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747-400 MAINTENANCE MANUAL
6) Small fractions of the beams (less than 0.2%) pass through one of the mirrors and are combined by optical means. When the laser beam frequencies differ, an interference pattern of alternating dark and light stripes is created. Photodiodes sense the rate and direction of movement of the interference pattern. The frequency and relative phase of the two diode outputs indicate the magnitude and direction of the gyro's rotation.
7) At low rotation rates, the small frequency difference between the laser beams causes the frequencies to lock together at a single false value. To compensate for this effect a piezoelectric dither motor vibrates the laser block through the lock-in region. Dither vibration produces no net inertial rotation. The dither motor vibration can be felt on the IRU case and produces an audible hum.
8) Changes in temperature cause errors in the laser gyro outputs. For this reason, each laser gyro has a temperature sensor which is used by the IRU processor to correct the laser gyro data. Errors are also caused by any cone-shaped motion of the gyros. The IRU contains circuits which compensate for these errors. Correction values, which detail the gyro's unique bias, thermal characteristics and alignment characteristics, are stored in the IRU's permanent memory. Data from this memory is routed to the processor when the IRU is turned on.
(5) Power Supply
(a) The power supply receives 115v ac, 400Hz power and a 28v dc backup power from the airplane. The 115v is stepped down to 29v, rectified and then diode ORed with the 28v backup power. The power supply furnishes the control currents for the IRU at various voltages. The main supply provides two drive signals for a high voltage power supply (-950v and +3500v dc), two mode selection signals to the input electronics, and a power-up initialization signal for the entire IRU.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400
MAINTENANCE MANUAL
(6)
Inertial Computer
(a)
The IRU computer is a 16 bit microprocessor running at 3.84MHz. The computer receives inputs from the inertial sensor assembly and the I/O computer, calculates inertial parameters and sends these parameters to the I/O computer. At turn on, the inertial computer receives data concerning biases, misalignments and temperature compensation coefficients from the accelerometers, gyros and nonvolatile memory. These are used to continually compensate the sensor inputs along with the effects of gravity and Earth's rotation. The compensated inputs are then used to calculate airplane attitude referenced to local level by integrating the rotation rate measured by the gyro. This produces the angular amount the airplane has rotated through, which when added to previous attitude gives present attitude. The inertial computer performs this calculation 50 times per second. When the accelerations, which can now be referenced to local level, are integrated they give the airplane's horizontal and vertical velocity. A second integration gives the distance travelled, which when added to previous position gives present position.
(b)
An algorithm uses a look up table to calculate magnetic variation as a function of position on the Earth. This magnetic variation is added to heading and track referenced to true North to produce heading and track referenced to magnetic North. Above 73 degrees North latitude and below 60 degrees South latitude heading and track referenced to magnetic North are not computed.
(c)
AIRPLANES WITH EXTENDED MAGVAR TABLES; The range for magnetic variation calculation is extended to 82 degrees South latitude and 82 degrees North latitude, except in the magnetic polar regions.
(d)
Altitude received from the I/O computer is used to stabilize the inertial vertical speed and inertial altitude which by themselves would slowly change due to positive feedback in the vertical acceleration calculation. True airspeed from the I/O computer is used to calculate wind speed and direction. Altitude rate from the I/O computer is used to initialize vertical speed in ATT mode.
(e)
The inertial computer also performs BITE monitoring of the entire IRU and sends status information to the I/O computer.
(7)
I/O Computer
(a) The I/O computer is an 8 bit microprocessor that reads the ARINC 429 busses and sends set latitude, set longitude, set heading, pressure altitude, true airspeed and altitude rate to the inertial computer. Discrete data from the input electronics is also received and passed to the inertial computer. The I/O computer also receives the parameters calculated by the inertial computer and puts them in ARINC 429 format. The data is then transmitted to the output drivers eight bits at a time.
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