Two degree of freedom vibration based electromagnetic energy harvester for bridge health monitoring system

This paper presents an electromagnetic energy harvester to extract low frequency and low acceleration vibration energy available in a bridge environment. The developed harvester is a multi-mode oscillator with dual electromagnetic transduction mechanisms. The harvester consists of two cantilever beams. The first cantilever beam is split into two equal pieces along its length and the second beam placed in between them coming back to the fixed end and attached at outer end to the first beam. This way instead of a long conventional cantilever beam a compact harvester is fabricated. Two magnets as proof masses are attached to each free end of the beam making it a two degree of freedom system (2-DOF). The magnets are positioned to oscillate inside hand wound coils during operation. An analytical model was developed and COMSOL multiphysics was used to simulate the mode shapes of the harvester. The mathematical model was simulated for open circuit voltage in MATLAB and showed closely matching results with the experimental values. The harvester is characterized in lab for its performance under sinusoidal vibrations at low frequency (3 Hz–15 Hz) and low acceleration (0.01–0.09 g) levels. The 2-DOF harvester has two resonant frequencies of 4.4 Hz and 5.5 Hz and a volume of 333 cm³. It produces maximum voltage of 0.6 V at first resonance on coil-1 and maximum voltage of 1.2 V on coil-2 at second resonance at 0.09 g. At acceleration of 0.09 g the harvester produced 2.51 mW at first resonant frequency and 10.7 mW at second resonance. Moreover, the AC output voltage of the harvester is rectified to DC voltage by a three-stage Cockcroft-Walton multiplier type circuit. The DC power output at 0.05 g was 0.939 mW at first resonance and 0.956 mW at second resonance while maximum voltages of 5.4 V on coil-1 and 4 V on coil-2 were produced.