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

W.H. de Vries, H.E.J. Veeger, C.T.M. Baten, F.C. Helm

    Research output: Contribution to JournalArticleAcademicpeer-review

    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°, decreasing to 3° 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 20-30 s). 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 20-30 s in a heavily distorted earth magnetic field. © 2008 Elsevier B.V. All rights reserved.
    Original languageEnglish
    Pages (from-to)535-541
    JournalGait and Posture
    Volume29
    Issue number4
    DOIs
    Publication statusPublished - 2009

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