Bio-inspired Flapping Wing Micro Research Articles

Fluid-structure interaction for aerodynamic performance evaluation of flapping wing with passive pitching motion

The aim of this study is to investigate the impact of locust-like wing flexibility which is modelled with Mylar and carbon fibre pultruded rods on aerodynamic performance. An in-house 3D Navier Stokes solver coupled with a structure solver has been used to evaluate passive pitching motion and aerodynamic performance. The study has focused on two effects which are variations in the diameter of root and spar distribution. All wings have been analysed for a prescribed flapping motion in a horizontal plane, with a peak-to-peak stroke amplitude of 120°, flapping frequency of 25 Hz, and Reynolds number of 13,500. The flow has been assumed to be laminar, viscous, and incompressible. The diameter of the wing root segment attached to the leading edge has been varied to control the extent of the passive pitching motion. The effect of spar distribution on the aerodynamic performance of the flexible flapping wing has also been investigated. The results revealed that careful selection of wing root parameters and spars distribution can achieve a passive pitch angle with an amplitude of 45° and the highest power economy of 1.26. Moreover, a comparison of the flexible wing with its rigid counterpart suggested that flexible wings modelled with a simple passive pitching mechanism can be an efficient solution for bio-inspired flapping-wing micro aerial vehicles.

Wing Kinematics-Based Flight Control Strategy in Insect-Inspired Flight Systems: Deep Reinforcement Learning Gives Solutions and Inspires Controller Design in Flapping MAVs.

Flying insects exhibit outperforming stability and control via continuous wing flapping even under severe disturbances in various conditions of wind gust and turbulence. While conventional linear proportional derivative (PD)-based controllers are widely employed in insect-inspired flight systems, they usually fail to deal with large perturbation conditions in terms of the 6-DoF nonlinear control strategy. Here we propose a novel wing kinematics-based controller, which is optimized based on deep reinforcement learning (DRL) to stabilize bumblebee hovering under large perturbations. A high-fidelity Open AI Gym environment is established through coupling a CFD data-driven aerodynamic model and a 6-DoF flight dynamic model. The control policy with an action space of 4 is optimized using the off-policy Soft Actor-Critic (SAC) algorithm with automating entropy adjustment, which is verified to be of feasibility and robustness to achieve fast stabilization of the bumblebee hovering flight under full 6-DoF large disturbances. The 6-DoF wing kinematics-based DRL control strategy may provide an efficient autonomous controller design for bioinspired flapping-wing micro air vehicles.

Vorticity dynamics of fully developed leading-edge vortices on revolving wings undergoing pitch-up maneuvers

In nature, birds and insects usually execute pitch-up maneuvers, which is either an active perching or a passive response against gusts. During such maneuvers, their wings flap with a concomitant nose-up rotation around an axis, and thus, both vortex structures and aerodynamic forces of the wings are influenced. This research focuses on the impact of pitch-up maneuvers on the evolution and underlying vorticity dynamics of a fully developed leading-edge vortex (LEV), which has received limited interest in previous research. Based on data obtained from numerical simulations, an analysis of vortex dynamics and vorticity transport is conducted at different pitch rates and pitch axis locations. Our findings show that an increase in pitch rate and a shift of pitch axis toward the trailing edge can both terminate the growth of LEV and then initiate its movement toward the trailing edge via strong downward convection. However, the contributions of spanwise convection and vortex stretching (or compression) are distinct in these two scenarios, leading to different lift generations. Other vortex-tilting-based mechanisms, e.g., the planetary vorticity tilting and the dual-stage radial-tangential vortex tilting, are reduced during pitch-up maneuvers. Moreover, a rapid pitch-up around the leading edge is encouraged to maximize the lift during the maneuver, although this should be accompanied by constraints in flight height and kinetic energy when being applied to guide the perching of bio-inspired flapping wing micro air vehicles.

Effect of Wing Membrane Material on the Aerodynamic Performance of Flexible Flapping Wing

Flexible deformation of the insect wing has been proven to be beneficial to lift generation and power consumption. There is great potential for shared research between natural insects and bio-inspired Flapping wing Micro Aerial Vehicles (FWMAVs) for performance enhancement. However, the aerodynamic characteristics and deformation process of the flexible flapping wing, especially influenced by wing membrane material, are still lacking in-depth understanding. In this study, the flexible flapping wings with different membrane materials have been experimentally investigated. Power input and lift force were measured to evaluate the influence of membrane material. The rotation angles at different wing sections were extracted to analyze the deformation process. It was found that wings with higher elastic modulus membrane could generate more lift but at the cost of more power. A lower elastic modulus means the wing is more flexible and shows an advantage in power loading. Twisting deformation is more obvious for the wing with higher flexibility. Additionally, flexibility is also beneficial to attenuate the rotation angle fluctuation, which in turn enhances the aerodynamic efficiency. The research in this paper is helpful to further understand the aerodynamic characteristics of flexible flapping wing and to design bio-inspired FWMAVs with higher performance.

Should friction losses be included in an electromechanical model of a bioinspired flapping-wing micro aerial vehicle to estimate the flight energetic requirements?

The paper aims to examine the effects of mechanical losses on the performance of a bioinspired flapping-wing micro aerial vehicle (FWMAV) and ways to mitigate them by introducing a novel electromechanical model. The mathematical model captures the effect of a DC gear motor, slider-crank, flapping-wings aerodynamics, and frictional losses. The aerodynamic loads are obtained using a quasi-steady flow model. The parameters of the flight mechanism are estimated using published experimental data which are also used to validate the mathematical model. Incorporating the flapping mechanism friction losses into the mathematical model enables capturing the physics of the problem with higher accuracy, which is not possible with simpler models. It also makes it possible to estimate the aerodynamic energetic requirements. Moreover, the model enabled evaluations of the effects of adding bioinspired elastic elements on the efficiency of the system. Although it is established through experimental studies that the addition of a bioinspired elastic element can improve system efficiency and increase lift generation, the existing mathematical models fail to model and predict such effects. It has been demonstrated that the addition of an elastic element can reduce friction losses in the system by decreasing the internal forces. Optimised parameters for a FWMAV incorporating elastic elements are also obtained.

Aerodynamic analysis of hummingbird-like hovering flight

Flapping wing micro aerial vehicles are studied as the substitute for fixed and rotary wing micro aerial vehicles because of the advantages such as agility, maneuverability, and employability in confined environments. Hummingbird’s sustainable hovering capability inspires many researchers to develop micro aerial vehicles with similar dynamics. In this research, a wing of a ruby-throated hummingbird is modeled as an insect wing using membrane and stiffeners. The effect of flexibility on the aerodynamic performance of a wing in hovering flight has been studied numerically by using a fluid–structure interaction scheme at a Reynolds number of 3000. Different wings have been developed by using different positions and thicknesses of the stiffeners. The chordwise and spanwise flexural stiffnesses of all the wings modeled in this work are comparable to insects of similar span and chord length. When the position of the stiffener is varied, the best-performing wing has an average lift coefficient of 0.51. Subsequently, the average lift coefficient is increased to 0.56 when the appropriate thickness of the stiffeners is chosen. The best flexible wing outperforms its rigid counterpart and produces lift and power economy comparable to a real hummingbird’s wing. That is, the average lift coefficient and power economy of 0.56 and 0.88 for the best flexible wing as compared to 0.61 and 1.07 for the hummingbird’s wing. It can be concluded that a simple manufacturable flexible wing design based on appropriate positioning and thickness of stiffeners can serve as a potential candidate for bio-inspired flapping-wing micro aerial vehicles.

Bio-Inspired Rapid Escape and Tight Body Flip on an At-Scale Flapping Wing Hummingbird Robot Via Reinforcement Learning

Insects and hummingbirds are capable of acrobatic maneuvers such as rapid turns and tight 360 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> body flips. It is challenging for bio-inspired flapping wing micro aerial vehicles to achieve animal-like performance during such maneuvers due to their limitation in mechanism sophistication and flight control. Besides being significantly underactuated compared to their natural counterparts, flight dynamics is highly nonlinear with rapidly changing, unsteady aerodynamics which remains largely unknown during aggressive maneuvers. As a result, conventional model-based control methods are inadequate to address such maneuvers effectively due to the lack of control references and aerodynamic models. In addition, during acrobatic maneuvers such as body flips, conventional control methods with underlying stabilization mechanisms would temporarily contradict the maneuvering requirements when the vehicle undergoes a full body flip including turning upside-down. In this article, reinforcement learning (RL) has been used to complement model-based control to enable animal-like maneuverability. The learned control policy serves in two different ways to either aid or even completely takeover the conventional stabilization controller in certain cases. We experimentally demonstrate animal-like maneuverability on an at-scale, dual-motor actuated flapping wing hummingbird robot. Two test cases have been performed to demonstrate the effectiveness of such integrated control methods: 1) a rapid escape maneuver recorded from hummingbirds, 2) a tight 360 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> body flip inspired by houseflies. By leveraging RL, the hummingbird robot demonstrated a shorter completion time in escape maneuvers compared to the traditional control-based method. It also performed 360 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> body flip successfully within only one wingspan vertical displacement.

The modeling and numerical solution for flapping wing hovering wingbeat dynamics

The production of wingbeat motion of flapping wing hovering flight are determined by the actuating, aerodynamic and inertia forces/moments, which influence the dynamic unsteadiness and controllability of flapping wing flying. This paper presents the feasible solution for cracking the problem of two degrees of freedom (two DoFs, namely, flapping and pitch motion, respectively) highly coupled nonlinear hovering wingbeat dynamics. Firstly, two DoFs nonlinear hovering wingbeat dynamic ordinary differential equations (ODEs) are derived on basis of the extended quasi-steady aerodynamic and inertial forces/moments model. Then, we perform their numerical solution by using tractable ODEs numerical algorithm, boundary value problem-solving format, and least square method. The numerical results have a good consistency with those measured by Dr. Muijres. Moreover, the adjustable rule of phase offset of wing pitch angle relative to the flapping angle is quantificationally studied by introducing frequency ratio between pitch frequency and flapping frequency. We find that the phase offset can be directly regulated by wing pitch hinge stiffness or indirectly modulated by frequency ratio, and the peak value of wing pitch angle monotonously decreases with the increase of wing pitch hinge stiffness, opposite to the angle of attack (AoA). This adjustable rule paves a useful way for the bio-inspired flapping wing micro aerial vehicle (FWMAV) featuring passive or semi-passive pitch flexible hinge to maintain high variable AoA.

Untethered Flight of an At-Scale Dual-motor Hummingbird Robot with Bio-inspired Decoupled Wings

In this letter, we present the untethered flight of an at-scale tailless hummingbird robot with independently controlled wings. It represents the first untethered stable flight of a two actuator powered bio-inspired Flapping Wing Micro Air Vehicle (FWMAV) in both indoor and outdoor environment. The untethered flight of such FWMAVs is a challenging task due to stringent payload limitation from severe underactuation and power efficiency challenge caused by motor reciprocating motion. In this work, we present the detailed modeling, optimization, and system integration of onboard power, actuation, sensing, and flight control to address these unique challenges of such FWMAV during untethered flight. We performed untethered flight experiments in both indoor and outdoor environment and demonstrate sustained stable flight of the robot.

PAC: A novel self-adaptive neuro-fuzzy controller for micro aerial vehicles

There exists an increasing demand for a flexible and computationally efficient controller for micro aerial vehicles (MAVs) due to a high degree of environmental perturbations. In this work, an evolving neuro-fuzzy controller, namely Parsimonious Controller (PAC) is proposed. It features fewer network parameters than conventional approaches due to the absence of rule premise parameters. PAC is built upon a recently developed evolving neuro-fuzzy system known as parsimonious learning machine (PALM) and adopts new rule growing and pruning modules derived from the approximation of bias and variance. These rule adaptation methods have no reliance on user-defined thresholds, thereby increasing the PAC’s autonomy for real-time deployment. PAC adapts the consequent parameters with the sliding mode control (SMC) theory in the single-pass fashion. The boundedness and convergence of the closed-loop control system’s tracking error and the controller’s consequent parameters are confirmed by utilizing the LaSalle–Yoshizawa theorem. Lastly, the controller’s efficacy is evaluated by observing various trajectory tracking performance from a bio-inspired flapping wing micro aerial vehicle (BI-FWMAV) and a rotary wing micro aerial vehicle called hexacopter. Furthermore, it is compared to three distinctive controllers. Our PAC outperforms the linear PID controller and feed-forward neural network (FFNN) based nonlinear adaptive controller. Compared to its predecessor, G-controller, the tracking accuracy is comparable, but the PAC incurs significantly fewer parameters to attain similar or better performance than the G-controller.

Experimental study of a bio-inspired flapping wing MAV by means of force and PIV measurements

Abstract This study explores the aerodynamic performance of an X-Wing bio-inspired flapping wing micro air vehicle (MAV) underlying clap-and-fling motion by means of force and flow field measurements. Wind tunnel tests were first conducted to identify the operation region of current flapping wing MAV. The effects of flapping frequency and wing flexibility on force generation and power loading were evaluated using a miniature six-component force transducer. Flow visualization in the chordwise wake of the flapping wings is measured by means of phase-lock particle image velocimetry (PIV) measurement, results revealed the shedding behavior of the vertical structures by the wings during clap-and-fling motion showed a momentum surplus. Additionally, the in-ground-effect (IGE) is also examined aiming to understand the aerodynamic characteristics during take-off and landing of flapping wing MAV.

Generic Evolving Self-Organizing Neuro-Fuzzy Control of Bio-Inspired Unmanned Aerial Vehicles

In recent times, with the incremental demand for fully autonomous systems, research interests are observed in learning machine-based intelligent, self-organizing, and evolving controllers. In this paper, a new evolving and self-organizing controller, namely generic-controller (G-controller), is proposed. The G-controller works in a fully online mode with minor expert domain knowledge. It is developed by incorporating the sliding mode control (SMC) theory with an advanced incremental learning machine, namely generic evolving neuro-fuzzy inference system. The controller starts operating from scratch with an empty set of fuzzy rule, and therefore, no offline training is required. To cope with the changing dynamic characteristics of the plant, the controller can add or prune the rules on demand. Control law and adaptation laws for the consequent parameters are derived from the SMC algorithm to establish a stable closed-loop system, where the stability of the G-controller is guaranteed by using the Lyapunov function. The uniform asymptotic convergence of tracking error to zero is witnessed through the implication of an auxiliary robustifying control term. In addition, the implementation of the multivariate Gaussian function helps the controller to handle the nonaxis parallel data from the plant and consequently, enhances the robustness against uncertainties and environmental perturbations. Finally, the controller's performance has been evaluated by observing the tracking performance in controlling simulated plants of unmanned aerial vehicle, namely bio-inspired flapping wing micro air vehicle and hexacopter for a variety of trajectories.

Effect of clap-and-fling mechanism on force generation in flapping wing micro aerial vehicles

The clap-and-fling effect, first observed in a number of insects, serves as a lift-enhancing mechanism for bio-inspired flapping wing micro aerial vehicles (MAV). In our comprehensive literature survey, we observe that the effect manifests differently in insects and contemporary MAVs; insects have active control over the angle of attack and stroke plane of the wing, whereas a number of kinematic parameters of an MAV’s flexible wings are determined passively. Although there is consensus that flinging motion significantly enhances aerodynamic lift, the effect of clapping motion is not well-studied. To address this gap, we experimentally quantify the contribution of clapping motion using force measurement and particle image velocimetry. No significant enhancement in lift was observed due to clapping motion, because the momentum jet was too weak. However, the kinematics and flow conditions in our study were notably different from those in the previous studies on insect models. The wings of the MAV are flexible, and deform passively. Hence, the clapping of the trailing edges, and the appearance of a trailing edge momentum jet, was delayed and significantly suppressed. Using force measurement and CFD simulations, it was also found that the lesser the distance between the leading edges of the wings at the end of clap, the higher is the lift due to the subsequent fling.

Bioinspired Flapping-Wing Robot With Direct-Driven Piezoelectric Actuation and Its Takeoff Demonstration

In this letter, we present the design and the performance of a prototype bioinspired flapping-wing micro aerial vehicle with piezoelectric direct-driven actuation. This type of actuation is superior in terms of structural simplicity because it does not include any transmission mechanism. Instead, a unimorph piezoelectric actuator directly drives the wing. We designed and fabricated a two-wing prototype with a wingspan of 114 mm and successfully demonstrated takeoff using an external power source under a one-degree-of-freedom constraint. The total mass of the prototype was 598 mg, and the maximum measured lift force was 6.52 mN (665 mgf) at a driving voltage of 100 V. We achieved tethered flight using the proposed direct-driven piezoelectric actuator considering the constraint.

Control-Oriented Modeling of Coupled Electromechanical-Aeroelastic Dynamics for Flapping-Wing Vehicles

A computationally tractable nonlinear aeroelastic model of a bioinspired flapping-wing micro air vehicle, which includes motor dynamics and is suitable for control evaluation, is developed in this paper. The structural model accounts for geometrically nonlinear deformations using an implicit condensation technique. The aerodynamic loads are computed as a superposition of quasi-steady translation and rotation as well as added mass effects with an unsteady correction. The aeroelastic model is compared to experiments and higher-fidelity computations and shows acceptable agreement. The model is subsequently implemented in open-loop control simulations and demonstrates that motor dynamics and wing aeroelasticity have a significant impact on the loads generated by the wings, potentially introducing challenges in vehicle control.

A review of compliant transmission mechanisms for bio-inspired flapping-wing micro air vehicles

Flapping-wing micro air vehicles (FWMAVs) are a class of unmanned aircraft that imitate flight characteristics of natural organisms such as birds, bats, and insects, in order to achieve maximum flight efficiency and manoeuvrability. Designing proper mechanisms for flapping transmission is an extremely important aspect for FWMAVs. Compliant transmission mechanisms have been considered as an alternative to rigid transmission systems due to their lower the number of parts, thereby reducing the total weight, lower energy loss thanks to little or practically no friction among parts, and at the same time, being able to store and release mechanical power during the flapping cycle. In this paper, the state-of-the-art research in this field is dealt upon, highlighting open challenges and research topics. An optimization method for designing compliant transmission mechanisms inspired by the thoraxes of insects is also introduced.

A novel methodology for wing sizing of bio-inspired flapping wing micro air vehicles: theory and prototype

To design efficient flapping wing micro air vehicles (FWMAVs), a comprehensive sizing method based on theoretical and statistical analyses is proposed and experimentally verified. This method is composed of five steps including defining and analyzing the MAV mission, determining the flying modes, defining the wing shape and aspect ratio of the wing, applying the constraint analysis based on the defined mission, and estimating the weights of the electrical and structural components of the bio-inspired flapping wing micro air vehicle. To define the vehicle mission and flight plan, path analysis is performed based on the defined mission, the speed of cruise and turning, the turning radius and climatic conditions in the flight area. Following the defined mission analysis, the appropriate modes of flying (i.e., flapping, gliding, hovering, bounding, and soaring) for the flapping wing bird are recognized. After that, the wing shape and the wing aspect ratio are determined based on the defined flight modes. To estimate the wing loading, a constraint analysis is exploited in which flight equitation is simulated based on the modes and missions of the flight. Along with the four listed steps, a statistical method is employed to estimate the FWMAV weight for a well-defined mission. Based on the offered method for wing sizing of flapping wings, a FWMAV named Thunder I has been designed, fabricated, and tested. This developed methodology is very beneficial by giving guidelines for the design of efficient bio-inspired FWMAVs.

Optimization of lift force for a bio-inspired flapping wing model in hovering flight

The lift force is an important component that affects the aerodynamic performance of bio-inspired flapping-wing micro aerial vehicles. However, there is a lack of endeavors in the optimization of the flapping wing parameters that affect the lift force of micro aerial vehicles. This research is therefore to establish a methodology that combines computational fluid dynamic simulation for the evaluation of the lift generation and wing parameters. The Taguchi’s framework for design of experiments, combined with the computational fluid dynamics simulations, is performed on a simplified robotic fly wing model used in the experiment found in Dickinson et al. (1999), under the hovering flight mode, to identify the most influential parameters on the lift generation of micro aerial vehicles. A commercial computational fluid dynamics code, ANSYS/FLUENT, along with a three-dimensional Navier–Stokes solver is used to simulate the unsteady flow field. With the optimization of the time-averaged lift force as the optimization objective, five typical parameters (flapping frequency, maximum angle of attacks during the upstroke and downstroke motions, stroke amplitude, and rotation type) in the flapping trajectory equation are selected as the input factors with each having four levels. Specific computational fluid dynamics cases are simulated in accordance with the chosen orthogonal arrays. By conducting statistical analyses with analyses of means and variance, the flapping frequency and the stroke amplitude are determined to be the two most influential parameters. The response surface of the time-averaged lift force and the power consumption contour are constructed with respect to these two parameters to determine the optimal combination for the generation of lift forces under a specific power constrain and provide a guideline for bio-inspired micro aerial vehicle designs.

Experimental Structural Dynamic Characterization of the Hawkmoth (Manduca Sexta) Forewing

While many bio-inspired flapping wing micro air vehicle wing designs continue to be conceived and studied in earnest, a general consensus of which physical attributes of the biological entity are important for flight is still at-large. It is proposed herein that the eigenstructure of the wing should figure prominently among rigorous engineering metrics for guiding flapping wing micro air vehicle wing designs at the scales of large insects. With virtually no compelling work done in this area to date, the method and results of system identification tests for the forewings of a representative sample of hawkmoth ( Manduca Sexta) are presented, revealing the underlying structural nature of this incredibly agile flyer's wings. Despite their inherent biological variability, these wings show very little variability in eigenstructure which may suggest it as a critical attribute for robust flight. Further supporting this hypothesis, the wings of four other insect species are briefly examined and show remarkable similarity with the hawkmoth wing's eigenstructure.