A two-dimensional electromagnetic vibration energy harvester with variable stiffness

This work investigates the performance of an electromagnetic vibration harvester for two-dimensional vibra-tions with variable magnetic stiffness and electromagnetic damping. The device consists of a free-to-move cylindrical magnetic structure with a set of bearings located on top and bottom, a couple of coils located on top and bottom of the device and finally a fixed system of disk-shaped magnets placed inside a ring holder. The number of disk-shaped magnets in the ring holder can be varied to change the magnetic stiffness of the system. The performance of the device is characterized experimentally for nine different configurations of disk-shaped magnets, exploring both symmetric and asymmetric designs. Using an XY-shaker to vibrate the system in two dimensions in frequencies from 1 Hz to 10 Hz and with motion amplitude of 2 mm on both axes, a maximum power of 27 mW was harvested. This occurs for an asymmetric device, i.e. with different magnetic stiffnesses along its two axis. For symmetric devices the power is lower by a factor of two. Finally, varying the electromagnetic damping, which is controlled by varying the coil dimensions, can further increase the power to 42 mW.