Magnetometer

A magnetometer is a device that measures magnetic field, typically expressed in Tesla T or Gauss G. A triaxial magnetometer measures that strength in three directions along the X, Y, and Z axis. You may use a magnetometer to measure local magnetic fields, or you can use it to detect the Earth's magnetic field.

In order to use a magnetometer as a digital compass it is necessary to calibrate the device to compensate for hard iron distortion, soft iron distortion, and magnetic declination. Hard iron distortion is caused by permanent magnets. Soft iron distortion is caused by the presence of iron, steel and other ferrous materials near the sensor that influence magnetic fields themselves (not the magnetic field source). The soft iron distortion is dependent upon the orientation of the magnetometer relative to the source of the soft iron distortion. Magnetic declination is the difference between the magnetic poles of the earth and the geographic poles.

Magnetometer Calibration

Hard iron distortions cause an additive offset to magnetometer readings. To correct for them, you take a series of X,Y,Z readings while moving the magnetometer in all directions, and then calculate the midway point between the minimum and maximum values for each axis. The midway point calculated for each axis is the offset correction to be applied to subsequent readings.


offset_x = (max(x) + min(x)) / 2
offset_y = (max(y) + min(y)) / 2
offset_z = (max(z) + min(z)) / 2

corrected_x = sensor_x - offset_x
corrected_y = sensor_y - offset_y
corrected_z = sensor_z - offset_z

Soft iron distortions are more difficult to remove because they are best performed with the use of a 3x3 transformation matrix. However, it is possible to use scale biases to achieve a reasonably good correction factor.


avg_delta_x = (max(x) - min(x)) / 2
avg_delta_y = (max(y) - min(y)) / 2
avg_delta_z = (max(z) - min(z)) / 2

avg_delta = (avg_delta_x + avg_delta_y + avg_delta_z) / 3

scale_x = avg_delta / avg_delta_x
scale_y = avg_delta / avg_delta_y
scale_z = avg_delta / avg_delta_z

corrected_x = sensor_x * scale_x
corrected_y = sensor_y * scale_y
corrected_z = sensor_z * scale_z

The pseudocode below applies the hard iron and soft iron correction factors to the measurement values (sensor_x sensor_y sensor_z).


corrected_x = (sensor_x - offset_x) * scale_x
corrected_y = (sensor_y - offset_y) * scale_y
corrected_z = (sensor_z - offset_z) * scale_z

Magnetic declination must be compensated for if you intend to use a magnetometer to provide a geographic heading. It is important to note that the magnetic declination changes based on your geographic position (latitude, longitude) on the earth, and it varies from year to year. See NOAA Magnetic Declination Estimated Value. It is possible to use an API to fetch the declination values based on latitude / longitude from NOAA's website.

Links

Magnetometer Calibration

Digi-Key How to Calibrate a Magnetometer

How to Calibrate a Magnetometer?

Simple and Effective Magnetometer Calibration

A way to calibrate a magnetometer

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