Automatic crosswind flight of tethered wings for airborne wind energy: modeling, control design and experimental results

Abstract: An approach to control tethered wings for airborne wind energy is proposed. A fixed length of the lines is considered, and the aim of the control system is to obtain figure-eight crosswind trajectories. The proposed technique is based on the notion of the wing's "velocity angle" and, in contrast with most existing approaches, it does not require a measurement of the wind speed or of the effective wind at the wing's location. Moreover, the proposed approach features few parameters, whose effects on the system's behavior are very intuitive, hence simplifying tuning procedures. A simplified model of the steering dynamics of the wing is derived from first-principle laws, compared with experimental data and used for the control design. The control algorithm is divided into a low-level loop for the velocity angle and a high-level guidance strategy to achieve the desired flight patterns. The robustness of the inner loop is verified analytically, and the overall control system is tested experimentally on a small-scale prototype, with varying wind conditions and using different wings.

• The paper presents a novel control system for airborne wind energy tethered wings that achieves automatic crosswind flight without requiring direct wind speed measurements, relying on a simplified model and 'velocity angle'.
• A hierarchical three-loop control strategy was developed and validated experimentally across varying wing types and wind conditions (2-6 m/s), demonstrating robust and consistent path tracking.
• This research significantly advances airborne wind energy by simplifying control requirements, increasing the potential feasibility and deployment of tethered wing systems in diverse environments.

Automatic Crosswind Flight of Tethered Wings for Airborne Wind Energy: An Overview

The paper "Automatic crosswind flight of tethered wings for airborne wind energy: modeling, control design and experimental results" introduces a comprehensive approach to the control of tethered wings used in airborne wind energy systems. The control system is uniquely designed without the need for direct wind speed measurements, relying instead on a simplified model based on the concept of a wing's "velocity angle."

Summary of Model and Control Approach

The authors propose a control strategy for a fixed tether length, aiming to manage figure-eight crosswind trajectories of the wings. They present a novel approach that does not necessitate measurements of wind speed at the wing's location, contrasting with existing methods that often rely on complex sensors and measurements. This simplification is achieved through defining the velocity angle, a parameter derived from the wing's velocity vector projected on local north and east axes.

A streamlined model for steering dynamics is formulated from first principles and validated through experiments with prototype systems. The authors derive explicit relationships between this control-oriented model and the fundamental characteristics of the system, such as wing size, efficiency, and mass. This link bridges theoretical models with practical application, providing a robust framework for control design.

The proposed control system consists of three hierarchical loops:

1. Position Control Loop: Using actuator feedback to manage the motor position.
2. Velocity Angle Control Loop: Based on a simple static gain, leveraging the control-oriented velocity angle model to achieve robust path tracking.
3. Guidance Strategy: Involves a switching strategy based on the wing's position to maintain desired flight paths, avoiding reliance on pre-computed trajectories.

The robustness of the velocity angle control loop is analytically verified, allowing for operation across varying wind conditions and different wing characteristics. Parameters are tuned quickly due to their intuitive impact on system behavior.

Experimental Results

The experimental validation shows the effectiveness of the control system across different wings and wind conditions. The authors highlight the consistency of flight paths even when conditions change, demonstrating the approach's versatility and adaptability. The robustness is affirmed by testing with wind speeds ranging from 2 m/s to 6 m/s and identifying that the control system stabilizes well within the anticipated operational bounds.

Implications and Future Work

The implications of this research are significant for the development of airborne wind energy systems. By reducing the complexity of feedback requirements and relying on a simplified control structure, the feasibility of deploying these systems in varied environments increases.

Future work is likely focused on integrating the control strategy with energy generation systems, executing complete generating cycles, and conducting comparative studies with established theoretical predictions to refine power generation capabilities.

This paper makes a substantial contribution to the field of sustainable energy technologies by pushing the boundaries of control design and implementation for airborne wind energy systems. The next steps of integrating energy conversion mechanisms and validating theoretical predictions will be crucial in advancing this promising technology.