(3)
Linear Variable Differential Transducer (LVDT)
(a) There are two LVDTs on each servo that provide electrical feedback of servo and surface position to the FCC. Both LVDTs are variable reluctance transformers with an excitation of 26vac and an output that varies linearly with motion of the component to which it is attached. One LVDT is attached to one end of the autopilot actuator piston to measure servo position, and the other is attached to the internal crank to provide surface position feedback.
(4)
Pressure Regulator and Relief Valve
(a) This valve regulates and limits the hydraulic pressure applied to the detent pistons of the autopilot actuator piston. This pressure relief function is what allows manual control wheel inputs to override autopilot control if neccesary.
(5)
Autopilot Actuator Piston
(a)
The autopilot actuator piston moves in response to changes in hydraulic pressure caused by the EHSV. The cavity has centering springs which center the piston when pressure is released.
(b)
There are two detent pistons within the autopilot actuator piston, which clamp the piston to the internal crank when the autopilot is hydraulically engaged. When the engage solenoid valve is energized, hydraulic pressure is ported to the detent pistons which causes the pistons to extend and clamp onto the roller at the end of the internal crank. The detent pistons are retracted by springs when hydraulic pressure is relieved.
3. Operation_________
A. Synchronization
(1) Prior to hydraulic engagement of a servo it is synchronized to the position of the control surface and a pre-engage test is performed. The sequence begins when the FCC energizes the ARM solenoid valve. This allows hydraulic pressure to flow to the EHSV and to the ENGAGE solenoid valve (this valve is still deenergized). The command to the EHSV is the difference between the surface position and the servo position, so the autopilot actuator piston will move to a point were the internal crank is centered between the detent pistons.
B. Pre-engage Tests
(1) Single channel Engage Tests
(a) Normal warning tests: This test verfies that the normal warning driver interfacing with MAWEA is functioning properly so that upon engagement the driver will not issue a false warning.
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(b)
Battery warn inhibit test: This test is similar to the normal warning test. It verifies that the respective warning is not set.
(c)
Aileron exceedance monitor test: The hardware aileron exceedance monitor is designed to inhibit engagement of the aileron autopilot servo when the servo deflection exceeds a predefined level. The aileron servo authority limit is + or -18 degrees in single channel operation.
(d)
The engage inhibit function of the aileron exceedance detector is verified by software during the single channel pre-engage test. Software commands the aileron servo to either + or - 30 degrees and then verifies that engagement is inhibited. The threshold + 30 or - 30 is determined at random. If the aileron surface position exceeds the equivalent of 20 degrees wheel input, a fault flag is set and the test is terminated.
C. Servo Hydraulic Engage
(1) After successful completion of synchronization and the pre-engage test, the FCC energizes the ENGAGE solenoid valve. This allows hydraulic pressure to clamp the detent pistons to the internal crank roller and so clamps the autopilot actuator piston to the output crank.
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