Note: Descriptions are shown in the official language in which they were submitted.
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EDDY CURRENT SENSOR ASSEMBLY
BACKGROUND OF THE INVENTION
The present invention generally relates to a sensor for non-destructive
inspection of magnetically permeable objects such as wire cables, rods, pipes
and the
like. The invention is concerned more particularly with an eddy current sensor
assembly for detecting structural faults in the objects.
Eddy current (EC) devices are known for inspecting elongated
magnetically permeable objects such as wire cables, rods, pipes, and the like
for
structural defects such as stress-corrosion cracks. One such device shown in
my U.S.
Patent 5,751,144 includes magnet means which induces a magnetic flux which
magnetically saturates the object along a longitudinal section. The magnet
means and
the induced magnetic flux move progressively and longitudinally relative to
the object
whereby a longitudinal section of the object experiences a changing magnetic
flux
which induces eddy currents. An eddy current change detecting means is
provided on
the magnet means and is to be positioned adjacent to the relatively moving
object to
detect changes in eddy currents which are representative of structural faults
in the
object.
Eddy current inspection and sensing methods for faults in objects made
of ferromagnetic materials suffer from two critical problems. First, the high
permeability of the ferromagnetic material acts as a shield because of the
frequency-
permeability-conductivity term that appears in skin depth calculations. Thus,
full-
wave penetration of the object wall is difficult to achieve. Second, the
permeability,
coercive force and remanence of steel are influenced by material properties,
by
internal stresses and by structural conditions. These magnetic influences
depend on
the selection of the initial materials and the melting, foundry, rolling and
annealing
processes. Because these magnetic characteristics are not well controlled
during
manufacture and handling, magnetic properties can vary in a random fashion
along
the length of the elongated object. Localized permeability variations, in the
absence
of auxiliary magnetization, usually lead to noise levels that prevent
sufficiently high
sensitivities during testing.
There are three ways to increase the through-wall penetration depth and
the signal-to-noise ration of EC inspections. First, the object can be
magnetically
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2
saturated to decrease its magnetic permeability and, thereby, increase skin
depth.
Magnetic saturation also decreases localized permeability variations, which in
turn
decreases distortions of the inspection signals, thereby improving the signal-
to-noise
ratios. Second, the skin depth can be increased by lowering the excitation
frequency.
And, third, the strength of the excitation signal can be increased.
For through-wall inspections, it is therefore necessary to magnetically
saturate the material to be inspected by a DC magnetic field. When
ferromagnetic
material is magnetically saturated, its relative permeability approaches a
value of
one (i.e., that of air). When thus saturated, the material behaves like a non-
ferromagnetic material and permeability variations will not affect the EC
inspection.
The low relative permeability decreases the background noise and improves the
signal-to-noise ratio so that discontinuity signals can be sensed. In addition
to noise
reduction, the DC magnetization method decreases the skin effect which is
otherwise problematic when applying an alternating magnetic field associated
with
conventional eddy current methods.
Feasibility experiments have shown that a simple DC magnetic
saturation method is easily implemented. However, for a reliable through-wall
detection of structural faults such as axial slits, two necessary conditions
must be
simultaneously satisfied. First, the magnetically permeable elongated object
must be
2 0 magnetically saturated or nearly saturated. Second, significant eddy
currents must
be induced so that eddy current changes representative of structural faults
can be
readily detected. The previous two conditions will hereinafter be referred to
as the
"necessary conditions".
Unfortunately, the necessary conditions are somewhat incompatible
2 5 with each other because the magnet means moving relative to the tested
object
induces a changing magnetic field which, according to Lenz's law, excites eddy
currents together with an associated magnetic field that opposes the change in
the
magnetic field which produced these eddy currents. Therefore, the induction of
eddy currents retards the diffusion of the magnetic flux through the object
wall so as
3 0 to oppose magnetic saturation of the pipe. These motion induced eddy
currents will
be called Self-Excited Eddy Currents (SEECs) hereafter. Full magnetic
saturation (or
near saturation) is achieved only after the eddy currents (the SEECs) have
decayed
toward zero magnitude. In other words, for a simple implementation of an SEEC
method, the independent control of the magnetic saturation together with the
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3
simultaneous induction of strong SEECs is not possible. As such, prior SEEC
apparatus and methods suffer from the difficulty of simultaneously achieving
the
necessary conditions for reliable structural fault detection.
A solution for improving the reliability of structural fault detection is to
provide a magnetic inspection device having two primary and opposite poles
which
induce a magnetic flux to place the object at least near magnetic saturation.
At least
one auxiliary pole is positioned on the inspection device between the primary
poles
and serves to boost the level of the magnetic flux induced by the primary
poles so as
to strengthen induced eddy currents during relative movement of the inspection
device and the object. At least one sensor positioned on the inspection device
detects eddy current changes which are representative of structural faults. A
drawback with current auxiliary pole methods for inducing eddy currents is
that the
eddy current sensors and auxiliary poles are lifted off of the pipe wall when
the
magnetic inspection device contacts a bead of a girth weld or longitudinal
weld
connecting adjacent pipe sections. The lifting of the eddy current sensor and
auxiliary poles cause a distortion in the readings of the eddy current sensor
which
can cause a structural fault to be undetected if a structural fault is located
near or at a
girth weld.
Accordingly, it is a general object of the present invention to provide
2 0 an eddy current sensor assembly that eliminates or otherwise minimizes
distortion
in detecting changes in eddy currents when a magnetic inspection device
contacts a
girth weld or other projection or obstruction in an inspected pipe wall.
SUMMARY OF THE INVENTION
2 5 One aspect of the present invention resides in an eddy current sensor
assembly of a magnetic inspection device for nondestructive detection of
structural
faults in an elongated magnetically permeable object. The sensor assembly has
magnet means including first and second magnetic poles oppositely polarized
relative to each other and spaced from one another for positioning and
movement
3 0 longitudinally relative to an elongated magnetically permeable object to
be tested.
A ferromagnetic member couples the first and second magnetic poles. Compliant
pole pieces such as magnetically permeable brushes are coupled to the poles to
be
interposed between the poles and the object to be inspected. An eddy current
sensor is disposed between the first and second magnetic poles and includes a
sensor
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4
body and a means coupled to the ferromagnetic member for urging the sensor
body against an opposing surface of the magnetically permeable object to be
inspected.
Another aspect of the present invention resides in a magnetic
inspection device for nondestructively detecting structural faults in
magnetically
permeable elongated objects. The inspection device includes first and second
primary magnetic poles oppositely polarized relative to each other and spaced
from
one another for positioning and movement longitudinally relative to an
elongated
magnetically permeable object to be tested. The first primary magnetic pole is
positioned upstream of the second primary magnetic pole relative to magnetic
pole
movement. A ferromagnetic member magnetically couples the first and second
primary magnetic poles. First and second auxiliary magnetic poles are
magnetically
coupled to the ferromagnetic member and interposed between the first and
second
primary magnetic poles. The first auxiliary magnetic pole is positioned
upstream of
the second primary magnetic pole. 'The first and second auxiliary magnetic
poles
respectively have the same poling as the first and second primary magnetic
poles.
The primary magnetic poles induce a static magnetic flux through a
longitudinal
section of an object extending between the primary magnetic poles to at least
a near-
saturation level. The auxiliary magnetic poles boost the magnitude of the
induced
2 0 static magnetic flux in a portion of the object adjacent to the auxiliary
magnetic
poles. The primary magnetic poles and the auxiliary magnetic poles induce
circumaxial eddy currents and associated opposing magnetic fluxes in moving
portions of the object adjacent to the first primary magnetic pole and the
first
auxiliary magnetic pole. Compliant pole pieces are coupled to the primary and
2 S auxiliary magnetic poles and are to be interposed between the poles and
the object
to be inspected. An eddy current sensor is disposed between the first and
second
auxiliary magnetic poles. The sensor includes a sensor body and a means
coupled to
the ferromagnetic member for urging the sensor body against an opposing
surface
of the magnetically permeable object to be inspected.
3 0 An advantage of the present invention is that when the eddy current
sensor assembly contacts a weld bead or other projection in the inspected
object the
distance between the magnetic poles of the sensor assembly and the object is
substantially maintained to prevent distortion of the eddy current signal
generated
by the sensor assembly.
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Another advantage of the present invention is that the sensor body of
the eddy current sensor is biased toward the object to be inspected in order
to
maintain continuous contact with the object as the sensor body moves over a
weld
bead or other projection for further preventing distortion of the eddy current
signal.
5
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A schematically illustrates a known magnetic inspection device
including an eddy current sensor assembly.
FIG. 1B schematically illustrates the eddy current sensor assembly of
FIG. 1A as it moves over a weld bead.
FIG. 2 graphically illustrates various signals detected by the magnetic
inspection device of FIG. 1.
FIG. 3A schematically illustrates an eddy current sensor assembly
embodying the present invention employed in a magnetic inspection device.
FIG. 3B schematically illustrates the eddy current sensor assembly of
FIG. 3A as its magnetic pole moves over a weld bead.
FIG. 4 schematically illustrates an eddy current sensor assembly in
accordance with another embodiment of the present invention.
FIG. 5 graphically illustrates various signals detected by a magnetic
2 0 inspection device of FIGS. 3A and 3B.
FIG. 6A schematically illustrates an eddy current sensor assembly
including a coiled spring and chamfered sensor body in accordance with the
present
invention.
FIG. 6B schematically illustrates the chamfered sensor body of FIG. 6A
2 5 as it contacts a weld bead.
FIG. 7 schematically illustrates an eddy current sensor assembly
including a elastic block and chamfered sensor body in accordance with the
present
invention.
FIG. 8A is a schematic, side elevational view of an eddy current sensor
3 0 assembly including guide plates and angled coil springs in accordance with
the
present invention.
FIG. 8B is a schematic, front elevational view of the eddy current
sensor assembly of FIG. 8A.
FIG. 9 schematically illustrates an eddy current sensor assembly
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6
including guide plates and upright coil springs in accordance with the present
invention.
FIG. 10 schematically illustrates an eddy current sensor assembly
including link members coupled to a chamfered sensor body in accordance with
the
present invention.
FIG. 11 schematically illustrates an eddy current sensor assembly
including a leaf spring and chamfered sensor body in accordance with the
present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference to FIGS. 1A and 1B, a known magnetic inspection
device for nondestructively inspecting a magnetically permeable elongated
object
such as a pipe P for structural faults is schematically illustrated and
generally
designated by the reference number 10. For clarity, only the bottom portion of
the
device 10 is shown and only the bottom longitudinal section of the pipe wall W
is
illustrated as being inspected by the device. Preferably, the inspection
device 10
extends substantially circumaxially about the object and may be located either
internally of the object (as shown) or externally of the object.
The inspection device 10 includes a permanent magnet having first and
2 0 second primary poles 12 and 14 for inducing a primary magnetic flux in a
pipe wall
W of the pipe P. Each of the primary poles 12 and 14 is oppositely poled
relative to
one another and is radially polarized relative to the pipe P. The inspection
device as
shown in FIGS.1A and 1B moves leftwardly relative to the pipe P as indicated
by
the arrow 16 whereby the first primary pole 12 leads the second primary pole
14
2 5 relative to inspection device movement. The primary poles 12 and 14 are
magnetically coupled by a primary ferromagnetic member such as a ferromagnetic
bar 18 which completes a magnetic flux circuit having a forward portion
through a
longitudinal section of the pipe P extending between the primary poles 12 and
14,
and a return portion through the bar 18.
3 0 The inspection device 10 further includes a flux-changing means
embodied as an auxiliary magnet assembly 20 for inducing substantial eddy
currents, whereby eddy current changes representative of structural faults can
be
readily detected. The auxiliary magnet assembly 20 is located between the
first and
second primary poles 12, 14, and includes a housing 22 for supporting an
auxiliary
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magnet and an eddy current sensor 24. T'he auxiliary magnet has first and
second
auxiliary poles 26 and 28 for inducing the substantial eddy currents in a
portion of
the pipe wall W between the auxiliary poles. The eddy current sensor 24 is
located
between the first and second auxiliary poles 26 and 28. The housing 22 is
adjustably
coupled to the ferromagnetic bar 18 of the inspection device 10 by means of a
link
member 30 having first and second longitudinal ends which are respectively
pivotally coupled to the housing 22 and the bar 18 by means of hinge members
32,
34. As the inspection device 10 contacts a projection 36 such as a weld bead
from a
girth weld or longitudinal weld in the pipe wall W, the housing 22, along with
the
eddy current sensor 24 supported therein and the auxiliary poles 26, 28, will
be
moved by the weld bead or other projection or obstruction radially away from
the
pipe wall W (see FIG. 1B). As the auxiliary poles 26, 28, the eddy current
sensor 24
and any magnetic flux leakage sensors are moved off of the pipe wall W, the
magnetic flux pattern will become distorted which, in turn, causes a massive
distortion of self excited eddy current and magnetic leakage flux signals
respectively
detected by eddy current and magnetic flux leakage sensors. This makes
structural
fault inspection in the vicinity of welds impossible. Since structural faults
such as
stress-corrosion cracking and weld seam corrosion frequently occur close to
and
inside girth and longitudinal welds, structural fault inspection capability
near these
2 0 welds is highly desirable.
FIG. 2 illustrates signal distortion received by such a conventional
inspection device near a girth weld. Signal 40 is the difference of signals
detected by
an eddy current sensor and magnetic flux leakage sensor (not shown in FIG. 1);
signal 38 is the eddy current signal, and signal 42 is the magnetic flux
leakage signal.
2 5 All three signals 38, 40, 42 show an abrupt distortion at 44 corresponding
to where
the sensors are being moved away from a pipe wall W as the sensor housing
contacts a girth weld.
Turning now to FIGS. 3A and 3B, an eddy current sensor assembly in
accordance with a preferred embodiment of the present invention is generally
3 0 designated by the reference number 50. The sensor assembly 50 is part of a
magnetic inspection device 52 for nondestructively detecting structural faults
in an
elongated magnetically permeable object, such as a pipe wall W.
The magnetic inspection device 52 includes a permanent magnet
having first and second primary poles 54 and 56 for inducing a primary
magnetic
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flux 58 in the pipe wall W. Each of the primary poles 54 and 56 is oppositely
poled
relative to one another and is radially polarized relative to the pipe P. A
magnetic
flux leakage sensor 57 may be located between the primary poles 54 and 56 for
detecting magnetic leakage flux caused by variations in metallic cross-
sectional area
of the pipe wall W. The inspection device 52 as shown in FIG. 3 moves
leftwardly
relative to the pipe P as indicated by the arrow 60 whereby the first primary
pole 54
leads the second primary pole 56 relative to inspection device movement. The
primary poles 54 and 56 are magnetically coupled by a primary ferromagnetic
member such as a ferromagnetic bar 62 which completes a magnetic flux circuit
having a forward portion through a longitudinal section of the pipe P
extending
between the primary poles 54 and 56, and a return portion through the bar 62.
The primary poles 54, 56 are magnetically coupled to the pipe wall W
by means of compliant pole pieces such as magnetically permeable brushes 55,
preferably steel brushes, attached to the primary poles 54, 56 and extending
radially
therefrom to the pipe wall W. The magnetic force of attraction pulls the
primary
poles 54, 56 toward the inside of the pipe wall. The brushes 55 couple the
magnetic
flux from the magnetic poles into the pipe wall. Brushes provide less
variation in
magnetic coupling than steel blocks or plates, leading to more consistent flux
levels
in the pipe. The lack of brushes with the auxiliary poles 26 and 28 of the
auxiliary
2 0 magnet assembly 20 of FIG. 1 causes the distortion shown in FIG. 2. This
means
that, as the auxiliary poles move across the bead of a girth weld 36, its
magnetic
poles are lifted off of the pipe wall causing a distortion of the magnetic
flux. The
brushes 55 not only couple the magnetic field into the pipe, but they also act
as an
integral part of the mechanical dynamics of the magnetization system. The
brushes
2 5 55 absorb the shock that can result at internal penetrations such as weld
roots,
restrictions, and dents in the pipe wall. For magnetic inspection devices with
solid
magnetic shoes, the dynamic performance is not as smooth, but the magnetic
coupling is better. Spring mounting systems (not shown) may be used to attach
the
magnets to the body of the magnetic inspection device. These systems help
center
3 0 the device in the pipe, and they absorb and damp vibrations.
The eddy current sensor assembly 50 is an auxiliary magnet assembly
for inducing substantial eddy currents, whereby eddy current changes
representative of structural faults can be readily detected. The auxiliary
magnet
assembly 50 is located between the first and second primary poles 54, 56. The
eddy
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current sensor assembly includes an auxiliary magnet having first and second
auxiliary poles 64 and 66 for inducing the substantial eddy currents in a
portion of
the pipe wall W between the auxiliary poles. Preferably, as shown in FIGS. 3A
and
3B, the auxiliary poles 64, 66 are closer to the second primary pole 56 which
lags the
first primary pole 54 relative to inspection device movement, and the second
auxiliary pole is adjacent to or is part of the second primary pole 56. The
second
primary pole 56 and the second auxiliary pole 66 may be structurally distinct
from
one another or may form a single pole serving both the primary and auxiliary
magnetization of the object to be inspected. If the primary and auxiliary
poles are a
single pole, the size of such pole should be the total size of the primary and
auxiliary
poles that would be employed separately from each other. As shown in FIGS. 3A
and 3B, the first auxiliary pole 64 is magnetically oriented in the same
direction as the
first primary pole 54, and the second auxiliary pole 66 is magnetically
oriented in the
same direction as the second primary pole 56, whereby the auxiliary poles 64,
66
induce an auxiliary magnetic flux 68 which reinforces the primary magnetic
flux.
The reinforcement of the primary magnetic flux results in substantial eddy
currents
so that any changes to the eddy currents which are representative of
structural faults
are more readily detectable. The auxiliary poles 64, 66 are magnetically
coupled to
the pipe wall W by means of magnetically permeable brushes 55 attached to the
2 0 auxiliary poles 64, 66 and extending radially therefrom to the pipe wall
W.
The eddy current sensor assembly 50 further includes an eddy current
sensor 70 located between the first and second auxiliary poles 64 and 66. The
eddy
current sensor 70 includes a sensor body 72 and a means 74 for attaching the
sensor
body 72 to the ferromagnetic bar 62 and for urging the sensor body 72 radially
2 5 toward and into contact with an opposing portion of the pipe wall W. The
eddy
current sensor assembly 50 may also include an additional eddy current sensor
76,
including a sensor body 72 and urging means 74. As shown in FIG. 3A, the
resilient
means 72 is a resilient member such as a coil spring in compression, but may
take
other practical forms as will be discussed with reference to the following
figures.
30 The additional eddy current sensor 76 is particularly useful in situations
where it is likely that the eddy current sensors will also detect a signal
component
due to other types of faults such as loss of cross-sectional area or other
localized
faults. The two signals generated by the eddy current sensors 70 and 72 may
then
be solved as two simultaneous equations having two variables to determine the
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component due to structural faults. A digital processor 77, such as a
computer,
communicates with the sensors 57, 70 and 76 for determining the location and
magnitude of structural faults along the object to be tested in response to
signals
generated by the sensors when detecting changes in eddy currents.
5 In operation, as the magnetic inspection device 52 moves leftwardly as
shown in the direction of the arrow 60, the sensor body 72 of the additional
eddy
current sensor 76 contacts a bead of a girth weld or projection 36 which moves
the
sensor body away from the pipe wall W in the radial direction, thereby further
compressing the resilient means 74. The compressed resilient means 74
continues to
10 urge the sensor body 72 of the additional sensor 76 against the girth weld
as the
sensor body moves thereover so that the sensor body is maintained in constant
contact with the pipe wall W. The constant contact with the pipe wall W of the
sensor body 72 of the additional eddy current sensor 76 as it moves past the
girth
weld 36 permits the eddy current sensor 76 with accuracy to detect changes in
eddy
currents due to structural faults occurring at or near the girth weld in the
pipe wall
W.
As the inspection device 52 continues to move leftwardly, the girth
weld bead 36 contacts the brushes 55 of the first auxiliary magnetic pole 64.
As
shown in FIG. 3B, the brushes 55 absorb the shock caused by such contact in
order
2 0 to minimize any movement of the eddy current sensor assembly 50 such as
the
auxiliary magnetic poles 54, 56 relative to the pipe wall W. Minimizing
relative
movement between the auxiliary magnetic poles 54, 56 and the pipe wall W
substantially maintains the level of the magnetic flux induced in the pipe
wall, and
consequently prevents distortion in the detection of eddy current changes by
the
2 5 eddy current sensors 70 and 72. The inspection device 52 further moves
leftwardly
such that the eddy current sensor assembly 50 is in continuous contact with
the girth
weld 36 as it contacts the eddy current sensor 70 and the second auxiliary
magnetic
pole 66 in a way similar to the previously described contact with the sensor
76 and
the first auxiliary pole 64.
3 0 FIG. 4 schematically illustrates an eddy current sensor assembly 53 in
accordance with a further embodiment of the present invention. For simplicity
of
illustration, the digital processor for processing the sensor signals is not
shown in
this and the following embodiments. With respect to further embodiments, like
elements with previous embodiments are labelled with like reference numbers.
The
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sensor assembly 53 is similar to the sensor assembly 50 of FIGS. 3A and 3B
except
for the location of the sensor assembly relative to the primary magnets 54 and
56 of
the inspection device. As shown in FIG. 4, the eddy current sensor assembly 53
is
disposed further from the second primary pole 56 and closer to the first
primary
pole 54 in comparison with the position of the eddy current sensor assembly 50
of
FIGS. 3A and 3B. The location of the eddy current sensor assembly 53 further
upstream of the second primary pole 56 relative to the location in FIGS. 3A
and 3B
may not be as accurate as the location in FIGS. 3A and 3B because the eddy
currents
generated by the first primary pole 54 may not have fully decayed toward zero
preventing the pipe wall W from complete magnetic saturation.
FIG. 5 illustrates the same pipe section as was inspected in FIG. 2, with
the eddy current sensor assembly 50 of FIGS. 3A and 3B. Signal 78 indicates
the axial
component and signal 80 indicates the radial component of the magnetic field
that is
caused by SEECs. In contrast to FIG. 2, the signals 78 and 80 of FIG. 5 do not
show
the distortion near the weld bead (corresponding to the ordinate distance of
120
inches) that is otherwise caused by magnet lift-off described with respect to
FIGS. 1
and 2.
FIGS. 6A and 6B schematically illustrates an eddy current sensor
assembly 100 in accordance with another embodiment of the present invention.
2 0 Ideally, an eddy current sensor moving in the axial direction of the pipe
P should
easily move in the radial direction if forced away from the pipe wall W by a
girth
weld or other obstruction. The eddy current sensor assembly 100 is similar to
the
assembly 50 shown in FIG. 3, except for the shape of a sensor body 102. The
sensor
body 102, coupled to the ferromagnetic bar 62 by means of the coil spring 74,
2 5 defines a chamfer 104 at a leading edge to oppose a pipe wall W of a pipe
P to be
inspected. As shown in FIG. 6B, as the sensor body 102 moves leftwardly
relative to
the pipe P as shown by the arrow 16, the weld bead 36 or other projection
asserts a
force (shown by the arrow N) against the sensor body at the chamfer 104. The
chamfer 104 facilitates the translation of the force N into horizontal and
vertical
3 0 components (shown by the arrow H and the arrow V respectively) for
movement
of the sensor body primarily in the radial direction in order to move the
sensor
body easily over the weld bead. A projection or stop 105 extending downwardly
from the ferromagnetic bar 62 and interposed between the sensor body 102 and
the
second auxiliary pole 66 prevents the sensor body 102 from contacting and
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12
damaging the second auxiliary pole as it partly moves in a horizontal
direction upon
contacting a weld bead 36. The sensor body 102 urged against the pipe wall W
by
the coil spring 74 maintains continuous contact with the pipe wall as it
travels aver
the weld bead 36 to prevent distortion of the eddy current changes detected by
the
sensor body.
FIG. 7 schematically illustrates an eddy current sensor assembly 300 in
accordance with a further embodiment of the present invention. The eddy
current
sensor assembly 300 is similar to the eddy current sensor assemblies of FIGS.
5 and
6, except that the resilient member is in the form of an elastic or
compression block
302, preferably made of rubber. Advantages of employing the rubber block 302
for
the resilient member is its low cost, simple construction and durability.
FIGS. 8A and 8B schematically illustrate a portion of an eddy current
sensor assembly 400 in accordance with a further embodiment of the present
invention. The assembly 400 includes a mounting plate 402 coupled to the
ferromagnetic bar 62. Two guide plates 406, 406 are coupled to the mounting
plate
402, and each extend from a supported end 410 at the mounting plate to a free
end
412 to oppose a pipe wall W of a pipe P to be inspected. As shown in FIG. 8A,
the
guide plates 406, 406 are spaced from one another and cooperate with the
mounting
plate 402 to form a channel 414 extending along the direction of sensor
movement.
2 o A sensor body 102 having a supported end 418 and a free end 420 is
partially
straddled between the guide plates 406, 406 within the channel 414 such that
the free
end 420 of the sensor body extends beyond the free ends 412, 412 of the guide
plates
406, 406 to oppose the pipe wall W. At least one resilient member, such as two
coiled springs 421, 421 shown in FIG. 8B, couples the supported end 418 of the
2 5 sensor body 102 to the mounting plate 402. As best shown in FIG. 8B, the
rniled
springs 421, 421 extend at an oblique angle a relative to the direction of
sensor
movement generally from a leading, supported end of the sensor body 102 toward
a lagging end of the mounting plate 402. The sensor body 102 at a leading
portion
of its free end 420 defines a chamfer 422 for engaging a weld bead 36 or other
30 projection or obstruction 37 in the pipe wall W. Each of the plates defines
slots 428
aligned with one another in the transverse direction in relation to sensor
movement,
and each slot exkends at an oblique angle a relative to the direction of
sensor
movement generally from a leading, free end 412 of the guide plate 406 toward
a
lagging, supported end 4i0. A mounting pin 430 extends through the sensor body
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13
416 in the transverse direction and its ends 432, 432 are supported in an
associated
slot 428 of the guide plates 406. When the weld bead 36 or the obstruction 37
exerts
a normal force against the chamfer 422 of the sensor body 416, the free end
420 of
the sensor body 102 at its leading portion is moved away from the pipe wall W
in a
direction limited and directed by movement of the pin 430 along the slots 428,
428.
FIG. 9 schematically illustrates a portion of an eddy current sensor
assembly 500 which is similar to the eddy current sensor assembly 400 of FIGS.
8A
and 8B, except for the resilient means, such as the two coiled springs 502,
502. The
coiled springs 502, 502 are longitudinally oriented in a direction
perpendicular to
sensor movement from the supported end 418 of the sensor body102 to the
mounting plate 402.
FIG. 10 schematically illustrates an eddy current sensor assembly 600 in
accordance with another embodiment of the present invention. The assembly 600
includes a mounting plate 602 coupled to the ferromagnetic bar 62. The sensor
body
102 is radially adjustably coupled to the ferromagnetic bar 62 by means of at
least
one link member, such as the parallel link members 604, 604 each having first
and
second longitudinal ends which are respectively pivotally coupled to the
supported
end 418 of the sensor body 102 and the ferromagnetic bar by means of hinge
members 606, 606. The link members 604, 604 are rotationally biased by means
of
2 0 springs (not shown) to urge the sensor body 102 against the pipe wall W.
FIG. 11 schematically illustrates an eddy current sensor assembly 700 in
accordance with another embodiment of the present invention. The assembly 700
includes a leaf spring 702 as the resilient member coupled at a first end to
the
ferromagnetic bar 62 and at a second end to the supported end 418 of the
sensor
2 5 body 102. The shape of the leaf spring 702 facilitates the translation of
a normal
force exerted by a weld bead 36 against the chamfer 104 of the sensor body 102
into
radial movement of the sensor body away from the inspected object and over the
weld bead. A stop 105 interposed between the sensor body 102 and the second
auxiliary pole 66 prevents the sensor body 102 from contacting and possibly
30 damaging the second auxiliary pole.
While the present invention has been described in several
embodiments, it will be understood that numerous modifications and
substitutions
can be made without departing from the spirit and scope of the invention.
Accordingly, the present invention has been described in several preferred
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14
embodiments by way of illustration, rather than limitation.
