Optimizing Yarn Quality
A combination of different modular systems can provide optimal solutions for foreign matter detection in ginning and spinning operations.
TW Special Report
The nature and extent of foreign matter contamination are strongly dependent on the origin of the cotton. US and Australian cotton, which are 100-percent machine-picked, do not have significant problems with foreign matter. Cotton from Turkey contains many red ribbon-shaped contaminants. In China, mills are battling threads from bleached cotton as well as white fluorescent polypropylene (PP) ribbons. Cotton from Central Asia is contaminated with white, non-transparent packaging residues. These are just a few examples.
Battle Of The Systems
A number of foreign matter separators, each operating on different detection principles, are available on the market. In addition to different kinds of sensors, material presentation to the sensors and the method of illumination are approached differently. Each system is distinguished by a combination of these three characteristics.
Photo sensors are relatively cheap sensors that are arranged in-line and detect differences in brightness in the passing flow of fibrous tufts.
Ultrasonic sensors, also arranged in-line, detect foreign parts with solid, sound-reflecting surfaces but cannot detect foreign fibers, threads and strings.
Color sensors, or 1-CCD (charged couple device) cameras, are line-scan cameras with a single CCD chip. These systems are used in office scanners. Sensitivity depends on the resolution of these cameras and the scanning width. Because these cameras work with three adjacent scan lines - red, green and blue - with a certain offset, the color recognition of moving objects is limited and results in a so-called color noise effect.
Much more effective, although more expensive, are 3-CCD cameras. Today, such systems are used in TV cameras. The three basic colors -- red, green and blue -- are separated by a prism and simultaneously directed onto three CCD chips. This system is also called a true-color system. Thanks to this simultaneous process, the variable speed of objects in the material flow no longer has a negative effect. Currently, 3-CCD cameras represent the high-end approach to foreign matter detection.
The application range of different foreign matter systems is a function of object size and optical properties.
Another important factor in determining object detectability is the type of illumination. Cameras, as well as the human eye, can detect only objects that distinguish themselves in color, contrast, structure or luster from cotton tufts. For this reason, the type of illumination applied in foreign matter separators plays an essential role. Today's standard is illumination units with fluorescent tubes operating in reflected light mode.
Ultraviolet (UV) light sources make white and other colorless foreign objects with stronger UV reflection clearly visible. The phenomenon is called fluorescence. Such objects may include pieces of polyester (PET), PP, or even bleached cotton treated with optical brighteners. With polarized reflected light and the corresponding camera filters, differences in surface luster of foreign objects can be detected. The system reaches its limits with dull objects.
Polarized transmitted light is the ideal system for detecting transparent and semi-transparent objects, such as polyethylene (PE) foil or PP fabric from bale packaging. The presence of these particles often results in the dreaded foreign fiber claim.
The presentation of the fibrous material to the sensors also affects the performance of foreign matter separators. Almost all systems on the market monitor the tuft flow in a rectangular chute. One major disadvantage is the undefined velocity of cotton tufts and foreign objects. Because the velocity is not constant, the downstream separation nozzles must be activated for a longer period of time. This inevitably results in an increasing loss of good fibers. However, one advantage that should not be underestimated is the gentle treatment of cotton fibers, which are not mechanically stressed. Systems that feature detection on or close to the surface of a rotating needle roll have three very important advantages: First is the accurately defined material velocity and, hence, the minimal loss of good fibers during removal. Second, accurately detecting the position of foreign objects is advantageous, as there are no problems due to differences in illumination intensity depending on chute depth, as is the case with chute-based systems. The third advantage lies in the high degree of material opening and the associated excellent exposure of the foreign objects. Figure 1 shows a schematic comparison of these different systems.
Solution: Modular Systems
This close examination of the different sensors, illumination systems and methods of material presentations clearly shows there is no single ideal system. However, by using systems precisely adapted to the actual requirements, one can come very close to this ideal. An optimum solution consists of an intelligent combination of different systems. Trützschler has developed three modules that, individually or in combination, are integrated in different machines.
The Colour Module features high-resolution 3-CCD cameras and can be applied in conjunction with a chute as well as a rotating cylinder. In the latter case, even white, non-transparent PP objects can be detected. The high resolution of the cameras also ensures reliable detection of tiny threads and ribbon-shaped objects. The PP Module reliably detects transparent and semi-transparent objects in transmitted light mode. Here, the patent-pending polarized light method is applied. The module is said to be ideal if cotton is contaminated with PE foil or packaging residue from PP fabric. The Trützschler UV Module complements the other two systems if the cotton is contaminated with fluorescent objects from bleached cotton threads, PP or PET.
Figure 2 shows a comparison of the different modules, with four scenarios. The combination of these three modules with the cylinder and/or chute system led to the development of a new range of foreign matter separators.
Comparison of Trützschler’s foreign matter detection modules: Each of the four pictures shows the same foreign objects and a cotton tuft in the center. By applying different sensors and illumination systems, the different objects can be detected.
For Different Requirements
Four solutions, each designed for specific applications, are available. The right decision depends on the particular requirements. The most important decision criteria are:
• nature and extent of foreign matter contamination;
• plant configuration;
• dust removal required;
• required detection and separation efficiency;
• investment costs;
• operating costs; and
• waste handling.
The foreign matter separator must fit into the plant concept. An optimal cleaning line is more than just individual units put together. Consequently, foreign matter separation must become an integral part of the cleaning line concept in a cotton gin or a spinning mill.