Unified strategy for overall impedance optimization in vibration-based electromagnetic energy harvesters

Abstract

Vibration-based electromagnetic energy harvesters involve an apparent coupling effect between the dynamics of mechanical structures and electric circuit. Such a coupling effect complicates the impedance optimization of energy harvesting circuit in the design of energy harvesters. The classical impedance matching, which ignores the coupling effect, becomes inapplicable. This paper proposes a unified overall impedance optimization strategy for the harvesting circuit to achieve the maximum output power and power efficiency, and such a unified strategy is applicable to a number of cases with different structural complexity, different types of excitations, and different levels of coupling effects between mechanical and electrical systems. By converting the mechanical structures of single-degree-of-freedom (SDOF) and multiple-degree-of-freedom (MDOF) energy harvesters into equivalent circuit models, the electro-mechanical coupling is simplified as the coupling effect inside an electric circuit. This conversion provides insight into the overall impedance optimization framework from the pure electric circuit perspective. Different optimal impedance values of energy harvesting circuits under different excitation types (harmonic and random) are derived within the proposed overall impedance optimization framework. The optimal impedance values for the maximum output power depend on the circuit dynamics, structural characteristics, and excitation types. Meanwhile, the optimal impedance values for the maximum power efficiency are related to the inherent damping of the structure and transducer but independent of excitation types. Numerical simulations of various cases were conducted, including resonant, non-resonant, and random excitation, in the SDOF and MDOF harvesters. Simulation results successfully validate the effectiveness and accuracy of the proposed overall impedance optimization strategy for enhancing the harvesting performance of vibration-based electromagnetic energy harvesters.