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Hardware in the loop based testing for autonomous airborne wind energy systems


D. Aregger, H. Hesse, F. Gohl, J. Heilmann, C. Houle, R. S. Smith, R. H. Luchsinger

Airborne Wind Energy Conference (AWEC), Delft, Netherlands

In the emerging field of airborne wind energy a crucial part of the system development is the validation of analytical results and simulation data. Often there exists a gap between theory and real life test data cause of e.g. unsteady wind conditions or hardware constraints. At present, classic design and testing strategies for airborne wind energy system development include analytical calculations, modelling, simulation, wind tunnel testing, tow testing or field testing. While e.g the simulator approach allows for full system observability, it often lacks exact modelling of system constraints and dynamics. In field testing problems can be for instance wind availability or limited repeatability of similar test conditions. A possible solution for those problems is the integration of real hardware subsystems into simulation, called hardware in the loop (HIL) testing. In a hardware in the loop simulation one splits the system into two parts. One subpart is referred to as the system under test (SUT). This part can be a controller or an actuator that exists in reality. The other subpart is referred to as the plant simulation. This part is a mathematical model of a dynamic system that interacts with the SUT.

A ground generation based airborne wind energy system consists in general of three main components: The groundstation, the tether and the kite. For the particular experimental HIL setup a testing method including two groundstations was developed. The two groundstations are directly connected by means of two tethers. Therefore a physical force interaction between the two groundstations is possible, see Figure 1 in attached abstract. One groundstation acts as the SUT whereas the other groundstation acts as a kite emulator. Steering inputs to the groundstation (SUT) result in certain tether displacements. This tether movements are fed to the simulator, which in turns calculates the resulting tether forces at every simulation time step. The simulator uses the vortex lattice method combined with a tether sag model to calculate the virtual line force values [1]. These virtually calculated forces are then translated into physical tether pulling forces by the second groundstation. The physical line forces in turns affect again the line movement on the groundstation (SUT) which closes the hardware loop.

The setup finally results in a weather independent laboratory test environment for extensive groundstation testing. It is used from simple groundstation parameter tuning, up till autonomous pumping cycle flight path strategy improvement.

[1] F. Gohl, R. H. Luchsinger, Simulation Based Wing Design for Kite Power, Airborne Wind Energy Green Energy and Technology ed. Ahrens U, Diehl M and Schmehl R (Berlin, Heidelberg: Springer Berlin Heidelberg) chap 18 ISBN 978-3-642-39964-0

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