Resonant Frequency Reduction of Vertical Vibration Energy Harvester by Using Negative-Stiffness Magnetic Spring

Abstract

This work studies the resonant frequency reduction of vibration energy harvesters so as to adapt low-frequency ambient vibrations. In a typical electromagnetic vibration energy harvester, it is common sense that the resonant frequency decreases with the spring stiffness. However, the mass gravity will result in a large initial deformation of the spring with small stiffness when the energy harvester is vertically installed. To solve the problem, this work proposes a method of resonant frequency reduction by adding a negative-stiffness magnetic spring which is arranged symmetrically at the two sides of the equilibrium point. The theoretical expression of resonant frequency is derived based on magnetic field calculation. Comparisons between the energy harvesters with and without negative-stiffness magnetic spring are implemented through simulations and experiments. The results show that the resonant frequency of the vertical energy harvester can be reduced as expected without an increase of initial spring deformation by using the negative-stiffness magnetic spring. This work can provide an effective approach to designing energy harvesters for applications with low-frequency vibrations.