April 2019

47 www.drivesncontrols.com April 2019 MACHINE VISION n Vision cameras test the ‘wettability’ of surfaces B efore surfaces can be painted, bonded, or treated in other ways in industrial processes, it is useful to know their“wettability”– a measure of the ability of a liquid to maintain contact with the surface. Developers in Germany have come up with a technology that uses machine vision cameras for automated, non-contact testing of wettability which, they claim, offers numerous benefits over previous techniques. Potential applications for the new inline process include testing the tackiness of automotive bodywork and aerospace surfaces. Reliable adhesive bonds are particularly important in carbon-fibre-reinforced plastic components, which are often safety-critical. The composite quality depends on the correct activation of the composite – by plasma blasting, for example. The surfaces must fulfil certain criteria to enable a secure bonded joint to be made. The first users of the new systemwere bonding specialists at Fraunhofer IFAM (Institute of Manufacturing Technology and Applied Materials Research), who used it to test the wettability of surfaces following atmospheric pressure plasma activation. They were able to detect contamination such as fingerprints and release agent residues consistently and automatically, as well as spotting inadequate activations. Ultrasonic atomiser Various methods already exist for testing the wettability of surfaces, such as special inks, “water-break”tests, and contact angle measurements. In the new approach, an ultrasonic atomiser generates an aerosol of high-purity water, which is sprayed onto the test surface with a width of about 10cm. At the same time, the sample is moved under the atomiser to ensure an even application of the aerosol. The relative movement of the sample and aerosol can be achieved either by using a linear drive moving at about 100mm/s, or by guiding a robot over the test component. These techniques result in 1,000–2,000 droplets with an average diameter of 100µm being applied to every square centimetre of the surface. The droplets form patterns that depend on the surface energy of the test surface. The patterns are captured by a camera system and evaluated within a few seconds by image- processing software. On the basis of the droplet distribution, it is possible to determine whether a surface meets the criteria for subsequent processes. More than 30,000 droplets are created every second and the vision system has to detect and evaluate them rapidly because the surfaces have to dry quickly to enable other processing operations to be carried out further down the line. Images of the droplet patterns are captured by a monochrome GigE linescan camera with a 2k pixel resolution. A telecentric lens provides images with a high depth-of-field. By specifying target values, it is possible to evaluate the effectiveness of a pre-treatment – such as cleaning or surface activation – or to detect contamination on the surface. Such contamination can be caused by release agent residues, oils, grease, or by fingerprints, and can lead to bonds failing or paints peeling off prematurely. Possible consequences include visual or functional defects in the end-product, such as paint blisters or labels not adhering properly on medication bottles. In extreme cases, a bonding fault can endanger life. Unlike traditional methods, the new system allows fast, automatic measurements over a wide area on a variety of surfaces, including plastics, metals and fibre-reinforced composites. Because the water droplets evaporate so quickly, the tested component can undergo further processing immediately after the test, allowing the technology to be integrated into production processes. This method of evaluating the wettability of surfaces is based on a patent held by Fraunhofer IFAM. An exclusive licence for the use of this patent is held by Stemmer Imaging in conjunction with AutomationW+R, which developed the inspection system, known as bonNDTinspect. Because the required light intensity could not be achieved with standard lighting, AutomationW+R developed special dome lighting. The images are evaluated by Stemmer’s Common Vision Blox in combination with imaging software from AutomationW+R. The complete test system consists of a PC and monitor, evaluation software, the test head with an ultrasonic atomiser, and a control cabinet. It is designed for in-line use and can operate continuously 24 hours a day, seven days a week. It can also be connected to external PLCs and higher-level systems. n An machine-vision-based system has been developed in Germany for automated, inline testing of the “wettability”of material surfaces to assess their readiness to be painted, bonded or treated in other ways. The droplet distribution is analysed using special image-processing software The wettability measuring technique is sensitive enough to detect a ngerprint in the aerosol pattern

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