Trajectory Generation and Tracking Control for Aggressive Tail-Sitter Flights

We address the theoretical and practical problems related to the trajectory generation and tracking control of tail-sitter UAVs. Theoretically, we focus on the differential flatness property with full exploitation of actual UAV aerodynamic models, which lays a foundation for generating dynamically feasible trajectory and achieving high-performance tracking control. We have found that a tail-sitter is differentially flat with accurate aerodynamic models within the entire flight envelope, by specifying coordinate flight condition and choosing the vehicle position as the flat output. This fundamental property allows us to fully exploit the high-fidelity aerodynamic models in the trajectory planning and tracking control to achieve accurate tail-sitter flights. Particularly, an optimization-based trajectory planner for tail-sitters is proposed to design high-quality, smooth trajectories with consideration of kinodynamic constraints, singularity-free constraints and actuator saturation. The planned trajectory of flat output is transformed to state trajectory in real-time with consideration of wind in environments. To track the state trajectory, a global, singularity-free, and minimally-parameterized on-manifold MPC is developed, which fully leverages the accurate aerodynamic model to achieve high-accuracy trajectory tracking within the whole flight envelope. The effectiveness of the proposed framework is demonstrated through extensive real-world experiments in both indoor and outdoor field tests, including agile SE(3) flight through consecutive narrow windows requiring specific attitude and with speed up to 10m/s, typical tail-sitter maneuvers (transition, level flight and loiter) with speed up to 20m/s, and extremely aggressive aerobatic maneuvers (Wingover, Loop, Vertical Eight and Cuban Eight) with acceleration up to 2.5g

Design & Implementation of an Electric Fixed-wing Hybrid VTOL UAV for Asset Monitoring

Fixed-wing unmanned aerial vehicles (UAVs) offer the best aerodynamic efficiency required for long-distance or high-endurance applications, albeit their runway requirement for take-off and landing in comparison with quadcopters, helicopters, and flapping-wing UAVs that can perform vertical take-off and landing (VTOL). Integrating a multirotor system with a fixed-wing UAV imparts VTOL capabilities without significantly compromising fixed-wing aerodynamic efficiency, endurance, payload capacity or range. Documented system design approaches to address various challenges of such fusion processes are sparse. This research proposes a holistic approach for designing, prototyping, and testing an electric-powered fixed-wing hybrid VTOL UAV. The proposed system design approach augments the standard aircraft design process with additional steps to integrate VTOL capabilities. Separate fixed-wing and multirotor designs were derived from the frozen mission requirements, which were then fused. The process used simulation for modeling and evaluating alternatives for the hybrid UAV created using standard aircraft design equations. We prototyped and instrumented the final design to validate operational capabilities through test flights. Multiple flight trials identified the ideal combination of Lithium-Polymer (Li-Po) batteries for VTOL (8000mAh) and fixed-wing (14000mAh) modes to meet the endurance and range requirements. The redundant power supplies also increased the survivability chances of the hybrid UAV during failures

Preliminary Design of an eVTOL Aircraft

The growing demand for alternative clean energy sources for civil aviation industry, from transport to surveillance, prompts new aircraft configurations for a different range of future applications and replacement of outdated concepts of aircraft still in service. The electrification of aircraft is the future of aviation. The electrical Vertical Take-off and Landing (eVTOL) type of aircraft is growing in demand as an alternative for Urban Air Mobility (UAM), in short to mid-range transportation of goods and passengers. In this dissertation, an eVTOL aircraft concept and preliminary design are proposed. The design is a small UAV optimized for mpay CL CD aiming at longer flight ranges and carrying capacity. An Excel spreadsheet developed to proceed with the parametric study. The parameters considered are: Winspan, mean aerodynamic chord (MAC), and lift coefficient as the main design parameters for the aircraft wing. Multiple parameters combinations were tested and the selection criteria were the objective function and MAC. A compromise between the two criteria was achieved by selecting a wing with a slightly less score in objective function but a mean chord value higher than the optimal chord, which was better for the structure. Prop Selector was used in combination with spreadsheets for the preliminary propeller design, where a variable pitch configuration with the known wing main characteristics was adopted, in hover and cruise conditions to screen the best propeller diameter using the hover lift performance (H) and global efficiency (?global) as the main performance parameters do optimize the propulsion of the aircraft. The selected diameter is studied in the climb condition and the ideal pitch of the propeller is predicted for the relevant flight conditions. The performance parameters such as the rate of climb, cruise speed, and range were estimated, showing promissing results. With XFOIL/XFLR5 software the static longitudinal stability was studied, and Open VSP and CATIA V5 CAD models of the aircraft, and tilt-rotor mechanism were made. Finally, the mission profile is devised, the battery mass fraction, and energy consumption for each flight condition is calculated, thus highlighting the capabilities and limitations of the aircraft.O crescimento no incentivo de fontes de energia descarbonizada na indústria da aviação civil, desde a aviação de transporte até vigilância, leva a que novos conceitos de aeronaves sejam desenvolvidos para uma gama diferente de aplicações futuras e actuais como substuição de conceitos ultrapassados de aeronaves ainda em uso. A electrificação é o futuro para propulsão de aeronaves. A aeronave do tipo eléctrico com descolagem e aterragem vertical (eVTOL) tem amadurecido como alternativa para Mobilidade Aérea Urbana (MAU), no transporte de mercadorias e passageiros para curto e médio alcance. Nesta dissertação, é proposto um projeto conceptual e preliminar de uma aeronave eVTOL. O design é um pequeno UAV otimizado para maximizar uma função objetivo, Fobj= mpay CL CD para um maior alcance e capacidade de carga. Utilizou-se uma folha de cálculo Excel para fazer um estudo paramétrico da asa com a envergadura, corda aerodinâmica média (CAM) e coeficiente de sustentação como principais parâmetros de projeto da asa da aeronave. Foram testadas múltiplas combinações dos parâmetros e os critérios de seleção foram a função objetivo e CAM. Um compromisso entre os dois critérios foi alcançado ao selecionar uma asa com uma pontuação um pouco menor na função objetivo, mas um valor de CAM superior à corda ótima, isso foi a melhor escolha para a estrutura. O Prop Selector foi utilizado em combinação com a folha de cálculo para o projeto preliminar da hélice, implementou-se uma configuração de passo variável com as características principais da asa já conhecida. Estudou-se o desempenho da aeronave em condições de voo pairado e cruzeiro para selecionar o melhor diâmetro da hélice com os critérios desempenho da eficiência de tração em voo pairado (H) e efficiência global (?global) para quantificar o passo apropriado a cada condição de voo e apurar a previsão do desempenho global da aeronave. A previsão de desempenho, razão de subida, velocidade de cruzeiro, e alcance, revelou resultados promissores. O software XFOIL/XFLR5 foi utilizado para estudar a estabilidade longitudinal estática, e em seguida desenhou-se com Open VSP e CATIA V5 modelos CAD da aeronave e do mecanismo basculante dos rotores. Finalmente, foi feito o perfil da missão, em que é de interesse a fração de massa da bateria, e o consumo de energia para cada condição de voo, e com essas métricas as capacidades e limitações da aeronave são reconhecidas

Model-free control algorithms for micro air vehicles with transitioning flight capabilities

Micro air vehicles with transitioning flight capabilities, or simply hybrid micro air vehicles, combine the beneficial features of fixed-wing configurations, in terms of endurance, with vertical take-off and landing capabilities of rotorcrafts to perform five different flight phases during typical missions, such as vertical takeoff, transitioning flight, forward flight, hovering and vertical landing. This promising micro air vehicle class has a wider flight envelope than conventional micro air vehicles, which implies new challenges for both control community and aerodynamic designers. One of the major challenges of hybrid micro air vehicles is the fast variation of aerodynamic forces and moments during the transition flight phase which is difficult to model accurately. To overcome this problem, we propose a flight control architecture that estimates and counteracts in real-time these fast dynamics with an intelligent feedback controller. The proposed flight controller is designed to stabilize the hybrid micro air vehicle attitude as well as its velocity and position during all flight phases. By using model-free control algorithms, the proposed flight control architecture bypasses the need for a precise hybrid micro air vehicle model that is costly and time consuming to obtain. A comprehensive set of flight simulations covering the entire flight envelope of tailsitter micro air vehicles is presented. Finally, real-world flight tests were conducted to compare the model-free control performance to that of the Incremental Nonlinear Dynamic Inversion controller, which has been applied to a variety of aircraft providing effective flight performances

Conception, modélisation, et commande d'un mini-drone convertible

There is a growing interest to design convertible aerial vehicles that can hover like helicopters and fly forward efficiently like airplanes. This thesis is devoted to the conception, modeling, and control of such a convertible mini-UAV (Unmanned Aerial Vehicle). The main contributions of this work are threefold. Firstly, we design a novel UAV structure by adding to each side of a quadrotor one wing that can rotate around an axis belonging to the propellers' plane. Our prototype has many advantages over existing convertible structures: simple mechanical concept since inspired by a classical quadrotor, flexibility for selecting different components (wings, propellers), flexibility for the control design, etc. Secondly, we provide an energy modeling of this type of convertible UAVs, taking into account their characteristics as compared to full-scale helicopters (large variation of aerodynamic forces, performance degradation at low Reynolds number, etc.). Finally, as for the control design, the degrees of freedom of the wings permit the decoupling between propellers and wings' orientations. This greatly enhances the control flexibility as compared to traditional aircraft. Relying on this feature, several control approaches are proposed. In particular, using a specific geometrical design, we show that an efficient control of our UAV can be obtained without air-velocity measurements. Simulation results confirm the soundness of our control design even in the presence of strong and varying wind. En route to validate the theory, a mechanical prototype of the UAV was constructed in our laboratory and preliminary flight tests were performed.Cette thèse concerne les drones dits "convertibles", qui allient capacité au vol stationnaire et efficacité énergétique en vol de croisière. Les principales contributions de ce travail comportent trois volets. D'abord, nous concevons une nouvelle structure de drone en ajoutant de chaque côté d'un quadrirotor une aile qui peut pivoter autour d'un axe appartenant au plan des hélices. Notre prototype a de nombreux avantages par rapport aux structures convertibles existantes: conception mécanique simple car dérivée d'un quadrirotor classique, flexibilité pour le montage de différents composants (ailes, hélices), etc. Deuxièmement, nous proposons une modélisation énergétique de ce type de drone convertible, en tenant compte de ses caractéristiques par rapport aux hélicoptères avec pilote à bord (grande variation des forces aérodynamiques, dégradation des performances à faible nombre de Reynolds, etc.). Finalement, concernant la conception de la commande, les degrés de liberté des ailes permettent le découplage entre les orientations des hélices et celle des ailes. Cela augmente considérablement les possibilités de contrôle par rapport aux aéronefs traditionnels. S'appuyant sur cette caractéristique, plusieurs approches de contrôle sont proposées. En particulier, en utilisant une conception géométrique spécifique, nous montrons qu'un contrôle efficace peut être obtenu sans mesures de la vitesse air. Les résultats de simulation confortent cette stratégie de contrôle, même en présence de vent fort et variable. Afin de valider la théorie, un prototype mécanique du drone a été construit dans notre laboratoire et des essais en vol préliminaires ont été effectués

Intelligent control of a ducted fan VTOL UAV with conventional control surfaces

Utilizing UAVs for intelligence, surveillance, and reconnaissance (ISR) is beneficial in both military and civil applications. The best candidates for successful close range ISR missions are small VTOL UAVs with high speed capability. Existing UAVs suffer from the design tradeoffs that are usually required, in order to have both VTOL capability and high speed flight performance. In this thesis, we consider a novel UAV design configuration combining several important design elements from rotorcraft, ducted-fan, tail-sitter, and fixed-wing vehicles. While the UAV configuration is more towards the VTOL type, high speed flight is achieved by performing a transition maneuver from vertical attitude to horizontal attitude. In this unique approach, the crucial characteristics of VTOL and high speed flight are attained in a single UAV design. The capabilities of this vehicle come with challenges of which one of the major ones is the development an effective autonomous controller for the full flight envelope. Ducted-fan type UAVs are unstable platform with highly nonlinear behaviour, and with complex aerodynamic, which lead to inaccuracies in the estimation of the vehicle dynamics. Conventional control approaches have limitations in dealing with all these issues. A promising solution to a ducted-fan flight control problem is to use fuzzy logic control. Unlike conventional control approaches, fuzzy logic has the ability of replicating some of the ways of how humans make decisions. Furthermore, it can handle nonlinear models and it can be developed in a relatively short time, as it does not require the complex mathematics associated with classical control theory. In this study, we explore, develop, and implement an intelligent autonomous fuzzy logic controller for a given ducted-fan UAV through a series of simulations

Design, construction and flight control of a quad tilt-wing unmanned aerial vehicle

Unmanned Aerial Vehicles (UAVs) are flying robots that are employed both in civilian and military applications with a steeply increasing trend. They are already used extensively in civilian applications such as law enforcement, earth surface mapping and surveillance in disasters, and in military missions such as surveillance, reconnaissance and target acquisition. As the demand on their utilization increases, novel designs with far more advances in autonomy, flight capabilities and payloads for carrying more complex and intelligent sensors are emerging. With these technological advances, people will find even newer operational fields for UAVs. This thesis work focuses on the design, construction and flight control of a novel UAV (SUAVI: Sabanci University Unmanned Aerial VehIcle). SUAVI is an electric powered compact size quad tilt-wing UAV, which is capable of vertical takeoff and landing (VTOL) like a helicopter, and flying horizontally like an airplane by tilting its wings. It carries onboard cameras for capturing images and broadcasting them via RF communication with the ground station. In the aerodynamic and mechanical design of SUAVI, flight duration, flight speed, size, power source and missions to be carried out are taken into account. The aerodynamic design is carried out by considering the maximization of the aerodynamic efficiency and the safe fiight characteristics. The components in the propulsion system are selected to optimize propulsion efficiency and fulfill the requirements of the control for a stable flight in the entire speed range. Simulation results obtained by ANSYS and NASA FoilSimII are evaluated and motor thrust tests are conducted during this optimization process. The power source is determined by taking the weight and flight duration into account. The wings and the fuselage are shaped iteratively in fluid flow simulations. Additionally, the verification of aerodynamic design and maneuverability are assessed in the wind tunnel tests on the half-body prototype. The mechanical structure is designed to be lightweight, strong and protective, and to allow easy assembly and disassembly of SUAVI for practical use. The safety factors in the mechanical system are determined using FEM analysis in ANSYS environment. Specimens of candidate composite skin materials are prepared and tested for lightness, strength and integrity in mechanical tests. The ready for flight prototype SUAVI is produced from the selected composite material. Dynamical model of SUAVI is obtained using Newton-Euler formulation. Aerodynamic disturbances such as wind gusts are modeled using the wellknown Dryden wind turbulence model. As the flight control system, a supervisory control architecture is implemented where a Gumstix microcomputer and several Atmega16 microcontrollers are used as the high-level and low- level controllers, respectively. Gumstix computer acts as a supervisor which orchestrates switching of low-level controllers into the system and is responsible for decision making, monitoring states of the vehicle and safety checks during the entire flight. It also generates attitude references for the low-level controllers using data from GPS or camera. Various analog and digital filters are implemented to smooth out noisy sensor measurements. Extended Kalman filter is utilized to obtain reliable orientation information by fusing data from low-cost MEMS inertial sensors such as gyros, accelerometers and the compass. PID controllers are implemented for both the high-level GPS based acceleration controller and the low-level altitude and attitude controllers. External disturbances are estimated and compensated by a disturbance observer. Real-time control software is developed for the whole fiight control system. SUAVI can operate in semi-autonomous mode by communicating with the ground station. A quadrotor test platform (SUQUAD: Sabanci University QUADrotor) is also produced and used for the initial performance tests of the fiight control system. After successful fiight tests on this platform, the control system is transferred to SUAVI. Performance of the flight control system is verified by numerous simulations and real flight experiments. VTOL and horizontal flights are successfully realized

Experimental Investigation of Shrouded Rotor Micro Air Vehicle in Hover and in Edgewise Gusts

Due to the hover capability of rotary wing Micro Air Vehicles (MAVs), it is of interest to improve their aerodynamic performance, and hence hover endurance (or payload capability). In this research, a shrouded rotor conguration is studied and implemented, that has the potential to oer two key operational benets: enhanced system thrust for a given input power, and improved structural rigidity and crashworthiness of an MAV platform. The main challenges involved in realising such a system for a lightweight craft are: design of a lightweight and stiff shroud, and increased sensitivity to external flow disturbances that can affect flight stability. These key aspects are addressed and studied in order to assess the capability of the shrouded rotor as a platform of choice for MAV applications. A fully functional shrouded rotor vehicle (disk loading 60 N/m2) was designed and constructed with key shroud design variables derived from previous studies on micro shrouded rotors. The vehicle weighed about 280 g (244 mm rotor diameter). The shrouded rotor had a 30% increase in power loading in hover compared to an unshrouded rotor. Due to the stiff, lightweight shroud construction, a net payload benefit of 20-30 g was achieved. The different components such as the rotor, stabilizer bar, yaw control vanes and the shroud were systematically studied for system efficiency and overall aerodynamic improvements. Analysis of the data showed that the chosen shroud dimensions was close to optimum for a design payload of 250 g. Risk reduction prototypes were built to sequentially arrive at the nal conguration. In order to prevent periodic oscillations in flight, a hingeless rotor was incorporated in the shroud. The vehicle was successfully flight tested in hover with a proportional-integral-derivative feedback controller. A flybarless rotor was incorporated for efficiency and control moment improvements. Time domain system identification of the attitude dynamics of the flybar and flybarless rotor vehicle was conducted about hover. Controllability metrics were extracted based on controllability gramian treatment for the flybar and flybarless rotor. In edgewise gusts, the shrouded rotor generated up to 3 times greater pitching moment and 80% greater drag than an equivalent unshrouded rotor. In order to improve gust tolerance and control moments, rotor design optimizations were made by varying solidity, collective, operating RPM and planform. A rectangular planform rotor at a collective of 18 deg was seen to offer the highest control moments. The shrouded rotor produced 100% higher control moments due to pressure asymmetry arising from cyclic control of the rotor. It was seen that the control margin of the shrouded rotor increased as the disk loading increased, which is however deleterious in terms of hover performance. This is an important trade-off that needs to be considered. The flight performance of the vehicle in terms of edgewise gust disturbance rejection was tested in a series of bench top and free flight tests. A standard table fan and an open jet wind tunnel setup was used for bench top setup. The shrouded rotor had an edgewise gust tolerance of about 3 m/s while the unshrouded rotor could tolerate edgewise gusts greater than 5 m/s. Free flight tests on the vehicle, using VICON for position feedback control, indicated the capability of the vehicle to recover from gust impulse inputs from a pedestal fan at low gust values (up to 3 m/s)