## 6 Conclusions  
In this paper, a multi-channel and multi-function SHM system was developed. The system is lightweight, energy efficient, and can be applied on aerospace structures. This SHM system can perform impact detection and damage quantification based on the EMI method, while both functions use the same set of PZT sensors. The key part of the hardware used to achieve the multi-function characteristic is the scheduling module. Figure 16: Damage quantification using the electromechanical impedance (EMI) method: (**a**) the conductance in the health case; (**b**) the conductance in damage case 1; (**c**) the conductance in damage case 2; (**d**) the difference between \(\text{C}_{\text{H}}\) and \(\text{C}^{\prime}_{\text{H}}\); (**e**) the difference in conductance between damage case 1 and the health case; (**f**) the difference in conductance between damage case 2 and the health case. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Damage** & **RMSD (\%)** & **[MAPD]** & **[Cov]** & **[CC]** \\ \hline Case 0 (repeat) & 0.0729 & \(0.709\times 10^{-4}\) & \(5.776\times 10^{-21}\) & \(3.461\times 10^{-27}\) \\ Case 1 & 0.1286 & \(1.232\times 10^{-4}\) & \(4.726\times 10^{-21}\) & \(2.821\times 10^{-27}\) \\ Case 2 & 0.1658 & \(2.506\times 10^{-4}\) & \(0.195\times 10^{-21}\) & \(0.117\times 10^{-27}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The value of EMI indicators at different damage levels. Experiments were conducted to verify the developed system. In terms of impact detection, the system can record the impact response signal, and, based on this, the system can also effectively identify the location of the impact. In terms of impedance detection, the experiments also confirmed that the electrical impedance changes with the change in the main structure. The indicator method has also proven to be effective on damage detections. The future topics are the following. The proposed architecture and interface can integrate vibration-based damage assessment. The vibration-based method is based on measurements from accelerometers. Regarding algorithms, introducing a more advanced impact detection method to improve the precision in detecting the impact position and impact amplitude could also be considered. In addition, more studies should be conducted to evaluate the performance of the proposed SHM system under changing environment. Furthermore, the reliability of the whole system through long-term monitoring should be investigated. A wireless system could also be considered for communication and energy harvesting in the next step. Conceptualization, T.H. ; methodology, T.H. ; software, Z.G. ; experimental design, Z.G. ; validation, Z.G. ; writing, original draft preparation, T.H. and Z.G. ; writing, review and editing, T.H. and Z.G. ; supervision, T.H. ; and resources, K.-U.S. All authors have read and agreed to the published version of the manuscript. This research was funded by China Scholarship Council grant number 201506830047. The authors gratefully acknowledge the sensor technology support from the Research Center of Structural Health Monitoring and Prognosis and the State Key Laboratory of Mechanics and Control of Mechanical Structures at Nanjing University of Aeronautics and Astronautics. The authors are also grateful to Flugwissenschaftliche Vereinigung Aachen (FVA) for the experimental support. The authors gratefully acknowledge the financial support from the CHINA SCHOLARSHIP COUNCIL for the first author. The authors declare no conflict of interest. * _[PERSON] et al. (2018)_ [PERSON]; [PERSON] A review of the piezoelectric electromechanical impedance based structural health monitoring technique for engineering structures. _Sensors_**2018**, _18_, 1307. [CrossRef] [PubMed]
* _[PERSON] et al. (2018)_ [PERSON]; [PERSON]; [PERSON] Structural Health Monitoring (SHM) of Civil Structures. _Appl. Sci._**2018**, 789.