This paper describes an approach to harvesting electrical energy from a mechanically excited piezoelectric element. A vibrating piezoelectric device differs from a typical electrical power source in that it has a capacitive rather than inductive source impedance, and may be driven by mechanical vibrations of varying amplitude. An analytical expression for the optimal power flow from a rectified piezoelectric device is derived, and an "energy harvesting" circuit is proposed which can achieve this optimal power flow. The harvesting circuit consists of an ac-dc rectifier with an output capacitor, an electrochemical battery, and a switch-mode dc-dc converter that controls the energy flow into the battery. An adaptive control technique for the dc-dc converter is used to continuously implement the optimal power transfer theory and maximize the power stored by the battery. Experimental results reveal that use of the adaptive dc-dc converter increases power transfer by over 400% as compared to when the dc-dc converter is not used.

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In this study, we investigated the capability of harvesting the electrical energy from mechanical vibrations in a dynamic environment through a ''cymbal'' piezoelectric transducer. Targeted mechanical vibrations lie in the range of 50-150 Hz with force amplitude in the order of 1 kN (automobile engine vibration level). It was found that under such severe stress conditions the metal-ceramic composite transducer ''cymbal'' is a promising structure. The metal cap enhances the endurance of the ceramic to sustain high loads along with stress amplification. In this preliminary study, the experiments were performed at the frequency of 100 Hz on a cymbal with 29 mm diameter and 1 mm thickness under a force of 7.8 N. At this frequency and force level, 39 mW power was generated from a cymbal measured across a 400 k resistor. A DC-DC converter was designed which allowed the transfer of 30 mW power to a low impedance load of 5 k with a 2% duty cycle and at a switching frequency of 1 kHz.

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