Wireless Magnetic Position-Detection System With Four Excitation Coils

For tracking position or motion of objects in small or shielded spaces, our wireless position-detection system using an LC resonant marker is considered to be effective. Generally, detection of the accurate position of the marker is difficult when reduction of the S/N ratio occurs due to the marker posture. To solve this problem, we propose a system with four excitation coils to generate four-directional magnetic fields whose components are different from each other. To evaluate this system, simulations and experiments were conducted for a single excitation coil and for four excitation coils. In the single excitation coil system, position detection was difficult for some postures of the LC marker due to the greatly reduced excitation efficiency of the marker. In contrast, in the four excitation coil system, the excitation strength of the marker was kept for every marker posture, and position detection was performed with high accuracy even when the marker was placed where the excitation was weakest. Therefore, it is possible to detect the position of the marker accurately regardless of its posture by using four excitation coils. By this improvement, our position-detection system is considered to be useful for tracking finger motion in some applications.

I. Introduction

Video tracking systems, which can sense targets correctly in a large space, are used extensively in fields such as sports, robotics, and virtual reality [1]–[3]. For example, a motion capture system to measure the motion of targets has been applied to a digital archive of body motion [4], [5]. However, video tracking systems suffer from occlusion, which hides a target from a camera. Solving this problem is difficult even if many cameras are used. Consequently, such video systems are unsuitable for detecting the motion of fingers in small or shielded spaces. The tracking method with data gloves brings highly accurate measurement of finger motion without occlusion, but it is difficult for the user to carry out dexterous motion because he or she loses the subtle sense of the hand [6], [7]. Magnetic tracking systems can supply non-occlusion detection [8]–[12], so commercial products such as Polhemus and Ascension [13], [14] have become widely used. However, these products require markers to be attached to targets with electric wiring or a battery to generate or receive the magnetic field. Consequently, cabled or large-sized markers can disturb the motion of targets.