Alternate Strategy to Auto-focusing for Inspection of Large Curved Surfaces

摘要

Inspection of surfaces under large magnification usually requires fine positioning of the optics because the depth of focus (DOF) is usually very small (i.e. less than 0.1 mm). The inspection of large curved surfaces may require inspection to be carried out at many locations of the surface. This imposes stringent requirements on the numerical control platform that positions the optics for inspection. Here, a liear slider is mounted to the primary motion platform to enable fine motion of the camera along the direction normal to the inspected surface. Instead of using feedback control for fast and accurate positioning of the camera, the strategy is to take a series of images of the surface as the camera travels in-and-out of the focusing range, and select the best as the result image. Realization of this strategy requires consideration of the shutter opening time and frame rate of the camera, the DOF and the normal velocity of the linear slide.Two imaging functions have been successfully implemented: (ⅰ) capturing an image at a specified location of the curved surface; and (ⅱ) capturing images along a specified path on the curved surface while the robot is in continuous motion to track the path.