AUTOMATED PRECISION-MEASUREMENT OF SURFACE PROFILE IN CAD-DIRECTED INSPECTION

This paper presents an optimal match scheme that aligns the measurement data with the design data in CAD-directed dimensional inspection. The principle of this scheme is to determine the actual measured points on the nominal surface by minimizing the sum of the squared distances of the measurement data from the surface with respect to the parameters of a rigid body transformation. For implementing the proposed method, its computational efficiency and robustness against singularity were discussed. Two important applications of this scheme were examined. The first of these is the application in determining the design coordinate system with respect to a machine's coordinate system. The second is the application in comparative analysis for precision measurement. For determining the design coordinate system, a sensitivity measure is proposed and used to estimate the joint effect of the selected measurement locations and the total number of measurement points on the accuracy of coordinate transformation. This measure provides a mathematical index that can be used for selecting a set of minimum number of measurements for achieving the best possible accuracy. Experimental results were given to confirm the proposed measure in the selection of measurement locations. For error comparative analysis, a statistical model is developed to determine the minimum number of required measurement points so that the sampled points can closely represent the entire population based on manufacturing process capability (accuracy) and tolerance specification. An example was then used to demonstrate the statistical results of determining the minimum number of measurement points and the importance of using optimal match algorithm in precision surface profile measurement.