On the use of low-cost 3D stereo depth camera to drive robot trajectories in contact-based applications

In production systems characterized by small batches and high customization levels, operations are required to be flexible to adapt to different products within the shortest possible time and with the minimum effort for system setup. Contact-based operations such as surface finishing, polishing, deburring, and material deposition are mandatory in the fabrication of numerous products. To maintain consistent performance over time, many of these operations require a high level of accuracy, both in end-effector positioning and contact force level. This paper proposes a robotic solution to generate the robot working trajectory for contact-based operations over the external surface of unknown objects of which a digital model is not available or different from the actual state of the workpiece. The paper introduces the integration process of RGBD images to construct a 3D model and its elaboration to extract the workpiece. Different searching subroutines have been developed to select different areas of the workpiece based on the operation to be carried on and generate the related trajectory. The evaluation metric of the proposed robotic solution is given by the stability of the contact force exerted by the robotic tool and the error between generated and the actually followed trajectory due to the depth estimation of a low-cost camera. A few millimeters of the inaccuracy of the trajectory are obtained; these inaccuracies are compensated using force control. Different tests with different nominal values of the force control loop are carried out. Statistical analysis shows that the mean values of the contact force obtained coincide with the nominal value of the single tests.