EM Tracker Coil Characterization

EM Tracker Coil Characterization

When we test a tracker for accuracy, we generally check using point sets that make mechanical sense. For example, we may use a 3D robot to move the receiver all over the working volume (also called the motion box).

In 6DOF trackers, the coils must be precisely characterized for their electromagnetic properties: gains, non-orthogonalities, non-concentricities, and finite-size effects. Since characterization is measuring electromagnetic properties, the point set chosen should make electromagnetic sense rather than mechanical sense.

The coils can only be manufactured to so much precision. Characterization measures the actual properties of each coil, to improve the tracker accuracy.

One important property of electromagnetics, is the boundary-condition property: If we know the magnetic field everywhere on the boundary of the working volume, we can calculate the field inside the working volume.

For coil characterization, the boundary-condition property suggests characterizing coils using only the the plane of the working volume closest to the transmitter. Farther-away data points provide no characterization-dependence not already in the closest points, so farther-away points can confuse characterization fitters.

Pete believes (though has not verified) that the scribble-test data-collection method in this paper would serve for coil characterization:

• C.A. Nafis, V. Jensen, L. Beauregard, P.T. Anderson, "Method for estimating dynamic EM tracking accuracy of Surgical Navigation tools", SPIE Medical Imaging Proceedings, 2006. Reports low-cost accuracy-measuring techniques and results for various trackers.

The scribble data-collection method involves mounting the receiver on a small flat slider, and slowly sliding the receiver around (and rotating the receiver) on a known-flat surface (such as a granite surface plate). We know that the points mechanically are all on a plane (though we do not know exactly what plane), and that the receiver orientations must all be the same within rotations about the axis perpendicular to the plane.

For coil characterization, the transmitter is mounted under the center of the known-flat surface, as close as the minimum range (minimum transmitter to receiver distance) of the tracker permits.