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Keeping an eye on flying sausages

22 November 2019

A high-speed vision inspection and sorting system is inspecting sausages for surface contaminants as small as 1mm² as the products pass across a gap between two conveyor belts. 

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One of the products manufactured by German company, Wolf, is the Berner sausage. These Frankfurter-style sausages contain a portion of cheese and are wrapped in smoky bacon. 

The Berner is created by filling a synthetic membrane made from cellulose with sausage meat which is then smoked and cooked. A peeling machine then removes the membrane before the sausage is wrapped in smoky bacon. Although the cellulose membrane is edible and poses no health risk, its residue on the product may be mistaken for plastic or dirt, so it essential that they cannot reach the consumer. Machine vision allows 100% inspection of the sausages.

A flying vision system
After detailed discussions with Wolf to find a suitable machine vision solution, Stemmer Imaging recommended the use of Austrian company BT-Anlagenbau, a specialist in the development of sorting and quality control systems that incorporate machine vision. BT-Anlagenbau’s recommendation was to use 360° camera technology, rather than to inspect the sausages with a line scan camera as they passed by on a conveyor belt, as this solution would not allow the underside of the sausages to be checked. 

The 360° camera technology had already proven itself in many sorting systems and it is able to inspect the sausages in flight using two line scan cameras positioned opposite each other. In this way virtually the entire surface of each sausage can be examined for the widest variety of defects. 

Working with BT-Anlagenbau to provide the machine vision solution for the system, Stemmer Imaging carried out feasibility studies to evaluate the most suitable machine vision components to meet the specific performance and cost requirements and to supply a complete subsystem that would comply with the strict standards and cleaning requirements in the food industry. The final system consisted of two monochrome Linea line scan cameras from Teledyne DALSA with heat sinks, red LED lighting from Metaphase and lenses from Kowa equipped with polarising and bandpass filters. The cameras are enclosed in specialist IP65-rated housings. The vision system was integrated with smart imaging software and a precise sorting system from BT-Anlagenbau. Computing power is provided by an industrial PC with a real-time operating system.

In action
The peeled sausages are aligned via a conveyor belt and flexible cascades to ensure they are longitudinal to the direction of travel and cannot roll around. They are then accelerated to about 2 m/sec and, at the end of the belt, fly past the cameras integrated above and below which inspect them in free flight. Currently sausages with lengths of 90mm and 160mm are inspected by the system. The image capture, residue detection and removal of contaminated sausages using compressed air jets takes place within 115 milliseconds. The rest are gently caught and fed to the next stage of processing. The system can process up to 30 sausages per second. An air curtain in front of the camera housings ensures that any accompanying traces of the product cannot prevent the acquisition of sharp camera images over the course of time. A sophisticated rejection system was developed to fit into the existing available production space. This has 80 very fast special valves using only sterilised compressed air to comply with food regulations, which can cover the entire belt width of 400mm. The nozzles are installed in a suspended position so that any condensed water runs off and cannot cause contamination.

The system has been in operation since mid-2018 and has reliably detected undesired membrane residues down to 1mm². After sorting, the good products have a purity of 99.999%, depending on the defect size, meeting all of Wolf’s requirements. 

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