FIELD OF THE INVENTION
The present invention generally relates to a device for web inspection of cold
rolled coils in cold roll mills. More particularly, the present invention relates to
a method in a device for improved web inspection of rolling coils in rolling
mills. The improvement constitutes in that the inspection process takes the
rolling speed of the web as input and provide scan rate, which is number of
lines captured per second, feedback to the camera instantaneously so that
the vertical resolution of the image can be made consistent. Further it also
takes the average gray level of the acquired image frame as input and
provide exposure time to the camera.
BACKGROUND OF THE INVENTION
Inspection devices are known which have an electronic camera or an array of
photosensitive components, generally linear, the optical field of which is
constant, in width and time frequency of view capture.
The duration between the start of two successive view captures is therefore
independent of possible changes in speed of running of the material or
changes in pitch of the lattice borne by successively inspected materials. The
two lattices, that of the individual inspection areas, linked to the
optoelectronic system, and that of the textile, interfere and prevent any fine
inspection.
This is because, on the one hand, when the textile lattice has a mesh which is
smaller than that of the optoelectronic system, a defect on only one element
of the textile lattice cannot be detected by the inspection system since it

causes too small a variation in signal. On the other hand, when the textile
lattice has a mesh which is larger than that of the optoelectronic system, the
latter detects each space between the elements of the textile lattice and
interprets it as a perforation in the lattice.
French patent application FR 93 02279 teaches a method and device for
woven textile fabric image processing. Each fabric having a repetitive design
and being defined by its period, the method of this document consists of
acquiring an image and filtering it by subtracting, at each digital illumination
value of a point of the image corresponding to a point of the fabric, the digital
illumination value of another point of the image which is substantially
displaced by one period of the cloth structure. This is intended to avoid the
effects due to the cloth framework, that is to say the design which is
periodically reproduced on the cloth, during weaving.
The filtering thus performed is purely electronic and is performed a posteriori
on an image which, at view capture, has no particular characteristic apart
from being in synchronised with the running of the cloth so that each view
capture corresponds to the same distance travelled by the cloth.
Since the image capture performed by this camera is synchronised with the
speed of running of the fabric, between two view captures, the same length
of fabric is passed under the camera, whatever the speed of movement of the
fabric. However, between two points of one image or between two points of
two images, the number of threads or the number of lattice pitches or the
number of framework periods which have passed in the camera field between
two view captures is not an integer number or the inverse of an integer
number, but only a constant number, unvarying with the speed of the cloth.

Thus the lattices of the points observed during view captures and that of the
cloth threads have the same defect as described above: two points at which
the illumination values are subtracted, since they are approximately one
period apart, may correspond one to an interstice between threads and the
other to the centre of a thread. The result of the difference then gives a high
value even though the cloth has no weaving defect. Conversely, a defect,
such as a thick thread which fills up the said interstice, may correspond to a
small difference in illuminations. It is then not detected according to the
method of the document. Again, the smallest defects may not be detected by
this method.
US 5,990,468 discloses a device for optical inspection of moving material is
further known. Such a device is applicable to all materials which bear a lattice
and in particular textiles, cloth, wallpaper, plastic, printed materials or dyed
weaves.
According to a second aspect, US 5,990,468 proposes a device for inspecting
material having a lattice of regular pitch, including a means for moving the
said material and at least one electronic camera having a photosensitive point
sensor and supplying a signal representing an image formed on the said
sensor, a camera in the optical field of which the said material moves in the
course of inspection. A synchronization means is adapted so that, in the
direction of running of the material, the pitch between the areas observed by
the photosensitive points of the camera is a multiple of the pitch of the
material lattice, each said pitch of the said observed areas corresponding
substantially to an integer number of pitches of the said material lattice.

US 5,040,057 teaches mode selection switches which selectively interconnect
a sensor line shift timing generator with one of three signal sources a
conventional raster scan timing signal from a raster scan sync generator
variable speed external timing signals from a tachometer fixed speed timing
signals developed from the horizontal timing signals of the raster scan sync
generator. In a time delay and integration mode, the sensor line shift timing
generator causes the CCD arrays of an image section and storage section to
shift pixel values down the CCD arrays at a rate commensurate with the
external timing signal. As a spot of light emanating from a portion of an
object moving through an examination region moves along the CCD array, a
corresponding pixel charge is shifted through the CCD array at the same
speed such that the same pixel charge integrates light from the same spot as
it moves the entire length of the CCD array. An output register timing
generator generates timing signals for output registers such that an output
signal is produced that is compatible with television signal standard. In the
conventional raster scan mode, the camera works as a conventional video
camera. When connected to the variable frequency timing signal, the video
picture scrolls down the TV monitor clocked by the tachometer in proportion
to the speed of the conveyor. In the calibration mode, the image scrolls down
the video screen at a fixed rate.
EP1030173 describes a method for automatic inspection of the surface of a
moving object, the method comprising the steps of illuminating a region on
the object's surface from at least two different illumination directions, in each
of which there is at least one light source, and imaging said object's
illuminated surface region with a camera to provide image information for
analysis. The light sources in the different illumination directions are pulsed to
illuminate the object's surface region at different times, the pulsing frequency
being >lkHz, and the object's illuminated surface region is imaged as lines
with a line scan camera in sync with the above pulsing.

US2009303484 teaches Systems and methods for calibrating a web inspection
system.
As the vertical resolution of the line scan camera depends upon the speed of
an object under inspection (Figure 1), and further for the reason that the
rolling speed of coil at CRM (Cold Roll Mill) is not constant, capturing image
with a constant line-rate as of the prior art, introduces
elongation/compression in the image.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method in a device for
improved web inspection of rolling coils in rolling mills, which improves quality
of captured images for accurate defect detection.
Another object of the invention is to propose a method in a device for
improved web inspection of rolling coils in rolling mills, which allows
provisioning of continuous feedback for current line rate and exposure time.
A still another object of the invention is to propose a method in a device for
improved web inspection of rolling coils in rolling mills, which is implemented
independent of the main controller of the cold roll mill.
Yet another object of the invention is to propose a device for improved web
inspection of rolling mills.

SUMMARY OF THE INVENTION
Accordingly, in a first aspect of the invention, there is provided in a device for
improved web inspection of rolling coils in coils in cold Rolls Mills. The method
comprising the steps of:
- determining the rolling Speed from the Analog voltage signal
representing a speed signal;
- inputting the analog signal into a microcontroller for converting and
outputting corresponding digital signal with Ex-Synch cycles;
- establishing a relation between current rolling speed and line rate;
- calculating a current line rate based on the established relation;
- updating at least one of two cameras corresponding to the current line
rate when it differs from the provisions line rate beyond a threshold
value;
- providing a display device to exhibit the current line rate including the
cameras exposure time;
- automatically controlling including adjusting the camera exposure time
based on the average gray value of the image;
- generating a sync cycle in different modes in the timing cycles through
two each channels of at least solid two cameras; and
- providing independent switches to manually adjust the exposure for
the lighting conditions;
The device is operally connected to a known inspection table for web
inspection of coils in cold roll mill.
The inspection table is of known type. It allows running of HR coil in front of
the eyes of a user, said coil being illuminated by at least one light source. The
light sources are continuous or high-frequency light sources, that is to say

ones of which the light emission periods follow one another with a frequency
very much higher than that of a camera. By way of example, a frequency of 2
kHz allows the capture of a hundred images per second by the camera each
image corresponding to about ten flashes emitted by the light source
operating at that frequency. It should be noted that this light source is
modified compared to that which is traditionally found on inspection tables,
for which the operating frequencies are of the order of a few tens or a few
hundreds of hertz, frequencies adapted to the retinal persistence period.
According to a second aspect of the invention, there is provided a device that
takes the rolling speed of the web as input and provide scan rate, which is
number of lines captured per second, feedback to the camera instantaneously
so that the vertical resolution of the image can be made consistent. The
device that takes the average gray level of the acquired image frame as input
and provide exposure time, which is the image sensor active time in
microsecond, feedback to the camera instantaneously so that gray level
across the frames can be made consistent. The device that ensures consistent
image quality even if the reflectance of the web surface changes significantly,
and further ensures consistent image quality by accounting for light variations
due to aging. The device, is independent of the main web inspection system,
that allows the web inspection computational resources to be dedicated for
image processing and analysis tasks only. The device can control multiple
cameras by generating adequate Ex-Synch cycles (square wave with variable
frequency & duty cycle) depending on web rolling speed and acquired gray
level respectively, and provides feedback to the user using LCD display for the
current gray value. The device takes the input using the push-button type
switches for the target grey value.

According to the type of material which is being inspected and according to
the type of defect which is being looked for, the inspection computer jointly
controls the levels of the light intensities continuously emitted simultaneously
by the lower light sources which are located below the inspection table and
which illuminate the coil by transparency, by a low-angled upper light source
which illuminates the coil at an incidence which is oblique and preferentially
close to a right angle, and by a head-on upper light source which illuminates
the coil at an angle of incidence close to the view capture angle used by the
camera. The head-on upper light source is provided with a polarizing filter.
The camera is provided with a polarizing filter, the polarization axis of which
is, according to the types of material and the types of defect looked for, either
parallel, or perpendicular (in order to detect marks on a reflective, for
example metallic, material), to the axis of the polarizing filter.
A coil passage is an area where the coil run between the light source and the
camera on the one hand, and the eyes of a user, on the other hand.
In the area of the coil passage, a coil tensioning means is located. The
tensioning means may be composed of assemblies of cones between which
the coil is gripped and which are driven in rotation by the coil itself, around an
axis which is oblique with respect to the direction of movement of the coil.
This tensioning means is well known to persons skilled in the art of equipment
for the textile industry.
An unwinder composed of an axis of rotation is provided, around which turns
a roller over which the coil unwinds, and a brake which slows down the
rotation of this roller. An winder composed of an axis of rotation, is also
provided around which turns a roller onto which the coil winds, and a motor.

The motor is of known type. It activates the winder in order to wind the cloth
on. It is controlled by a motor control.
A storage computer is a computer of known type which stores on the one
hand the positions and on the other hand the types of defects encountered
on the coil. The positions are given by an encoder and by an inspection
computer according to techniques known in the inspection field. The types of
defect are given by the inspection computer.
The camera is of known type. It has either a matrix image sensor, which
simultaneously captures illuminations on a number of lines of photosensitive
points of the image sensor and supplies a signal representing illuminations
received by the said photosensitive points, or a linear image sensor which
simultaneously captures the illuminations on a single line of photosensitive
points of the sensor, points which follow one another in a straight line. The
camera supplies an electrical signal representing illuminations which arrive at
each of the photosensitive points of its electronic sensor.
A digitizer receives the illumination-representing signal coming from the
camera and supplies a digital signal representing the said illuminations.
A vertical differentiation means compares, by subtraction, the digital values of
the illumination-representing signals originating from the same photosensitive
point of the image sensor, between two successive view captures.
The vertical differentiation means thus receives the image-representing
signals coming from the electronic camera and sends a signal representing
the difference, for each photosensitive point of the camera, of the image-
representing signals originating successively from the said point.

These illumination-representing signals are supplied by the digitizer. The
vertical differentiation means supplies a signal representing this difference. It
is of known type, being composed for example of either a correspondence
table or a comparator. Preferentially, it adds a constant value to the said
difference so that all the values obtained remain positive.
A horizontal addition means adds the successive differences coming from the
vertical differentiation means. The number of differences added is one, two or
a number of units and is supplied by the inspection computer.
A threshold means sends threshold-crossing signals for the values coming
from the horizontal addition means which exceed, plus or minus, values
supplied by the inspection computer.
A fringe extractor means separates the threshold-exceeding signals for the
points near the edges of the coil, at a distance less than a value given by the
inspection computer. The fringe extractor applies, to the signals coming from
the camera image sensor photosensitive points which receive an image from
these fringes, particular processing in order to detect therein specific defects.
The horizontal differentiation means compares, by subtraction, the digital
values of successive points of each line of illumination values on the line of
the image sensor, values supplied by the digitizer. It supplies a signal
representing this difference.
The vertical addition means adds the successive differences coming from the
horizontal differentiation means for each image sensor point. The differences
added therefore always correspond to the same pair of photosensitive points
of the image sensor. The number of differences added is one, two or a
number of units and is supplied by the inspection computer 17.

The threshold means sends threshold-crossing signals for the values coming
from the vertical addition means which exceed values supplied by the
inspection computer.
The inspection computer receives the input information, and processes it in
order to supply at least one list of defects of each coil length, said list
including the positions located on the one hand in the direction of movement,
also called "length", and on the other hand in the direction perpendicular to
this movement, also called "width", and the types of defect.
An encoder continuously supplies data from the length of the coil which has
already passed under the camera. It can be reset to zero either by the user,
or by the inspection computer.
A frequency detection means composed of a resonant phase-locked loop
circuit which receives the signal coming from an optical sensor which
observes an area of the moving material which is smaller than the pitch of the
lattice on which the camera is synchronised. For example the optical sensor is
a photodiode, in the optical field of which the threads of the inspected cloth
or the points of a lattice of coating points run past one by one.
The optical sensor sends a signal representing the speed of movement of the
material by its frequency. The resonant phase-locked loop circuit constitutes
an example of a means for automatically controlling the phase of the signal
coming from the optical sensor.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows schematically a process of inspection of a moving object.
Figure 2 - shows a schematic diagram of the apparatus implementing the
method.
Figure 3 - shows four modes of sync cycles generated in two channels of
each of the two cameras according to the invention.
DETAILS DESCRIPTION OF THE INVENTION
The apparatus (Figure 2) is implemented on AVR family of Micro-controllers
which has analog input channels for performing analog to digital conversion.
The Analog voltage signal from the plant containing analog speed signal is
digitalized which is used to calculate a current line rate using an established
relation. If the current line rate differs from the previous line rate by a pre-
decided tolerance margin, the camera is being updated with the new line
rate. This operation is repeated at a rate of 2 KHz. The apparatus is also
provided with a LCD Display which continuous displays the current line rate
and exposure time. Manual switches are provided to set the exposure time
externally.
Apart from the manual exposure control, the apparatus is also capable of
controlling exposure time automatically by taking the average gray value for
an image as input and then it adjusts the exposure time accordingly, the
apparatus comprises a synchronization means capable of generating a sync
cycle in four different modes in two individual channels (i.e. for top and

bottom camera independently). In the timing cycles shown in figure 3, the
total time period which is also known as line capture period is denoted by TP,
the exposure time by TEX and line transfer time by TLT
Figure - 3 shows the following possible modes:
Case 1 : Uniform Exposure and Variable Line Transfer Time
Case 2 : Uniform Exposure and Variable Line Transfer Time
Case 3 : Variable Exposure and Uniform Line Transfer Time
Case 4 : Variable Exposure and Variable Line Transfer Time
The apparatus computes the line capture time from the speed signal. While
generating these sync, overriding of line capture cycles is automatically
eliminated. Thus, the new cycle only begins when the previous cycle ends or
starts.
Benefits of the invention
> Improved image quality in terms of less blur/distortion thereby increasing
efficiency of defect detection.
> The independent of main program hence CPU time of the CRM is saved.
> Operating at speed (typically 05 mS)
> Continuous feedback for current line rate and exposure time is possible
> Independent exposure control for different lighting condition can be
maintained.

WE CLAIM
1. A method in an apparatus for improved web inspection of rolling coils in cold
roll mills. The method comprising the steps of:
- determining the rolling Speed from the Analog voltage signal representing a
speed signal;
- inputting the analog signal into a microcontroller for converting and
outputting corresponding digital signal;
- establishing a relation between current rolling speed and line rate;
- calculating a current line rate based on the established relation;
- updating at least one of two cameras corresponding to the current line rate
when it differs from the previous line rate beyond a threshold value;
- providing a display device to exhibit the current line rate including the
cameras exposure time;
- automatically controlling including adjusting the camera exposure time based
on the average gray value of the image, if the previous line rate deviates
from a threshold tolerance valve;
- generating a sync cycle in different modes in the timing cycles through two
each channels of at least solid two cameras.
2. The method as claimed in claim 1, wherein at least one camera comprises a
top and a bottom camera, and wherein the cameras have each different
exposure time.

3. The method as claimed in claim 1, wherein a new line capture cycle begins
only after the completion of the previous line captured cycle in order to
eliminate overlapping cycle.
4. The method as claimed in claim 1, wherein the camera exposure can be
manually fixed
5. The method as claimed in claim 1, wherein the exposure control can be
implemented in automatic mode by taking average grey value as the
feedback.

ABSTRACT

The invention relates to a method in an apparatus for improved web inspection of
rolling coils in rolling mills. The method comprising the steps of determining the
rolling Speed from the Analog voltage signal representing a speed signal; inputting
the analog signal into a microcontroller for converting and outputting corresponding
digital signal; establishing a relation between current rolling speed and line rate;
calculating a current line rate based on the established relation; updating at least
one of two cameras corresponding to the current line rate when it differs from the
provisions line rate beyond a threshold value; providing a display device to exhibit
the current line rate including the cameras exposure time; automatically controlling
including adjusting the camera exposure time based on the average gray value of
the image, if the previous line rate deviates from a threshold tolerance valve;
generating a sync cycle in different modes in the timing cycles through two each
channels of at least solid two cameras.