Strongly coupled piezoelectric energy harvesters: Optimised design with over 100 mW power, high durability and robustness for self-powered condition monitoring

Highlights: • Strongly coupled piezoelectric energy harvester with two power peaks at resonance. • Piezoelectric material pre-stressed to exploit the high compressive strength. • Peak stress in the structure below half of the fatigue limit to ensure longevity. • Plate springs to increase the stability and robustness in harsh environments. • 140 mW of power with a 10-mW-bandwidth of 24 Hz when actuated at 0.5 g, 157 Hz. Harvesting ambient vibration energy is a promising method to realise self-powered wireless sensors. However, most of the energy harvesters developed to date are not suitable for real-world applications because of low power output and/or poor durability and robustness. To overcome these challenges, this work develops a strongly coupled piezoelectric stack energy harvester (PSEH) with design considerations not just on the power output but also on the durability and robustness. The PSEH took advantages of the force amplification capability of an optimised mechanical transformer and the high coupling coefficient of a 33-mode multilayer piezoelectric stack to achieve strong coupling and therefore high-power generation. To increase the durability, the piezoelectric stack was pre-compressed to prevent the development of tensile stress, to exploit the high compressive strength of piezoelectric ceramics; the maximum dynamic stress in the mechanical transformer was kept below half of the material's fatigue limit. Plate springs were used to guide the motion of the PSEH and prevent undesired vibration to enhance robustness. A finite element model was developed for design optimisation, which links the design parameters directly to the full performance matrix including maximum power generation. When actuated at 0.5 g, 157 Hz in the lab tests, the PSEH produced a maximum average power of 140 mW with a 1-mW-bandwidth of 72 Hz and 10-mW-bandwidth of 24 Hz. The PSEH showed no performance degradation after continuously actuated at 0.3 g, 157 Hz for 7.9 h. In addition to the lab tests, on-site tests were performed by installing the PSEH in two locations of a screw air compressor. On-site tests showed that the PSEH was able to produce average power of 15.95 ± 2.3 mW and 43.19 ± 1.52 mW when the acceleration produced by the air compressor was 0.125 ± 0.012 g and 0.259 ± 0.004 g, respectively.