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The bio-inspired aerial–aquatic vehicle offers attractive perspectives for future intelligent robotic systems. Cormorant’s webbed-feet support water-surface takeoff is a typical locomotion pattern of amphibious water birds, but its highly maneuverable and agile kinetic behaviors are inconvenient to measure directly and challenging to calculate convergently. This paper presents a numerical Computational Fluid Dynamic (CFD) technique to simulate and reproduce the cormorant's surface takeoff process by modeling the three-dimensional biomimetic cormorant. Quantitative numerical analysis of the fluid flows and hydrodynamic forces around a cormorant’s webbed feet, body, and wings are conducted, which are consistent with experimental results and theoretical verification. The results show that the webbed feet indeed produced a large majority of the takeoff power during the initial takeoff stage. Prior lift and greater angle of attack are generated to bring the body off the water as soon as possible. With the discussion of the mechanism of the cormorant’s water-surface takeoff and the relevant characteristics of biology, the impetus and attitude adjustment strategies of the aerial–aquatic vehicle in the takeoff process are illustrated.

Although the biologically flexible flippers of the cormorant (Phalacrocorax) are believed to be one of the most important features to achieve optimal swimming performance before take off, studies on a deformable bionic flipper with a non-uniformly distributed stiffness are rare. In this letter, to gain the unsteady hydrodynamic distribution between flexible aquatic animals and the ambient medium during the power stroke and recovery stroke, the numerical physical model solutions which coupled fluid-structure interaction based on computational fluid dynamics (FSI-CFD) are presented. We quantified the three-axis component distribution, and the results show that the horizontal force of the fluid will not provide positive thrust during the initial take-off stage. Greater lift and forerake moment are generated, which brings the body off the water as soon as possible and reduces the angle of attack. As the angle of attack decreases, the positive thrust will be generated, and the forward velocity and the lift will be further increased. As the draft area of the cormorant is reduced, the wings will start flapping, further increasing the lift and thrust, and thus taking off. The solver in the letter will serve as a framework for the future bio-inspired studies involving active and passive control associated with complex structural materials.

Aquatic unmanned aerial vehicles (AquaUAV) have aroused much attention from researchers, though no fully-featured aerial-aquatic UAV exists so far. The assistance of webbed foot hydroplaning can accomplish rapid take-off of a cormorant. A significant impact force and moment can be generated due to the webbed foot propulsion in the water-to-air transition. However, the change law of force and moment experienced by the cormorant during take-off has not been captured. Based on previous achievements in the biological investigation, we developed a biomimetic prototype with curve fitting model and parameter optimization to attain specific movements to imitate cormorant's hydroplaning strategy. The bionic webbed foot considers the elastic mechanics, and the forepart is regarded as flexible material for fluid-structure interaction (FSI). Dynamic process of rapid take-off in the aspects of flow characteristics and mechanical properties can be estimated by computational fluid dynamics (CFD) in our proposed FSI model, which establishes a foundation for further applications in the design of the assisted propulsion system of aerial-aquatic UAV.

Lower-limb exoskeletons are of great interest in the robotics community because of their various applications in enhancement and rehabilitation. In this paper we present an autonomous exoskeleton platform for ankle plantarflexion assistance. The untethered exoskeleton has a high efficiency transmission system with reduction ratio of 27.4:1. This allows relocating the actuator to the wearer’s hip, which reduces device inertia. A feed-forward controller based on field oriented control was implemented to control the brushless DC motor on the exoskeleton. Through various performance tests, the exoskeleton was shown to provide a torque control bandwidth of 17.5Hz and can effectively track biological torque profiles. The augmentation factor (AF) of the exoskeleton is 64.7W, implying potential to reduce walking metabolic cost. This exoskeleton establishes an autonomous platform for experiments involving ankle assistance.

The process of taking off from the surface of the water for cormorant is a combination of flapping its wings and its webbed-feet periodically. To look into the contribution of the latter that helps in taking-off and to compute each period in the fixed quantity of the amount of force in each flapping action, this article explores the contribution of webbed-feet power to the process taking-off as well as the reasons of the need of leg strength assistance. To investigate this transition, we reproduced the take-off process of the cormorant through Computational Fluid Dynamics(CFD) and integrated the calculation of body, webbed-feet and wings forces during the process of take-off.

The amazing multi-modal locomotion of flying squid helps to achieve fast-speed migration and predator-escape behavior. Observation of flying squid has been rarely reported in recent years, since it is challenging to clearly record the flying squid’s aquatic-aerial locomotion in a marine environment. The existing reports of squid-flying events are rare and merely record the in-air motion. Therefore, the water-air locomotor transition of flying squid is still unknown. This paper proposes the idea of using CFD to simulate the process of the flying squid (Sthenoteuthis oualaniensis (S. oualaniensis)) launching from water into air. The results for the first time reveal the flow field information of squid in launching phase and show the kinematic parameters of flying squid in quantification. Both a trailing jet and pinch-off vortex rings are formed to generate launching thrust, and the formation number L ω /D ω is 5.22, demonstrating that the jet strategy is to produce greater time-averaged thrust rather than higher propulsion efficiency. The results also indicate that the maximum flying speed negatively correlates with the launch angle, indicating that a lower launch angle could result in a larger flying speed for the flying squid to escape. These findings explore the multi-modal locomotion of flying squid from a new perspective, helping to explain the trade-off strategy of water-to-air transition, and further enhance the performance of aquatic-aerial vehicles.

The aquatic unmanned aerial vehicle (AquaUAV), a kind of vehicle that can operate both in the air and the water, has been regarded as a new breakthrough to broaden the application scenario of UAV. Wide application prospects in military and civil field are more than bright, therefore many institutions have focused on the development of such a vehicle. However, due to the significant difference of the physical properties between the air and the water, it is rather difficult to design a fully-featured AquaUAV. Until now, majority of partially-featured AquaUAVs have been developed and used to verify the feasibility of an aquatic–aerial vehicle. In the present work, we classify the current partially-featured AquaUAV into three categories from the scope of the whole UAV field, i.e., the seaplane UAV, the submarine-launched UAV, and the submersible UAV. Then the recent advancements and common characteristics of the three kinds of AquaUAVs are reviewed in detail respectively. Then the applications of bionics in the design of AquaUAV, the transition mode between the air and the water, the morphing wing structure for air–water adaptation, and the power source and the propulsion type are summarized and discussed. The tradeoff analyses for different transition methods between the air and the water are presented. Furthermore, it indicates that applying the bionics into the design and development of the AquaUAV will be essential and significant. Finally, the significant technical challenges for the AquaUAV to change from a conception to a practical prototype are indicated.

In this paper, we studied the wing root pivot joint’s radial load of a submersible airplane which imitates the locomotion of gannet’s Morus plunge-diving, by implementing a test device name Mimic-Gannet. The housing of the device was designed by mimicking the morphology of a living gannet, and the folding wings were realized by the mechanism of variable swept back wing. Then, the radial loads of the wing root were obtained under the conditions of different dropping heights, different sweptback angles and different water-entry inclination angles (i.e., the angle between the longitudinal body axis and the water surface), and the relationships between the peak radial load and the above three parameters were analyzed and discussed respectively. In the studied areas, the minimum peak radial load of the pivot joint is 50.93 N, while the maximum reaches up to 1135.00 N. The largest peak load would be generated for the situation of vertical water entry and zero wing sweptback angle. And it is of great significance to choose the three parameters properly to reduce the pivot joint’s radial load, i.e., larger wing sweptback angle, smaller dropping height and water-entry inclination angle. It is also concluded that the peak radial load on the wing root is closely linear with the water-entry dropping height and the wing sweptback angle with a significant correlation. Eventually, the relationship between the wing load and the dropping height, water-entry inclination angle or wing sweptback angle, could be used to calculate the wing load about plunge-diving of a submersible aircraft, and the conclusions reveal the wing load characteristic of the gannet’s plunge process for the biologists.

In this paper, we present the design, fabrication and the mathematical model of an entire soft robotic arm with three-dimensional (3D) locomotion. We first describe the design of the soft arm based on 3D printed channels that were spatially distributed at the interface of two different silicone elastomeric materials which enable complex 3D motion of the soft arm. Then we demonstrate the workspace of motion at different air pressure levels, and the ability of the mathematic model to predict the three-dimensional movement in free space. We further demonstrate that modifying the texture of the surface of the soft arm can constrain the radial expansion. Finally, we estimate the workspace, location repeatability of the soft arm via actual tests.

to reveal the flippers propulsion and the self-adjustment mechanism, which can be used to inspire the design of submersible aircraft. This article makes some research on the bionic observation on the movement of cormorant's flippers during the surface take-off process, bringing forward a flippers propulsion theory, which there is a new undulate propulsion besides the common flapping propulsion. Observation use the video image processing technology to analyze the trajectory of cormorant' flippers and the forced characteristics, revealing the mechanism of effective propelling force of cormorant's flippers. It provides a new train of thinking for people to develop the submersible aircraft with high efficiency.

This paper conducted two biological observation experiments on cormorants, whose data concerns cormorants' weight, volume, area of webbed feet and area of their wings, and other data on density, wing loading and webbed feet loading is calculated as well. The average density is 0.738g/cm3, the average wing loading is 1.58 g/cm2 and the average webbed feet loading is 25.3 g/cm2. The flight performance of cormorants is negatively correlated to the wing loading, and the ability of underwater locomotion and fishing capacity are closely relative to the webbed feet loading. Furthermore, the average webbed feet loading of cormorants is an order of magnitude larger than the wing loading, which indicates that respectively designing corresponding propulsion systems of aquatic unmanned aerial vehicles (AquaUAV) in both water and air is essential and significant, including geometry, dimensions, kinematics, and the rated thrust power, etc. These conclusions are useful for the development of AquaUAV.

A bionic gannet was developed based on the analysis of the body configuration and skeleton structure and the motion pattern of wings of a gannet in plunge-diving. In the current prototype, adjustable sweptback wings were implemented so as to achieve different body shapes for entering water. The impact acceleration in the longitudinal body axis direction and the axial overload on the body were investigated through the falling-down experiments under different conditions including dropping height, water-entry inclination angle, and wing sweptback angle. It is found that when the above three key parameters are 10 m for dropping height, 0° for wing sweptback angle, and 90° for water-entry inclination angle, the maximum peak impact acceleration and overload are −167.20 m·s−2 and 18.06 respectively. Furthermore, the variation of peak impact acceleration with the three key parameters were also analyzed and discussed.

This paper presents an online nonlinear actuator fault detection and isolation system, aiming at monitoring monitor the health of a novel ducted fan UAV. In order to cover wider flight envelope, an unscented multiple model adaptive estimation method is introduced as an extension of the EKF-based method. The proposed method provides a bank of UKFs running in parallel, each of which is responsible for completely monitoring corresponding actuator's health. Simulation results shows that locked-in-place and floating faults of the control vanes and failures of the auxiliary ducted fans can be handled with small ambiguity or false detection, rapid response and low computational load, indicating a more efficient operation than EKF-based method. Then, the robustness of the fault detection system is further enhanced by the usage of auxiliary excitation signals.

In this study, we built a four-fingered soft robotic gripper with tunable effective finger lengths. This robotic model is purely made of soft materials and allows two working modes that require very simple control: 1) deflate the soft fingers for bending to one direction therefore to open the gripper “claw” and 2) inflate the fingers with compressed air for bending to the reverse direction, therefore to grip objects reversibly. Systematic tests of the gripping performance of the soft robotic model were conducted for 5 effective finger lengths ranging from 30 mm to 100 mm. Under each effective length, we measured the pull-off force of 8 sphere-shaped objects with diameters from 20 to 90mm, and five typical geometric shaped objects including sphere, cubic and cylinder etc. We also measured the pull-off force of gripping objects with different stiffness. Notably, we found that each object with different size prefer a “sweet” effective finger length for generating maximum pull-off force. We show that tunable effective finger length for the soft robot can significantly improve the performance when gripping multiple objects. Current soft robotic prototype exhibits a simple-control, low-cost approach of grasping objects with different size, weight, and shape as well as material stiffness, and may open up new avenues for future industrial gripping.

Two distinguishing features, inclining backward rotor disk and unpowered rotor, make fluid field of autorotating rotor in gyroplane level flight different from helicopter rotor in forward flight. Unsteady flow past autorotative rotor in gyroplane level flight is time-accurately simulated by solving URANS based on multi-block structured grid. The methods of the sliding mesh and the Full-Approximation Storage Multigrid are utilized. Autorotating speed, the solution of the BET model of gyroplane level flight, is assigned to an initial condition of unsteady numerical simulation after fine tune. At several advanced ratios, the agreement between simulation results and Eugene E. Niemi's wind tunnel test data is good. The aerodynamics of the autorotative rotor is investigated by using these numerical results.

A real submersible flying boat which is both flight-capable and submersible has hardly been reported yet. In this paper, the Flying Fish created by Beihang University will be introduced to verify the feasibility of this kind of novel aircraft. The Flying Fish achieves its ability to transfer between the air and the water by imitating the flying fish and aquatic birds. The morphological characteristics of the flying fish and the variable density method of the aquatic birds are imitated in the design of this novel aircraft. Moreover, the technological deficiencies of this vehicle have also been concluded through the experiments, which can be referred to by the relative future studies.

A submersible unmanned aerial vehicle (UAV) is proposed firstly, which is capable of operating in both air and water. One of the outstanding characteristics of the UAV is that the air-water transition imitates that of a gannet, i.e., plunge-diving. In this paper, the plunge-diving process of this UAV is simplified as a water-entry problem with a certain initial velocity, and the impact force is calculated by the method of the computational fluid dynamics (CFD). The Volume of Fluid is coupled with the 3-D Navier-Stokes equations to establish the model of the flow field, and the equations are solved in Fluent 6.3. The phase distribution and the pressure distribution during water-entry are presented and analyzed. Furthermore, the effects of the dropping height and the wing's sweptback angle on the impact force are investigated and discussed.

Skid-steering mobile robots, both wheels and tracked vehicles, are widely used because of their simple mechanism and robust. However, due to the complex wheel-ground interactions and the kinematics constraints, it is a challenge to understand the kinematics and dynamics of such a robotic platform. In this paper, we develop a kinematics modeling estimation scheme to analyze the skid-steering wheeled mobile robot based on the boundedness of the Instantaneous Centers of Rotation (ICRs) of treads on the 2D motion plane, and the parameters of this model are determined based on experimental analysis. We study the relationship between the ICRs of the robot treads and two physical factors, i.e., the radius of the path curvature and the robot speed. Moreover, an ICRs coefficient and a nondimensional variable are introduced. An approximating function is used to describe the relationship. To validate the obtained results, the proposed model has been applied to a popular research robotic platform, Pioneer P3-AT. It is empirically demonstrated that the developed model improves dead-reckoning performance of this skid-steering robot.

In this paper, a novel method was presented to investigate the hydrodynamics of a robotic fish at different Reynolds number. The ionic polymer-metal composite (IPMC) was used as the soft actuator for biomimetic underwater propulsion. A hydrodynamic model based on the elongated body theory was developed. Based on image analysis, the kinematic parameters of the robotic fish were identified. To obtain the hydrodynamic thrust performance of the robotic fish, we implemented a novel experimental apparatus. Systematic tests were conducted in the servo towing system to measure the self-propelled speed and thrust efficiency at viscous and inertial flow. The robotic fish's thrust efficiency was compared at different body and caudal fin (BCF) swimming modes, i.e. anguilliform, carangiform and thunniform. The thrust performance of the robotic fish is determined by the kinematics and Reynolds number. We show that at high Reynolds number, thunniform kinematics is the most efficient, while anguilliform kinematics produces relatively poor thrust efficiency. At low Reynolds number, the fish has the highest thrust efficiency with the anguilliform type. It is less efficient with the thunniform type.

This paper presents the construction and control of a turtle-like underwater vehicle, which uses four single axis oscillation flippers for swimming and attitude control. The maneuvers are realized by the composition of appropriate angular offset, vibration amplitude and vibration frequency of the flippers. After analyzing the actuated flippers' motion and the swimming gait, a CPG-based (central pattern generator) control architecture is introduced. Such a CPG model has several nice properties, i.e. limit cycle behavior, closed form solution, no discontinuities nor jerks performance. To estimate the utility and stability of the vehicle, experiments were also carried out in the Mochou Lake of Antarctic Zhongshan Station following the Chinese 29th Antarctic expedition. The experimental result proves that the vehicle is applicable to the underwater detection.

Plunge diving is the most commonly used feeding method of a gannet, which can make the gannet transit from air to water rapidly and successfully. A large impact acceleration can be generated due to the air-to-water transition. However, the impact acceleration experienced by the gannet during plunge diving has not been studied. In this paper, this issue is investigated by using the CFD method. The effect of the dropping height and the water-entry inclination angle on the impact acceleration is considered. The results reveal that the impact acceleration along the longitudinal body axis increases with either of the two parameters. The peak time decreases with the dropping height. A quadratic relation is found between the peak impact acceleration and the initial water-entry velocity. According to the computation, when the dropping height is 30 m (most of gannets plunge from about this height), the peak impact acceleration can reach about 23 times the gravitational acceleration, which will exert a considerable force on the gannet body. Furthermore, the pressure distribution of different water-entry inclination angles indicates that the large pressure asymmetry caused by a small oblique angle may lead to a large impact acceleration in the direction perpendicular to the longitudinal body axis and cause damage to the neck of the gannet, which partly explains the reason why a gannet performing a high plunge diving in nature enters water with a large oblique angle from the perspective of impact mechanics. The investigation on the plunge-diving behavior in this paper will inspire and promote the development of a biomimetic amphibious robot that transits from air to water with the plunge-diving mode.

This paper reproduces the plunge-diving process of a biomimetic gannet through the CFD (Computational Fluid Dynamics) simulation. The numerical investigation of impulse force and pressure distribution on the designed biomimetic gannet when interacting with water is also presented. Three-dimensional computational transport equations based on the complete set of Navier-Stokes equations are solved by using the FLUENT 6.2 solver. Numerical simulations have been performed to examine the flow characteristics on the interface between air and water, where the interface is tracked by coupling the VOF (Volumn of Fluid) method. The biomimetic gannet is developed by imitating the natural gannet, the morphology and water-entry posture of which are the same as those of the gannet in nature. For different given dropping heights, the effect of initial water-entry velocity on the axial impact force acting on the gannet's body is investigated and numerically examined.