Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PATENTS, APPLICATIONS AND PUBLICATIONS PERTINENT TO
THE INVENTION
As far as we know, there is available the follow-
ing prior art document pertinent to the present invention:
Japanese Patent Provisional Publication
No.60-11,157 dated January 21, 1985.
The contents of the above-mentioned prior art
document:will be discussed hereafter under the heading of
the "BACKGROUND OF THE INVENTION".
FIELD OF THE INVENTION
The present invention relates to an apparatus for
detecting an inner surface flaw of each pipe constituting
a pipeline.
BACKGROUND OF THE INVENTION
S'~
15 ~ n apparatus for detecking c~ fl~w o.E ~ E~ipe with
the use of electromagnetic inductlon is publicly known.
For example, an apparatus for detecting an outer surface
flaw of a pipe with the use of electromagnetic induction is
disclosed in Japanese Patent Provisional~Publication
No.60-11,157 dated January 21, 1985, which comprises: at
least one cylindrical primary coil, a high frequency
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electric current generator, a plurality of probe coils,
i.e., a plurality of cylindrical secondary coils, a
multiplexer and a signal processing circuit (hereinafter
referred to as the "prior art").
The at least one primary coil surrounds a pipe
to be inspected, and is coaxial with the pipe. In other
words, the pipe is coaxially inserted into the at least
one primary coil. The inner peripheral surface of the
at least one primary coil is spaced apart from the outer
peripheral surface of the pipe by a prescribed distance.
The high frequency electric current generator
supplies high frequency electric current to the at least
one primary coil.to cause the at least one primary coil
to produce an AC magnetic field, and the magnetic flux
density of the AC magnetic field varies.in response to
an outer surface flaw of the pipe.
The plurality of secondary coils are arranged
along the outer surface of the pipe at prescribed intervals
in the circumferential direction oE the pipe in the close
vicinity of the at.least one primary coil. The axis of
each of the pl.urality of secondary.coils is arranged at
right angles to the axis of the at.least one primary coil.
Each of the plurality of secondary coils produces an ~C
voltage proportional to the density of a.component
-- 3
~2~
parallel to the axial direction of each of the plurality
of secondary coils, of the magnetic flux interlinking
with each of the plurality of secondary coils, of the AC
magnetic field of the at least one primary coil. The
plurality of secondary coils constitute, together with
the at least one primary coil, a detecting probe, and the
detecting probe is moved relative to the pipe in the axial
direction of the coil.
The multiplexer repeatedly takes out the AC
voltage signals from the plurality of secondary coils
sequentially in the order of arrangement of the plurality
of secondary coils at a prescribed sampling cycle period T.
The signal processing circuit comprises a
synchronous detector, a delay.circuit and an adder.
The synchronous detector sequentially and
synchronously. detects the AC voltage signals from the
plurality of secondary coils, taken out by the multiplexer,
with the high frequency electric current from the high
frequency electric current generator a~ the re.~erence
signal, thereby eliminat.in~ noise signals from -the AC
voltage signals Erom the plurality oE secondary coils, and
at the same time, converting the AC voltage signals into
DC voltage signals. Each value of the thus converted DC
voltage signals is proportional to the depth of an outer
~s~
surface flaw of the pipe.
The delay circuit causes delay of the DC voltage
signals from the synchronous detector by a period of time
equal to the above-mentioned sampling cycle period T.
The adder adds the thus delayed DC voltage signal
from the delay circuit to a DC voltage signal from the
synchronous detector in the next sampling cycle period for
each of the plurality of secGndary coils, thereby obtaining a
DC voltage signal with a minimized detection error in the
pipe axial direction of the outer surface flaw of the pipe
for each of the plurality of secondary coils.
In the above-mentioned prior art, it is possible to
detect the presence and the depth of the outer surface flaw
of the pipe with a minimized detection error in the axial
direction of the pipe, by sequentially detecting a
differential voltage signal proportional to the depth of the
outer surface flaw of the pipe, between the DC voltage signal
from the adder for each of the plurality of secondary coils,
on the one hand, and the bias voltage signal thereof
resulting from the inclination or other cause of each of the
plurality of secondary coils, on the other hand.
According to the prior art, it is possible to detect
an outer surface flaw of the pipe without overlooking any
other surface flaw in the pipe axial direction, even when
carrying
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out detecting operation of the outer surface flaw of the
pipe while moving, relative to the pipe, the detecting
probe comprising the :at least one primary coil and the
plurality of secondary-coils at a high speed in the axial
direction of the pipe.
The above-mentioned prior art, which relates to
the detection of an outer surface flaw of a pipe, is also
applicable to the detection of an inner surface flaw of
each pipe constituting a pipeline, by causing the de-tect-
ing probe comprising the at least one primary coil and the
plurality of secondary coils to travel through the pipe-
line. However, when detecting any of the outer surface
flaw or the inner surface.flaw of the pipe, the prior art
has the following drawbacks.
More specifically, the magnetic flux of the AC
magnetic.field of the at least.one primary coil, which is
distributed in the axial direction of the pipe in the
space near the outer surface or the inner surface of the
pipe, comes into an outer or.inner surface flaw oE the
p.ipe, if any, and as a result, the magnetic flu~ density
in the space near the pipe.portion containing the outer
or inner surface flaw shows a normal.distribution having
. a peak of.the lowest density at the position of the flaw
i center. This means.that, the magnetic flux has the lowest
density at the position of. the flaw center,
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~i;5~35
and consists only of a componen-t parallel to the axial
direction of the at least one primary coil. On the other
hand, the magnetic flux has the highest density at the
position distant from the flaw center, and consists only
of a component paralleito the axial direction of the at
least one primary coil. In the middle between the posi-
tion of the flaw center and the position distant from the
flaw center, the magnetic flux density becomes higher
according as the distance from the position of the flaw
center increases. The magnetic flux is analyzed into a
component parallelto the axial direction of the at least
one primary coil and a component at right angles to the
axial direction of the at least one primary coil, and the
latter component increases according as.the.distance from
the position of the flaw center increases to reach the
maximum, and then decreases. Therefore, in a space near
the pipe portion containing an outer or inner surface flaw,
the highest density of the component of the magnetic flux,
which component is at right angles to the axial direction
of the at least.one primary coil, exists in the middle
between the position of the flaw ccnter and the pO9itlOn
distant from the Elaw center.
The difference in the magnetic.flux density
between the lowest density at the position of the flaw
center and the highest density at the position distant
-- 7 --
~2&~9S
from the flaw center corresponds to the depth of the
flaw. The highest density of the component of the magnetic
flux, which component is at right angles to the axial
direction of the at least one primary coil,at a position
between the position of the flaw center and the position
distant from the flaw center also corresponds to the depth
of the flaw. Since, in the above-mentioned prior art, the
plurality of secondary coils are arranged so that the axis of
each of the plurality of secondary coils is at right angles
to the axis of the at least one primary coil, each of the
plurality of secondary coils.senses a component at right
angles to the axial direction of the at least one primary
coil, i.e., a component parallel to the axial direction of
each of the plurality of secondary coils r of the magnetic
flux of the AC magnetic field of the at least one primary
coil, which magnetic flux interlinks with each of the
plurality of secondary coils, and.produces an.AC voltage
proportional.to the density of the above-mentioned component.
Therefore, it-is possible to detect the depth of the outer
surface flaw.or the inner surface 1aw of the pipe, by
processing the AC voltage signal produced by each Oe the
plurality of secondary coils.
However, when a first flaw, a second flaw and
a third flaw each having a respective depth are present
in this order on the outer or inner surface of the pipe
at close intervals in the axial direction of the pipe,
the density of the magnetic flux in the axial direction
of the pipe, of the AC magnetic field of the at least one
primary coil, in the space near the pipe portion containing
these flaws, shows a distribution in which three normal
distributions of the magnetic flux density corresponding
respectively to these three flaws partly overlap in the
axial direction of the pipe. In such a.distribution of
the magnetic flux density, a distribution of the magnetic
flux density at a position between the center position of
the first flaw and a position opposite to the second flaw
relative to the first flaw, and a distribution of the
magnetic flux density at a position between.the center
position.of the third flaw and a position opposite to the
second flaw relative to.the third flaw., are not affected by
the distribution of the~magnetic flux density corresponding
to the second flaw. Therefore, the highest densities of
the components at right angles to the axial direction of
~ the at least one primary coil.of the magnetic flux in these
two intermediate positions correspond respectively to the
depth of the first Elaw and the depth of the third :Elaw.
On the contrary, a distribution of the magnetic flux density
at a positio~ between the.center..po~ition of the first
flaw and the.center position of the second flaw, and a
distribution of the magnetic flux density at a position
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between the center position of the second flaw and the
center position of the third flaw, are a~fected by the
distributions of the magnetic flux density corresponding
respectively to the first flaw and the third flaw. There-
fore, the highest densities of the components at right
angles to the axial direction of the at least one primary
coil of the magnetic flux in these two intermediate posi-
tions do not accurately correspond to the depth of the
second flaw. Thus, the depth of the second flaw cannot be
accurately detected by the prior art.
Also when four or more flaws are present on the
outer or inner surface of the pipe at close intervals in
the axial direction of the pipe, the same problem as
described above is posed for the flaws other than those
at the both ends.
Under such circumstances, there is a strong
demand for the development of an apparatus for detecting,
with the use of electromagnetic induction, an inner surface
flaw of each pipe constituting a pipeli.ne, which, when
detecting an inner surfac~ ~law oE each pipe cons~ituting
the pipeline, permits accurate de-tection of the depth of
each of three or more inner surface.flaws of the pipe even
when these inner surface flaws exist on the inner surface
of the pipe at close intervals irl the axial direction of
5~
the pipe, but an apparatus provided with such properties has
not as yet been proposed.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to
provide an apparatus for detecting, ~ith the use of
electromagnetic induction, an inner surface flaw of each pipe
constituting a pipeline, which, when detecting an inner surface
flaw of each pipe constituting the pipeline, permits accurate
detection of the depth of each of three or morè inner surface
flaws of the pipe even when these inner surface flaws exist on
the inner surface of the pipe at close intervals.
In accordance with one of the features of the present
invention, there is provided an apparatus *or detecting a
surface flaw of a pipeline using electromagnetic induction,
which comprises:
a pig capable of travelling along a plurality of pipes
forming a pipeline; at least one primary coil mounted on the
pig, the primary coil being arranged to interact electro-
magnetically with the pipe, and a peripheral surface of the
primary coil being spaced apart from a confronting peripheral
surface of the pipe by a prescribed distance to form a
clearance space therebetween; a high frequency electric current
generator mounted on the pig, for ~upplying high Erequenc~
electric current to the primary co.il to cause the primary coil
to produce an AC magnetic field, a magnetic flux density of the
AC magnetic field varying in response to a surface flaw of the
pipe; a plurallty of secondary coils arranged at prescribed
intervals in the circumferential direction of the pipe in the
MLS/lcm 11
clearance space, so that each of the secondary coils produces
an AC voltage signal proportional to a magnetic flux density
of an interlinking magnetic flux component, the interlin~ing
magnetic flux component being of the AC magnetic field produced
by the primary coil; a multiplexer mounted on the pig, the
plurality of secondary coils being coupled to the multiplexer,
and the multiplexer taking out sequentially the AC voltage
signals from the secondary coils in the order of arrangement
of the secondary coils; and a signal processing circuit,
mounted on the pig, comprising a synchronous detector, a moving
average circuit and a flaw detecting circuit; the synchronous
detector detecting the AC voltage signals from the plurality
of secondary coils, taken out by the multiplexer, and
converting the AC voltage signals into DC voltage signals; the
moving average circuit moving-averaging the DC voltage signals
in a prescribed number from the s~vnchronous detector for each
of the secondary coils to obtain bias voltage signals contained
in the DC voltage signals for each of the secondary coils; and
the flaw detecting circuit detecting a differential voltage
signal proportional to a depth of the surface flaw of the pipe
as a function of the DC voltage signals; the improvement
wherein: the signal processlng circuit ~urther includes a
detectlon error correcting circuit for ampllPying the
differential voltage signal from the flaw detecting circuit for
each oP the plurality of secondary coils, wherein the detection
error correcting circuit includes means for setting an
amplification factor of the detection error correcting circuit
at a value inversely proportional to a value of the bias
, . ~
~ MLS/lcm 12
....... . .
~5~
voltage signals from the moving average circuit for each of the
secondary coils, whereby a detection error in the differential
voltage signal detected by the flaw detecting circuit, caused
by a fluctuation in distance between each of the secondary
coils, and the confronting peripheral surface of the pipeline,
is corrected.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram illustrating the basic
structure of an embodiment of the apparatus of the present
invention for detecting an inner surface flaw of each pipe
constituting a pipeline;
Fig. 2 is a longitudinal sectional view illustrating
the positional relationship between the detecting probe which
is one of the components of the apparatus of the present
invention shown in Fig. 1 and a pipe to be inspected;
Fig. 3 is a transverse sectional view illustrating the
arrangement of the pair of primary coils and the
; MLS/lcm 13
plurality of secondary coils, which constitute -the detec-
ting probe shown in Fig. 2;
Fig. 4 is a timing chart illustrating the timing
for taking out an AC voltage from the plurality of
secondary coils by the multiplexer which is one of the
components of the apparatus of the present invention shown
in Fig. l;
Fig. 5 is a graph illustrating the relationship
- between the output voltage from the synchronous detector
of the signal processing circuit, which is one of the
components of the apparatus of the present invention shown
in Fig. 1, on the one hand, and the distance (~) between
the secondary coil and the inner surface of a pipe to be
inspected, on the other hand;
Fig, 6 is a graph illustrating the relationship
between the output voltage from the synchronous detector
of the signal processing ~ircuit, which is one of the
components of the apparatus of the present invention shown
in Fig. 1, on the one hand, and the depth (d) of an inner
surface flaw of a pipe to be inspected, on the other hand;
Fig. 7 is a graph illustrating the relationship
between the relative value of the output voltage from the
synchronous detector of the signal processing circuit,
which is one of the components of the apparatus of the
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present invention shown in Fig. 1, on the one hand, and
the distance (~) between the seco~dary coil and the inner
surface of a pipe to be inspected,; on the other hand; and
Fig. 8 is a graph illustrating the relationship
between the relative value of the output voltage from the
detection error correcting circuit of the signal processing
circuit, which is one of the components of the apparatus
of the present invention shown in Fig. 1, on the one hand,
and the distance (~) between the secondary coil and the
inner surface o a pipe to be inspected, on the other hand.
DETAILED DESCRIP~ION OF PREFERRED EMBODIMENTS
From the above-mentioned point.of view, extensive
studies were carried out to develop an apparatus for
detecting, with the use of electromagnetic. induction, an
inner surface flaw of each pipe constituting a pipeline,
which, when detecting an inner surface flaw of each pipe
constituting the pipeline, permits accurate detection of the
depth.of each of three or more.inner surac~ flaws of the
pipe even when these inner sur~ace flaws exist on the
inner surface of the pipe at close intervals in the axial
; direction of the pipe.
As a result, the following findings were obtained:
When a first flaw, a second flaw and a.third flaw each
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having a respective depth are present in this order on
the inner surface of the pipe at close intervals in the
axial direction of the pipe, the density of the magnetic
flux in the axial direction of the pipe, of the AC magnetic
field of the at least one primary coil, in the space near
the pipe portion containing these inner surface flaws,
shows a distribution in which three normal distributions of
the magnetic flux density corresponding respectively to
these three inner surface flaws partly overlap in the
axial direction of the pipe. Even in such a distribution
of the magnetic flux.density, the magnetic flux densities
at the center positions of the first, the second and the
third inner surface flaws correspond respectively to these
respective flaws. Therefore, the magnetic flux densities
at the center positions of the first, the second and the
third inner surface flaws present the lowest values corres-
ponding to the respective flaws, and each magnetic flux
consists only of a component parallel to the axial direc-
tion of the at least one primary coil. -The difference in
the magnetic flux density between the lowest density of
the magnetic 1ux at the center position o~ each of the
first, the second and the thirdinner surface flaws, on the one
hand,.and the highest density oL the.ma~netic flux at a
position distant from each of these flaws, on the other
hand, corresponds to the depth of each of the first, the
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second and the third inner surface flaws. Therefore, by
arranging a plurality of secondary coils so that the
axis of each of the plurality of secondary coils is
parallel to the a~is of the at least one primary coil,
each of the plurality of secondary coils senses a component
parallel to the axial direction of each of the plurality
of secondary coils, of the magnetic flux of the AC magnetic
field of the at least one primary coil, which magnetic fiux
interlinks with each of the plurality of secondary coils,
and produces an A~ voltage proportional to the density of
the above-mentioned component. Thus, it is possible to
accurately detect the depth of each of the first, the
second and the third inner surface.flaws of the pipe, by
processing the AC voltage signal produced by each of the
plurality of secondary coils.
The present invention was made on the basis of
the above-mentioned. findings. Now, an embodiment of the
apparatus of the present invention for detecting an inner
surface flaw of each pipe constituting a pipeline is
described with reference to the drawings.
. Fig. 1 is a block diagram illustrati~g the basic
structure of an embodiment o:E the apparatus of the present
invention for detecting an inner surface flaw.of each pipe
constit.uting a pipeline. As shown in Fig. 1, the apparatus
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5~i~5
of the present invention basically comprises a pig not
shown, a pair of cylindrical primary coils Pc, which may
be only one primary coil, mounted on the pig, a high
frequency electric current generator 2 mounted on the
pig, a plurality of cylindrical secondary coils Sl;
S~ mounted on the pig, a multiplexer 3 mounted on the
pig, and a signal processing circuit 4 mounted on the pig.
The pair of primary coils Pc and the plurali-ty of secondary
coils Sl, ... , SN form a detecting probe 1.
The pig is capable of travelling through a
pipeline in the axial direction of each pipe constituting
the pipeline.
The pair of primary coils Pc forming part of
the detecting probe 1 are arranged.at a prescribed
interval in the axial. direction. of.the pipe 14 as shown in
Fig. 2. The.pair of primary coils Pc are coaxial with
the pipe 14, and the outer peripheral.surfaces of the
pair of primary coils Pc are.spaced apart from the inner
peripheral surface of the.pipe 14 by a.prescribed distance.
The primary coils Pc are arranged in a p~:ir at a prescribed
interval in the axial direction of the pipe 14 for the
purpose of causing..the magnetic.~lux of the AC magnetic
field produced by the.pair.of primary coils Pc to be
distributed in.the axial direction.of.the pipe 14 in.the
space between the outer peripheral surfaces of the pair
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of primary coils Pc and the inner peripheral sur~ace of
the pipe 14. This allows the magnetic flux to effectively
interlink with the plurality of secondary coils Sl, ... .
SN, each of which senses a component parallel to the axial
direction of each of the plurality of secondary coils Sl,
..., SN, of the magnetic flux of the AC magnetic field
of the pair of primary coils Pc, and produces an AC
voltage proportional to the density of the above-mentioned
component. Therefore, by increasing the coil width of
the primary coil Pc, it suffices to provide only one
primary coil Pc.
The high frequency electric current generator 2
supplies high frequency electric current to the pair of
primary coils Pc to cause the pair of primary coils Pc to
produce an AC magnetic field. The magnetic flux density
of the AC magnetic.field of.the pair of primary coils Pc
varies in response to an inner surface flaw 14a of the
pipe 14. With.a view to increasing the detection~sensi-
tivity of the inner surface flaw of the.pipe 14 by the
plurality of secondary.coils Sl, .... , SN., it is desirable
to concentrate the magnetic flux of the AC magnetic field
of.the pair of primary coils Pc into.the space between the
outer peripheral surfaces of.thë pair of primary coils Pc
and the inner peripheral surface of the pipe 14 50 that
the magnetic flux penetrate9.only i~to the inner surface
.
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~5S95
portion of the pipe 14, thereby increasing the density of
the magnetic flux in that space. In general, the penetra-
tion depth d' of the magnetic flux of the magnetic field
of the primary coil into the material is determined from
the frequency f of high frequency electric current supplied
to the primary coil, electric conductivity ~ of the
material and the magnetic permeability ,u of the material,
and is expressed by the formula: ~ = 1/ ~ . It suffices
therefore to cause the high frequency electric current
generator 2 to generate a high frequency electric current
having a proper frequency in response to the material of
the pipe 14 so that the magnetic flux of the AC magnetic
field of the pair of primary coils Pc penetrates only
into the inner surface portion of the pipe 14.
As shown in Fig. 3, the plurality of secondary
coils Sl, ... , SN are arranged at prescribed intervals
in the circumferential direction of the pipe 14 between
the outer peripheral surfaces of the pair of primary coils
Pc and the inner peripheral surface of the pipe 14. The
axis of each of the plurality of secondary coils Sl, ....... .
SN is parallel to the axis of each of the pair of primary
coils Pc so as to permit an accurate detection by the
plurality of the secondary coils Sl, ..~ , SN, even when
three or more inner surface flaws 14a each having a
respective depth are present on the inner surface of the
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pipe 14 at close intervals in the axial direction of the
pipe 14, of the depth of each of these inner surface flaws
14a. Even when three or more inner surface flaws 14a each
having a respective depth are present on the inner surface
of the pipe 14 at close intervals in the axial direction
of the pipe 14, the lowest density of the magnetic flux of
the AC magnetic field of the pair of primary coils Pc at
the center position of each of these inner surface flaws
14a in the space near the portion of the pipe 14 contain-
ing these inner surface flaws 14a corresponds only to each
of these lnner surface flaws 14a, and the magnetic flux
consists only of a component parallel to the axial direction
of the pair of primary coils Pc. The difference in the
magnetic flux density between the lowest density of the
magnetic flux at the center position of ~ach of these
inner surface flaws 14a, on the one hand, and the highest
density of the magnetic flux at a position distant from
each of these inner surface flaws 14a, on the other hand,
corresponds to the depth of each of~these inner surface
flaws 14a. Therefore, by arranging the pl~rality of
secondary coils Sl, ... , SN as descxibed above, each of
the plurality of secondary coils Sl, ..., SN senses a
component parallel to the axial direction oE each of the
.plurality of secondary coils Sl, ... , SN, of the magnetic
flux of the AC magnetic field o the pair of primary coils
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~2~5595
P , which magnetic flux interlinks with each of the
plurality of secondary colls Sl, ..., SN, and produces
an AC voltage proportional to the density of the above-
mentioned component, thus, permitting an accurate detection
of the presece and the depth of each of these inner surface
flaws 14a.
When there is no inner surface flaw 14a on the
inner surface of the pipe 14, there is no change in the
density of the component parallel to the axial direction
of each of the plurality of secondary coils Sl, ... , SN, of
the magnetic flux of the AC magnetic field of the pair of
primary coils Pc, which magnetic flux interlinks with each
of the plurality of secondary coils Sl, ..., SN. Therefore,
each of the plurality of secondary coils Sl, ..., SN
produces a constant AC voltage. On the other hand, when
a distance between the outer peripheral surfaces of the pair
of primary coils Pc and the inner peripheral surface of
the pipe 14 is changed, the density of the above-mentioned
component parallel to the axial direction of each of the
pLurality of secondary coils Sl, .... SN, of the magnetic
flux of the AC magnetic field of the pair of primary coils
Pc is changed in response to the thus changed distance.
Therefore, each o~ the pluralit~ of secondar~ coils Sl, ...
SN produces an AC voltage corresponding to the thus changed
2~ distance.
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~65595
As is clear from the foregoing, when the detect-
ing probe 1 travels through the plpe 14 in the arrow
direction in Fig. 2 along with the travel of the pig until
the pair of primary coils Pc reach the inner surface flaw
S 14a of the.pipe 14, a secondary coil Sl closest to the
inner surface flaw 14a, for example, among the plurality
of second~ry coils Sl, ..., SN produces an AC voltage
corresponding not only to the depth of the inner surface
flaw 14a, but also to the.distance between the outer
peripheral.surfaces of the pair of primary coils Pc and
the inner peripheral surface of the pipe 14, i.e., between
the secondary coil Sl and the inner peripheral surface of
the pipe 14.
The multiplexer 3 repeatedly.takes out the AC
voltage signals from the plurality of secondary coils Sl,
SN sequentially in the order of.arrangement thereof at
a prescribed sampling cycle period ~ as shown in Fig~ 4.
Operations of the multiplexer 3 are controlled by control
signals from a multiplexer controller 6. The control
signal is created., as shown in Fig. 1, in the multiplexer
controller 6 on the basis o~ the high frequency electric
current from the high ~requency electric current gener~tor
2, which has been divided by a frequency ~ivi~er 5 into a
frequency having a prescr.ibed value.
.
The sampling cycle period rc of the AC voltage
. _ - 23 -
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~5S~S
signals from the plurality of secondary coils Sl, ..., SN
is set in accordance with conditions for the detection of
the inner surface flaw 14a, and usually ranges from 1/104
to 1/10 seconds.
S The signal processing circuit 4 basically
comprises, as shown in Fig. 1, a synchronous detector 9,
a moving average circuit 11, a.flaw detecting circuit 10
and a detection error correcting circuit 13, and has, in
addition, an amplifier 7, a noise filter 8 and a moving
average circuit controller 12.
~ The AC voltage signals from the plurality of
qsecondary coils Sl, ... , SN, taken out by the multiplexer 3
are amplified by the amplifier 7, then entered into the
synchronous detector 9 after.the preliminary elimination of
noise signals by the noise filter 8.
The synchronous detector 9 sequentially and
synchronously detects the AC voltage signals from the
plurality of secondary coils Sl, ..., SN, which have passed
through the amplifier 7 and the noise.filter 8, with the
high frequency electric current from.the high ~requency
electric current generator 2.as the reference signal,
thereby eliminating noise signals from the AC voltage
signals from the plurality of secondary coils Sl, ..., SN,
and at the same time, converting the AC voltage signals
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into DC voltage signals.
As described above, when.the pair of primary
coils Pc do not reach the inner surface flaw 14a of the
pipe 14, each.of the plurality of secondary coils Sl,
SN produces an AC voltage corresponding to the distance
between the outer peripheral surfaces of the pair of primary
coils Pc and the inner peripheral surface of the pipe 14,
i.e., between each of the plurality of secondary coils Sl,
~ SN and the inner peripheral surface of-the pipe 14.
When.the pair of primary coils Pc reach the inner surface
flaw 14a along with the travel.of the.detecting probe 1,
the secondary coil Sl closest to the inner surface flaw 14a,
for example, among the plurality of.secondary coils Sl,
SN produces an AC voltage corresponding not only to the
depth of the inner surface-flaw 14a, but also to the
distance between the secondary coil Sl and the inner
peripheral surface of the pipe 14. Therefore, the distance
between the secondary coil Sl and the inner peripheral
surface of the pipe ].4 appears as a bias voltage signal
of the DC voltage signal converted.by the synchronous detec-
tor 9 from the AC voltage signal froM the secondary coil Sl.
More specifically, when the pair of primary coils Pc do not
reach the inner.surface flaw 14a, the above-mentioned bias
voltage signal.for the secondary coil Sl is equal to the
DC voltage signal from the secondary coil Sl. The depth
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~iS59~
of the inner surface flaw 14a appears as a differential
voltage signal between the DC voltage signal from the
synchronous detector 9 cor^esponding to the AC voltage
signal from the secondary coil Sl closest to the inner
surface flaw 14a and the bias voltage signal therefor.
Fig. 5 is a graph illustrating the relationship
between the output voltage from the synchronous detector,
on the one hand, and the distance (~) between the outer
peripheral surfaces of the primary coils and the inner
peripheral surface of the pipe, i.e., between the secondary
coil and the inner peripheral surface of the pipe 14, on
the other hand. In Fig. 5, the ordinate represents the
value of the DC voltage signal from the synchronous detector
9, i.e., the value of the bias voltage signal, corresponding
to the value of the AC voltage signal from the secondary
coil Sl, for example, of the plurality of secondary coils
Sl, ...~ SN in the case where there is no inner surface
flaw 14a on the inner surface of the pipe 14. The abscissa
represents the distance ( Q ) between that secondary coil
Sl and the inner peripheral surface of the plpe 14. The
output voltage from the synchronous detector ~ represented
on the ordinate shows values wi.th the OtltpUt voltage at a
distance (Q ) of 30 mm as zero V. The high frequency
electric current supplied to the pair of primary coils Pc
has a frequency of 100 kHz.
- 26 -
~' .
~S~5
As is clear from Fig. 5, the output voltage from
the synchronous detector 9 in the case where there is no
inner surface flaw 14a decreases almost linearly according
as the distance ( Q ) between the secondary coil Sl and the
inner peripheral surface of the.pipe 14 increases. This
demonstrates that the value of the bias voltage signal
from the synchronous detector 9 varies in proportion to
the distance ( Q ) .
Fig. 6 is a graph illustrating the relationship
between the output.voltage from the synchronous detector
and the depth (d) of an inner surface flaw of the pipe.
In Fig. 6, the ordinate represents the value of the DC
voltage si~nal from the synchronous detector 9, correspond-
ing to the value of the AC.voltage signal from the secondary
coil Sl closest to the inner surface flaw 14a of the pipe
14, for example, of the plurality of.secondary coils Sl,
SN, in the case where the pair of pr.imary coils Pc reach
the inner surface flaw.14a. The abscissa represents the
depth (d) of the inner surface flaw 14a. The output voltage
from the synchronous detector 9 represented on the ordinate
shows values with the output voltage in the case where the
pair of primary coils Pc do not reach the inner surface
flaw 14a as zero V. The inner surface flaw 14a was
artificially made.by a drill. The inner surface flaw 14a
has a diameter of 30 mm... The distance (Q ) between the
~6S~5
secondary coil Sl and the inner peripheral surface of the
pipe 14 is 30 mm, and the high frequency electric current
supplied to the pair of primary colls Pc has a frequency
of 100 kHz.
5 As is clear from Fig.. 6, the output voltage from
the synchronous detector 9 decreases according as the
depth (d) of the inner surface flaw 14a increases. There-
fore, the depth (d) of the inner surface flaw 14a can be
detected ~rom the value of the DC voltage signal from the
synchronous detector 9.
However, as is clear from Figs. 5 and 6, the
variation in the output voltage.from the synchronous
detector 9 corresponding to the variation in the depth (d)
/~ ~ of the inner surface flaw 14a issmaller than the variation
in the output voltage from the synchronous detector 9
corresponding to the variation in the distance (~ ) between
the secondary coil Sl and the inner surface of the pipe 14.
Therefore, detection of the differential.voltage signal
between the DC voltage signal and the bias voltage signal
by detecting the variation in voltage oE the DC voltage
signal ~rom the synchronous detector 9 not only gives a
low detection.sensi.tivity of the depth (d) of the inner
surface flaw 14a, but also causes the risk of taking the
variation in the distance (~ ) for the depth (d) of the
inner surface flaw 14a. Therefore, it is necessary to
- 28 -
, .
~ . ~
~;~6SS~S
obtain the dif~erential voltage signal proportional to
the depth (d) of the inner surface fla~ 14a, between the
DC voltage signal which is entered from the synchronous
detector 9 directly into the flaw detecting circuit 10,
5 on the one hand, and the DC voltage signal which has been
entered from the synchronous detector 9 into the moving
average circuit 11 and has been moving-averaged there and
is then entered from the moving avexage circuit 11 into
the flaw detecting circuit 10, on the other hand.
More specifically, the moving average circuit
11 moving-averages the DC voltage signals in a prescribed
number from the synchronous detector 9 for each of the
plurality of secondary coils Sl, ..., SN, thereby sequen-
tially taking out bias voltage signals from the DC voltage
signals for each of the plurality of secondary coils Sl,
..., SN.
The number.of DC voltage signals from the syn-
chronous detector 9 to be moving-averaged:is usually 3
to 10.. Operation of the moving average circuit 11 is
controlled by control signals from the moving average
circuit controller 12. The control signa~ is created, as
shown in.Fig. 1, in the moving average circuit controller
12 on the basis of the high frequency.electric current
from the high frequency electric current generator 2,
which has been divided by the frequency divider 5 into a
_ - 29 -
,
, '' ' ' ' :
~265~i95
frequency having a prescribed value. According to the
above-mentioned takeout of the bias voltage signals by
the moving average circuit 11, it lS possible to obtain
the bias.voltage signals from which the noise signals
caused, for example, by the inclination of each of the
plurality of secondary coils Sl, ..., SN have been
eliminated.
The flaw detecting.circuit 10 sequentially
detects a differential voltage signal proportional to the
depth (d) of the inner surface flaw 14a of the pipe 14,
between the bias voltage signal.from the moving average
clrcuit 11 for each of the plurality of secondary coils
Sl, ..., SN, on.the one hand, and a DC voltage signal,
which.immediately follows the moving-averaging by the
moving avera.ge.circuit 11., from the-synchronous detector
9 for each of the plurality of secondary coi.ls Sl, ..., SN,
on the o.ther.hand. By detecting the above-mentioned
~ differential voltage:signal, it is possible to detect the
depth (d) of the inner surface flaw 14a at ahigh detection
sensitivity and without confusing with the change in the
distance ( R ) between each.of the plurality of secondary
coils Sl, ..., SN an~ the inner peripheral surface of the
pipe 14.
However, along with the. change in the distance
- 3a -
,
, ,
, :
! ' ,
,.,'
~2~S595
(Q) between each of the plurality of secondary coils Sl,
~ SN and-the inner surface of the pipe 14, there occurs
a change in the portion of the AC voltage from each-of
the secondary coils SI, ..., SN, corresponding to the
depth .(d) of the inner surface flaw 14a. More particularly,
not only the bias voltage signal of the DC voltage signal
from the synchronous detector 9, but also the differential
voltage signal between the DC voltage signal and the bias
voltage signal are affected by the change in the distance
(~ ). As a.result,.the above-ment oned differential voltage
signal detected by the flaw detecting circuit 10 does not
accurately correspond to the depth (d) of the inner surface
flaw 14a, but originally contains an error signal. As
described above with.reference to Fig... 5, the value of the
.15 bias voltage signal.of.the.DC.voltage signal from:the
synchronous detector 9 varies in.proportion to the distance
(~). Therefore, the value of the DC voltage signal from
~! the synchronous detector 9 and the value of the differen-
tial voltage signal from the fl~w detection circuit 10
also vary in proportion to the distance. ( Q ) . Therefore,
if the chan.ges in these signal values.caused.by the change
in the distance (~ ) are not affected by the difference
in the depth.~d) of the inner surf~ce flaw 14a, it would
be possible to correct a detection error of the differential
25 voltage signal resulting from the change in.the distance
- 31 -
: - ,
.
.
s
( Q ) by amplifying the differential voltage signal from
the flaw detecting circuit 10 at an amplification degree
inversely proportional to the value of the bias voltage
signal from the moving average circuit 11.
Fig. 7 is a graph illustrating the relationship
between the relative value of the output voltage from the
synchronous detector of the signal processing circuit, on
the one hand, and the distance ( Q ) between the secondary
coil and the inner surface of the pipe, on the other hand.
In Fig. 7, the ordinate represents the relative value of
the output voltage from the synchronous detector 9, corres-
ponding to the value of the AC voltage signal from the
secondary coil Sl closest to the inner surface flaw 14a of
the pipe 14, for example, of the plurality of secondary
coils Sl, ... , SN, in the case where the pair of primary
coils Pc reach the inner surface flaw 14a. The abscissa
represents the distance ( Q ) between that secondary coil S
and the inner surface of the pipe 14. The above-mentioned-
relative value of t~le output voltage from the synchronous
detector 9 represented on the ordinate was obtained by
dividing the value o~ the DC voltage signal from the
synchronous detector 9 by the vaLue o the DC voltage signal
. . .
from the synchronous detector 9 for a distance (Q ) of 25 mm
between the secondary coil Sl and the inner surface of the
pipe 14. The inner surface flaw 14a was artificially
- 32 -
.~
;S~35
made by a drlll.
As is clear from Fig. 7,. according as the distance
( R ) between the secondaryicoil Slland the inner surface
of the pipe 14 varies from 25 mm to 30 mm, and from 30 mm
to 35 mm, the relative value of the output voltage from the
synchronous detector 9 decreases for all the depths (d)
of the inner surface flaw 14a of.3 mm, 5 mm and 7 mm,
whereas the-extent of this decrease.is the same for the
same distance (Q), and:the ratio of the decrease ln the
above-mentioned relative value to the distance (Q ) remains
the same without difference dependent on the depth (d)
of the inner surface flaw 14a. More particularly, while
the value of the DC voltage signal itself from the synchro-
nous detector 9 contains a di.fference dependent on the
depth (d) of the inner surface flaw 14a, the change in the
output voltage from the synchronous.detector 9 caus.ed by
the change in the distance (Q ) remains the.same irrespec-
tive of the depth (d) of the inne~r surface ~law 14a. As
described above, therefore., it is possibLe to correct a
detection error of the differential voltage signal resulting
from the change in the distance ( R ) by amplifying the
dif.ferential voltage.slgnal from the~1aw detecting circuit
}0 at,an Amplification degree inversely prop,ortional to
the value.of the bias voltage.signal. from the moving average
25 ~r~ circuit
_ , ~ 33 -,
..
': '
~65~
More specifically, the detection error correcting
circuit 13 amplifies the differential voltage signal from
the:flaw detecting circuit lO for each of the plurality
of secondary coils Sl, ..., SN at an amplificatio~ degree-
inversely proportional to the value of the bias voltage
signal from the moving average circuit ll for each of the
plurality of secondary coils Sl, ..., SN, thereby correct-
ing a detection.error of the differential voltage signal
caused by a fluctuation in the distance (~) between each
of the plurali.ty of secondary coils Sl, .. , SN and the
inner surface of the pipe 14.
Fig. 8 is a graph illustrating the relationship
between the relative value of the output voltage from the
detection error correcting circuit.of the.si.gnal processing
circuit, on the one.hand,. and the distance (Q ) between
the secondary coil and the inner surface of the pipe, on
the other hand. In Fig. 8,. the ordinate represents the
relative value of the output voltage from:the detection
error correcting circuit 13 corresponding to the value of
the differential voltage signal of the secondary coil Sl
closest to the inner surface flaw 14a of the pip~ 1~, Eor
example,.o~ the.plurality.of secondary coils S1, ..., SN,
in the case.whe.re the pair of primary coils Pc reach the
inner sur~ace flaw 14a. The abscissa represents the dis-
tance (~) between that secondary coil S1 and the inner
- 34 -
~2~S595i
surface of the pipe 14. The above-mentioned relative
value of the output voltage from the detection error
correcting circuit 13 represented on the ordinate was
obtained by dividing the value of the DC voltage signal
from the detection error correcting ci-rcuit 13 by the value
of the DC voltage signal from the detection error correct-
ing circuit 13 for a distance (~) of 25 mm between the
secondary coil Sl and the inner surface of the pipe. The
inner surface flaw 14a was artificially made by a drill.
As is clear from Fig. 8, the relative value of
the output voltage from the detection error correcting
circuit 13 shows the same value if the distance (~ )
between the secondary coil Sl and the inner surface of the
pipe 14 remains the same, irrespective of the depth (d)
lS of the inner surface flaw 14a, and the ratio of the
decrease in the above-mentioned relative value to the
distance (Q ) is very small. More specifically, the DC
voLtage signal from the detection error correcting circuit
13 shows a value corresponding to the depth (d) of the
inner surface flaw 14a even when the distance ( Q ) varies,
and in the DC voltage signal from the detection error
correcting circuit 13, the detection error of the differen-
tial voltage signal from the flaw detecting circuit 10
caused by the change in the distance ( Q ) has been corrected.
Therefore, it is possible to accurately detect the depth (d)
~65595
of the inner surface flaw 14a by using the above-mentioned
DC voltage signal from the detectlon error correcting
circuit 13.
According to the apparatus of the present
invention, as described abcve in detail, in which the
plurality of secondary coils forming part of the detecting
probe are arranged at prescribed intervals in the circum-
ferential direction of a pipe to be inspected between the
outer peripheral surface of the.at least one primary coil
forming part of the detecting probe and the inner peripheral
surface of the pipe so that the.axis of each of the plurality
of secondary coils is parallel to the axis of the at least
; - one primary coil, it is possible.to accurate.ly detect the
presence and the depth of each of three or more inner
; 15 surface flaws of each pipe constituting a.plpeline even
when these inner surface flaws are.present.at close
intervals in the axial direction.of the pipe, thus providing
industrially useful effects.
.. .
: _ - 36 -
'; ~., ' ,
