Kinematics of a 5-axis hybrid robot near singular configurations

Abstract

• A method based on kinematics modification for singularity avoidance is proposed. • A criterion for choosing the appropriate inverse kinematic solution is presented. • A semi-analytical algorithm for forward kinematic analysis is developed. • An algorithm is developed by modifying C -axis instructions. • Both simulations and experiments are carried out to verify the proposed approach. This paper proposes a method for non-singular tool path generation based on inverse kinematics modification in joint space to deal with the singularity problem of a 5-axis hybrid robot in machining process. Firstly, a criterion for choosing the appropriate solution of inverse displacement analysis is presented, that enables the avoidance of mechanical interference and enlargement of the task workspace. As a counterpart of inverse displacement analysis, a semi-analytical algorithm for forward displacement analysis is developed with high computational efficiency. These works lay a comprehensive and solid foundation for motion control and tool path simulation of the hybrid robot. Then, it reveals that at a singular configuration that varies with the system configurations, sudden changes occur in both rotation of the C -axis of the 2-DOF wrist and lengths of three telescopic legs within the 3-DOF parallel mechanism. This finding leads to an effective method for non-singular tool path planning by directly modifying the C -axis commands in the joint space using linear interpolation technique. Finally, both simulation and experimental results on a prototype machine show that in the neighborhood of singularity, continuous and smooth joint motions and acceptable tracking accuracy can be ensured after the modification.