ACCURACY OF AN ELECTROMAGNETIC TRACKING DEVICE - A STUDY OF THE OPTIMAL OPERATING RANGE AND METAL INTERFERENCE

The positional and rotational accuracy of a direct-current magnetic tr acking device commonly used in biomechanical investigations was evalua ted. The effect of different metals was also studied to determine the possibility of interference induced by experimental test fixtures or o rthopaedic implants within the working field. Positional and rotationa l data were evaluated for accuracy and resolution by comparing the dev ice output to known motions as derived from a calibrated grid board or materials testing machine. The effect of different metals was evaluat ed by placing cylindrical metal samples at set locations throughout th e working field and comparing the device readings before and after int roducing each metal sample. Positional testing revealed an optimal ope rational range with the transmitter and receiver separation between 22 .5 and 64.0 cm. Within this range the mean positional error was found to be 1.8% of the step size, and resolution was determined to be 0.25 mm. The mean rotational error over a 1-20 degrees range was found to b e 1.6% of the rotational increment, with a rotational resolution of 0. 1 degrees. Of the metal alloys tested only mild steel produced signifi cant interference, which was maximum when the sample was placed adjace nt to the receiver. At this location the mild steel induced a position al difference of 5.26 cm and an angular difference of 9.75 degrees. Th e device was found to be insensitive to commonly used orthopaedic allo ys. In this study, the electromagnetic tracking device was found to ha ve positional and rotational errors of less than 2%, when utilized wit hin its optimal operating range. This accuracy combined with its insen sitivity to orthopaedic alloys should make it suitable for a variety o f musculoskeletal research investigations. (C) 1996 Elsevier Science L td.