Piezoelectric energy harvesting (PEH) systems, as a kind of electromechanically coupled system, are composed of two essential parts: the piezoelectric structure and the power conditioning interface circuit. Previous studies have shown that the energy harvesting capability of a piezoelectric generator can be greatly enhanced by up to several hundred percent by using synchronized switch harvesting on inductor (SSHI) interface circuits, the most extensively investigated family of synchronized bias-flip interface circuits. After SSHI, some other bias-flip circuit topologies, which utilize active approaches for PEH enhancement, have been proposed sporadically. Yet, how active is active enough for harvesting as much energy as possible was not clear. This paper answers this question through the generalization and derivation of existing bias-flip solutions. The study starts by analyzing the energy flow in existing featured interface circuits, including the standard energy harvesting (bridge rectifier) circuit, parallel-SSHI, series-SSHI, pre-biasing/energy injection/energy investment scheme, etc. A synchronized multiple bias-flip (SMBF) model, which generalizes the bias-flip control and summarizes the energy details in these circuits, is then proposed. Based on the topological and mathematical abstraction, the optimal bias-flip (OBF) strategy towards maximum harvesting capability is derived. A case study on the series synchronized double bias-flip (S-S2BF) circuit shows that the potential of the PEH interface circuits can be fully released by using the OBF strategy. The proposed SMBF model and OBF strategy set the theoretical foundation and provide a new insight for future circuit innovations towards more powerful PEH systems.
Piezoelectric energy harvesting interface circuits for energy autonomous sensors
