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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
(k) Laser intensity monitor (LIM) - The LIM monitors the intensity of the laser beams in the ring laser gyro (RLG). There are two levels of an IRU failure found by the LIM. 1) The first level of an IRU failure occurs when the intensity
of a laser beam decreases to a low level where the RLG still can operate correctly. These annunciations do not occur until landing roll-out when the ground speed of the airplane is less than 40 knots. a) The FAULT light on the mode select unit (MSU) comes on
for that IRU. b) The IRS master caution light comes on. c) The FAULT light on the MSU and the IRS master caution
light will automatically go off after a normal power up sequence and the IRU is aligned.
NOTE: If the first level of an IRU failure occurs, the
____ maintenance personnel can make the decision to replace or not to replace the IRU. If the IRU maintenance is not available, the airplane can be sent to where the IRU maintenance is available since the IRU is still serviceable.
2) The second level of an IRU failure occurs when the intensity of a laser beam decreases to a low level where the RLG cannot operate correctly. All IRU outputs are invalid and these annunciations occur immediately when the failure is found by the LIM: a) The FAULT light on the MSU comes on for that IRU. b) The IRS master caution light comes on.
NOTE: The IRU must be replaced.
____
(l) Accelerometer electronics - These circuits consist of three accelerometers, temperature sensors and wave-shaping circuits. This portion of the IRU outputs analog and digitized acceleration. The output of each accelerometer is an analog current proportioned to acceleration. The current is integrated and digitized and the digital counts are stored. They are then used to produce a component of velocity and change in velocity (acceleration) for each axis.
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. AIRPLANES WITH EFIS ú
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
(m) (n) (o) (p) The accelerometer electronics also contains a 7.68 MHz precision system clock. The clock outputs are divided down to 3.84 MHz and 640 kHz. The clock signals control the timing of data transfer and computer program execution.Sensor electronics and A/D multiplexer - The A/D multiplexer converts analog input signals from the sensors into digital words. The input signals are accelerometer analog currents and sample and hold signals, pitch rate, temperature sensor signals, and BITE monitor signals.Acceleration and gyro data are received under timing control of a clock signal. The accelerometer pulses are accumulated as velocity and change in velocity. The gyro pulses are summed as changes in angular rate and direction. Coning circuits correct the gyro data for errors developed by the cone-shaped motion of the gyros. Sensor data is then routed to the common CPU bus.IRU sensor data is supplied by the laser gyros, accelerometers, and their temperature sensors. Signal outputs of these devices are formatted into data words for manipulation by the IRU computer. A common data bus links the following blocks of the IRU:
(q) 1) Input electronics 2) Sensor electronics and A/D multiplexer 3) CPU 4) Output electronics 5) BITE electronicsInput Electronics - The IRU receives altitude, altitude rate, and true airspeed (TAS) from the digital air data computer (DADC). The flight management computer (FMC) and the ISDU each send initial latitude/longitude and heading (in the ATT mode only) to the IRU. This data is placed in a buffer which is accessible to the CPU. Mode 1 and Mode 2 discretes, switched
from the MSU, select the operating mode using open/ground logic as shown on the schematic.
(r) CPU - The IRU computer is controlled by a stored program. The computer uses the sensor data to perform calculations.
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