In order to provide a serious alternative to chemical batteries for the energy supply of isolated sensors, bistable generators have been enthusiastically highlighted in recent years for their ability to harvest vibration energy on a wider frequency range compared to linear generators. Nevertheless, these bistable harvesters are generally characterized through a frequency sweep which does not reveal all the steady-state behaviors they can reach and therefore their full energy harvesting potential. Among such behaviors, subharmonic motions are hidden by this classical characterization and therefore had not received a lot of attention. This study proposes an original complete analytical analysis of subharmonic orbits for energy harvesting to predict their contribution to the global bandwidth of bistable generators. In addition, a new criterion, referred as stability robustness, is introduced to estimate the sensitivity of those behaviors to disturbances of different levels, allowing to finely and accurately estimate suitable behaviors for energy harvesting purposes in realistic conditions (behaviors easy to reach and maintain in time). Experimental results conducted with a buckled beams based electromagnetic generator confirm the pertinence of this criterion showing good agreement with the analytical predictions. Subharmonic behaviors finally appears, both theoretically and experimentally, to be of significant interest, as exploiting them leads to a 180% increase of the global operating frequency range of the considered bistable energy harvester, for which more than 100 µW are generated on a 70 Hz bandwidth at 0.5 g.

In the aim of giving an alternative to chemical batteries for energy supply of wireless autonomous devices, the present work focuses on ambient vibration energy harvesting using harvesters taking benefit of energy stored in an oscillating mass subjected to acceleration. More particularly, the attention is put on nonlinear bistable harvesters which offer a wider bandwidth than linear ones. However, due to their nonlinearities, these bistable harvesters exhibit different coexisting behaviors (orbits) on their operating frequency range. Only some of them are interesting for energy harvesting because of their high amplitude of oscillations inducing high energy levels (high orbits). Nevertheless, those high orbits are coexisting with low orbits (i.e., low harvestable energy) on a major portion of their frequency range and thus are not automatically reached. This work hence introduce new strategies to experience orbit jumps from low to high orbits playing with different parameters of the bistable harvester. Preliminary analyses based on energy considerations demonstrates that the most technically achievable strategy, adopted for the experimental analysis, is the orbit jump with fast modifications of its buckling level. More particularly, the bistable harvester is first quickly over-buckled at a particular instant and then quickly released to its initial buckling level when the mass reaches a maximum of displacement (in order to maximize the potential energy brought to the mass). Two elements of this strategy were adjustable: the amplitude of the buckling level variation and the instant at which this change starts. Experimental results show that choosing a good combination of those two elements leads to a high probability to jump from low to high orbits on the whole frequency range concerned by high orbits (from 70% chance to 100% chance to jump). Thanks to this orbit jump technique, the high orbits can be ensured for the considered bistable harvester on a continuous wide frequency range of 50 Hz (from 20 Hz to 70 Hz) on which the mean harvested power varies from 20 µW to 500 µW. Finally, it is shown that the energy consumed to ensure the orbit jump can be recovered within 2 seconds.