On-line optical measurement and monitoring of yarn density in woven fabrics Page: 4 of 11
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while the second signifies the same quantity in the warp direction (machine direction). At different points in the
process of fabric manufacturing and because of a number of different but known causes (e.g., changing tension in
the take-up activity or the process of sanforization), the pick and warp densities can undergo subtle, as well as dras-
tic, variations. To ensure that these variations are within an allowable tolerance and to create the opportunity for
automatic process control, accurate on-line measurement and monitoring of yarn density is the necessary first step.
This paper describes a vision-based monitoring system that accomplishes the above task on-line. The system is
described in terms of its two principal modules, namely, the image acquisition and the image analysis subsystems.
The image acquisition subsystem, described in Section 3.1, is implemented with standard components on a low-
cost personal computer platform. The main function of this subsystem is to acquire high-resolution, vibration-free
images of the fabric that are suitable for processing by the ensuing image analysis subsystem. This subsystem
implements a novel algorithm (Section 3.2) that utilizes the discrete Fourier transform to generate a 1-D signature
representing each fabric. Simple statistics of the obtained signatures are the basis for characterizing the fabric in
terms of its yarn density. The obtained results from testing the proposed monitoring system on seven fabrics with
common properties but varying yarn densities are reported in Section 4.
3. VISION-BASED MONITORING SYSTEM
The proposed yarn density monitoring system is described below in terms of its two major modules. First, the
image acquisition subsystem, whose main function is to acquire high-resolution, vibration-free images of the fab-
ric, is outlined. Then the image analysis module is delineated in terms of an algorithm that efficiently estimates the
yarn density of the fabrics from the Fourier spectrum of their images.
3.1 Image acquisition subsystem
The task of on-line (or in this case, on-loom), real-time image acquisition that produces high-quality, high-reso-
lution images of fabrics can present three major difficulties, namely,
1. the isolation of the mounting components from the considerable vibration produced during loom oper-
ation,
2. the decoupling of the true forward motion of the fabric from its irregular movement shortly after its
construction, and
3. the placement of the monitoring system components so that they do not interfere with loom operation
or with operator access.
Moreover, these challenges are combined with the requirement that an inspection device must meet a cost require-
ment in order to justify its use on an individual loom. Each of the above problems has been addressed in the devel-
opment of the proposed image acquisition subsystem.
The proposed subsystem is implemented with standard components on a low-cost personal computer. These
components, shown in block diagram form in Fig. 1, consist of (1) a line-scan camera that is synchronized to the
moving fabric by means of an incremental encoder, (2) a source of illumination for backlighting the fabric, (3) a
TMS320C40, DSP-based image acquisition and processing card, and (4) a host personal computer. These compo-
nents are used to acquire high-resolution, vibration-free images of the fabric under construction and to store them
on the on-board memory of the acquisition card. A custom interface between the camera and the encoder extracts
the true forward movement of the fabric from the highly oscillatory motion and enables an accurate image line trig-
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Sari-Sarraf, H. & Goddard, J.S. Jr. On-line optical measurement and monitoring of yarn density in woven fabrics, article, December 31, 1996; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc676911/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.