auf Funktions-Know-How, langjährige Erfahrung und eine breite Palette verschiedener Konfigurati-onen achten. Hersteller wie Mettler-Toledo beraten bei der Auswahl des passenden Systems. Ralph Swinka, Leiter Marketing bei Mettler-Toledo Produktinspektion Deutschland Continued from page 14 device diodes and a computer. The diodes con-vert x-ray radiation into an electrical signal. When using a diode 0.8mm in size, for example, every product movement of 0.8 mm generates a new line of image data, which is used to make up the full picture. The software analyzes and compares the image with a pre-defined standard. In the event of contamination, the rejection system removes the product from the line. For the detection of contaminants using an x-ray inspection system to be successful, three condi-tions must be fulfilled: 1. The detection device must have sufficient spatial resolution. 2. The contaminants must absorb more x-ray radiation than the product it is surrounded by (radiographic contrast). 3. The detection algorithms must be able to distinguish the contaminant from the product to avoid product effects. Spatial resolution The size of the diode affects the spatial res-olution, or pixel density, of the x-ray image. The smaller the diode, the higher the spatial resolution and therefore the probability that con-taminants will be detected. The diode sizes most often used in the food industry are between 0.4mm and 0.8mm. However, to maintain image quality, smaller diodes require stronger x-ray radiation and a lower belt speed, which in turn results in higher energy consumption. The spatial resolution also depends on the position of the contaminant in relation to the diode. The further away it is, the less accurately it is depicted by the diode, which is why a larger diode is recommended for thicker products. Radiographic contrast When a product is passing through the x-rays, only the residual energy reaches the diodes. Foodstuffs such as meat and sausage products consist of ingredients with low density, like water, whereas contaminants normally have quite a high density. As the density and thickness of the product increas-es, more x-ray energy is required to penetrate the product. This lowers the contrast to contaminants and makes them harder to detect. One of the reasons for this is the signal-to-noise ratio, which is the unit of measurement for signal strength in relation to the background noise of an electronic device. The stronger the signal in comparison to the noise, the better the signal-to-noise ratio. A larger diode leads to a stronger signal and is there-fore particularly recommended for thick products and products with high densities e.g. meat with a high muscle percentage. Product characteristics It is not only thickness and density that play an important role in the selection of the correct diode, but also other product properties. For homogene-ous products, like a packet of Teewurst (spreading sausage), the decision is easier: small to medi-um- sized diodes deliver good results. For heteroge-neous products, selecting a diode is more difficult: Differences in product thickness, like a particularly thick schnitzel inside schnitzel packaging, present the system with more of a challenge than clear changes or transitions inside a product that are caused by a high-density object. To achieve a good detection performance, manufacturers should carefully consider how to adapt their inspection conditions for products with thickness and density fluctuations. The characteristics of the contaminant are also an important factor. If the product is thin and the contaminants are expected to be high-density, like metal, then a small diode is sufficient. A larger diode should be used for small contaminants with lower densities, like glass, stones or bones. Thanks to the improved signal it can deliver high image quality, even with less x-ray energy. The individual characteristics of a product can also include natural density fluctuations. These lead to product effect: a packet of mince meat, for example, causes a strong product noise because of the attenuation difference inside the product. This makes detecting a very small contaminant all the more challenging. A small diode is less suitable here, because the signal could be too weak. Conclusion Contrary to the popular belief that small diodes lead to higher detection sensitivity, large diodes are often more suitable. Those who want to inspect primarily thick, heterogeneous high-density products should use a larger diode. It produces a stronger signal, consumes less energy and is therefore cost-effective. For thin, homogeneous products such as fine liver sausage, small diodes really are better suited to x-ray inspection because they offer higher spatial resolution. Generally speaking, the system must be tailored as precisely as possible to the product conditions in order to guarantee efficient production with the greatest possible safety. When choosing a manu-facturer, those who want to be absolutely certain of product safety and compliance with all legal requirements should look for system expertise, long-standing experience and a broad palette of different configurations. Manufacturers such as Mettler-Toledo provide advice about the selection of a suitable system. Ralph Swinka, Marketing Manager, Mettler-Toledo Product Inspection Division KLEIN ODER GROSS? EIN SPICKZETTEL ZUR DIODENWAHL Kleine Dioden Große Dioden Räumliche Auflösung Hoch Niedriger Benötigte Röntgenenergie Höher Niedriger Durchsatz & Bandgeschwindigkeit Muss reduziert werden, sonst Hoch kostenintensiv und ineffizient Radiografischer Kontrast Niedriger Höher, dadurch bessere Bildqualität Signal-Rausch-Verhältnis Höhere Röntgenstrahlung nötig Gut, da stärkeres Signal Reduzierung von Produkteffekten Weniger geeignet Besser geeignet, da stärkeres Signal SMALL VS. LARGE: A CHEAT-SHEET FOR DIODE SELECTION Small diodes Large diodes Spatial resolution High Lower X-ray energy required Higher Lower Throughput & belt speed Must be reduced, otherwise High costly and inefficient Radiographic contrast Lower Higher, producing better image quality Signal-to-noise ratio Higher x-ray radiation required Good, due to the stronger signal Reduced product effect Less suitable More suitable due to the stronger signal 16 2015 AUTOMATION AUTOMATISIERUNG
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