A singularity handling algorithm based on operational space control for six-degree-of-freedom anthropomorphic manipulators

Zhi-Hao Kang,Han-Pang Huang,Ching-An Cheng

doi:10.1177/1729881419858910

Abstract

In this article, we analyze the singularities of six-degree-of-freedom anthropomorphic manipulators and design a singularity handling algorithm that can smoothly go through singular regions. We show that the boundary singularity and the internal singularity points of six-degree-of-freedom anthropomorphic manipulators can be identified through a singularity analysis, although they do not possess the nice kinematic decoupling property as six-degree-of-freedom industrial manipulators. Based on this discovery, our algorithm adopts a switching strategy to handle these two cases. For boundary singularities, the algorithm modifies the control input to fold the manipulator back from the singular straight posture. For internal singularities, the algorithm controls the manipulator with null space motion. We show that this strategy allows a manipulator to move within singular regions and back to non-singular regions, so the usable workspace is increased compared with conventional approaches. The proposed algorithm is validated in simulations and real-time control experiments.

Highlights

Humanoid robots and service robots have been gaining popularity
We show that the boundary singularity and the internal singularities of these 6-DOF anthropomorphic manipulators can be identified, despite lacking kinematic decoupling
We find that the determinant of the Jacobian of 6-DOF anthropomorphic manipulators possesses a certain algebraic structure, which allows the identification of different singular conditions and the corresponding singular directions

Summary

Introduction

Humanoid robots and service robots have been gaining popularity. These robots are usually equipped with anthropomorphic manipulators that are mechanically designed to mimic human arms, with links corresponding to a human’s upper arm and forearm, and joints corresponding to the shoulder, elbow, and wrist, so that they can execute tasks with human-like motion. We find that the determinant of the Jacobian of 6-DOF anthropomorphic manipulators possesses a certain algebraic structure, which allows the identification of different singular conditions and the corresponding singular directions Based on this discovery, we design a singularity handling algorithm following the type-II method.. The manipulator enters the singular region (configuration 2) and the boundary singularity is detected by equation [28] At this moment, the Jacobian matrix becomes illconditioned, so it cannot derive the generalized force and the joint torque properly. We follow the design of the singularity robust algorithm, which chooses the potential function to minimize the jerk generated by the controller based on the collapsed Jacobian (this controller attempts to minimize the tracking error; as it uses a collapsed Jacobian matrix, it is often that jerky motion is generated at the boundary of the singular region). Ð35Þ where kN > 0 is a proportional gain, xðqÞ is the associated joint corresponding to the direction of the null space motion, and x0 is the joint corresponding to the singular

Compute the control input

Conclusions