Video-Tachometer Methodology for Wind Turbine Rotor Speed Measurement

Abstract

The measurement of the rotational speed of rotating machinery is typically performed based on mechanical adherence; for example, in encoders. Nevertheless, it can be of interest in various types of applications to develop contactless vision-based methodologies to measure the speed of rotating machinery. In particular, contactless rotor speed measurement methods have several potential applications for wind turbine technology, in the context of non-intrusive condition monitoring approaches. The present study is devoted exactly to this problem: a ground level video-tachometer measurement technique and an image analysis algorithm for wind turbine rotor speed estimation are proposed. The methodology is based on the comparison between a reference frame and each frame of the video through the covariance matrix: a covariance time series is thus obtained, from which the rotational speed is estimated by passing to the frequency domain through the spectrogram. This procedure guarantees the robustness of the rotational speed estimation, despite the intrinsic non-stationarity of the system and the possible signal disturbances. The method is tested and discussed based on two experimental environments with different characteristics: the former is a small wind turbine model (with a 0.45 m rotor diameter) in the wind tunnel facility of the University of Perugia, whose critical aspect is the high rotational speed (up to the order of 1500 RPM). The latter test case is a wind turbine with a 44 m rotor diameter which is part of an industrial wind farm: in this case, the critical point regards the fact that measurements are acquired in uncontrolled conditions. It is shown that the method is robust enough to overcome the critical aspects of both test cases and to provide reliable rotational speed estimates.

Instantaneous Rotational Speed Measurement of Wind Turbine Blades using a Marker-Tracking Method

“…3, the vector 𝑣 ⃗ 𝑖,𝑗 is formed from the center of the rotation to the marker coordinate; with 𝑖 ∈ {1, … ,3} as blade index and 𝑗 ∈ {1, … , 𝑘} as the frame number. The rotation angle (θ 𝑖 ) during the sampling interval is then calculated using the geometric definition of vector dot product according to (1).…”

“…This is because the rotational speed of blades is related not only to the wind turbines' performance but also to their structural health. Therefore, accurate and real-time measurement of instantaneous speed of wind turbine blades is helpful to detect potential faults in advance and prevent major failures, thus reducing the downtime of wind turbines and decreasing maintenance costs [1].…”

“…It is mentioned in their paper that the data collection can only be performed around sunset so that the retroreflectors are bright enough for the cameras to pick up [10]. In addition, recently, Natili et al [1] analyzed the frequency response of image correlation values. Although this method has proven robust and produces accurate results, it relies on precisely tuned parameters and many padded zeros used in the image correlation algorithm.…”

Vibration damping of a vertical axis wind turbine in operating conditions

“…This method of analysis has been applied to the in field measurement, both for rooftop and pole installation during some startup events of the turbine, as to obtain a linearly increasing rotational speed signal from the machine. The basis of this spectral analysis lays on the Short Time Fourier Transfor (STFT), explained in detail in [7]: it is the application of the Fast Fourier Transform (FFT) on contiguous segments of a signal; this approach is particularly useful for the study of non-stationary signals, in order to highlight how the spectral components evolve in time.…”

RFTacho: Non-intrusive RF monitoring of rotating machines

“…In addition, speed information enables the detection of local incipient internal component faults, such as gear teeth wear or cracks, which are undetectable using vibration sensors alone [11]. As a result, the rotating machine's speed information is crucial to understanding nominal performance or to predict possible future failures [6,29]. Tachometers are state-of-art for accurate rotation speed measurement.…”