The desire to reduce power consumption of current integrated circuits has led
design engineers to focus on harvesting energy from free ambient sources such as
vibrations. The energy harvested this way can eliminate the need for battery
replacement, particularly, in low-energy remote sensing and wireless devices. Currently,
most vibration-based energy harvesters are designed as linear resonators, therefore,
they have a narrow resonance frequency. The optimal performance of such harvesters is
achieved only when their resonance frequency is matched with the ambient excitation. In
practice, however, a slight shift of the excitation frequency will cause a dramatic
reduction in their performance. In the majority of cases, the ambient vibrations are totally
random with their energy distributed over a wide frequency spectrum. Thus, developing
techniques to extend the bandwidth of vibration-based energy harvesters has become
an important field of research in energy harvesting systems.