A bi-stable mechanism is a system that has two distinct stable equilibrium positions within its range of operation. It is capable of steadily staying at the positions without external power input and its snap-through behavior reduces the energy consumption required to move the position. Diverse applications including switches, deployable structures, and reconfigurable robots can benefit from bi-stability using these characteristics. Conventional methods for implementation of bi-stable mechanisms utilize composite laminates, SMAs (Shape Memory Alloys) and compliant mechanisms. However, due to the complexity of implementation and the limitation of structure configuration, conventional bi-stable mechanisms are difficult to apply to the structures which require rotational bi-stability. In this study, an implementation method of rotational bi-stable mechanism using cylindrical permanent magnets is proposed.

The proposed bi-stable revolute joint consisting of two links (one is rotation link and the other is fixed link) has rotational bi-stability through the magnetic force relationship between the magnet arrays on each link. Two cylindrical permanent magnets are attached on the surface of the rotation link in a line. On the other hand, the fixed link has an annular array of cylindrical permanent magnets. The two links are assembled using a shaft on the center point. The characteristics of a rotational bi-stability of the proposed revolute joint are dependent on not only the number of magnets but also the diameter of each magnet in the annular array. To identify the characteristics of the proposed bi-stable mechanism, a cylindrical permanent magnet is considered as an electromagnet model which consists of one ring with a virtual electric current. In that case, the magnetic field of the cylindrical permanent magnet can be calculated by Biot-Savart law. Based on the electromagnet model, the magnetic force between two cylindrical permanent magnets is calculated by Lorentz force law and verified by the load cell experiment according to different diameters.

The proposed bi-stable revolute joint has been designed to have a stable equilibrium position at every 180 deg rotation and the criteria of the magnet array for symmetric bi-stability is described. The potential energy diagram of the rotation link is considered as the performance criterion of the proposed revolute joint. The proposed bi-stable mechanism has been applied to the prototype of a deployable structure consisting of two links. Moreover, the load testing of the structure against external torque was performed to find the degree of the static stability. The proposed bi-stable revolute joint can also be easily designed to have a multi-stability by changing the magnet arrays on the fixed link. Finally, an additional multi-stable revolute joint has been applied to the prototype and the degree of static stability at each equilibrium position was identified.


Design of Revolute Joint With Bi-Stability Using Permanent Magnets

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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