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Delegates are invited to meet and discuss with the poster presenters during the poster presentation sessions between 10:30-11:30 and 16:00-17:00 on Thursday, 19 November 2015.

Lead Session Chair:
Stephan Barth, ForWind - Center for Wind Energy Research, Germany
Christoph Heilmann BerlinWind GmbH, Germany
Co-authors:
Christoph Heilmann (1) F Anke Grunwald (1) Michael Melsheimer (1) Martin Peters (1) Jan Mueller (1)
(1) BerlinWind GmbH, Berlin, Germany

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2015 and are published here unedited

Dr. Christoph Heilmann has been working in the wind industry for fiveteen years. He is currently Head of the R&D department at BerlinWind GmbH, Germany. He studied mechanical engineering at the Technical University of Berlin, Germany. After finishing his doctorate in 2005, he was for five years Project Manager at Deutsche WindGuard Dynamics GmbH leading R&D projects and wind energy training courses. Since 2009 he is at BerlinWind GmbH, developing wind turbine rotor balancing and load measuring systems, as well as photometric and laser-based blade angle measuring systems. Additionally, he is involved in consulting.


Poster


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Abstract

Development and testing of video-based motion analysis at wind turbine

Introduction

Load and vibration measurements at wind turbines (WT) are necessary for design simulation validation. But as well during WT operation it is useful to measure and analyse the vibrational behaviour, e.g. verifying the tower’s natural frequency since the rotor speed or blade passage may not excite it (resonance). For special cases, measuring the clearance between blade tip and tower is of interest. Furthermore, a share of 45% of WT is affected by rotor imbalance (RI) limit exceedence, which causes accelerated fatigue through increased loads. RI is typically determined through measuring the axial, lateral and torsional tower-nacelle vibrations using accelerometers installed in the nacelle. This requires time consuming climbing, equipment craning and installation. Therefore, video-based vibration measurements are promising to save time and have the advantage that in a 2D image vibration can be tracked at several points in two linear directions and object rotation.

Approach

In a research project funded by the German government, the measurement system and evaluation software for video-based motion analysis at WT was developed. For uncertainty analysis, simultaneous measurements with accelerometers were carried out at a special high-precision linear motion lab test stand. Several applications were field tested successfully at Multi-MW WT.

Main body of abstract

In order to get accurate quantitative results there are several challenges:
Motion tracking requires suitable tracking objects throughout the entire film. Installing special markers at nacelle, blade or tower is very time consuming but an additional option.
Infavourable weather and illumination conditions on site (low clouds, rain or fog) hinder or blur filming, while they not disturb a measurement using accelerometers. Changing light and moving shadows disturb evaluation. Applying filters during image processing may help but may falsify measurement results. Wind-induced camera vibration is an issue.
For good image resolution and quality, a high-quality camera system and accurate camera setup are necessary since sub-pixel accuracy is required for measuring e.g. imbalance-related amplitudes of 2…20 mm at the rotor’s rotational frequency despite large hug height and gust-related displacements of more than 2 m.
Careful image calibration is needed for every film to transform displacements from Pixel to "real-world-length" .
A reliable coordinate transformation from the steady camera coordinates to the yawing nacelle coordinates is necessary for RI measurements. Blocking the yaw produces falsification by inclined inflow.
Simultaneous multi-point tracking reduces significantly the evaluation time and allowas order analysis using a generated rotor speed signal from tracking of re-appearing objects at the rotor.
Handling and storage of large files, i.e. a 20 min full HD video file (30 fps) of 2 GB is huge compared to a 4 MB file from a simultaneous acceleration measurement with 4 sensors.

In first applications during root cause analyses natural frequencies and eigen modes of blade and tower were successfully measured. Nacelle and blade displacements of several meters during gusty full-load operation and emergency shut-downs were evaluated . RI-related amplitudes in the range of 1 mm were successfully determined at WTs with hub height up to 140 m. Video-based strain gauge calibration for load measurements was performed, too.


Conclusion

Video-based motion analysis by filming from the ground is an innovative powerful tool feasible to investigate quickly and reliably the structural behaviour during WT design, prototype testing and daily operation. The expert method provides for various applications simultaneous multi-point motion measurements without sensor installation.


Learning objectives
What are the advantages and applications of video-based motion analysis at WT?
What are the major challenges for this method if applied to measure in the field WT and its components?
What is the benefit of this method for designand testing but as well for inspections and root cause analysis during WT operation?