An Elastic Shoulder Joint for Humanoid Robotics Application

Designing multiple DOFs joints having a parallel kinematic structure brings additional challenges, if compared with classical single DOF joints. Generally, it is more difficult to formalize the forward kinematic model for a parallel kinematic chain, in addition, while performing a target motion, the actuators have to be carefully coordinated to avoid wasting energy or cause damages to the system. The problem is becoming even more complex if instead of using a rigid structure, elastic elements are incorporated in the design. With this work, we present how to make usage of the method of Finite Element Analysis (FEA) in order to compare alternative designs for the central leg of a shoulder joint having a parallel kinematic architecture. The optimization process of the joint's design was defined by providing 2 different scenarios: 1) bending and 2) twisting. Moreover, we introduce how to model the kinematics of an elastic legs, by replacing the bending object with a combination of 3 universal joints and 2 links. In order to increase the payload-to-weight ratio of the shoulder joint, a high power-to-weight ratio servomotor was integrated by using a tendon-driven mechanism.