Calibration of Tri-Axial Sensor Coil for Magnetic Tracking

Six degree of freedom (6DoF) magnetic tracking is increasing in popularity for close range tracking owing to the advantage of non-line-of-sight operation. For ac magnetic field-based systems, a tri-axis sensor coil is typically used to sense the 3-D magnetic field. However, tri-axis coils have the issue of cross-coupling between the individual sensor coils, which is typically not taken into account in magnetic tracking algorithms. This can lead to inaccuracies in the estimated position and orientation. In this article, a method to calibrate tri-axis sensors using a Helmholtz coil is presented. The method also calibrates for the sensitivity mismatch between the three coils, cross-coupling between them, as well as the gain mismatches in the analog front ends (AFEs) for the three sensor coils at the specified frequency. For systems that also use a magnetometer along with a tri-axis sensor, a procedure to align the tri-axis coil with the magnetometer is also discussed. The distortion in tracking is characterized using circle tests for a representative magnetic tracking system using an off-the-shelf 1.5-cm tri-axis coil sensor cube. An improvement of about 54% in the circularity is seen compared to an uncalibrated tri-axis sensor coil, for the magnetic tracking system considered. A calibrated tri-axis sensor coil will provide a more ideal operation, improving the tracking accuracy for any magnetic tracking system, although by a different amount.