Singularity analysis and treatment for a 7R 6-DOF painting robot with non-spherical wrist

Singularity is a fundamental problem in robotics which seriously affects the motion control, path planning, and off-line programing of the robot. In this paper, the singularities of a 7R 6-DOF painting robot with non-spherical wrist are analyzed in detail, and a new avoidance method is proposed. Firstly, the singularities are decoupled into the position singularities and the orientation singularities by introducing a virtual wrist center. All singularity conditions for both cases are identified based on a rational transformation between the 7R 6-DOF robot and the equivalent 6R robot with spherical wrist. Secondly, the position singularities are regarded as inescapable singularities, and unusable regions are defined in the workspace to deal with these singularities. Thirdly, the orientation singularities are regarded as escapable singularities. Based on the definition of functional redundancy for painting application, the Twist-decomposition approach (TWA) is extended to handle the orientation singularities. As the main improvement, a new and simple singularity criterion is proposed to overcome the drawback of the existing algorithm. In the end, two simulations for a 7R 6-DOF painting robot are implemented to demonstrate the effectiveness of the proposed method. (C) 2018 Elsevier Ltd. All rights reserved.