Quality assurance of wind turbines using displacement sensors

Paul Boughton

The use of modern testing methods for wind turbines provides high quality and reliable operation. Erich Winkler reports.

Since the last amendment of EEG 2009, wind turbines have had an enormous market potential as the latest addition stipulates that electricity generation targets using renewable energies will increase from 25 to 30 per cent. The relatively new technology of modern systems still provides a lot of optimisation potential. The use of modern testing methods provides high quality and reliable operation. Therefore, displacement sensors are being utilised more and more for the development, production and operation of wind turbines.

The sensor system for controlling wind turbines is critical as the movement of the gondola and the rotors is performed automatically and must therefore be measured. Repair and maintenance work for land-based systems already have a high-risk potential. In the case of offshore systems, this high risk potential is also combined with enormous technical costs as special ships for complex tasks have long waiting times. The latest sensors help to prevent breakdowns and optimise maintenance scheduling. This starts with the design of the components.

Rotors under load

Thorough test methods are increasingly required for the development of new geometries and production technologies for wind turbine rotors. Therefore, some test rigs have been developed for load testing of rotor blades, where actual loads caused by wind and weather conditions can be accurately simulated. It is important here that the expensive rotor is not destroyed during testing. Standard rotors are currently between 40m and 60m in length and are manufactured in a half shell sandwich arrangement made from glass fibre reinforced plastic.

The Fraunhofer Institut IWES, Bremerhaven has developed a test rig that can be used to test rotor blades up to 70m in length. The tip of the rotor blade can be distorted by up to 10m due to mechanical loads. For this, the rotor blade is mounted horizontally in the test rig. Steel cables are attached to the rotor using guide pulleys and connected to the rotor blade at various positions either directly or via mechanical clamps. Twelve draw-wire sensors are used on the test rig for measuring the distortion. Two sensors per traction point measure the deflection and torsion of the rotor blade. For this, the sensors are mounted on rails on the ground; the measuring wire is attached to prefabricated eyes on the terminals. The simple handling and the robust design of the sensors are particularly impressive. The draw-wire sensors in this application operate with measuring ranges between 3m and 10m. The digital signal output is directly included for further simulations.[Page Break]

Inspected weld seam

The tower of a wind turbine is usually considered as the least noticeable part. However, with a height of 130m and a weight of several hundred tonnes, the tower is the largest and heaviest part of a wind turbine. The unnoticeable yet extremely important task of supporting the gondola and resisting all types of weather conditions requires a high degree of quality and reliability. Tubular steel towers are a very frequent structure to use and usually consist of two to five segments, each one up to 30m in length. A segment of a tower is rolled from steel sheet 20mm to 40mm thick and then welded. The individual segments are bolted or welded together, whereby a flange has to be welded onto the segment for the bolted variant. In doing so, each weld seam must withstand the very high loads on the tower in the finished condition. For safety reasons, a quality inspection of the weld seams is therefore a critical step.

Laser scanners from Micro-Epsilon are already being used in many other industry sectors for the automatic, precision inspection of weld seams. The weld seam inspection of pipelines provides a suitable comparison.

A laser projects a spot or a line onto an object. The diffuse reflected light there is recorded using the light-sensitive receiving unit (CMOS). If the object departs, the angle of light incidence changes as well as the illuminated point on the receiving element.

During a measurement, the diffusely scattered light of the laser line is detected by a highly sensitive CMOS chip, which produces a precise image of the surface profile. Each change of the profile changes the displayed line, resulting in an altered image on the chip. Each line can be considered as a series of points.

If the scanner traverses over the weld seam, a 3D image of the object is produced by apposing the individual line profiles. A so-called 'point cloud' is formed because the image is composed of thousands of individual measurement points. Using this method, a faultless weld seam is guaranteed for wind turbines.[Page Break]

Profile scanners

After several years in use, components are dismantled and routinely inspected for wear or damage. For this, scanCONTROL profile scanners are also used in order to produce a 3D image of the gear wheel.

Several laser triangulation sensors that establish a movement of the gear wheel under load are used for testing the stiffness of gear wheels for pitch control. These sensors measure the distance to the gear wheel with one laser spot; in doing so, each movement is registered.

Erich Winkler is product manager, optical sensors, MICRO-EPSILON MESSTECHNIK GmbH & Co KG, Ortenburg, Germany. www.micro-epsilon.com.

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