Aerial-Underwater Systems, a New Paradigm in Unmanned Vehicles

Unmanned Aerial-Underwater Vehicles (UAUVs) arise as a new kind of unmanned system capable of performing equally well in multiple mediums and seamlessly transitioning between them. This work focuses in the modeling and trajectory tracking control of a special class of air-underwater vehicle with full torque actuation and a single thrust force directed along the vehicle's vertical axis. In particular, a singularity-free representation is required in order to orient the vehicle in any direction, which becomes critical underwater in order to direct the thrust force in the direction of motion and effectively overcome the increased drag and buoyancy forces. A quaternion based representation is used for this purpose. A hierarchical controller is proposed, where trajectory tracking is accomplished by a Proportional-Integral-Derivative (PID) controller with compensation of the restoring forces. The outer trajectory tracking control loop provides the thrust force and desired orientation. The latter is fed to the inner attitude control loop, where a nonlinear quaternion feedback is employed. A gain scheduling strategy is used to deal with the drastic change in medium density during transitions. The proposed scheme is studied through numerical simulations, while real time experiments validate the good performance of the system.