Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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OPTICAL IMAGING SYSTEM FOR FABRIC SEAM DETECTION
BACKGROUND OF THE INVENTION
The present invention is directed to an optical im:~ging system for fabric seam
detection in which there is no contact between the sensor and the web of material being
monitored.
During the course of manufacturing various webs of material, for example,
textiles, such materials are typically processed in the form of long strips. Since it is often
time consuming and costly to start up equipment and machinery for processing such long
webs of material, such machinery frequently operates continuously for long periods of time
during which it is desirable that the web being processed be formed into one continuous
element. However, such a continuous web of material is invariably fabricated by piecing
together shorter lengths of such web material. For example, in textile manufacturing, the
continuous web of fabric processed by the textile m~çhinery is formed by sewing together
shorter lengths of fabric.
The result of such a procedure for forming a single continuous web of material
is that seams or other periodic patterns are introduced into the web at periodic patterns are
introduced into the web at periodic but unpredictable intervals. The detection of the seams
or other periodic patterns is of great concern to the web manufacturers, since such seams or
discontinuities are generally somewhat thicker than the web of material being processed and
can frequently damage the processing machinery.
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An example of the prior art approach to the detection of seams or other
protrusions on a running web is shown in United States Patent No. 3,748,414, issued on July
24, 1973, to Holm. The device disclosed in that patent utilizes a sensor which bears against
the web surface. A pair of spaced parallel rollers mounted on a common pivoted bracket
engage opposite sides of the web. One roller is positioned opposite the sensor and the other
roller is positioned opposite a fixed position roller. A mechanism is provided for urging the
pivoted rollers against the web surfaces such that a change in web thickness will
automatically calibrate the sensor to the roller gap by self-adjustment of the pivoted rollers.
The system of Holm has the disadvantage that it is a contacting type of web
measurement system with the attendant drawbacks that such a contacting system ismechanical in nature and thus prone to frequent adjustment and/or malfunction and that
such mechanical systems can have the effect of ~ ging or discoloring the web they are
used to monitor.
United States Patent No. 2,091,522, issued on August 31, 1937, to Perry,
shows a seam or defect detector for sheet material for use in connection with a measuring
table over which the sheet material is drawn and utilizes a mechanical system which
contacts the web and indicates when a seam or defect has been detected. The seam detector
of Perry, however, suffers from the same drawbacks as that of Holm, because it is a
mechanical contact type system and therefore requires frequent adjustment. It also uses
operating parts which may become worn or inoperative.
A non-contacting approach for detecting splices in the web of a printing press
is shown in United States Patent No. 4,901,577, issued February 20, 1990, to Roberts. In
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that patent, the web in a high speed printing press passes between a transmitter and receiver.
The transmitter produces ultrasonic sound which is received by the receiver after attenuation
by the web. The received ultrasonic sound is converted into an oscillating electrical signal
of corresponding frequency which, after being amplified and filtered, is impressed upon a
peak detector and comparator. As long as no splices are found in the web, the reference
potential remains undisturbed. In the event a splice is detected, a periodic pattern is created
by the comparator in its oscillating signal. That periodic pattern is detected and is utilized
to trigger a m~rking device for hllpalling a suitable mark to the web both before and after
the splice.
A method and apparatus for analyzing the formation of a web of material by
generating a formation index lltili~ing a linear array CCD camera is shown in United States
Patent No. 4,857,747, issued August 15, 1989, to Bolton et al. In that patent, a light source
is located beneath and perpendicular to the direction of movement of a web of material and
the web passes between the light source and the CCD camera. The camera generates signals
corresponding to the light intensity across the entire width of the web as compared to a
single point for sample measurements. The signals are then processed to generate a full
width formation profile of the web in real time. Utilizing a pulse generator which is coupled
to a roller which engages the web, the sc:~nning rate of the camera is regulated such that it
corresponds to the speed of the web.
The Bolton et al. system is directed to a web formation system, and not a web
inspection system, as is the present invention. In the Bolton et al. system, all light intensity
variations are measured for a given area and not just the areas that exceed preset threshold
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limits.
Thus, in the Bolton et al. system, the light intensities across the entire web are
converted into analog electrical signals which are in turn converted into digital signals.
The Bolton et al. system is not directed to a web inspection system and can
only be used in instances where the web allows light to pass through it. Thus, for many
applications in which the web is not transparent, the Bolton et al. system is not operative.
Furthermore, the Bolton et al. system is complex and utilizes a large amount of electronic
equipment for operation.
SUMMARY AND OBJECTS OF THE INVENTION
In view of the foregoing, it should be apparelll that there still exists a need in
the art for a system for detecting a seam or other periodic pattern or closely spaced periodic
objects in a moving web in which an optical mech:~ni~m is utilized in a simple and precise
manner to accomplish the detection. It is, therefore, a primary object of this invention to
provide a system for detecting a seam or other periodic pattern in a moving web of material
which is characterized by simple electronic circuitry and which has particular application as
a fabric seam detector.
More particularly, it is an object of this invention to provide an optical
im~ging system for detection of a seam or other periodic pattern which has simple and
reliable electronic circuitry which does not require frequent alignment or costly
components.
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Still more particularly, it is an object of this invention to provide an opticaldetector for detecting seams or other periodic patterns in a web of material in which a CCD
camera is utilized to produce an alternating current signal which varies depending upon
whether a seam or other periodic pattern is present.
Another object of the present invention is to provide a reliable and relatively
inexpensive optical im:~ging system for detecting a fabric seam for use as a peripheral sensor
for textile machinery.
Briefly described, these and other objects of the invention are accomplished by
providing a charge-coupled-device (CCD) line scan camera which captures the seam or
other periodic pattern image of a moving web. The web is illllmin~ted from the side in
order to create maximum contrast between the seam or periodic patterns and their shadows.
The video output from the line scan camera is processed by an analog band pass filter to
produce an alternating current signal which has a large magnitude when a seam or other
periodic pattern is present and a small magnitude when a seam or periodic pattern is absent.
The AC signal from the filter is then converted to a DC signal having a level proportional to
the magnitude of the filtered AC signal. This DC signal is compared to a predetermined
threshold value in order to determine the presence or absence of a seam or other periodic
pattern. When a seam or other periodic pattern is detected, the comparison circuitry drives
the ~pl~pliate output circuitry, such as a bell, relay, etc. for interfacing with the web
processing machinery controls.
Accordingly, the invention herein comprises a method for detecting a seam or
other closely spaced periodic object in a moving web having top and bottom surfaces,
comprising the steps of: illumin~ting the top surface of said moving web; receiving light
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reflected by said moving web as a result of said step of illumin~ting using camera means
aligned across the width of said moving web; converting said reflected light into
corresponding electrical signals; reading out said electrical signals from said camera means
for processing; filtering the converted corresponding electrical signals to produce an
alt~rn:~ting current signal having a magnitude proportional to the presence of a seam or other
closely spaced periodic object; processing said altçrn:~ting current signal to produce a direct
current signal corresponding to said reflected light; and comparing said direct current signal
to a predetermined reference value to determine whether a seam or other closely spaced
periodic object has been detected.
The invention further comprises an apparatus for detecting a seam or other
closely spaced periodic object in a moving web having top and bottom surfaces comprising:
a source of illumination for illllmin~ting the top surface of said moving web, a camera
aligned across the width of said moving web for receiving light reflected off of said top
surface of said moving web for converting said light into electrical signals which are read
out for processing; filter means for filtering said read out electrical signals to produce an
altçrn~ting current signal having a magnitude proportional to the presence of a seam or other
closely spaced periodic object; cil~;uill y for processing said alternating current signal to
produce a direct current signal proportional to said received light; and comparison circuitry
for comparing said direct current signal to a predetermined reference value to determine if a
seam or other periodic object has been detected.
With these and other objects, advantages and features of the invention that
may become hereinafter apparent, the nature of the invention may be more clearlyunderstood by reference to the following detailed description of the invention, the appended
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claims and to the several drawings attached herein.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic diagram showing the application of the present
invention to a fabric seam detector using non-contacting optical im~ging techniques;
5Figure 2 is a schematic diagram of the lateral cross-section of a fabric seam
showing lighting contrast for the seam thread;
Figure 3 is a block diagram showing the a~paldLus of the present invention;
Figure 4a shows a typical video signal of the line scan imager where a seam or
other periodic pattern has been detected;
10Figure 4b shows a typical video signal of the line scan imager in which a seamor other periodic pattern has not been detected; and
Figure 5 shows a lighting configuration for the present invention when used
with a fabric web having a seam.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now in detail to the drawings wherein like parts are de~ign~tecl by
like reference numerals throughout, there is shown in Figure 1 a schematic diagram of the
application of the present invention as a fabric seam detector. It should be understood that
the system of the present invention can be used in applications where the image to be
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detected exhibits a linear periodicity, such as in the case of textile seam, laser weld,
diffraction gratings or corrugated object. In addition, the system of the present invention
can be used to detect a two-dimensional periodicity such as would be needed to evaluate the
density of a two-dimensional weave, the texture of paper or defects in plywood. However,
for purposes of explanation, the present invention is described in the context of a fabric
seam detector using non-contacting optical im:~ging techniques.
Accordingly, there is shown in Figure 1 a web of fabric material 10 which is
moving in one of two directions denoted by the arrow past a camera 12 which may
preferably be a line scan CCD camera. However, it should be understood that other types of
video cameras can be utilized, including, but not limited to, cameras lltili7ing tubes, such as
VIDICON cameras, visible and non-visible light sensitive cameras and image intensified
cameras. Visible light sensitive cameras are sensitive to the visible light range (red, orange,
yellow, blue, violet) and are typically used in home video apparatus. Non-visible light
sensitive cameras are sensitive to light energy that falls beyond the visible limit of red
(infra-red) and/or beyond the visible limit of violet (ultra-violet). Non-visible (infra-red)
cameras are typically used in aerial agricultural surveys to locate concentrations of diseased
foliage due to their lower emission of infra-red energy.
Image intensified cameras can operate at extremely low light levels due light
amplification which occurs within a vacuum tube image inten~ifier placed at the front of the
camera. Such cameras are used for night time surveillance. Current military night vision
goggles utilize image int~n~ifiers which also operate in the non-visible, infra-red light range.
The camera 12 is situated in a plane parallel to that of the moving fabric 10
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such that it can readily detect a seam 14 which extends across the web of fabric 10
perpendicular to the direction of travel of the fabric 10. Typically, such seams 14 include a
plurality of threads 16 which are used to secure two pieces of fabric into a larger piece of
fabric. The output form the line scan camera is fed to the image processing circuilly shown
in Figure 3.
Figure 2 shows the position of a light source 22 to one side of the fabric web
10 such that, when such light impinges on the plurality of threads 16, a shadow 20 is
formed.
The light source 22, as is also shown in Figure 5, is set at an angle oblique tothe top surface of the moving web of fabric 10 so as to create the maximum contrast
between the seam threads 16 and their shadows.
Referring now to Figure 3, there is shown the image processing Cil1Uill.y for
use with the present invention. The video output from the CCD camera 12 is processed by a
filter 30, which may be an analog bandpass filter. The filter 30 is tuned to the expected or
predetermined spatial frequency of the seam. Since the camera converts spatial frequency to
a time frequency, the filtering is accomplished utili~ing standard electronic audio frequency
or radio frequency methods. The signal which is output from the filter 30 is an alternating
current signal which has a large magnitude when a seam is present and a small magnitude
when a seam is not present. At this point in the processing circuitry, signal variations due to
the printed pattern of the fabric, fabric thread defects, such as weave or knit and the
brightness or reflectivity of the fabric are elimin~tecl or have been significantly attenuated.
The AC signal output from the filter 30 is then converted to a DC signal with a
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- 10-
level proportional to the magnitude of the filtered AC signal by passing the AC signal
output from the filter 30 through a rectifier 32. The rectifier 32 is preferably a half-wave
rectifier (although it could also be full wave rectifier) which creates a time varying signal
that is always positive (or negative) with respect to ground. The rectified signal is then fed
to an integrator 34 which serves to produce a signal that represents a summation of all of the
individual imaged threads 16 which make up a seam 14. If there are few or no seam threads
16 present, then the magnitude of the signal produced by the integrator 34 is low.
Otherwise, the magnitude of the signal produced by the integrator 34 is high.
The output from the integrator 34 is then applied to a comparator 36 where it
is compared to a manually or automatically adjustable reference voltage. The result of this
comparison is used to drive an alarm or display 38 or to provide a signal for interfacing with
the controls of the fabric processing machinery.
Figure 4a shows a typical video output from the line scan camera 12 which
identifies the presence of a seam.
Figure 4b shows a typical video output from the camera 12 in which no seam
has been imaged.
Figure 5 shows the lighting configuration for the present invention in which
the camera 12 is used to capture the seam image. The camera 12 is placed perpendicular to
the direction of motion of the fabric 10. The linear imager of the camera 12 is arTanged to
be parallel to the axis of the seam, as shown in Figure 1. The optical image entering the
camera 12 is filtered through a long pass infrared filter in order to allow the camera 12 to be
"color blind" and to prevent the seam detector system from incorrectly identifying a densely
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printed fabric pattern within the visible light range. Fabric dyes are generally selected to
impart a certain visible color or hue to the fabric. These dyes typically do not impart
"color" beyond the visible range. Therefore, fabric which has been printed with a visible
pattern will have little or no pattern when observed by an infra-red sensitive camera.
It will be apparent to those of ordinary skill in the art that because the optical
im~ging system described herein relies upon electronic filtering of a one-dimensional signal,
it can be used to provide an inexpensive, rapid, non-contacting method of seam detection for
textile manufacturing, apparel production, industrial weaving or other webs in which the
location of seams is required.
The system described herein performs its detection function based upon web
thread topography and ignores color variations. However, such configuration may be
unacceptable for use with textured fabric. In order to use the system of the present
invention for the detection of seams in textured fabric, the infrared long pass optical filter
used with the camera 12 may be replaced by an infrared short pass filter. The system of the
present invention, when thus configured, will detect seams based upon color variations but
will be insensitive to texture variations.
The system described herein can be further improved by the addition of a
cylindrical lens between the web and camera lens or between the camera lens and the
camera. This cylindrical lens would minimi7e false triggering due to periodic texture
(holes) present in some fabric such as some double knits. The effect of this lens is to
defocus or blur these small holes in a direction parallel to the seam threads while
m~int~inin~; sharp focus perpendicular to the threads. This technique would result in the
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blurring of holes without the blurring of seam threads.
Although only a preferred embodiment is specifically illustrated and described
herein, it will be appreciated that many modifications and variations of the present invention
are possible in light of the above teachings and within the purview of the appended claims
without departing from the spirit and intended scope of the invention.
