(b)
DADC - The indicator is set when the yaw damper module determines that data from any one air data computer is invalid (the indicator may be latched if it is set while the airplane is in the air).
(c)
AIR/GRD - The indicator is set when an air/ground discrete is invalid (the indicator may be latched if it is set while the airplane is in the air).
(d)
HYD PRESS - The indicator is latched if the hydraulic pressure high signal disagrees with the hydraulic pressure low signal for more than two seconds.
(e)
LVDT - During normal operation, the indicator is set if the LVDT common mode voltage is low. During the preflight test, the indicator is set only if the common mode voltage is low and a position test fault is detected (valid high hydraulic pressure must be present).
(f)
YD ACT - During normal operation, the indicator is latched when a servo loop fault is detected. During the preflight test, the indicator is set only if a servo loop fault and position test fault are detected. To receive a YD ACT fault indication, valid high hydraulic pressure must be present and the LVDT common mode voltage must be normal.
(g)
YDM - The indicator sets with the following faults: power up test fault, preflight test fault, channel code input fault, A/C type code input fault, control or monitor channel command coincidence fault, or computing channel fault.
When the RESET switch is pressed, all yaw damper module monitors and control logic is reset. The fault ball indicators are also reset to black. The RESET switch may need to be pressed a second time to reset a YD ACT faultball to black.
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22-21-00
ALL ú ú 03 Page 10 ú Sep 20/91
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL
C. AIRPLANES WITH -122 YDM AND SUBSEQUENT YDM DASH NUMBERS; Yaw Damper Module
(1)
Each yaw damper module consists of two digital computing channels (control and monitor) and a servo amplifier. The module consists of the following functional components:
(a)
Fault Display - provides the logic to display fault messages on the front panel of the module.
(b)
Monitor CPU - provides the digital computing function for the yaw damper, duplicating the computing of the control channel.
(c)
Monitor Receiver - provides three high-speed and two low-speed ARINC 429 receiver interfaces for the control and monitor microprocessors.
(d)
Analog - provides the output to the yaw damper servos and analog to digital and digital to analog signal conversion for both channels.
(e)
Input/Output - provides input and output discrete signal conditioning for the control and monitor microprocessors.
(f)
Control CPU - provides the digital computing function for the yaw damper.
(g)
EMI Filter - provides EMI filtering for signal inputs, signal outputs and electrical power inputs for the control and monitor channels.
(2)
The servo amplifier provides rudder command signals to the yaw damper electrohydraulic servo-valve. Feedback from the Linear Variable Differential Transducer (LVDT) position sensor is received by the actuator position demodulator assembly to close the servo loop. The assembly also provides servo loop monitoring which operates the automatic solenoid disengagement relay when a failure affecting operation is detected.
(3)
One connector with three inserts is on the rear panel of each module. Two inserts contain the YDS signal pins and low voltage power pins. One insert contains the ground reference pins. Cooling is provided by air from the rack plenum being forced from the bottom to the top of the modules. A hinged front provides access to the interior of the module.
(4)
The 28v dc engage servo solenoid power is routed from the control panel, through the module auto-disengage relay contacts, to the servo. Module operating power (+5v dc and |15v dc) and the 26v ac LVDT demodulator reference is provided by the Control System Electronics Unit (CSEU) power supply modules (Ref 27-09-00).
(5)
The fault detection logic in the modules isolates faults to the LRU level within the YDS and interfacing LRUs. The faults are divided into FAULTS NOW and LAST LEG FAULTS. FAULTS NOW corresponds to the fault monitors currently indicating a fault. LAST LEG FAULTS are the faults that occurred prior to the last air/ground transition.
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