detent pistons. The pressure relief function allows manual inputs from the control linkage to override autopilot control.
(6)
Actuator Piston Assembly
(a)
The actuator piston translates autopilot input commands through the EHSV into mechanical positioning of the control surface. 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 pressurization 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 command signal from the FCC to stop movement of the actuator piston.
(7)
Cranks
(a) The internal crank roller is clamped by the detent pistons when the autopilot is engaged. The crank moves with the actuator piston. Motion of the internal crank moves the output position LVDT. One end of the output crank is directly connected to the internal crank at a common pivot point. The other end of the output crank is connected to the control surface linkage.
(8)
Mechanical Control Sequence - Autopilot Disengaged
(a) Initially, the actuator piston is fixed by the centering springs. With the autopilot not engaged (armed), SV1 is open, SV2 is closed; detent pistons are disengaged, and the internal crank is free to move within the piston cavity. The output position LVDT provides internal crank position to the FCC for autopilot synchronization to surface position. The FCC commands through the EHSV cause the actuator piston to follow the internal crank so that the crank remains centered within the cavity. The actuator piston position LVDT nulls the command signal to stop the actuator piston.
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(9)
Autopilot Control Sequence - Autopilot Engaged
(a) With the autopilot engaged, SV1 and SV2 are open, the detent pistons are pressurized and the internal crank is clamped in the center of the actuator piston. When the EHSV receives a command from the FCC, hydraulic pressure is ported to one end of the actuator piston. The detent pistons carry the internal crank with the actuator piston to its commanded position. The output crank moves the linkage to the surface power control actuators and the output position LVDT sends position information back to the FCC to null the command signal and stop surface movement.
(10)
Camout
(a) Camout occurs when the output crank position (surface position) does not correspond to the actuator piston position (servo postion) as sensed by differing LVDT outputs. Causes of camout are mechanical jamming of output linkage, pilot override of an engaged autopilot channel, or mechanical override of one autopilot channel by the remaining channels during multi-channel operation.
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