# Vibration Energy Harvesting from Plates by Means of Piezoelectric Dynamic Vibration Absorbers
## 4 Experimental Equipment  
The experimental tests were performed in the Laboratory of Vibration Mechanics of Padova University in the absence of ambient vibrations and wind. The experimental equipment was developed with the aim of evaluating the performance of the harvesting system in different load conditions. To make the clamps, the two shorter sides of the aluminum plate were fixed to a very stiff steel base by means of screws (Figure 10). The CPDVA used in experiments was built using a commercial cantilever harvester (PEH M2814P2C-01) made by Smart Materials GmbH. The base to attach the harvester to the plate was 3D printed using polylctide (PLA) filament. Table 3 reports the parameters of the harvester, and Figure 11 shows the CPDVA attached to the backside of the plate. The CPDVA base was bonded to the center of the plate (\(x=a/2\), \(y=b/2\)) by means of structural adhesive. In order to verify the advantages of the CPDVA, experiments were also performed with a piezoelectric patch in place of the CPDVA. In particular, the same piezoelectric patch used in the PEH M2814P2C-01 cantilever harvester was bonded under the center of the plate, as shown in Figure 12. The piezoelectric patch was bonded with the piezoelectric patch length oriented along the \(x\)-axis of the plate. The patch was bonded with an offset of 5 cm in the \(y\)-axis with respect to the center of the plate, whereas no offset was used in the \(x\) direction. Since the first mode of vibration is predominant, this offset does not affect the results much. During tests, the OC voltage generated by the piezoelectric layer and the acceleration of the center of the plate were measured. The latter measurement was carried out using a small PCB 352C22 accelerometer (sensitivity 10.48 mV/g, measurement range \(\pm\)4900 m/s\({}^{2}\) and frequency range 1.0 to 1000 Hz), which was attached by means of wax. Since the mass of this accelerometer is less than 1 g (value from datasheet: \(5\times 10^{-4}\) kg), it does not affect the dynamics of the plate. Two load cases were considered in the framework of this research. In the first, the center of the plate was excited by means of a hammer for modal testing PCB 086C03 (sensitivity 2.25 mV/N, measurement range \(\pm\)2224 N and hammer mass 0.16 kg). Since in this case, measurement of the exciting force was possible and reliable, the FRFs of the system were calculated. It is worth noting that the simulations presented in Section 3 show that there are small differences between the general trend of the FRFs obtained with uniform pressure load and the FRFs obtained with concentrated force acting at the center of the plate (e.g., compare Figure 6 to figures of Section 5.1). Figure 11: Picture of the CPDVA. The tip mass is on the left-hand side, and the base glued to the plate is on the right-hand side. Figure 10: Clamped plate and fan for excitation of the plate via air jet. In the second load case, a fan was installed over the plate at a distance of 1.2 m, see Figure 10. The plate was excited by the pressure generated by the air jet impinging on the plate surface. The fan diameter is 0.148 m, and the maximum flow rate is 595 m\({}^{3}\)/h, which corresponds to a Reynolds number \(Re\approx 10^{5}\). Experimental tests dealing with turbulent jets impinging on flat surfaces were presented in [37; 38; 39]. The results show axial-symmetric distributions of both mean velocities and fluctuating components, which reach maximum values at small distances from the jet axis: that is, at the center of the plate. The pressure fluctuations caused by the above-mentioned pressure distributions can excite the first mode of the plate but cannot excite the modes with an odd number of nodal lines. The voltage generated by the CPDVA and the signals of the sensors were acquired through an NI9230 board. Measurements were performed in the range \(0\div 1024\) Hz, and typically, 4096 samples were collected, with a sampling frequency of 2048 Hz.