Design and Control of Flapping Wing Micro Air Vehicles

Flapping wing Micro Air Vehicles (MAVs) continues to be a growing field, with ongoing research into unsteady, low Re aerodynamics, micro-fabrication, and fluid-structure interaction. However, research into flapping wing control of such MAVs continues to lag. Existing research uniformly consists of proposed control laws that are validated by computer simulations of quasi-steady blade-element formulae. Such simulations use numerous assumptions and cannot be trusted to fully describe the flow physics. Instead, such control laws must be validated on hardware. Here, a novel control technique is proposed called Bi-harmonic Amplitude and Bias Modulation (BABM) which can generate forces and moments in 5 vehicle degrees of freedom with only two actuators. Several MAV prototypes were designed and manufactured with independently controllable wings capable of prescribing arbitrary wing trajectories. The forces and moments generated by a MAV utilizing the BABM control technique were measured on a 6-component balance. These experiments verified that a prototype can generate uncoupled forces and moments for motion in five degrees of freedom when using the BABM control technique, and that these forces can be approximated by quasi-steady blade-element formulae. Finally, the prototype performed preliminary controlled flight in constrained motion experiments, further demonstrating the feasibility of BABM

DESIGN AND CONTROL OF A HUMMINGBIRD-SIZE FLAPPING WING MICRO AERIAL VEHICLE

Flying animals with flapping wings may best exemplify the astonishing ability of natural selection on design optimization. They evince extraordinary prowess to control their flight, while demonstrating rich repertoire of agile maneuvers. They remain surprisingly stable during hover and can make sharp turns in a split second. Characterized by high-frequency flapping wing motion, unsteady aerodynamics, and the ability to hover and perform fast maneuvers, insect-like flapping flight presents an extraordinary aerial locomotion strategy perfected at small size scales. Flapping Wing Micro Aerial Vehicles (FWMAVs) hold great promise in bridging the performance gap between engineered flying vehicles and their natural counterparts. They are perfect candidates for potential applications such as fast response robots in search and rescue, environmental friendly agents in precision agriculture, surveillance and intelligence gathering MAVs, and miniature nodes in sensor networks

Principle Of Bio-Inspired Insect Wing Rotational Hinge Design

A principle for designing and fabricating bio-inspired miniature artificial insect flapping wing using flexure rotational hinge design is presented. A systematic approach of selecting rotational hinge stiffness value is proposed. Based on the understanding of flapping wing aerodynamics, a dynamic simulation is constructed using the established quasi-steady model and the wing design. Simulations were performed to gain insight on how different parameters affect the wing rotational response. Based on system resonance a model to predict the optimal rotational hinge stiffness based on given wing parameter and flapping wing kinematic is proposed. By varying different wing parameters, the proposed method is shown to be applicable to a wide range of wing designs with different sizes and shapes. With the selected hinge stiffness value, aspects of the rotational joint design is discussed and an integrated wing-hinge structure design using laminated carbon fiber and polymer film is presented. Manufacturing process of such composite structure is developed to achieve high accuracy and repeatability. The yielded hinge stiffness is verified by measurements. To validate the proposed model, flapping wing experiments were conducted. A flapping actuation set up is built using DC motor and a controller is implemented on a microcontroller to track desired wing stroke kinematic. Wing stroke and rotation kinematic were extracted using a high speed camera and the lift generation is evaluated. A total of 49 flapping experiments were presented, experimental data shows good correlation with the model\u27s prediction. With the wing rotational hinge stiffness designed so that the rotational resonant frequency is twice as the stroke frequency, the resulting wing rotation generates near optimal lift. With further simulation, the proposed model shows low sensitivity to wing parameter variation. As a result, giving a design parameter of a flapping wing robot platform, the proposed principle can predict the rotational hinge stiffness that leads to near optimal wing rotation. Further iteration can be done around the selected value and achieve the optimal lift generation

The wake dynamics and flight forces of the fruit fly Drosophila melanogaster

We have used flow visualizations and instantaneous force measurements of tethered fruit flies (Drosophila melanogaster) to study the dynamics of force generation during flight. During each complete stroke cycle, the flies generate one single vortex loop consisting of vorticity shed during the downstroke and ventral flip. This gross pattern of wake structure in Drosophila is similar to those described for hovering birds and some other insects. The wake structure differed from those previously described, however, in that the vortex filaments shed during ventral stroke reversal did not fuse to complete a circular ring, but rather attached temporarily to the body to complete an inverted heart-shaped vortex loop. The attached ventral filaments of the loop subsequently slide along the length of the body and eventually fuse at the tip of the abdomen. We found no evidence for the shedding of wing-tip vorticity during the upstroke, and argue that this is due to an extreme form of the Wagner effect acting at that time. The flow visualizations predicted that maximum flight forces would be generated during the downstroke and ventral reversal, with little or no force generated during the upstroke. The instantaneous force measurements using laser-interferometry verified the periodic nature of force generation. Within each stroke cycle, there was one plateau of high force generation followed by a period of low force, which roughly correlated with the upstroke and downstroke periods. However, the fluctuations in force lagged behind their expected occurrence within the wing-stroke cycle by approximately 1 ms or one-fifth of the complete stroke cycle. This temporal discrepancy exceeds the range of expected inaccuracies and artifacts in the measurements, and we tentatively discuss the potential retarding effects within the underlying fluid mechanics

Bioinspired fluid-structure interaction problems: gusts, load mitigation and resonance

Mención Internacional en el título de doctorNature often serves as a reference for the design and development of sustainable solutions in numerous different fields. The recent development of small-scale robotic vehicles, asMicro-Air Vehicles (MAVs), is not an exception, and has had an increasingly important impact on society, proposing new alternatives in areas as surveillance or planetary exploration. Trying to mimic the flight of insects and small birds, these devices try to offer more efficient designs and with higher manoeuvrability abilities than the already existing designs. It happens similar with robotic swimmers, with many different existing prototypes. Indeed, it is even possible to find designs of bioinspired small-scale wind turbines based on auto-rotating seeds looking for a more efficient energy harvesting. Besides, in order to develop sustainable designs, increasing their lifetime and reducing the maintenance costs are crucial factors. Depending on the device to design, different methodologies may be followed in order to achieve these two goals while meeting the design requirements. One clear example can be found in the development of wind turbines. Their blades must be designed to withstand not only maximum loads and stresses but also the fatigue caused by the fluctuations around the load required to operate correctly. Reducing fatigue issues by limiting the amplitude of those fluctuations using passive or active control is a viable option to improve their lifetime. The aimof this dissertation is to contribute to the understanding of the underlying physics in biolocomotion. To this end, direct numerical simulations of different examples and problems at low Reynolds number, Re, have been performed using an existing fluid-structure interaction (FSI) solver. This FSI solver relies on the coupling of an incompressible-flow solver with robotic algorithms for the computation of the dynamics of a system of connected rigid bodies. The particularities of this solver are detailed in the thesis. The second part of the thesis includes the analysis of these examples and problems mentioned above.More in detail, the aerodynamic and aeroelastic behaviour of airfoils and wings at Re Æ 1000 in various conditions and environments has been analysed. Natural flyers and swimmers are immersed in turbulent and gusty environments which affect their aerodynamic behaviour. The first problem that has been studied is that of the unsteady response of airfoils impacted by vortical gusts. This first example focuses on how the impact of viscous vortices of different size and intensity on two-dimensional airfoils modify their response. Although in a simplified framework, this analysis allows to gather relevant information about the aerodynamic performance of the airfoils. This aerodynamic response is seen to be self-similar, and the work proposes a semi-empirical model to determine the temporal evolution of the lifting forces based on an integral definition of the vertical velocity induced by the gust, which can be known a priori. The target of the second problem is to analyse the load that can be mitigated in airfoils undergoing oscillations in the angle of attack using passive-pitching trailing edge flaps. This corresponds, for example, to a simplification of the problem of load mitigation in small-scale wind turbines. The use of passive-pitching trailing edge flaps is a strategy that has recently been recently proposed for large-scale wind turbines. Here, we investigate the validity of this strategy on a completely different scenario. Contrary to what happens in experiments at higher Reynolds numbers, whose results match the predictions of a quasi-steady linear model when the kinematics are within the range of applicability of this model, the load mitigation obtained in this work differs from the values of this theory. The load mitigated is larger or smaller than the predicted values depending on the amplitude of the oscillations in the angle of attack. However, the results of this work show that an increase in the length of the flap while the chord of the airfoil is kept constant leads to an equal change in the reduction of load, in line with the predictions of the quasi-steady model. The development of vortical structures is clearly affected by the flap when it is sufficiently large, which also involves changes in the dynamics of the flap and the forces seen by the airfoil. The repercussion that several of the variables defining the parametric space have on the aerodynamic behaviour of the foil and the dynamics of the flap are analysed. This allows to gather more information for an appropriate selection of those variables. Finally, the third and fourth problems involve the study of the effects of spanwise flexibility on both isolated wings and pairs of wings arranged in horizontal tandem undergoing flapping motions. The wings are considered to be rectangular flat plates, and the spanwise flexibility is modelled discretizing these flat plates in a finite number of rigid sub-bodies that are connected using torsional springs. The wings are considered to be rigid in the chordwise direction. Isolated spanwise-flexible wings find an optimal propulsive performance when a fluid-structural resonance occurs. At this flexibility, the time-averaged thrust is maximum and twice the value yielded by the rigid case, and the increment in efficiency is around a 15%. Flexibility and the generation of forces are coupled, such that the structural response modifies the development of the vortical structures generated by the motion of the wing, and vice versa. The optimal performance comes from a combination of larger effective angles of attack, properly timed with the pitching motion such that the projection of the forces is maximum, with a delayed development of the vortical structures. Besides, while aspect ratio effects are important for rigid wings, this effect becomes small when compared to flexibility effects when the wings become flexible enough. In fact, while the increase in thrust coefficient for rigid wings with aspect ratio 4 is 1.2 times larger than that provided by rigid wings with aspect ratio equal to 2, the value of this coefficient for resonant wings is twice the value yielded by rigid wings of aspect ratio 4. While forewings of the tandem systems are found to behave similarly to isolated wings, the aeroelastic response of the hindwings is substantially affected by the interaction with the vortices developed and shed by the forewings. This wake capture effect modifies the flexibility at which an optimal propulsive behaviour is obtained. This wake capture effect is analysed through an estimation of the effective angle of attack seen by both forewings and hindwings, linking the optimal behaviour with the maximisation of the effective angle of attack at the right instants. Based on the obtained results, a proof-of-concept study has been carried out analysing the aerodynamic performance of tandem systems made of wings with different flexibility, which suggests that the latter could outperformsystems of equally flexible wings.This thesis has been carried out in the Aerospace Engineering Department at Universidad Carlos III de Madrid. The financial support has been provided by the Universidad Carlos III de Madrid through a PIPF scholarship awarded on a competitive basis, and by the Spanish Ministry of Economy and Competitiveness through grant DPI2016-76151-C2-2-R (AEI/FEDER, UE).Programa de Doctorado en Mecánica de Fluidos por la Universidad Carlos III de Madrid; la Universidad de Jaén; la Universidad de Zaragoza; la Universidad Nacional de Educación a Distancia; la Universidad Politécnica de Madrid y la Universidad Rovira i VirgiliPresidente: José Ignacio Jiménez González.- Secretaria: Andrea Ianiro.- Vocal: Manuel Moriche Guerrer

Closed-Loop Control of Constrained Flapping Wing Micro Air Vehicles

Micro air vehicles are vehicles with a maximum dimension of 15 cm or less, so they are ideal in confined spaces such as indoors, urban canyons, and caves. Considerable research has been invested in the areas of unsteady and low Reynolds number aerodynamics, as well as techniques to fabricate small scale prototypes. Control of these vehicles has been less studied, and most control techniques proposed have only been implemented within simulations without concern for power requirements, sensors and observers, or actual hardware demonstrations. In this work, power requirements while using a piezo-driven, resonant flapping wing control scheme, Bi-harmonic Amplitude and Bias Modulation, were studied. In addition, the power efficiency versus flapping frequency was studied and shown to be maximized while flapping at the piezo-driven system\u27s resonance. Then prototype hardware of varying designs was used to capture the impact of a specific component of the flapping wing micro air vehicle, the passive rotation joint. Finally, closed-loop control of different constrained configurations was demonstrated using the resonant flapping Bi-harmonic Amplitude and Bias Modulation scheme with the optimized hardware. This work is important in the development and understanding of eventual free-flight capable flapping wing micro air vehicle

Power Requirements for Bi-harmonic Amplitude and Bias Modulation Control of a Flapping Wing Micro Air Vehicle

Flapping wing micro air vehicles (FWMAV) have been a growing field in the research of micro air vehicles, but little emphasis has been placed on control theory. Research is ongoing on how to power FWMAVs where mass is a major area of concern. However, there is little research on the power requirements for the controllers to manipulate the wings of a FWMAV. A novel control theory, BABM, allows two actuators to produce forces and moments in five of the FWMAV\u27s six DOF. Several FWMAV prototypes were constructed and tested on a six-component balance. Data was collected for varying control parameters and the generated forces were measured. The results mapped control parameters to different degrees of freedom. The force required to generate desirable motion and power required to generate that motion was plotted and evaluated. These results can be used to generate a minimum power controller in the future. The results showed that BABM control required a 26% increase in power in order to increase lift by 22%. The lift increase was accomplished by increasing the amplitude by 10% over the established baseline. The data also showed that varying some parameters actually decreased the power requirements, allowing other parameters to increase which in turn would enable more complex maneuvers. For instance, an asymmetric change in split-cycle shift of + or - 0.25 decreased the power required by 14% and decreased the lift by 25%. Changing the stroke bias to + or - 0.75 had a negligible effect on power but decreased the lift by 27%. Furthermore, the data identified certain parameter combinations which resulted in other forces and moments. These results identified how BABM be used as a control theory for the control of FWMAVs

Roll, Pitch and Yaw Torque Control for a Robotic Bee

In the last decade, the robotics community has pushed to develop increasingly small, autonomous flapping-wing robotic vehicles for a variety of civilian and military applications. The miniaturization of these vehicles has pushed the boundaries of technology in many areas, including electronics, artificial intelligence, and mechanics; as well as our understanding of biology. In particular, at the insect scale, fabrication, actuation, and flight control of a flapping-wing robot become especially challenging. This thesis addresses these challenges in the context of the “RoboBee” project, which has the goal of creating an autonomous swarm of at-scale robotic bees. A 100mg robot with a 3cm wingspan capable of generating roll, pitch and yaw torques in the range of $\pm 1\mu Nm$ by using a large, central power actuator to flap the wings and smaller control actuators to steer is presented. A dynamic model is used to predict torque generation capabilities, and custom instrumentation is developed to measure and characterize the vehicle’s control torques. Finally, controlled flight experiments are presented, and the vehicle is capable of maintaining a stable pitch and roll attitude during ascending vertical flight. This is the first successful controlled flight of a truly insect-scale flapping-wing robot.Engineering and Applied Science

Design and Performance of Insect-Scale Flapping-Wing Vehicles

Micro-air vehicles (MAVs)—small versions of full-scale aircraft—are the product of a continued path of miniaturization which extends across many fields of engineering. Increasingly, MAVs approach the scale of small birds, and most recently, their sizes have dipped into the realm of hummingbirds and flying insects. However, these non-traditional biologically-inspired designs are without well-established design methods, and manufacturing complex devices at these tiny scales is not feasible using conventional manufacturing methods. This thesis presents a comprehensive investigation of new MAV design and manufacturing methods, as applicable to insect-scale hovering flight. New design methods combine an energy-based accounting of propulsion and aerodynamics with a one degree-of-freedom dynamic flapping model. Important results include analytical expressions for maximum flight endurance and range, and predictions for maximum feasible wing size and body mass. To meet manufacturing constraints, the use of passive wing dynamics to simplify vehicle design and control was investigated; supporting tests included the first synchronized measurements of real-time forces and three-dimensional kinematics generated by insect-scale flapping wings. These experimental methods were then expanded to study optimal wing shapes and high-efficiency flapping kinematics. To support the development of high-fidelity test devices and fully-functional flight hardware, a new class of manufacturing methods was developed, combining elements of rigid-flex printed circuit board fabrication with "pop-up book" folding mechanisms. In addition to their current and future support of insect-scale MAV development, these new manufacturing techniques are likely to prove an essential element to future advances in micro-optomechanics, micro-surgery, and many other fields.Engineering and Applied Science