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b737 ng irs

Old 2nd May 2018, 17:09
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b737 ng irs


i have a question for the technically more inclined or whoever can help me out with the issue.
as far as a i know the IRU portion of the ADIRU provides position, attitude and heading information and when integrated with the data from the ADC vertical speed, ground speed and track as well, by means of three strap down ring [email protected] gyros one for each axis of the airplane sensing angular rates providing an indication of the direction of displacement from a reference frame and of three accelerometers sensing linear accelerations that when integrated with time twice will provide an indication of the distance travelled from the reference frame along the respective axis.
Ring [email protected] gyros are based on the Sagnac effect: two light beams travelling in a circular tube will reach the reference point at the same time being the speed of light a constant, whenever the system is at rest, however as the system is rotated along an axis perpendicular to the plane containing the beams, the light beam traveling in the same direction as the rotation will reach the reference point at a later time than the beam travelling in the direction opposite the rotation as the reference point moves away from it and thus it will have to cover a longer distance, being the speed of light constant, resulting in a difference in frequency. In the ring [email protected] gyros the light beams are replaced by two [email protected] beams obtained by ionizing a mixture of helium and neon via a cathode while the circular tube is replaced by a triangular shaped tube with mirrors in each corner reflecting the beams around with one mirror silvered allowing a small portion of the [email protected] beam through, then fed through a prism to generate on a detector a fringe pattern. When the system is stationary the fringe pattern is stationary as well as the [email protected] beams reach the detector at the same time while as a rotation is sensed along an axis perpendicular to the lasing plane as the airplane manoeuvres around, the difference in frequency due to the [email protected] beams reaching the detector at different times, will cause the fringe pattern to shift either left and right as detected by the photocells, indicating the presence of an angular rate and its direction and thus providing an indication of the direction of displacement from the reference frame allowing to compute the attitude and when integrated with the inputs from the accelerometers, the position of the aircraft. At low speeds as the [email protected] beams tend to lock making it difficult to detect the difference in frequency and thus the shift in the fringe pattern, a piezo-electric dither motor is used to vibrate the [email protected] block through the locking region, preventing locking from occurring.
The accelerometers whereas consist of a proof mass maintained centred by two springs when the system is at rest while as the airplane accelerates along its input axis, due to Newton s first law of motion stating that a body at rest maintains its state unless acted upon by an external force, the proof mass appears to lag behind the reference frame as detected by a pick off generating an electrical error signal proportional to the change in position, fed to an amplifier which in turn evens the signal out and returns the mass to its neutral position via a re-centring coil: the current in the coil needed to null the error signal is used as a measurement of the linear acceleration, integrated with time once to provide the velocity, and a second time to compute the distance travelled from the reference frame.
Integrating the angular rates from the ring [email protected] gyros with the linear accelerations from the accelerometers, the IRU computes the shift in position from the reference frame of the respective axis and thus the new aircraft position/attitude.
The reference frame is provided to the IRU during the initialization process as the on mode selector on the MSU is set to ATT and during which the IRU determines the local vertical first on the premise that when the aircraft is stationary the only vertical force acting upon the system is the earth gravity always perpendicular to the local horizon and used to erect the attitude data and then the geographical position computing the latitude on the premise that when the aircraft is stationary the only motion acting upon the system is due to earth rotation, sensing the earth rotational velocity, and recovering the longitude from the last position stored in a non-volatile memory. Integrating the local vertical with the geographical position the IRU also determines the true north and applying the value of the variation from its own database to the true north, it computes the magnetic north as the earth rotational velocity varies with latitude being greatest at the equator and smallest at the poles, the alignment time varies between 10 at the equator to up to 17 at 78.15N/S representing the maximum latitude for alignment and beyond which the magnitude of the earth rotational velocity is too small, while the accuracy of the alignment process can be greatly improved by providing the system with the updated geographical position longitude - from the GPS via the FMC. At the completion of the alignment process the ALIGN light will extinguish.

Do i get the IRS right?
How does the IRS compute the true north by integrting the geographical position with the local vertical?

Many thanks
barrichello72 is offline  
Old 2nd May 2018, 22:43
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Something like that 😳
Matey is offline  
Old 2nd May 2018, 23:24
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How does the IRS compute the true north by integrting the geographical position with the local vertical?

Something needs to provide a heading input to the IRUs during the align process. Since local vertical can be deduced in a stationary plane (vehicle) and lat/long is available (most likely from GPS), you still need to tell the units which way the airframe is pointing. Most likely from a magnetic compass and a database of magnetic north variation for geographic positions.

Once you are moving, a GPS can calculate your course, but not your exact heading due to crosswinds.
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Old 3rd May 2018, 02:25
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When it detects a rotation with the aircraft assumed stationary, that rotation can only be the earth's rotation. The IRU knows which way the rotation is relative to the IRU (after all, it's an IRU, that's what it does) and it knows which way it's mounted to the airplane. Therefore, it's known which way the rotation is relative to the airplane. No magnetic or GPS motion info is needed.

For example, if it's on the equator and it's rolling right, it's pointed North.

If it's pitching down, it's pointed East.


If it's not on the equator, then there are rotations in multiple axes at once, and the components are figured out with trig.

(This isn't about Boeing equipment, just genetic on what information is available and what can be inferred)

Last edited by Vessbot; 3rd May 2018 at 02:38.
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Old 3rd May 2018, 08:03
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Vessbot thanks for your explication - it clicked and it makes perfect sense.
to the other users, thanks for your contributions
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Old 3rd May 2018, 08:38
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Vessbot is correct, in terms of basic theory and generic mechanical internal platforms, INS 101, you do not need to enter present position or use GPS to align north/south...a basic platform will probably simply use a process called gyro compassing.

At the equator if the platform correctly levelled (first part of the align process) and by luck happens to be directional aligned correctly and then if the platform is uncaged the north/south axis will not sense a tendency to “tip” as the earth rotates under it, but the east/west will (the east end will have a tendency rise relative to the local horizontal, the western end will have a tendency to dip.). However if it is not aligned correctly then the north/south ends will have a tendency to rise/ the platform is motored/slew/rotated until the north/south axis is stable, and bingo, you are GPS or magnetic info required. As V has said it is slightly more complex away from the equator but the same logic applies. You can use an analogous process (using fancy maths) to align strap down platforms, through the motoring/slewing in that case will be virtual....ultimately of course you do need to enter start position at some point to get the aligned platform to produce meaning output...

This used to be accompanied by lots of flashing lights (and cursing from the rear seat on some aircraft)...nowadays it’s press “on” and enter GPS PPOS....

Last edited by wiggy; 3rd May 2018 at 09:05.
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