Hybrid orientation/force control for robotic polishing with a 2R1T force-controlled end-effector.

For robotic polishing of curved surfaces with compliant polishing disc, it is critical to accurately control the tilt angle of the polishing disc and normal contact force between the disc and the workpiece surface simultaneously. As conventional industrial robots lack force control capability, a 3-DOF two-rotational-one-translational (2R1T) force-controlled end-effector based on the 3-PPS parallel mechanism is developed for robotic polishing. During the polishing process, the changes of the tilt angle of the end-effector's moving platform will result in displacement variations of the contact point, owing to the inherent parasitic motion of the end-effector module, the geometry of the disc, and the compliance of the disc. Without proper compensation of the displacement variation, the control accuracy of contact force will be significantly decreased. To tackle this problem, a parametric model is established to predict the contact point variation. A hybrid orientation/force control architecture with compensation of the contact point variation is proposed. Based on the kinematic analysis, orientation control is achieved through a position controller. The force tracking control considering the uncertainty of the environment is achieved through an admittance controller. By introducing the contact point compensation model into the control architecture, the control accuracy of the contact force is significantly improved. The proposed control architecture is evaluated on a macro-mini manipulator consisting of a 6-DOF industrial robot and the 2R1T force-controlled end-effector. The experimental results indicate that the orientation control is accurate and the mean force errors with three different kinds of tilt angle references are reduced by 78.9 % , 81.1 % and 72.3 % compared to the conventional hybrid orientation/force control, respectively, which validate the effectiveness of the proposed control method. [ABSTRACT FROM AUTHOR]