It is useful to know how works the magnetic fields in order to properly use magnetometers. This article presents general notions about the Earth magnetic field and how it is used to get true north. It also gives a quick overview of the magnetic disturbances and how to prevent it.
Earth Magnetic field
On the globe
The Earth magnetic field is often used to get the heading angle (direction to North Pole).
The Earth behaves like a giant magnet, it generates its own magnetic field that goes from South to North pole.
Poles are not exactly aligned with the geographic North-South axis. There is an angle offset between magnetic heading and geographic heading. This difference is called declination. There are also local differences in both magnitude and direction, as the magnetic field is not perfectly uniform around the globe. This is presented in the following picture:
Note that local declinations may range from a few tenths of degrees to more than 20°. Moreover, those errors may vary over time.
All these effects combined together make it very difficult to find True North using a simple magnetic compass.
The American NOAA agency updates every 5 years a global map of the Earth magnetic field.
This map can be used, given a specific location and date, to know what is the declination in that specific place.
Fortunately, SBG Systems embeds this map into their Inertial navigation systems, enabling automatic True North heading while using magnetometers.
Magnetic compass accuracy
The magnetic field vertical component becomes stronger than the horizontal components when getting close to the poles. When operating a magnetic compass at very high latitudes, it might be impossible to detect a heading angle due to the low signal/noise ratio. Typically, a magnetic compass will be able to operate at latitudes lower than 70°.
Local magnetic disturbances
Some materials can influence the magnetic field of the Earth, they are separated in 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 by 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:
Calibration for constant disturbances
The SBG Systems calibration is able to compensate disturbances in 2D and in 3D. It aims to compensate permanent changes in Earth Magnetic field, due to sensor close environment, it handles both hard and soft iron effects. 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 or not be compensated by 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 can not 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 sensor.
- Reducing emissions: Taking care of other equipments emissions is often the best solution.
- Shield: Shielding is in general 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.
Usually, problematic disturbances (= not compensated either by calibration or gyroscopes for a short time) are weak (such as cables) so keeping some distance can solve the problem.
The strength of the generated magnetic field is directly proportional to the distance. So being at 10cm instead of 5cm of a cable divide by 2 the disturbance.
Many equipment can generate their own magnetic field, in many cases it is possible to reduce those disturbances with clever designs. For instance power cables can generate magnetic fields. The direction of the magnetic field will depend on the current direction. On the following image we see the magnetic field represented by the vector B in function of the current I:
Shielding the sources of magnetic emissions can be done with ferromagnetic material, however it is not recommended, as it is complex to model the actual effect it will have. Furthermore, shielding the source of the magnetic emissions without affecting the Earth magnetic field can be tricky.
By using gyroscopes in combination with a compass, it is possible to combine the advantages of both systems: the gyroscopes are good in short-time, but will drift on long-term, while the magnetometers can be subject to local disturbances, but will be reliable on long term (if calibrated and in a proper environment).
Our Inertial systems are able to estimate in real time wether the magnetic field is reliable or not. The use of gyroscopes during periods of unreliable magnetic field allows maintaining a highly accurate heading solution in most situations.