M
odern manufacturing processes must produce right-first-time. This is of the highest
importance in fiber opening and preparation. In spinning, second-quality or contaminated yarns must
be avoided. One obstacle for top-quality yarn is the increasing amount of foreign matter and
particles. This article, based on experience and information of Germany-based machinery
manufacturer Trützschler GmbH & Co. KG, presents a modular approach to solving the problem of
foreign matter in ginning and spinning.
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.
Sensors
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.
Illumination
Another important factor in determining object detectability is the type of illumination.
Cameras, as well as the human eye, can only detect 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.
Material Presentation
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. Since
it 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.
Figure 1: The application range of different foreign matter systems is a function of object
size and optical properties.
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.
Figure 2: 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.
Different Modules
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.
October/November/December 2009