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Automatic Weft Detection

Weft distortion presents a challenge for textile finishers, but new automated detection technologies make the problem easier to handle.

Udo Skarke

F ashion trends are production challenges faced every day by a textile finisher. The goal is to produce first-quality goods only. This is especially difficult when weft distortion has been introduced to the fabric during various process stages.

Uneven tension distribution across the weft is the main cause for distortion. There are many causes for this defect, including tension variations, equipment lineup, improper roller adjustments and direct contact of machinery with the material.

Dealing With Weft Distortion

Mechanical forces are commonly employed to correct the geometry of a weft that has been distorted during the finishing phase. Both mechanical weft straighteners and differential drive tenters are commonly applied technologies.

By adjusting the speeds of differential tenter chains, or by applying linear tension and passing material across rollers that can be pivoted around the center point of the fabric width, the weft lines can be moved and realigned.

Mechanical weft correction devices have always been used to correct weft distortion, primarily on materials where distortion is more visible and becomes an issue for the end-user. However, as tolerances grow tighter, production speeds increase, and the fabrics become more complex, automatic weft straightening is a must in today's modern textile finishing operations.
 
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Using photoelectronic detection in oscillation removes possible mechanical difficulties.
Straightening Principles

The history of automatic weft straightening goes back more than 40 years. Generally, there are two common automatic weft-straightening principles used today: automatic straightening by means of mechanics; and automatic straightening by means of electromechanics.

Automatic straightening by means of mechanical force is based on the parallelogram effect. When the distortion of the fabric is along a diagonal axis, the warp and weft geometry, initially orthogonal, assumes a parallelogram configuration. When the two selvages are pulled out to stretch the cloth to its full width, the weft is tightened, generating complex forces in a parallelogram configuration to establish a square structure. This straightening force can be exploited for restoring proper weave geometry if the selvages, in spite of their lateral tensions, are sufficiently free to move in the direction of the warp.

Automatic straightening by means of electromechanics is the most common method employed today. Weft geometry is automatically detected using a mechanical or electrical sensor. That information is then transformed into a signal that displaces rollers, making the correction as necessary. The weft-straightening effect here is the result of the difference in distance traveled by one selvage in relation to the other over the rollers in the weft-straightening system.

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Sensing Technology

In order to have an effective automatic weft-straightening system, it is important for the sensing device to detect all the possible fabric styles, designs, colors and structures. With today's complex textile products, that is not always an easy task. The ideal detection system is capable of sensing denim, sheeting, jacquard, apparel and automotive fabrics; fiberglass, carpet, lace, terry towels and a large variety of technical textiles, among other products.

In the early 1960s, two major detection systems on the market were used for detecting weft distortion in fabric. One system involved the use of mechanical principles, and the other utilized photoelectronic sensors.
 
Mechanical Detection Principle

The mechanical principle is based on the use of two wheels mounted on a freely moving mounting pin. Under normal conditions, both wheels are turned by the passing fabric, resulting in a homogenous signal. When a distortion is sensed, the tension created by the skewed weft causes the freely mounted wheels to turn left or right depending on the distortion. A non-proportional signal is generated by a differential amplifier, sending correction signals to a correction device. As long as the wheels in continuous contact with the material remain in the same good working condition, the measurement results remain satisfactory. Any uneven fabric surface challenges the mechanical principle and generates asymmetrical distortion. This system has not been further developed in its design and has been discontinued because of limited detection abilities and hardware reliability.

Photoelectronic Detection Principle

The photoelectronic sensor is based on a signal modulation created by the passing web structure. One of the first photoelectronic sensors was based on several photo elements located on one side of the web and a light source on the opposite side of the web. When the weft line was parallel and straight in front of the sensor, a somewhat equally strong signal was generated in both channels.

If the weft line was not parallel to the sensor, one pair of photo elements generated more signal than the other, and this determined the distortion. That type of sensor could only differentiate between left or right distortion and was not able to quantify the amount or angle of distortion. Therefore, a proportional correction based on the physical distortion was not possible.

The next generations of optical sensors utilized only one photo element by letting it freely oscillate between known limits. That improved the linearity, resolution and accuracy of each sensor, enabling it to detect more complex fabric structures.

With microcomputers entering the industrial arena, it was not long until the first microcontroller-based detection system was introduced. Stepper motor technology for linear movement replaced the free-oscillation detection lens and improved lighting principles such as infrared light-emitting diodes and reflex light sources, bringing the system more up-to-date.

The photoelectronic detection technology is utilized in various executions such as oscillation, rotation or shifted dual-rotating mode. By rotating the lens 360º instead of always oscillating on the same position, one could avoid possible mechanical difficulties. However, there is some valuable time lost by scanning areas of the fabric that are unrelated to the weft line. The shifted dual-rotating mode reduces such time delays in collecting valuable weft distortion information with a two-lens system.

camera
The latest CCD matrix camera from Erhardt + Leimer GmbH employs high-resolution auto-focus and auto-zoom technologies.

Weft Detection Using Cameras

The rapid reductions in the prices of computer systems and digital cameras combined with advanced image-processing techniques have led to the introduction of several vision systems to the textile manufacturer. There are two types of camera systems that can be used to create an image: line scan camera technology; or charged-couple device (CCD) matrix camera technology.

The line scan camera in combination with an encoder creates an image while the web passes by. After capturing the image, the evaluation software uses special algorithms to process the digital information.

The line scan camera technology typically is used to detect the full width of the web, detecting and evaluating patterns rather than the weft line. That can be realized with either one or two cameras, depending on the area of interest. However, if there is no pattern in the web, the line scan technology is not able to detect any distortion in a web.

A matrix camera takes images similar to any conventional digital camera on the market. The web is imaged several times per second, whether running or not, on a 2-D area. The camera looks at multiple weft lines simultaneously, providing a high data rate of the passing weft structure. With sophisticated software tools and mathematic calculation, a 3-D image is processed, detecting not only the weft, but also the warp of a web.

The matrix camera technology is used for a closer look at the individual weft and utilizes the weft structure in its calculation of the residual web distortion. Whether with several cameras evenly distributed over the full width of the web, or just one scanning camera taking images across, a large cross-direction and machine-direction profile of the passing web is captured. A reflecting circumference infrared illumination guarantees the best uniform imaging results.

The latest camera introduced from Germany-based Erhardt + Leimer GmbH employs high-resolution auto-focus and auto-zoom technologies that allow the camera to view an optimum evaluation area and the largest 2-D area possible. The camera is mounted 250 millimeters away from the web, moving it away from sometimes harsh existing environmental conditions. The image is then processed within a 3-D space and filtered through a fast Fourier transformation (FFT), resulting in spectral data. Separating the noise from the actual high-level spectral data leaves the positioning information of the weft and the warp.

A major breakthrough of the 2-D image detection system using CCD matrix camera technology makes detection on many difficult webs such as carpet, jacquard and thicker wefts a problem of the past. Whereas conventional photoelectronic systems detect one weft at a time, the CCD matrix camera has the ability to evaluate a large number of weft lines simultaneously, even if the web is not moving. And whereas conventional photoelectronic sensors are required to have a minimum speed for detection, the speed of the process has no influence on the final result or the quality of the evaluated detection area when using the CCD matrix camera.

In addition to increasing the number of weft lines being detected, the system with its wide field of view also has increased drastically the evaluation area in the cross direction of the weft. That makes it more accurate and reliable because it sees far more than conventional detection systems see.

Many disadvantages associated with conventional detection systems can be overcome by using image-processing techniques to monitor webs of all kinds. New camera measurement systems have improved the range of use from unstructured, homogenous webs to complex, patterned fabrics.

Higher-quality products and more satisfied customers result from more accurate and reliable technologies.


Editor's Note: Udo Skarke is manager of Germany-based Erhardt + Leimer GmbH's Textile Division in Duncan, S.C.
July/August 2006

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