Robust tracking of bicycle crank angles using magneto

The crank angle is an important outcome in biomechanical analyses of cycling. Wireless inertial and magnetic measurement systems are unobtrusive and have the potential to measure crank angles more advantageously than ergometers, encoders or cameras. However, magnetic field disturbances and large centripetal accelerations during pedaling introduce tracking errors. The aim of this study was to validate two magnetometer-free sensor-to-body frame alignment methods for tracking the bicycle crank angle using wireless inertial and magnetic measurement systems. A passive complementary filter is presented for tracking the crank angle using an inertial and magnetic measurement system mounted on the bicycle frame and another on the crank arm. Sensor-to-body frame alignment is performed for both inertial and magnetic measurement systems using functional calibration techniques that do not require magnetometer measurements. The filter also performs dynamic tracking of the crank arm inertial and magnetic measurement system without magnetometer data by exploiting domain constraints and compensating for centripetal accelerations. The filter was validated at a slow, medium and fast pedaling cadence using stereophotogrammetry. The filter produced absolute errors of 1.3°？±？0.9° or less in all three tests. In contrast, large and variable absolute errors (11.6°？±？7.6°, 14.2°？±？10.7° and 14.0°？±？10.2°, respectively) were found with a standard passive complementary filter using a traditional static pose calibration that relies on magnetometer data. The proposed filter operated with low and consistent errors despite the presence of magnetic interferences, whereas traditional magnetometer-based approaches produced unacceptable results. This study contributes toward the ultimate goal of outdoor cycling analysis using inertial and magnetic measurement system technology by accomplishing magnetometer-free frame alignment and centripetal acceleration compensation when tracking crank angles