## 3 Field Modal Testing for PV Support Structure
### Field Modal Testing Scheme  
Field modal testing integrates two complementary measurement modalities: (1) a vision-based measurement system employing camera sampling at 30 frames per second, with paper markers positioned on PV modules as measurement targets, as shown in Figure 7a; (2) velocity sensors with frequency range from 1 Hz to 100 Hz sampled at 50 Hz, as shown in Figure 7b. The instrument and its main technical parameters are shown in Table 1.  
For the first-order mode, the camera and paper markers were arranged as shown in Figure 8a. The three measurement points were clustered in a centralized area, which did not fully capture the overall structural behavior--primarily due to limitations in camera placement. The flexible PV structure was situated on a hillside, where the sloped terrain and the short spacing between rows of PV supports constrain the available space for camera positioning. For the second-order mode, the camera and paper markers were arranged as shown in Figure 8b. In this case, the markers were distributed both along the spanwise and vertical directions to more effectively capture the spatial characteristics of the mode shape. For the third- and fourth-order modal tests, velocity sensors were employed due to the low and weak vibration energy associated with these modes, which requires sensors with higher resolution to accurately record the signals, whether the excitations are artificial  
Figure 6: The first four mode shapes of the PV support structure.  
Figure 7: Attachment of the paper markers and velocity sensors to the PV support structure.  
or ambient. The velocity sensor arrangement for third- and fourth-order modal testing is shown in Figure 7(c).  
The optimal location for excitation was determined based on the FE modal analysis results, with the theoretical ideal position corresponding to the maximum response in the mode shape profile. During field testing, it was observed that the second-order torsional mode was difficult to excite using an artificial method. Here, artificial excitation refers to manually applying loads to the photovoltaic support structure--such as pushing the PV panel up or down--to induce vibrations. Therefore, ambient excitation was utilized as a supplementary approach to activate this mode. In this study, ambient excitation refers to the vibrations generated in the structure as a result of environmental loads, specifically the wind forces acting on the PV panels. For the tests with manual excitation, A duration of  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Instruments** & **Model** & **Technical Parameters** \\ \hline CMOS camera (Baumer & Camera chip: PYTHON5000 \\ VCXU-53C, Frauenfeld, Switzerland) & Maximum resolution: 2592 \(\times\) 2048 \\  & Pixel size: 4.8 mm \\  & Chroma: RGB \\ \hline Lens (Computar V3528-MPY, Mebane, NC, USA) & Focal length: 35 mm \\  & Relative aperture: F = 1:2.8 \\  & Variable aperture range: 2.8–16 \\  & TV distortion: \(-\)0.1\% \\ \hline Velocity Sensor (EY521V) & Sensitivity: 5.71 m/s \\  & resolution: 4 \(\times\) 10\({}^{-7}\) m/s \\  & frequency range: 0.5–100 Hz \\  & Maximum speed: 0.3 m/s \\ \hline \hline \end{tabular}
\end{table}
Table 1: Test instruments and their main technical parameters.  
Figure 8: Sensor placements.  
5 min was chosen to ensure not only complete coverage of the free decay stage but also to provide sufficient redundant data for multiple trials, enabling the selection of high-quality free decay segments for analysis. For tests with ambient excitation, a longer signal duration is required for accurate modal identification. Considering both this requirement and the limitation of the mobile power supply at the experimental site, the test duration was set to 120 min.  
The test conditions are summarized in Table 2.