Wireless magnetic motion capture system using multiple LC resonant magnetic markers with high accuracy

Abstract

The present paper reports the principle of a wireless magnetic motion capture system that uses multiple LC resonant magnetic markers and demonstrates its application. Small and lightweight markers (4 mm in diameter, 10 mm in length, and 0.63 g in weight) use a soft ferrite core and a coil, representing a minimal LC circuit with no battery, driven wirelessly by electromagnetic induction. The markers are given respective resonant frequencies ranging from 150 to 450 kHz. The magnetic signal of the marker is detected by a pick-up coil array consisting of 25 pick-up coils. The markers are excited by a superposed wave corresponding to all of the resonant frequencies, while the voltage signals induced through pick-up coils are separated in a frequency spectrum by FFT analysis. Regardless of the number of markers, the voltage amplitude for each resonant frequency can be easily obtained simultaneously, and thus the proposed system can detect multiple markers. In addition, the positional error of the system caused by a mutual inductance between the exciting coil and the LC marker was examined. The impedance change of the exciting coil due to a resonance of the LC marker was found to perturb the strength of the magnetic field used for marker excitation. The closer a marker approaches the exciting coil, the larger it becomes. This fluctuation induces an error in the marker signal, which is measured by the pick-up coils and is necessary for positional calculation. Then, considering the mutual inductance, a compensatory process was employed for positional calculation in order to improve the positional accuracy. After compensation, the absolute positional accuracy was determined to be less than 2 mm within 140 mm of the pick-up coil array.