A nonlinear terrain-following controller for a VTOL unmanned aerial vehicle using translational optical flow

This paper presents a nonlinear controller for terrain following of a vertical take-off and landing vehicle (VTOL). The VTOL vehicle is assumed to be a rigid body, equipped with a minimum sensor suite (camera and IMU) along with a measure of the forward speed from another sensor such as global positioning system, maneuvering over a textured terrain made of planar surfaces. Assuming that the forward velocity is separately set to a desired value, the proposed control approach ensures terrain following and guaranties the vehicle does not collide with the ground during the task. The proposed control acquires an optical flow from three spatially separate observation points, typically obtained via three cameras or three non collinear directions in a unique camera. The proposed control algorithm has been tested extensively in simulation and then implemented on a quadrotor UAV to demonstrate the performance of the closed loop system.