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ALL ú ú 10 Page 30 ú Jun 20/92
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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757
MAINTENANCE MANUAL
(c) Computing Channels 1) Each yaw damper module consists of two independent microprocessors with dissimilar software and instruction sets. Each microprocessor and associated software form a separate computing channel. These are a MONITOR channel and a CONTROL channel. Each microprocessor is programmed with the following software functions: a) Executive b) Power-up test and preflight test c) Input, output, and initialization d) Foreground software that includes input monitors, signal selectors, redundancy management, comparison monitors, degraded mode logic, control laws, and annunciation logic. e) Background monitors that conduct watchdog monitor tests and Read Only Memory (ROM) tests. 2) Control Law Computations a) The control law computations are enabled only if the APL type code is valid. The code ensures that the software in the module is correct for the airplane in which it is installed. The control law continually computes output commands. The output is zero on the ground (except during the preflight test) due to the disabled easy-on/easy-off function. b) The control law computes output commands based on the data from the ADCs and IRUs. The command outputs from the two microprocessors are compared in coincidence monitors to detect when the two channel outputs do not agree. If non-coincidence is detected, the YDM fault is set.
c) Each computing channel of a yaw damper module performs self monitoring. The channel self monitoring consists of a background ROM check, executive activity monitor, and a watchdog monitor. The watchdog monitor checks for total Central Processing Unit (CPU) loss, operating speed, and erroneous ROM sum check values. If the watchdog monitor detects a failure, the YDM fault is set.
(d) Authority Limiting
1) The control law output command may never exceed |3 degrees rudder. The authority is further limited as the number of valid ADC and IRU inputs decreases.
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22-21-00
ALL ú ú 09 Page 31 ú Jun 20/92
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL
(e) Easy-On/Easy-Off
1) The control law output command is routed through a ramp gain schedule stage. The gain is zero when on the ground and one when in-air. With an air/ground relay indication that the airplane has taken off, the gain linearly rises from zero to one in 5 seconds. With an air/ground relay indication that the airplane has landed, the gain linearly decreases from one to zero in a 5-second time period. If an in-air power-up occurs, the fade-in begins immediately following the power-up.
(f) Command Output 1) The control microprocessor output is converted to analog, smoothed, and routed to the yaw damper servo for rudder control. The output command operates the yaw damper electrohydraulic servo valve (EHSV). The EHSV directs hydraulic pressure flow to the servo actuator piston in proportion to the magnitude and sign of the command signal. 2) The EHSV receives hydraulic pressure when the engage solenoid is actuated by 28 vdc from the control panel ON switch (via the engage relay in the yaw damper module). The engage relay is closed unless an auto-disengage signal is generated, or there is a loss of operating power to the yaw damper module. 3) A Linear Variable Differential Transducer (LVDT) is physically attached to the actuator piston. The LVDT uses 400 Hz, 26 vac power for excitation. The LVDT output voltage reflects the position of the servo actuator piston. This data is routed back to the yaw damper module to complete the servo loop. 4) The LVDT feedback signal and the command output signal are compared in the yaw damper module. If the LVDT signal does not track the command signal, the servo fault latch is set.
5) AIRPLANES WITH YDM DASH NUMBERS -101 THRU -121; This sets the YD ACT faultball. 6) AIRPLANES WITH -122 YDM AND SUBSEQUENT YDM DASH NUMBERS; This causes a YD ACT fault to be recorded.
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