Improved Adjusting Capacitor Method for Piezoelectric Frequency Tuning and Maximum Energy Harvesting

Energy harvesting refers to conversion of ambient energy into electrical energy. A typical way to accomplish this conversion is through the use of a piezoelectric harvester. This device produces a maximum power when its natural resonance frequency matches that of the ambient vibration. This property is the main limitation to developing many application. To address this restriction, it has been proposed by several investigators that a capacitor be connected in parallel to a piezoelectric cantilever as a method of electrical tuning. When such passive element is connected, the power decreases from its original value. In this paper an improvement to this approach is proposed. Once the tuning capacitor is connected, an inductor value is chosen such that conjugate impedance matching becomes reasonable and plugging this component can give an improvement for both the voltage and power generated. Capacitors of 0.2 μF to 1.5μF values were connected in parallel to a piezoelectric unimorph Type TH-7R with an inherent capacitance of 166 nF and a 208 Hz resonance frequency to develop a tuning range of four Hz. The harvested power during the tuning was proved to be correlated inversely to the shunt capacitor value. By connecting a 700 mH and 2 H inductors in parallel to the system, a significant improvement in power was obtained. In addition, a correlation between the resonance frequency and optimal load resistance with the shunt capacitor value has been studied. The results show that this innovative method is an efficient method for frequency tuning and maximum power extraction.