Precise Position Control of a Multi-rotor UAV with a Cable-suspended Mechanism During Water Sampling

This paper addresses the problem of water sampling by using a multirotor UAV with a cable-suspended mechanism. In order to ensure the safe execution of the sampling procedure and the stabilization of the vehicle, the disturbances, induced by the water flow and transferred through the cable, have to be identified. Specifically, an estimate of the disturbances is extracted by integrating a depth sensor, a load cell, an ultrasonic sensor and a downward-looking camera into the UAV's sensor suite and fusing the respective measurements. Gaussian Processes are afterwards employed so as to learn the uncertain disturbances in real time and in a non-parametric manner. The predicted disturbances are incorporated into a geometric control scheme which is capable of stabilizing the UAV above the desired sampling position while compensating for the aforementioned disturbances. The performance of the proposed control strategy is demonstrated through both simulation and experimental results.