In this paper, a wide-band vibration energy harvester using a nonlinear hardening oscillator with self-excitation circuit is
presented. A vibration energy harvester is one of the energy-harvesting devices that collects unused energy from vibrating
environment. For the conventional linear vibration energy harvester, the resonance frequency is matched to the source
frequency, and the mechanical Q factor is designed as large as possible to maximize the oscillator's amplitude. The large Q
factor, however, bounds the resonance in a narrow frequency band, and the performance of the vibration energy harvester
can become extremely worth when the frequency of the vibration source fluctuates. As is well known, the resonance
frequency band can be expanded by introducing a hardening (or softening) nonlinear oscillator. However, it is difficult
for the nonlinear vibration energy harvester to maintain the regenerated power constant because such nonlinear oscillator
can have multiple stable steady-state solutions in the resonance band. In this paper, a control law that switches the load
resistance between positive and negative values according to the instantaneous displacement and the velocity is proposed
to give the oscillator a self-excitation capability, which ensures the oscillator entrained by the excitation only in the largest
amplitude solution. Moreover, an adaptive adjustment of the control law is proposed to quicken the entrainment process.
Numerical analysis shows that the nonlinear vibration energy harvester with resistance switching can maintain the large
amplitude response even when the excitation frequency abruptly changes.