Design and testing methodologies for UAVs under extreme environmental conditions

L'abstract è presente nell'allegato / the abstract is in the attachmen

Experimental and numerical analysis of hovering multicopter performance in low-Reynolds number conditions

Unmanned Aircraft Systems (UAS) are state of the art in the aerospace industry and are involved in many operations. Although initially developed for military purposes, commercial applications of small- scale UAS, such as multicopters, are abundant today. Accurate engineering tools are required to assess the performance of these vehicles and optimize power consumption. The thrust and power curves of the rotors used by small-scale UAS are essential elements in designing efficient aircraft. The scarcity of experimental data and sufficiently accurate prediction models to evaluate rotor aerodynamic performance in the flight envelope are primary limitations in UAS science. In addition, for small-scale rotors at usual rotation rates, chord-based Reynolds numbers are typically smaller than 100,000, a flow regime in which performance tends to degrade. In this paper, experimental data on small-scale multicopter propulsion systems are presented and combined with a Computational Fluid Dynamics (CFD) model to describe the aerodynamics of these vehicles in low Reynolds numbers conditions. We use the STAR-CCM+ software to perform CFD simulations adopting both a dynamic-grid, time-accurate analysis and a static-grid, steady- state technique that solves the Navier-Stokes equations in a suitable framework. Comparing numerical simulation results on a conventional UAS propeller with related experimental data suggests that the proposed approach can correctly describe the thrust and torque coefficients in the range of Reynolds numbers characterizing the UAS flight envelope

Design and Realization of an Unmanned Aerial Rotorcraft Vehicle Using Pressurized Inflatable Structure

Unmanned aerial rotorcraft vehicles have many military, commercial and civil applications. There is a necessity to advance the performance on several ranges of rotorcraft for using these vehicles successfully in the expanded future roles. A lower flight time, noise disturbance and safety issues remain the key obstacles in increasing the efficiency of the rotorcraft for various applications. This work presents the design and realization of a rotorcraft using pressurized inflatable structure filled with lighter than air gas such as helium or hydrogen to provide lift assistance for the vehicle. Two iterative design procedures were developed for designing the vehicle. One is based on the net weight of the vehicle and the other based on the diameter of the pressurized structure. Fabrication of a design based on the diameter of the pressurized structure is analysed and evaluated. Gross static lift, the correlation between the size of the inflatable structure and lift force produced, lifting gas properties in the flight range, stress on the structure, and the maximum achievable altitude is also discussed. The vehicle possesses the potential to overcome some inherent limitations of the current unmanned aerial rotorcraft vehicles. This work holds an excellent prospect for future research and more isolated development in all the applications this particular system can be employed

멀티로터형 비행체의 공력소음: 비행 제어 시스템과 공기역학적 상호작용의 영향

학위논문(박사) -- 서울대학교대학원 : 공과대학 항공우주공학과, 2022. 8. 이수갑.Multirotor configurations using a distributed electric propulsion (DEP) system have different aerodynamic and aeroacoustic characteristics from conventional rotorcrafts. Generally, DEP systems use electric motors to control the rotational speed (revolutions per minute, RPM) of individual rotors to perform flight control. Besides, aerodynamic interactions between multiple rotors occur significantly. The main objective of this study is prediction-based evaluations of RPM-controlled multirotor noise. Therefore, three numerical studies are conducted from the perspective of the flight control system and aerodynamic interactions. First, a comprehensive multirotor noise assessment (CONA) framework is developed for real-time noise prediction and psychoacoustic analyses of RPM-controlled multirotor configurations. The CONA framework utilizes flight control, aerodynamics, tonal and broadband noise prediction, and psychoacoustics modules. By this framework, it is possible to conduct the real-time noise assessment in actual flight environments considering the mission profile and gusty wind conditions. A high-resolution time-frequency analysis technique is introduced to analyze the frequency and amplitude modulation characteristics of rotor tonal noise. The Griffin-Lim algorithm is used for the phase reconstruction for time signal synthesis of predicted rotor broadband noise in the 1/3 octave band. Using the CONA framework, the noise of quadrotor configurations is analyzed in representative mission profiles, such as cruise, takeoff, and loitering flights. As flight parameters, flight speed, wind speed, and quadrotor flight type are selected, and the effects of each parameter on acoustic signatures are evaluated. Second, wake interaction effects of multirotor configurations are analyzed by developing the MultiPA framework based on the free-wake vortex lattice method. The aerodynamic and aeroacoustic performance of individual rotors is compared with that of a single rotor with RPM, forward velocity, and incidence angle as variables in two flight types of the quadrotor. Besides, induced circulation is introduced to analyze wake interactions quantitatively. Wake interaction effects are divided into wake-, rotor-, and motion-induced circulation. By circulation analyses, it is quantitatively confirmed that wake effects depend on the flight conditions and rotor topology. Finally, numerical techniques are developed to simulate the torque ripple in the hovering flight of multirotor configurations. In the MultiPA framework, a periodic RPM signal is applied to the numerical analysis. In the CONA framework, a statistical technique that introduces a periodic random RPM signal is used to implement uncertainties in torque ripple numerically. Based on the results of each framework, the effects of torque ripple are illustrated in aerodynamic and aeroacoustic characteristics. The implications can be derived that torque ripple should be considered in the noise assessment of RPM-controlled multirotor configurations using an electric motor. The frameworks developed in this study are specialized in analyzing the unique aerodynamic and aeroacoustic characteristics according to the flight control system and wake interaction effects of DEP systems. The entire process of the CONA framework can be utilized for various multirotor configurations to perform real-time noise prediction and noise impact assessment. The MultiPA framework and induced circulation concepts can be utilized to analyze the wake interaction effect and develop efficient wake models of multirotor configurations. This study illustrates the effects of the flight control system and wake interactions from various perspectives. It is expected that the research of low-noise and high-efficient urban air mobility will be possible through perception-based evaluations using the developed frameworks.멀티로터형 비행체는 분산 전기 추진(Distributed electric propulsion, DEP) 시스템을 활용하여 기존 회전익기와는 다른 공력 및 공력 소음 특성을 가진다. 일반적으로 DEP 시스템은 전기 모터를 이용하여 개별 로터의 회전속도(revolutions per minute, RPM)를 제어하여 비행 제어를 수행하며, 다수의 로터 사이에 공기역학적 상호작용이 뚜렷하게 발생한다. 본 연구의 주된 목적은 RPM 제어 멀티로터 소음의 해식 기반 평가이다. 따라서, 세 가지 수치적 연구가 비행 제어 시스템과 공기역학적 상호작용의 측면에서 수행되었다. 먼저, RPM 제어 멀티로터의 실시간 소음 예측과 심리음향학적 분석을 위한 CONA (Comprehensive multirotor noise assessment) 프레임워크를 개발하였다. CONA 프레임워크는 비행 제어, 공기역학, 로터 톤 및 광대역 소음 해석, 심리음향 해석 모듈을 활용하며, 멀티로터형 비행체의 임무 형상과 대기 바람 조건을 부여한 실제 비행 환경에서의 실시간 소음 해석이 가능하다. 로터 톤 소음의 주파수 및 진폭 변조 특성을 분석하기 위해 고해상도 시간-주파수 분석 기법을 도입하였고, 1/3 옥타브 밴드로 해석되는 로터 광대역 소음을 시간 신호로 변환하기 위하여 그리핀-림 알고리즘을 활용한 음원 합성을 수행하였다. 개발한 CONA 프레임워크로 대표적인 임무 형상인 순항, 수직 이륙, 선회 비행에서 쿼드로터의 소음 특성을 분석하였다. 비행 변수로 비행 속도, 바람 속도, 쿼드로터 비행 타입을 선정하여 각 변수의 공력 소음 특성에 대한 영향을 평가하였다. 두 번째로, 자유 후류 와류 격자 기법 기반의 MultiPA 프레임워크를 개발하여, 멀티로터형 비행체의 후류 상호작용 효과를 분석하였다. 쿼드로터의 두 가지 비행 타입에서 RPM, 전진 속도, 전진각을 변수로 하여 개별 로터의 공력 및 공력 소음 성능을 단일 로터와 비교하였다. 또한, 후류 상호작용 효과를 정량적으로 분석할 수 있는 유도 순환 지표를 도입하였다. 후류 상호작용 효과는 유도 순환 지표를 통해서 후류-유도 순환, 로터-유도 순환, 그리고 로터의 구동에 따른 순환으로 구분되며, 비행 조건과 로터 배치에 따라 후류 영향이 달라짐을 정량적으로 확인하였다. 마지막으로, 멀티로터형 비행체의 제자리 비행 시 발생하는 토크 리플의 수치적 모사 기법을 고안하였다. MultiPA 프레임워크에서는 주기적인 RPM 신호를 해석에 적용하였고, CONA 프레임워크에서는 불확실성이 강한 토크 리플을 수치적으로 구현할 수 있도록 주기적인 무작위 RPM 신호를 도입한 통계적 기법을 활용하였다. 각각의 해석 결과를 바탕으로 토크 리플에 의한 공력 및 공력 소음 특성을 분석하였고, 전기 모터를 활용한 RPM 제어 비행체의 소음 평가 시 토크 리플을 고려해야 한다는 시사점을 도출하였다. 본 연구에서 개발한 프레임워크들은 DEP 시스템의 비행 제어 시스템과 후류 상호작용 효과에 따른 독특한 공력 및 공력 소음 특성을 분석하는 데 특화되어 있다. CONA 프레임워크의 전체 해석 프로세스는 다양한 멀티로터형 비행체에 활용되어 실시간 소음 해석과 소음 영향 평가를 수행할 수 있다. MultiPA 프레임워크와 유도 순환 지표는 후류 상호작용의 효과를 분석하고, 멀티로터형 비행체의 효율적인 후류 모델을 개발하는 데 활용될 수 있다. 본 연구에서는 다양한 관점으로 비행 제어 시스템과 후류 상호작용 효과를 분석하였으며, 개발한 프레임워크를 활용해 인지-기반 평가를 통한 저소음 고효율 도심 항공 모빌리티의 연구를 수행할 수 있을 것으로 기대된다.1 Introduction 1 1.1 Background 1 1.1.1 Multirotor configurations 1 1.1.2 Noise assessment of novel aerial vehicles 3 1.2 Frequency-modulated multirotor noise 5 1.2.1 Frequency and amplitude modulation 5 1.2.2 Time-frequency analysis of the noise signal 7 1.2.3 Perception-influenced evaluation of aerial vehicles 8 1.3 Wake interactions in multirotor configurations 9 1.3.1 Wake interaction phenomena 9 1.3.2 Previous research on wake interactions 10 1.4 Research objectives and scope 12 1.4.1 Real-time noise prediction 13 1.4.2 Wake interactions in multirotor configurations 15 1.5 Dissertation organization 16 2 Real-time noise prediction framework 19 2.1 Flight control module 21 2.2 Aerodynamics module 22 2.2.1 HBEM with aerodynamic models 22 2.2.2 Beddoes wake models 24 2.2.3 Unsteady aerodynamic corrections 26 2.3 Time reconstruction module 28 2.4 Tonal noise prediction module 29 2.5 Time-frequency analysis module 31 2.6 Broadband noise prediction module 34 2.6.1 Semi-empirical model 34 2.6.2 Amiet's theory with wall-pressure spectrum models 35 2.7 Phase reconstruction module 36 2.7.1 Step 1: Narrowband spectrogram synthesis 36 2.7.2 Step 2: Time signal synthesis 37 2.8 Psychoacoustics module 38 3 Free-wake vortex lattice method solver 41 3.1 Aerodynamic and aeroacoustic solver 41 3.1.1 Free-wake vortex lattice method 41 3.1.2 Additional aerodynamic models 43 3.1.3 Acoustic analogy 44 3.2 Quantification factors for wake interaction 45 3.2.1 Wake interaction relations 45 3.2.2 Concepts of induced circulation 46 3.2.3 Quantification factors derived by induced circulation 47 4 Verification, validation, and numerical setup 49 4.1 CONA framework verification and validation: Flight control, aerodynamics, and tonal noise 49 4.1.1 UAV: Single rotor hovering flight 49 4.1.2 UAV: Single rotor forward flight 52 4.1.3 UAV: Quadrotor forward flight 57 4.1.4 UAM: Quadrotor forward flight 63 4.2 CONA framework verification and validation: Broadband noise and psychoacoustics 66 4.2.1 UAV: Airfoil self-noise 66 4.2.2 UAV: Single rotor hovering flight 69 4.2.3 UAV: Single rotor forward flight 73 4.2.4 UAV: Quadrotor forward flight 75 4.2.5 UAV: Quadrotor hovering flight 77 4.3 Free-wake vortex lattice method solver 79 4.3.1 Reference model for wake interaction analyses 79 4.3.2 Test matrix and target outputs 82 4.3.3 Solver validation 83 4.4 Torque ripple modeling 87 4.4.1 Sinusoidal RPM signal approach 89 4.4.2 Random periodic RPM signal approach 90 5 Frequency-modulated multirotor noise 93 5.1 Flight simulation for quadrotor configurations 93 5.1.1 Mission profile and numerical settings 93 5.1.2 Flight control results 96 5.2 High-resolution time-frequency analyses 101 5.2.1 Flyover noise 102 5.2.2 Takeoff noise 104 5.2.3 Loitering noise 107 5.3 Prediction-based psychoacoustic analyses 109 5.3.1 Auralization process 109 5.3.2 Flyover noise 114 5.3.3 Takeoff noise 119 5.3.4 Loitering noise 122 6 Wake interactions in multirotor configurations 125 6.1 Wake interactions in quadrotor hovering flight 125 6.2 Performance of quadrotor forward flight 129 6.2.1 Aerodynamic and aeroacoustic performance 129 6.2.2 Comparison of polynomial regression 133 6.3 Physics of quadrotor forward flight 140 6.3.1 Wake dynamics of quadrotor configurations 140 6.3.2 Distribution of induced circulation 146 6.3.3 Temporal characteristics of wake interaction 161 7 Torque ripple modeling 167 7.1 Sinusoidal RPM signal approach 167 7.1.1 Aerodynamic characteristics 167 7.1.2 Aeroacoustic characteristics 173 7.1.3 Effects of the angular frequency of RPM variations 176 7.2 Random periodic RPM signal approach 180 7.2.1 Effects on noise spectrum 180 7.2.2 Effects on noise directivity 182 8 Conclusions and recommendations 185 8.1 Conclusions 185 8.2 Recommendations for future work 187 8.2.1 Applications of the CONA framework 187 8.2.2 Applications of the MultiPA framework 189 Bibliography 191 Appendix A Control outputs of the CONA framework 205 국문초록 215박

Investigating Forward Flight Multirotor Wind Tunnel Testing in a 3-by 4-foot Wind Tunnel

Investigation of complex multirotor aerodynamic phenomena via wind tunnel experimentation is becoming extremely important with the rapid progress in advanced distributed propulsion VTOL concepts. Much of this experimentation is being performed in large, highly advanced tunnels. However, the proliferation of this class of vehicles extends to small aircraft used by small businesses, universities, and hobbyists without ready access to this level of test facility. Therefore, there is a need to investigate whether multirotor vehicles can be adequately tested in smaller wind tunnel facilities. A test rig for a 2.82-pound quadcopter was developed to perform powered testing in the Cal Poly Aerospace Department’s Low Speed Wind Tunnel, equipped with a 3-foot tall by 4-foot wide test section. The results were compared to data from similar tests performed in the U.S. Army 7-by 10-ft Wind Tunnel at NASA Ames. The two data sets did not show close agreement in absolute terms but demonstrated similar trends. Due to measurement uncertainties, the contribution of wind tunnel interference effects to this discrepancy in measurements was not able to be properly quantified, but is likely a major contributor. Flow visualization results demonstrated that tunnel interference effects can likely be minimized by testing at high tunnel speeds with the vehicle pitched 10-degrees or more downward. Suggestions towards avoiding the pitfalls inherent to multirotor wind tunnel testing are provided. Additionally, a modified form of the conventional lift-to-drag ratio is presented as a metric of electric multirotor aerodynamic efficiency

Physics-Based Modeling for Control and Autonomous Operation of Unmanned Aerial Vehicles

UAS are widely employed in commercial and military applications, and their utilization is growing at a rapid pace. Effective predictive models for aeromechanics, body dynamics and control are critical in trajectory planning and optimization, autonomous operations, and decision-making. Aeromechanical and wind models that are currently used in the control and guidance of UAS are typically simplistic and often do not represent the essential physics to an adequate degree. Therefore, the performance and versatility of such vehicles may be limited in extreme flight conditions. At the other end of the spectrum, there exist high fidelity models that are computationally expensive, and thus not applicable in path planning, optimization, and onboard flight controllers. The major goal of this dissertation is to bridge the gap between physics-based models and onboard decision-making. Multi-disciplinary models of appropriate fidelity are developed and integrated into a comprehensive flight simulation software suite. These models are experimentally validated and utilized in trajectory planning, optimization, onboard control and autonomous flight. Studying the impact of models of different fidelity for the environment and the aerodynamics determines the impact of modeling uncertainty on system-level goals. A vortex-based model for lifting surfaces is developed, using which control surfaces and couplings therein can be efficiently represented. Using this model, the interaction of the propeller wake with a downstream wing is studied, and it is demonstrated these models are effective tools in predicting the propeller-induced span-wise loading. Such a model is beneficial for trajectory planning and optimization applications to improve flight stability and trajectory tracking. Next, a novel HBEM model is developed to predict rotor forces over a wide range of flight conditions. The HBEM model is self-contained and combines blade element theory, momentum theory and a linear inflow model to determine the {em unique} inflow that is {em consistent with all theories}. The model utilizes the blade geometry and the flight condition as inputs to determine the relationship between the forces/moments and the rotor RPM. A detailed set of wind tunnel experiments is conducted to validate the model across a very wide range of flight regimes. Further, a semi-empirical model for the RIPF is developed using experimental data. It is noted that these models can be executed in real-time which makes them useful for implementation in flight software. A custom quadrotor is built and equipped with an ultrasonic wind sensor and RPM sensors. The HBEM and RIPF models are embedded in quadrotor flight software, and it is illustrated these models are fully integrable and efficient enough to run on a typical onboard compute module. To evaluate the ability of these models to function in harsh environmental conditions, motion capture aided autonomous flight is realized in the presence of strong wind gusts generated by a large industrial fan. A feedforward controller is designed to incorporate physical insight into flight mechanics and to provide estimates of the state. Flight tests are conducted in and out of strong crosswind conditions to further show the impact of computationally efficient models that are capable of being executed onboard in real-time. It is shown that the wind sensing and physics-based models along with the feedforward controller improve trajectory tracking in extreme environmental conditions.PHDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/169719/1/davoudi_1.pd

Experimental investigations on the aerodynamic and aeroacoustic characteristics of small UAS propellers

Unmanned aerial system (UAS) is a hot topic in both industry and academia fields. As a popular planform, the rotary-wing system gains more attentions. The small UAS propeller is the most important component in this system, which transfers electric energy into kinetic energy to accomplish fly missions. In the present work, several experimental studies have been performed to investigate the aerodynamic and aeroacoustic characteristics of small UAS propellers. First of all, by conducting force and flow filed measurements, the unsteady dynamic thrust and the wake structure of the propeller has been studied to explore the fundamental physics to help researchers and engineers to obtain a better understanding. Secondly, two kinds of bio-inspired the propellers have been designed and manufactured. Through a set of force, sound, and flow filed measurements, the aerodynamic and aeroacoustic performance of these propellers has been compared to the baseline propeller to evaluate the effects of aerodynamic efficiency and noise attenuation. It was found that the serrated trailing edge propeller could reduce the turbulent trailing edge noise up to 2 dB, and the maple seed inspired propeller could reduce the noise up to 4 dB with no effect on the aerodynamic performance. In addition, since the rotary-wing system consists more than one propeller, the rotor to rotor interaction on the aerodynamic and aeroacoustic performance also has been studied. By enlarging the separation distance between two propellers, the thrust fluctuation and noise generation could be restricted. Not only the design of the device itself has effect on the flying performance, the extreme weather also would affect it. Therefore, an icing research study on the small UAS propeller has been conducted to illustrate how does the ice formed on the propeller and how does the icing influence the aerodynamics performance and power consumption. During these experimental studies, the force measurements were achieved by a high sensitive force and moment transducer (JR3 load cell), which had a precision of ñ0.1N (ñ 0.25% of the full range). The sound measurements were conducted inside of the anechoic chamber located in the aerospace engineering department at Iowa State University. This chamber has a physical dimensions of 12ÃÂ12ÃÂ9 feet with a cut-off frequency of 100 Hz. The detailed flow structure downstream of the propeller was measured by a high-resolution digital PIV system. The PIV system was used to elucidate the streamwise flow structure downstream of the propeller. Both “free-run” and “phase-locked” PIV measurements were conducted to achieve the ensemble-average flow structure and detailed flow structure at certain phase angles

Low Speed Flap-bounding in Ornithopters and its Inspiration on the Energy Efficient Flight of Quadrotors

Flap-bounding, a form of intermittent flight, is often exhibited by small birds over their entire range of flight speeds. The purpose of flap-bounding is unclear during low to medium speed (2 - 8 m/s) flight from a mechanical-power perspective: aerodynamic models suggest continuous flapping would require less power output and lower cost of transport. This thesis works towards the understanding of the advantages of flap-bounding and tries to employ the underlining principle to design quadrotor maneuver to improve power efficiency. To explore the functional significance of flap-bounding at low speeds, I measured body trajectory and kinematics of wings and tail of zebra finch (Taeniopygia guttata, N=2) during flights in a laboratory between two perches. The flights consist of three phases: initial, descending and ascending. Zebra finch first accelerated using continuous flapping, then descended, featuring intermittent bounds. The flight was completed by ascending using nearly-continuous flapping. When exiting bounds in descending phase, they achieved higher than pre-bound forward velocity by swinging body forward similar to pendulum motion with conserved mechanical energy. Takeoffs of black-capped chickadees (Poecile atricapillus, N=3) in the wild was recorded and I found similar kinematics. Our modeling of power output indicates finch achieves higher velocity (13%) with lower cost of transport (9%) when descending, compared with continuous flapping in previously-studied pigeons. To apply the findings to the design of quadrotor motion, a mimicking maneuver was developed that consisted of five phases: projectile drop, drop transition, pendulum swing, rise transition and projectile rise. The quadrotor outputs small amount (4 N) of thrust during projectile drop phase and ramps up the thrust while increasing body pitch angle during the drop transition phase until the thrust enables the quadrotor to advance in pendulum-like motion in the pendulum swing phase. As the quadrotor reaches the symmetric point with respect to the vertical axis of the pendulum motion, it engages in reducing the thrust and pitch angle during the rise transition phase until the thrust is lowered to the same level as the beginning of the maneuver and the body angle of attack minimized (0.2 deg) in the projectile rise phase. The trajectory of the maneuver was optimized to yield minimum cost of transport. The quadrotor moves forward by tracking the cycle of the optimized trajectory repeatedly. Due to the aggressive nature of the maneuver, we developed new algorithms using onboard sensors to determine the estimated position and attitude. By employing nonlinear controller, we showed that cost of transport of the flap-bounding inspired maneuver is lower (28%) than conventional constant forward flight, which makes it the preferable strategy in high speed flight (≥15 m/s)

Geometric Active Disturbance Rejection Control for Autonomous Rotorcraft in Complex Atmospheric Environment

This dissertation presents several novel robust tracking control schemes of rotorcraft unmanned aerial vehicles under realistic atmospheric turbulence. To achieve fast converging and stable performance of the rotorcraft control scheme, a new H\ {o}lder-continuous differentiator, similar to the super-twisting algorithm used in the second-order sliding model control scheme, is proposed with guaranteed fast finite-time stability. Unlike the super-twisting algorithm, which uses a sliding-mode structure to achieve finite-time stability, the proposed differentiator maintains its fast finite-time stability with H\ {o}lder continuity, theoretically eliminating the harmful chattering phenomenon in practical control applications. Perturbation and noise robustness analyses are conducted for the proposed differentiator. The dissertation formulates the rotorcraft tracking control and disturbance estimation problems separately. The rotorcraft aerial vehicle is modeled as a rigid body with control inputs that actuate all degrees of freedom of rotational motion and only one degree of freedom of translational motion. The motion of the aircraft is globally represented on \TSE, which is the tangent bundle of the special Euclidean group \SE. The translational and attitude control schemes track the desired position and attitude on \SE. The disturbance estimation problem is formulated as an extended states observer on \TSE. Next, two rotorcraft control schemes on \SE with disturbance rejection mechanisms are presented. The proposed disturbance rejection control systems comprise two parts: an extended states observer for disturbance estimation and a tracking control scheme containing the disturbance rejection term to track the trajectory. The first disturbance rejection control scheme comprises an exponentially stable extended states observer and an asymptotically stable tracking control scheme. The second system comprises a fast finite-time stable extended state observer and a fast finite-time stable tracking control scheme. The fast finite-time stable extended state observer uses the \textup{H\ {o}}lder-continuous differentiator to estimate the resultant external disturbance force and disturbance torque acting on the vehicle. It ensures stable convergence of disturbance estimation errors in finite time when the disturbances are constant. Software-in-the-loop simulation is carried out for the active disturbance rejection control scheme with an open-source autopilot and a physics-based simulation tool. The simulation utilizes simulated wind gusts, propeller aerodynamics, actuator limitation, and measurement noise to validate the disturbance rejection control systems in a simulated environment with high fidelity. Two sets of flight experiments are conducted to investigate the autonomous rotorcraft flight control performance under turbulent income flows. A wind tunnel composed of fan arrays is involved in both experiments to provide different turbulent incoming flows by adjusting the duty of individual fans. The first set of experiments conducts income flow measurements for wind tunnel calibration. For the turbulent flows generated by different fan configurations, their steady velocity field and unsteady turbulence characteristics are measured by a pressure scanner and hot-wire anemometer. The second set of experiments involves flight tests of a rotorcraft within the turbulent environment measured and calibrated in the first experiment set. The proposed extended states observer is implemented onto a rotorcraft by customizing an open-source autopilot software. With this implementation, the flight control performance of the proposed disturbance rejection control schemes is presented and compared with the autopilot without customization. The experimental results show that the proposed disturbance rejection control scheme enhanced by the disturbance estimation schem

UAV Selection Methodology and Performance Evaluation to Support UAV-Enabled Bridge Inspection

This project performed preliminary work to support use of Unmanned Aerial Vehicles (UAV)-based for bridge inspections, providing an economical and safer alternative to conventional inspection practices. The main challenge is that most existing technologies rely on general-purpose UAV platforms and there is no verified methodology for UAV-enabled bridge inspection principles and relevant considerations to reliably obtain inspection data. There have been some efforts to use general-purpose commercially available UAVs for bridge inspection. However, the turbulent environment that often exists around bridges requires customized and enhanced UAV platforms with a higher level of robustness, taking into account the bridge type and structure as well as the weather conditions around the bridge. Additionally, the data-acquisition capabilities of commercially available UAVs need to be compared to those required for bridge inspection. Previously, there has not been a study to quantify the gap between the performance of the commercially available UAVs and ideal desired performances. In this multidisciplinary project, a comprehensive set of experiments were developed for selection, testing, and evaluation techniques of candidate UAVs, the complex nature of flying UAVs in close proximity to bridges was explored, and the limitations of UAV flight due to turbulent flows around bridge components and nearby terrain was assessed. Commercially available platforms for bridge inspection were selected, tested, and evaluated. Deliverables from this project include: (1) measurable metrics to evaluate the performance of UAVs for bridge inspection, (2) experiments to test the suitability of UAVs for bridge inspection, and (3) a comprehensive analysis near-bridge environment flow field. Computational analysis of air flow patterns near bridge elements shows that the bridge geometry creates areas of turbulence and flow variation which impact the control requirements of the UAV. Local weather conditions can amplify these areas. Test flights were performed at selected structures to provide additional insight into the flight and data collection capabilities of the UAVs under consideration. Findings and deliverables from this project will help NCDOT justify capital purchases made to support UAV-assisted inspection, as well as additional research needed to integrate UAVs into their current bridge inspection processes. Ultimately, this work supports a follow-up project to develop workflows and implementation tools for efficient UAV-enabled bridge inspection