Rotationally symmetrical objects - this term covers many everyday products. These are objects that can be completely imaged when they are rotated around their own axis, such as spray cans, ampoules or canned goods. They are often found in cylindrical form in the pharmaceutical sector and in the packaging industry for food and beverages - sensitive sectors in which it is important to precisely monitor the quality and safety-related issues, some of which are prescribed by law, in order to reliably detect possible damage to screw connections, deformation of the packaging, missing spray nozzles or missing labels or even incorrect contents.

Up to now, approaches to inspecting rotationally symmetrical products have focused on rotating the objects once completely around their own axis on a defined path at a constant speed and with an exact exposure time in front of a camera. The image is then processed and checked using image processing. "However, the seam areas between the individual images are often unclean, so that in some cases only serial defects, i.e. defects across several products, can be detected in one stream," says Stefan Tukac, authorized signatory at Industrielle Sensorsysteme Wichmann (ISW), explaining the disadvantages of previous inspection systems. It would be better to have a 100% inspection system that reliably detects even one-off product defects while remaining in the production cycle.

This led to the development of an inspection system called '4ninety'. The name says it all: the system uses four Baumer cameras to cover 90° of the circumference of a round body, i.e. exactly the 360° that also includes an unwinding. The four individual images are combined and evaluated by the software without overlapping to form an overall image.

Easy to integrate

ISW's approach is simple and can be integrated into existing production processes with little effort: the inspection cell can be placed above any conveyor belt. With the help of various axes, cameras and lighting can be quickly adapted to different conveyor belts and products. Once positioned, correctly aligned and set up on the software side, the system takes on a wide range of inspection tasks. The shapes of products can be checked, for example for integrity, dimensional compliance or lid placement. The presence and position of features such as the correct placement of labels can also be checked; it is also possible to check applied 1D and 2D codes and texts (OCR/OCV). "Our standard library consists of several thousand fonts. This allows us to assign clear customer fonts to our standard fonts such as 'Document', 'Universal' or 'Pharma' for reliable readability," says Tukac, explaining the OCR process. The location and position of features are measured in world coordinates so that faulty products can be sorted out based on the measured deviations in millimetres or degrees thanks to the definition of fixed threshold values. For label detection, ISW uses a grayscale and contour algorithm that compares the recorded labels with a master. If there are any differences, the product is ejected.

The heart of image processing

The products to be inspected are fed into the inspection box by a conveyor belt. An integrated belt takes over and transports them through the testing process. The system is trained to the desired parameters.

© Baumer

The core of the inspection system lies in the specially developed image processing software. The basis for this is the recording of images with very good image quality. The system uses cameras from Baumer's CX series: four megapixel CX.I cameras with a Sony Pregius IMX264 sensor. Polarization filters suppress the reflections on the products caused by the lighting. With the modular tube system patented by Baumer, the camera and lens are additionally protected against dust, for example, by a variable number of intermediate rings. At the same time, protection class IP65/IP67 is achieved. Combined with the hard-anodized surface, the cameras are thus equipped to withstand the cleaning often required in food environments. The cameras have a standard-compliant GigE Vision interface, are bright and deliver low-noise images. In particular, they support the detection of small deviations and the correct seam formation between the individual images. ISW uses a powerful computer, multithreading, a very good graphics card for outsourcing computationally intensive operations and clever programming to solve the problem that the back calculation of the four individual images, each with a resolution of 5 megapixels, into one plane and the calculation of the seams is very processor-intensive. If products with multiple labels that have strong light and dark contrasts are to be inspected, the system can also be operated with four LXG models with a resolution of 4 megapixels and HDR function.

The applicability of the inspection system is not limited to cylindrical objects - other geometric shapes such as hexagonal packaging can also be inspected with '4ninety'. An adjustment in the software is sufficient to compensate for possible imaging errors due to the object geometry.