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Autonomous Pumping Cycles of Tethered Wings


H. Hesse, T. A. Wood, A. Millane, A. Zgraggen, R. S. Smith

Airborne Wind Energy Conference (AWEC), Delft, Netherlands

The focus of this work is on autonomous pumping operation of rigid tethered wings for ground-based power generation. Twings, an acronym for tethered wings, provide increased aerodynamic performance without significant weight penalties. In this work we focus on control of Twings developed at TwingTec and operated on the two-line, ground-based Airborne Wind Energy (AWE) system developed at Fachhochschule Nordwest-schweiz (FHNW). In a two-phase pumping operation Twings have the inherent advantage that during the retraction phase they function like a glider and can be stabilised even in the absence of tether forces.

To achieve autonomous pumping operation of Twings we adopt the control strategy in [1] using the velocity vector orientation as feedback variable. To achieve figure-eight trajectories during the traction phase we use a target switching strategy. During retractions the wing is actively depowered using an elevator and guided towards the side of the wind window where the tethers are reeled in under low tension. During this aggressive manoeuvre, however, the Twing can reach high flight velocities. In this talk we therefore outline how the two-phase strategy in [1] has been adapted to achieve reliable pumping operation with rigid Twings on the FHNW platform including a torque-based reeling control strategy.

Tether dynamics during operation at high altitudes have been found in experiments to introduce significant time delay in the dynamics of the velocity vector orientation. Such system delay causes reduction in control performance during traction and retraction phases. This talk therefore further gives an overview of our parallel development to either incorporate system delay in the control design or reduce estimation delay through sensor fusion. The latter uses an estimator which fuses measurements from range sensing, based on novel ultra-wideband radios, and inertial readings from an inertial measurement unit [2]. The resulting approach improves the estimation during retraction and eliminates lag introduced from tether dynamics. In contrast, we can also model the steering dynamics of the tethered wing as a delayed dynamical system [3]. The parameters of the delayed model, which are identified online from measured data, are then explicitly utilised to account for system delay in the control design. The talk presents experimental results which demonstrate the improved control performance.

[1] Zgraggen A., Lorenzo F., Morari M.: Automatic Retraction and Full Cycle Operation for a Class of Airborne Wind Energy Generators. arXiv:1409.6151, September 2014.
[2] Millane A., Hesse H., Wood T.A., Smith R.S.: Range-Inertial Estimation for Airborne Wind Energy. IEEE Conference on Decision and Control, Osaka, Japan, December 2015, (submitted).
[3] Wood T.A., Hesse H., Zgraggen A., Smith R.S.: Model-based Identification and Control of the Velocity Vector Orientation for Autonomous Kites. American Control Conference, Chicago, USA, July 2015.

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