A rapid method for magnetic tracker calibration using a magneto-optic hybrid tracker

Electromagnetic trackers are greatly affected by magnetic field distortion resulting from metal objects and electronic equipment in close proximity. In this paper, we propose a rapid method for magnetic tracker calibration using a magneto-optic hybrid tracker. Although the calibration data can be acquired by freehand using the hybrid tracker, two problems arise as a result of freehand acquisition. One is the error caused by the time delay between the measurements of optical and magnetic trackers. The other is the inconsistency of calibration accuracy resulting from variations on uniformity and density of the freehand acquisition of calibration data. To overcome the problem of error, we developed a temporal calibration procedure to estimate the time delay and apply it accordingly. For the problem of inconsistent calibration accuracy, we formulated a distortion model selection method using a cross-validation technique with five distortion models obtained by 0-th to 4-th degree polynomial fitting to the calibration data. We experimentally evaluated the method in a setting affected by an actual operating table. By combining the temporal calibration and model selection methods, the error caused by the magnetic distortion was reduced from around 40 mm to less than 2 mm, and 30 seconds were needed to obtain the calibration data for a 200 × 200 × 20 mm³ volume.