| Summary: | 博士 === 國立臺灣科技大學 === 機械工程系 === 99 === This research presents a novel multi-beam laser probe that can be used to measure the position and orientation (normal vector) of a freeform surface. The probe system comprises a five-laser-beam projector and a charge-coupled device (CCD) camera. The developed probe is integrated with a four-axis platform to do the automatic measurement. Five designed laser beams are projected onto a measuring surface, where five light spots are observed and captured by CCD. The 3D coordinates of the five light spots can be then computed. The normal vector at the central spot is determined by calculating the cross product of two tangent vectors along two crossed curves passing through the coordinates of these five light spots.
A new algorithm of normal vector estimation using moving mask method was developed for this probe. This algorithm can also be employed for the stylus radius compensation of coordinate measurement machine (CMM). For the normal vector calculation, the grid points within the moving mask are configured as two categories - cross-curve type of five (+5) or nine (+9) points and grid type of 3x3 or 5x5 points. Different geometries, such as a line, a curve, a plane, or a surface fitting to these grid points within the moving mask, have been constructed to calculate the normal vectors. Three surface categories - spherical surfaces, cosine-like surfaces, and shoe-last surface have been adopted as the test surfaces to evaluate the accuracy of normal vector computation. The Bezier interpolation method of a +5- or +9-point cross-curve with chord ratio parametric value is a simple and efficient computing methodology to obtain a very accurate normal vector. Thus, the Bezier method was chosen to compute the normal vector for this developed multi-beam laser probe.
A scheme for calibrating and making measurements using this five-laser-beam probe has been proposed and verified experimentally. Experimental results demonstrate that this five-laser-beam probe system can be employed to measure the position and orientation of a freeform surface. The range of depth that can be measured using this probe is 2.4mm and the range of angle is 40°. The positional measuring accuracy of the complete system is approximately 30μm, while the orientational accuracy is 1.8° without compensation and 1° with compensation.
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