A command-filtered backstepping approach to autonomous inspections using a quadrotor

This paper applies command-filtered backstepping for position control of an underactuated quadrotor. First the dynamics are differentiated until the rotational and translational dynamics become coupled, then the paper applies command filtered backstepping to control the underactuated rigid body with constrained actuation. The thrust, roll and pitch torques are used for position control and designed through the backstepping, while the yaw torque is a available for another control objective. The yaw torque is therefore designed using a PD+ controller for target tracking, enabling the quadrotor to track any point (stationary or moving) with a sensor that is assumed to be fixed to the x-axis of the body frame. The paper applies these results to perform autonomous inspections of a wind turbine using a combination of waypoints and a desired trajectory to cover the whole structure while pointing the sensor towards the wind turbine.