THESIS
2017
xiv, 122 pages : illustrations ; 30 cm
Abstract
Five-axis continuous inspection is an emerging technology for freeform surface inspection which, unlike the traditional discrete inspection that works in a point-by-point manner, keeps the stylus tip in constant contact with the surface during the scanning and thus could tremendously improve the inspection efficiency. However, at present its planning of inspection path is mostly accomplished manually, because existing automatic inspection path planning algorithms are mainly design for the traditional three-axis and “three + two” axis discrete inspection process, which is time-consuming and unable to be utilized for the much more efficient five-axis continuous inspection due to the drastically different working patterns between them.
This study focuses on automatic generation of five-ax...[
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Five-axis continuous inspection is an emerging technology for freeform surface inspection which, unlike the traditional discrete inspection that works in a point-by-point manner, keeps the stylus tip in constant contact with the surface during the scanning and thus could tremendously improve the inspection efficiency. However, at present its planning of inspection path is mostly accomplished manually, because existing automatic inspection path planning algorithms are mainly design for the traditional three-axis and “three + two” axis discrete inspection process, which is time-consuming and unable to be utilized for the much more efficient five-axis continuous inspection due to the drastically different working patterns between them.
This study focuses on automatic generation of five-axis continuous inspection for arbitrary free-form surfaces, while taking into consideration the unique kinematical characteristics of the inspection system. Firstly, we conduct physical experiments to investigate on the kinematical characteristics of the system. The result shows that it is crucial to limit the speed of the three linear axes and to utilize the rotary axes as much as possible under high speed scanning in order to ensure the inspection quality. Based on the experimental results, we propose a framework for the generation of five-axis continuous inspection path. Instead of adopting the methodologies of traditional five-axis machining tool path generation, in which the cutter contact curve is firstly generated and then the corresponding tool orientation is assigned, we firstly generate the trajectory of the probe head, and then obtain corresponding sweeping inspection curve on the surface.
In the framework, we develop an algorithm using a concept called guiding surface, based on which the trajectory of the probe head is generated with its accessibility region being portioned on the original surface. For each partitioned surface patch, we carefully design oscillating patterns for the sweep curve while at the same time trying to shorten and smoothen the trajectory curve as much as possible, with the given inspection sample accuracy always upheld. This algorithm is particularly suitable for inspecting large free-form surfaces, such as those in automobile and aerospace industry.
In addition, we propose an alternative algorithm to generate inspection path for arbitrary free-form surfaces regardless of their size. The algorithm first evaluates the Directional Curvature
Distribution (DCD) of surface to determine the Principal Scanning Direction (PSD). After PSD is determined, a series of parallel planes that are perpendicular to the PSD is applied to intersect with the surface and get intersection curves. A partitioning process is then performed on each intersection curves to get partition boundary points on the curves. All partition points of a corresponding partition together construct the boundary curves of that partition on the surface. The corresponding probe head trajectory and sweeping curves are then generated for each partition.
Both computer simulation and physical experiments are conducted on free-form surfaces with different shapes to validate the algorithms. The algorithms are able to automatically divide the surfaces into partitions, and to generate sweep inspection paths for each partition that altogether cover the entire surface. The experimental results show that the proposed inspection paths are able to scan the surface in a much shorter time comparing to traditional methods such as the Zig-Zag method, while at the same time keeps the velocities and accelerations of the three linear axes of CMM at very lower level, which is critical for precision inspection.
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