Articles | Volume 8, issue 1
Research article
22 Mar 2017
Research article |  | 22 Mar 2017

Design and Modelling of a Cable-Driven Parallel-Series Hybrid Variable Stiffness Joint Mechanism for Robotics

Cihat Bora Yigit and Pinar Boyraz

Abstract. The robotics, particularly the humanoid research field, needs new mechanisms to meet the criteria enforced by compliance, workspace requirements, motion profile characteristics and variable stiffness using lightweight but robust designs. The mechanism proposed herein is a solution to this problem by a parallel-series hybrid mechanism. The parallel term comes from two cable-driven plates supported by a compression spring in between. Furthermore, there is a two-part concentric shaft, passing through both plates connected by a universal joint. Because of the kinematic constraints of the universal joint, the mechanism can be considered as a serial chain. The mechanism has 4 degrees of freedom (DOF) which are pitch, roll, yaw motions and translational movement in z axis for stiffness adjustment. The kinematic model is obtained to define the workspace. The helical spring is analysed by using Castigliano's Theorem and the behaviour of bending and compression characteristics are presented which are validated by using finite element analysis (FEA). Hence, the dynamic model of the mechanism is derived depending on the spring reaction forces and moments. The motion experiments are performed to validate both kinematic and dynamic models. As a result, the proposed mechanism has a potential use in robotics especially in humanoid robot joints, considering the requirements of this robotic field.

Short summary
A new joint mechanism design is presented in this study which has a potential use in robotics, particularly in humanoid robot designs, because of its variable stiffness characteristics, lightweight and remotely-actuated structure. A helical spring inside the mechanism brings compliance which increases the safety. The spring is analyzed with a method proposed in the paper and verified by FEM. The kinematic and the dynamic models are obtained and experimentally validated.