Current Research Projects

Aerodynamics of Flexible Wings

Despite the recent surge of interest in flapping flight mechanics, very little is known about how wing flexion influences aerodynamic forces on flapping wings. We investigated this question using a dynamically scaled mechanical model of flapping wings and systematically varied the flexural stiffness of wings over two orders of magnitude and constructed the resulting aerodynamic polar plots. These experiments reveal the strong influence that wing flexion exercises upon the magnitude and direction of forces during flapping. While translating at constant angles of attack, the net aerodynamic force coefficient on a flapping wing increased as the wing got stiffer. Furthermore, we explored the structural advantages of wing veins in aerodynamic force production by reinforcing the most flexible wing with a rudimentary framework of veins. Even a very basic framework of appropriately placed veins substantially enhanced the aerodynamic performance of flexible wings. Flow vorticity and velocity fields were obtained through particle image velocimetry (PIV) experiments. This study characterizes several variables including material properties, kinematics, aerodynamic forces and center of pressure and can be hence useful for validation of computational models of aeroelastic flapping wings and in design of small robotic insect wings. It also provides insight about the function of actual insect wings. (pdf)

Video of dragonfly model wings in motion

Video of flow and vorteces in dragonfly hovering

Wing-Wing Interactions in Dragonflies

Dragonflies move each wing independently and therefore may alter the phase difference between the forewing and hindwing stroke cycles. They are observed to employ certain phase difference under different flight conditions. We investigated the aerodynamic effect of phase difference during hovering and forward flight with highly inclined stroke planes and drag-based lift wing kinematics, characteristic of dragonfly flight. This is achieved using a pair of dynamically scaled robotic dragonfly model wings. Aerodynamic forces were measured while phase difference was systematically varied. The results showed that, i) for hovering flight, 0° enhances the lift force on the forewing by 17%; a phase difference of 180° reduces the total lift, but is beneficial for vibration suppression and body stabilization. As observed in real dragonflies, 0° is used in acceleration mode while 180° is used in hovering mode. ii) For forward flight, wing-wing interaction always enhance forewing lift while reduce hindwing lift; the total lift was only slightly reduced for 0~90° and significantly decreased by 38% at 270° phase differences . This result may explain why dragonflies employ 50~100° during forward flight, while 270° is never favored. We further investigated this using flow visualization and PIV system and the results show large downwash brought by forewing which is responsible for the lift reduction on hindwing and an upwash brought by hindwing which enhance the lift on forewing. (pdf)

Dynamics and Passive Damping in Flapping Flight

We investigated the effect of body rotation on the aerodynamic force and torque generation on flapping wings during fast turning maneuvers (body saccades) in the fruit fly Drosophila. A quasi-steady aerodynamic analysis with symmetrically flapping wings under turning maneuvers showed that body rotation causes a substantial aerodynamic counter torque, termed flapping counter torque (FCT), which acts in the opposite direction of turning. From the force measurements on a dynamically scaled robotic wing with fruitfly saccade wing kinematics, we found that passive aerodynamic damping due to FCT account for a large part of the deceleration during saccade, while active yaw torque from asymmetric wing motion may help to terminate the turning. We estimated the damping coefficient s for the principle turning axes in stroke plane coordinate frame and body centered coordinate frame. Our results show that FCT induced passive damping exist in all these axes, and the aerodynamic effect of body velocity cannot be ignored in the analysis of free flight dynamics. Furthermore, our results suggest that aerodynamic damping in flapping flight exists in both rotational and translational dynamics, and it works to reduce the relative velocity between the fly and air. (pdf)

Furthermore, we found that flying animals of body sizes ranging from fruit flies to large birds benefit from substantial damping of angular velocity through FCT. Our FCT model predicts that isometrically scaled animals experience similar damping on a per-wingbeat timescale, resulting in similar turning dynamics in ‘wingbeat time’ regardless of body size. The model also shows how animals may simultaneously specialize in both maneuverability and stability (at the cost of efficiency) and provides a framework for linking morphology, wing kinematics, maneuverability and flight dynamics across a wide range of animals spanning insects, bats and birds. (pdf and supplementary materials)

Video of first generation dragonfly robot

Video of third generation dragonfly robot

Fourth generation 20 Hz with wings

Fifth generation 40 Hz with wings

(under test, email if interested)

Dragonfly Robot Development

Dragonflies demonstrate unique and superior flight performances than most of the other flyinginsect species. They are equipped with two pairs of independently controlled wings. The high level of dexterity in wing motion of the dragonfly allows it to hover, fly fast forward, make turns rapidly, fly sideways, and even glide. A dragonfly-inspired robot which could effectively mimic those kinematics would potentially exhibit superior flight performance than existing designs of insect robots. So far, we have developed three generations of robotic dragonfly prototypes with preliminary experiments on kinematics and aerodynamic force measurements. The phase difference of the forewing and hindwing can be varied by a coupler link in the robot mechanism. To simplify the design, one motor is driving both wings to achieve the desired flapping angle, while angle of attack is achieved through wing passive rotation. Kinematics and aerodynamic forces are calculated analytically and measured on robot experiments.

Current work include minimizing the size and weight of the robot and increasing the flapping frequency, in order to achieve lift generation which can sustain its weight. Our third generation model is 4 grams including batteries and electronics and beats at 10Hz. Long term goals of the robot prototype development also include dynamic variable phase difference design during flight. (pdf)

Our latest model (fifth generation) is 2 grams and is up to 90 Hz without wings and 40 Hz with wings. Video available soon.

Hydrodynamics, Stability and Control of Boxfish Robot

Boxfish with multiple fins can maneuver in confined spaces with a near zero turning radius and it has been found that its unusual boxy shape is responsible for a self correcting mechanism that makes its trajectories immune to water disturbances. We developed an autonomous boxfish robot with multiple fins to serve as a platform to study its stability and controls. Micro underwater vehicles with these characteristics have a variety of applications, such as environmental monitoring, ship wreck exploration, inline pipe inspection, forming sensor networks, etc. Fin hydrodynamics have been investigated experimentally using robotic flapper mechanisms to arrive at a caudal fin shape with optimal shape induced flexibility. Fluid simulation studies were utilized to arrive at the body shape that can result in self correcting vorticity generation. Finally the robotic ostraciiform prototype was designed based on the above results. Ostracifform locomotion is implemented with a pair of 2-DOF pectoral fins and a single DOF tail fin. The finalized body shape of the robot is produced by 3d prototyping two separate halves. (pdf)

Our current research on the second generation robotic boxfish involves developing a tethered fish with sensors such as gyroscope and accelerometers onboard to investigate sensory feedback controls. In this fish we use flexible pectoral fins. Attitude stabilization and control is achieved via asymmetric fin motions. Body shape induced dynamic stability is observed from the counter rotating vortex generation and shedding around the keels of the boxfish through flow visualization and PIV results.

Video of fly recovering from upside-down

Flight Control Algorithms

By subtly adjusting wing kinematics, insects are able to conduct elaborate flight maneuvers and achieve fast stabilizations of its body posture. It is challenging to study the control mechanisms adapted by insects and design bio-inspired flight controllers for man-made systems. Insect flight control involves fast synthesis of various sensory information, lower level sensorimotor reflex, and higher level central nervous system. Previously, we have designed linear controllers based on the linearized average system near trim conditions, using the averaging theory in nonlinear system analysis. Currently, we developed a neural adaptive controller for the attitude control of flapping wing micro aerial vehicles inspired by insect flight. Lyapuov based stability analysis shows asymptotic convergence of control outputs. Simulation results based on a 6 DOF nonlinear time-varying dynamic model are presented in position regulation and trajectory tracking cases. Model parameters range from fruit fly Drosophila melanogaster to honey bee Apis mellifera morphological data, resembling the size and morphological parameters of robotic insects currently under development. Trajectory tracking was demonstrated by two cases: a saccadic turning and a sinusoidal variation of yaw angle. The proposed controller successfully regulates flight by generating desired torques using proper parameterized wing motions. Furthermore, our results show similarities between the simulated turning and free-flight turning in real insects, revealing some inherent properties of insect flight dynamics and control. (pdf)

Bio-inspired Multimodal Sensor Fusion

We developed a sensor fusion algorithm for the attitude estimation of Micro Aerial Vehicles (MAVs) inspired by insect flight. First, a dynamic observer is proposed which estimates attitude based on kinematic data available from different and redundant bio-inspired sensors such as halteres, ocelli, gravitometers, magnetic compass and light polarization compass. In particular, following a geometric approach, the traditional structure of complementary filters, suitable for multiple sensors fusion, is specialized to the Lie group of rigid body rotations SO(3) and almost global asymptotic stability is proved. Then, the filter performance is experimentally tested via a 3 DOF robotic flapper and robotic fish and a custom-made set of inertial/magnetic sensors. Experimental results show good agreement, upon proper tuning of the filter, between the actual kinematics of the robotic flapper and the kinematics reconstructed from the inertial/magnetic sensors via the proposed filter. Finally, we develop flight control based on the sensor feedback. The dynamic orientation estimation problem is decoupled from the attitude control problem via the separation principle. Similarly, we adopt a geometric approach which allows operating directly on the Lie Group of rigid body orientations SO(3) and proving almost-global stability of the proposed controller. Simulation results highlight the robustness of the proposed attitude controller in the case of sudden disturbances affecting the orientation sensors. (pdf)