Shape Optimization of Cantilevered Devices for Piezoelectric Energy Harvesting

Abstract

Cantilevered piezoelectric devices under transverse base excitations, for generating usable power from ambient vibrations is a highly researched topic over the past decade. The commonly used rectangular shaped bimorphs require a large proof mass to drive sufficient power, and suffer from having a large stress concentration near the fixed end of the device. Tapering geometry provides a constant axial strain through the length of a triangular cantilever, and therefore provides the opportunity for more reliable operation due to enhanced efficiency. However, in order to make fair comparisons for power output, it is important to compare devices with matching resonance frequency, device volume, and inertial loads to study the effect of geometry. This study takes an experimental approach for designing such devices, and evaluates the effects of shape change with and without the presence of proof masses. While a mass-less triangular device does not outperform a rectangular counterpart for power output generation, tapering the geometry does increases the k₃₁ electromechanical coupling coefficient, while the damping ratios are nearly the same. The addition of a nominal 2 g proof mass increases the output power by an order of magnitude, and the triangular device outperforms its rectangular counterpart by 40 %; and a subsequent 30 % with 4 g of proof mass. With the addition of proof masses, the electromechanical coupling and damping ratio also increase, which are always greater in the case of the cantilevered triangular bimorphs, and these important parameters may be used as design parameters for better device design.