## 4 WSN Platforms and Energy Harvesting Techniques Investigations
### Energy Harvesting Techniques
#### 4.2.1 Vibration Energy Harvesting Systems  
There are mainly two types of mechanical vibration energy harvesting: (1) piezoelectric material (PZT) vibrations and (2) electromagnetic generator vibrations. Piezoelectric energy is produced by the mechanical rotation or vibration of piezoelectric patches. The electromagnetic vibration energy is generated by the relative movement of the magnet and coil. A mechanical vibration energy harvester is usually attached to ambient vibrating or moving objects. Typical examples of these moving objects are motors, machines, vehicles, and even human bodies. This is regarded as a very efficient approach to harvesting energy [24].  
Tables 10 and 11 summarize the electromagnetic-based and PZT vibration energy harvesting techniques. The second column depicts the peak output voltage. The total volume of the electromagnetic circuit is presented in the third column. The resonant frequency of the electromagnetic circuit is expressed in the fourth column. The fifth column presents the minimum value of acceleration (peak/RMS) for the conversion process. The generated output power (peak/average) of the conversion process is presented in the sixth column. The seventh and eighth columns express the rectification and integration processes. As can be seen in Table 10, the output power is directly proportional to the volume of the electromagnetic circuit used in the systems. For example, the system proposed by [PERSON] et al. [45] delivers the maximum output peak power and average power of 2229 \(\upmu\)W and 1147 \(\upmu\)W, respectively with an electromagnetic circuit volume of 686.9 cm\({}^{3}\).  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & **Peak** & **Volume** & **Frequency** & **Acceleration (m/s\({}^{2}\))** & \multicolumn{2}{c}{**Power (mW)**} & \multirow{2}{*}{**Rectification**} & \multirow{2}{*}{**Integration (Storage Capacitor)**} \\ \cline{4-4} \cline{6-9}  & **Voltage (V)** & & & & & & & & & \\ \cline{1-7} \cline{10-10}  & & & & & & & & & & \\ \hline
[39] & - & - & 6.4 & - & - & - & - & - & - \\ \hline
[42] & 10 & - & 3.1 & 3.79 & 2.68 & 12 & - & - & - \\ \hline
[45] & 2.27 & 686.9 & 3.05 & 1.31 & 0.18 & 2.229 & 0.1147 & \
\begin{tabular}{c} AC-DC and \\ Boost-up \\ converters \\ \end{tabular} &
\begin{tabular}{c} Battery \\ (Stick-and-detect) \\ \end{tabular} \\ \hline
[47] & 2 & 68 & 2 & - & 0.34 & 0.0125 & 0.00502 &
\begin{tabular}{c} AC-DC \\ converter \\ \end{tabular} & Capacitor \\ \hline
[67] & - & 85 & 1.9 & - & 0.0546 & - & 0.00947 & - & - \\ \hline
[70] & 1 & - & 3.6 & - & - & 0.3545 & - & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 10: Electromagnetic-based vibration energy harvesting systems.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & **Voltage** & **Frequency** & **Power (mW)** & **Rectification** & **Integration** \\ \hline
[73] & 3.5 & - & 400 & AC-DC converter &
\begin{tabular}{c} Supercapacitor \\ (0.1 F) \\ \end{tabular} \\ \hline
[74] & 15 p-p & 23.2 & 0.00060 & AC-DC converter & - \\ \hline
[75] & 1.8 & 42.2 & 3.8 &
\begin{tabular}{c} AC-DC converter \\ and boost converter \\ \end{tabular} & Capacitor \\ \hline
[76] & 9.6 & 40 & 0.2 & - & - \\ \hline
[77] & 0.81 & 7.8 & - & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 11: PZT-based low-frequency vibration energy harvesting systems.  
The second column in Table 11 identifies the output voltage. The resonant frequency of the PZT circuit is expressed in the third column. The fourth column presents the generated output power of the conversion process. The fifth and sixth columns express the rectification and integration processes. As can be seen in Table 11, the AC-DC converter is the most used rectification technique to generate the DC voltage needed to recharge the integration capacitors.