A Theory for Energy-Optimized Operation of Self-Adaptive Vibration Energy Harvesting Systems with Passive Frequency Adjustment

Abstract

Self-adaptive vibration energy harvesting systems vary their resonance frequency automatically to better exploit changing environmental conditions. The energy required for the adjustment is taken from the energy storage of the harvester module. The energy gained by an adjustment step has to exceed the energy expended on it to justify the adjustment. A smart self-adaptive system takes this into account and operates in a manner that maximizes the energy output. This paper presents a theory for the optimal operation of a vibration energy harvester with a passive resonance-frequency adjustment mechanism (one that only requires energy for the adjustment steps proper, but not during the hold phases between the steps). Several vibration scenarios are considered to derive a general guideline. It is shown that there exist conditions under which a narrowing of the adjustment bandwidth improves the system characteristics. The theory is applied to a self-adaptive energy harvesting system based on electromagnetic transduction with narrowband resonators. It is demonstrated that the novel optimum mode of operation increases the energy output by a factor of 3.6.

A Self-Adaptive and Self-Sufficient Energy Harvesting System

“…At , the harvested and adaptation power would have been equal. The fit in Figure 11 corresponds to Figure 1 in [ 31 ].…”

“…The average adaptation width results from the spacing between the frequencies. With uniformly distributed random frequencies within the adaptation range, one would have [ 31 ]. Furthermore, in the case of a periodic adaptation obeying Equation (4), the average angle shift is (see Appendix A ).…”

“…Less net power can be extracted from such an environment. The authors have already dealt with this issue theoretically in [ 31 ] and derived the equation: where is the average energy value per adaptation.…”

“…Theoretical considerations about the influence of the dynamics of frequency changes on the usable harvested power [ 31 ] could be experimentally corroborated with this setup. The self-adaptive system was tested with different acceleration amplitudes between 0.8 and and time durations between two frequency changes from 2 to 10 min.…”

“…This semi-active adjustment is inferior to a passive adjustment, where the system only needs energy for the transition to a new resonance frequency, but not to hold the frequency. The term “passive” does in this context not mean an adapting system totally without the need of adjustment energy [ 31 ]. Environments with a low vibration do not offer much energy to harvest, so it is vital to save as much energy as possible.…”