Resonance tuning of piezoelectric vibration energy scavenging generators using compressive axial preload

Vibration energy scavenging, harvesting ambient vibrations in structures for conversion into usable electricity, provides a potential power source for emerging technologies including wireless sensor networks. Most vibration energy scavenging devices developed to date operate effectively at a single specific frequency dictated by the device's design. However, for this technology to be commercially viable, vibration energy scavengers that generate usable power across a range of driving frequencies must be developed. This paper details the design and testing of a tunable-resonance vibration energy scavenger which uses the novel approach of axially compressing a piezoelectric bimorph to lower its resonance frequency. It was determined that an axial preload can adjust the resonance frequency of a simply supported bimorph to 24% below its unloaded resonance frequency. The power output to a resistive load was found to be 65 - 90% of the nominal value at frequencies 19 - 24% below the unloaded resonance frequency. Prototypes were developed that produced 300 - 400 mu W of power at driving frequencies between 200 and 250 Hz. Additionally, piezoelectric coupling coefficient values were increased using this method, with k(eff) values rising as much as 25% from 0.37 to 0.46. Device damping increased 67% under preload, from 0.0265 to 0.0445, adversely affecting the power output at lower frequencies. A theoretical model modified to include the effects of preload on damping predicted power output to within 0 - 30% of values obtained experimentally. Optimal load resistance deviated significantly from theory, and merits further investigation.