# Snap-Through Buckling Mechanism for Frequency-up Conversion in Piezoelectric Energy Harvesting
## 4 Results
### Experimental Characterization of the STB-EH under Sinusoidal Excitation  
Finally, to explore the response in case of an external acceleration history, the prototype is tested on an electrodynamic shaker (Bruel and Kjaer Vibration Exciter 4808). The acceleration necessary for the STB from the reversed to the initial configuration is excessive for the shaker capability, since, for model B, it exceeds 20 g. To carry out the experimental test, it is possible to introduce a modification of the prototypes to increase the overall mass without affecting the dynamic response of the cantilever. Consequently, a metal washer, with mass equal to 3.8 g, is added in a symmetric fashion on the central block of the bistable mechanism: the final experimental configuration is shown in Figure 15. Focus is again restricted on model B, modified with the addition of the washer. The additional mass does not alter the vibration of the piezoelectric cantilever; moreover, it does not affect significantly the first eigenmode. The mass is increased in order to reduce the acceleration necessary for the STB from the reversed to the initial configuration. On the other hand, the introduction of the washer is detrimental for the performance of the STB mechanism, since a part of the released elastic energy is converted into kinetic energy of the mass which do not participate in the motion of the piezoelectric cantilever. As a consequence, performances are expected to be lower than reported in Figure 11.  
The prototype is pre-deformed so that the reversed configuration is achieved. Subsequently, the structure is excited with different time histories of acceleration, changing the frequency, the amplitude, and the shape of the signal. In the case of sinusoidal acceleration at frequencies well below the first eigenmode of the device, as long as the peak amplitude is lower than 2 g,  
Figure 14: Voltage measured across the storage capacitor 0.2 s after the snap (**a**). Stored energy calculated 0.2 s after the snap (**b**). Figure 15: Experimental setup used to characterize Model B with the additional mass on the electrodynamic shaker. the measured voltage is negligible. The peak amplitude of 2 g is sufficient to pass over the energy barrier of the bistable mechanism and to trigger the FuC. This value of acceleration is lower than expected: such a difference can be explained in view of the uncertainties in the constitutive law of FR4 and the relevant properties. Nonetheless, the low-frequency, low-amplitude applied acceleration history is enough to trigger the STB of the bistable mechanism from its reversed configuration to the initial undeformed one. Hence, the bistable mechanism is less stable in its reversed configuration than assessed numerically, namely it is characterized by a lower absolute value of \(F_{\text{bot}}\) than the one indicated in Figure 6.  
Figure 16 shows the open-circuit voltage, for sinusoidal accelerations with frequency 20 Hz, 25 Hz and 30 Hz, with different peak amplitude, namely 1 g, 2 g, and 3 g. When the peak acceleration is larger than or equal to 2 g, FuC is triggered and the open-circuit voltage attains peaks that are comparable with the case of Figure 11, where STB is provoked manually. The achieved results confirm that, if the input signal is able to pass over the energy barrier of the second stable configuration, the occurence of STB adds high frequency components in the frequency spectrum of the output signal, in spite of the low-frequency input excitation, as shown in Figure 17. The output voltage is distributed over a frequency band that is broader than the previous cases. Other tests are carried out, by attaching the piezoelectric material to a resistive load \(R_{\text{L}}=400\,\text{k}\Omega\), so that the extracted power can be computed on the basis of the recorded voltage and of Equation (4). Figure 18 shows the average power that is obtained for different frequencies and peak accelerations.