Kurt Geebelen

Real-time control of a kite-model using an auto-generated nonlinear MPC algorithm

Abstract Airborne wind energy systems like power-generating kites promise to become a sustainable and safe alternative for todays fossil fuel based energy production. Controlling these systems is still a challenge due to the fast and highly nonlinear dynamics. A real-world prototype, which has been build at the K.U. Leuven, consists of a carousel which drives an airplane being attached at one of its arms. In the first part of this paper, we propose a nonlinear grey box model which is developed for controlling this kite carousel. In the second part, we present a code generation tool for nonlinear real-time MPC algorithms which exports plain C-code tailored to particular model dynamics. Numerical closed-loop simulations of the kite carousel show that auto-generated code allows to solve the resulting dynamic optimization problems within less than 900 microseconds.

Moving Horizon Estimation with a huber penalty function for robust pose estimation of tethered airplanes

This paper presents a Moving Horizon Estimator (MHE) for estimating the position and orientation (pose) of moving objects, in particular for tracking tethered airplanes for Airborne Wind Energy systems. In this application, absolute pose measurements are captured by a marker based stereo vision system. These measurements are fused with measurements of the angular velocity and linear acceleration from an Inertial Measurement Unit (IMU). In our MHE, the IMU measurements in the intervals between camera frames are modeled as samples of a superposition of orthonormal polynomial basis functions. This results in a MHE formulation that requires fewer optimization variables, enabling faster solution. In order to achieve robustness to marker detection errors, a formulation based on the Huber penalty is also presented. We show that our robust MHE formulation outperforms a MHE formulation using the ℓ2-norm and a traditional extended Kalman filter.

An experimental test set-up for launch/recovery of an Airborne Wind Energy (AWE) system

This paper describes an experimental set-up for AWE systems. A novel approach of launching a rigid wing tethered airplane, using a rotational start-up, is presented. A test set-up performing this rotational start-up has been designed and built at KU Leuven. A scale analysis of this novel approach is conducted and provisions are made to accommodate future modifications to the experimental set-up. A sensor suite consisting of a stereo vision system and an inertial measurement unit are used for sensor fusion with an extended kalman filter (EKF). This EKF estimates the position, velocity and orientation of the airplane. Numerical and experimental validation of the EKF are presented in this paper as well.

An Experimental Test Setup for Advanced Estimation and Control of an AirborneWind Energy System

This chapter gives a detailed description of a test setup developed at KU Leuven for the launch and recovery of unpropelled tethered airplanes. The airplanes are launched by bringing them up to flying speed while attached by a tether to the end of a rotating arm. In the development of the setup, particular care was taken to allow experimental validation of advanced estimation and control techniques such as moving horizon estimation and model predictive control. A detailed overview of the hardware, sensors and software used on this setup is given in this chapter. The applied estimation and control techniques are outlined in this chapter as well, and an analysis of the closed loop performance is given.

Optimal periodic control of power harvesting tethered airplanes: How to fly fast without wind and without propellor?

This paper proposes reversed pumping as a strategy to keep tethered kites airborne in a wind-free condition without any on-board propulsion. In the context of kite power, it is shown to be a necessary addition to a rotational startup scheme. The enhanced scheme ultimately allows for injection into power harvesting orbits at high altitude, in case no wind is present near ground level. The need for reversed pumping is advocated with scaling laws, and is demonstrated in simulation by solving a proposed open-loop periodic optimal control problem. The obtained periodic orbits can be used as reference for feedback control.

In-flight Estimation of the Aerodynamic Roll Damping and Trim Angle for a Tethered Aircraft based on Multiple-shooting

Abstract The Airborne Wind Energy paradigm proposes to generate energy by flying a tethered aircraft across the wind flow. Accurate models for tethered flight are essential for the control and optimization of airborne wind energy systems. This paper proposes an estimation of the aerodynamic roll damping of a tethered aircraft based on Inertial Measurement Unit data only, gathered at a high aircraft angular velocity. The resulting system dynamics are nonlinear and the estimation problem is non-convex. Because the aircraft is subject to disturbances, the perturbations are estimated alongside the uncertain parameters. In order to handle the unstable aircraft dynamics, a discretization of the model equations based on multiple-shooting is proposed.