The optimization of magnet and coil configurations in a planar electromagnetic energy harvester for low-frequency, low-acceleration vibrations

Electromagnetic vibration energy harvesters (EM-VEHs) are a potential sustainable alternative to batteries to power Wireless Sensor Nodes (WSNs) for Internet of Things (IoT) applications. This paper presents an innovative planar EM-VEH suitable for the low-frequency (< 15 Hz) and low-acceleration (0.05 g to 0.2 g, where g = 9.81 m/s²) in-plane vibrations prevalent in many IoT applications. The structure consists of three fixed-free beams, each attached at one end to the harvester’s housing and at the other to a shared single mass that acts as a holder for an array of magnets. A set of coils is located on a fixed frame in close proximity above the magnets. In-plane excitations induce oscillations of the beams in the in-plane direction and, hence, relative movement of magnets and coils. In this paper, using a combination of simulation and experimentation, eight different magnet configurations, three different magnet sizes, five different coil configurations and three different wire diameters are analyzed in order to determine an optimum magnet and coil combinations for the planar EM-VEH. The optimum design features an 8 × 3 Halbach magnetic array in combination with three round coils, each 3 mm thick, formed from wire of 0.1 mm diameter. With the coils connected in series / anti-series to an optimum load resistance of 1500 Ω, the harvester produces a power level of 3.35 mW at a resonance frequency of 15.2 Hz under acceleration levels of 0.2 g, yielding a projected Normalized Power Density (NPD) of 1950 μW/cm³·g².