Airborne wind energy (AWE) is a novel technology that aims at accessing wind resources at higher altitudes which cannot be reached with conventional wind turbines. This technological challenge is accomplished using tethered aircraft or kites in combination with either onboard or ground-based generators. In the former case, the kinetic energy of the air flow is transformed into electricity and transmitted via a conductive cable to the ground. In the latter case, the aerodynamic force of the aircraft or kite is translated into tether tension. The pulling force uncoils the tether from a drum which turns a generator and hence transforms the mechanical torque into electrical power on the ground. In this case two operational modes are required: In the first mode, the tether is reeled out until the maximum length is reached. It follows a reeling in phase where the aircraft or kite glides back towards the ground station and a fraction of the generated power is used to wind the tether again onto the drum of the winch. The cycle restarts as soon as the minimum tether length is reached. These two modes combined constitute a so-called pumping cycle. Reliability is a key system property that will decide over the success of AWE as a commercially feasible technology. To reach this goal, a well designed control system is required that can achieve the nominal control objectives as well as handle disturbances such as atmospheric turbulence and mismatches between the model used for the controller derivation and the real plant. In light of these challenges, the present work tries to make a contribution to bring AWE closer to commercial success. More specifically, a workflow to design a modular control architecture for a rigid wing AWE system operated in pumping cycle mode is presented. The thesis introduces models of different fidelity that are either directly used for the controller synthesis or in order to verify if the designed controller is able to meet its objectives. A quasi-stationary analysis is performed to describe the operational flight envelope and to derive linear state space models for the longitudinal and lateral flight controller synthesis. A generic outer loop controller, independent of the specific aircraft actuation, is designed which guides the system along the traction and retraction phase reference flight paths. A ground based winch controller is used to track the tether tension and hence the radial motion of the aircraft. To track the outer loop guidance commands several linear and nonlinear inner loop flight controllers are proposed. All controller designs are verified in detail using Monte Carlo simulations. The resulting distributions of critical metrics are used to quantify performance as well as robustness of the controllers in the presence of stochastic variations in the wind field and model uncertainties. In the last part of this thesis a methodology is proposed that can be used to systematically generate conditions in which the AWE control system is failing. The generated knowledge can be leveraged to create an analytic model that is able to predict during operation a critical flight state. Ultimately, this allows to trigger a mitigation maneuver to avoid the failure. Different prediction strategies are presented and eventually the methodology is specifically applied to the case of tether rupture condition generation, prediction and avoidance. @en

Airborne wind energy systems convert wind energy into electricity using tethered flying devices, typically flexible kites or aircraft. Replacing the tower and foundation of conventional wind turbines can substantially reduce the material use and, consequently, the cost of energy, while providing access to wind at higher altitudes. Because the flight operation of tethered devices can be adjusted to a varying wind resource, the energy availability increases in comparison to conventional wind turbines. Ultimately, this represents a rich topic for the study of real-time optimal control strategies that must function robustly in a spatiotemporally varying environment. With all of the opportunities that airborne wind energy systems bring, however, there are also a host of challenges, particularly those relating to robustness in extreme operating conditions and launching/landing the system (especially in the absence of wind). Thus, airborne wind energy systems can be viewed as a control system designer’s paradise or nightmare, depending on one’s perspective. This survey article explores insights from the development and experimental deployment of control systems for airborne wind energy platforms over approximately the past two decades, highlighting both the optimal control approaches that have been used to extract the maximal amount of power from tethered systems and the robust modal control approaches that have been used to achieve reliable launch, landing, and extreme wind operation. This survey will detail several of the many prototypes that have been deployed over the last decade and will discuss future directions of airborne wind energy technology as well as its nascent adoption in other domains, such as ocean energy.@en

Airborne wind energy (AWE) systems are tethered flying devices that harvest wind resources at higher altitudes, which are not accessible to conventional wind turbines. To become a viable alternative to other renewable energy technologies, AWE systems are required to fly reliably and autonomously for long periods of time while being exposed to atmospheric turbulence and wind gusts. In this context, the present paper proposes a three-step methodology to improve the resilience of an existing baseline control system toward these environmental disturbances. In the first step, upset conditions are systematically generated that lead to a failure of the control system using the subset simulation method. In the second step, the generated conditions are used to synthesize a surrogate model that can be used to predict upsets beforehand. In the final step an avoidance maneuver is designed that keeps the AWE system operational while minimizing the impact of the maneuver on the average pumping cycle power. The feasibility of the methodology is demonstrated on the example of tether rupture during pumping cycle operation. As an additional contribution a novel transition strategy from retraction to traction phase is presented that can reduce the probability of tether rupture significantly.

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In this paper, we present the design and computational model of a representative multi-megawatt airborne wind energy (AWE) system, together with a simulation framework that accounts for the flight dynamics of the fixed-wing aircraft and the sagging of the tether, combining this with flight control and optimisation strategies to derive the power curve of the system. The computational model is based on a point mass approximation of the aircraft, a discretisation of the tether by five elastic segments and a rotational degree of freedom of the winch. The aircraft has a wing surface area of 150 m² and is operated in pumping cycles, alternating between crosswind flight manoeuvres during reel out of the tether, and rapid decent towards the ground station during reel in. To maximise the net cycle power, we keep the design parameters of the aircraft constant, while tuning the operational and controller parameters for different wind speeds and given contraints. We find that the presented design can generate a net cycle power of up to 3.8 megawatts.

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Airborne wind energy is an emerging technology that uses tethered unmanned aerial vehicles for harvesting wind energy at altitudes higher than conventional towered wind turbines. To make the technology competitive to other renewable energy technologies an automatic control system is required that allows autonomously operating the system throughout all phases of flight. In this study a modular control system is presented, adapting the underlying kinematic and dynamic framework from conventional aerospace terminology and applying this to tethered crosswind flight with varying tether length. The high level control strategy in form of a state machine as well as the cascaded flight control structure consisting of path-following guidance and control, attitude and rate loop is presented along with the winch controller. The present work is a first step towards a methodology for the systematic development of reliable and high-performance control solutions for airborne wind energy systems. Models for the airborne system, ground station, as well as the tether connecting the ground system with the airframe will be presented. Results from a simulation study in a realistic wind field will be used to demonstrate the feasibility of the proposed concept and to identify particularly challenging situations in the operational envelope. @en

Airborne wind energy is an emerging technology that uses tethered unmanned aerial vehicles for harvesting wind energy at altitudes higher than conventional towered wind turbines. To make the technology competitive to other renewable energy technologies a reliable control system is required that allows autonomously operating the system throughout all phases of flight. In the present work a cascaded nonlinear control scheme for reliable pumping cycle control of a rigid wing airborne wind energy system is proposed. The high-level control strategy in the form of a state machine as well as the flight controller consisting of path-following guidance and control, attitude, and rate loop is presented along with a winch controller for tether force tracking. Amathematical model for an existing prototype will be derived, and results from a simulation study will be used to demonstrate the robustness of the proposed concept in the presence of turbulence and wind gusts. @en

Reliable autonomous operation of Airborne Wind Energy (AWE) systems requires control algorithms that are able to attenuate the effect of stochastic disturbances on the control performance in continuously changing wind conditions. Assessing the stability and robustness of the control system is in general carried out using simplified system models where the real stochastic nature of the control problem is neglected. Therefore, a direct Monte Carlo approach is used in practice to increase the confidence in the control system’s reliability. However, this approach performs poorly if it is used to estimate the effect and the probability of rare events such as strong gusts. Statistically, these events are located at the tails of the underlying joint probability density function. Consequently, only a few samples leading to rare events can be identified in a reasonable amount of time which leads to a biased probability estimate. In addition, it is difficult to recognize and leverage patterns if only a small set of samples is available that lead to a violation of a critical control requirement @en

In this work, a novel vertical takeoff and landing methodology for flexible wing kite power systems is presented. Starting from a basic mast-based launching and landing concept, the operational envelope will be enlarged using the external assistance of a multicopter. The multicopter is used to drag the kite along a specified launching path until the operational altitude is reached, where the kite is detached and steered to its characteristic parking position while the multicopter lands. The landing of the kite will be conducted without multicopter assistance, and solely the winch will be used to pull the kite toward the ground station. For all maneuvers, flight control algorithms are presented, and the feasibility of the proposed methodology is analyzed using a developed simulation environment incorporating models for the kite, multicopter, ground station, and the tethers that connect the individual subsystems.
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A key enabler of success in the field of airborne wind energy (AWE) is the ability to operate the AWE system in a reliable manner throughout every phase of flight. In this contribution a modular path following flight control architecture will be presented that allows to control a rigid wing AWE systemduring all phases of the flight, including vertical takeoff, power generation and vertical landing. It is a first step towards a flight control architecture with increased reliability and robustness in nominal aswell as in adverse conditions compared to existing approaches. The cascaded control structure consists of two separate outer loop controllers connected with a switching logic which is controlled by a high-level state machine. The flight path controller for vertical takeoff and landing (VTOL) guides the system along a predefined flight path up to the operational altitude or lands the system. To guide the kite along the path during VTOL mode common thrust vectoring is used. For improved control effectiveness the control surfaces of the wing are used along with the thrusters using a pseudo-inverse control allocation method to generate the required moments. This allows to exploit the complementary control effectiveness of thrusters aswell as control surfaces with respect to the current flight condition. For the power generation mode the second outer loop controller is activated. A novel path following controller for AWE systems has been developed that can be applied to soft was well as rigid wing kites. Different flight patterns can be generated consisting of circle and great circle segments. This allows to solve the path following control problem on a sphere similarly to straight line and orbit following control problems of untethered aerial vehicles. Since during power generation mode the thrusters are not in operation, only the lift vector is controlled using the orientation of the kite with respect to a tether reference frame. This allows to generate the required centripetal force to guide the kite along the curved paths on the tangential plane. In this way the model dependency of the controller is reduced to a minimum, while at the same time the control method is intuitive from a flight physical point of view. To enhance the disturbance compensation capabilities the baseline controller is enhanced with an adaptive part, which allows to recover a defined reference behavior in case of large disturbances or failures. So far simulation results using a generic rigid wing kite model demonstrate the feasibility of the proposed control approach. Due to the modularity of the control architecture, the low computational demand and the reduced model dependency the proposed flight control architecture can be easily tested on different platforms in the future. @en