How to Handle Magnetic Disturbances?

Soft and Hard Iron Correction

Some materials can influence the magnetic field of the Earth. Theses influences can be separated into two categories:

  • Hard Iron effect is caused by any magnet, natural or electric. It generates its own magnetic field which is added to the Earth magnetic field.
  • Soft Iron effect influences the existing magnetic field, and is harder to compensate through calibration. The soft iron is caused by ferromagnetic objects (any object attracted by a magnet). It changes the direction of an existing magnetic field (such as the Earth magnetic field) depending on its orientation.

The following picture summarize the effect of both hard iron and soft iron effect:

>>The SBG Iron Calibration Tool is intended to compensate both Soft AND Hard iron effects using a powerful calibration algorithm.

Calibration limitations

However, it has limitations: it can only compensate disturbances that are constant in time and in space. The following image shows an example of the typical perturbation that can and cannot be compensated through calibration:

• Fixed Magnet and Structure are magnetic disturbances that stay constant and don’t vary in time. They can be compensated by calibration.
• Metallic Moving Parts, Engine and Electric Wires don’t have the same constant effect on the magnetic field: they are modifying it in different level of intensity / direction over time. They cannot be compensated for soft and hard iron effects.

Prevention for dynamic disturbances

When using magnetometers to detect the Earth magnetic field, every other field around is a disturbance. In order to avoid it:

  • Keep distance: increase distance from disturbances around the inertial sensor.
  • Reducing emissions: Taking care of other equipment emissions is often the best solution.
  • Shield: Shielding is generally not advised, but it is sometimes the last solution.
  • Combine with inertial systems: It prevents temporary short-term disturbances.
  • When designing a system, choosing non-ferromagnetic materials, such as aluminium or plastic, helps to reduce disturbances.

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