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Claims  |
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What is claimed is:
1. A method of inspecting the surface of a running object comprising the
steps of scanning the surface of a running object with a spot light source
through an optical scanning means, condensing said light reflected on said
surface of said running object, detecting a regularly reflected component
of said light at the point of said condensation, simultaneously detecting
irregularly reflected light components at a plurality of predetermined
positions in the vicinity of said condensation point, summing the detected
light components derived at said respective positions, comparing said sum
with a predetermined reference value, and producing an output representing
a defect when said total deviates from said reference value, thereby
making it possible to detect said defect discriminatively without being
affected by the manner in which said surface of said object is finished.
2. A method of inspecting the surface of a running object comprising the
steps of scanning the surface of a running object with a spot light
source, condensing said light reflected on said surface of said running
object, detecting a regularly reflected component of said light at the
point of said condensation, simultaneously detecting irregularly reflected
light components at a plurality of predetermined positions in the vicinity
of said condensation point, comparing a predetermined reference value with
the absolute value of the difference between the output current of at
least two of said irregularly reflected light receiving photoelectric
converter elements disposed punctually symmetric with respect to said
condensation position, and producing an output representing a defect when
said difference exceeds said reference value, thereby making it possible
to detect a defect discriminatively.
3. A method of inspecting the surface of a running object comprising the
steps of scanning the surface of a running object with a spot light
source, condensing said light reflectd on said surface of said running
object, detecting a regularly reflected component of said light at the
point of said condensation, simultaneously detecting irregularly reflected
light components at a plurality of predetermined positions in the vicinity
of said condensation point, comparing individual components of said
irregularly reflected light with a predetermined reference value, and
producing a signal representing a defect when an individual value of a
light component exceeds said reference value without any change in the
absolute value of the light components at least at two of said irregularly
reflected light positions which are symmetric with respect to said
condensation point, thereby making it possible to detect a defect
discriminatively.
4. An apparatus for inspecting the surface of a running object for defects
comprising
means for continuously scanning a spot of light over the surface of said
running object,
means for condensing the light reflected from said surface as a result of
such scanning,
a first detector positioned at a point at which regularly reflected light
will be projected by said condensing means,
a plurality of second detectors disposed at predetermined positions in the
vicinity of said first detector so as to detect irregularly reflected
light, and
control means responsive to the outputs from said first and second
detectors for generating a flaw indicating signal,
said control means including first means for detecting a difference between
the outputs of at least two of said second detectors which are disposed
symmetrically on diametrically opposite sides of said first detector and
second means responsive to an output of said first means in excess of a
predetermined threshold for generating a first flaw indicating signal.
5. An apparatus as defined in claim 4 wherein said first means comprises a
subtractor receiving the output of two of said second detectors.
6. An apparatus as defined in claim 5 wherein said second means comprises a
first comparator.
7. An apparatus as defined in claim 4 wherein said control means further
includes third means for individually comparing the outputs of at least
one of said second detectors with a predetermined threshold, fourth means
for providing an output in response to the detection by said third means
of the output of said second detector exceeding said predetermined
threshold, and fifth means responsive to the output from said fourth means
and no output from said second means for generating a second flaw
indicating signal.
8. An apparatus as defined in claim 7 wherein said third means comprises a
second comparator connected to the output of said second detector.
9. An apparatus as defined in claim 7 wherein said third means comprises a
plurality of second comparators each connected to an output of a
respective one of said second detectors.
10. An apparatus as defined in claim 9 wherein said fourth means comprises
an OR gate having inputs connected to the outputs of said second
detectors.
11. An apparatus as defined in claim 10 wherein said fifth means comprises
an AND gate having one input connected to the output of said OR gate and
an inhibiting input connected to the output of said second means.
12. An apparatus as defined in claim 7 wherein said control means further
comprises sixth means for adding the outputs of at least two detectors of
the group of first and second detectors, seventh means for comparing the
output of said sixth means with a predetermined reference value, and
eighth means responsive to the output of said seventh means and no output
from said second means for generating a third flaw indicating signal.
13. An apparatus as defined in claim 12 wherein said sixth means comprises
a signal summing circuit connected to the outputs of said first detector
and a second detector.
14. An apparatus as defined in claim 12 wherein said sixth means comprises
a signal summing circuit connected to the outputs of said first detector
and at least two second detectors.
15. An apparatus as defined in claim 4 wherein said control means furter
comprises third means for adding the outputs of at least two detectors of
the group of first and second detectors, fourth means for comparing the
output of said third means to a predetermined reference value, and fifth
means responsive to the output of said fourth means and no output from
said second means for generating a second flaw indicating signal.
16. An apparatus as defined in claim 15 wherein said third means comprises
a signal summing circuit connected to the outputs of said first detector
and a second detector.
17. An apparatus as defined in claim 15 wherein said third means comprises
a signal summing circuit connected to the outputs of said first detector
and at least two second detectors.
18. An apparatus for inspecting the surface of a running object for defects
comprising
means for continuously scanning a spot of light over the surface of said
running object,
means for condensing the light reflected from said surface as a result of
such scanning.
a first detector positioned at a point at which regularly reflected light
will be projected by said condensing means,
a plurality of second detectors disposed at predetermined positions in the
vicinity of said first detector so as to detect irregularly reflected
light, and
control means responsive to the outputs from said first and second
detectors for generating a flaw indicating signal,
said control means including first means for comparing the output of at
least one of said second detectors with a predetermined threshold and
third means for generating a first flaw indicating signal in response to
the detection by said first means of the output of said second detector
exceeding said predetermined threshold.
19. An apparatus as defined in claim 18 wherein said first means comprises
a plurality of first comparators each connected to an output of a
respective one of said second detectors and said second means comprises an
OR gate having inputs connected to the outputs of said first comparators.
20. An apparatus as defined in claim 19 wherein said control means further
comprises third means for adding the outputs of at least two detectors of
the group of first and second detectors, fourth means for comparing the
output of said third means to a predetermined reference value, and fifth
means responsive to the output of said fourth means and no output from
said second means for generating a second flaw indicating signal.
21. An apparatus as defined in claim 20 wherein said third means comprises
a signal summing circuit connected to the outputs of said first detector
and a second detector.
22. An apparatus as defined in claim 20 wherein said third means comprises
a signal summing circuit connected to the outputs of said first detector
and at least two second detectors.
23. An apparatus for inspecting the surface of a running object for defects
comprising
means for continuously scanning a spot of light over the surface of said
running object,
means for condensing the light reflected from said surface as a result of
such scanning,
a first detector positioned at a point at which regularly reflected light
will be projected by said condensing means,
a plurality of second detectors disposed at predetermined positions in the
vicinity of said first detector so as to detect irregularly reflected
light, and
control means responsive to the outputs from said first and second
detectors for generating a flaw indicating signal,
said control means including first means for adding the outputs of at least
two detectors of the group of first and second detectors and second means
for comparing the output of said first means to a predetermined reference
value to generate a first flaw indicating signal when said output exceeds
said reference value.
24. An apparatus as defined in claim 23 wherein said first means comprises
a signal summing circuit connected to the outputs of said first detector
and a second detector.
25. An apparatus as defined in claim 23 wherein said first means comprises
a signal summing circuit connected to the outputs of said first detector
and at least two second detectors.
26. An apparatus as defined in claim 23 wherein said control means further
includes third means for detecting the difference between the outputs of
at least two of said second detectors which are disposed symmetrically on
diametrically opposite sides of said first detector and fourth means
responsive to an output of said third means in excess of a predetermined
threshold for generating a second flaw indicating signal.
27. An apparatus as defined in claim 26 further including fifth means for
blocking said first flaw indicating signal in response to generation of
said second flaw indicating signal.
28. An apparatus as defined in claim 27 wherein said fifth means comprises
an AND gate. |
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Claims |
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Description |
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FIELD OF THE INVENTION
This invention relates to a method and an apparatus for automatically
detecting by optical means flaws, color shading, and the like on the
surface of metal plates, plastic boards, and the like which may occur
during the manufacturing processes thereof.
DESCRIPTION OF THE PRIOR ART
A well-known conventional device for detecting the defects on the surface
of metal by flying spot scanning is so constructed that the surface of a
running object is scanned in the transverse direction with a beam from a
spot light source through a rotary mirror or vibrating mirror, and the
light beam reflected from the mirror, after being again reflected on a
cylindrical concave mirror, is converged and converted into an electrical
signal by means of a photoelectric element for the purpose of detection of
the flaws in the running surface. In such a device, if the object is a
mirror-polished lustrous one, the light beam is reflected thereon
regularly and converged at a point thereby to detect in the form of
variations in quantity of regularly reflected light any defects that might
exist on the surface of the running object.
On the other hand, if the surface of the running object is grind-finished
in the form of hair lines or has a dull finish, the reflected light does
not converge a single point, but is scattered over a certain area.
Further, in such a case, a considerable texture signal is produced in the
device for receiving the regularly reflected light. As a result, the
disadvantage of such a device is that defects that may exist on the
surface of the running object may not be detected by the regularly
reflected light receiver because of the presence of the texture signal,
depending on the nature of the defects.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method
of and apparatus for detecting with high sensitivity all the defects that
may exist on the surface of an object regardless of whether such surface
is mirror-polished or dull.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram showing a plan view of an embodiment of the
present invention;
FIG. 2 is a side view of the embodiment of FIG. 1;
FIG. 3 is a diagram showing the distribution of light reflected and
received in the presence of various types of defects for various types of
finished surfaces of an object to be inspected;
FIG. 4 is a block diagram showing a circuit for discriminatively detecting
various types of defects according to the present invention; and
FIG. 5 is a diagram showing waveforms produced from various parts of the
circuit of FIG. 4.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the drawings, in FIGS. 1 and 2 reference numeral 1 designates
the surface of an object running at a predetermined speed and numeral 2
designates a polyhedral rotary mirror the axis of which is positioned
perpendicular to the surface of the running object so as to scan it in the
transverse direction by means of a light beam from the spot light source
7. Numeral 3 designates a scanned locus and numeral 4 designates the
scanning light beam reflected on the surface of the running object 1 which
is applied to a cylindrical concave mirror 5 having its rotational axis
positioned perpendicular to the surface of the running object and in
coincidence with the central axis of the same mirror 5, so that light beam
4 reflected from a mirror surface will return to point P.sub.1 on the
central axis of the polyhedral rotary mirror 2.
As will be understood from the device illustrates in FIGS. 1 and 2 of the
drawings, if the running object is lustrous, i.e., has a mirror finish,
the entire light beam reflected after the scanning operation will converge
at point P.sub.1, and therefore, it is possible to detect in the form of
current the light beam reflected on the surface of the object by means of
a photoelectric element positioned at point P.sub.1. In this way, any
defects on the surface of the object are detected in the form of
variations in detected current as the result of variations in the
reflection factor of the surface and reflection angle.
Further, according to the present invention, provision of one or a
plurality of photoelectric elements at such points as P.sub.2 and P.sub.3
at some distance from P.sub.1 permits the reception of scattered light due
to changes in texture or irregularities on the surface of the object, and
thereby makes it possible to detect flaws which otherwise would be hard to
detect only with regularly reflected light. This advantage is further
emphasized in the case of objects having hairline finish or other types of
dull surfaces. In other words, since a significant amount of light
reflected from a dull surface of an object will be scattered over a wide
area without being converged at point P.sub.1 in spite of the cylindrical
concave mirror 5, the provision of a photoelectric element only at point
P.sub.1 will not detect sufficient variations in the amount of light to
indicate the presence of flaws because a considerable texture signal is
produced due to the quality of the surface of the object, thereby to
reduce the detecting sensitivity.
By providing additional independent photoelectric elements at points
P.sub.2, P.sub.3 and so forth to receive the irregularly reflected light,
it is possible to collect a relatively large amount of light reflected
from various defects which otherwise would be hard to detect with means
for reception of only regularly-reflected light; whereas, scattered light
due to the irregularities of the surface is collected in a relatively
small amount, resulting in a wide variety of defects being detected.
Explanation will be made here of an exemplary method and circuit for
processing a signal for flaw detection according to the present invention
with reference to FIGS. 3, 4, and 5.
The distribution of light reflected on various types of surfaces to be
inspected is shown in FIG. 3. Symbol A.sub.1 of diagram (a) of FIG. 3
shows the distribution of light reflected normally on a mirror-finished
surface, while symbol B.sub.1 illustrates the distribution of light
reflected normally on a dull surface, such as a dull hair-line finished
surface. The shadowed areas under curves A.sub.1 and B.sub.1 represent the
total quantity of reflected light which are shown to be equal to each
other, the small absorption of light in the inspected surface being
neglected to simplify the description. As will be seen, the light
reflected on the dull finished surface, even if normal, is scattered so
that the light-receiving level is not only low but varies continuously
from one detecting position to the next, with the result that variations
in the output current of a regularly-reflected light receiving element do
not provide an exclusively effective means for flaw detection.
In order to obviate such a disadvantage, if a plurality of
irregularly-reflected light receiving elements Ph.sub.2, Ph.sub.3, . . .
are provided at points P.sub.2, P.sub.3, . . . , shown in FIG. 2, a more
accurate measure of the total amount of reflected light can be obtained in
conjunction with the measure of reflected light at point P.sub.1. By
combining the light levels detected at the various detecting points,
smaller variations in the current output of the light detecting system
result from variations in the quality of the surface to be inspected, thus
making it easier to detect any variations in output level due to a flaw.
Referring to diagram (b) of FIG. 3, showing the displacement of the
reflected light due to the shape of defects, the peak of the reflected
light B.sub.1, which would normally be received at point P.sub.1, is
transferred to the neighborhood of point P.sub.2. The shifting of the
point of receipt of the light reflected from the defect occurs due to the
shape of the defect which causes a change in the angle of reflection of
the light so as to transfer the reflected light B.sub.2 from the receiving
point P.sub.1 to the receiving point at P.sub.2 or thereabouts. This
applies to the case where the flaw under consideration has a larger
diameter than the scanning beam spot. In such a case, displacement may
alternatively occur toward point P.sub.3 or laterally. In the case of a
mirror finished surface where the light level A.sub.1 is shifted with
respect to the point P.sub.1, the fact that the output current of the
regularly-reflected light receiving element is considerably reduced, may
permit detection of such a defect in shape larger in diameter than the
beam spot; however, in the case of a dull finished surface, it will be
seen from FIG. 3 of the drawing that the light level B.sub.1 is already so
low that variations in the output current of the light receiving element
resulting from a shift of the light level from B.sub.1 to B.sub.2
represents such a small difference in detected light that such variations
are not easily discriminated from the variations in light receiving level
due to the surface finish. Further, the output current of the light
receiving element also may be reduced by other defects in the surface of
the object or defects of the element.
However, it will be noted from diagram (b) of FIG. 3 that a significant
difference in light level is detected at points P.sub.2 and P.sub.3 when
the light level is shifted from B.sub.1 to B.sub.2, so that defects in
shape can be detected by detecting the difference in output current of the
light receiving elements Ph.sub.2 and Ph.sub.3 disposed at point P.sub.2
and P.sub.3, that is, .vertline.I.sub.2 - I.sub.3 .vertline.. In other
words, in the case of a defect in shape having a diameter larger than that
of the light beam spot, an increased amount of light received by any of
the irregularly-reflected light receiving elements Ph.sub.2, Ph.sub.3, . .
. in the neighborhood of the regularly-reflected light receiving element
Ph.sub.1 results in a reduced amount of light received by an
irregularly-reflected light receiving element disposed symmetric with
respect thereto. Thus, in the event that there is no abnormality, the
difference between the output currents of the irregularly-reflected light
receiving elements, such as elements P.sub.2 and P.sub.3, which are
symmetric with each other is zero; whereas, there is a difference
therebetween if there is a defect in shape, thus making it possible to
easily detect such a defect in shape.
Generally, the scanning beam has a spot diameter of about 1mm to 5mm and in
diagram (c) of FIG. 3 the curve B.sub.3 shows the distribution of the
amount of light received by the light receiving elements from a dull
surface when there is a scratch or other defect on the surface in the form
of an uneveness having a diameter not larger than that of the scanning
light beam spot. Light reflected on such a small defect is generally
scattered substantially equally right and left or upward and downward. In
the case under consideration, the amount of light received is reduced at
point P.sub.1, while light received at points P.sub.2 and P.sub.3 is
increased by substantially the same amount. The difference between the
output current of the light receiving elements Ph.sub.2 and Ph.sub.3, that
is, .vertline.I.sub.2 - I.sub.3 .vertline. is almost zero. For this
reason, if the difference between the output currents at points P.sub.2
and P.sub.3 is determined to be almost zero in coincidence with a separate
detection of an increase in the output of the irregularly-reflected light
receiving elements Ph.sub.2 and/or Ph.sub.3, it indicates the presence of
a scratch or other defect in the form of uneveness on the surface of the
inspected object.
The diagram (d) in FIG. 3 shows the distribution of light received from a
defect such as a smear, stain, or other color spot. Generally, the curve
representing the distribution of received light due to increased light
absorption by a defect is similar to, but uniformly lower in level than,
the normal light distribution curve. Such a defect in the form of a color
spot may be discriminated when it is found that the total output of the
light-receiving elements Ph.sub.1, Ph.sub.2, Ph.sub.3 and so forth, or the
output of the element Ph.sub.1 in the case of mirror finished surface, has
decreased and that the difference between the outputs of the symmetric
elements Ph.sub.2 and Ph.sub.3, that is, .vertline.I.sub.2 - I.sub.3
.vertline. is substantially zero.
Even though the above description involves a pair of irregularly-reflected
light receiving elements arranged vertically upwardly and downwardly of
the regularly reflected light receiving element, as shown in FIG. 2,
accurate flaw detection also may be achieved by following the same
processes through use of similar irregularly-reflected light receiving
elements (Ph.sub.4, Ph.sub.5, . . . ) positioned horizonally or at
different angles. Also, an optimum spaced relationship between the
light-receiving elements depends on the diameter of expansion of the beam
spot, the size of the apparatus or the light receiving elements and the
distance between the reflection and receiving points, all of which are
factors contributing to variations in the amount of received light shown
in FIG. 3.
The manner in which the above-mentioned various types of defects are
detected discriminatively will be described below with reference to the
circuit shown in the block diagram of FIG. 4.
Symbols P.sub.1 to P.sub.5 represent points at which the light receiving
elements Ph.sub.1 to Ph.sub.5 are positioned, which elements may be
arranged vertically horizontally or at other angles symmetrically with
respect to the regularly-reflected light receiving element Ph.sub.1.
Comparators CP.sub.1 to CP.sub.6 compare the analog signals or output
current received from the light receiving elements with reference levels
being set for each comparator so as to produce a signal with the logic
level or "1" when the analog signal exceeds a certain threshold voltage.
Symbols S.sub.1 to S.sub.5 and S.sub.2 ' to S.sub.5 ' represent switches
for selectively switching a given one or plurality of light receiving
elements depending on the state of the surface finish or the kind of
defects to be inspected. An output is produced at the terminal 1 when a
defect in shape larger in diameter than the beam spot is discovered; an
output is produced at the terminal 2 in the presence of a color spot
defect; and an output is produced at the terminal 3 in the presence of a
scratch or other uneveness smaller in diameter than the beam spot.
The diagram of FIG. 5 shows waveforms produced at various sections of the
circuit of FIG. 4 during the period of one transverse scanning of the
surface of the object to be inspected. In the drawing, waveform (a)
represents the output current I.sub.1 produced from the
regularly-reflected light receiving element Ph.sub.1 ; waveform (b)
represents the output current I.sub.2 produced from the light receiving
element Ph.sub.2 ; waveform (c) represents the output current I.sub.3
produced from the light receiving element Ph.sub.3 ; waveform (d)
represents the output I.sub.2 - I.sub.3 produced from the subtractor 11,
and waveform (e) represents the output of synthetizer 10 which serves to
sum the outputs of the detectors to produce a signal in the form of
I.sub.1 + I.sub.2 + I.sub.3 + . . . . On vertical line D.sub.1 for each
waveform there is shown the effect on the signal of a defect in shape
larger in diameter than the beam spot. On line D.sub.2 a color spot defect
affects the signal, and on line D.sub.3 an uneveness defect including a
scratch affects the signal.
First of all, a description will be made of the operation for detecting a
defect in shape. The difference between the output currents I.sub.2 and
I.sub.3 of the light receiving elements Ph.sub.2 and Ph.sub.3 positioned
to be symmetric with respect to the light receiving element Ph.sub.1 is
detected by the subtractor 11. When the difference .vertline.I.sub.2 -
I.sub.3 .vertline. exceeds a certain level set by the comparator CP.sub.6,
an output signal is produced by the comparator at terminal 1. In the
process, the absolute input value .vertline.I.sub.1 - I.sub.3 .vertline.
may be obtained either by extracting the absolute value of the result of
the subtraction I.sub.2 - I.sub.3 or by applying the result of the
subtraction I.sub.2 - I.sub.3 and by defining the upper and lower limits
of signal variations by a window comparator. In any event, it will be
obvious from FIG. 5 that output variations of the symmetrically disposed
light receiving elements Ph.sub.2 and Ph.sub.3 are in opposite phase and
therefore the other defects are offset in the direction of
.vertline.I.sub.2 - I.sub.3 .vertline., with the result that only the
defect in shape is emphasized to facilitate the direction thereof.
Secondly, the operation for discriminating and detecting a color spot
defect, such as a smear or stain, will be explained. The switches S.sub.1
to S.sub.5, or only S.sub.1 in the case of mirror finished surface, is
closed and the outputs I.sub.1, I.sub.2, . . . are added by the
synthetizer 10. The reference level of the comparator CP.sub.1 is set in
such a manner that a defect signal is applied to AND gate 14 and produced
at the terminal 2 when the output in the form of I.sub.1 + I.sub.2 +
I.sub.3 + . . . of the synthetizer is reduced below the highest level
associated with the absence of any defect.
The composite output signal (I.sub.1 + I.sub.2 + I.sub.3 . . . ) from
comparator CP.sub.1 is reduced most in the case of a color spot defect, as
seen in diagram (d) in FIG. 3, whereas it undergoes little change as it is
offset in the case of a defect in shape or other small uneveness, as shown
by waveform (e) of FIG. 5, the last color spot detect being inhibited by
the defect-in-shape output from comparator CP.sub.6 at the input to AND
gate 14. In other words, a color spot defect signal is produced from the
output of AND gate 14 at the terminal 2 when the composite output is
reduced and the output I.sub.2 - I.sub.3 is zero.
Finally, explanation will be made of the operation for discriminating and
detecting other small uneveness defects, such as scratches. First of all,
at least one of the irregularly-reflected light receiving elements
Ph.sub.2, Ph.sub.3, . . . , other than the light receiving element
Ph.sub.1, are selected by the switches S.sub.2 ' to S.sub.5 ' and the
reference levels of the comparators CP.sub.2 to CP.sub.5 are set in such a
manner as to produce a defect output when the output current I.sub.2 to
I.sub.5 of the light receiving elements is higher than in the absence of a
defect.
As a result, a defect output is produced from the OR circuit 12 when any of
the selected outputs of the light receiving elements Ph.sub.2, Ph.sub.3,
Ph.sub.4 and so on are increased. In view of the fact that the
above-mentioned defect output may be also produced in the case of defect
in shape, discrimination therefrom is made by inhibiting the last small
uneveness defect output by use of a defect-in-shape output from comparator
CP.sub.6 to the input of AND gate 13. In other words, when any of the
outputs I.sub.2, I.sub.3, I.sub.4 and so on exceeds a certain level and
the equation .vertline.I.sub.2 - I.sub.3 .vertline. .apprxeq. 0 is
satisfied, a small uneveness defect is detected discriminatively from the
output of AND gate 13 at terminal 3.
It will be understood from the above explanation that according to the
present invention almost all the defects that may exist on the surface of
a running object can be detected without fail and also it is possible to
inspect successfully the dull surface of the object.
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