A considerable amount of ambient vibration energy spreads over an ultra-low frequency spectrum. However, conventional resonant-type linear energy harvesters usually operate within high and narrow frequency bands, which cannot match the frequencies of many vibration sources. If the excitation frequency deviates a bit from the natural frequency of an energy harvester, the energy harvesting performance will deteriorate drastically. Because of the ultra-low frequency characteristic, it is challenging to reliably harvest energy from the ambient vibrations. To address this mismatching issue, the ultra-low frequency ambient vibrations are converted into high-frequency oscillations using certain mechanical mechanisms, which are termed frequency up-conversion techniques. This paper reviews the existing approaches that can realize frequency up-conversion for enhancing energy harvesting from low-frequency vibration sources. According to their working mechanisms, the existing methods are classified into three categories: impact-based, plucking-based, and snap-through-based approaches. The working principles of the three approaches are explained in detail. Representative designs from all categories are reviewed. This overview on the state-of-the-art frequency up-conversion technology would guide the better design of future kinetic energy harvesting systems.

This paper proposes a novel phononic crystal sandwich beam (PCSB) for low-frequency and broadband vibration suppression. The representative volume element (RVE) consists of two hourglass truss unit cells with the same span but different rod radii. After validating the modeling method, a model of the PCSB is established to calculate band structure and transmittance response, and the results show good agreement. It is found that the PCSB can open wider and lower band gaps compared to a traditional sandwich beam (TSB). The band-folding mechanism is applied. The PCSB breaks the spatial symmetry, becomes diatomic, and opens the folding points, finally leading to two band-folding-induced gaps. The experiment is conducted on the PCSB, and the vibration band gap property is confirmed. Subsequently, the impacts of geometric parameters on the PCSB’s band gaps are investigated in detail. Design guidelines for tuning the geometric parameters toward lower frequency and broadband band gap are provided based on the parametric study results. In addition, the higher-order band-folding strategy is proposed. It is shown that a multi-folding PCSB can produce more band gaps. However, through two examples, i.e., second-folding and third-folding PCSBs, it is known that simply increasing the folding order may not be effective and even could deteriorate the vibration attenuation ability. In summary, this work explores a general strategy for designing sandwich beams with low-frequency and broadband vibration suppression ability.

The nonlinear behaviors and energy harvesting of an unsymmetric cross-ply square composite laminated plate with a piezoelectric patch is presented. The unsymmetric cross-ply square composite laminated plate has two stable equilibrium positions by applying thermal stress, thus having snap-through with larger amplitude between the two stable equilibrium positions relative to the general laminated plate. Based on the von-Karman large deformation theory, the nonlinear electromechanical coupling equations of motion of the unsymmetric composite laminated plate with a piezoelectric patch are derived by using Hamilton’s principle. The influence of the base excitation amplitude on nonlinear behaviors and energy harvesting are investigated. For different base excitation amplitudes, the motions of the system demonstrate periodic motion, quasi-periodic motion, chaotic motion and snap-through, and two single-well chaotic attractors and a two-well chaos attractor coexist. Moreover, the power generation efficiency is optimal when the excitation amplitude is in a certain range due to its own unique nonlinear characteristics. The unsymmetric cross-ply square composite laminated plate subjected to thermal stress can actually be called a kind of bistable composite shell structure that has a broad application prospect in combination with morphing aircraft, large deployable antenna and solar panel, which are very likely to have nonlinear vibration.