Although wireless sensors show extensive promise across a wide range of applications, one requirement necessary for widespread deployment is a suitable long-life power source. Self sustainable powering techniques allow for efficient use of these sensors, whose potential life is usually longer than that of the power sources. Vibration energy harvesting techniques offer to have the potential to be employed in powering these devices. The most important requirement of vibration energy harvesting devices is that they be in resonance to harvest energy efficiently. Most of the vibration energy harvesting devices built, irrespective of the mechanism involved, are based on a single resonance frequency, with the efficiency of these devices is very much limited to that specific frequency. In this paper, a frequency tunable mechanism is presented which allows the energy harvesting device to generate power over a wide range of frequencies. External magnetic forces have been used to induce additional stiffness which is variable depending on the distance between the magnets. This technique allowed us to tune the resonance frequencies to have +/− 20% of the original (untuned) resonant frequency. Further, the device can be tuned to higher and lower frequency with respect to the untuned resonance frequency by using attractive and repulsive magnetic forces, respectively. As a proof-of-concept, a piezoelectric cantilever-based energy harvesting device with a natural frequency of 26 Hz was fabricated whose resonance frequency was successfully tuned over a frequency range of 22 Hz to 32 Hz, enabling a continuous power output of 240 μW to 280 μW over the entire frequency range. The tuning mechanism can be employed to any vibrating structure.
A Theory for Energy-Optimized Operation of Self-Adaptive Vibration Energy Harvesting Systems with Passive Frequency Adjustment
