Real-time freeform surface and path tracking for force controlled robotic tooling applications

Modern force controlled robotic manipulators in combination with additional sensor configurations allow a wide range of operational scenarios for flexible production processes. This paper considers a sensor-based geometric approach for tracking curved freeform surfaces with the focus on robotic tooling applications such as polishing and grinding. A local approximation of the surface geometry is created from sparse distance measurements between the robotic tool and the workpiece maintaining a desired distance and orientation relative to the surface. The approach only relies on current sensor data and does not require any a priori model of the geometry from CAD data or previous 3D scans of the workpiece. The considered surface tracking controller allows the additional tracking of a planned path. The presented control is extended by a force controller to achieve a desired interaction force between the manipulator and the workpiece at a specified angle. To provide a flexible automation solution for the tooling of freeform surfaces, a path planning method is presented to cover a larger area based on the teaching of the area's boundary. The considered approaches are realized on an experimental setup with the robotic manipulator KUKA LBR iiwa 14, where the surface tracking capabilities, as well as the performance of the force controller, are evaluated. The surface tracking and force controller are implemented directly on the robot controller while the sensor data processing is performed on a PLC which is connected to the robot controller via EtherCAT. The setup uses the position-based control interface for the integration of the surface tracking and the force control which introduces some limits on the performance of the applied control schemes but preserves the safety features and the collaborative behavior of the robotic manipulator.