Null Space-Based Control Using Sliding Mode for a UAV Formation

Abstract

This paper proposes a controller combining null space and sliding mode control to guide a formation of two UAVs (unmanned aerial vehicles) carrying a cable-suspended load, taking the load to a desired position. Two hierarchically organized subtasks are considered: keeping the formation shape, the priority, and moving the formation to the desired position, a secondary task. The load carried by the UAVs exerts a force that drags each UAV towards the other, thus disturbing the system. Keeping the formation shape is the priority aimed at avoiding this. The control architecture involves a kinematic loop for the formation and a dynamic compensator for each UAV, based on sliding mode, aiming to reject disturbances. The basis is the virtual structure paradigm, with the null space-based controller performing the formation control, generating velocity references for the two UAVs, to which sliding mode-based dynamic compensators are included. Experimental results show that the proposed controller safely drives the UAV-UAV-payload system, taking the payload to the desired position with small errors.