Wide Bandwidth Piezoelectric Energy Harvester With Transverse Movable Mass

Novelty / Progress Claim(s)

High Q-Factor vibrational energy harvesters are ideal as they maximize power generation, but a narrow bandwidth limits the potential use in most applications, and it is a major challenge that has not been resolved. Numerous designs have been investigated to solve this challenge but most of the attempts are based on frequency sweeping mechanism or require complex design/fabrication which are not practical. This paper reports for the first time a transverse or vertical moving mass method to widen the bandwidth which is independent of frequency sweeping. Ultra-wide bandwidth was achieved for low (0.5g) and high (1g) accelerations. This transverse method of widening the bandwidth is potentially scalable to MEMS devices.

Background / State of the Art

Numerous attempts have been made to widen the bandwidth of piezoelectric energy harvesting cantilevers. The most common method uses duffing resonators, but these suffer from hysteresis caused from frequency sweeps. Recent attempts to widen the bandwidth include investigating a center of gravity change using a movable mass. Researchers have attempted to create a movable mass using liquid sloshing effects or rolling masses, which has demonstrated modest bandwidth widening. However, these rely on high displacement oscillations to alter the center of gravity in order to increase the bandwidth. This paper demonstrates a vertical movable mass that changes the overall mass of the system during oscillation and widens the bandwidth based on 1) varying resonant frequency due to change in mass and 2) impact of movable mass on cantilever creates non-linear effects.

Description of the New Method or System

The device consists of a piezoelectric cantilever with a custom-made 3D printed proof mass. The proof mass includes a vertical cavity or multiple cavities where stainless steel masses can be inserted. As the cantilever oscillates due to external vibration source the spheres are free to move in a transverse or out-of plane movement, this causes the overall mass of the system to change at any given point in time, which thus changes the resonant frequency. Therefore, the resonant frequency is constantly changing giving the effect of widening the bandwidth. Testing was performed using a vibration shaker with controlled acceleration feedback. A variable load resistor and oscilloscope were used to measure power. A comparison study using a control cantilever (no movable mass) and movable mass with varying acceleration, number of spheres, and size of spheres was conducted. A capping layer was used (not shown). Random frequencies were chosen instead of performing a frequency sweep to avoid potential hysteresis. 

Experimental Results

Power vs. frequency curves were demonstrated to determine the bandwidth effects using different size spheres and by altering the number of spheres in a cavity. The control cantilever had a bandwidth of 3.9 Hz. A significant increase in bandwidth was demonstrated with values of 49.3 and 56 Hz for the 1 and 3 spheres per cavity respectively. Both devices had a significant decrease in peak power, but the 3 sphere per cavity device demonstrated a ~2x increase in power compared to the single sphere device. This study also varied the size of the spheres to determine if a single large mass gave the same results as numerous small masses with same overall mass. A single sphere resulted in high bandwidth with a low power section combined with a large peak power, which results in an output with high power and high bandwidth. The 10 mm sphere systems resulted in high overall power compared to the 5mm spheres but with less bandwidth. We also investigated the effects of varying acceleration to validate that the bandwidth increase was demonstrated with low acceleration. The error bars represent the standard deviation for a given frequency as the device was going in and out of resonance, due to the non-linear dynamics of the system.