Using the magnetic norm to identify magnetic distortions

The MTi's magnetometer sensors are used to measure the Earth's magnetic field, which is exploited as a reference to estimate the heading (North referenced yaw). When there are additional magnetic sources in the environment, these will distort the Earth's magnetic field measured by the magnetometer. 

Causes of distortion can be divided into two kinds:

  1. Distortion caused by an object attached to the MTi (the MTi moves with the object), for example when strapping the MTi onto a vehicle with a ferromagnetic structure. This type of distortion is commonly known as hard and soft iron effects. The Magnetic Field Mapper (MFM) or In-Run Compass Calibration (ICC) tools have been developed and can be used to calibrate the MTi’s magnetometers in order to cancel out these distortions. Refer to the Magnetic Calibration Manual for more information. 
  2. Disturbance caused by objects in the environment near the MTi, like file cabinets or vehicles that move independently, with respect to the MTi. This type of disturbance are difficult to cancel. However, the amount of error caused by the disturbance can be reduced by optimally using the available sensor information and valid assumptions about the application. This is the task accomplished by the Xsens Kalman Filter (XKF). For example, an MTi can track and handle temporary (~30 s) magnetic disturbances while keeping an accurate heading. However, when environmental disturbances are more structural, i.e. when the MTi stops next to a file cabinet, the MTi may lose track of the true magnetic north. In this case, Active Heading Stabilization (AHS) is an option.

The magnetic field norm (Mag Norm) can be used as an indicator for magnetic distortions. The Mag Norm can be visualized in MT Manager in the magnetic field data plot, found under the inertial data view (Figure 1). Although the magnetic field norm is not available as a direct output of the MTi, its value can very easily be calculated using a formula found here. If the magnetometers of the MTi are correctly calibrated and the MTi is used in an environment without magnetic distortions, then the Mag Norm will be constant while moving the MTi (and it will be approximately equal to 1 after an MFM calibration), see the black line in Figure 1. In this case, the Earth’s magnetic field that is measured by the MTi is constant, and it can be used for estimating an accurate heading. 

If magnetic distortions are present in the environment where the MTi are used, then the Mag Norm will vary depending on the location and orientation of the device (see Figure 2). In this case, the magnetic field that is measured by the MTi is not homogeneous. It results in a disturbed Earth's magnetic field, and it cannot be used properly for computing an accurate heading.


Figure 1. An MTi is rotating in an environment without any magnetic distortions. The magnetic field norm (black line) is constant while the sensor is moving.



Figure 2. An MTi is rotating in an environment with strong magnetic distortions. The magnetic field norm (black line) is not constant due to the magnetic disturbances.

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