Discrete Inputs
(a)
The status of input discretes for mounting position and interface test mode are also sent from the input electronics to the I/O computer.
(b)
Mounting position #1 discrete is always grounded since the IRUs are always mounted with handles facing forward in the airplane.
(c)
The interface test input discrete is paralleled with the test switch on the front of the IRU.
(d)
Mode 1 and Mode 2 discretes, switched from the IRS mode select
panel, select the operating mode of the IRU.Output
(a)
Bus transmitters format and transfer CPU data to the ARINC 429 busses. Three ARINC 429 busses supply identical information to the interfacing systems.
(b)
If the IRU detects a fault, an analog fault signal is sent to each EIU.
(c)
If the IRU is operating on dc power, an ON DC signal will be
sent through the air/ground relay to the ground crew call horn.Sensors
(a) Accelerometer (Fig. 4) 1) There are three accelerometers in each IRU. The accelerometers sense changes in velocity and output an analog current proportional to acceleration. 2) The figure illustrates the construction of the accelerometers. A proof mass is mounted on a pivot between two capacitor plates. The proof mass suspends a common capacitor plate forming two capacitances. During an acceleration, the proof mass begins to pivot causing the capacitances to become unequal. A servo loop senses the unequal capacitances. As a result, it forces current through an electromagnetic torque coil which drives the proof mass to null. The changing servo current is proportional to acceleration. 3) The acceleration signal is integrated once to give velocity and integrated a second time to give distance.
4) Changes in temperature create errors in the accelerometer outputs. For this reason, each accelerometer has a temperature sensor which is used by the IRU processor to correct the accelerometer data.
(b) Laser Gyro (Fig. 5)
1) There are three laser gyros in each IRU. The laser gyros detect pitch, roll, and yaw rates and provide this data to the IRU computer for computation and navigation.
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A
747-400
MAINTENANCE MANUAL
UPPER MAGNET STRUCTURE
-+
ACCELERATIONFORCE
SERVO AMP
CAPACATIVE PICKOFF PLATE CAPACATIVE PICKOFF GAP TORQUER COIL FLEXURE
PROOF MASS SUBASSEMBLY THIN FILM PICKOFF AND TORQUE LEADS
MAGNET FLEX LEADS LEAD SUPPORT POSTS
PROOF MASS LOWER MAGNET STRUCTURE SENSOR ELECTRONIC CONNECTOR
VELOCITY (GROUND SPEED)
DISTANCE
INTEGRATOR 1 INTEGRATOR 2
EXCITATION
POWER
TEMPERATURE SENSOR TEMPERATURE
Accelerometer
Figure 4
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A 747-400 MAINTENANCE MANUAL
2) The ring laser gyros use laser light to measure the angular rotation rates. Each gyro is housed in a triangular container part of which is filled with helium-neon gas. Mirrors are mounted at each corner, and high voltage electrodes are mounted in each leg. The cavity and mirrors form the positive feedback, resonant system required for laser action.
3) The laser gyro is started by the application of -950V dc and +3500V dc from the HV power supply. The high voltage causes the helium-neon gas to ionize, producing clockwise and counter-clockwise beams of laser light. After startup, the 3500v dc is removed and the lasing is sustained on -950v dc. The two laser beams are reflected around the cavity by the mirrors.
4) The path length around the cavity is carefully monitored and adjusted so that it is an integral multiple of the peak power laser wavelength.
5) When the IRU is at rest, the frequencies of the two opposite travelling laser beams are equal. When the IRU is rotating about an axis perpendicular to the lasing plane of a laser gyro, one laser beam has a longer than average apparent distance to travel while the other beam has a shorter than average apparent distance to travel. This distance difference implies a frequency difference between the two beams.
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