Magnetic immunity of the MTi AHRS

Xsens produces various motion trackers in the MTi product range that are able to output a North-referenced Heading estimate. The MTi AHRS (MTi-3, MTi-30, MTi-300, MTi-630) and MTi GNSS/INS (MTi-7, MTi-670, MTi-680G, MTi-G-710 when using the GeneralMag filter profile) are able to achieve this by measuring the Earth’s magnetic field using 3-axis magnetometers. In other words, the magnetic field measurements are used as a compass reading. If the local Earth magnetic field is disturbed temporarily, the MTi’s on-board filters will identify this disturbance instead of incorrectly assuming that it represents the Earth's magnetic field. As demonstrated in the video above, the Yaw remains stable and North-referenced, despite a magnetic name tag moving around the device. The MTi outputs a Yaw value of 90 degrees, as its x-axis is pointing North. 

In case of structural magnetic disturbances, the computed Heading will slowly converge to a solution using the 'new' local magnetic North. Note that this has no direct effect on the estimates of Roll and Pitch. The period after which the Heading will start converging can be 10 to 30 seconds, depending on the filter profile selected for the MTi.

The same procedure of convergence takes place when the magnetic disturbance is removed. After the filters have converged back to the original magnetic environment, the Heading will be correctly North-referenced again.

It is important to note that indoor environments are often magnetically distorted areas. For example, most tables and desks are composed of ferromagnetic materials. The magnetic norm can be used as an indicator to determine whether magnetic distortions are present in the MTi's environment. 

We conclude that in order to obtain an accurate North-referenced Yaw estimate, it is important to calibrate the MTi’s magnetometers for any ferromagnetic materials that are attached to the device. Xsens provides several calibration methods and tools for this purpose. For more information, refer to the Magnetic Field Mapper Documentation.

Prolonged magnetic disturbances that are not attached to the MTi, such as the name tag in the video, cannot be calibrated for. If these disturbances are present in the environment of the MTi, then the use of Active Heading Stabilization or the FixedMagRef filter profile (newer AHRS products only) can be a solution. 

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