Unscented External Force Estimation for Quadrotors and Experiments

In this paper, we describe an algorithm, based on the well-known Unscented Quaternion Estimator, to estimate external forces and torques acting on a quadrotor. This formulation uses a non-linear model for the quadrotor dynamics, naturally incorporates process and measurement noise, requires only a few parameters to be tuned manually, and uses singularity-free unit quaternions to represent attitude. We demonstrate in simulation how the proposed algorithm outperforms existing methods, namely a non-linear observer, and how our approach can be used to generate force and torque profiles from experimental data that are useful in controller design. Finally, we show how the resulting controllers enable a quadrotor to track the center of wind induced by a fan, and avoid catastrophic loss of lift due to downwash.