Analysis of magnetic force and potential energy function of multi-stable cantilever beam with two magnets
Authors: Sun Shuai-Ling , Leng Yong-Gang , Zhang Yu-Yang , Su Xu-Kun , Fan Sheng-Bo ,   School of Mechanical Engineering, Tianjin University, Tianjin 300350, China
Published: December 2020

<sec>Multi-stable structures are deformable structures that can have large deformations under external excitation. Generally, multi-stable structures have at least two stable points and can jump from one to another. Because multi-stable structures have excellent nonlinear characteristics, they are widely used in many fields. In the field of energy harvesting, multi-stable structures are often obtained by means of cantilever beams. This is because the cantilever beam is simple to make, low in stiffness, and high in sensitivity, and can undergo large deformations under small excitation forces. Besides, by simply sticking magnets on its free end and its outside, various kinds of multi-stable characteristics can be constructed, such as bi-stable characteristics, tri-stable characteristics, quad-stable characteristics, etc. Furthermore, the cantilever beam and the magnet at its end can generally be simplified into an equivalent mass-spring-damping mechanical model, which is convenient for the analysis of system potential function and dynamics.</sec><sec>In recent years, many vibration energy harvesters proposed by researchers have adopted the conventional multi-stable cantilever beams, which involve many bi-stable cantilever beams and tri-stable cantilever beams. However, if the cantilever beams need to introduce more stable points, the number of magnets required will also increase accordingly. As a result, the adjustable parameters are continuously increasing, which increases the complexity of structural optimization and the tediousness of dynamic analysis. In order to make up for the shortcomings of conventional multi-stable cantilever beams, in this paper we present a multi-stable cantilever beam with only two magnets, a ring magnet and a rectangular magnet. By changing the size of the rectangular magnet and the distance between the two magnets, this cantilever beam can have mono-stable, bi-stable, tri-stable or quad-stable characteristics. This multi-stable cantilever beam greatly simplifies the complexity of the system design, dynamic analysis, debugging and installation, and provides new ideas and technical methods for the design and application of the vibration energy harvester realized by the multi-stable cantilever beam.</sec><sec>In this paper, firstly, the magnetizing current method is used to analyze the magnetic induction intensity of the ring magnet at any point in the three-dimensional coordinate system, and the simulation and experimental results prove its correctness. Secondly, two methods of calculating the position of the rectangular magnet at the free end of the cantilever beam are compared. Thirdly, the magnetic force between the ring magnet and the rectangular magnet is calculated and verified in experiment. Fourthly, the system potential functions under different structural parameters are analyzed and it is found that the change of the number of the stable points of the system is caused by the change of the magnetic force between the two magnets. Finally, the correctness of the number of stable points of the system under different parameters is verified in experiment and by dynamic simulations.</sec>