6DOF Electromagnetic Tracker Construction HOWTO

A basic 6DOF (six degrees of freedom) electromagnetic tracker contains the following parts:

• Transmitter contains three colocated orthogonal coils.

• Receiver contains three colocated orthogonal coils.

• Driver electronics provides three sinewaves at distinct frequencies through three series-tuning capacitors to the three transmitter coils.

• Operating frequencies are typically 30 Hz to 15000 Hz. 1000 Hz, 1300 Hz, and 1600 Hz are a good starting point. Higher frequencies give higher induced voltages, lower frequencies reduce error-causing eddy-current effects.

• Data-acquisition electronics measures the currents in the three transmitter coils, and measures the voltages induced in the three receiver coils. The voltage preamps should have 2 nV/sqrt(Hz) or lower input noise. The ADC sampling rate must be high enough to capture the driver frequencies.

• A six-ADC electronics can measure all the currents and voltages continually and simultaneously.

• A four-ADC electronics can use one channel to measure the three currents periodically over time (The currents change slowly as the transmitter coils warm up.), and three channels to measure the three voltages continually and simultaneously.

• A single-ADC electronics can measure the currents and voltages sequentially, but this gives poor dynamic performance due to inconsistent data sets.

 Signal-processing software converts the current and voltage measurements
 into measurements of the HFluxPerI coupling from each transmitter
 coil to each receiver coil.  This gives a 3x3 matrix HFluxPerIMeasured.

 Each component of HFLuxPerIMeasured is the H flux through one
 receiver coil, divided by the current I in one transmitter coil.
 HFLuxPerIMeasured has units of meters, and is a geometrical property of
 the coils' sizes, shapes, number of turns, positions, and orientations.

 Algorithm software converts HFluxPerIMeasured to estimated receiver
 position and orientation, using direct-solution algorithms in
 Raab's 1981 paper:

 Frederick H. Raab, "Quasi-Static Magnetic-Field Technique for
 Determining Position and Orientation", IEEE Transactions on
 Geoscience and Remote Sensing, Vol. GE-19, No. 4, October 1981,
 pages 235-243.

 More elaborate fitter-based algorithms in U.S. Patents ABC provide
 higher accuracy at the expense of much more computation.

Much elaboration and extension is needed to give high accuracy with high convenience, but the above is the basic idea.

Better than 1 millimeter P95 accuracy is achievable, as reported in this paper:

 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.