## 3 Materials and Methods
### Design and Manufacturing
#### 3.1.5 Energy-Harvesting System  
The implemented strategy to harvest energy was based on the approach of having piezoelectric sensors attached to a thin steel cantilever beam [44]. The plate was embedded in one side of the structure that held the sensor, and the other end was free to vibrate. This array created vibrations that, in turn, generated voltage through the piezoelectric transducers. Within this baseline design, different types of materials and cantilever structures of a diverse range of lengths were tested to generate energy. All harvesting experiments utilized the flexible piezoelectric vibration sensor by Measurement (MEAS) Specialties, as shown in Figures 10 and 11. Subsequent storage of the harvested energy in this work was not addressed, as we focused on the energy-harvesting system. A mixture of rubber and catalyst was used to cover the piezoelectric sensor. The combination of materials generated plates that were placed on a protective case. Each hole was designed for a specific screw and its metallic safety nut. This configuration allowed the piezoelectric-sensor and rubber-plate movement by the vibrations to be provided. The rubber plates were connected to the energy-harvesting system that was located below them on the same case. To create an adequate and efficient way to generate voltage from the piezoelectric element, a rubber cantilever beam was manufactured to increase the vibrations in the piezoelectric sensor. The first step was to create an adequate mold for the rubber structure. The clay mold used for this experiment is shown in Figure 10. After the clay cast for the rubber plate was created, the piezoelectric sensor was positioned to enhance voltage production. The selected position was the edge of the mold, where output cables came out of the mold to connect it to the circuit. Figure 9: From measurement reading and local processing to online data visualization; dashboard available in the cloud for users. A rubber-based solution needs to have the correct amount of each component so that it can be poured into the mold without having excess solution. This is very important due to the restriction posed by the rubber plate's width. If too much solution was poured into the mold, it would result in an increase of the desired width of the rubber plate. After several tests, it was determined that the best components and quantities were white rubber and chemical catalyst, with a composition of 80% rubber and 20% catalyst and a total mass of 20 g. The components were poured onto a Petri dish placed on a digital balance to eliminate excess materials. After the masses were verified, elements were mixed with a sterile flat-end laboratory spatula. Afterwards, the mixture was poured into the mold, maintaining a uniform mixture distribution around the piezoelectric module to ensure that there was an equal amount of rubber around the module to avoid incorrect performance. After the solution was poured, the mix in the mold needed around 2 to 3 h to fully dry. When this process was completed, the rubber plate was taken out of  
Figure 11: (**a**) mold filled with rubber-based mixture; (**b**) MEAS piezovibration sensor inside an empty mold. Figure 10: Two implemented clay molds for rubber plates. the mold for the testing experiments. The mold when it was full of the mixture and the commercial piezoelectric module is shown in Figure 11.  
The harvesting system required a signal-conditioning stage to drive the gathered energy towards the sensor. Currently, proposals for gathering the generated energy from piezoelectric devices have been widely explored and solved, for example, see [45; 46]. In this work, for the purpose of analysis, the output of the system, a simple circuit using a 1000 \(\upmu\)F capacitor and a 1N4007 diode conditioned the voltage signal as shown in Figure 12. The output voltage (Vout) was measured between the capacitor terminals that temporarily stored the generated energy.