A Virtual Mechanism Approach for Exploiting Functional Redundancy in Finishing Operations

We propose a new approach to programming by the demonstration of finishing operations. Such operations can be carried out by industrial robots in multiple ways because an industrial robot is typically functionally redundant with respect to a finishing task. In the proposed system, a human expert demonstrates a finishing operation, and the demonstrated motion is recorded in the Cartesian space. The robot’s kinematic model is augmented with a virtual mechanism, which is defined according to the applied finishing tool. This way, the kinematic model is expanded with additional degrees of freedom that can be exploited to compute the optimal joint space motion of the robot without altering the essential aspects of the Cartesian space task execution as demonstrated by the human expert. Finishing operations, such as polishing and grinding, occur in contact with the treated workpiece. Since information about the contact point position is needed to control the robot during the operation, we have developed a novel approach for accurate estimation of contact points using the measured forces and torques. Finally, we applied iterative learning control to refine the demonstrated operations and compensate for inaccurate calibration and different dynamics of the robot and human demonstrator. The proposed method was verified on real robots and real polishing and grinding tasks.
Note to Practitioners
—This work was motivated by the need for automation of finishing operations, such as polishing and grinding, on contemporary industrial robots. Existing approaches are both too complex and too time-consuming to be applied in flexible and small-scale production, which often requires the frequent deployment of new applications. Our approach is based on programming by demonstration and enables the programming of finishing operations also for users who are not specialists in robot programming. Programming by demonstration is especially useful for teaching finishing operations because it enables the transfer of expert knowledge about finishing skills to robots without providing lengthy task descriptions or manual coding. Besides the human demonstration of the desired operation, the proposed approach also requires the availability of the kinematic model for the machine tool applied to carry out the finishing operation. We provide several practical examples of grinding and polishing tools and how to integrate them into our approach. Another feature of the proposed system is that user demonstrations of finishing operations can be transferred between different combinations of robots and machine tools.