The investigation of viscous and structural damping for piezoelectric energy harvesters using only time-domain voltage measurements

Knowing the nature of damping in piezoelectric energy harvesters can lead to proper damping and electromechanical models and designing highly efficient harvesters with less damping. As an attempt toward a better understanding of damping in piezoelectric energy harvesters, this paper presents experimental results for structural and viscous air damping coefficients extracted directly from voltage measurements under shock induced tests. Free-vibration excitations are analyzed using the modified Short-Term Fourier Transform and Resampling method. Seven cases are studied, namely Macro Fiber Composite with different substrate shims, different bonding layers, and with or without a tip mass. The damping coefficients can be reliably extracted using an up-chirp driving signal and analyzing the system's decay curve, without the need for full measurement of harmonic response over a wide frequency range. The results also indicate that the damping coefficient is not independent of the base excitation amplitude and can increase up to 30%. The relative significance of viscous air damping and structural damping mechanisms is identified in each case. The dependency of viscous air damping on the base excitation amplitude is also evaluated. The experimental results highlight the significance of the bonding layer in structural damping, which can account for approximately 60% of the total damping. In the absence of a substrate shim and bonding layer, the main contribution to energy dissipation is viscous air damping. While an added tip mass increases the output power, it also escalates the viscous air damping to approximately 40% due to increased beam tip deflection.