This paper proposes a key feature for vision based automatic landing of a UAV: the addition to a given control law of the constraint that a given ground point be maintained in the camera field of view (FoV). This feature is an application case of an output constraint method recently developed and available for any nonlinear system. This method is based on an observation made for relative degree one systems, and extended iteratively to any nonlinear system. The landing aircraft application case considered in this article consists of the design of a control law for an aircraft to maintain a given ground point in the camera field of view for visual servoing purpose. Simulation results confirm the effectiveness of the approach.

The paper addresses the control of Vertical Take Off and Landing (VTOL) Underactuated Autonomous Vehicles (UAVs) in hover flight, based on measurements provided by an on-board video camera and rate gyros. The objective is to stabilize the vehicle to the pose associated with a visual image of a planar target. By using the homography matrix computed from the camera measurements of the target, stabilizing feedback laws are derived. Explicit stability conditions on the control parameters are provided. It is shown that very good robustness and performance can be achieved without any apriori information on the visual target (like geometry, or orientation), by a proper tuning of the control parameters. Simulation results confirm the effectiveness of the approach.