Normal direction measurement in robotic drilling and precision calculation

Abstract

The perpendicularity of bolting holes in large assemblies has an obvious effect on the fatigue life and the fluid dynamic configuration of the surface. Because the large and complexly curved workpiece often differs from the computer-aided design (CAD) model because of manufacturing errors, normal direction measurement is necessary in robotic drilling. One useful approach is to arrange several displacement sensors and calculates the normal direction at the center, whereby the curved measuring area is simplified as a small plane. However, the simplification results in measurement errors with respect to the normal direction and the measuring precision are difficult to be calculated. This study presents a new method for normal direction measurement by arranging four displacement sensors in a rhombus layout, which has several advantages compared with other layouts. A novel algorithm for calculating the measuring precision using the proposed method is introduced in this study. The algorithm makes it possible to predict the performance of the normal direction measurement during the design phase, thus saving substantial time and money in performance tests and equipment redesign. Lastly, an experiment is introduced in which the proposed normal direction measurement is applied. In the experiment, a precision of ±0.15° and ±0.3° is achieved when measuring different workpieces. These results, in which the experimental precision is within the calculated precision, indicate the applicability of the proposed algorithm for calculating the measuring precision.