## 4 Test and Analysis
### Segmentation Test  
In order to explore more details about the PVDF characteristics applied in flapping wing, the PVDF wing skin was divided into 4 rows and 4 columns to study the relationship of flapping deformation and electric data. These divided areas were named as \(V_{ij}\) (\(I\) = 1-4, \(j\) = 1-4) respectively and 16 areas of the PVDF film were obtained totally. Given the size of small area of \(V_{44}\), \(V_{44}\) was merged into \(V_{43}\) part. There was also one minor thing that should be considered, that all grid line area was measured into relative parts, considering area factors. In the test, we took 10 Hz as the test frequency and conducted low-pass filtering on the obtained signal. To make the image more clearly shown in one picture, the real time voltage data was processed in advance. The voltage value of the first row of PVDF divided areas was reduced to half, the voltage value of third and fourth row multiply 2 and 3 respectively, the processing result of these areas was shown in Figure 11. It was obvious that the real time voltage signal of all these grid blocks displayed trigonometric function waveform and the first-row grid blocks had the maximum voltage value compared with others in column direction. According to the Equation \(U=4\pi hQ/\epsilon S\), we could know that the area size would have effect on the value of output voltage and power. In order to investigate more piezoelectric characteristics of different PVDF areas, the area size factor should be eliminated. So, the area normalization method was proposed and \(V_{11}\) was set as the standard block area,  
Figure 11: Output voltage waveforms for each row of grid blocks. Figure 10: The voltage and power versus frequency. then others compared with the reference grid block getting their area normalization scale factors. The area normalization scale factors for each grid block can be found in Table 1.  
Combining the factors in Table 1 and the real voltage of each part in Figure 11, we could get the modified RMS output voltage value and piezoelectric level of each block shown in Table 2 and Figure 12 respectively. By analyzing the data obtained from the piezoelectric experiments, we found that the piezoelectric voltage of wing-root area far exceeds the voltage of other parts and whole PVDF film wing shown in Figure 7. It was not hard to deduce that wing-root area also has the largest amplitude of deformation with the fastest variation rate at the same time. Through further analysis, it was found that both of real time voltage amplitude and RMS voltage decline numerically from the wing-root area to the edge area. Along the wingspan direction, the amplitude of deformation of the PVDF wing showed the similar law that the curving degree or deformation curvature decreased gradually from wing root to wingtip verified by the camera. Because of the comprehensive effect of wing inertia and air-resistance, the area near the wing root apparently bent more easily and rapidly. According to the Equations (14) and (15), wing root area had the largest output voltage and power which was verified in Figures 11 and 12. In conclusion, the RMS voltage level could reflect the curving degree of PVDF wing directly in situ way, which provided the feasibility to detect the flutter condition without equipping other sensors and adding extra weight. Expect for the amplitude of voltage, the phase was also very critical for signal analysis. With the help of MATLAB, the primary data was processed quickly and accurately. Firstly, the primary voltage data was filtered by the Butterworth low-pass filter, then the voltage \begin{table}
\begin{tabular}{c c c c c} \hline \hline
\begin{tabular}{c} **Column** \\ \end{tabular} & **Row** & **1** & **2** & **3** & **4** \\ \hline
1 & 1 & 0.824 & 0.677 & 0.412 \\
2 & 1 & 1 & 1 & 0.913 \\
3 & 1 & 1 & 1 & 0.741 \\
4 & 0.882 & 0.967 & 0.708 & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Area normalization scale factors of different wing blocks. Figure 12: Comparison of the voltage output on each grid for 10 Hz.