A wearable magnetometer-free motion capture system: innovative solutions for real-world applications

Although inertial and magnetic wearable sensors are promising tools to develop novel technologies for human motion capture, their diffusion is being limited by their fair accuracy. In indoor applications, most of the inaccuracy comes from the magnetic disturbances contained in the magnetometer data. Besides, non-technicians might easily fail in properly calibrating the magnetometers. Hence, a novel magnetometer-free motion capture system was developed in this work, in the attempt to increase sensors accuracy and usability. Innovative strategies, invisible to the user, were introduced to compensate for the lack of the magnetometer: (a) an accurate linear Kalman filter- based 3D orientation estimator; (b) a simple sensor-to-segment calibration procedure; (c) efficient error compensations for soft- tissue artifact; (d) a simple solution to guarantee a common reference to all body segments. Ten expert Yoga practitioners were asked to perform a common sequence (sun salutation) while inertial sensor data and ground-truth data were acquired for the upper-body through a customized body area network and a stereophotogrammetric system, respectively. As expected, the overall joint angles accuracy proved to be dependent on the acquisition time window. The achieved worst-case accuracy at 90 s was 6 and 14 deg, depending on the main rotational axis involved (vertical or horizontal axis, respectively). Given the large and unconstrained angular workspace explored at each joint (up to 200 deg), these errors make the proposed solution (which can easily be extended to the full human body) an eligible alternative to current motion capture systems relying on magnetometers.