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Automatic Monitoring of Food Quality

1 Mar 2012

An automatic test device has been developed for random sample testing of chocolate bars. A particular challenge in this task is, on the one side, the hygiene demands within the sector and, on the other, the irregular forms of the objects.

In the food industry, imaging and machine vision has for some time now been part of the established techniques for product control and quality assurance. The particular challenges in this sector arise not only from the hygienic conditions, but also from the irregular shapes of the objects being examined.

The vision specialist Kdorf Automation, based in Kempen, includes in its skills focus the testing of objects in the food industry. In a current application, Kdorf CEO Detlef Klüssendorf, in collaboration with the Lower Rhine University, had the task of surveying chocolate bars.

The concern of the sweet manufacturer is primarily to carry out checks on the geometry of the bars, thus ensuring that his bars conform to the specifications. With the machine, already in service, this is straightforward, Klüssendorf tells us. He describes the test procedure as follows: “For measuring the chocolate bars are laid on the device’s plastic-coated stainless-steel carriage, which provides the adhesive friction necessary for transporting the bars and is very easy to clean. The operator can then choose the type of bar in a drop-down menu, which automatically sets all specific parameters for the bar, e.g. dimensions, tolerances and greyscale transitions. After closing the security flap, the measuring program in the application software is then started via a button and the bar is automatically pulled into the test device.”

Line scan cameras with telecentric lenses record the images

In the process, an integrated linear system accelerates the bar within a few centimetres, after which it is then moved with constant speed between two line scan cameras. “After the images are recorded by the two line scan cameras with telecentric lenses, the system then determines the measured values of length, breadth and height of the chocolate bar as quality assurance,” as the Kdorf CEO describes the continuance of the process. Because of the irregular contours of the bars, the test task represents, in Klüssendorf’s words, a particular challenge for the hardware and software of the measuring system.

At the end of the test process, the carriage returns to the security flap and the bar can again be removed. While the bar is returning to the starting position, the screen already shows the plan and side views along with all measurements of the bar.

Besides giving the measurement results with an accuracy of 100 µm, the maximum values recorded for length, breadth and height respectively of the bar show clearly, by red or green markings, whether the test results lie within the required tolerances or not. “Because of the irregular surface structures, several hundred points on the bar are measured, depending on the size of the bar, and the maximum measurements derived from these.” as Klüssendorf explains one of the special features of the development.

Inexact insertion of the test objects is compensated for by the software: “If a bar is laid askew on the tray, the software used provides an exact electronic orientation of the recorded image in plan view before carrying out the measurements,” Klüssendorf adds. “If the test object is in a skewed position, a signal warns the operator that he must re-insert the bar in the correct position.”

System saves complete test protocol

The camera system and the linear drive are located in an enclosed housing. Opening the safety flap during measuring leads automatically, for safety reasons, to an immediate emergency stop of the transport carriage. The measurement must then be repeated.

For statistical evaluation, the text results are passed on to a further program for quality assurance. The complete test protocol with all measured values and the recorded images of plan and side view are saved by the system for documentation purposes.

The imaging components employed in the system were obtained by Klüssendorf entirely from STEMMER IMAGING in Puchheim. “We had already realised several projects with this partner in the past and were therefore sure of optimum service there,” says Klüssendorf. He particularly emphasises the good cooperation with his vision supplier during the development phase: “Due to the irregular contours of the chocolate bars, some trials were necessary before we found the optimum combination of imaging components for these demanding tasks. During the trials, the know-how of the experts at STEMMER IMAGING helped enormously in assembling the right hardware and software elements quickly, thus realising the system in a short time.”

In the meantime, the system has successfully entered service with customers, as Klüssendorf reports. In addition, further systems have arisen out of the successful project and are in use in measuring the volume of other sweets and even of meat. In the latter case, this method also enables the indirect determination of the weight of the meat, which opens up interesting possibilities for large butcheries, for example.

Special features of the installation

  • Precise recorded images of the chocolate bars using two high-quality line scan cameras, telecentric precision lenses and three LED lighting fields.
  • Imaging on an optimised industrial PC using PCI CameraLink Frame Grabbers from Teledyne DALSA and the Common Vision Blox Foundation Package software from STEMMER IMAGING.
  • Triggering of the line scan image recording by an exactly calibrated linear drive.
  • Automatic orientation of the recorded image in the event of skewed placing of the bar on the transport carriage.
  • High measurement accuracy despite the irregular surface structure of the bars.

Puchheim, Germany

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 80,000 imaging applications in various industries.