Null Space-Based Control Embedding an Adaptive Sliding Mode Term Applied to a UAV-UAV Formation Carrying a Load

This paper proposes a null space-based behavioral controller embedding an adaptive sliding mode control law to guide the formation of two UAVs (unmanned aerial vehicles) carrying a cable-suspended load when tracking a trajectory. This controller is based on hierarchically organizing the two subtasks moving the formation accordingly and keeping the formation shape. The influence of the load on the UAVs, including the force dragging each UAV towards the other, is dealt with as a perturbation, which the sliding mode term allows to reject. The proposed controller guides the UAVs as a virtual structure corresponding to the straight line linking them, keeping both at the same altitude, preserving the distance between them, and rejecting the disturbance caused by the load. Keeping the formation's shape is the priority, and moving the formation is the secondary task. This approach modifies the control signal for the UAVs, ensuring a rigid shape for the formation, thus preserving the distance between the UAVs, keeping them at the same altitude, and providing additional energy to reject the load disturbance, hence improving the performance of the UAV-UAV-load system. Therefore, the whole system is dealt with as a virtual structure for which the null space-based controller, the formation controller, generates velocity references for the two UAVs, whereas a dynamic compensator embedding an adaptive sliding mode control law works as an individual controller for the drones, reducing the disturbance caused by the load.