# Mini-Crack Detection of Conveyor Belt Based on Laser Excited Thermography
## 4 Experimental Setup, Procedure, Results, and Analysis
### Experimental Results  
Figure 10 shows a temperature images of three surface conditions of the conveyor belt, and the temperature image with a size of 150 \(\times\) 150 mm\({}^{2}\) comes from the original image with a size of 500 \(\times\) 625 mm\({}^{2}\). The first one is the healthy conveyor belt. The second one is the conveyor belt with a mini-crack with length of 100 mm and width of 1 mm, and the third one is the conveyor belt mini-crack with length of 100 mm and width of 5 mm. It takes 3 s for each conveyor belt mini-crack case with the actual size of 150 \(\times\) 150 mm\({}^{2}\) to be scanned based on the conveyor belt running speed of 0.05 m/s, and the temperature of the thermal images of four groups of mini-cracks were collected at four time points of 0 s, 1 s, 1.8 s, and 2.4 s respectively. By fusing the surface temperature thermogram of key feature regions of each frame during the whole scanning process, the mini-crack feature images can be obtained. Figure 11a shows the original images of mini-cracks with length of 100 mm and width of 5 mm, and Figure 11b reveals the mini-crack feature image. Figure 11c shows the gray mini-crack feature image. When the surface temperature is above the threshold, the surface appears white, while other areas are shown in black. In order to identify the size of the mini-crack more quickly, the gray feature image is enhanced, as shown in Figure 11d. As shown in Figure 11e, the binary image processing is adopted to achieve feature segmentation. Finally, the mini-crack size is identified by Hough transform. The length and width of the mini-crack are measured as 99.41 mm and 4.71 mm respectively, as shown in Figure 11f. The error of mini-crack size estimation is about 3.24%. Figure 9: Mini-crack category of conveyor belt (length = 150 mm; width = 250 mm). Figure 10: Surface temperature thermographs of three surface conditions of the conveyor belt. Table 1 shows the measured results of three sizes of mini-cracks based on laser excited thermography. The accuracy of mini-crack detection results is listed, which are calculated by the equation  
\[accuracy=1-\frac{1}{2}[\frac{(width_{observed}-width_{actual})}{width_{actual}}+ \frac{(height_{observed}-height_{actual})}{height_{actual}}] \tag{8}\]  
It can be seen that the mini-crack width of 1 mm only has a correct rate of about 60%, while the mini-crack width of more than 2 mm has a correct detection rate of about 95%. In this experiment, the running speed of the conveyor belt is 0.05 m/s, which is mainly \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **True** \\ **Value/mm** \\ \end{tabular} } & \multicolumn{4}{c}{**Length\({}_{observed}\) (50)**} & \multicolumn{4}{c}{**Length\({}_{observed}\) (100)**} & \multicolumn{4}{c}{**Length\({}_{observed}\) (150)**} \\ \cline{2-9}  & **Length\({}_{actual}\)** & **Width\({}_{actual}\)** & **Accuracy (\%)** & **Length\({}_{actual}\)** & **Width\({}_{actual}\)** & **Accuracy (\%)** & **Length\({}_{actual}\)** & **Width\({}_{actual}\)** & **Accuracy (\%)** \\ \hline Width(1) & 50.00 & 1.76 & 62.00 & 100.50 & 1.76 & 61.75 & 150.59 & 1.76 & 61.80 \\ \hline Width(2) & 50.00 & 2.35 & 91.25 & 98.24 & 2.35 & 90.37 & 150.59 & 2.35 & 91.05 \\ \hline Width(3) & 50.00 & 3.53 & 91.17 & 99.41 & 3.53 & 90.87 & 148.82 & 3.53 & 90.77 \\ \hline Width(4) & 50.00 & 4.12 & 98.50 & 100.00 & 3.53 & 94.12 & 150.59 & 3.53 & 93.93 \\ \hline Width(5) & 49.41 & 4.71 & 96.51 & 99.41 & 4.71 & 96.81 & 150.59 & 5.29 & 96.90 \\ \hline Width(6) & 48.82 & 5.88 & 97.82 & 98.82 & 5.88 & 98.41 & 150.00 & 5.88 & 99.00 \\ \hline Width(7) & 50.00 & 6.47 & 96.21 & 99.41 & 6.47 & 95.91 & 150.59 & 6.47 & 96.02 \\ \hline Width(8) & 49.41 & 8.24 & 97.91 & 100.00 & 7.65 & 97.81 & 150.00 & 7.65 & 97.81 \\ \hline Width(9) & 49.41 & 8.82 & 98.41 & 99.41 & 8.82 & 98.71 & 150.00 & 8.82 & 99.00 \\ \hline Width(10) & 49.41 & 10.00 & 99.41 & 99.41 & 10.0 & 99.71 & 150.59 & 9.41 & 96.85 \\ \hline \hline \end{tabular}
\end{table}