風力發電機的設計規範與維護程序必須確保零組件未超過其機械極限，當風力發電機運轉時，風力會刺激機體振動，使塔身產生擺動，在特定自然頻率下甚至可造成旋轉葉片高達1公尺的偏移。一般風力發電機的監控方式是在驅動軸上裝設感測器，提供運轉中的軸承狀態資訊，然而運轉中的葉片狀態監控需要更加複雜的遙測技術。這篇文章說明如何使用雷射測振儀進行非接觸式且長距離的量測轉動葉片，甚至不需在葉片表面塗佈反射膠條也能輕易量得葉片振動。
Wind power plant vibrations must be monitored during operation to optimize the simulation models used for design and construction, and to ensure faultless day-to-day operation by recognizing excessive material stress and fatigue prior to failure. Such preventative maintenance, or condition monitoring, is often done with the aid of vibration sensors which are placed along various sections of the drive shaft. These sensors can then monitor vibrations and provide information on the status of bearings in the power transmission. To monitor the rotor blades is much more difficult, particularly during operation, since measuring vibrations with contact sensors is only possible when using elaborate telemetry systems.
Laser vibrometry is a non-contact, optical technique for measuring vibration with zero-mass loading. The laser probe permits a long standoff distance (remote) from the measurement point, and, in the ideal case, there is little surface preparation prior to the measurement. This investigation tested the suitability of using laser vibrometers for non-contact, remote measurement of vibrations in wind power plants. The study was within the framework of a much larger project to research sensorenabled operational monitoring systems. This scientific work is part of the research network CEwind, in which the activities of many German universities are grouped together with the goal of elaborating and solving fundamental issues concerning future wind power plants, parks and infrastructure.
Experimental Issues
The wind power plant examined was an Enercon model E-30 with a nominal rating of 300 kW and a hub height of 50 m. Various vibrometer systems were placed at ground level and used for measurements on the tower shaft and on the rotor blades. In the experiment, apart from the eigen frequencies, the signal-to-noise ratio and transmission functions should be determined for both unprepared surfaces and those prepared with reflective film. Both a rotor blade and the hub had reflective film bonded in specified places in advance (Fig. 1).
For high resolution and reference measurements, an OFV-505 single point vibrometer was available. The OFV-5000 vibrometer controller was equipped with a high-resolution, digital VD-09 velocity decoder. Other measurement points were acquired using a PSV-400 Scanning Vibrometer. The scanner mirrors made it easy to align the laser to the measurement locations and with the aid of the integrated geometry scanner, it was possible to determine the coordinates automatically.
Results
Vibration measurements with a good signal-to-noise ratio can be made easily on the prepared surface, even at standoff distances of 90 m. The first eigen frequencies are at 0.47 Hz for the tower or the hub and at 1.85 Hz for the rotor blade. Without averaging, harmonics of these frequencies can be seen up through 50 Hz. On the surfaces that have not been prepared, good measurements are also possible using
the OFV-505 sensor head with an SLR Super Long Range lens. The measurement values must be limited to less than 5 Hz with a low pass filter (Fig. 2). To align and monitor the measurement spot without reflective film, a telescope with a narrow bandfilter for the laser wavelength, or other optical aid, is strongly recommended. To determine the transmission function, measurements were carried out with several vibrometers at the same time on a fixed reference position, and on various points on the rotor blade. The measurements result in noise levels of 1 μm/s for the velocity signal (at 4 mHz resolution), or respectively 0.1 μm for the displacement signal. Displacement amplitudes of up to 8 mm were observed, at moderate wind forces (12 … 28 km/hr.) during the measurements.
The equipment worked well and first attempts to make measurements during operation (rotating blades) were successful. The tower vibrations are superimposed with the periodicity occurring as a result of the rotor rotation (Fig. 3, left).
After a Fast Fourier Transformation (FFT) of the signal and applying a low pass filter, the first eigen frequency of the tower can clearly be seen at 0.47 Hz (Fig. 3, right). To be able to acquire the vibrations of the rotor blades during operation as well, the measurement would have to be made closer to the hub. There, the duty cycle for the retention period of the laser spot on the rotor blade is more favorable. With the aid of a time resolved FFT, it would then be possible to separate the tower and rotor vibrations from each other.
Summary and Outlook
Laser vibrometers are a powerful tool for remote, non contact vibration monitoring of wind power plants or other large engineered structures. Equipped with the appropriate measurement technology and a suitable measurement setup, the measurements are easily made from the ground with the plant operating or stationary even without applying reflective film.
Source: InFocus – Optical Measurement Solutions – Issue 2/2008 – ISSN 1864-9203.
HALT/HASS加速試驗技術對產品設計的重要性 
