## 4 Analysis of Low Velocity Impact Experimental Results
### Analysis of Low Velocity Impact Energy Recognition  
Combining with the impact energy feature sets of three impact energy, the impact energy eigenvalue ranges of the variable thickness composite laminate at 1, 2, 3 J are (55.83, 87.93), (290.74, 340.25) and (387.45, 453.98) respectively, while the impact energy \begin{table}
\begin{tabular}{l c c c c c c c c c c c} \hline \hline
\begin{tabular}{c} **Energy** \\ \end{tabular} & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **10** & **11** & **12** \\ \hline
1 & 49.25 & 52.15 & 59.05 & 69.24 & 78.80 & 88.72 & 53.02 & 56.02 & 61.23 & 71.18 & 85.54 & 91.38 \\
2 & 272.12 & 282.25 & 306.39 & 307.66 & 310.74 & 345.08 & 284.60 & 293.77 & 300.11 & 321.70 & 335.98 & 345.35 \\
3 & 362.63 & 376.66 & 407.97 & 409.83 & 415.20 & 461.33 & 379.24 & 390.94 & 399.58 & 429.72 & 447.73 & 459.18 \\ \hline
\begin{tabular}{c} **Energy** \\ \end{tabular} & **13** & **14** & **15** & **16** & **17** & **18** & **19** & **20** & **21** & **22** & **23** & **24** \\ \hline
1 & 63.86 & 66.21 & 69.52 & 71.10 & 85.59 & 94.51 & 74.98 & 80.53 & 83.78 & 83.74 & 80.58 & 88.75 \\
2 & 303.98 & 301.98 & 327.70 & 314.53 & 336.19 & 346.04 & 333.77 & 337.42 & 331.70 & 343.98 & 338.05 & 344.87 \\
3 & 405.58 & 402.03 & 436.70 & 419.37 & 448.03 & 461.76 & 444.79 & 449.63 & 441.42 & 458.82 & 449.93 & 458.33 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Impact energy eigenvalues at 1–3 J. \begin{table}
\begin{tabular}{l c c c c c c c c c c c c} \hline \hline
\begin{tabular}{c} **Energy** \\ \end{tabular} & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **10** & **11** & **12** \\ \hline
1 & 55.83 & 58.73 & 65.63 & 69.42 & 73.53 & 82.14 & 59.60 & 62.60 & 66.23 & 71.92 & 80.27 & 84.80 \\
2 & 308.46 & 318.32 & 340.25 & 307.22 & 290.74 & 320.46 & 319.91 & 327.84 & 324.32 & 325.84 & 315.29 & 319.77 \\
3 & 411.09 & 424.20 & 453.98 & 409.38 & 387.45 & 427.02 & 426.35 & 436.89 & 432.23 & 434.19 & 420.16 & 426.10 \\ \hline \hline
\begin{tabular}{c} **Energy** \\ \end{tabular} & **13** & **14** & **15** & **16** & **17** & **18** & **19** & **20** & **21** & **22** & **23** & **24** \\ \hline
1 & 63.86 & 66.21 & 69.52 & 71.10 & 85.59 & 94.51 & 74.98 & 80.53 & 83.78 & 83.74 & 80.58 & 88.75 \\
2 & 303.98 & 301.98 & 327.70 & 314.53 & 336.19 & 346.04 & 333.77 & 337.42 & 331.70 & 343.98 & 338.05 & 344.87 \\
3 & 405.58 & 402.03 & 436.70 & 419.37 & 448.03 & 461.76 & 444.79 & 449.63 & 441.42 & 458.82 & 449.93 & 458.33 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Impact energy eigenvalues of the variable thickness composite laminate at 1–3 J.  
Figure 9: Spectrum of the third-order component of the signals at the impact point a (60 mm, 160 mm) using whitening EMD algorithm. By analyzing the experimental results, the following problems can be further discussed. (1) The relationship between the thickness of the plate and the number of sensors required. For the composite plate with general uniform thickness, a slight increase in the thickness of the plate does not need to add more sensors, but for the variable thickness plate, because the impact signal aliasing in its interior is more serious, which can be seen from the same sensor network, the recognition error of the large thickness area is greater than that of the small thickness area. (2) The relationship between sensor location and impact energy monitoring and identification results. For this variable thickness specimen, two 3-sensor combinations in sensor network were selected as examples to compare impact energy monitoring and identification results. As shown in Table 8, the average identification error of sensor combination in small thickness area is 7.73, which is better than that in large thickness area by 9.12. This is because the greater the thickness, the higher the degree of signal aliasing and attenuation. This shows that the position of the sensor will affect the monitoring and recognition results. (3) How to find the best sensor position related to the impact point.