Extending kalibr: Calibrating the extrinsics of multiple IMUs and of individual axes

An increasing number of robotic systems feature multiple inertial measurement units (IMUs). Due to competing objectives-either desired vicinity to the center of gravity when used in controls, or an unobstructed field of view when integrated in a sensor setup with an exteroceptive sensor for ego-motion estimation-individual IMUs are often mounted at considerable distance. As a result, they sense different accelerations when the platform is subjected to rotational motions. In this work, we derive a method for spatially calibrating multiple IMUs in a single estimator based on the open-source camera/IMU calibration toolbox kalibr. We further extend the toolbox to determine IMU intrinsics, enabling accurate calibration of low-cost IMUs. The results suggest that the extended estimator is capable of precisely determining these intrinsics and even of localizing individual accelerometer axes inside a commercial grade IMU to millimeter precision.