Electromechanical coupling effects in tapered piezoelectric bimorphs for vibration energy harvesting

Energy harvesting from vibrations utilizing the d₃₁ mode of operation via cantilevered bimorphs has been the subject of significant research over the past decade. The concept of tapering cantilevered rectangular bimorphs into triangular shapes to evenly distribute the axial strain along the surface of the cantilevered bimorphs has previously been presented in literature. However, an extensive experimental characterization of tapered and comparable rectangular bimorphs, with varying sizes, with and without the presence of a tip mass from an electronic standpoint has been elusive. In this embodiment, rectangular and triangular bimorphs of various sizes, designed with matching resonance frequencies have been investigated. It is shown that the resonance frequencies of triangular devices, with and without a proof mass match that of a rectangle when the length (i.e. altitude for isosceles triangles) to clamping widths aspect ratios match. Moreover, triangular devices with matching resonance frequencies and volumes when compared with rectangular counterparts provide enhanced electromechanical coupling coefficients, which translate into lower optimal load resistances at the resonance (also the short-circuit resonance) frequency, and a higher optimal load resistances at the anti-resonance (also the open-circuit resonance) frequency. With increasing bimorph sizes, and constant tip masses, resulting in lower maximum strains, the k²Q values improve, which shift the overall device impedance values to lower values, a desired effect for circuitry applications.