Harvesting energy from ambient vibrations, such as those created by walking or the shaking of moving vehicles, is one way to obtain low-cost and renewable power for small electronic devices. Most examples so far convert vibrations into electrical energy using 'linear' oscillators, which miss out much of the typically broad frequency spectrum of vibrations.
Nonlinear oscillators — for example, bistable ones with two stable oscillating states — can do a better job, according to Francesco Cottone at the University of Limerick in Ireland and his colleagues. Their proof of principle is an inverted pendulum comprising a piezoelectric beam, which produces electricity when it bends. This generates four to six times more power from ambient vibrations when it oscillates in a bistable rather than a linear manner.