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Gait Posture. 2009 Jun;29(4):535-41. doi: 10.1016/j.gaitpost.2008.12.004. Epub 2009 Jan 15.

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

Author information

  • 1Department of Biomechanical Engineering, Faculty of Mechanical, Maritime & Materials Engineering, Delft University of Technology, Delft, The Netherlands. w.devries@rrd.nl

Abstract

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.

PMID:
19150239
[PubMed - indexed for MEDLINE]
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