Flapping-wing System Research Articles

Harvesting the wind energy through an actively controlled pitch-plunge flapping wing

A different approach for harvesting the wind energy with an actively controlled pitch-plunge aeroelastic system is investigated. The paper present some theoretical considerations and results regarding an oscillating wing. The performances of the flapping wing system were investigated numerically and experimentally for the power extraction process.

Identification and Robust Control of a Flapping Wing System Using Fuzzy Wavelet Neural Networks

Identification and Robust Control of a Flapping Wing System Using Fuzzy Wavelet Neural Networks

Dynamics modeling and experiment validation for piezoelectric-transmission-wing system

Aiming at the challenge of predicting the dynamic behavior of the flapping wing in the piezoelectric-transmission-wing (PTW) wingbeat system, a lumped-parameter model coupling piezoelectric structural mechanics and flapping-wing aerodynamics is proposed to address the challenge caused by the fluid-structure interaction (FSI). Firstly, a model describing the piezoelectric mechanical response is established, which has the ability to better predict the mechanical response of the piezoelectric actuator under high electric field and low-frequency driving conditions. In order to verify the validity and accuracy, tip deflection, stiffness and blocking force experiments as well as data comparisons are performed to substantiate the findings. Secondly, a comprehensive and reasonable lumped-parameter dynamic model is constructed based on the analysis of the transmission mechanics based on pseudo-rigid-body theory and the improved aerodynamic modeling of the wing, along with the aforementioned piezoelectric mechanical modeling. Thirdly, simulation and dynamic response experiments of the wingbeat system are carried out. Results show that the simulation data are in good agreement with the experimental values, which indicates the accuracy of the model in predicting the dynamic behavior of the flapping wing powered by the piezoelectric actuator. Moreover, some interesting phenomena such as the pitch oscillation of the wing and the dependence of the flapping motion on the driving frequency have been demonstrated in the simulation and the wingbeat experiment, respectively. The research findings of this article offer three types of assistance: (1) providing a parameter design method for piezoelectric actuators to achieve the desired mechanical response; (2) predicting and explaining the dynamic behavior of the PTW wingbeat system; and (3) providing a theoretical foundation for the parameter design and optimization of the wing-actuator pairing in the flapping wing system.

Closed-loop nonlinear optimal control design for flapping-wing flying robot (1.6 m wingspan) in indoor confined space: Prototyping, modeling, simulation, and experiment

The flapping-wing technology has emerged recently in the application of unmanned aerial robotics for autonomous flight, control, inspection, monitoring, and manipulation. Despite the advances in applications and outdoor manual flights (open-loop control), closed-loop control is yet to be investigated. This work presents a nonlinear optimal closed-loop control design via the state-dependent Riccati equation (SDRE) for a flapping-wing flying robot (FWFR). Considering that the dynamic modeling of the flapping-wing robot is complex, a proper model for the implementation of nonlinear control methods is demanded. This work proposes an alternative approach to deliver an equivalent dynamic for the translation of the system and a simplified model for orientation, to find equivalent dynamics for the whole system. The objective is to see the effect of flapping (periodic oscillation) on behavior through a simple model in simulation. Then the SDRE controller is applied to the derived model and implemented in simulations and experiments. The robot bird is a 1.6 m wingspan flapping-wing system (six-degree-of-freedom robot) with four actuators, three in the tail, and one as the flapping input. The underactuated system has been controlled successfully in position and orientation. The control loop is closed by the motion capture system in the indoor test bed where the experiments of flight have been successfully done.

A Framework for Characterizing Flapping Wing Systems

Flapping wing systems are being developed by various institutions and research groups around the world with many systems developed that are capable of full flight. However, while instrumentation has been developed that is capable of measuring some of the characteristics of these systems, there is no complete solution. This paper seeks to take the first step toward instrumentation that could be applied to any flapping wing system. This first step is to identify and characterize the forces that are operating on flapping wing systems. This paper presents, in premiere, a systematic analysis of all cases that can create useful or parasitic aerodynamic loads along with the other major loads that would be experienced by these cases and methodology for how these can be measured with the ambition that it can become a framework to be used to characterize any flapping wing system.

Modeling and Analysis of a Simple Flexible Wing-Thorax System in Flapping-Wing Insects.

Small-scale flapping-wing micro air vehicles (FWMAVs) are an emerging robotic technology with many applications in areas including infrastructure monitoring and remote sensing. However, challenges such as inefficient energetics and decreased payload capacity preclude the useful implementation of FWMAVs. Insects serve as inspiration to FWMAV design owing to their energy efficiency, maneuverability, and capacity to hover. Still, the biomechanics of insects remain challenging to model, thereby limiting the translational design insights we can gather from their flight. In particular, it is not well-understood how wing flexibility impacts the energy requirements of flapping flight. In this work, we developed a simple model of an insect drive train consisting of a compliant thorax coupled to a flexible wing flapping with single-degree-of-freedom rotation in a fluid environment. We applied this model to quantify the energy required to actuate a flapping wing system with parameters based off a hawkmoth Manduca sexta. Despite its simplifications, the model predicts thorax displacement, wingtip deflection and peak aerodynamic force in proximity to what has been measured experimentally in flying moths. We found a flapping system with flexible wings requires 20% less energy than a flapping system with rigid wings while maintaining similar aerodynamic performance. Passive wing deformation increases the effective angle of rotation of the flexible wing, thereby reducing the maximum rotation angle at the base of the wing. We investigated the sensitivity of these results to parameter deviations and found that the energetic savings conferred by the flexible wing are robust over a wide range of parameters.

Derivative-Free Observability Analysis for Sensor Placement Optimization of Bioinspired Flexible Flapping Wing System.

Observability analysis of a bioinspired flexible flapping wing system provides a measure of how well the states of flexible flapping wing micro-aerial vehicles can be estimated from real-time measurements during high-speed flight. However, the traditional observability analysis approaches have trouble in terms of lack of quantitative analysis index, high computational complexity, low accuracy, and unavailability in stochastic systems with memory, including bioinspired flexible flapping wing systems. Therefore, a novel derivative-free observability analysis method is proposed here based on the generalized polynomial chaos expansion. By formulating a surrogate model to represent the relationship between the cumulative measurement and the random initial state, the observability coefficient matrix is calculated and the observability rank condition is stated. Consequently, several observability indices are proposed to quantity the observability of the system. Altogether, the proposed method avoids the disadvantages of the traditional approaches, especially in assessing the observability degree of each state and the effect of stochastic noise on observability. The validation of the proposed method is first provided by demonstrating the equivalence between the traditional and proposed methods and subsequently by comparing the observability of the Lorenz system calculated via three different approaches. Finally, the proposed method is applied on a bioinspired flexible wing system to optimize the placement of sensors, which is consistent with the natural configuration of campaniform sensilla on the wing of the hawkmoth.

Bionic flutter wing piezoelectric-electromagnetic composite energy harvesting system

The bionic structure has great potential in the field of energy harvesting. Inspired by the fluttering structure of dipteran insects, this paper proposes a novel bionic multi-domain energy harvester based on the mechanism of fluttering behavior of dipteran insects. The device is designed concerning the skeletal and muscular structures of dipteran insects, using a flexible cantilever beam and a rigid linkage to simulate the fluttering wings and skeletal structures of dipteran insects, while the reciprocating motion generated by the central permanent magnet with the cooperation of gravity and a limiting spring simulates the muscle contraction of dipteran insects. In the process of simulating the fluttering motion, the piezoelectric sheet installed on the cantilever beam can harvest the vibration energy, while the magnetic induction coil arranged on the outside of the central permanent magnet can harvest the electromagnetic energy generated by the vertical reciprocating motion, providing a second channel of energy harvesting for the bionic fluttering wing piezoelectric-electromagnetic composite energy harvesting (BCEH) system. Based on the electromechanical coupling characteristics of the energy harvesting device, a centralized parametric dynamics model of the system is established. The modal analysis of the bionic structure is carried out by using the finite element method, and the natural frequency and mode shape of the structure are determined. The key structural parameters are optimized through theoretical calculation and simulation. After optimization, the output voltage and output power of the proposed energy harvester have been effectively improved. The output voltage of the piezoelectric port has been increased by up to 347%, and the output power has been increased by up to 1789%. The output voltage of the electromagnetic port has been increased by up to 40%. The output power has been increased by up to 137%. So the experimental results show that the bionic flapping-wing composite energy harvesting system can achieve better energy harvesting.

인장스프링을 활용한 모터 직접 구동형 양방향 날갯짓 장치 개발

This paper proposes a direct-driven bi-directional actuation flapping wings system using an extension spring instead of a torsional spring. Then, the proposed flapping-wing system’s mathematical model is derived and analyzed. A prototype is fabricated using a DC motor, an extension spring, pinion gear, spur gear, and a flapping wing. The performance of the proposed wing is measured using an oscilloscope, a load cell, and a high-speed camera. The results demonstrate that the system works well without fatigue failure or the spring’s mechanical deformation. Additionally, it can generate 11.1 g of thrust when 24 Hz driving signal is applied using 12 V PWM, which is sufficient to fly 20 g micro air vehicles using dual flapping-wing systems. The wing flapping system’s energy consumption is 13.95 W, which means that more than a 1-minute flight is possible using a 7.4 V battery with 70 mAh capacity.

Effect of incoming gravity waves on the energy extraction efficiency of flapping wing hydroelectric generators

The energy extraction performance of flapping wing hydroelectric generators is subject to the effects of the free surface and incoming gravity waves. The present work addresses this issue by conducting transient numerical simulations of a two-dimensional flapping wing system. The effects of the wave wing phase difference, encounter frequency, and wing submergence depth on the energy extraction performance are investigated. The results indicate that the phase difference is the primary factor affecting the energy extraction efficiency. The incoming gravity wave has the most positive effect on the energy extraction efficiency when it's in the range of 0°–90°. In addition, the energy extraction efficiency increases with the increase in the encounter frequency. Moreover, the periodicity is best when the oscillation frequency of the wing is equal to the encounter frequency. Finally, the influence of the submerged depth on the energy extraction efficiency is also discussed.

Probabilistic uncertainty and sensitivity analysis of a vibratory-based actuation mechanism for a bio-inspired flapping-wing system

Probabilistic uncertainty and sensitivity analysis of a vibratory-based actuation mechanism for a bio-inspired flapping-wing system

Interaction of the wakes of two flapping wings on control forces and moment

Control forces and moments of a flapping-wing system are highly affected by wakes and their interaction, but most studies have relied on estimations with an aerodynamic model, which cannot cover the effects of the wakes and the interaction. In this study, the effect of the wakes on the longitudinal control force and moment was investigated. A mean sweeping angle in various sets of wingbeat motion profiles was considered as a control input, and the forces and moments obtained from a rigid wing with an aspect ratio of four, which were controlled by a scaled-up robotic model, were converted to that on a virtual body. The results showed that adjusting a mean sweeping angle, which is originally expected to produce a nose-down moment and a consequent thrust to move forward by tilting a lift force, rather produced a considerable backward force due to the interaction of the wakes. This backward force was larger than the estimations, and the difference gradually increased with an increase in the mean sweeping angle in most of the motion profiles. The force was equivalent to 12.7% of the lift force at maximum, sufficient to resist an initial transition of a flapping-wing system during rapid maneuvers, which needs a strong control force. This suggests that at least for a mean sweeping angle, the effect of wakes must be taken into account.

Entropy and fractal perspectives of a flapping wing subjected to gust

Studies on entomopter’s performance under the influence of gust have received impetus in the past decade. There exists a dire need to ascertain the threshold of the frontal gusty conditions which would destabilize these anthropogenic flyers. This would help to devise methods to mitigate the detrimental effects of gust. In light of this aspect, the present study aims at analyzing the onsets of instability in a flapping wing system subjected to temporal gust by employing recurrence period density entropy (RPDE) and detrended fluctuation analysis (DFA). Simulation of the flapping wing along inclined stroke is carried out for a Reynolds number of 150. This Reynolds number lies in the typical operating regime of fruit flies and entomopters like the Pico aerial vehicle. Numerical simulations are carried out to solve the laminar, unsteady, and incompressible Navier–Stokes equations. The dynamic meshing technique is employed to model flapping kinematics. Nine gusts with a combination of frequency and velocity ratios of 0.1, 0.5, and 1.0 are considered. Instantaneous horizontal and vertical forces are estimated. Time series of these forces are analyzed using RPDE and DFA paradigms. These analyses indicate that gust frequency of an order of magnitude higher than flapping frequency and gust amplitude of the order of magnitude as the wing’s root mean square velocity induces a possible onset of instability.

Phenomenology and scaling of optimal flapping wing kinematics

Biological flapping wing fliers operate efficiently and robustly in a wide range of flight conditions and are a great source of inspiration to engineers. The unsteady aerodynamics of flapping wing flight are dominated by large-scale vortical structures that augment the aerodynamic performance but are sensitive to minor changes in the wing actuation. We experimentally optimise the pitch angle kinematics of a flapping wing system in hover to maximise the stroke average lift and hovering efficiency with the help of an evolutionary algorithm and in situ force and torque measurements at the wing root. Additional flow field measurements are conducted to link the vortical flow structures to the aerodynamic performance for the Pareto-optimal kinematics. The optimised pitch angle profiles yielding maximum stroke-average lift coefficients have trapezoidal shapes and high average angles of attack. These kinematics create strong leading-edge vortices early in the cycle which enhance the force production on the wing. The most efficient pitch angle kinematics resemble sinusoidal evolutions and have lower average angles of attack. The leading-edge vortex grows slower and stays close-bound to the wing throughout the majority of the stroke-cycle. This requires less aerodynamic power and increases the hovering efficiency by 93% but sacrifices 43% of the maximum lift in the process. In all cases, a leading-edge vortex is fed by vorticity through the leading edge shear layer which makes the shear layer velocity a good indicator for the growth of the vortex and its impact on the aerodynamic forces. We estimate the shear layer velocity at the leading edge solely from the input kinematics and use it to scale the average and the time-resolved evolution of the circulation and the aerodynamic forces. The experimental data agree well with the shear layer velocity prediction, making it a promising metric to quantify and predict the aerodynamic performance of the flapping wing hovering motion.

Towards Bio-Inspiration, Development, and Manufacturing of a Flapping-Wing Micro Air Vehicle

Throughout the last decade, there has been an increased demand for intricate flapping-wing drones with different capabilities than larger drones. The design of flapping-wing drones is focused on endurance and stability, as these are two of the main challenges of these systems. Researchers have recently been turning towards bioinspiration as a way to enhance aerodynamic performance. In this work, the propulsion system of a flapping-wing micro air vehicle is investigated to identify the limitations and drawbacks of specific designs. Each system has a tandem wing configuration inspired by a dragonfly, with wing shapes inspired by a bumblebee. For the design of this flapping-wing, a sizing process is carried out. A number of actuation mechanisms are considered, and two different mechanisms are designed and integrated into a flapping-wing system and compared to one another. The second system is tested using a thrust stand to investigate the impact of wing configurations on aerodynamic force production and the trend of force production from varying flapping frequency. Results present the optimal wing configuration of those tested and that an angle of attack of two degrees yields the greatest force production. A tethered flight test is conducted to examine the stability and aerodynamic capabilities of the drone, and challenges of flapping-wing systems and solutions that can lead to successful flight are presented. Key challenges to the successful design of these systems are weight management, force production, and stability and control.

Extended State Observer based Attitude Control of a Bird-like Flapping-wing Flying Robot

The attitude control system of a flapping-wing flying robot plays an important role in the precise orientation and tracking of the robot. In this paper, the modeling of a bird-like micro flapping-wing system is introduced, and the design of a sliding mode controller based on an Extended State Observer (ESO) is described. The main design difficulties are the control law and the adaptive law for the attitude control system. To address this problem, a sliding mode adaptive extended state observer algorithm is proposed. Firstly, a new extended state approximation method is used to estimate the final output as a disturbance state. Then, a sliding mode observer with good robustness to the model approximation error and external disturbance is used to estimate the system state. Compared with traditional algorithms, this method is not only suitable for more general cases, but also effectively reduces the influence of the approximation error and interference. Next, the simulation and experiment example is given to illustrate the implementation process. The results show that the algorithm can effectively estimate the state of the attitude control system of the flapping-wing flying robot, and further guarantee the robustness of the model regarding error and external disturbance.

Scaling of the performance of insect-inspired passive-pitching flapping wings.

Flapping flight using passive pitch regulation is a commonly used mode of thrust and lift generation in insects and has been widely emulated in flying vehicles because it allows for simple implementation of the complex kinematics associated with flapping wing systems. Although robotic flight employing passive pitching to regulate angle of attack has been previously demonstrated, there does not exist a comprehensive understanding of the effectiveness of this mode of aerodynamic force generation, nor a method to accurately predict its performance over a range of relevant scales. Here, we present such scaling laws, incorporating aerodynamic, inertial and structural elements of the flapping-wing system, validating the theoretical considerations using a mechanical model which is tested for a linear elastic hinge and near-sinusoidal stroke kinematics over a range of scales, hinge stiffnesses and flapping frequencies. We find that suitably defined dimensionless parameters, including the Reynolds number, Re, the Cauchy number, Ch, and a newly defined 'inertial-elastic' number, IE, can reliably predict the kinematic and aerodynamic performance of the system. Our results also reveal a consistent dependency of pitching kinematics on these dimensionless parameters, providing a connection between lift coefficient and kinematic features such as angle of attack and wing rotation.

Aerodynamic characteristics of flapping wings under steady lateral inflow

This experimental study investigates the effect of a uniform lateral inflow on the aerodynamic characteristics of flapping wings. Seven designated sideward ratios in the hovering condition and in the presence of a contralateral wing and a body were taken into account as variables in order to secure a better understanding of wing–wing and/or wing–body interactions under the lateral inflow. Our results from the single-wing cases clarified that an inflow running from the wingroot strengthened the leading-edge vortex, thereby augmenting the aerodynamic force/moment. The inflow running in the opposite direction drastically bent the leading-edge vortex to the trailing edge, but the cycle-averaged aerodynamic force/moment was barely changed. This led to substantial imbalances in the force/moment on the two wings. The roll moment on a centre of gravity and the static margin suggested flight instability in the lateral direction, similar to previous studies. We found that the wing–wing interaction was not completely negligible overall under a lateral inflow. A massive downwash induced by the wing on the windward side nearly neutralized the aerodynamic force/moment augmentations on the other wing with lower effective angles of attack. The wing–wing interaction also gave rise to a low-lift high-drag situation during the pitching-up wing rotation, resulting in greater side force derivatives than the theory of flapping counterforce. Further calculations of the roll moment and the static margin with the centre of gravity showed that the wing–wing interaction can improve static stability in the lateral direction. This mainly stemmed from both the attenuation of the lift augmentation and the elimination of the positive roll moment of the flapping-wing system.

Design and demonstration of a bio-inspired flapping-wing-assisted jumping robot

Jumping insects such as fleas, froghoppers, grasshoppers, and locusts take off from the ground using a catapult mechanism to push their legs against the surface of the ground while using their pairs of flapping wings to propel them into the air. Such combination of jumping and flapping is expected as an efficient way to overcome unspecified terrain or avoid large obstacles. In this work, we present the conceptual design and verification of a bio-inspired flapping-wing-assisted jumping robot, named Jump-flapper, which mimics jumping insects’ locomotion strategy. The robot, which is powered by only one miniature DC motor to implement the functions of jumping and flapping, is an integration of an inverted slider-crank mechanism for the structure of the legs, a dog-clutch mechanism for the winching system, and a rack-pinion mechanism for the flapping-wing system. A prototype of the robot is fabricated and experimentally tested to evaluate the integration and performance of the Jump-flapper. This 23 g robot with assisted flapping wings operating at approximately 19 Hz is capable of jumping to a height of approximately 0.9 m, showing about 30% improvement in the jumping height compared to that of the robot without assistance of the flapping wings. The benefits of the flapping-wing-assisted jumping system are also discussed throughout the study.

Bio-Inspired Flexible Flapping Wings with Elastic Deformation

Over the last decades, there has been great interest in understanding the aerodynamics of flapping flight and development of flapping wing Micro Air Vehicles (FWMAVs). The camber deformation and twisting has been demonstrated quantitatively in a number of insects, but making artificial wings that mimic those features is a challenge. This paper reports the development and characterization of artificial wings that can reproduce camber and twisting deformations. By replacing the elastic material at the wing root vein, the root vein would bend upward and inward generating an angle of attack, camber, and twisting deformations while the wing was flapping due to the aerodynamic forces acting on the wing. The flapping wing apparatus was employed to study the flexible wing kinematics and aerodynamics of real scale insect wings. Multidisciplinary experiments were conducted to provide the natural frequency, the force production, three-dimensional wing kinematics, and the effects of wing flexibility experienced by the flexible wings. The results have shown that the present artificial wing was able to mimic the two important features of insect wings: twisting and camber generation. From the force measurement, it is found that the wing with the uniform deformation showed the higher lift/power generation in the flapping wing system. The present developed artificial wing suggests a new guideline for the bio-inspired wing of the FWMAV.