Resonant frequency tunable vibration energy harvesting device

Vibration energy harvesting is an attractive technique for the potential powering of wireless sensors and low power devices. While the technique can be employed to harvest energy from ambient vibrations and vibrating structures, a general requirement independent of the mechanical to electrical energy transfer mechanism is that the vibration energy harvesting device operate in resonance at the excitation frequency. However, many of the energy harvesting devices developed to date are designed based on a single resonance frequency, thereby hindering their commercial emergence. To address this limitation, in this paper the design and testing of a resonance frequency tunable energy harvesting device using magnetic forces is presented. Attractive and repulsive magnetic forces are used to tune the device to resonance frequencies that are either greater or less than the untuned resonance frequency of the device. It is found that these magnetic forces can alter the resonance frequency of a cantilevered beam to +/- 20% of the untuned resonance frequency. In particular, the vibration energy harvesting cantilever beam with a natural frequency of 26 Hz has been successfully tuned over a frequency range of 22-32 Hz, enabling a continuous power output of 240-280 mu W over the entire frequency range. The experimental results are found to agree with the appropriate theoretical models. In addition, damping in the system was found to increase with the increase in magnetic force used to tune the resonance frequency. Optimal resistance values are experimentally determined and compared with the theoretical values. These results show that using this tuning technique, a wide frequency working range vibration energy harvesting is feasible without sacrificing the output power. Even though tuning through the use of magnetic forces is demonstrated here for a cantilevered beam geometry, the technique can be generalized to tune the resonance frequency of any vibration structures.