Vision system ensures carton caps are spot on
Vision technology from Cognex has replaced a simple pass/fail height sensor of a capping machine to provide more accurate information not only on whether or not the cap is present, but also on the actual position of the cap, giving increased quality.
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. 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.
Caps placed in the wrong position became an item high on the agenda. Plant manager Patrick van Erp comments: "The plastic screw caps were not always installed at the right place on the package. The final inspection, using an optical 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."
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 and your products."
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.
In practice the vision system works virtually without mistakes so that the retailer no longer receives defective packages. However, there are other advantages, as Patrick van Erp explains: "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."
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