(2)
When opened, the ARM solenoid ports hydraulic fluid to both the servovalve and the ENGAGE solenoid. When opened, the ENGAGE solenoid ports hydraulic fluid to the detent pistons. The detent pistons, when pressurized, clamp the actuator piston to the output crank, The autopilot commands can then be translated to the elevator control surface.
F. Electrohydraulic Servovalve (EHSV)
(1)
One EHSV is mounted on each EAS. The EHSV is mounted with four mounting bolts and sealed with a gasket plate and O-ring. Electrical connectors mate when the valve is bolted into position. The EHSV contains a sealed torque motor, feedback spring, projector jet and an internal piston.
(2)
The EHSV distributes hydraulic fluid to the autopilot actuator piston at a rate, and in a direction, proportional to the electrical current flow in either of the two motor windings. Thus, the polarity on the motor windings determines the direction of flow of hydraulic fluid (toward C1 or C2) and the magnitude of the current determines the flow rate of hydraulic fluid.
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G. Linear Variable Differential Transducer (LVDT)
(1)
Each EAS has 2 LVDTs: one is the autopilot LVDT, the other is the output (surface position) LVDT. The autopilot LVDT senses position of the actuator piston. This electrical position signal is fed back to the FCC to complete the control loop for EHSV command signals and to detect servo failure. The output LVDT is operated by an internal crank which is connected to the external crank, thus providing the FCC with elevator surface position. This electrical position signal is compared with the autopilot LVDT for synchronization and failure detection.
(2)
Both LVDTs are variable reluctance transformers with an output that varies directly with linear mechanical motion. Each LVDT is excited by 26 volts ac that is supplied by its associated FCC. LVDTs are not line replaceable units (LRUs).
3. Operation (Fig. 5)
_________
A. Autopilot Engagement Control Sequence
(1) Single Channel Engagement
(a)
Pushing a CMD switch/light on the MCP provides an engage request to the appropriate FCC. There are three phases to the autopilot engage sequence: 1) Engage Request 2) Arm 3) Engage
(b)
During the engage request phase, the FCC verifies the engage request and provides the appropriate voltage to energize the ARM solenoid on the EAS. The opened ARM solenoid valve provides hydraulic pressure to the electrohydraulic servovalve (EHSV) on the EAS.
(c)
The arm phase includes two major functions: synchronization and a pre-engage test. During the arm phase, the FCC also provides a discrete voltage to illuminate the CMD switch/light on the MCP. 1) Synchronization of the servo command signals is needed to
provide for transient-free engagement. During synchronization, the FCC receives surface position from the output LVDT and autopilot actuator position is provided by the autopilot LVDT. The two signals are compared to provide the appropriate commands to the EHSV. These command signals direct the actuator piston to move to the position of the output crank. When the difference between the two LVDT signals is zero, the output crank will remain centered in the piston cavity as the piston moves in response to commands given to the EHSV and output crank follows the surface.
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