Vibration-to-electric energy conversion using a mechanically-varied capacitor

Abstract

Past research in vibration energy harvesting has focused on the use of variable capacitors, magnets, or piezoelectric materials as the basis of energy transduction. How- ever, few of these studies have explored the detailed circuits required to make the energy harvesting work. In contrast, this thesis develops and demonstrates a circuit to support variable-capacitor-based energy harvesting. The circuit combines a diode-based charge pump with an asynchronous inductive flyback mechanism to re- turn the pumped energy to a central reservoir. A cantilever beam variable capacitor with 650 pF DC capacitance and 347.77 pF zero-to-peak AC capacitance, formed by a 43.56 cm2 spring steel top plate attached to an aluminum base, drives the experimental charge pump near 1.56 kHz. HSPICE simulation confirms that given a maximum to minimum capacitance ratio larger than 1.65 and realistic models for the transistor and diodes, the circuit can harvest approximately 1 lW of power. This power level is achieved after optimizing the flyback path to run at approximately 1/4 of the mechanical vibration frequency with a duty ratio of 0.0019. Simulation also shows that unless a source-referenced clock drives the MOSFET, spurious energy injection can occur, which would inflate the circuit's conversion efficiency if the harvester is driven by an external clock. A working vibration energy harvester comprising a time varying capacitor with a capacitance ratio of 3.27 converted sufficient energy to sustain 6 V across a 20 MQ load.

(cont.) This translates to an average power of 1.8 pW. Based on a theoretical harvesting limit of 40.67 luW, the prototype achieved a conversion efficiency of 4.43 %. Additional experiments confirm that the harvester was not sustained by clock energy injection. Finally, the harvester could start up from a reservoir voltage of 89 mV, suggesting that the circuit can be initiated by an attached piezoelectric film.