AHRS - Attitude and Heading Reference System

today we will talk about the attitude and heading reference system which like the air data computer has helped the transition from the conventional analog instruments to the new digital systems however before going into detail with this system we must see how conventional analog instruments work in the case of gyroscopic instruments each one has its own gyroscope specifically designed to measure a certain parameter this means that each instrument has its own gyro gimbals gears and other moving parts this results in more complex isolated systems with errors and inaccuracies due to mechanical imperfections friction gyroscopic drift among others and in addition to this in the case of the heading indicator the crew must manually correct the indication using the magnetic compass as a reference and to do so the aircraft must be flying in straight flight with a constant speed as these are the only conditions under which the compass gives a correct heading indication all this results in higher workloads and lower accuracy as we have seen these conventional gyroscopic instruments tend to have low accuracy and several disadvantages so in order to improve the accuracy and functionalities of the system the attitude and heading reference system was developed which is abbreviated as ahars this unit has gyroscopes accelerometers and magnetometers oriented in the three axes these sensors calculate the aircraft attitude and heading accurately in real time based on the measurement of the movements of pitch bank and yaw then the information is sent electronically to the relevant instruments and systems now since the resulting parameters are generated electronically these are easily integrated with different types of instruments for example the electronic data generated by the unit can be used to feed analog instruments that are adapted to receive electronic inputs or they can also be sent to electronic flight instrument systems with digital presentations the instruments normally fed by the ahrs are the attitude indicator the heading indicator the turn and slip indicator and the turn coordinator and apart from this the system can also feed instruments that previously used a remote indicating compass such as the hsi or the rmi here we see the instruments in their conventional analog presentation however normally this unit is used in conjunction with digital instruments as we can see here here we can see the attitude indicator the slip and skid indicator the hsi and the rmi and although the main purpose of the ahars is to feed the flight instruments the information can also be sent to other systems that require it such as the autopilot and flight director system the yaw damper the weather radar the monitoring and warning system the flight management system the flight data recorder and the a cars now that we have seen the instruments and systems that use the information generated by the ahars let us now look at its sensors the ahars has a series of sensors contained within an inertial measurement unit abbreviated as imu this unit is responsible for measuring changes in the aircraft's attitude and heading and then sending this raw information to a kalman filter and a processor to integrate and process it after this the electronic data is sent to the flight instruments and systems now this was a basic overview of how the imu works let us now take a closer look at how the imu detects changes in attitude and heading normally this unit has three accelerometers three gyroscopes and three magnetometers oriented on the three axes this axes are known as x y and z respectively for the movements of roll pitch and yaw this means that there is one accelerometer for each axis which allows to calculate the accelerometer's derived attitude the same happens with the gyroscopes there is one for each axis which allows to calculate the gyro derived attitude and finally there is also one magnetometer for each axis which allows to calculate the magnetometer's derived heading with this in mind and taking into account the capabilities of each type of sensor we would obtain the following data the accelerometers are capable of measuring the attitude of the aircraft in terms of pitch and bank the gyros are capable of measuring the pitch bank and heading and the magnetometers are capable of measuring only the magnetic heading all this information derived from the sensors is sent to a kalman filter which integrates and complements this information in order to obtain a sensor fusion this sensor fusion improves the integrity and accuracy of information to then calculate the estimated pitch bank and heading values now it is important to mention that each imu is different since each unit is designed depending on needs applications and budget therefore there are different technologies used in inertial sensors for example laser and fiber optic sensors are normally used in advanced inertial reference systems while on the other hand microelectromechanic sensors are widely used in attitude and heading reference systems and therefore in this video we will focus on this type of sensors the microelectromechanics system abbreviated as mems is a compact relatively simple inexpensive and accurate system used to measure the attitude and heading that is why they are easily incorporated into wireless and mobile devices for example to provide the compass or inclinometer functions of some smartphones with this in mind let's now see how each of the sensors of a mems system works starting with the accelerometers a mems accelerometer consists of a mass attached to calibrated springs that moves back and forth depending on the acceleration it experiences this way the displacement of the mass can be measured and interpreted as a measure of acceleration the information from the accelerometers is vital for example for the initial alignment of the system since they can detect the direction of gravity and use it as a reference to establish the position of the horizon apart from this accelerometers are also useful for measuring changes in the attitude of the aircraft in terms of pitch and bank now a mems gyroscope consists of a mass attached to calibrated springs that constantly moves back and forth this way when a rotation around the measured axis occurs the mass is displaced to a side due to the coriolis force this displacement of the mass can be measured and the angular rate can be determined just like when the precession is measured in a conventional gyroscope in essence then a mems gyro is not actually a real gyro since instead of measuring rotation on an axis using the gyroscopic principles the mem system calculates it based on the resultant forces when a rotation occurs however it is called a gyro because the information it provides is basically the same finally a mems magnetometer consists of a plate with high electrical conductivity attached to a direct current circuit in this case the earth's magnetic field will influence the behavior of the electrons as they flow through the plate this way if the voltage between two point in the plate is measured the resulting value will depend on the strength and direction of the magnetic field thus allowing to measure it in the three axes however in aviation we have a big problem with this type of magnetometers since they are affected by electromagnetic interferences from the avionics or the engines so to solve this instead of using the magnetometers of the imu the ahars can receive information from a remote flux valve which we have already discussed in detail in the video about the remote indicating compass and just like in a remote indicating compass some ahars incorporate a free mode that allows the magnetometers to be disconnected from the system in areas where magnetic heading information is unreliable as for example near the magnetic poles in this case then the heading information will only be provided by the mems gyros not the magnetometers now since we have seen how the sensors of a mems system work we must say that the kalman filter allows for error compensation since by using the joint data derived from the accelerometers gyroscopes and magnetometers in all three axes errors due to gyroscopic drift and other imperfections can be almost completely eliminated this substantially increases the accuracy of information and reduces crew workloads since it is not necessary to perform manual corrections now in order for the ahar's sensors to function properly an initial alignment is required the purpose of this initial alignment is to determine the attitude of the aircraft when it is on the ground since changes in attitude will be measured in relation to the attitude detected during this initial alignment this process normally takes a few seconds however it can take up to a couple of minutes depending on the manufacturer of the ahars system an important consideration during this initial alignment is that the aircraft must remain stationary on a level surface so that the initial reference attitude is as accurate as possible now this alignment is required only upon power up however if there is a problem with the unit during flight a realignment may be required in this case the aircraft must remain in straight and level flight at constant speed and follow the instructions of the manufacturer for the realignment now in the event of a total failure of the ahars an alert will be displayed on the affected instruments as we can see here in this situation in aircraft equipped with a single ahars computer the crew must use the standby analog instruments these standby instruments use conventional mechanical gyroscopes independent of the ahars sensors so they are not affected by the failure of the unit now talking about the system layout smaller aircraft with simple systems usually have only one ahrs computer in this case the imu sensors send the raw data to the kalman filter and processor of the ahrs and then the unit sends the process to electronic data to the relevant instruments and separately the standby instruments are driven by mechanical gyroscopes now an aircraft with two sets of instruments two ahrs are normally installed each fed by its own imu with dedicated sensors this means that there are two ahars one for the captain's instruments and another for the first officer's instruments and apart from this there are also the standby instruments driven by mechanical gyros and as with the air data computers when there is more than one ahrs unit installed although each one is independent they are interconnected so that the information calculated by each unit can be compared to detect failures or inaccuracies of the sensors and in addition to this there is also the possibility to select the ahars that feed a certain group of instruments for example if here we have this configuration and the ahar's number two fails then the crew can select the ahar's number one to also feed the first officer's instruments so they can work properly now as with the er data computer in more complex and modern aircraft the ahars is integrated with other computers and units to complement the information and calculate other useful parameters for example it can be integrated with an air data computer obtaining the air data attitude and heading reference system this new unit allows increasing the accuracy of attitude information gives a more stable vertical speed indication and compacts two units into one it is also common to find this unit integrated with a global navigation satellite system which allows to calculate other parameters such as wind direction and speed the ground speed the wind correction angle and the magnetic track an example of the use of this type of unit is the garmin g1000 which although it does not use the units combined it does use the information derived from each one to calculate all the parameters finally with the advent of smaller and cheaper mems sensors portable ahars units that transmit information wirelessly through wi-fi or bluetooth became available in the market these can be used to provide accurate data to mobile navigation applications such as for flight or air navigation pro and all those smartphones and tablets already incorporate mems sensors their accuracy and reliability is much lower compared to the portable ahars units i hope the information presented in this video was useful if so don't forget to share like subscribe and leave a comment down below thanks for watching

Channel: Aviation Theory

Views: 40,559

Keywords: Aviation, Theory, ATPL, Pilot, Training, PPL, CPL, IFR, Preparation, Test, Study, Guide, Tutorial, Explanation, Explained, Aircraft, Airplane, Pilots, Instruments, AHRS, Attitude, Heading, Reference, System, IRS, Inertial, IMU, MEMS, Gyros, Magnetometer, Flux Valve, Avionics, Glass cockpit, Accelerometer, PFD, Attitude Indicator, Heading Indicator, Turn Coordinator

Id: _-bP_N9Xt38

Length: 14min 3sec (843 seconds)

Published: Sun Sep 12 2021