On-Line Pose Measurement and Adjustment of End Effector Based on Laser Tracker for Robotic Drilling System

Automatic robotic drilling is a widely used way of fastening in the field of aircraft assembly and is worth studying continuously. Drilling accuracy is one of the most remarkable properties of the system, which is directly related to the absolute positioning accuracy of the robot end effector. Due to the kinematic errors and gravity of the robotic system itself, the nominal pose and the actual pose of the end effector are no longer consistent with each other. It is necessary to keep the high positioning accuracy of the system. In this paper, an automatic robotic drilling system with high positioning accuracy is proposed. Generally, there are two methods to improve positioning accuracy: off-line calibration and on-line adjustment. An off-line calibration based on the DH method is proposed to identify and modify the parameter errors of the robot. A 6- degree of freedom industrial robot is integrated into the drilling system. After the kinematic error modeling of the robot is established, the sample points in the reachable workspace of the robot are measured, the kinematic errors are identified and compensated, and the calibration process is achieved. Then an on-line pose measurement and adjustment of the end effector based on a laser tracker are developed. Transforming relations between coordinate systems of all parts of the drilling system are firstly obtained accurately. With the robotic drilling end-effector as the working module and the laser tracker as the measuring module, A closed-loop control system for positioning accuracy is established. By measuring the actual pose of the end-effector and comparing it with the nominal pose, the pose of the end-effector is adjusted in real-time. The experiment results indicate that after pose measurement and adjustment the positioning accuracy of the end effector meets the accuracy requirements.