1.1 challenges due to high productivity as well

 

1.1  Background of Fabric inspection systems

 

The textile industry is a most important and rapidly growing
sector that makes a considerable impact to the economical state of Sri Lanka.
Quality is a major parameter in textile, thus good quality products increase
the profit of the industry as well as the customer satisfaction. If the defects
in the fabric are not detected properly significant financial loses can be
occurred. Hence, the fabric inspection process should carry out vigilantly. The
key issue, therefore, is how and under what circumstances fabric inspection
will lead to quality improvement. As a result, the automated fabric inspection
systems have been designed to improve the efficiency of the inspection process.

Until, recently,
the fabric inspection is still undertaken manually by skilled staff with a
maximum accuracy of only 60%-70%. The modern fabric manufacturing industry
faces a lot of challenges due to high productivity as well as high quality
manufacturing environment. Because the production speeds are faster than ever
and because of the increase in roll sizes, manufacturers must be capable of
identifying defects, locating the sources of defects, and making the necessary
corrections in less time so as to lessen the amount of low quality fabric. This
in turn places a greater strain on the inspection departments of the
manufacturers. Due to the factors such as tiredness, boredom and carelessness,
the staff performance is often unreliable. Therefore, the best reliable
evaluation is through the application of an automated inspection system.

From the early
beginning, the target is to achieve optimum potential benefits such as high
quality, low cost, comfort, accuracy and speed in the manufacturing process. As
the technology is revolutionized the fabric inspection process has been
developed from manual to automated machinery to help achieve all those benefits
in the manufacturing process. The application of automated fabric inspection
would seem to offer a number of advantages, such as improved quality, reduced
labor costs, the elimination of human errors and increase the profit of the
industry. Therefore, the automated visual inspection is gaining progressive
importance in fabric manufacturing industry.

An automated
inspection system usually consists of a computer-based vision system. Because
they are computer-based, these systems do not undergo the drawbacks of human
visual inspection. The application of digital image processing is useful in
textile manufacturing and inspection. In recent years, it has proven to be the
most promising, rapid and reliable solution for the development of automated
fabric inspection systems. Considerable efforts have been taken to develop
and/or improve the task of automated fabric inspection systems. As all fabric
has the periodic regular structure, analyzing the parameters such as variance,
intensity of the fabric presents a possible way to predetermine the occurrence
of defects in the fabric.

The described
method in this thesis represents an effective and accurate approach to
automatic defect detection. It is capable of identifying various types of
defects by monitoring the fabric structure. Presence of a defect over the
periodical structure of a fabric causes changes in the variance and intensity
parameters of the fabric. By thresholding the defected area to create a binary
image, it is possible to identify the exact location of the defect. Further,
for accurate detection of the defects the noise removal, morphological erosion
and dilation, connected component analysis are carried out.

The fabric defect
could be simply defined as a change in or on the fabric construction.

Fabric defects can occur due to
machine faults, color bleeding, yarn problems, scratch, poor finishing, dirt
spots, excessive stretching, and crack points. Because of the wide variety of
defects, it will be useful to apply the study on the most major fabric defects.
The chosen major fabric defects are: hole, missed-yarn, oil mark, knitting
fault and pin holes.

A fabric fault detection system designed with MATLAB is
used for this procedure. It is implemented on the above mentioned chosen fabric
defects as well as the defect-free samples to identify as defect or non-defect.
To verify the success of the defects it is implemented on one hundred and fifty
samples.

 

1.2  Fabric inspection

 

Fabric inspection or defect detection is a process of
identifying and locating defects in fabric. A fabric defect is a result of the
manufacturing process. The textile industry is very concerned with quality. It
is desirable to produce the highest quality goods in the shortest period of
time possible. Fabric quality inspection process is very important for
manufacturing industry.

Mainly, fabric defect detection uses two type of inspection
model. The first one is the Human based Inspection Systems (HIS).The second
system is Automated based Inspection Systems (AIS).

 

1.2.1    Human based inspection systems(HIS)

 

In modern textile industry, fabrics are manufactured in
complex forms with high quality and high production line. Thus, the inspection
process becomes more difficult and complicated. Therefore, industrial fabric
inspection process has extremely high requirements.

 

 

Figure 1.1 Manual fabric inspection system

 

 

            Traditionally, Human based
inspection systems are based on human. This process is performed by
well-trained human inspectors. Usually, after the produced fabric is doffed
from the weaving machine, it is batched into large rolls and sent to the
inspection department. Most industries have power driven fabric inspection
machines where the manufactured fabric rolls are unrolled on an inspection
table(under adequate light) at a relatively higher speed of 8-20 meters per
minute9. When a defect is found on the moving fabric, the inspector stops the
machine to record the defect and its location, and starts the motor again. For
each inspected fabric roll, the number of defects per meter length is
calculated and the fabric is classified. Extra ordinary defect rate or
repeating defects are informed to the production department so that appropriate
actions can be taken to reduce the defect rate.

 

1.2.2    Drawbacks of human based inspection systems

 

While,
modern fabric industries demonstrate a high productivity and high quality
manufacturing environment, human based inspection systems have no ability to satisfy
today’s requirements due to limitations based on the physiological nature of
human. Therefore, Human based inspection systems (HIS) suffers from many
drawbacks described as follows:

1)      Training
phase takes much long time to be a skilled inspector.

2)      The
continue inspection becomes difficult and inefficient due to limitations in
human behavior such as boredom and tiresomeness.

3)      Human
perceiving speed is slower when compared to machines hence the process takes
longer time.

4)      The
human based inspection system could never reach 100% of defect detection
accuracy.

5)      The
inspector sometimes not capable of identifying the level of defects that is
acceptable.

6)      The
low fabric inspection is not compatible with high speed production line.

7)      Human
inspectors have to deal with extensive variety of defects.

8)      Human
based inspection systems are limited in finding defects and hence cause to
reduce the profit of the fabrics.

 

As a
result of these drawbacks several attempts are made to replace the manual
inspection systems by automated systems to increase the accuracy and efficiency
of the process by increasing the quality level of fabrics.

 

1.2.3    Automated fabric inspection systems

 

The shortcomings such as low profit, low accuracy and very
slow performance of manual fabric inspection process have led to the necessity
of automated inspection systems. Many researches are carried out in recent
years to develop automated inspection systems with high accuracy, stability and
speed with respect to manual inspection systems. Beside this, automated inspection
systems are provide high defect detection rates, reduce labor costs, improve
product quality and increase manufacturing efficiency.

            The automated inspection systems are
generally consists of four phases: image acquisition, image pre-processing,
feature extraction and decision making. All the phases in the system need to
work in the best way to achieve the accurate and effective inspection software.

 

 

1.2.4    Requirements for automated fabric inspection systems

 

The main
detriments of existing inspection systems are the high cost for hardware and
software development, vast computational efforts are required, and the defect
detection ranges are limited. So, the researchers aim to design systems with
minimal cost. The developments in fabric inspection system software facilitate
for better extraction of defects and patterns. Hence, the main requirements of
automated inspection systems are:

1)     
System should be designed with minimum cost.

2)     
Automated inspection systems must provide the necessities for real-time
inspection.

3)     
The system must work for different types of fabric patterns.

4)     
The defect detection rate should be higher than the human based
inspection systems.

5)     
System should resist for poor conditions in the textile industries both
as hardware and software.

6)     
The system should have easy and understandable control mechanisms that
the workers are capable of operating. 

 

1.2.5    Advantages of automated fabric inspection systems

 

 

Automated inspection systems are designed
to fulfill the requirements of the modern competitive textile industries. In
recent years, various types of automated systems are proposed using digital
image processing techniques as they possess more advantages than human based
inspection systems. The advantages are summarized as follows:

1)     
Automated
systems increase the speed of the defect detection and also the reliability of
inspection.

2)     
As
fabrics have very sensitive structures, the noncontact capability in automated
fabric inspection prevents the fabric pattern from disturbances that may arise
during contact period.

3)     
Automated
inspection systems are more stable as the effect of external conditions is
less.

4)     
Automated
systems provide high detection rates.