Aqua Slide: An Underwater Leveling Motion Scheme for M-UAAV Utilizing Singularity

The underwater leveling motion control of a morphable unmanned aerial–aquatic vehicle (M-UAAV) is essential but lacks an elegant solution. In this article, a scheme named Aqua Slide is proposed for Mirs-Alioth, our predeveloped M-UAAV prototype, to achieve underwater leveling motions by utilizing singularity. For the first time, the singular thrust tilt angle (STTA), i.e., the tilt angle that causes a vehicle to reach singularity, is characterized and defined. For underwater leveling motion generation, STTA is utilized as a key tool. Crucial factors of these motions, i.e., the existence of control direction uncertainty, and coupling effects are identified. Aqua Slide is proposed to tackle the crucial factors in generating underwater leveling motions, consisting of an STTA controller, a primary-auxiliary switch, and an auxiliary controller incorporating the saturated Nussbaum function. A simulation environment, enriched with a detailed model, is established using a Gazebo and hardware-in-the-loop (HITL) configuration. Under similar control parameter settings, full-cycle Gazebo-HITL simulations and experiments reveal the effectiveness of the approach. The vehicle maintains the attitude within a maximum of 5<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${}^{\circ}$</tex-math></inline-formula> during the whole procedure. Meanwhile, the ablation studies in Aqua Slide highlight the indispensability of the submodules in the approach. The absence of any module results in undesirable or even divergent behavior of Mirs-Alioth during underwater leveling motions. The preliminary exploration in this article elegantly solves the issue of generating underwater leveling motion for an M-UAAV and provides a referential solution for transitioning from simulation to real-world applications. Video attachment: <uri xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">https://youtu.be/XpViXL0qcvk</uri>.