This letter introduces a small hybrid aerial underwater vehicle (HAUV), which we named Nezha-F, that can fly in the air, perform vertical profiling underwater, and vertically take off and land from both the water surface and ground. A foldable and self-deployable arm mechanism linked to and driven by a piston variable buoyancy system (PVBS) is proposed to reduce the excessive underwater drag caused by aerial structures. By having a compact size and successfully balanced aerial and underwater performance without adding excessive actuators, this design provides a feasible idea for the miniaturization of amphibious floats. The dynamic characteristics of the small PVBS are linear fitted, and modeled. The originally nonlinear actuator performance is linearized by the post-fitting mapping. Asymmetric dead zones of the actuator are removed by adding compensation to the algorithm. During a 10-day field test, the vehicle showed good aerial performance and underwater control effect. Several full mission cycle tests proved the vehicle's ability in semi-autonomous operation and robust domain crossing and verified the vehicle's endurance during each mission stage.

The agile and safe transdomain in waves is a promising feature but the primary bottleneck of the hybrid aerial underwater vehicle (HAUV). In this article, the surfing algorithm is proposed for Nezha-mini, our predeveloped HAUV prototype, to search for the dynamic window facilitating takeoff in waves and avoiding hazardous waves. For the first time, the cross-domain window, i.e., the vehicle is at the wave crest and heading downstream, is characterized and defined through the vehicle-wave coupled dynamic model. The novel surfing algorithm consists of the gradient perceptron, time-limited momentum gradient search, heading server, and initial conditions. Nezha-mini senses, searches, and tracks the dynamic window in real-time, until the takeoff decisions are triggered. Numerical simulations and experiments in regular and irregular waves reveal the effectiveness of the algorithm. The vehicle maintains a healthy initial attitude and inaccessible wave disturbance during takeoff, thus alleviating the thrust distraction from stability recovery and uncertainty. The average transition time and energy cost are reduced by 59.2% and 26.1% compared with random takeoff cases, and the locomotion is smooth, graceful, and low-risk. The computation and cost are low as the algorithm only requires the basic flight controller and the data from the inertial measurement unit instead of the prior parameters of the HAUV and waves. In comparison with the adaptive robust controller, which resists wave disturbance directly, this article provides an enlightening strategy from the perspective of harnessing waves.

This paper presents the design and fabrication of a profiling float primarily used for thermocline observations and tracking, with an emphasis on depth control performance. The proposed float consists of a frame-type electronic chamber and a variable buoyancy system (VBS) actuator chamber. Components or sensors can be added or removed according to specific requirements. All components were off the shelf, which lowered the cost of the float. In addition, a segment PD control method is introduced. Simulink results showed that there was no need to change any parameter when carrying out tasks at different depths. This method is superior to the traditional PD control and sliding mode control (SMC). In the process of diving, the speed could be well controlled to less than 0.2 m/s. We completed depth determination and control method validation in Qiandao Lake. The final results were consistent with the simulation results, and the maximum depth retention error was less than 0.3 m. Field tests also demonstrated that the prototype float can be used for thermocline observations in the upper layer of seawater or lake water.

Hybrid aerial underwater vehicles (HAUVs) provide unique capabilities for aerial and underwater observation and sampling, with a variety of configurations already developed. However, challenges in maneuverability, payload integration, and practicality persist, while their specific applications remain underexplored, limiting the full realization of their potential. This letter introduces an aerial and underwater observation and sampling (AUOS) system, featuring a Nezha-H HAUV and an autonomous water sampler. The Nezha-H fuses an H-shaped quadrotor configuration with a T-shaped quad-thruster mechanism, enabling mobility across air, surface, and underwater environments, with vertical profiling sampling capabilities. The static and cruise stability of Nezha-H on the surface and underwater are also analyzed through theory and experiment. The intelligent multi-water sampler automatically detects the thermocline and collects water samples at specific depths. This feature enables speedy and precise observations and sampling in a wide variety of aerial-aquatic environments. Successful trials in the field demonstrate its versatility in vertical sampling, thermocline identification, and air-water interface observation.

As emerging multirotor-type hybrid aerial underwater vehicles capable of both flight and underwater operations, numerous conceptual prototypes have been proposed in recent years. However, existing designs lack a balanced consideration of cross-medium stability, underwater maneuverability, and mode-switching reliability within a single configuration, and there remains no unified evaluation criterion for cross-domain performance. To compensate for the limitations of conventional optimization-based design, this work leverages key constraints identified through experiments and employ a multiobjective optimization approach to achieve a balanced design among these competing requirements. A lightweight and reliable foldable-arm system is further developed to support the implementation of the optimized configuration. In addition, through systematic integration of aerial propulsion bench test data with vehicle design parameters, we develop novel static quantitative metrics (cross-domain thrust margin@50%) to comprehensively assess overall design rationality. Finally, simulation and field tests show that the underwater damping of the final prototype has decreased by 55.2%. Compared with the previous generation, the cross-domain average power consumption is reduced by 68.9%.

Hybrid aerial underwater vehicles (HAUVs), capable of autonomous operations in both air and water with rapid, repeated transitions, have garnered significant attention for versatile marine applications. Yet, current HAUVs often fail to operate and survive in challenging sea states, particularly during robust submergence and cross domain maneuvers. Crucially, the success of cross domain transitions is tightly coupled to the robustness of submergence control. To overcome these limitations, this study introduces an adaptive angular velocity compensator that ensures precise and robust submergence. Building on this, a strategy for temporal window awareness for wave-adaptive cross-domain is developed and rigorously analyzed from a statistical perspective through simulations. Field experiments under Sea State 2, conducted with HAUVs of varying scales but identical configurations, validate the approach. Results demonstrate that the control precision of submergence depth and attitude angle reaches 0.1% and 7.5% throughout 0 to 100 m with the compensator. Furthermore, the wave-adaptive cross-domain control is achieved within 1.5 s, with root mean square errors of 4.1° in course tracking and 3° in attitude angle, in cooperation with the temporal window awareness. This study provides a foundation for enhancing the resilience and mission endurance of HAUVs, paving the way for their reliable deployment in increasingly challenging marine environments.