Enhanced electromagnetic wrist-worn energy harvester using repulsive magnetic spring

While wrist-worn electronics have great potential in a variety of areas, their applications are currently constrained by the limited working time of their electrochemical batteries. Harvesting energy from arm swinging motions provides a promising means to address this issue. In this work, we introduce a tiny repulsive magnetic spring to improve the performance of a wrist-worn inertial energy harvester. In contrast to conventional inertial rotational energy harvesters, the proposed device contains a repulsive magnetic spring that consists of two pairs of repulsive magnets. The existence of the magnetic spring lowers the potential energy well depth of the energy harvester by reducing the system's stiffness, thereby enhancing power generation. An analytical model is built to predict the system performance and investigate the effects of the magnetic spring air gap. To validate this design, a prototype is constructed and tested using a bench-top excitation source that emulates the swinging motion of the human arm. The experiments show that, in the tested excitation frequency (0.7-1.3 Hz) and air gap (2.2-3.6 mm) ranges, the average output power is significantly enhanced by the magnetic spring and smaller air gap of magnets results in more improvement, which agrees well with the simulation results. With the requirement of a minimal amount of space, the magnetic spring helps the energy harvester achieve maximum output power of 151 mu W with an air gap of 2.6 mm at 1.3 Hz. Additionally, a maximum power improvement of 425% for an air gap of 2.2 mm at 0.7 Hz is achieved compared with the conventional wrist-worn inertial energy harvester. (C) 2020 Elsevier Ltd. All rights reserved.