FIELD OF INVENTION
The present invention relates to an illumination method for automatic surface
inspection. More particularly it relates to a method of illumination of cold rolled
sheets for automatic surface inspection.
BACKGROUND OF THE INVENTION
Quality control of production has always been a necessity in steel mills. The steel
industry sets ever increasing requirements on product quality. The purpose of a
surface inspection system is to detect and classify surface defects that impair
product quality with regard to the requirements set by the user. Critical defects
are also those which cause production disturbances.
Automatic inspection systems for metal surfaces have been available for some
time, but not until recently, the development of electronics and information
technology, have made the actual image analysis applications possible. The
current systems use CCD-sensorsto produce grey scale images of the defects.
These images prOVidenew opportunities for defect classification.
There are many types of metal surface imperfections. In general, large defects
are more critical than small ones, but steel users ma.yalso be concerned about
defects whose diameter is only tenths of a millimeter. There are two main
reasons for steel mills to strive for defect free material: internal and external.
Internal reasons arise while producing a continuous strip of sheet at high
speeds; the imperfections in the strip quality often create problems in
production. External reasons arise from the customers where out-of-quality end
products may cause returns or even result in losing a customer.
Worst problems that material defects may cause in a steel mill are coil breaks.
With the help of surface inspection system coils with critical defects, that can
cause coil breaks in the rolling mill, can be blocked. Such critical defects are for
example edge cracks, holes and heavy silvers. Steel producers wan~ to know
emergence of such defects that may prevent running the process smoothly and
to make sure that the product quality meets the customer requirements. The
causesof different defects should be located and removed as soon as possible. If
defects are present, then these should be recorded for different types of
statistical quality reports. Without a surface inspection system, surface defect
identification and root cause tracing can easily take a long time. During this delay
the problem may repeat causing even more downgraded production.
Illumination plays a very vital role in these automatic surface inspection systems
where the image quality depends on the type of illumination used. The choice of
illumination technique is primarily problem dependent and hence there exists a
need for special illumination techniques for Cold Rolled (CR) sheets, whose
surface are shiny and highly reflective.
There have been many prior attempts on obtaining the perfect illumination for
cold rolled sheets. For this purpose, many different light sources such as those
patented in [2] and [3] of patent references given below were used. But they
prove to be very expensive and highly difficult to manufacture. This uses a
hemispherical arrangement by which the complete sheet can be illuminated for
the camera to capture. Hence, the number of light sources needed increases
thereby increasing the cost of the arrangement.
OBJECTSOF THE INVENTION
It is therefore an object of the invention to propose a method of illumination of
cold rolled sheets for automatic surface inspection which is capable of revealing
even the minor defects of the rolled strips.
Another object of the invention is to propose a method of illumination of cold
rolled sheets for automatic surface inspection which improves the quality of
image received by the camera.
SUMMARY OF THE INVENTION
An illumination design is developed for the automated surface inspection of cold
rolled sheets which depends on the properties and texture of the sheets.
Illumination plays a very vital role in these automatic surface inspection systems
where the image quality depends on the type of illumination used. The choice of
illumination technique is primarily problem dependant and hence the need for
special illumination techniques for Cold Rolled (CR) sheets, whose surfaces are
shiny and highly reflective in nature. The lamps to be used in this design are
automobile fog lamps. The different parameters governing illumination which are
addressed are, choice of lamps, the arrangements of the lamps, number of
lamps to be used and the spacing between the lamps. These vital parameters
govern the type and quality of the illumination and also the quality of the images
received by the camera.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 - shows the determination of the cone angle of the light source (Fog
lamp)
Fig. 2 - shows the Gaussiancurve fitting for intensity of the fog lamp
Fig. 3 - shows the graph for determination of number of light sources needed
Fig. 4 - shows the side view for the fog lights and its mounting
Fig. 5 - shows the arrangement of lights and the mounting arrangement of the
Lights.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE
INVENTION
The Lighting arrangement consists of automobile fog lights in which the cutoff
regions of light illumination is sharp and the spread is optimal. Hence, the
intensity is higher than halogen lamps, on shiny, highly reflective surfaces. The
parameters defined and evaluated for the arrangement are: (i) The optimal
distance of the lights from the ground (ii) Cone Angle (Hi) The distance between
the two lamps and (iv) The number of lamps needed and (v) the arrangement of
the lamps.
A. Optimal distance of the lights from the group and Cone angle:
The optimal distance of the lights from the ground was found to be 650 mm for
maximum intensity of light to reach the camera, while the shadow of the
filament isn't captured by the camera..Once the distance is reduced more, the
shadow of the filament is also captured by the camera, thereby producing errors.
Figure 1 shows this arrangement that includes the vertical distance between the
light and object and the spread. Using these measurements, the cone angle can
be calculated to be as:
Coneangle = 2 tan-1 (270/650)
e = 45° (apprOXimately)
B. Distance between two lamps and number of lamps needed
The intensity of the lights at a distance of 650mm was verified experimentally
using a LUXmeter to verify the workable region of the light spread.
It can be seen that this takes the form of a Gaussiancurve with a spread of 540
mm. Hence, by using the mean and standard deviation calculated from the
samples above, the Gaussian curve fitting for the samples above and this curve
is given in Figure 2. If total length of the illumination required is 1750 mm, then
to have an illumination of 1750 mm, we would need to have a workable length of
light of about 2000mm. The distance between every light bulb used depends on
the ripples factor when many such Gaussiansare placed close to each other in
order to form an approximate straight line.
The number of lights for the distance required was found to be 14. The most
appropriate distance between two lights was found to be 10 mm which would
mean the difference between the centers of two lights would be 160mm length
of each light being 150 mm. The plot to explain this is given in Figure 3. It can
be seen from the plot that the magnitude of the ripple is to the order of 10-3•
c. Arrangement of Lamps
The lights are arranged based on the number of lights and the distance between
them obtained in the previous experiments. They are arranged on a metal
mounting in such a way that there will be vertical movement. This proves
necessary because for a Iinescan camera to obtain a straight line image, the
illumination needs to be in an arc by which the perfect illumination is provided
for the line which is to be scanned. Hence, a vertical movement for the lights is
provided in which the center light is the closest and the farther lights are moved
to a maximum distance of about 15 mm from the center, thus forming an arc.
Thus, the invention provides an optimal design for the arrangement of lights to
perfectly illuminate the surfaces of cold rolled sheets. This design consists of fog
lamps in which the cutoff regions of light illumination are sharp and the spread is
optimal. The parameters defined and evaluated for the arrangement are the
optimal distance of the lights from the ground. ConeAngle, the distance between
the two lamps and the number of lamps needed and the arrangement of the
lamps.
WE CLAIM:
1. A method of illumination of cold rolled sheets for automatic surface
inspection comprising the steps of:
finding the optima' distance of a plurality of automobile fog lamps from the
ground for maximum intensity of light;
determining the cone angle;
verifying experimentally the workable region of the light spread;
finding the magnitude of ripples;
ascertaining the number of lights to be adapted in the spread available;
arranging a plurality of said lights on a metal mounting proViding a vertical
movement;
characterized in that the lights are arranged in the outward semicircular arc
by which the center lamp is at the center and the corner lights are at a
maximum distance of 15 mm from the center.
2. A method as claimed in claim 1, wherein the spread available is 540 mm.
3. A method as claimed in claim 1, wherein the optimal distance of the lights
from the ground is 650 mm.
4. A method as claimed in claim 1, wherein the workable length of light is 2000
mm.
5. A method as claimed in claim 1, wherein the cone angle is 450
.
6. A method as claimed in claim 1, wherein the number of lights arranged is 14.
7. A method as claimed in claim 1, wherein the distance between two lights is 10
mm.
8. A method as claimed in claim 1, wherein the length of each light being 150
mm, the difference between the centers of two lights is 160 mm.
9. An illumination design as claimed in claim 1 wherein the magnitude of the
ripples is in the order of 10-3.