An Observability Constrained Downward-Facing Optical-Flow-Aided Visual-Inertial Odometry

Visual-Inertial Odometry (VIO) has been widely used by autonomous drones as an onboard navigation method. However, it suffers from drifts especially in scenarios where the environments have few texture features such as an empty room with solid color walls. Optical flow sensors are another type of onboard sensor used by drones that face downward and measure the velocity by detecting changes in pixels between consecutive images, which don’t introduce accumulative error. In this work, we present an efficient tight-coupled estimator to improve the accuracy of VIO by fusing the measurements of a downward-facing optical flow sensor into the VIO framework consistently. We further analyze the observability of the estimators and prove that there are four unobservable directions in the ideal case and then we utilize OC-EKF to maintain the consistency of the estimator. Furthermore, we extend an adaptive weighting algorithm to the proposed method, which can better adapt to the scenes where feature tracking is less accurate. Finally, both simulation and real-world experiments demonstrate the feasibility of the proposed method.