3-D motion estimation and online temporal calibration for camera-IMU systems

When measurements from multiple sensors are combined for real-time motion estimation, the time instant at which each measurement was recorded must be precisely known. In practice, however, the timestamps of each sensor´s measurements are typically affected by a delay, which is different for each sensor. This gives rise to a temporal misalignment (i.e., a time offset) between the sensors´ data streams. In this work, we propose an online approach for estimating the time offset between the data obtained from different sensors. Specifically, we focus on the problem of motion estimation using visual and inertial sensors in extended Kalman filter (EKF)-based methods. The key idea proposed here is to explicitly include the time offset between the camera and IMU in the EKF state vector, and estimate it online along with all other variables of interest (the IMU pose, the camera-to-IMU calibration, etc). Our proposed approach is general, and can be employed in several classes of estimation problems, such as motion estimation based on mapped features, EKF-based SLAM, or visual-inertial odometry. Our simulation and experimental results demonstrate that the proposed approach yields high-precision, consistent estimates, in scenarios involving both constant and time-varying offsets.