Figure 5 Tip marker displacement measurements in flapwise direction (measurement 1). Initial analyses of the measurements revealed that the deformation of the turbine can be measured with an average accuracy of ± 25 mm from a distance of 220 meters. It was also found that this error, in fact, consists of 2 different parts, namely the random and the systematic error parts. The random error is in range of ± 5mm and can be corrected by using higher resolution cameras. Systematic errors were investigated further in frequency domain to find the underlying reasons. The 3D frequency distribution of the calculated measurement errors is displayed in Figure 6. The frequencies, normalized with respect to the rotational frequency, are displayed along the X-axis. The Y-axis represents the marker number. The numbering of the markers was depicted in Figure 7. The Z-axis represents the amplitude of the Fourier transform. As can be seen from Figure 6, the systematic error also has two important components mainly caused by different physical factors having different frequencies and patterns. A dominant 1P component where P denotes the rotational frequency of the rotor is determined for markers 1 and 2. These 2 markers are placed on curved surfaces close to the root of the blade. The measurement errors calculated for these markers are expected to be mainly related to the reflection quality and marker orientation varying during the rotation. A variation in the reflection quality appears once per cycle when the marker passes through the problematic region. The 1P component of the measurement error can easily be eliminated by using a spatial filter based on the expected deformation modes (for example based on simple beam theory). 262
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