Stiffness Optimization of a Robotic Drilling System for Enhanced Accuracy in Aerospace Assembly

The low stiffness of robots significantly limits their applicability within the aerospace assembly and manufacturing sectors. The majority of existing research focuses on optimizing robot posture; however, the efficacy of these approaches is constrained in situations with minimal posture variation. To address this challenge, this study examines a robotic drilling system designed for use in confined spaces. An in-depth analysis of its stiffness model is conducted, and the system's stiffness limitations are identified using the stiffness ellipsoid evaluation method. Based on the mechanical analysis of the drilling state, a stiffness enhancement method grounded in the local force closure of the end effector is proposed. This method involves locking the end effector's expansion module with the substrate during the drilling process, thereby enabling the axial drilling forces to be jointly borne by the expansion module and the robot's base joints. Consequently, the system's stiffness, particularly in the axial direction, is substantially improved. A series of experiments rigorously validate the effectiveness of the proposed stiffness enhancement method. The experimental results demonstrate that the stiffness-optimized robot reduces axial deformation during drilling by a factor of ten and significantly improves hole quality and exit burr height.