simultaneously by using 4 CCD (charge coupled device) cameras while the turbine was rotating. The response data obtained from the tower markers are not presented or used in this article since tower measurements were interrupted each time a blade passed in front of it. The utilized photogrammetric measurement setup can be seen in Figure 4. Initial results show that the deformations on the turbine can be measured with an average accuracy of ±25 mm from a measurement distance of 220 m. Preliminary analyses of the measurements also show that some of the important turbine modes can be identified from photogrammetric measurement data. More detailed information about these measurements and the corresponding data analyses can be found in (22) and (23). Figure 4 Photogrammetric measurement setup. A third system, which has already been installed in the turbine to monitor the wind loading on the structure, consists of 6 strain gauges placed at the root region of the three blades (2 strain gauges per blade) and 2 strain gauges located at the tower base. These strain gauges are used to monitor the flapwise and edgewise vibration of the blade and the tower and have a sampling frequency of 32 Hz. The strain gauge data used in this work was provided by ECN turbine monitoring program. Table 1 summarizes the main features of measurements taken by these 3 different measurement systems. Measurement type fs [Hz] Parked turbine Rotating turbine Measurement blocks Strain gauge 32 X X continuous Photogrammetry 28 - X 21 secs Laser Doppler vibrometer 1280 X - 294 secs Table 1 Main features of the measurements 260
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