SCARA+ System: Bin Picking System of Revolution-Symmetry Objects

The bin picking system integrates manipulators with cameras to automatically pick randomly piled objects. It has the commercial potential to help streamline production and is evaluated by its work efficiency and purchase cost. For universal object picking, the system requires the manipulator to cover six degrees of freedom (DoFs). If the picking objects have a uniform shape, this requirement may be relaxed to achieve higher efficiency with cheaper low-DoF manipulators. On the one hand, the pose of revolution-symmetry (RS) objects is 5-DoF for rotating around their revolution axes without changing their poses. On the other hand, the widely used selective compliance assembly robot arm (SCARA) can achieve 4-DoF kinematics due to four joint actuators. Inspired by the fact that the DoF of the RS pose is exactly the same as adding one DoF to SCARA, we develop a SCARA+ system of a SCARA with an additional revolute joint and explore the possibility of integrating it with a 3-D camera to achieve bin picking of RS objects. Toward this end, we first discuss the SCARA+ kinematics with the modified Denavit-Hartenberg (DH) parameters. Then, to calibrate the additional DH and the hand-eye parameters in the kinematics, we construct an axis-point model and provide an iterative solution without singularity. Finally, comprehensive experiments verify the superiority of the SCARA+ system. When compared with the state-of-the-art systems, our system achieves a significant efficiency improvement with relatively lower costs. It has also been successfully applied in the spinning industry for practical bobbin loading.