(8)
When the RESET button on the faceplate is pressed, all yaw damper module monitors and control logic are reset. To inhibit the LAST LEG FAULT messages from being displayed, the DISPLAY and RESET buttons are pressed at the same time.
D. Yaw Damper Servo
(1)
Each yaw damper servo drives the mechanical linkage to the power control actuators. Servo weight is approximately 7.5 pounds and the piston stroke is |0.75 inches. Normal operating pressure at the pressure port is 3000 psi. The maximum return pressure is 100 psi. The servo rated load at 3000 psi is 1700 pounds maximum with the piston extended or retracted.
(2)
Servo component parts consist of the input filter, electrohydraulic solenoid valve, electrohydraulic servo valve, output piston and shaft, and LVDT. A single electrical connector completes the servo engage voltage, command signal voltage, LVDT excitation voltage, and LVDT feedback voltage.
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(3) Yaw Damper Servo Schematic (Fig. 5)
(a)
Hydraulic pressure is maintained at the servo input port by the airplane hydraulic system. The hydraulic fluid is filtered by a filter screen before entering the servo.
(b)
Solenoid Valve
1) The solenoid valve is an electrically operated open-close valve that completes hydraulic pressure through the servo when the yaw damper system is armed. The 28 vdc valve-actuating voltage is controlled by the auto-disengage relay in the yaw damper module. The valve is installed with 4 bolts and sealed with a gasket plate.
(c)
Electrohydraulic Servo Valve (EHSV)
1) One EHSV is installed on each yaw damper servo. It 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 supplied, 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 causing it to move. This opens the corresponding output port to complete hydraulic fluid to the servo. When the error signal is nulled out, 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 controlled by a rudder command signal from the yaw damper modules. In response to this command, the EHSV ports hydraulic pressure to the right or left side of the actuator piston. Feedback from the LVDT nulls out the module error signal and piston movement stops. The EHSV is installed with 4 bolts and sealed with a gasket plate.
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DIFFERENTIAL TRANSDUCER (LVDT)
Yaw Damper Servo Schematic
Figure 5
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(d)
Actuator Piston Assembly
1) The actuator piston translates yaw damper input commands through the EHSV into mechanical positioning of the rudder. The actuator piston is normally centered by two springs and is moved right or left by hydraulic pressure from the EHSV. The springs center the piston when pressurization is released. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：757 AMM 飞机维护手册 自动飞行 AUTOFLIGHT 2(27)
