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Putting a lid on tilted bottle-tops

10 April, 2013

Monitoring the positions of bottle caps is a challenge for the beverage industry, usually demanding intricate inspection systems. A German company has developed a system that uses just one smart camera, thus cutting costs and simplifying installation, while achieving a detection rate of almost 100%.

“Applying plastic screwcaps onto PET bottles during beverage filling involves certain difficulties which make the subsequent monitoring of the screwcap for presence and correct position very demanding,” says Bernhard Voigt, a German specialist in beverage industry installations with decades of experience.

One of the Voigt’s latest projects is an innovative idea for monitoring twist caps on PET bottles for the mineral water producer, Aqua Römer.

Variations in bottle tolerances and environmental conditions, coupled with high process speeds, are some of the main obstacles that such applications have to overcome. PET bottles can exhibit variations in height of several millimetres, especially after several washes. Any shrinkage is not necessarily symmetrical, so skewing can occur on the bottle’s neck or along the entire bottle.

Moreover, the bottle’s contents can affect to its height and width. And drops of moisture on the outside of the bottle caused by liquid sprays can make the detection of contours difficult. High production line speeds and variable conveyor conditions – from uneven belts to guide rail adjustments – can result in bottles not being aligned correctly when they pass a detector. Despite the use of fast, precise triggering systems, bottles can wobble or move. With throughputs of up to 40,000 bottles an hour, the range of these movements can be considerable.

To ensure accurate results in such applications, several detection technologies – such as X-rays, ultrasonics and image processing systems – are typically used together. Aqua Römer was using a combination of inspection systems at its large mineral water bottling plant in Baden-Württemberg, and asked Voigt to find a more economical and easier-to-use alternative. The main aim was to improve detection quality, thus reducing pseudo-scrap rates.

Voigt’s suggestion was to use a single smart camera to perform presence inspections and to monitor for tilted caps. Using “blob” (binary large object) analysis, the camera can handle variations in the PET bottles and simultaneously detect caps tilted in any direction, 360° around the neck of the bottle.

Blobs are contiguous areas of pixels whose light intensities lie between defined boundaries. By restricting blob features such as length, width, height, surface, shape factor or circumference, individual objects or groups of objects can be detected and differentiated reliably on the basis of their geometric features – even when other processes are supplying incorrect results.

“The high process reliability when monitoring for tilted cap position is also largely based on the option of tracking test ranges” explains Voigt. The bottle cap is “searched” both horizontally and vertically with additionally-created test fields to track the actual test fields. This compensates for position shifts and height tolerances. The support ring in the bottle’s neck area is used as a reference (as shown in the screenshot above). The distances from both sides of this ring to the retaining ring under the cap, as well as height and width data of the cap, provide information about whether it is in the correct position.

The Voigt system consists of a Leuze LSIS 412i smart camera equipped with an opposing transmitted light source (shown above), instead of using its own illumination. A retro-reflective photoelectric sensor with a polarisation filter is used for triggering. This enables reliable detection of transparent media. The camera, with a 1kHz switching frequency and a 0.5ms response time of, is ideal for triggering mineral water bottles. An output signal from the camera controls a downstream diverter directly.

A big advantage of the LSIS 412i camera for Voigt is that it “unites all of the necessary components, from image processing to data memory, and display to interfaces, in one device”. Program memory and image processing are built into the camera, which is configured via a Web browser. No extra connections are needed. The robust metal housing incorporates scratch-resistant protective glass and is IP67-protected, thus meeting the demands of the beverage-filling industry.

“The new system for monitoring tilted cap position is considerably more economical and easier to use than conventional solutions,” says Voigt. “In addition, it does not require setting or any maintenance worth mentioning by the user. Furthermore, the detection rate is an excellent 99.8%.”

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