Henrik Hesse Postdoctoral Researcher
ETH Zurich Automatic Control Laboratory
Physikstrasse 3, ETL K22.2 8092 Zurich Switzerland hesseh@ethz.ch control.ee.ethz.ch
Autonomous Pumping Cycles of Tethered Wings
Henrik Hesse1, Tony A. Wood1, Alexander Millane1, Aldo U. Zgraggen1, Roy S. Smith1, Flavio Gohl2,4, Dino
Costa2,4, Rolf H. Luchsinger2,4, Damian Aregger3, Jannis Heilmann3, Corey Houle3,4 1Automatic Control Laboratory, ETH Zurich
2Center for Synergetic Structures, Empa 3Institute of Aerosol and Sensor Technology, FHNW
4TwingTec AG
The focus of this work is on autonomous pumping op-eration of rigid tethered wings for ground-based power generation. Twings, an acronym for tethered wings, pro-vide increased aerodynamic performance without signif-icant weight penalties. In this work we focus on con-trol 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 vec-tor orientation as feedback variable. To achieve figure-eight trajectories during the traction phase we use a tar-get switching strategy. During retractions the wing is ac-tively 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 opera-tion 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 perfor-mance during traction and retraction phases. This talk therefore further gives an overview of our parallel devel-opment 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 ultwideband ra-dios, and inertial readings from an inertial measurement unit [2]. The resulting approach improves the estima-tion during retracestima-tion and eliminates lag introduced from tether dynamics. In contrast, we can also model the steer-ing dynamics of the tethered wsteer-ing as a delayed dynam-ical 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 con-trol design. The talk presents experimental results which demonstrate the improved control performance. References:
[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 Esti-mation 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 Au-tonomous Kites. American Control Conference, Chicago, USA, July 2015
WIND ENERGY 2.0
