## 4 WSN Platforms and Energy Harvesting Techniques Investigations
### Energy Harvesting Techniques  
The major performance parameters for the analytical comparison between various energy harvesting systems include (1) the output voltage and power, (2) the rectification process, and (3) the integration process. There is a wide range of ambition energy sources such as solar, wind, and RF. The selection of the most appropriate energy harvesting technique for a specific SHM system depends on different factors such as the environmental conditions, the type of the bridge, and the availability of the source for radio frequency signals. Energy harvesting techniques have been employed in 18 research studies (9 in the second category and 9 in the combined category). The selected studies cover the following energy harvesting techniques: vibration, solar, wind, radio frequency (RF), and thermal. It is important to note that the eleven selected studies for vibration energy harvesting techniques are further categorized as six studies for electromagnetic-based vibration energy harvesting systems and five studies for piezoelectric material-based vibration energy harvesting systems. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref.** & **Bridge Type** & **Quantity** & **Accuracy** \\ \hline
[35] & - & Acceleration & The relative error between wired and wireless system natural frequencies estimation is 1.85\%. \\ \hline
[37] & Concrete box-girder & Acceleration & The Modal Assurance Criterion (MAC) coefficient is around 0.9 for different modes in modal analysis \\ \hline
[39] & Railway & Acoustic emission & The intensity difference between wired and wireless systems in event detection is 4 dB. \\ \hline
[43] & Concrete box-girder & Acceleration & The error of the acceleration time history collected by the proposed wireless SHM system is 1.1\%. \\ \hline
[49] & Footbridge & Acceleration & The system can locate signals up to 0.1 Hz with a 0.5\% error. \\ \hline
[51] & Steel-truss & Acceleration & The percentage discrepancies between the bridge frequencies obtained from the proposed wireless system and the two previous systems are below 5–6\%. \\ \hline
[52] & Steel-truss & Acceleration & The relative error between the wired and the proposed wireless system in natural frequency estimation is 4.7\%. \\ \hline
[54] & Cable-stayed & Acceleration & The measured tension force results using the proposed wireless system matched with the theoretical results with about 0.53\% difference; it is also close to the results presented by previous systems with a difference of less than 0.53\%. \\ \hline
[56] & Cable-stayed & Acceleration & On average, the difference between the existing wired and the proposed wireless system in cable tension measurement is less than 5 tons f/m. \\ \hline
[61] & Steel- girder & Strain & A \(\pm\) 0.1 microstrain difference in the strain reading produces less than a 2\% absolute difference from the calculated DF based on the acquired strains. \\ \hline
[62] & Steel-girder & Acceleration & The Modal Assurance Criterion (MAC) has been used to compare the mode shapes of the analytical analysis with the experimental study, with MAC values being within the range of 0.93 to 1.00. In addition, comparing the measured eigenfrequencies of the proposed system with those of commercial acceleration accelerometers presents a maximum difference of 0.87 Hz with a difference of 3.3\%. \\ \hline
[65] & Wooden model & Acceleration & The absolute average relative difference between the wired and the proposed wireless system for the identified natural frequencies was 0.422\%, and the average modal assurance criterion (MAC) value was 0.943. \\ \hline
[78] & Cable-stayed & Acceleration & The difference between the numerical and the experimental frequency values is 0.7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 9: The accuracy of proposed WSN systems. harvesting systems. There are four research studies that combine two different energy harvesting techniques. For example, the solar and wind techniques have been jointly deployed in two studies [38; 69].