FIELD OF THE INVENTION
The present invention generally relates to a method of cold-rolled scratch
detection on flat products in manufacturing processes by using surface
inspection system. More particularly, the invention relates to an improved
method for detection of cold-rolled scratches on flat products by a scratch
detection system.
BACKGROUND OF THE INVENTION
Scratches are mechanical damages in width, depth and length at the surface
primarily caused by friction between the stock and parts of the equipment due to
relative movements. Any kind of visible abnormalities (before painting) on the
surface is not acceptable to the automotive customers for the skin panel
applications. In some critical application scratch may act as a stress riser and
reduce the fatigue life of the parts. Overall the customer's satisfaction and
aesthetics value of the material surface is reduced drastically in presence of the
not easily detectable scratch defect while processing.
The detection of scratches at the earlier part of the flat product production chain
of the steel industry, prevents downgrading of the high valued product as its
final stage. Information of the occurrence of such defects is helpful for the
judgment of machine's health which is running continuously and optimizing the
maintenance schedule of quality response equipment. Similarly, bright scratches
may be generated at any of the stages such as rolling, cleaning, coating, coiling
and handling. It is very important to know the instance and the point of
generation of the scratch by close monitoring of the coil surface at every stage of
processing for proper strip path management as immediate remedial measures.
It is observed that the scratches appearing along the length, on the surface of
cold rolled sheets are not properly captured in the surface inspection systems.
(Reference 1); however, the scratches are very clearly visible to the human
observer.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved method in a scratch detection system
for detection of cold-rolled scratches on flat products, the system comprising at
least a plurality of area-scan cameras with point-light sources, a plurality of
rollers holding the flat-products for inspection, the area-scan cameras being
disposed over the rollers, the method comprising the steps of:
placing an inspectable flat-product below the detection system being
held by the plurality of rollers;
placing a plurality of area-scan cameras and a plurality of point-height
source placed at equal distances along the width of the product;
causing the light to fall upon the product-surface from the plurality of
light sources from an angle substantially parallel to the surface; and
capturing in the plurality of cameras the scattering produced by an
interaction between the light radiation and scratches leading to
dispersing in all directions and incident light ray falling on the cameras.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved method for
detection of cold-rolled scratches on flat products by a scratch detection
system.
Another object of the invention to propose an improved method for detection
of cold-rolled scratches on flat products, which is enabled to detect defects in
the order of 100 microns.
A still another object of the invention to propose an improved method for
detection of cold-rolled scratches on flat products, which eliminates the
problem of scratch appearing as broken images.
Yet another object of the invention to propose an improved method for
detection of cold-rolled scratches on flat products, which allows disposition of
the scanning means of the scratch detection system on the top of roller
surface of the system.
A further object of the invention to propose an improved method for
detection of cold-rolled scratches on flat products, which uses an improved
detection system with multiple area scan cameras with point-light sources.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The improved method in a scratch detection system according to the invention
enables detection of small micro scratches on the flat products.
The descriptions of the drawings provided are as given below:
Figure 1: shows a method for scratch detection in an operational mode for
known scratch detection system on a clean surface with no scratches.
Figure 2: shows the device of Fig.l, in an operation mode for scratch detection,
when scratches present on the surface.
Figure 3: shows the prior art device of Fig-1 exhibiting i that the resolution gets
poorer for the portion of a scratch detectable strip when the portions are away
from the camera.
Figure 4: shows an improved scratch detection method and system with multi-
camera according to the invention.
Figure 5: shows a scanning for defects on the upper part of roller surface to
avoid problems due to fluctuation of the metal sheet, according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
Scatter of radiation is caused by variations of the refractive index as light passes
through a material. Other reasons include foreign particles or voids, gradual
changes of composition, second phases at grain boundaries, and strains in the
materials. If radiation traverses a perfectly homogeneous medium, it is not
scattered.
When light is made to fall upon the surface from an angle, almost parallel to the
surface, most of the light gets reflected from the surface, following the law of
reflection, in absence of any surface irregularity like a scratch as shown in figure
1.If an area-scan CCD camera is placed alongside the light source, it won't be
able to see anything as virtually no light is falling back on to the camera sensor.
In presence of the surface irregularity like a scratch the interaction of the
radiation results in phenomena called scattering as shown in figure 2. When
scattering takes place a major component of the radiation disperses in directions
all around the point of contact. From all points on the scratch, part of the
radiation travels back in direction of the incident ray and falls onto the CCD
sensor and gets registered as prominent bright line in the image.
Since the camera and the light source is placed at one end of the strip almost
parallel to the surface, the resolution of the area mapped on the CCD varies
proportional to the cosine of the angle between the normal of the surface to the
principal axis of the camera. This effect is known as foreshortening. Hence,
resolution gets poorer for the part of the strip away from the camera, as shown
in figure 3. The solution to this problem is to use multiple cameras with multiple
light sources placed at equal distances along the width of the strip. This
arrangement also facilitates the precise localization of the scratch defect in the
strip as shown in figure 4.
In normal rolling conditions strip is held tight between the rollers. In this
arrangement the strip fluctuates perpendicular to the direction of rolling causes
the displacement of the strip in the image because of the relative movement
between the camera and the object. Hence, scratches appear in the image as
broken pieces. This problem was overcome by shifting the entire scanning
system at the top of the roller surface as shown in figure 5. This is the place
where the strip rests nicely and there is no hop or vibration present to distort the
image quality.
Owing to the fact that scratches are very fine and the scattering component is
almost negligible in comparison with the incident radiation, the point light source
and the line scanners are not a good choice for such applications. Though the
line scan cameras are good for precise acquisition of moving objects, they are
particularly unfit for this application because of extremely low exposure time.
Hence, low contrast evidences in the image. This problem was overcome by
replacing line scan cameras with area scan cameras and the point light sources
by automobile lamps for high intensity directional beams.
Other References
(2) A.K. Jain, Fundamentals of Digital Image Processing, Prentice Hall
Publishing.
(3) Jae S.Lim, Two dimentional signal and image processing, Prentice Hall
Publishing.
WE CLAIM:
1. An improved method in a scratch detection system for detection of cold-
rolled scratches on flat products, the system comprising at least a plurality of
area-scan cameras with point-light sources, a plurality of rollers holding the flat-
products for inspection, the area-scan cameras being disposed over the rollers,
the method comprising the steps of:
placing an inspectable flat-product below the detection system being
held by the plurality of rollers;
placing a plurality of area-scan cameras and a plurality of point-height
source placed at equal distances along the width of the product;
causing the light to fall upon the product-surface from the plurality of
light sources from an angle substantially parallel to the surface; and
capturing in the plurality of cameras the scattering produced by an
interaction between the light radiation and scratches leading to
dispersing in all directions and incident light ray falling on the cameras.
2 An improved method in a scratch detection system for detection of cold-
rolled scratches on flat products substantially as herein described and as
illustrated in the accompanying drawings.

An improved method in a scratch detection system for detection of cold-rolled
scratches on flat products, the system comprising at least a plurality of area-
scan cameras with point-light sources, a plurality of rollers holding the flat-
products for inspection, the area-scan cameras being disposed over the rollers,
the method comprising the steps of placing an inspectable flat-product below the
detection system being held by the plurality of rollers placing a plurality of area-
scan cameras and a plurality of point-height source placed at equal distances
along the width of the product; causing the light to fall upon the product-surface
from the plurality of light sources from an angle substantially parallel to the
surface; and capturing in the plurality of cameras the scattering produced by an
interaction between the light radiation and scratches leading to dispersing in all
directions and incident light ray falling on the cameras.