Precision Robotic Milling of Fiberglass Shims in Aircraft Wing Assembly Using Laser Tracker Feedback

During aircraft wing assembly, machined fiberglass shims are often used between mating parts to compensate for inherent geometric variability due to manufacturing. At present, fiberglass shims for large aerospace structures, such as shims attached to wing ribs, are manufactured either manually or by precision machining, both of which pose a challenge due to tight tolerance requirements and wide geometric variations in the aircraft structures. Relative to articulated arm industrial robots, gantry-style computer numerical control (CNC) machines are costly, consume large footprints, and are inflexible in the application. Therefore, industrial robots are viewed as potential candidates to replace these gantry systems to facilitate metrology, shim machining, and permanent joining of aircraft structure, with all these processes taking place in the assembly process step. However, the accuracy of articulated arm robots is limited by errors in kinematic calibration, gear backlash, joint compliance, controller performance, and mechanical deformation of the robot structure during machining. Therefore, industrial robots are currently unable to meet the strict accuracy requirements for aerospace parts without error compensation methods. This article presents a control architecture that utilizes real-time closed-loop position feedback derived from a high-accuracy laser tracker to improve the machining accuracy of articulated arm industrial robots. In addition, the article evaluates the performance of two closed-loop control methodologies in robotic milling, namely, controlling for path error versus controlling for trajectory error. The control methodologies are tested in robotic milling of fiberglass coupons along a curvilinear (sinusoidal) path. In addition, the best control methodology is tested in robotic milling of fiberglass shims installed on the mating surfaces of a 3.5 m aluminum aircraft wing rib. The dimensional accuracies and surface finish of the machined features using the proposed control methodologies are shown to be within acceptable tolerances for machined fiberglass shims.