High requirements for speed and accuracy make machine vision an ideal solution for quality control in the print and paper industries. The German company Roptec proves that this technology goes far beyond the mere production process. Their machine vision system is used for optical quality control in the handling of paper rolls.
With up to 2,000 metres of paper web passing modern systems each minute, the paper industry has to deal with extremely high production speeds. Paper rolls weighing up to 5,000 kg with widths of up to 5 m and diameters of 1.5 m are wrapped in order to prevent them from damage, and are then transported for further processing. On arrival they are unwrapped, placed in printing machines and are eventually changed at full production speed by an automatic paper roll changer. Even the smallest defects on these rolls can cause the tearing of the paper during paper roll changes resulting in production stops or printing errors.
Another source of error lies in the packaging of the paper rolls: their end faces and sides are wrapped in thin paper for shipment to protect them against damage during transportation and further handling and to ensure consistent quality in the subsequent printing process. Damage to this packaging can lead to expensive malfunctions in the automatic crane systems which are used to the handle the paper rolls during storage and retrieval due to their great weight. The necessary consequence is manual intervention, which involves additional risks and hazards: they can damage or even knock over other rolls, which must be avoided at all costs due to the danger to employees and for economic reasons.
Frank Rossbruch and his company have specialised in this industry segment and developed the so-called RCx Roptec Control System which uses machine vision to detect faulty wrapped paper rolls. Roptec Managing Director Frank Rossbruch explains: “The task was to check the packaging of the paper rolls’ end faces for possible defects and to automatically sort out the poorly packed ones and repack them. Apart from that, labels should be checked on the end faces of the rolls for presence, wrinkle-free application and positioning. The labels for the identification of the rolls are applied in moist condition. In some cases, labels might slip, wrinkle or even fall off when applied by the robot arm. If one of these errors occurs, the system stops immediately and allows the operator to apply a new label.“
According to Frank Rossbruch, the particular challenge is the fact that very large objects need to be evenly illuminated: „Besides that, the packaging line shouldn’t have to slow down or stop to carry out the evaluation. Stopping the line was only permitted in the case of an error detection.“ The enormous weight of the paper rolls required a robust steel construction to protect the mounting of the imaging system and the camera housing from damage caused by “derailed” rolls.
According to Mr Rossbruch, it was unusual that his customer was completely unaware of how many defective rolls were fed into production every day. He assumed a certain number, which was solely based on random observations. With the help of Roptec GmbH, this question can now be answered precisely. „First, we built a prototype of the system and used the identical hardware regarding camera, lens and illumination as in the final version. During a test run of three days, all paper rolls were initially recorded from one end face and evaluated manually. This enabled us to identify specific defects. Based on these data and images, we were able to work out the perfect machine vision solution“, Mr Rossbruch describes the further course of the project.
He is particularly proud that the system could be implemented as planned after a detailed specification in accordance with part 2 of VDI/VDE 2632 and additional discussions with the client. Actually, it passed the final inspection without any open issues well before the due date. „The perfect planning spared us unpleasant surprises during the implementation process“, Mr Rossbruch points out.
STEMMER IMAGING, Roptec’s machine vision partner, greatly contributed to the smooth running of the entire project. Mr Rossbruch praises the excellent support: „As a first step, we attended a basic machine vision training at the European Imaging Academy. Following this, we arranged an individual training session tailored to our specific questions on the system, which saved us a great deal of time and facilitated the successful implementation. For example, STEMMER IMAGING simulated our special application in order to examine questions on camera control in a practical way. At the end of the project, Florian Mayr, our sales partner at STEMMER IMAGING provided us with a detailed proposal to create our own machine vision system, perfectly matching our and our customer’s requirements.”
The selected components produced a system including one monochrome 1.2 MPixel Allied Vision Prosilica GC1290 camera per image acquisition station and a 6 mm Ricoh fixed focus lens. „We are aware that we opted for an older, but solid camera model. However, it was the right choice for this specific application”, Mr Rossbruch points out. When it comes to lighting, Roptec selected a strobe light from the Canadian company Papertech Inc., which is well established in the paper industry.
For the evaluation of the recordings, Frank Rossbruch relied on Florian Mayr’s recommendation to use the Teledyne DALSA Sherlock software which is available from STEMMER IMAGING. „We were particularly impressed how quickly we were able to turn a prototype into the final product”, Frank Rossbruch is elated. In addition to the cameras, optics and imaging software, STEMMER IMAGING also provided the industrial PC for the image analysis, as well as further components such as the necessary I/Os, connectors and cables.
Mr Rossbruch praises the cooperation with his machine vision partner: “I particularly appreciate their professional approach at all times, with the objective of exactly comprehending our problem definition and providing us with the appropriate support in a competent and constructive way. We could always count on the responsible support team at STEMMER IMAGING. It was there for us whenever we needed it!“
The Roptec customer is also highly satisfied with the resulting solution: Since he has been using the RCx Roptec Control System, only perfectly packed paper rolls reach his customers, which considerably enhanced the performance of the entire factory due to reduced downtimes. In the eyes of Frank Rossbruch, a further indicator of his customer’s satisfaction is the fact that he has recently requested for enhancing the system with the function to inspect also the side faces of the rolls for correct packaging before onward transportation. Up to now, only the end faces have been inspected and errors could only be detected if they appeared close enough to the roll’s end face in order to be recorded and evaluated by the machine vision system.
Example of error detection: Incorrect labelling.
© Roptec GmbH
Example of error detection: Incorrect wrapping.
© Roptec GmbH
Example of error detection: Displaced cover.
© Roptec GmbH
Example of error detection: Ruptured wrapping.
© Roptec GmbH
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STEMMER IMAGING has been one of the leaders in the machine vision market since 1987. It is one of Europe's largest technology providers in this field. In 1997 STEMMER IMAGING presented Common Vision Blox (CVB), a powerful programming library for fast and reliable development and implementation of vision solutions, which has been deployed successfully throughout the world in more than 40,000 imaging applications in various industries.
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