Distant vibration measurement of wind turbines in operation (WEALyR)

Vibrometry measurement setup with laser on a pan-tilt head — © Fraunhofer IOSB, indigo
The measurement setup with laser on a pan-tilt head enables distant vibration measurement on wind turbine rotors.

Measurement arrangement diagram — Schematic representation of the measurement arrangement

Initial situation

Based on the sounds our car, washing machine or PC fan makes, everyday experience already shows us how much information is contained in the vibration behavior of devices about their functional status. For this reason, condition monitoring systems for monitoring technical equipment often use vibration sensors, whose detection capacity usually covers a much wider frequency range than the human ear. Sensors in modern wind turbines (WTG) also provide valuable data on the condition and characteristics of a turbine based on its vibration behavior. In this way, possible damage can be detected during operation or dangerous resonances or disturbing noise emissions can be eliminated during plant design.

However, effort and costs limit the number of sensors with which a WTG can usefully be equipped. Measurement data is therefore limited to a limited number of selected positions. In addition, for continuous operation, permanently integrated vibration sensors, especially in the rotor blades, often cannot be replaced during later operation in the event of a failure.

Project idea and requirements

The Fraunhofer IOSB is developing a measuring system that can record the vibrations from a distance of 200 m to 500 m completely independently of the WTG. It neither requires contact with the plant nor does it require any modifications for the measurement. Laser Doppler Vibrometry (LDV) forms the basis of the method. With this method, a vibration signal can be picked up at any point on the outer surface of the WTG. If required, LDV can also be used to detect vibrations with very high spatial resolution.

The special focus of the project is the measurement of the vibrations of the rotating rotor blades during operation. For this purpose, the laser spot of the LDV has to follow the rotational movement of the desired measuring position for a few seconds at a time and stabilized there to an accuracy of a few centimeters. For this purpose, the laser Doppler vibrometer is mounted on a pan-tilt head (SNK), a carrier platform that can be rotated around horizontal and vertical axes with high angular precision.

The challenges in the project consist in the development of a vibrometer suitable for measuring a moving object on the one hand and a tracking method for detecting the rotor movement and controlling the SNK on the other hand.

The video illustrates the operating principle of vibration measurement from a distance.

Background info and details

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Tracking

The tracking system is based on a stationary camera that captures the rotor in full format. As a first step, the wind turbine must be separated from the background in the image of this camera. This is essentially done by creating a difference between successive images in the video stream. Stationary elements, and thus elements that are identical in two images, cancel each other out, leaving moving objects such as the rotor blades.

In the second step, the rotor blade tips are detected and the motion sequence of the rotor in space is determined from this. The entire image processing must run in real time. Due to the unavoidable latency of the data processing and the control system for the SNK control, the rotor blade position must be predicted for a small time interval into the future. For this purpose, a dynamic 3D model of the wind turbine is built up in the computer based on the detected rotor blade movements and continuously updated. The projection of the 3D model onto the detector plane yields the required azimuth and elevation angles for controlling the SNK.

In addition, the laser spot of the vibrometer is detected in the camera image and its actual position is compared with the desired target position. The deviation flows as a correction into the control of the SNK and compensates, among other things, for slight distortions caused by the camera optics, parallax errors and inertia and torques of the setup that are difficult to compensate for.

To enable detection of the measuring laser spot of the vibrometer, a SWIR camera (SWIR = Short Wave Infra Red) is used for the tracking procedure. Suitable optical filters (bandpass) ensure a balanced contrast ratio between laser and ambient brightness in the image, so that the laser spot and rotor blades can be detected equally.

Laser Doppler Vibrometry

The so-called Doppler effect describes the velocity-dependent frequency shift of a light wave when it is emitted or reflected by an object moving relative to the observer. A laser Doppler vibrometer uses this effect by comparing the frequency of its emitted laser beam with the light backscattered by an object to be measured. From the frequency shift, the relative motion between the measuring device and the object can be calculated.

A vibration is a periodic motion and thus causes a temporal modulation of the measured frequency shift. Similar to the music in FM radio reception, vibration information is obtained by demodulating the measured signal waveform.

In the interest of eye safety, commercial laser Doppler vibrometers do not have sufficient laser power for the measurement distance of several hundred meters required here. In addition, rotating rotor blades cannot be scanned with such vibrometers, because the inherent motion of the blades leads to an additional macroscopic Doppler shift. Not only is its magnitude many times greater than that of the vibrations to be measured, but its value and sign also change constantly with the rotation of the rotor at a high rate. On the one hand, this means that the signal exceeds the bandwidth of conventional receivers, and on the other, it cannot be easily demodulated due to the rapidly changing frequency offset.

The laser Doppler vibrometer built in the project operates at a laser wavelength of 1.5 µm. Thus, even a higher output power is still eye-safe, allowing larger measurement distances. Different concepts are used to handle the macroscopic Doppler shift due to the rotor's own motion. In the original development on the laboratory system, an electronic control loop continuously tracked the frequency offset of the receiver to the frequency shift caused by the target motion. In field measurements on large wind turbines, A/D converters with sufficient bandwidth were used to record even the macroscopic frequency shift. Digital signal evaluation subsequently enables the separation and analysis of proper motion and vibration information.

System properties

Vibration measurements during operation, under real environmental conditions.
Non-contact measurement of all visible turbine components
No precautions or modifications to the wind turbine necessary
Scanning of vibration modes with high spatial resolution
Acquisition of spectra in the range of 10 Hz to 1 kHz
Sensitive even to very small amplitudes down to a few 10 µm

Future application possibilities

High-resolution vibration data for validation of simulation models
Structural and aerodynamic optimization of rotor blades
Evaluation of structural plant condition - lifetime extension or repowering?
Detection of new, still hidden damage by regular control measurements
Localization and analysis of sources of changed vibration behavior/damage
Identification and quantification of noise emissions and their sources during operation

Participating departments / research groups of Fraunhofer IOSB

The Optronics department (OPT) is developing the laser vibrometer in the Quantum and Laser Sensor Systems group and is responsible for project coordination and the interaction of the components.

In the Object Recognition department (OBJ), the Tracker and Tracker Evaluation group develops the image processing methods and the real-time control of the pan-tilt head.

In both departments, methods for analyzing various aspects of the acquired vibration data are being investigated.

Publications

Here you will find an overview and links to publications in technical journals, conference papers and other publications from Fraunhofer IOSB on the topic of distant laser vibration measurement on wind turbines.

To the publications

Downloads

Laser-Doppler-Vibrometrie_en.indd [ PDF 1.38 MB ]

More information

Project duration: 12.2019 to 06.2023 (predecessor projects since 2011).

The project is funded by the German Federal Ministry for Economic Affairs and Energy.

Project history and future plans

Fraunhofer IOSB has been researching distant vibration measurement on wind turbines since 2010. The following link provides a brief outline of the development and an outlook on unresolved issues.

Review/Outlook

Optronics department (OPT)

The competences of this department cover the development, optimization and evaluation of active and passive optonic systems.

To OPT department

Object Recognition department (OBJ)

The department develops and evaluates algorithms for automatic object recognition and object tracking in sensor networks.

To OBJ department