Calibration of the 'Flock of Birds' electromagnetic tracking device and its application in shoulder motion studies

In this paper the applicability in terms of measurement accuracy of the 'Flock of Birds' six D.O.F. electromagnetic tracking device in shoulder research is investigated. Position measurements in a workspace of approximately 1 m³ were performed using a stylus. The andom error at the stylustip appeared to be 1.86, 1.98 and 2.54 mm for x-, y- and z-coordinate, respectively. The error caused by distortion of the magnetic field by metal in the concrete of especially the floor was 20.8, 22.2 and 20.4 mm for the x- , y- and z-coordinate, respectively. Calibration and leaving out the measurements closests to the floor lowered this error to 2.07, 2.38 and 2.35 mm. Orientation errors of the shoulder bones evolving from the measurement inaccuracy were estimated from repeated measurements of shoulder bony landmarks of ten subjects by means of the stylus. These errors were generally below 2°. This is lower than found for the same measurements using a spatial linkage digitizer. It is concluded that the 'Flock of Birds' is a useful tool for shoulder kinematic studies.