Magnetic distortion in motion labs, implications for validating inertial magnetic sensors.

BACKGROUND

Ambulatory 3D orientation estimation with Inertial Magnetic Sensor Units (IMU's) use the earth magnetic field. The magnitude of distortion in orientation in a standard equipped motion lab and its effect on the accuracy of the orientation estimation with IMU's is addressed.

METHODS

Orientations of the earth magnetic field vectors were expressed in the laboratory's reference frame. The effect of a distorted earth magnetic field on orientation estimation with IMU's (using both a quaternion and a Kalman fusing algorithm) was compared to orientations derived from an optical system.

FINDINGS

The magnetic field varied considerably, with the strongest effects at 5 cm above floor level with a standard deviation in heading of 29 degrees , decreasing to 3 degrees at levels higher than 100 cm. Orientation estimation was poor with the quaternion filter, for the Kalman filter results were acceptable, despite a systematic deterioration over time (after 20-30s).

INTERPRETATION

Distortion of the earth magnetic field is depending on construction materials used in the building, and should be taken into account for calibration, alignment to a reference system, and further measurements. Mapping the measurement volume to determine its ferromagnetic characteristics in advance of planned experiments can be the rescue of the data set.

CONCLUSIONS

To obtain valid data, "mapping" of the laboratory is essential, although less critical with the Kalman filter and at larger distances (>100 cm) from suspect materials. Measurements should start in a "safe" area and continue no longer than 20-30s in a heavily distorted earth magnetic field.