Linear stiffness compensation using magnetic effect to improve electro-mechanical coupling for piezoelectric energy harvesting

Abstract

Piezoelectric transducers have been widely used in vibration-based energy harvesting. The efficiency of piezoelectric energy harvesting depends to a large extent on the electro-mechanical coupling of the harvester. Improvement of the coupling effect is possible through a variety of means from the transducer material-level to the device-level. At the device-level, the coupling effect of a cantilever-type energy harvester is inversely proportional to its mechanical stiffness which consists of the stiffness of the cantilever beam and that of the piezoelectric transducer. The mechanical stiffness, however, cannot be easily reduced because of the necessary inclusion of the cantilever beam and the inherent stiffness of the transducer. Both the beam and the transducer cannot be made arbitrarily thin due to typical design considerations. This paper reports a device-level approach for improvement of energy conversion efficiency by directly compensating the effective stiffness of a cantilever-type vibration energy harvester through magnetic effect. Specifically, two sheet-type permanent magnets are placed in the vicinity of the piezoelectric composite beam, and another slender magnetic bar is attached onto the beam together with an iron proof mass where one of its poles is located between the two magnetic sheets. It is shown analytically and experimentally that, under such configuration, a linear magnetic field yielding linear force pointing toward the magnetic sheets can be produced, which results in the reduction of the effective stiffness of the energy harvester to improve the electro-mechanical coupling. The experimental case studies demonstrate that the electro-mechanical coupling coefficient can be increased by 65% with 44.1% stiffness compensated. Both the open-circuit voltage and the power output are enhanced.