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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.

Lower-limb assisted exoskeletons are widely researched for movement assistance or rehabilitation training. Due to advantages of compliance with human body and lightweight, some cable-driven prototypes have been developed, but most of these can assist only unidirectional movement. In this paper we present an untethered cable-driven ankle exoskeleton that can achieve plantarflexion-dorsiflexion bidirectional motion bilaterally using a pair of single motors. The main weights of the exoskeleton, i.e., the motors, power supplement units, and control units, were placed close to the proximity of the human body, i.e., the waist, to reduce the redundant rotation inertia which would apply on the wearer’s leg. A cable force transmission system based on gear-pulley assemblies was designed to transfer the power from the motor to the end-effector effectively. A cable self-tension device on the power output unit was designed to tension the cable during walking. The gait detection system based on a foot pressure sensor and an inertial measurement unit (IMU) could identify the gait cycle and gait states efficiently. To validate the power output performance of the exoskeleton, a torque tracking experiment was conducted. When the subject was wearing the exoskeleton with power on, the muscle activity of the soleus was reduced by 5.2% compared to the state without wearing the exoskeleton. This preliminarily verifies the positive assistance effect of our exoskeleton. The study in this paper demonstrates the promising application of a lightweight cable-driven exoskeleton on human motion augmentation or rehabilitation.

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.

The goal of robotic manipulators is to grasp objects of irregular shapes with better precision and efficiency. Universal grippers developed in recent years are designed with excessive complexity or lower resistance to damage. In this paper, we propose the idea of a universal parallel-fingered hand with soft fingertips based on jamming effect, in which negative pressure and particle flow are utilized to achieve variable stiffness. Then we set up a whole pick-and-place experiment system to evaluate the operational performance in the real serving situations. The results indicate that for most rigid objects, our prototype with soft fingertips, the stiffness of which is over the threshold to bear maximum task load after jamming, can demonstrate higher grasping stability than rigid ones, reduce around 40% motor drive power consumption and have a higher load capacity compared with the rigid-fingertip prototype under stable grasping, and have a relatively better operating precision of ± 0.62 mm compared to most of previous soft grippers when placing objects with equal weight. The findings can facilitate to improve the grasping precision of mechanical hands with soft fingertips.

Some animals have the ability to shuttle freely between water and air. These creatures have evolved special body structures to accommodate both underwater and airborne environments. Natural creatures have given us great inspiration to the variable structure design for aquatic-aerial multimodal vehicle. In this paper, we propose a new squid-like soft morphing fins according to the flying squid. This squid-like soft morphing fins can spread and fold for the adaptation of the aquatic-aerial multimodal locomotion. When the fin is spread, it can generate lift force for the aquatic-aerial vehicle. When the soft fin is folded, it can reduce the drag force during the underwater movement. An unique air cavity inside the fin was designed as actuator to bend the soft fin. The fabrication method of the soft fin was introduced. The bending performance of the soft fin was analyzed by FEA simulation and tested by several experiments. Then, morphology tradeoff strategy of aquatic-aerial vehicle with soft morphing fins was investigated by the wind and water tunnel. The results demonstrate the soft morphing fins have fast actuating response and good deformability, which verifies the soft fin can help the aquatic-aerial vehicle to realize multi-modal locomotion.

This article presented a four-fingered soft bionic robotic gripper with variable effective actuator lengths. By combining approaches of finite element analysis, quasi-static analytical modeling, and experimental measurements, the deformation of the single soft actuator as a function of air pressure input in free space was analyzed. To investigate the effect of the effective actuator length on the gripping performance of the gripper, we conducted systematical experiments to evaluate the pull-off force, the actuation speed, the precision and error tolerance of the soft gripper while grasping objects of various sizes and shapes. A combination of depressurization and pressurization in actuation as well as applying variable effective actuator length enhanced the gripper’s performance significantly, with no sensors. For example, with tunable effective actuator length, the gripper was able to grasp objects ranging from 2 mm – 170 mm robustly. Under the optimal length, the gripper could generate the maximum pull-off force for the corresponding object size; the precision and the error tolerance of the gripper were also significantly improved compared to those of the gripper with full-length. Our soft robotic prototype exhibits a simple control and low-cost approach of gripping a wide range of objects and may have wide leverage for future industrial operations.

Aquatic-aerial multimodal vehicle is a new concept aircraft that can freely shuttle between water and air. Some of the natural organisms provide the inspiration to realize this multi-locomotion. Most of current prototypes use rigid link mechanisms or hinges to morph the structure thus to adapt to the aquatic-aerial environment, which is commonly complicated and bulky. In this paper, we present a novel prototype with pneumatically-driven soft fins and arms that can fold and spread just like the flying squid. The fins and arms can augment the lift force during flying by spreading and reduce drag force during swimming by folding. The performance of the morphable structures was investigated in wind and water tunnel. The results explain the tradeoff strategies of multimodal-locomotion between water and air, and verify the feasibility of the novel aquatic-aerial vehicle with soft morphable structures.

Aquatic unmanned aerial vehicles (U AV) have aroused attention from researchers, though no fully- featured aerial-aquatic U AV exists so far. The biological prototypes of which includes the cormorants adopting a treading strategy. Based on previous achievements in biological investigation, we developed a biomimetic prototype mimicking the webbed-foot under similar movement and stress situation. Curve fitting modeling and parameter optimization using MATLAB contribute to the design of the linkage mechanism to attain specific movements. The kinematic and dynamic infinite analysis is conducted by ANSYS FLUENT, revealing key features of the velocity, and the force and torque exerted on the morphologic mimicking flipper. From the results, we evaluated the contributions of the webbed-feet on the take-off propulsion in the aspects of motor pattern and forced deformation, establishing a foundation for further applications in the design of propulsion system of aerial-aquatic UAV.

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.

Cormorants can take off from water surface by cooperatively regulating the propulsion respectively produced by wings and two webbed-feet, which can provide an inspiration for an aquatic unmanned aerial vehicle's (AquaUAV) takeoff from water. This paper adopts modeling and simulation methods to investigate the take-off process of a cormorant from water surface, and to explore how the wings and feet function to improve the takeoff performance on water. Quasi-steady blade element method and hydrodynamic equation were used to analyze the forces produced by flapping wings and slapping feet respectively. The dynamic model of the cormorant's take-off from water was established. Then video-based kinematic data were obtained to verify the accuracy of this model. Further, The effects of the flapping frequency, body angle attack and webbed feet area on takeoff distance were investigated and discussed based on simulation results. The results demonstrate that the flapping frequency and webbed feet area contributes substantially to reducing take-off distance.

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.

Remoras of the ray-finned fish family Echeneidae have the remarkable ability to attach to diverse marine animals using a highly modified dorsal fin that forms an adhesive disc, which enables hitchhiking on fast-swimming hosts despite high magnitudes of fluid shear. We present the design of a biologically analogous, multimaterial biomimetic remora disc based on detailed morphological and kinematic investigations of the slender sharksucker (Echeneis naucrates). We used multimaterial three-dimensional printing techniques to fabricate the main disc structure whose stiffness spans three orders of magnitude. To incorporate structures that mimic the functionality of the remora lamellae, we fabricated carbon fiber spinules (270 μm base diameter) using laser machining techniques and attached them to soft actuator-controlled lamellae. Our biomimetic prototype can attach to different surfaces and generate considerable pull-off force-up to 340 times the weight of the disc prototype. The rigid spinules and soft material overlaying the lamellae engage with the surface when rotated, just like the discs of live remoras. The biomimetic kinematics result in significantly enhanced frictional forces across the disc on substrates of different roughness. Using our prototype, we have designed an underwater robot capable of strong adhesion and hitchhiking on a variety of surfaces (including smooth, rough, and compliant surfaces, as well as shark skin). Our results demonstrate that there is promise for the development of high-performance bioinspired robotic systems that may be used in a number of applications based on an understanding of the adhesive mechanisms used by remoras.

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.

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.

This article presented the optimization parameter of a bidirectional soft actuator and evaluated the properties of the actuator. The systematic simulation was conducted to investigate the effect of the top wedged angle (the angle for the wedged shape of the actuator structure) of the chamber on the bending extent of the actuator when it is deflated. We also investigated the width of the actuator and the material combinations of the two layers with the relation to the deformation performance. A mathematical model was also built to reveal the deformation of the actuator as a function of the geometrical parameters of the inner chambers and the material properties. We quantitatively measured the bending radius and the actuation time of the actuator both in air and under water. Digital particle image velocimetry experiments were conducted under water to observe the flow patterns around the actuator. We found that the top wedged angle has a significant effect on the outward bending of the actuator when it is deflated, and 15° was found to be optimal for bending into a larger gripping space. The result shows that the actuator can deform much easier with a bigger width. Utilizing a soft gripper that was built by mounting four actuators to a three-dimensional-printed rigid support, we found that the prototype can grip objects of different sizes, shapes, and material stiffness in amphibious environments.

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.