A bistable electromagnetic energy harvester with cross-ring structure for ultra-low frequency applications

Ultra-low frequency energy harvesting technology offers an effective solution for capturing weak environmental vibrations, enabling the advancement of self-powered systems. This study presents a novel bistable electromagnetic energy harvester incorporating an optimized cross-ring structure designed for ultra-low frequency vibration environments. The nonlinear restoring force of the cross-ring configuration is analytically derived using a chained beam constraint model, enabling the determination of bistable structural parameters that produce the shallowest potential wells. The dynamic response of the harvester under a harmonic base excitation is modeled, and the steady-state displacement transmissibility and output voltage are analytically obtained via the harmonic balance method. The analytical outcomes are numerically validated through the direct integration of the governing dynamic equation. Experimental investigations, including the restoring force characterization and the energy harvesting performance tests, confirm the theoretical predictions. Results demonstrate that the proposed harvester achieves a peak output voltage of 1.74 V and a power output of 2.54 mW at an ultra-low frequency of 1.172 Hz. Furthermore, numerical simulations indicate that under optimal preload mass conditions, the device can generate 3.85 V at an even lower frequency of 0.502 Hz. Compared to equivalent linear harvesters, the cross-ring energy harvester exhibits a 2.18-fold wider operational bandwidth and 7.49-fold higher power output, attributed to its enhanced nonlinear characteristics. This work establishes both a practical prototype and a theoretical foundation for efficient ultra-low frequency energy harvesting via a cross-ring structure.