Development of In-Situ Hybrid 3-D Measuring Methodology for Critical Dimension Inspection in Large-Scale Robot Machining of Composite Materials

The article presents a hybrid 3-D measuring methodology to address a critical common problem in a large scale space of robot machining of composite materials, especially assembly holes or geometric features to be accurately machined on a free-form shell of composite materials. The referred machining volume can be ranged 1 m(3) up to 10 x 10 x 10 m(3) or larger, which is commonly needed in aerospace space industries. The developed hybrid 3-D measuring methodology integrates a highly accurate touch-triggered probe and an optical 3-D surface profilometer as its sensor fusion configuration. The accuracy of the robot machining space is pre-calibrated and compensated by the accuracy traceability transferred from a pre-calibrated coordinate measurement machine (CMM) by measuring the mold fixture attached to the machined part. More importantly, the key development in this research is the measurement integration of touch-triggered (for holes or other inaccessible features by the optical) and optical sensing (for freeform surfaces) by linking the geometric features with some pre-defined unique or invariant machined geometry, such as specific geometric centers or positions. The accurate extraction of the theses important positioning targets from both the measured data plays non-trivial key in achieving accurate measurement especially in a large-scale working volume. From our preliminary experimental test and verification, using the developed methodology, the maximum positioning error of the composite material shells machined by an industrial robot can be improved from several hundred micrometers down to less than 100 r m when a machining volume is up to 2 x 2 x 2 m(3). Three to five fold improvement of the machined accuracy is achievable.