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With the newly developed “VibroTrack” laser system, scientists from Fraunhofer IOSB are measuring the vibration behaviour of wind turbines from a distance of up to 1,000 metres.
© Gerisch/Fotolia
Wind turbines
31.01.2014

The laser tracks the rotor blade and remains focussed on an exact position. This enables wind turbine operators to measure the vibration patterns of the components – at any point required.
© Fraunhofer IOSB

Laser-based remote diagnosis of rotor blades

Wind turbine rotor blades and towers vibrate during normal operation and particularly when there are gusty winds. The analysis of these vibrations plays an important role in terms of the turbine development and maintenance. This was previously only possible at selected points directly on the turbines. For this purpose the Fraunhofer Institute of Optronics, System Technologies and Image Exploitation (IOSB) has developed a laser-based process that records vibrations on towers, nacelles and rotor blades during the ongoing operation of the turbines – and from a distance of up to one kilometre.

Owing to the very nature of their structure, wind turbines have components capable of vibrating by considerable amounts, such as the tower, rotor blades and parts of the drive train. The high vibration loads often, however, cause damage that can cause the wind turbines to fail. Recording and assessing vibrations therefore provide an indispensable basis for monitoring the operating conditions, diagnosis and structural development.

Condition monitoring systems based on sensors installed in the nacelle have now become standard in modern wind turbines. These systems monitor the vibration behaviour of the drive train. Systems are also being increasingly deployed that use sensors fixed to the tower or the rotor blades to record the vibration behaviour of these extended turbine components during operation. Non-contact, in particular laser-based measurement processes are becoming increasingly interesting that can also record vibrations on turbines from larger distances without having to fix sensors to the turbines.

“Wind turbines also vibrate with normal wind speeds by up to one metre. This represents a considerable load on the material, which can be damaged and in the worst case cause the turbine to fail,” explains Dr Ilja Kaufmann, a research assistant in the Optronics department at Fraunhofer IOSB in Ettlingen. Operators therefore continually check the severity of the vibration using sensors installed in the tower and rotor blades. The disadvantage: It is only possible to measure at the points where the sensors have been applied. “A comprehensive vibration pattern for the entire wind turbine is not possible with this technology,” says Kaufmann.

Laser measurement systems for recording vibrations

The “VibroTrack” system consists of a laser directed at the turbine which can measure vibrations at any point on the surface – and from a distance of up to 1,000 metres. To enable these measurements to be also made on movable components such as the rotor blades, the laser automatically tracks their movement.
A camera and the laser are mounted on a pan head so that these can track the movements of the rotor blades. The camera takes photos of the turbine and sends them to software that processes the images and, based on the data, creates a virtual model of the rotor blades. This information is used to control the pan head so that the laser can track the rotor blades. At the same time the camera collects data about the exact position of the laser point on the rotor blade, which is about 2-3 centimetres in size, in order to stabilise this on the rotating blades. The laser therefore follows the rotor blade and remains focussed exactly on the same position. This enables as many points on the turbine as required to be scanned during the ongoing operation. “In a short period of time this creates substantially more comprehensive analyses than are possible with the permanently installed sensors,” explains Kaufmann and adds: “The measurement duration can be varied: the slower the vibrations, the longer the laser measures.”

Prototype of the diagnosis model at CeBIT

The IOSB researchers are showing a prototype of this diagnosis system at CeBIT. On a two-metre-high model of a wind turbine at the Fraunhofer’s joint stand (Hall 9, Booth E40), visitors can track the path of the eye-safe laser beam as a green point on the rotor blades. The camera images and the vibration analysis can be viewed on two connected screens.

The German government is spending 690,000 euros on funding the project, entitled “Entwicklung von Verfahren zur lasergestützten Erfassung des Schwingungsverhaltens von Windkraftanlagen”, until the end of March this year.

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