barrichello72
2nd May 2018, 17:09
Hi
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 laser 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 laser 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 laser gyros the light beams are replaced by two laser 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 laser 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 laser 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 laser 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 laser 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 laser 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 laser 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 78.15°N/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
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 laser 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 laser 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 laser gyros the light beams are replaced by two laser 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 laser 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 laser 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 laser 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 laser 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 laser 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 laser 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 78.15°N/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