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(4)
Modal Suppression Accelerometers
(a) The modal suppression accelerometers measure the lateral acceleration of the forward and aft sections of the airplane. This information is provided to the corresponding yaw damper modules as an analog signal.
(5)
System Annunciation
(a) The yaw damper ON and INOP annunciation is on the passenger oxygen and yaw damper control panel. System failure status is provided to the integrated display system (Ref 31-61-00/001) and to the control display unit (Ref 34-61-00/001) for display.
(6)
Upper and Lower Rudder Power Control Module Schematic (Fig. 4)
(a)
Hydraulic pressure is maintained at the input port by the airplane hydraulic system. The hydraulic fluid is filtered by a 25 micron filter before entering the module.
(b)
Solenoid Valve
1) The solenoid valve is an electrically operated open-close valve that controls hydraulic pressure flow into the EHSV. When 28 volts dc is applied, the valve opens and arms the yaw damper system. The 28-volt dc valve-actuating voltage is controlled by the auto-disengage relay in the yaw damper module. The valve is installed with four bolts and sealed with a gasket plate.
(c)
Electrohydraulic Servo Valve (EHSV)
1) One EHSV is installed on each rudder power control module. 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.
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Upper/Lower Rudder Power Control Module Schematic
Figure 4
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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 yaw damper actuator piston. Feedback from the LVDT nulls out the module error signal and piston movement stops. The EHSV is installed with four bolts and sealed with a gasket plate.
(d)
Yaw Damper Actuator Piston Assembly
1) The actuator piston translates yaw damper input commands through the EHSV into mechanical positioning of the rudder. If the yaw damper actuator is moved due to a control input, the rudder power control module control valve will port control pressure to the rudder actuators and move the rudder surface. 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 within 2.3 seconds when pressure is released. The maximum operating stroke of the yaw damper actuator is | 0.132 inch of input lever travel which is equivalent to | 4.0 degrees of rudder travel. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：747-400 AMM 飞机维护手册 空调 AUTO FLIGHT(150)
