Linear flaw detector

A linear flaw detector for coated surfaces. An incident beam is provided and sensed above and below the theoretical specular angle. Where a flaw occurs, the reflected signal changes in intensity. By sensing above and below the specular angle, the signal corresponding to the flaw is accented in reference to the signals from the background. The signal corresponding to the flaw may then be separated from background noise and an output provided.

BACKGROUND OF THE INVENTION 
In coating moving surfaces, such as paper webs, certain defects are 
produced if the coating is not applied uniformly by the coating blade. 
Such coating flaws or linear flaws may be produced by an obstruction in 
the coating nip, dilatency of the coating or coating inconsistency. 
The present decection methods used are primarily the observations of the 
machine operators. This is totally unsatisfactory in that in the case of a 
paper web, it normally would move as fast as 2,000 ft. per minute and it 
is very difficult to detect and report coating defects. Electronic devices 
to detect flaws are available. One such device includes 
shoulder-to-shoulder silicon detectors. The detectors may or may not scan 
a predetemined portion of the width of the web. The sensing elements of 
such detectors generally provide an incident beam such as from a light 
emmitting diode and the reflected beam is collected by a photocell. When 
the beam of light traverses a gloss variation, the reflected signal is 
greatly reduced. The photocell senses this light level change at the 
specular angle, automatically compensating for gloss variations, and an 
output signal is provided. 
In coating paper webs, ideally a uniform coating is applied, resulting in 
uniform gloss over the entire coated surfaces. However, in practice in 
addition to the coating flaws described above other variations in gloss 
appear such as caused by variations in formation, flocculation, platelets, 
etc. Generally, it is only desired to detect linear flaws which require 
rejection of the coated web while overlooking variations in gloss which 
are not serious enough to require rejection of the coated web. 
The problem with sensing elements that detect variations in gloss is that 
variations relate not only to linear flaws, but also to most other 
variations in the paper stock being coated, and in the coating. 
It is important therefore to provide a sensing unit which would detect only 
those flaws which determine whether or not the coating being applied 
warrants that the coated stock be accepted or rejected. Thus, of the flaws 
which occur in a coating, rejectable flaws must be distinguished from 
non-rejectable flaws. Present sensors are not able to accomplish this 
since they "see" all flaws which results in an unacceptably high rate of 
rejection. That is, in an automatic inspection system, there is a tendency 
to report all variations in gloss as flaws. 
SUMMARY OF THE INVENTION 
In the present invention, a sensing unit is provided which selectively 
detects flaws on a coated surface, such as a paper web. The flaws to be 
detected may correspond to a first type of reflected energy from a diffuse 
surface in contrast to a second type of reflected energy such as from a 
background specular surface. 
Reflected light energy from a coated surface is sensed either above or 
below or both above and below the theoretical specular angle of 
reflection. (See Handbook of Pulp and Paper Technology, 2nd Ed., p. 658, 
for definition of reflective intensities and specular angles.) 
When sensing above the spectral angle, the reflection from the coated 
surface or background will be muted or diminished and the flaw will appear 
brighter. When sensing below the spectral angle, close to the angle of 
maximum reflection from the paper, the flaw will appear much darker than 
the background illumination. In the case of clay-coated stock, the 
background illumination is reduced also, but to a lesser degree than the 
flaw. The sensing unit of this invention discriminates among various flaws 
on a coated surface and provides a signal corresponding to "rejectable" 
flaws such as linear flaws. Normally, such rejectable flaws are diffuse, 
while the coated stock surface is specular. 
The invention in one aspect provides a sensing unit which detects the 
reflected energy offset from the theoretical spectral angle. The signal 
corresponding to the flaw, such as a diffuse reflection from a specular 
surface is accented with respect to the other reflected variations. The 
signal corresponding to the flaw is easily distinguished from other 
background signals. 
In another aspect of the invention, the sensing unit receives the reflected 
energy from either side of the theoretical spectral angle, converts the 
received reflected energy into signals and combines the signals to 
generate an output corresponding to the detected flaw. 
In a preferred embodiment of the invention, a sensing unit is provided to 
detect flaws on a surface such as a coated and/or calendared surface. 
The sensing unit includes two sensors adapted to receive reflected energy 
above and below the theoretical spectral angle. The surface per se with 
myriad small variations in gloss produces signals within a narrow range 
which signals may be considered noise. The flaw to be detected produces an 
accented signal when viewed at the proper angles, having a much sharper 
slope, either negative or positive, which differentiates from the noise. 
The signals are combined and distinguished from the noise and an output 
provided indicating a flaw.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention will be described in reference to the detection of linear 
flaws on the coated surface of a moving web, and more particularly to 
detecting fine scratches on 35-80 gloss magazine stock. 
As is well known, paper stock to be coated moves from roll to roll with the 
coating being applied such as by a doctor blade. To detect flaws which 
would require rejection of the stock, machines are presently available. 
The type stock and ultimate use will determine what is acceptable in terms 
of flaws, such as area flaws, small flaws, linear flaws, etc. Simply, all 
detection is based on gloss measurement made over the web of the moving 
coated paper by an array of sensors. The array scans the moving web and 
imbalances in gloss are sensed and a signal provided. The signal is 
processed such that an operator may identify the location, frequency, and 
length of the flaws if desired. 
The sensing unit of the present invention may be used with any flaw 
detection machine, such as a Leigh Control Systems Electronic Machine 131. 
Accordingly, only the sensing unit will be shown in detail. 
As shown in FIG. 1, a sensing head 10 is disposed over a coated web 12 
moving at 2000 feet per minute such as magazine stock of between 35-80 
gloss. The output from the sensing head is transmitted to a signal 
manipulation control and display unit 14, which typically provides 
information as to the length of the flaw, location, etc. Although only one 
sensing head will be described in detail, it is to be understood that 
normally a plurality of such sensing heads are arrayed across the web of 
paper on a suitable support. In this example, the sensing head transverses 
the web in a reciprocating manner approximately eight inches in one 
direction. 
In FIG. 2, a sensing unit is shown in greater detail and comprises a 
housing 18, a light source 20 such as a CM-20-3 manufactured by Chicago 
Miniature Lamp Works, Chicago, Ill., and photoresistors 22 and 24 such as 
CL705H1, manufactured by Clairex Corp., Mount Vernon, N.Y. The light 
energy from source 20 strikes as an incident beam the coated surface of 
the web 12 at an intensity of about 700 ft-candles, and is reflected. 
This incident beam strikes the coated surface in a direction parallel to 
the movement of the web 12. The photoresistors 22 and 24 receive the 
reflected energy at an intensity of about 30 and 200 ft-candles 
respectively. As shown, the photoresistors 22 and 24 lie in the same 
plane. The light source 20 and photoresistors 22 and 24 are fixedly 
secured in the housing 18 at the angles illustrated. As shown, they are 
moulded in place, but may be secured in any suitable manner. The bottom 
plane of the housing and the photoresistors are approximately 3/16 inches 
from the coated surface. 
Referring to FIG. 3, the sensing unit 18 is shown in dotted lines 
superimposed over a linear flaw or scratch in the coated surface. The 
function of the positioning of the photoresistors 22 and 24 is represented 
graphically in FIG. 4. 
FIG. 4 depicts the various signals corresponding to the coated surface of 
the web 12 including the linear flaw as shown in FIG. 3. In FIG. 4, the 
linear flaw of FIG. 3 is viewed at the specular angle. The signal 
corresponding to the linear flaw viewed at the spectral angle is difficult 
to distinguish from the other signals corresponding to variations in 
gloss. 
When viewed at the same incident intensity I above the spectral angle at 
67.degree.-68.degree., a bright signal against a generally dark background 
is provided. In FIG. 4b, the signal corresponding to the linear flaw is 
more accented. 
When viewed at the intensity below intensitybelow the spectral angle at 
56.degree.-57.degree., a very dark signal against a generally dark 
background is provided. In FIG. 4c, the signal corresponding to the linear 
flaw is more accented. 
As shown in FIGS. 4b and c, the background signals or noise remain 
substantially the same while the signal corresponding to the linear flaw 
changes considerably. In this illustration, the accenting of the signal 
corresponding to linear flow is essentially because the flaw represents a 
diffuse reflection from an otherwise specular background. The signals 
received by the photoresistors 22 annd 24 are algebraically combined 
electronically to provide a sharply differentiated signal from which is 
eliminated background noise, and processed as a signal corresponding to a 
detectable flaw, as shown in FIG. 4d. 
Referring to FIG. 2, the signals from the photoresistors 22 annd 24 are at 
a value of 1.0 to 2.0 volts, depending upon the actual value of the gloss. 
The signal-to-noise ratio is approximately 2:1 above the spectral angle 
and 4:1 below the spectral angle. The signals are algebraically summed and 
amplified in the summing circuit 30 and output in a range of from 2.0 to 
8.0 volts. The signal-to-noise ratio is now about 5:1. The lower voltage 
level will vary and the signal-to-noise ratio will vary such as to 16:1 
depending upon how the background noise cancels out. The level 
discriminator is set at 5 volts where only those signals over 5 volts will 
be output to signal manipulator and control 14. 
In this example, the signal input to be analyzed in the level discriminator 
has a positive going slope as shown in FIG. 4d. The signals-to-noise 
(background) ratio is approximately 5:1, and therefore easily handled. 
Depending upon the surface analyzed, the signal may be a negative going 
slope or both. 
The above invention has been described in reference to detection of linear 
flaws, which substantially diffuse. It has been particularly designed in 
reference to magazine stock of a 35-80 gloss and for the detection of fine 
scratches or linear flaws thereon, the incident angle preverably being 
62.degree. and the lower reflected angle being 56.degree.-57.degree.; and 
the higher reflected angle between about 67.degree.-68.degree.. Another 
incident angle and corresponding reflected angles that may be used are 
70.degree., 61.degree.-62.degree. and 72.degree.-73.degree. respectively. 
The specific angles set forth provide the highest signal-to-noise ratio. 
For the type of stock described in the preferred embodiment, the effect 
described occurs if the angle of incidense, hereafter referred to as `i`, 
is limited to angles between 60.degree. and 70.degree.. The corresponding 
low and high reflecting angles are then defined as i-8.degree. and 
i+2.degree. respectively. For different paper types and coating types, the 
range of i may be different, as may be the angular separation of the low 
and high reflecting angles from the theoretical specular angle. 
The invention may also be used to find holidays on a transparent release 
coating on a cheap grade of stock such as "Sly-Off 23" manufactured by 
Dow-Corning Corp., Midland, MI. on calendared kraft stock, wherein the 
incident angle would be approximately 70.degree. and the higher reflected 
angle 67.degree.-68.degree., and the lower reflected angle between about 
72.degree.-75.degree.. Also, it may be used to detect smudges and 
scratches on a heavy textured white art paper such as used for sketching, 
etc. where the incident beam would be approximately 70.degree. and the 
higher reflected beam would be about 40.degree. and the lower reflected 
beam about 72.degree.. 
Common to all of the above examples is that the flaw being detected and its 
reflective qualities differentiate substantially from the reflective 
qualities of the surface per se. In the preferred embodiment, it was 
described as detecting diffuse flaws from a specular surface. Preferably, 
the change in surface reflective characteristics would be at least 2:1 on 
a signal level. However, situations can exist in which the flaw is a 
relatively specular area on a generally specular background, such as a 
spot of oil on a coated surface. Contrariwise, the flaw may be a 
relatively diffuse area on a generally diffuse background, such as a 
mechanical cut on a matte surface. In cases such as these, the change in 
surface reflective characteristics between the flaw and the background can 
yield a ratio barely greater than unity (1:1) when viewed at the 
theoretical specular angle. In cases such as these, angles of incidence 
and low and high reflectance may be empirically determined in accordance 
with the present method to yield a usefully large signal difference. 
Further, the sensors of the present invention may be fixedly mounted above 
a moving web and the sensors adapted for limited oscillatory movement to 
scan the web. They may be fixed for reciprocating motion across the web 
and the sensors fixedly secured or a combination thereof. 
Although described in reference to detecting the flaws in a coated surface 
by moving the viewing angle away from the spectral angle, either above or 
below and combining the signals from both, it is obvious that one may be 
used either above or below or preferably both to get the greatest signal 
differentiation. 
In the illustrative embodiment, the direction of incident and reflected 
angles were described as parallel to the direction of the moving web. If a 
matte surface is scanned, the angles may be transverse to the direction of 
the moving web. The orientation will depend upon the surface scanned and 
can be determined simply by positioning the unit to detect the maximum 
signals received.