Article

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.

Langue : ANGLAIS