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

Past research in vibration energy harvesting has focused on the use of variable capacitors, magnets, or piezoelectric materials as the basis of energy transduction. However, 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 return 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 cm 2 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 ptW 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. This translates to an average power of 1.8 pW. Based on a theoretical harvesting limit of 40.67 pW, 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. Thesis Supervisor: Jeffrey H. Lang Title: Associate Director, Laboratory for Electronic and Electromagnetic Systems