The laser scanning projection system can accurately project a virtual laser template onto complex models and parts to guide operators through the process of part fabrication, and the template is the outlines of parts to be placed, aligned or nested. In order to project the virtual template onto a workpiece accurately, it is necessary to obtain the coordinates of six retroreflective targets at least, and the coordinates are in the projected coordinate system and the workpiece coordinate system, respectively. Then calculate the coordinate transformation matrix between the projector coordinate system and the workpiece coordinate system. The numbers and positions of the edge scanning points vary with different scanning methods, and thus different scanning methods will affect the accuracy of center extraction for retroreflective targets. In this paper, we studied different retroreflective target scanning methods, and proposed the sunflower-shaped scanning method and the epicycloid-shaped scanning method. The raster rectangular scanning method can extract a large number of scanning points corresponding to the high reflection area of retroreflective targets, and the center extraction results are reliable. But there are too many scanning points, which result in long scanning time and low acquisition probability. The scanning points of the epicycloid-shaped scanning method are sparsely distributed in the central area and densely distributed in the edge area. This method can effectively save the scanning time and improve the acquisition probability by reducing the number of scanning points in the central area. According to the experimental results, the acquisition probability of the epicycloid-shaped scanning method is 0.89%, which is 6.719 times that of the raster rectangular scanning method. It can achieve the desired effect that more edge scanning points but less total number of scanning points. However, there are obvious position deviations of the edge scanning points when scanning with the epicycloid-shaped scanning method, and that leads to a large error of center extraction. The average center extraction error of the epicycloid-shaped scanning method is 0.227 935 mm. The sunflower-shaped scanning method can also reduce the number of scanning points in the center area of retroreflective target and improve the acquisition probability effectively. According to the experimental results, the acquisition probability of the sunflower-shaped scanning method is improved to 0.96%, which is 6.231 times that of the raster rectangular scanning method , and the probability of obtaining edge scanning points is greater. Moreover, the scanning time of the sunflower-shaped scanning method is 0.055 710 mm , and the center extraction error is small. Therefore, the scanning time of the raster rectangular scanning method is 2.5 s, while the scanning time of sunflower-shaped scanning method is 0.36 s, and the scanning time of epicycloid-shaped scanning method is 0.612 s. Moreover, the acquisition probability of edge scanning points of the raster rectangular scanning method is 0.39% , while the acquisition probability of edge scanning points of the sunflower-shaped scanning method is 0.89%, and the acquisition probability of edge scanning points of the epicycloid-shaped scanning method is 0.96%. Both the sunflower-shaped scanning method and the epicycloid-shaped scanning method can save the scanning time and improve the acquisition probability of edge scanning points. The average center extraction error of the raster rectangular scanning method is 0.091 883 mm, and the average center extraction error of the sunflower-shaped scanning method is 0.055 710 mm. The accuracy of the retroreflective target's center coordinates is higher by using the sunflower-shaped scanning method. In conclusion, the sunflower-shaped scanning method can meet the demands for short scanning time, high acquisition probability and high center extraction accuracy. It could achieve more efficient and accurate scanning for retroreflective targets and improve the working efficiency and positioning accuracy of auxiliary assembly when applied to a laser scanning projection system.
