Design of a flexibly-constrained revolute pair with non-linear stiffness in multiple directions

Abstract

In order to synthesize a human-friendly flexible mechanism with a simple structure, a revolute pair with flexible kinematic constraint in multiple directions is proposed. It is called the multi-directionally flexibly constrained revolute pair (MFCRP). The structure of the MFCRP is composed of a link with two same spherical surfaces and a link with two same cam surfaces, and each set of the cam surface and the spherical surface is in contact at a point. The connection between the two links is kept by two linear springs arranged between the two links. The MFCRP can generate 1-axial relative rotation between the two links and relative motions in the other directions are flexibly constrained. In order for the MFCRP to have both flexibility for safety and rigidity for the force transmission, the specified non-linear stiffness can be implemented in the two relative translational directions between the two links. This flexible translational constraint is generated by the spring forces and the reaction forces between the two links. In this paper, two methods to design the cam surfaces to implement the specified non-linear stiffness are proposed. The validity of the proposed design methodology is confirmed by comparing measured stiffness characteristics between two links of some prototypes with the theoretical characteristics. As an application, a flexible closed-loop linkage with the MFCRP is fabricated and its flexibility and kinematic performance are investigated through some experiments.

An extended design method of flexibly-constrained pairs with non-linear stiffness in multiple directions

“…In this paper, concept of the multi-directionally flexibly constrained revolute pair (MFCRP) (Kimura et al, 2020) was extended as the "flexibly constrained pair (FCP)", which can allow not only 1-axial rotation but also other relative motions including such as 1-axial translation. It has a simple structure composed of several cam surfaces and several spherical surfaces kept in contact at a point with each other by some linear springs.…”

“…As a simple example, the MFCRP, which was designed in previous work (Kimura et al, 2020), was designed again by using the method proposed in this paper. The reference point is fixed at 1 p = [0 0 33] T [mm], and the rotation around the axis parallel to the x 1 -axis of Σ 1 is allowed.…”

“…Fig. 1 Multi-directionally flexibly constrained revolute pair (Kimura et al, 2020) leads lightweight because an additional actuator is not required. Non-linear stiffness has both flexibility and rigidity because it has both low and high stiffness area.…”

“…In order to solve this problem, the authors have proposed a passive revolute pair with multi-axial passive flexibility as shown in Fig. 1 (Kimura et al, 2020). This is called the multi-directionally flexibly constrained revolute pair (MFCRP).…”

“…This function can absorb an excessive load applying to the joint by the displacement between the links. The advantage of the idea of "flexible kinematic constraint" is that a mechanism with multi-directional flexibility can be achieved easily when an mechanism with multiple links is synthesized with conventional type synthesis by regarding the FCP as an joint to move only the main-directions (Kimura et al, 2020). About the FCP, this paper describes the following two contributions.…”