Modal Approach for Optimal Design of Two-Layer Piezoelectric Vibration Energy Harvesters

A modal approach for the development of optimal configurations of two-layer piezoelectric vibration energy harvesters is presented. The harvester comprises a primary cantilevered beam to which a secondary beam is attached via a mass that is located along the length of the primary beam. An additional mass is located along the secondary beam. The two masses are used to tune the natural frequencies of the composite system and one of them also serves as a spacer between the two beams. By varying the dimensions of the beams and masses, and the locations of the masses along the beams, the harvester can produce close resonance frequencies and significant power output. Thus, the frequency bandwidth of significant power generation by the harvester can be extended. To judge the performance of any harvester configuration requires a full analysis, using the coupled electromechanical equations of the piezoelectric harvester, to determine the electrical power output. However, the analysis is lengthy and time consuming. To hasten the process, a modal approach has been developed. The approach determines the modal performance by means of the mass ratio (which represents the influence of modal mechanical behaviour on the power density directly) and the modal electromechanical coupling coefficient. The modal parameters required by the approach are computed numerically by finite element analysis. The modal approach is used to select harvester configurations with optimal or near-optimal performance, which are harvester configurations with close resonances and moderate values of mass ratio. A full analysis is subsequently performed to determine the power outputs of these harvester configurations.