## 5 Results
### Energy Harvesting--Powering Up Smart Sensor  
For the experimental setup described in Section 4.2, the cantilever beam that included the piezoelectric sensors was tested. The tests started with a cantilever length equal to 5.5 cm with increments of 0.5 cm. Although the tests were performed with lengths between 5.5 and 8.0 cm, only results between 6.5 and 8.0 cm are shown here because the highest voltage levels were generated with these lengths. For each frequency and cantilever length, three experiments were carried out. The justification was to avoid depending on a single result, and checking if there was congruence between the outcomes of the different lengths and frequencies that formed the three replicates of the experiment. The experiment consisted of applying a sinusoidal signal of a certain frequency and taking 500 readings (voltage measurements with a sampling time equal to 0.1 s) of the voltage between the piezoelectric-array terminals. Findings showed that the cantilever length was directly proportional to the generated voltage. Moreover, when the input-signal frequency was between 7.0 and 8.0 Hz, an even greater voltage was produced. The results are shown in Figure 23.  
Figure 23 shows that reducing the cantilever length reduced the generated voltage. Moreover, it was detected that, around 7 Hz, the generated voltage had maximal values. Variation was observed in the measured data, and this could have been due to the noise affecting the data-acquisition system. Due to the length and geometry characteristics of the cantilever beam, it was estimated that there was a resonance frequency around 7 Hz. Furthermore, by increasing the cantilever length, there was a longer lever arm, which contributed to generating a greater piezoelectric reaction. As a consequence, the piezoelectric material underwent a greater degree of deformation and delivered more voltage to the energy-harvesting system. Figure 22: Compression tests. Electrical resistance vs applied force for three manufactured MRE transducers shown in Figure 3. Two tests (Replicas 1 and 2) were carried out for each transducer. (**a**) Four lines in parallel; (**b**) four lines in 90\({}^{\circ}\); and (**c**) three lines in 90\({}^{\circ}\)/45\({}^{\circ}\). For each of the 3 experiments carried out, the cantilever length was set and a signal was applied so that the metallic beam started to vibrate. Considering the length and frequency ranges specified in Section 4.2, for each replica, the results of the series of experiments are depicted in Figures 24-26. Of the three experiments, the maximum generated voltage occurred when the cantilever beam length was 7.5 cm and the excitation signal had a frequency equal to 8.0 Hz. Figure 23: Generated voltage for different cantilever-beam lengths. Three replicas were applied for each test (length and frequency), and 500 measurements were gathered per tested frequency: (**a**) 6.5 cm; (**b**) 7.0 cm; (**c**) 7.5 cm; and (**d**) 8.0 cm. Figure 24: Energy harvesting for Replica 1. The maximum generated voltage was 1.625 V.  
The energy harvesting outcomes, which are depicted in Figures 24-26, were concentrated in the 2D graphs shown in Figure 27. The graph of the generated voltage, which is function of the beam length and the input signal frequency, shows the maximum generated voltage with an input signal around 8.0 Hz and with a length equal to 7.5 cm. For all the tests run, these two combinations of independent variables contributed to harvest the highest amount of energy with values between 1.20 and 1.625 V.  
The generated voltage by the cantilever beam went through a diode for rectification, and it was temporarily stored in a capacitor.