Industrial Image Processing

When processing cotton fibers, it is important that foreign particles are detected and removed with as little loss of fiber as possible, to ensure optimum preparation for spinning. The new particle separator from the Trützschler company in Mönchengladbach, SECUROPROP SP-FP, features an illumination unit with polarized light. The cotton fibers are transported past special cameras, which scan the fibers for foreign particles. These cameras are made by LINOS.  

Foreign particles in cotton processing fall into two distinct categories: on the one hand, there are particles that differ significantly from the cotton in color, contrast and structure. The second category – usually polypropylene or polyethylene film particles – is made up of light-colored or transparent materials which can hardly be distinguished from the cotton by color, and are thus invisible to conventional foreign particle separators. The scanning cameras use polarized light, taking advantage of the physical properties of plastics to make these materials appear in color. This ensures reliable detection and removal of such particles.

Color separation prisms

Conventional cameras with trilinear sensors for red, green and blue are used in many machine vision applications. A color line camera with a trilinear sensor is not practical for this application, because the cotton fibers do not travel at a unified speed over the entire breadth of the scanning field, and the individual color channels of the trilinear sensor are spatially separated by some 30 µm to 40 µm, and the three channels are not directed at the same point. This is why it is essential to use a camera system with color separation prisms.

LINOS as partner

Because the standard, commercially available cameras with color separation prisms cannot meet the special requirements of this application, Trützschler began searching for a company to join them in developing a customized camera system. They found a partner in LINOS. Following a feasibility study to determine the precise requirements, Trützschler decided to entrust the development of the camera system to LINOS. One of the main objectives in the development of the system was a specially optimized design to enable correction of the optical imaging errors, or chromatic aberrations, that occurred with the 3-channel prisms.

Lateral chromatic abberation

In the various approaches for the design, special attention was given to lateral chromatic aberration, to ensure that the three color channels were focused on the same point. Just as important is the position of the image edge over the three colors, because the edges must overlap within an extremely narrow tolerance range to enable sufficient reconstruction of the object color without color fringes. In this application, the appearance of color fringes in the image indicates the presence of foreign particles: objects that are grayish-white appear in color, and good cotton material is separated. Another significant factor is the correct execution of color separation in the three-fold prism, thanks to dielectric color separation layers. The quality of the color separation layers has a significant effect on the reconstruction of an object's color from the three color channels. Two color separation layers are required in the system: The first layer reflects blue and transmits red and green, while the second layer reflects red and transmits green. The dielectric layers must have as steep an edge gradient as possible. LINOS has extensive experience in developing this type of layered system. Multiple simulation repetitions resulted in the development of a suitable layer design. Since the manufacture of customized prism systems is also part of the daily routine at LINOS, this important part of the camera system was developed here as well.

Micrometer precision

Once the lens and prism system had been defined and developed for the application at hand, it was just a matter of integrating these elements, together with three linear sensors, in a camera system. As described above, the exact reproduction of color over the entire image field is essential in the implementation of this system. Thus extremely strict specification of the relative positioning of the three sensors is necessary, because a misalignment in either the lateral or transverse direction of the cotton during inspection leads to imaging errors, or color fringes. It was necessary to position the three sensors precisely and fasten them securely. This enabled a positioning accuracy of the three sensors to within < 2 µm, as required by the tolerance limits. This completed the development of the customer's system, and nothing more stood in the way of its production.

Complete solution from LINOS

The cameras were installed, adjusted and secured entirely by LINOS, using special tools and adjustment software. The end product, delivered to the customer, could be put to use immediately. 

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The authors

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Georg Zeitelhack and Thomas Schäffler, Business Unit Vision Technology at LINOS in Germany.