Dynamics and control of hybrid aerial underwater vehicle subject to disturbances

Hybrid aerial underwater vehicle (HAUV) has attracted increasing interest for its potential of operating in two distinct mediums, air and underwater. However, stable transition between medi-ums in realistic environment remains a challenge. This paper presents the dynamic model and control strategies of a quadcopter-like HAUV to enable cross-domain locomotion under ocean disturbances. An integrated model considering the complete hydrodynamic changes (added mass, inertia and damping) through the loss coefficients is formulated for the first time, which is suitable for aerial flight, underwater locomotion, and transition. Then a propulsion model about aerial propellers working at the water-air interface is established. Disturbance considered in this work was periodic wave force acting on the fuselage and instantaneous impact on propellers, the latter of which includes the impact force and the ambient density change of the propellers. Second-order sliding mode controller (SMC) is developed for the cross-domain trajectory track-ing control and is compared with the cascade proportional integral-derivative controller (PID). Simulation results show that both controllers can track the trajectory of water egress and wa-ter ingress. However, the proposed Second-order SMC achieves faster convergence speed and greater robustness. A set of representative simulations were run to analyze the performance of these controllers with the effects of ascending speed and wave properties during the transition. The results demonstrate the inherent superiority of the proposed Second-order SMC in enabling a stable and accurate aerial-aquatic locomotion for the HAUV. Detailed analysis of several typ-ical cases illustrates the referential conditions for smooth cross-domain processes.