## 5 Experimental Validation of the Designed SHM System
### Damage Evaluation Using the EMI Method  
Experiments to verify the EMI method were also conducted. The purpose of experiments was to verify the effectiveness of the EMI method for damage detection, especially to verify whether the system  
Figure 15: PZT sensor time-domain response signal (impact is introduced near No. 6 sensor).  
can detect the changes in EMI signals of PZT sensors on the irregularly shaped experimental objects, and meanwhile to verify the robustness and repeatability of the hardware system being designed.  
The impedance measurement module of the hardware system can measure the impedance with a frequency range from 10 Hz to 100 kHz. Considering that the proposed SHM system is designed for real-time monitoring and with low power consumption, the difference between the upper and lower limits of the measurement frequency range is set to be 5 kHz. In addition, based on a pre-experiment, the impedance from 10 kHz to 15 kHz is more sensitive to damage. Thus, the impedance of each PZT sensor is measured at the frequency of from 10 kHz to 15 kHz with a step of 10 Hz, and the excitation signal is in sinusoidal form with an amplitude of 1.98 V. It should be emphasized that for different structures, the range should be changed accordingly, and the designed impedance measurement system can alter the measurement frequency range and the amplitude of excitation signal with the software configuration. Furthermore, only the conductance (the real part of admittance, which is the reciprocal of impedance) was selected for damage detection in the experiment, because the real part of admittance is more robust to the environment than the imaginary part of admittance [34]. In this study, the damage is simulated with different weights of mass blocks. Each mass block weighed 5 g, and it was glued to the structure surface with resin.  
Note that in this section, a baseline-based method was used to detect damage. That is, first, the conductance of a PZT sensor in the health state was measured as the baseline data, then the conductance of different damage levels was measured, finally the damage level was evaluated by analyzing the difference of conductance between the damaged one and the health one.  
The detailed experiment process is shown below.  
1. Measure the conductance of one PZT  
The noise for impendence measurement can be quantified by calculating the SNR of the conductance difference. Based on Equation (10), the SNR of the conductance difference is 20 dB.  
To conclude, these results demonstrate that the patterns of conductance of various damage levels are different. Compared with the conductance in health state, the conductance of the structure also changed. The conductance of the same damage level is almost the same, and this demonstrates the repeatability and robustness of the EMI method. The results also verified the effectiveness of the system on the damage detection by applying baseline method on EMI signal.  
The indicators of the EMI method introduced in Equations (6)-(9) were applied on the data and the results are listed in Table 5. As can be seen from the table, these indicators can distinguish different damage levels, and the difference between the values of the indicators between the same damage level is also much smaller than that between different damage levels. This means the indicators can well represent the information showing in Figure 16, and show the possibilities of engineering applications.