Micro electromagnetic vibration energy harvester based on free/impact motion for low frequency–large amplitude operation

• New non-resonant micro-electromagnetic energy harvester based on combined free/impact motion is presented.
• It shows a unique dynamic behaviour.
• The power generated increases with both input amplitude and frequency.
• The harvester simple design promotes size minimization.
• The harvester is well suited for human-powered micro devices.

This paper presents design, simulation, and experimentation of a novel micro-electromagnetic vibration energy harvester based on free/impact motion. Power harvesting is simply achieved from relative oscillation between a permanent magnet allowed to move freely inside a tube-carrying electrical coil with two end stoppers and directly connected to the vibration source. The proposed harvester with free/impact motion shows a non-resonant behaviour in which the output power continuously increase with the input frequency and/or amplitude. In addition, the allowable free motion permits significant power scavenging at low frequencies. Hence, the proposed harvester is well suited for the applications involved variable large amplitude–low frequency vibrations such as human-powered devices. A nonlinear mathematical model of the proposed harvester including electromagnetic and impact characteristics is derived and used further for a case study model prediction. A unique way of oscillation is observed, in which four modes of magnet/tube relative motion appear over the range of exciting amplitudes and frequencies. Two experiments are conducted on different fabricated prototypes. The first shows the effect of different magnet shapes on the harvesting performance, and the second is carried out to investigate the performance of two different size prototypes with variable large amplitude-low frequency vibrations. A harvester with cylindrical total size of D9 × L12 mm can generate RMS power of 71.8 μW at (2.5 Hz and 5.2 ms−2) and 113.3 μW at (3.33 Hz and 12.38 ms−2). Another of D7 × L12 mm size can generate RMS power of 28.4 μW at (2.5 Hz and 5.2 ms−2) and 82.9 μW at (3.33 Hz and 12.38 ms−2). Comparison with some previously fabricated low frequency energy harvesters is made which shows the advantageous of the new harvester in size minimization as well as the significant power raise with the input amplitude.