# A Circularly Polarized Millimeter Wave Radar for Wind Turbine Sensing
## 5 Radar Experiments
### Radar Experiments on a Practical Wind Turbine  
Practical experiments are performed in a wind turbine using an mmWave radar sensor to verify the proposed method and to analyze the performance of the blade tip clearance detection. The circularly polarized radar is installed on the top of the wind turbine through a bracket, as shown in Figure 15a. The radar is initially vertically oriented towards the ground and then rotated at a specific angle towards the blade tip, such that the radar is pointed toward the center of the clearance area. The mmWave FMCW radar uses high frequency chirp signals to scan the scene. As the chirp signal encounters a blade tip target, the signal is reflected and picked up by the receiving antenna. The RF-integrated chip in the antenna extracts the echo, which contains valuable target information. The W-band RF signal obtained is then fed into a  
Figure 14: Measurement data of millimeter wave radar. Figure 15: Radar installation position. mixer to obtain the intermediate frequency (IF) signal, which is further processed for signal analysis. The signal processing techniques utilize the analog-to-digital converter (ADC) data to generate range-Doppler (RD) maps by executing a 2D-FFT in every discrete frame to calculate the distance R and speed V of the target. Subsequently, the constant false alarm rate algorithm (CFAR) computes the noise level in the RD matrix and extracts the blade tip signal. Afterward, we use the direction of arrival (DOA) algorithm to determine the income wave direction \(\theta\). Ultimately, the target's three-dimensional information (R,V,\(\theta\)) is translated into the clearance value by the formula clearance value \(\Delta\)Y = Rsin\(\theta\) shown in Figure 15b. To analyze the turbine's performance, the range-Doppler spectrum after CFAR of the radar is extracted and displayed in Figure 16. When the turbine is functioning properly, the blade tip bends inward, and both the radar root and tip area are visible. At a distance of 73.4 m, the blade tip moves at around 1.8 m per second. The background and signal intensity are 27 dB and 34 dB, respectively, resulting in an SNR of 7 dB. The radar system used in the wind turbine can operate continuously and capture the blade tip target to record the clearance curve and rotor speed curve accurately. The clearance value is calculated by filtering out the tip point of the blade, and the real-time rotational speed of the turbine is calculated based on the time difference between two adjacent clearance data. The collected data, presented in Figure 17, displays the blade clearance values and the wind turbine rotor speed curves of the target detection radar, represented by black and red lines, respectively. During testing, the wind turbine operates at a speed range of 6 to 12 rounds per minute (RPM), with a clearance range of 6 to 12 m. As clearance measurement is a relatively new concept with limited reference standards, it is imperative to ascertain the reliability of the clearance value. This paper proposes two ways of checking the accuracy of the results. In the first place, to calibrate the maximum and minimum clearance value accurately, manual measurement can be obtained at these limit positions. Table 4 provides the calibration values for maximum and minimum rotor speeds. At a minimum wind turbine rotor speed of 6 RPM, the manual calibration value is 12 m, and the clearance value measured by radar is 11.534 m. Similarly, at a maximum rotor speed of 12 RPM, the manual calibration value is 5.5 m, and the clearance value measured by radar is 5.85 m. The deviation of \(\pm\)0.5 m between the two measurement methods demonstrates the high accuracy of the proposed scheme at 0.5 m. Secondly, the inverse relationship between rotor speed and clearance also reflects the accuracy of the results. This is because when the blade rotor speed is higher, there is a greater wind speed  
Figure 16: Range-Doppler spectrum. and pressure on the wind blade, which results in a smaller blade-tip clearance to the tower.