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Measurement & Instrumentation

Vision System Ensures Carton Caps Are Spot On

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At Refresco Bodegraven in The Netherlands, nine thousand one-and-a-half litre packages filled with soft drinks or fruit drinks leave the conveyor belt of one of eleven production lines every hour. It is one of the four Combibloc lines, where one-litre packs are also produced at the same speed. In addition, there are five Tetra-Pak lines, completely specialized in packages of 20 and 25 ml. Even though the Tetra-Pak lines that have been in use the longest have a capacity of 7500 per hour, two high-speed lines have been in operation since 2008, producing 24,000 20-ml packages, which are still growing in popularity. There are also two canning lines that produce 60,000 15 and 25-cl cans and 90,000 standard 33-cl cans, respectively. All together good for a 2009 production of more than 325 million litres of soft drinks and fruit drinks.

Incorrect cap position
The Combiblock packages are fitted with the familiar plastic screw cap. After filling by a special machine, the cap applicator pushes this cap precisely into the right place on the package. Until recently, only the presence of the cap was checked automatically. This was done by using a photo sensor which actually only detected whether the signal was interrupted at a certain height. If this was not the case, the product was rejected. In addition, the operators made a visual inspection and moreover random inspections were made by means of a torque meter measuring the force required to screw the cap off.

There was no inspection of the position of the cap. In theory, the supplier of the cap applicator specified that this machine should be able to place the cap virtually without errors, but this turned out to be otherwise in practice. Sometimes there were minor deviations, damaging the shape of the packaging as well as its appearance. Sometimes the problems were more serious: the cap was placed on the wrong side because the package was twisted somehow or other in the line, and ended up back to front in the cap applicator. This resulted in undesired production failure and, in some cases, the packs were returned by retailers if the error had not been detected.

Continuous improvement
Caps placed in the wrong position became an item high on the agenda since the implementation of the world-class manufacturing program ‘Refresco Bodegraven – leading manufacturer’, an ambitious improvement philosophy aimed at turning Refresco Bodegraven into the best performing producer of these products. Initiator and stimulator of this sizable trajectory is Patrick van Erp, who has been working as the plant manager at Refresco for two years. “We started by deciding what we find important and formulated goals in the area of quality, hygiene, output and SHE, (safety, health and the environment). We also made a joint decision on how we can meet those goals. Doing this together is crucial: you have to share the vision, not just impose something from above and hope it will happen. The fact that everyone is proud of his or her work station, the factory and the products is an important value in our company. Everyone is doing important things and valuable work for his or her own sake. In that framework, we strive for more personal responsibility and ownership. For example: an operator not only works on a line, but is the owner of that line. A job such as cleaning, which is very essential in the food industry, consists not only of implementing a number of required procedures, but also of inspecting the line, always being alert to things that could result in failures in the long run, not waiting for a failure and only then taking action. ‘Cleaning is inspection’ is one of our most important principles for a good reason.”

Focused improvement results in vision
Focused improvement, or improvement with a focus, puts the initiated process of constant improvement into practice.  When pursuing quality, one of the four chief goals, an analysis was first made of where the biggest problems present themselves, or more clearly stated: where the greatest benefits are. Multidisciplinary groups then searched for solutions. “The challenge we tackled first had to do with the plastic screw caps, which were not always installed at the right place on the package. The final inspection, using an optic sensor, was not sufficient: it only checked whether the cap was present. We soon started looking in the direction of vision, but we were a little reluctant in light of our ideas about vision: expensive and complicated systems with PLC-like programming techniques: not something we relished doing. On the contrary, we wanted a simple system that was easy to implement, and with minimum impact on the machine,” says Patrick van Erp in regard to the starting points in the search for the right vision solution.

Checker surprised us
All doubt about vision systems disappeared when Van Erp made contact with Digitron Industrial Systems in Uden, which sells the Cognex Checker vision systems in the Netherlands. “When they visited our company, I found it absolutely amazing that within half an hour we came to a functioning solution that did exactly what it was supposed to do. Final implementation did take more time, of course, but the improvement compared to the former situation is absolutely sensational: our first time right percentage changed from 93.7 to 98.8% partly due to this. This is an enormous improvement, not only in the cost-benefit sense, but also the pride in your work, your production line, your products and that then has an effect on the total improvement philosophy in our company,” according to a satisfied Patrick van Erp.

Fast photos
The Checker 3G1 can make and process up to 25,000 images per minute, more than sufficient for the 9000 packages passing along the Combibloc lines every hour. The system is implemented in such a way that the output to the machine’s controls is exactly the same as that of the optic sensor that was used previously. For this reason no adjustments have had to be made to the controls of the cap-applicator, which of course is a great advantage when the line is already in operation.

The Checker is triggered by a fork sensor which announces that a package is coming and that a photo will have to be made within an experimentally determined period by means of a pulse. This photo shows almost the entire top of the package, but the vision software is only interested in a small part of it: the position of the lip used to attach the screw cap to the package in relation to the fold of the cardboard packaging. When this space is outside the pre-set specifications, it indicates that something is wrong and the machine is instructed to remove the package. The cap may be missing but it also may have been attached wrongly.

Quick adjustment to new products
In practice this vision system proves to works virtually without mistakes so that the retailer no longer receives defective packages. However, there are other advantages. Patrick van Erp: “The batches we run per line differ from each other quite a bit; sometimes we only change products four times per week, but sometimes four times per day. It depends on all kinds of factors, for example orders, supplies, shelf life of the product and processing time. Scheduling runs with maximum length is a real art, but when the product has to be changed the conversion time has to be limited to the minimum. The vision system is very suitable for this, since the vision system is very simple to train. The operator signals that a new training set is coming via a button on the control panel. After three or four packages of the new batch – rejected as a standard procedure – the system knows what it must pay attention to and production can be resumed at full speed. The colour of the cap, the design on the packaging, it is all the same to the vision system.” Variations in lighting hardly throw off the vision system either. “We do production runs here seven days per week, 24 hours per day. There are significant differences between daylight, which shines in through the skylights in copious amounts, and the fluorescent lighting during the night hours. Despite those big differences, adding some extra light in the machine was sufficient to create equivalent light conditions for the Checker; elimination of external light sources, much more time-consuming, proved to be unnecessary.”

Follow-up

After the successful implementation of the Checker in the Combibloc lines, it is conceivable that this system will be utilized at other Refresco concern plants. Patrick van Erp: “Horizontal expansion is indeed an option certain to be explored in the near future. However, our priority for the present is solving the next challenge. We have now shown that our approach works in the framework of world class manufacturing. We all agree: the results are good, so we will continue on this path!”

For further information please contact;

Cognex UK Ltd,
Sunningdale House, 43 Caldecotte Lake Drive, Caldecotte Lake Business Park, Milton Keynes, Bucks MK7 8LF
Tel: 01908 206000  Fax: 01908 392463
www.cognex.co.uk

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