Unmanned Aerial Underwater Vehicles Research Articles

Experimental study on the hydrodynamic of foldable-wing unmanned aerial-underwater vehicles during water-exit process

The Foldable-Wing Unmanned Aerial-Underwater Vehicle (F-UAUV) demands swift movement over water surfaces at significant pitch angles during the water-exit procedure, a phase highly influenced by strong, non-linear, time-varying hydrodynamic effects. To tackle these complexities, we devised a model experiment scheme mirroring the actual water-exit process. Crafted through structural drainage comparisons, high-fidelity models were developed, complemented by a test platform featuring a double track for accurate evaluations. Subsequent water-exit tests, conducted at various speeds and angles, unveiled a consistent increase in water-exit resistance with speed, peaking at the moment of exit. Concurrently, resultant torque escalated inversely with both speed and angle. Notably, the heightened resistance due to structural drainage exhibited a tendency to stabilize during the latter half of the water-exit process. Utilizing the foundational formula for resistance in a single medium, we proposed an enhanced formula for predicting maximum water-exit resistance, a tool instrumental in facilitating F-UAUV design and powertrain selection. This paper offers an exhaustive analysis of hydrodynamic influences during the water-exit process, serving as a pivotal reference for Unmanned Aerial-Underwater Vehicles development.

Efficient prediction method for the water-exit characteristics of unmanned aerial–underwater vehicles

In the design of unmanned aerial–underwater vehicles (UAUVs), the performance during the transition from water to air is a crucial aspect requiring considerable attention. During water exit, UAUVs transition from water to air, crossing two vastly different mediums. Even after emerging from the water, the vehicle is still subjected to the forces exerted by the attached water. These challenges pose significant difficulties in predicting the water-exit performance. This study investigated a UAUV equipped with foldable wings and introduced a novel method for forecasting its water exit and flight characteristics using relay calculations. The method considered the complex force changes when crossing the water–air interface, as well as the force changes caused by wing unfolding and the influence of water attached to the vehicle after exiting the water. Additionally, an efficient aerodynamic coefficient identification method was adopted, and a novel approach for capturing and describing the state of the attached water was introduced. The accuracy and efficiency of the proposed method were demonstrated through comparisons with computational fluid dynamics methods. This method can be applied to the rapid iterative design of new UAUVs and provides a foundation for investigating water-exit motion control.

Constrained multi-objective trans-media maneuver trajectory optimization for morphing unmanned aerial-underwater vehicles

In this study, the challenge of optimizing trans-media maneuver trajectories is addressed through the introduction of a Constrained Multi-Objective Grey Wolf Optimizer (CMOGWO). The objective function has been refined to assess solutions more accurately based on their feasibility and adherence to constraints. Three distinct Archives have been employed for categorizing and optimizing dominant, non-dominant, and feasible non-dominant solutions, each benefiting from specifically tailored optimization strategies. A hybrid search method, complemented by a watch-based strategy, is proposed for effectively guiding the population towards the Pareto front, ensuring the discovery of potential optimal solutions. Investigations have been conducted into the impact of initial altitude on trajectory optimization, with comparative simulations revealing the effectiveness of the proposed CMOGWO in managing unmanned aerial-underwater vehicle trajectories. The results demonstrate the potential of the CMOGWO in enhancing the reliability and feasibility of trajectory optimization in complex environmental conditions.

Design and field test of a foldable wing unmanned aerial–underwater vehicle

AbstractThis paper presents the design and field test of a foldable wing unmanned aerial–underwater vehicle (UAUV). The vehicle can complete diving and air operations, and still have the ability of multiple trans‐medium water egress and ingress under the condition of carrying mission load during a single flight. The wings can be folded back for drag reduction in underwater sailing and water egress, and unfolded to provide lift for flight after water egress. This paper presents the major components and system design of the vehicle, including the overall structure of the vehicle, the design of the mechanism for self‐locking and quick‐folding, the design of the electrical system, the selection of the propulsion system, and analyzes the performance gains from foldable wings and foldable propellers. Then the trans‐medium field test was introduced, and the analysis and discussion were conducted for the four typical operating conditions of the vehicle, including water sailing, aerial flight, water egress and water ingress. Furthermore, the endurance performance under the corresponding operating conditions is estimated based on experimental data and the current battery energy carried. The presence of foldable wings and a large pitch angle water egress strategy reduces the thrust‐to‐weight ratio requirement for the vehicle. The results of the field tests can provide important experience and data support for the subsequent application‐oriented UAUVs.

Disturbance Observer-Based Robust Take-Off Control for a Semi-Submersible Permeable Slender Hybrid Unmanned Aerial Underwater Quadrotor

The development of hybrid unmanned aerial underwater vehicles (HAUVs) compatible with the advantages of the aerial vehicles and the underwater vehicles is of great significance. This paper presents the first study on a new HAUV layout using four rotors to realize the medium crossing motion of a transverse slender body similar to the fuselage of a missile or a submarine, that is, the hybrid aerial underwater quadrotor (HAUQ). Then, a robust control strategy is proposed for the take-off HAUQ on the water in the presence of unknown disturbances and complex model dynamic uncertainties. As a semi-submersible HAUQ rises straight from the water, the inside of the slender fuselage placed horizontally is filled with water. The center of the mass, the moment of inertia, and the arm of the force of the HAUQ will change rapidly in the take-off phase from the water because of the rapid nonuniform change in mass caused by the passive fast drainage. It is difficult to establish an accurate mathematical model of the complex dynamic changes caused by the multi-media dynamics, the fast changing buoyancy, and the added mass crossing the air–water surface. Therefore, an uncertain kinematic and dynamic model is established through the passive, fast, nonuniform change and the complex dynamics are considered as the unknown terms, and the external disturbances of gust and other factors are assumed as the bounded disturbance input. A robust design approach is introduced to deal with the fast time-varying mass disturbance based on the input-to-state stability (ISS) theorem. The complex dynamics are estimated using the basis function and the unknown weight parameters, and the adaptive laws are adopted for the on-line estimation of the unknown weight parameters. Considering the residual disturbance of the uncertain nonlinear system as a total disturbance term, a disturbance observer is introduced for disturbance observation. The numerical simulation shows the feasibility and robustness of the proposed algorithm.

Dynamic step selection algorithm for piecewise linear approximation of complex control trajectories

The paper proposes a method for replacing a complex function used as a target for automatic control of unmanned vehicles with a simplified piecewise linear function, which requires much less computing resources. A numerical method has been developed for adaptive selection of a variable step for approximating a nonlinear one-dimensional function, the analytical expression of which is not given, by a piecewise linear function. The article shows that selection of approximation step (grid) is an important task for minimizing the calculations required number, if the computing device on board the unmanned vehicle must be miniature. The developed algorithm includes the calculation of successive intervals lengths that eventually cover the entire domain of the function with a predetermined approximation accuracy. The determination coefficient is used as a measure of accuracy. Numerical experiments are presented for calculating the linear-piecewise function parameters for the approximating a target trajectory problem with complex configuration for unmanned aerial underwater vehicle automatic control. The proposed method efficiency is compared with methods with a constant step, as well as with methods for calculating the dynamic step in various approaches. The advantage of the developed method from the point of view of computational costs is proved with the same accuracy, the limitations of the applicability of the method are determined.

DDQN path planning for unmanned aerial underwater vehicle (UAUV) in underwater acoustic sensor network

DDQN path planning for unmanned aerial underwater vehicle (UAUV) in underwater acoustic sensor network

Numerical study on the water entry of two-dimensional airfoils by BEM

Hydrodynamics of the traditional wedges, cylinders, and cones entering water have been heavily studied. However, the hydrodynamic characteristics of airfoils are practically unknown; the problem is inspired by the recently widely developed fixed-wing unmanned aerial-underwater vehicle, which can fly in the air, cruise in water and frequently enters and exits free water surface. In this study, the hydrodynamic characteristics and fully nonlinear free surface deformation for the water entry of airfoils have been investigated by the time domain boundary element method. First, convergence study based on the NACA0012 airfoil is considered. The time step and element size are adjusted, and the results show that the free surface elevation and pressure distribution agree well with those obtained through Reynolds-averaged Navier–Stokes equations. The results show that the higher the entry velocity, the greater the peak hydrodynamic force. Furthermore, the higher thickness ratio causes greater added mass. Gravity and velocity are found to significantly influence the hydrodynamic behavior, including the time-varying force, while thickness ratio has a great impact on added mass and pressure.

Numerical Study on the Water Entry of a Freely Falling Unmanned Aerial-Underwater Vehicle

The unmanned aerial–underwater vehicle (UAUV) is a new type of vehicle that can fly in the air and cruise in water and is expected to cross the free water surface several times to perform continuous uninterrupted observation and sampling. To analyze the hydrodynamic and motion characteristics of the vehicle, the whole water-entry process of a multi-degree-of-freedom UAUV with various velocity and pitch angle was investigated through a Reynolds-averaged Navier–Stokes method. The computational domain was meshed by trimmer cells. The relative movement between the vehicle and fluid domain was simulated using moving reference frame overset mesh to delineate the interaction region around vehicle body. To reduce the computational cost, adaptive mesh refinement and adaptive time-stepping strategy were used to capture the slamming pressure accurately with reasonable computational effort. First, convergence study is considered. Simulations of the vehicle with various initial velocities and different pitch angles were performed. The variable physical properties were analyzed, and detailed results through the time-varying force and velocity were shown. Initial velocity and pitch angle are found to significantly influence hydrodynamic behavior, including the time-varying force, while thickness ratio has a great impact on added mass and pressure. The results show that higher entry velocity results in greater peak vertical force. The transverse hydrodynamic characteristics for oblique water entry of the vehicle with varies pith angle are quite different.

Optimization of water-entry and water-exit maneuver trajectory for morphing unmanned aerial-underwater vehicle

In this study, the problem of time-optimal trans-media maneuver trajectory design for the morphing unmanned aerial-underwater vehicles (MUAUVs) is considered. A the three-degree MUAVU dynamic model to plan the optimal flight and navigation trajectory is established. To solve the trajectory optimization model, the improved Radau pseudospectral method (RPM) is used to transform the optimal control model into a nonlinear programming problem (NLP). A double-layer hybrid solver based on the improved grey wolf algorithm and sequential quadratic programming (IGWO-SQP) is designed to solve the NLP under complex constraints. The outer layer employs an improved grey wolf optimization algorithm based on scouting strategy and the inner layer adopts a gradient-based SQP algorithm, which overcomes the disadvantage of sensitive initial value of traditional algorithm. The simulation results demonstrated that the proposed method is effective and feasible to solve the trajectory optimization of MUAUVs. A comparative study between air-water and water-air trans-media optimal trajectory verifies the proposed method which is usually applied to the aircraft is feasible to solve the underwater navigation trajectory of the MUAUV.

Experimental study on trans-media hydrodynamics of a cylindrical hybrid unmanned aerial underwater vehicle

This paper presents the experimental investigation of the hybrid unmanned aerial underwater vehicle (HAUV) hydrodynamics during the water-air trans-media process. A novel experimental platform that can control the water-air transition profile of HAUV with different velocities, accelerations, and attitudes is firstly developed with the capability of measuring the precise hydrodynamic forces of the vehicle. This technique makes it possible to perform a series of constrained model experiments to obtain the hydrodynamic forces of HAUV during the water-air transition. The hydrodynamic forces on the HAUV changing with the velocity, acceleration, and attitude of the vehicle during the transition are measured. Analysis of the test data shows that when the velocity is more than 0.1 m/s, the suction force caused by the free surface effect increases first and then remains constant with the velocity increasing. The maximum value of the suction force is about 10% of gravity. At low accelerations, less than 0.15 m/s2, the vertical force is approximately proportional to the square of time. When the attitude is less than 15°, the hydrodynamic moment first decreases and then increases to a certain value. When the attitude increases, the hydrodynamic moment decreases directly to a certain value. The influence of the moment caused by hydrodynamic forces other than time-varying buoyancy cannot be ignored. These results contribute to the design, theoretical modeling, numerical simulation, and control of HAUV.

Lifting‐principle‐based design and implementation of fixed‐wing unmanned aerial–underwater vehicle

AbstractThis paper presents the implementation of a novel proof‐of‐concept design of a fixed‐wing unmanned aerial–underwater vehicle (UAUV). The UAUV is designed on the basis of the lifting principle and has an overall density between those of water and air. During air flight, fixed‐wings are deployed to create lift to overcome gravity, similar to ordinary fixed‐wing aircraft. Moreover, the fixed‐wings generate sufficient downward lift to overcome the large net buoyancy (FB), allowing the vehicle to dive during underwater cruising. Owing to the lack of buoyancy regulation and emergency load rejection systems, the proposed UAUV is relatively simpler than those of ordinary autonomous underwater vehicles or other UAUVs. Water exit of the proposed UAUV is driven by a combination of inertial and tractor propellers in conjunction with FB, which allows smooth maneuverability during water‐to‐air transition. A series of successful water‐exit tests are used to demonstrate that the proposed UAUV can rapidly exit from water with a wide range of pitch angles, indicating strong survival capability in the future bad sea condition. This paper presents the proof‐of‐concept, including the lifting‐based principle, general design, avionics and propulsion systems, and field tests in both air and water. The vehicle performance for endurance, duration, flight speed, attitudes during water‐to‐air transitions, and landing on the water surface are also analyzed and discussed.

Trajectory Planning for Hybrid Unmanned Aerial Underwater Vehicles with Smooth Media Transition

In the last decade, a great effort has been employed in the study of Hybrid Unmanned Aerial Underwater Vehicles, robots that can easily fly and dive into the water with different levels of mechanical adaptation. However, most of this literature is concentrated on physical design, practical issues of construction, and, more recently, low-level control strategies. Little has been done in the context of high-level intelligence, such as motion planning and interactions with the real world. Therefore, we proposed in this paper a trajectory planning approach that allows collision avoidance against unknown obstacles and smooth transitions between aerial and aquatic media. Our method is based on a variant of the classic Rapidly-exploring Random Tree, whose main advantages are the capability to deal with obstacles, complex nonlinear dynamics, model uncertainties, and external disturbances. The approach uses the dynamic model of the HyDrone, a hybrid vehicle proposed with high underwater performance, but we believe it can be easily generalized to other types of aerial/aquatic platforms. In the experimental section, we present simulated results in environments filled with obstacles, where the robot is commanded to perform different media movements, demonstrating the applicability of our strategy.

Trans-Media Kinematic Stability Analysis for Hybrid Unmanned Aerial Underwater Vehicle

In recent years, hybrid unmanned aerial underwater vehicles (HAUVs), which are capable of air–water trans-media motion, have been increasingly developed. For most HAUVs, air–water trans–media motion is a relatively dangerous and difficult process. Therefore, it is of great significance to study the particular process. This paper presents the first study on the kinematic stability of the air–water trans–media motion of HAUVs. First, a simplified dynamic model of HAUVs is proposed, including the hydrodynamic forces and the time–varying buoyancy. Then, based on the proposed model and the Hurwitz method, this paper derives the air–water trans–media kinematic stability criterion for HAUVs. This criterion can be applied to most air–water trans–media motions that satisfy the assumptions in this paper. Finally, this paper takes “Nezha”, a novel HAUV, as an example to analyze its air–water trans–media kinematic stability. The results show that the proposed criterion is effective in judging the vehicle’s design, including the geometry and thruster power, which are important factors in the performance of the trans–media process.

Multi-objective optimization of dive trajectory for morphing unmanned aerial-underwater vehicle

A well-planned dive trajectory is the key for the trans-media maneuver and flight control of morphing unmanned aerial-underwater vehicles (MUAUVs). The main focus of this paper is the application of a watch-based multi-objective grey wolf optimizer (MOGWO) to dive trajectory optimization problems. A multi-objective trajectory optimization model considering the change of sweep angle and static moment is developed and parameterized by the Gauss pseudospectral method, and a multi-objective nonlinear programming problem is constructed. To overcome the defect that, in the MOGWO, the rest wolves blindly follow the best wolves, the watch strategy is introduced, which provides the wolves with the ability of independent exploration. Subsequently, the watch-based MOGWO is employed to generate the Pareto front, which is compared with that obtained through other multi-objective techniques. The simulation results demonstrated that the proposed method is more reliable and can obtain more widely distributed non-dominated solutions, indicating that the watch-based MOGWO is effective and feasible in dealing with multi-objective trajectory optimization problems with complex constraints. In addition, comparative studies on optimal trajectories demonstrated that the maneuverability and gliding ability of the MUAUV are improved through cooperation between the angle-of-attack, bank angle, and sweep angle.

Survey of the emerging bio-inspired Unmanned Aerial Underwater Vehicles

Unmanned Aerial-Underwater Vehicles have been envisioned as a new kind of hybrid unmanned system capable of performing equally well in multiple mediums and seamlessly transitioning between them. Currently, bio inspired vehicles are leading the way in the development of such a vehicle, that can fly like an eagle and swim like a fish. This path-breaking amphibious aerial underwater vehicle is sought after because of its wide range of applications in military, commercial and civil environments. From aerial surveillance, deep water exploration to underwater non-destructive testing, this vehicle aims to do it all. However, the development of a fully functional version is hindered by the significantly different properties of air and water, considering the air water transition, different propulsion systems and stability underwater. We evaluate these available architectures that aim to accomplish the goal of creating a flawless multi-medium unmanned vehicle which has its limitations as one of the modes is dominant leaving the other to be at a lesser than expected measure. This paper presents a survey discussing the evolution of these systems by first looking at their parent systems and then evaluating the various existing types of UAUVs by understanding their working mechanisms. The paper aims to provide a comprehensive overview of the work done in the field so far which can provide direction to the further development of such vehicles.

Attitude and Altitude Control of Unmanned Aerial-Underwater Vehicle Based on Incremental Nonlinear Dynamic Inversion

A vehicle able to navigate both in the air and underwater is referred to an unmanned aerial-underwater vehicle (UAUV). Aiming at the attitude and altitude control of UAUVs, an incremental dynamic inverse control (INDI) method with second-order low-pass filters is proposed. Based on the underwater motion model of an UAUV and the incremental attitude control model, the relationship between thrust increment and angular acceleration increment is obtained through Taylor series expansion and angular acceleration feedback. Subsequently, attitude and altitude controllers based on nonlinear dynamic inversion (NDI) and INDI are designed and their robustness is analyzed. By introducing second-order low-pass filters at the part of control and feedback, the accuracy of the angular acceleration estimation is improved and the time synchronization of the control system is ensured. Simulation results demonstrated that when introducing external disturbances and uncertainties, the INDI control with second-order low-pass filters proposed in this article can track the reference signal faster and more accurately than the NDI control.

System Modeling and Simulation of an Unmanned Aerial Underwater Vehicle

Unmanned Aerial Underwater Vehicles (UAUVs) with multiple propellers can operate in two distinct mediums, air and underwater, and the system modeling of the autonomous vehicles is a key issue to adapt to these different external environments. In this paper, only a single set of aerial rotors with switching propulsion abilities are designed as driving components, and then a compound multi-model method is investigated to achieve good performance of the cross-medium motion. Furthermore, some additional variables, such as water resistance, buoyancy and their corresponding moments are considered for the underwater case. In particular, a critical coefficient for air-to-water switching is presented to express these gradually changing additional variables in the cross-medium motion process. Finally, the sliding mode control method is used to reduce the altitude error and attitude error of the vehicles with external environmental disturbances. The proposed scheme is tested and the model is verified on the simulation platform.

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.

Research on vertical air–water trans-media control of Hybrid Unmanned Aerial Underwater Vehicles based on adaptive sliding mode dynamical surface control

We address the problem of vertical air–water trans-media control of Hybrid Unmanned Aerial Underwater Vehicles in the presence of parameters uncertainty and disturbances. Hybrid Unmanned Aerial Und...