Towards Offshore Recovery: Tether Force Sensing for Admittance Guided UAV Landing

The use of a tether cable for Unmanned Aerial Vehicle (UAV) recovery on mobile platforms such as sea faring vessels is an underexplored area. This presents an opportunity to reduce dependence on conventional landing systems while increasing the range of environmental and weather conditions under which UAVs can be safely and autonomously recovered. While a UAV flying with a tether cable can be more limited than an unrestrained UAV, this physical link can be exploited as a safer tracking system. The tether provides a real-time physical reference to a moving landing point that conventional vision and GPS systems can fail to track. Building on previous simulation work that proposed the use of admittance control to generate a landing trajectory, we propose a lightweight onboard tether force sensing system capable of measuring the force vector required for the practical landing strategy. This approach improves existing landing systems that depend on external position estimates by using real-time tether tension feedback measured directly onboard. The force sensing system comprises of a passive 2-axis gimbal with two encoders and a strain gauge to measure the tether angles and tension, respectively, mounted on the UAV at the tether attachment point. An onboard companion computer resolves the measurements to a force vector, that the admittance controller uses to generate the setpoints which define the landing trajectory. The system was tested on a quadcopter with a companion computer used as an interface between the sensor and flight controller. Results demonstrate the ability of the system to accurately measure the tether forces and successfully guide the UAV to the landing platform using the generated trajectory. The proposed pairing of force sensing and onboard trajectory generation using only the tether as a guide is a novel contribution towards more robust autonomous UAV offshore recovery systems.