Development of FEM-based simulation methods for facilitating the design process of electromagnetic vibration energy harvesters

Abstract

Electromagnetic vibration energy harvesting is an emerging technology for extracting small amounts of electrical energy from vibrations. It can be used to power sensors and data transmission equipment for monitoring industrial processes and machines in hard-to-access-places without easy access to electricity, without requiring regular replacement like batteries. Methods, models and software to aid the design process of springs for electromagnetic vibration energy harvesters have been developed in this work. It has been carried out at ReVibe Energy, a company which develops such harvesters. Previous FEM simulations of eigenfrequencies in the harvesters showed poor correspondence with experiments. To investigate this, experiments to measure sti nesses of springs were set up. This was done by suspending weights in the springs. It was found that also the stiffness of the springs corresponded poorly to finite element simulations. There were also significant differences between spring individuals that were nominally identical. Dimensional measurements were conducted on the springs, and it was found that the spring arms were generally narrower than specifed in drawings for the components. There were also arm width variations between individuals, which correspond to and explain the between-individuals stiffness variations. When simulations were performed with corrections for arm width, the stiffness prediction error was reduced by approximately 50 %. The methodology employed in designing and manufacturing springs was studied, in an effort to improve spring design while making the design work less cumbersome. With the purpose of removing manual iterative design loops, which are very time-consuming, a method for automatically generating and evaluating different spring designs was conceived. It consists of automatic changes to a CAD- file in Autodesk Inventor, export of a STEP file containing the component geometry, meshing of the component with GMSH, finite element analysis on the mesh using Elmer, and finally evaluation of results. This was implemented in a software program written in Python. Keywords: Electromagnetic vibration energy harvesting, finite element method, beam elements, parametric simulations, open source, water jet cutting, tolerances