## 5 Experimental Results and Discussion
### Experiments of 2# Piezolectric Brick
#### 5.2.2 Average Powers from PZT4 Patches of 2#1-2#4 on PCSs (Close-Circuit)  
In the following experiments, let the amplitude and the frequency of footstep excitation be 500 N and 1 Hz respectively; the average powers from PZT4 patches of 2#1 on unimorph PCS1 and 2#3 on unimorph PCS2 of 2# piezoelectric brick are measured through adjusting the load resistance. From the experimental results shown as Figure 13, the average powers from PZT4 patches of 2#1 on unimorph PCS1 and 2#3 on unimorph PCS2 all show a trend of first increase and then decrease with the increase in the load resistance from 10 k\(\Omega\) to 900 k\(\Omega\). Specifically, for the force arm of 19.5 mm, 26.5 mm and 33.5 mm, as shown in Figure 13a the maximum average output powers from 2#1 PZT4 patch on unimorph PCS1 are 9.63 mW, 18.33 mW, and 41.23 mW corresponding to their optimal load resistances of 411.8 k\(\Omega\)  
Figure 11: Comparison of peak-to-peak voltages from PZT4 patches of 2#1 on unimorph PCS1 and 2#3 on unimorph PCS2 of 2# piezoelectric brick versus the amplitude of footstep excitation (\(f=1\) Hz). (**a**) \(L=19.5\) mm; (**b**) \(L=26.5\) mm; (**c**) \(L=33.5\) mm. Figure 12: Comparison of peak-to-peak voltages from PZT4 patches of 1#1 on unimorph PCS of 1# piezoelectric brick and 2#1 on unimorph PCS1 of 2# piezoelectric brick versus the amplitude of footstep excitation (\(f=1\) Hz). (**a**) \(L=19.5\) mm; (**b**) \(L=26.5\) mm; (**c**) \(L=33.5\) mm. 375 k\(\Omega\), and 333.3 k\(\Omega\) respectively; the maximum average powers from 2#3 PZT4 patch on unimorph PCS2 and their optimal load resistances are (10.07 mW, 411.8 k\(\Omega\)), (21.04 mW, 375 k\(\Omega\)), and (43.65 mW, 333.3 k\(\Omega\)), respectively, which is displayed as Figure 13b. Based on the aforesaid experimental results, it can be observed that with the increase in the force arm length, the average output powers from PZT4 patches of 2# piezoelectric brick increase, but the optimal load resistance matching maximum average powers decreases. This is because the increase in the force arm length would cause the increase in the bending moment exerted on LPCB and the decrease in the 1 st resonant frequency of the LPCB. The former would further cause the increase in the average power, and the latter could lead to the decrease in the optimal load resistance. In addition, the average output power from 2#3 PZT4 patch on unimorph PCS2 is slightly larger than the corresponding one from 2#1 PZT4 patch on unimorph PCS1. This phenomenon indicates that PCS1 and PCS2 of the LPCB of 2# piezoelectric brick basically have the same energy harvesting ability for their same uniform surface strain in the treading process. It is noteworthy that the 2#3 PZT4 patch (50 mm \(\times\) 50 mm \(\times\) 1 mm) on unimorph PCS2 can output an average power up to 43.65 mW (17.46 \(\mu\)W/mm\({}^{3}\)). This indicates the proposed piezoelectric bricks in this research are promising and cost-effective. The experiments of bimorph PCSs of 2# piezoelectric brick were conducted by pasting 2#2 and 2#4 PZT4 patches on the lower surfaces of PCS1 and PCS2 respectively. The average powers from PZT4 patches of 2#1-2#2 on bimorph PCS1 and 2#3-2#4 on bimorph PCS2 also show a change of first increase and then decrease with the increase in the load resistance, which can be seen from Figures 14 and 15. In addition, some other valuable findings can be obtained from Table 3. First, the optimal load resistances for 2#1-2#4 PZT4 patches are same when the force arm length is set; for example, the optimal load resistance keeps 444.4 k\(\Omega\), 411.8 k\(\Omega\), and 375 k\(\Omega\), respectively corresponding to force arm of 19.5 mm, 26.5 mm, and 33.5 mm for 2#1-2#4 PZT4 patches; second, the average output powers from 2#1 PZT4 patch are slightly larger than the corresponding ones from 2#2 PZT4 patch, the average output powers from 2#3 PZT4 patch are slightly larger than the corresponding ones from 2#4 PZT4 patches.