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.
