Nonlinear mechanism in mems devices for energy harvesting applications

This paper reports a novel bistable microelectromechanical system for energy harvestingapplications. In particular, we focus here on methodologies and devices for recovering energyfrom mechanical vibrations. A common energy harvesting approach is based on vibratingmechanical bodies that collect energy through the adoption of self-generating materials. Thisfamily of systems has a linear mass–spring damping behaviour and shows good performancearound its natural frequency. However, it is not generally suitable for energy recovery in a widespectrum of frequencies as expected in the vast majority of cases when ambient vibrationsassume different forms and the energy is distributed over a wide range of frequencies.Furthermore, whenever the vibrations have a low frequency content the implementation of anintegrated energy harvesting device is challenging; in fact large masses and devices would beneeded to obtain resonances at low frequencies. Here, the idea is to consider the nonlinearbehaviour of a bistable system to enhance device performances in terms of response toexternal vibrations. The switching mechanism is based on a structure that oscillates aroundone of the two stable states when the stimulus is not large enough to switch to the other stablestate and that moves around the other stable state as soon as it is excited over the threshold. Aresponse improvement can be demonstrated compared to the classical linear approach. Indeed,both a wider spectrum will appear as a consequence of the nonlinear term and a significantamount of energy is collected at low frequencies. In this paper the bistable working principleis first described and analytically modelled, and then a numerical study based on stochasticdifferential equations (SDE) is realized to evaluate the behaviour of a MEMS device. Amicromachined SOI prototype has been realized and a measurement campaign validated thenonlinear mechanism. As expected, the study shows that the nonlinear system exhibits a lowpass filter behaviour suitable for harvesting ambient energy at low frequency.