Singularities of ABB's YuMi 7-DOF robot arm

ABB's YuMi is a unique, innovative cobot but it is also one of the most challenging 7-degree-of-freedom (DOF) robot arms on the market, in terms of kinematics. Indeed, unlike some other 7-DOF robot arms with revolute joints, in YuMi, consecutive joint axes are normal to each other, but not intersecting. And despite being invented over a decade ago, there is surprisingly little information available about its kinematics. To effectively incorporate this manipulator into motion planning tasks, it is essential to have a comprehensive grasp of its arm angle and singularities. In this paper, we use the screw dependency approach with a novel combinatorial technique and Grassmann geometry of lines to identify and categorize, for the first time, the kinematic singularities of YuMi based on simple geometrical conditions. This methodology allows for a systematic and clear understanding of the robot's singular configurations. In addition, we provide the definition of the arm angle used by ABB and a formula for the angle calculation. Then, we describe the representation singularity, and explain the algorithmic singularities that are related to the arm angle.