Analysis of two dimensional, wide-band, bistable vibration energy harvester

Abstract In this paper a new strategy for developing broadband, bi-directional, vibration energy harvesters is presented. Often energy harvesting systems address unidirectional incoming energy, however this is a strong limitation when considering real applications; in fact it is very likely that incoming vibrations will not be aligned with the direction of motion of the harvester. Therefore the presence of irregular vibrations along several orientations reduces the energy conversion and the harvester efficiency. Furthermore, often the ambient vibrations come with energy distributed over a wide spectrum of frequencies, with predominance of low frequency components such as vibration generated by: highway traffic, human motions, trains, electrical machines and generic noise-induced vibrations. The classical approach is based on vibrating mechanical bodies (linear systems) able to collect energy through the adoption of smart materials, magnetic or electrostatic solutions. Moreover this family of systems performs poorly for off-resonance conditions. For this reasons suitable harvester architecture for collecting energy from vibrations having a broad spectrum and distributed along different directions is needed. This work addresses both these issues by proposing a double nonlinear bi-stable oscillator with a large amplitude response over a broad range of frequencies. Such system improves traditional scavengers based on linear mechanical principles, which only give a suitable response if the dominant ambient vibration frequency is tuned to the mechanical resonance. Moreover a suitable topology of the double bistable oscillator is proposed such to obtain a bi-axial vibrations energy harvester. The system proposed here is composed of two magnetically coupled bi-stable beams, having orthogonal directions of deflection, with piezoelectric output; in presence of mechanical vibrations the beams will respond to both the inertial excitation and the magnetic coupling. Therefore the total output power, being the sum of the two piezoelectric outputs, will be optimized independently of the direction of the incoming vibrations. In this paper the two dimensional wide-band energy harvester is first described and analytically modeled, extensive simulations have been performed to evaluate the behavior of the two magnetically coupled beams. A macro-prototype has been conceived and realized and a measurement campaign has been performed in order to validate the principle.