Radar scanner detects defects of rotor blades
Rotor blades are the centerpiece of a wind turbine, with their production and maintenance subject to rigorous testing procedures. Thanks to a radar scanner from the Fraunhofer Institute for Applied Solid State Physics IAF, defects in the material composition of the wind turbine blades can now be detected with far greater accuracy and visualized in a cross-sectional view.
The rotors, which are usually equipped with three blades, are the central component of all wind turbines. They convert wind into rotational energy, and then into electricity. Much like the wings on an aircraft, the blades are subjected to enormous external loads and therefore must be designed to be extremely robust.
Modern wind turbine blades are mainly constructed from glass fiber and carbon fiber reinforced plastics (GFRP / CFRP), so that they can elastically absorb the wind energy from strong gusts without breaking. For a single blade, up to 100 sheets of glass fiber webbing are layered on top of each other, shaped and then glued together with epoxy resin. Quality control is essential at this stage in production: “The difficulty lies in layering the glass fiber sheets flat before they are glued, without creating undulations and folds, and avoiding the formation of lumps of resin or sections of laminate which don’t set when applying the epoxy,” explains Dr. Axel Hülsmann, coordinator of the radar project and group manager of sensor systems at the Fraunhofer IAF.
These kinds of defects, as well as delaminations or fractures, can be identified on a large-scale using infrared thermography. “Our material scanner enables defects to be identified with even greater accuracy, as depth resolution is also possible with radar technology – even in places where ultrasound methods fail,” says Hülsmann.
At the core of the material scanner is a high frequency radar, which operates in the W band between 85 and 100 GHz with only very few watts of transmitting power. Specialized software is then used to process the transmitter and receiver signals and visualize the measurement results.
“This enables us to generate a cross-sectional view of the blade, in which defects can be identified in the millimeter range, and makes our material scanner significantly more accurate than conventional methods,” notes Hülsmann. The radar module is based on indium gallium arsenide semiconductor technology. It is extremely light and compact thanks to its monolithically integrated construction, in which different components and functions are integrated into a single chip. Measuring 42 x 28 x 79 mm, it is approximately the size of a pack of cigarettes and weighs a mere 160 grams. It has a low power consumption of approximately five watts and is fitted with an integrated microcontroller which emits measurement signals via an internet interface.