Abstract
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.
Highlights
In recent years, there has been an emergent need in robotics to develop new mechanisms that go beyond the conventional structures, focusing on compliant, lightweight and energy efficient designs
The experimental setup is equipped with Razor 9 degrees of freedom (DOF) inertial measurement unit (IMU) which is attached on the upper plate along the x axis
The commands are sent to cubic trajectory node which enquires necessary motor angles from inverse kinematic server
Summary
There has been an emergent need in robotics to develop new mechanisms that go beyond the conventional structures, focusing on compliant, lightweight and energy efficient designs. The upper plate is able to perform rotation in two axes providing roll and pitch angles This configuration is further supported by a concentric shaft with a universal joint in the middle, which passes through the mechanism restricting and defining the bending motion of the compression spring. Two rotations of upper plate (roll and pitch) are actuated in a cable-driven way, the motion is constrained by the shaft inside the spring and the universal joint. The solution proceeds in the appropriate direction (i.e. towards minimizing approximation error) for finding the lengths of the remaining cables to realize the roll and pitch angles given in the inverse kinematic problem According to this method, only two selected cables are manipulated at any motion command. The dynamic modelling is composed of three sequential steps: (i) reduction of 3-D model into a 2-D model without information loss, (ii) force analysis on a bending helical spring using Castigliano’s Theorem, and (iii) complete dynamic model
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