Magnetic tracker calibration for an augmented reality system for therapy

Magnetic trackers are widely used position/orientation measurement devices in interactive computer systems. These trackers output six-degree-of-freedom position and orientation of their receiver coordinates with respect to their transmitter coordinates. It is a potential solution for real-time position/orientation measurement in an augmented reality system for therapy (ART). However, an obstruction to be overcome is that a magnetic tracker is vulnerable to measurement inaccuracy caused by magnetic field distortions (including ferric and non-ferric distortions) when it is used in metallic environments. The purpose of this paper is to discuss calibration methods used to compensate distortions produced by metallic objects which are moving during the measurement procedure, but fixed with a receiver. Past work on the calibration of magnetic trackers has concentrated on static distortions, which are produced by stationary metallic objects in the environment. Compared with static distortions, in this paper a relatively dynamic situation is considered where the distortion is not constant with time. However, the location of the metallic objects is reflected in the outputs of the receiver so that the distortion can be compensated. This type of distortions, which is called 'quasi-static distortion' in this paper, is common in an ART. Compensation of quasi-static distortion is more complicated than that of static distortion and has not been mentioned in past publications. Some new compensation methods for quasi-static distortion will be introduced here. The results demonstrate that the methods are capable of significantly reducing quasi-static distortions.