Three-axis piezoelectric vibration energy harvester

This paper reports for the first time a piezoelectric inertial harvester for scavenging vibrational energy in all three dimensions. The harvester is formed of optimized bulk-PZT/Si crab legs such that the first three resonance vibrational modes are in-plane and out-of-plane translational modes with closely-spaced frequencies (387-398 Hz). Partitioned electrodes on the PZT arms collect mechanical energy in the transverse (d₃₁) piezoelectric mode, and have phase difference in their voltage outputs according to the axis of the applied vibration. The harvester is realized using a planar micro-fabrication method, with internal and packaged device volumes of 2.5 and 6.5 cm³, respectively. The device has generated 5-53 μW from 50 mg acceleration applied in in-plane and out-of-plane directions at the respective resonance frequencies, and has a half-power bandwidth of 8-16 Hz. The normalized power density (0.3-3.2 mW/cm³/g²) compares favorably with respect to previously reported multi-axis inertial energy harvesters.