Magneto: Joint angle analysis using an electromagnet-based sensing method

Abstract

Joint angle analysis facilitates research into injury prevention, rehabilitation, and activity monitoring. Sensors used in such analysis must be unobtrusive, accurate, and if used for motion, capable of monitoring fast-paced, dynamic motions. To effectively contribute to these applications, we created a body-mounted electromagnet-based sensing system for joint angle analysis called Magneto. Our system is wireless, features a high sampling rate, is not subject to drift, and is unaffected by outside magnetic noise. Magnetic field readings are influenced by noise due to magnetic interference from the Earth's magnetic field, the environment, and nearby ferrous objects. Magneto uses the combination of an electromagnet and magnetometer to remove environmental interference from a magnetic field reading. We evaluated this sensing method to show its performance when removing the interference in three-movement dimensions, in six environments, and with six different cycling rates. Then, we localized the electromagnet with respect to the magnetic field reader in any direction within a 13.8 cm range with a relative error of 2.3% for the distance and an average error of 3.43° for the orientation angle. We applied Magneto in a pilot study: calculating elbow flexion angles. In this study, we calculated elbow flexion angles to the nearest 15° with 93.8% accuracy.