Resonant Frequency Modulation in Inductive Vibration Power Generator via Resistive Impedance Control

Resonant frequency modulation is indispensable for vibration power generators using inductive coils to maintain maximum power extraction against environmental fluctuation. We have shown that by inserting a fixed reactance element in series with an inductive vibration power generator, its resonant frequency can be modulated only by controlling the resistive impedance of the power extraction circuit. The upper limit of the frequency modulation only by resistive impedance was analytically shown and verified by simulation and experiments. For the purpose of comparison between different devices, the figures of merit for electrical frequency modulation capability were also proposed, which are applicable both to resistive impedance control and to complex impedance control. The figure of merit can predict whether the specific harvester device is suitable for electrical frequency modulation or requires mechanical modulation, and the prediction was verified by experiments with two types of harvester devices. The resonant frequency modulation only by changing resistive impedance is more practical than the previously reported methods via complex impedance variation, and readily implemented by power extraction circuits with simple switching operation.