Bias-Flip Technique For Frequency Tuning Of Piezo-Electric Energy Harvesting Devices

Devices that harvest energy from vibration are often implemented using amechanical resonator. When the source vibrates at the mechanical resonancefrequency, motion is amplified by the resonator, and the strain on the PZ materialgenerates electrical power. However, these devices have a serious drawback. Inmany cases, the frequency of the source vibration is not stable or is not matched to theresonance frequency, and the output power is low. In fact, the frequency of thesource vibration is not usually matched to the resonant frequency of the energyharvesting device, and the source vibration may vary with time, and it may have abroad spectrum. Present piezo-electric vibration energy harvesting devices can onlyharvest the vibration of single frequency, and they need to customize according todifferent application. In order to break through this difficult problem, the studyaccomplished the optimized output power in range of0.9ω_m~1.1ω_m based on thetheory of frequency tuning and Bias-Flip technology.However, away from the mechanical resonance, the commonly-used Nortonequivalent-circuit does not apply. So this paper will start with the material equationsof a PZ material, and derive sinusoidal, steady-state linear equations for mechanicaldisplacement, output voltage, and output power. Then it replace the tunable inductorwith bias-flip techniques based on microelectronics, and show how output power canbe optimized across a wide range of frequencies above and below in this paper.Bias-Flip technique uses a very small inductor together with ULP microelectronicsto emulate a large (~25Hz), tunable inductor. It forms an electronic resonant circuittogether with the output capacitance of the PZ device, when a BF inductor is used atthe output of a piezo-electric energy harvesting device. Furthermore, this electronicresonant circuit couples to the mechanical resonator, and provides a mechanism fortuning of the combined resonances. The study replaced the tunable inductor withBias-Flip techniques based on microelectronics, and showed how output power can beoptimized across a wide range of frequencies above and below.In addition, the Bias-Flip rectifier is effective in recovering most of the lost outputpower. Output power after Bias-Flip rectifier is just degraded8%, furthermore forlarge the effective of Bias-Flip technical rectification effect is more obvious.