Wideband vibrational electromagnetic energy harvesters with nonlinear polyimide springs based on rigid-flex printed circuit boards technology

A wideband vibrational electromagnetic energy harvester employing nonlinear spring effects is proposed and demonstrated. The harvesters were designed and fabricated by commercial rigid-flex printed circuit boards technology. Rigid FR-4 boards were used for mechanical support and coil winding, whereas flexible polyimide films were patterned for mechanical springs and mass platforms. Two sets of coils were patterned and fabricated in the harvester with an internal coil resistance of about 16 Ω each. Two rare-earth magnets were attached to the central platform as shuttle mass. The total dimension of the harvester was 20 × 20 × 4 mm³. In vibration tests, nonlinearity could be observed even at 0.1 g_rms vibration level due to the spring hardening effect. The frequency for peak induced voltage increased from 187 Hz at low vibration to 382 Hz at 5 g_rms vibration. The effective half-power bandwidth increased from 8 Hz at 0.1 g_rms to 32 Hz at 1 g_rms and 52 Hz at 5 g_rms due to the hysteresis in frequency response. For a matched load and 1 g_rms vibration at 250 Hz, the maximum output power was 160 nW, corresponding to a power density of 100 nW cm⁻³. (paper)