Technology for Wood and Natural Fiber-Based Materials

Research Project

ThoR – Robot for the internal inspection of rotor blades of wind-energy turbines

The rotor blades of wind-energy turbines are subjected to high mechanical loads during operation. Material defects such as defective bonding and cracks can have severe consequences - and can even result in the total loss of the entire unit. The rotor blades must therefore be meticulously inspected, both from the inside and the outside. Until now, inspection from the inside has been possible to only a limited extent, as not all blade sections are accessible and not all defects are optically visible. In cooperation with a number of research and industry partners, we are therefore developing a thermography robot for internal inspection purposes. We are thereby contributing towards ensuring that wind turbines remain permanently safe and can be operated economically.

Device with a caterpillar drive, cameras and radiant heater on white background.
© IAI GmbH
This robot uses a thermographic camera to visualize damaged sections inside wind-energy rotor blades. The radiant heater is positioned at the very front (in this photograph: on the right side), with the thermographic camera at the back (in this photograph: on the left side).

The rotor blades of wind turbines are developing into ever more complex components. The constantly increasing rotor diameters are not the only reason for this: The blade geometries are also becoming more and more complicated in order to achieve better aerodynamic properties and lower noise radiation. A diverse range of materials is used during production, including glass-fiber- and carbon-fiber-reinforced plastics (GRP/CFRP) with polyester or epoxy resins as the basis, wood, foamed plastics and metal. Experience shows that in the past, insufficient attention had been paid to quality assurance in rotor-blade manufacture. As a result, material failures occur time and again, which lead, for example, to cracks in the flange or web. In the worst case, this ends in total failure of the rotor blade, the so-called blade loss. This is not only disastrous for the operator of the plant, who has to replace the blades, thereby incurring long downtimes: It also harms the image of wind energy, as pieces of blade flying through the air can be very dangerous for humans and animals. Furthermore, they could damage buildings and other material assets.

Foto einer Windenergieanlage (»Windrad«) auf einer grünen Wiese vor blauem Himmel.
© Photo of a wind-energy turbine in a green meadow against the backdrop of a blue sky.
The thermography robot can inspect the rotor blades of wind-energy turbines for internal damage.

The majority of rotor-blade manufacturers have now responded to this and are making efforts to improve quality control within the production process. In this context, the Fraunhofer WKI has already developed a system in which rotor blades can be examined for production defects by means of active heat-flow thermography. In this way, defects can also be detected which lie several centimeters below the surface and which are not visible from the outside. This method has proven successful and is already being utilized in industry.

However, solely defects close to the surface can be detected. For the detection of defects on the inside of the rotor blade, it must also be possible to carry out thermography from the inside.

Our project partners at the Harz University of Applied Sciences and at the Institut für Automatisierung und Informatik in Wernigerode are developing a mobile robot which is inserted into the inner cavity of the rotor blade and then moves lengthwise along the rotor blade via remote control. At the Fraunhofer WKI, we are developing a suitable thermography procedure for this purpose. With the aid of an infrared radiant heater, the rotor blade is heated whilst the robot travels along it. Damaged areas such as cracks, bonding defects or air pockets exhibit a deviating thermal conductivity and therefore alter the heat flow. The thermographic camera installed in the robot records these changes in the heat flow and converts them into images. Using special software, which was also developed at the Fraunhofer WKI, the images can be viewed and optimized on the computer (e.g. contrast improvement, noise suppression, background subtraction). The camera settings can also be adjusted via the software. Furthermore, the robot records high-resolution optical images for navigation purposes and for a better assessment of material defects visible on the surface.

The robot is to be used with both horizontally stored and already installed rotor blades. As a result, it could optimally complement quality assurance both in the production process and during operation.

Further information:

The illustration shows images of eight samples, two per row. The two images in the top row are greenish, whilst the three following rows are shaded in gray.  All images show up to six punctiform structures of differing sizes at the same respective position. The perceptibility and representation varies from image to image.
© Fraunhofer WKI | Jochen Aderhold
Samples of glass-fiber-reinforced plastic with artificial defects in the photo (upper row of images) and in the thermographic image under differing measuring conditions (lower rows of images).
Screenshot of the software developed by the Fraunhofer WKI.
© Fraunhofer WKI | Jochen Aderhold
With the software developed at the Fraunhofer WKI, the thermographic images produced by the robot can be viewed and optimized and camera settings can be adjusted.
Elongated radiant heater in operational state; the heating rod glows red-white.
© IAI GmbH
Infrared radiant heater for active heat-flow thermography.

Results

In collaboration with project partners, we succeeded in developing the thermography robot. A follow-up project for further developing is already planned. 

Project partners

  • The Experimental Factory, Magdeburg
  • Harz University of Applied Sciences, Faculty of Automation & Computer Sciences
  • IAI Institut für Automatisierung und Informatik GmbH
  • itv gesellschaft für industrie-tv mbH
  • a3Ds GmbH, automated 3D scanning

Funding

Funding body: German Federal Ministry for Economic Affairs and Energy (BMWi)

Funding reference: 0324137B

Project management: Projektträger Jülich (PTJ)

Duration: 1.3.2017 to 30.6.2020