DESIGN OPTIMISATION OF A PLANAR ELECTROMAGNETIC ENERGY HARVESTER SUITABLE FOR LOW FREQUENCY VIBRATIONS

Nowadays, most of the wireless sensor nodes (WSNs) used in Internet of Things (IoT) applications are powered by batteries. Despite the convenience of battery usage, the need for frequent battery replacement contributes to both direct and indirect environmental pollution. Vibrational energy harvesting using electromagnetic vibrational energy harvesters (EM-VEHs) is a more sustainable alternative to batteries as it harnesses the kinetic energy from ambient vibrations. Usually, the form factors of these devices render them difficult to integrate with printed circuit board and energy storage technologies since coils and magnets are generally arranged in a cylindrical form factor. Arranging coils and magnets in a planar configuration can result in a more compact device and improved integration. In this paper, an innovative planar EM-VEH design is proposed for harvesting kinetic energy from low frequency (10 Hz to 20 Hz), in-plane vibrations. The structure comprises three fixed-free beams, each of which is fixed on one side to the housing of the harvester while their free ends are attached to a single mass that acts as a magnet holder. Compared to previous studies where the beams are straight (90 degrees to the mass and to the fixed ends), the proposed fixed-free beams form an angle (86 degrees) to the mass and at the fixed end. Coils can be placed on top and bottom of the magnets or, in case of a Halbach array, coils can be placed on top or bottom of the magnets. In-plane excitations induce oscillations of the beams in the in-plane direction and, hence, relative movement of magnets and coils. Finite element analysis (FEA) using the stress and frequency analysis modules in SolidWorks was carried out to investigate and optimise different geometrical parameters of the design in order to increase the output power for low amplitude (up to 0.5 g, g = 9.81 m/s(2)) and low frequency (up to 20 Hz) vibrations. In particular, geometrical dimensions of the beams are optimised in terms of: (1) stress at the fixed end of the beam; (2) vertical deflection of the mass at free end; and (3) the movement of the magnet holder. The FEA results show a resonant frequency of 16.8 Hz, and relative displacements in x-axis of coil and magnet of 4.3 mm, 6.5 mm, 8.7mm and 10.8 mm at acceleration levels of 0.2 g, 0.3 g, 0.4 g, and 0.5 g, respectively.