Calibration and characterization of electromagnetic position and orientation trackers

An electromagnetic position and orientation tracker must be calibrated so that magnetic measurements can be converted into spatial data, and the accuracy must also be characterized to know what accuracy is obtained. We describe a magnetic calibration matrix model which is extensible to the common coil configurations. We detail data representations and discuss successful approaches to the calibration optimization. An accurate positioning device is necessary for both calibration and characterization. We discuss the effect of positioning error on calibration accuracy and some design tradeoffs for positioning. We characterize the calibrated accuracy with two different source coil designs and several calibration variations, with the best configuration achieving 208 μm/0.27° uncertainty, an accuracy exceeding published values for commercial EMTs (tested by other methods). We use local measurement nonlinearity to characterize the expected accuracy across small motions, including the cross-coupling between position and orientation. All software described is open source (Apache License 2.0).