spection systems: the outlet conveyor or, prior to that, a free necker tool plate. At first sight, the conveyor solution seems to be the more obvious approach. Single camera systems can be installed without great effort just as well as multiple-camera systems; the inspection unit is independent from any programmable logic controller (PLC) signals and seems to live an easy life, free from considerable machine vibrations, aluminum chips and other disturbances. Single-camera vision units at the outlet conveyor can use aspherical surfaces for top and side view of flange and shoulder in one image. There are two main reasons that inspired developers to integrate the inspection unit directly in the necker: the lack of can positioning accuracy at the outlet conveyor and the idea to dip the can or bottle into the lighting unit to get an optimal illumination, especially of the can shoulder and behind the flange. Inspection inside the necking machine While the inspection of cans inside the necker is no longer considered pie-in-the-sky today, the first solutions of that kind encountered a great deal of skepticism. Would high-end optical elements be robust enough to survive the tough environment and permanent movement of the machine? The answer is simple: they can. The compact vision system consisting of a lighting unit and a camera installed inside the necker tool plate uses the high can positioning accuracy of few 10 μm, which the machine naturally provides. Exposure time can be kept extremely short (e.g. a few 100 μs). Due to the periodic stroke of the necker, each can or bottle can enter the lighting unit to receive the optimal illumination of flange, neck and shoulder. With an aspherical optic, this method allows for looking at the most hidden corners of an aerosol can: the backside of the flange. In short… When trying to enhance the image quality, two facts should be considered: High Dynamic Range imaging is a good technique, but real HDR takes too much time for an inline application. Color cameras can be useful, but all-in-all, monochrome cameras provide higher sensitivity and resolution capabilities; they are still the best choice for can top inspection. In weighing the pros and cons of different can top inspection methods, one conclusion seems to be obvious: one-camera systems are superior to multiple-camera systems. The optimal position for the vision unit is inside the necker where all technical capabilities can be fully exploited, e.g. precise can positioning, best illumination, high resolution, flange backside visualization, magnified images and customized optics. The necker installation, however, may not always be practicable. In this case, excellent single-camera vision systems are available today for the outlet conveyor. SPRAY Flange and shoulder inspection for aerosol cans Continued from page 24 es. The smallest item that can be detected as an independent object marks the physical limit of the resolution. In most cases, the resolution is limited by the camera. Both the number and the size of camera pixels are relevant for the resolution of an inspection system. Resolution is only one vital factor of high inspection performance; another factor is contrast. Defects measuring just one pixel, but contrasting strongly against the background, can be detected easier than a very low contrast defect spreading ten times as wide. Although often demanded, stating the minimum defect size detectable by a vision system is not possible without information about the contrast. Knowing this keeps from falling into the trap of fabulous figures sometimes given when talking about the inspection performance. How to improve the image quality The dynamic range of each pixel and, as a consequence, the image contrast is limited. One idea for improvement is high dynamic range (HDR). The real HDR picture is composed of several separately acquired images. The idea behind it is to make dark areas more detailed and avoid bright areas being overexposed. The camera takes three pictures, one with short, one with middle and one with long exposure time. The composition of all three images makes the final, high-contrast HDR image. This concept is not really new; the human eye works similar to an HDR-camera. Unfortunately, for inspection systems running at line speed (more than a few cans per second) the exposure time is not sufficient to acquire a real HDR image. Consequently, HDR is a good approach but not an option for fast-running inline applications. Color or monochrome camera When comparing a color picture with a monochrome picture, the impression often suggests that the color spectrum provides more nuances to show quality variances in the image. This is not an illusion— human vision is adapted to color. However, it cannot be compared to the working principle of vision systems. RGB color cameras with a Bayer pattern use groups of four pixels: one red pixel, two green pixels and one blue pixel for full color information. Monochrome cameras can use each single pixel to picture a detail in the image, which means resolution is higher. They are also more sensitive. Image processing algorithms performing a homogeneity analysis based on gray scale values are very precise and a good method to detect surface defects. Color cameras make sense for complex print images. For a usual can top inspection, monochrome cameras are still the 26 Spray May 2016 best choice, even if a print inspection on the can shoulder or bottle neck is included, which are features that still only few vision systems are able to perform. State-of-the-art can top inspection Two locations in the production line are particularly suitable for can/bottle top in- Inspection with pericentric lens Necker integration Aspheric optic
Spray May 2016
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