## 2 Design of the Energy Harvester
### Energy Harvesting from Piezoelectric Material  
The proposed energy harvesting system operates under the principle that, when a force is applied to piezoelectric devices, mechanical stresses are accumulated. Under these conditions, when an electric current is passed through the polarity arrangement, an electric potential is generated at both terminals. Subsequently, the generated energy is passed through a rectifier circuit to transform the AC produced to a DC, which is then used for powering the operation of the battery and the connected electronics. For this system, an appropriate capacitor connection was essential to reduce ripple voltages at the output. The output was then transmitted to the Darlington circuit, which increased the current, in correlation with the boost converter and regulator, to maintain the expected output voltage serving the terminal devices.  
The overall system operation is depicted in Figure 4. Furthermore, the designed schematics for the functioning of the energy harvesting circuit during daytime and nighttime as well as the entire system combining these two circuits are presented in Figure 5.  
Initially, we assumed that using springs increased the harvested energy; when the first oscillating impulse moved further away, the spring would exert the reaction force back to the piezoelectric plate as if it were continuously excited. When the overall stiffness of the spring decreased, it would affect the natural frequencies of the system--the material would result in more extended oscillation periods, contributing to a more significant amount of harvested energy. Based on preliminary experiments, we determined that this assumption was accurate. Therefore, a miniature floor model was created; in this model, the excitation force from each piezoelectric device was phase-matched through the piezoelectric modules and an aluminum plate to deliver a constructive interference electric signal. The  
Figure 4: Overall working of piezoelectric energy harvesting system.  
Figure 3: Voltage and current characteristics according to mechanical force: (**a**) input voltage of oscillating piezoelectric module; (**b**) input current of oscillating piezoelectric module; (**c**) output voltage of oscillating piezoelectric module; (**d**) output current of oscillating piezoelectric module.  
preliminary test kit using springs for vibration energy enhancement is depicted in Figure 6. The installation location of the aluminum plate was also adjusted accordingly. Thus, a pressure applied at the tip of the piezoelectric modules could produce vibrations with the highest amplitudes; however, this position should not be subjected to excessive force since it could result in material damage. A trial installation area of 2.7 m\({}^{2}\) was determined, and the installation of a total of 15 electricity-generating floors (dimensions: 0.3 \(\times\) 0.3 m) was simulated, as shown in Figure 7.  
Figure 5: Proposed system for piezoelectric energy harvesting: (**a**) schematic of system for daytime operation; (**b**) schematic of system for nighttime operation; (**c**) schematic of entire system.  
Figure 6: Preliminary test kit using springs for the enhancement of the vibration energy.  
An observation of the areas with high traffic revealed that the ground generated energy every 1 s on average. Ten sets of mechanical forces were obtained simultaneously. Charging the lithium-ion battery required 3.5 s when eight piezoelectric plates were connected in parallel, serving as the energy source. The charging time for ten sets of ground power generators was 0.00683594 s, as shown in Table 2. Furthermore, a 2 mA lithium-ion battery could be charged within 0.01 s; this corresponds to a charging rate of 200 mA/s. Therefore, an 800 mAh lithium-ion battery could be fully charged within 4 h.