Note: Descriptions are shown in the official language in which they were submitted.
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Field of Invention:
This invention relates -to the non-destructive inspection
of pipelines and the like of ferromagnetic, or conductive
paramagnetic or diamagnetic materials, and in particular to
methods and apparatus for testing based on the detection of
changes in induced eddy currents.
Background of Invention:
Eddy current testing methods are well known in the
art and have been extensively employed for non-destructive
testing of tubes and relatively small diameter pipes. With
known methods, a coil or array is generally placed externally to
the tube or pipe. By passing an alternating current through
the coil, an eddy current is induced in the tube which in turn
produces an additional alternating magnetic field in the vicinity
of the tube. Discontinuities or inhomogeneities in the tube material
cause variations in the eddy current and hence changes in the
secondary magnetic field. These changes may be detected
electrically and measured by any of several known ways including
oscilloscopes and the like. Typically eddy currents on]y
penetrate a very short distance below the surface of the test
material and are generally not suitable for testing the entire
thickness of the material. They are, however, particularly
suitable for detecting and locating stresses, latent strains,
surface defects such as cracks, corrosion effects, hard spots
and the like.
There is frequently a need to detect and locate flaws
and in particular corrosion, on the internal surfaces of
large diameter steel walled oil and gas pipelines. Few of the
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known devices are, however~ suitable for operation inside a
pipeline and all suffer from certain disadvantages. Some are
not designed to operate efficiently with ferromagnetic material,
some are sensitive to the direction of detector scan or
alignment of flaw axes and some to "lift-off" or detector to
part gap. For example Neumaier in U.S. Patent 3,875,502 discloses
an eddy current flaw detector comprising an ~C excitation coil
surrounding at least one sensing coil which detects the
magnetic flux along its axis which is parallel to the workpiece
and orthogonal to the scan direction. The coils which are
connected in opposition so as to enhance signals from particular
depths in the workpiece, are loosely magnetically coupled to
the workpiece and have strong directional sensitivity which
necessitates an array and scanning coil system as described in
l~eumaier U.S. Patent 4,016,487.
Pratt in U.S. Patent 3,535,625 discloses a flaw
detector having an E-shaped core with sensor coils connected
in series on opposite core legs and a drive coil on the centre
leg to produce a diferential transformer. Multiple drive
coils are, therefore, required for multiple sensors and there
is a strong directional sensitivity (e.g. crac~s aligned with
the E core are not detected). This detector is particularly
suitable for detecting small flaws or flaws beneath the surface
but is not effective for inspecting relatively large surface
areas. Prattls detector operates by detecting differences in
the magnetic reluctance of the two magnetic paths and is
therefore very sensitive to lateral tilt of the detector.
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Objects and Summary of the Invention
It is an object of the present invention to provide an
eddy current flaw detector, suitable for use inside a steel
pipeline or the like, specifically adapted to examine relatively
large areas of the internal surfaces for flaws, cracks and
corrosion without marked directional sensitivity.
Thus, in accordance with one aspect of the invention
there is provided in an eddy current flaw detector, for detect-
ing flaws in a workpiece such as the s-teel walls o~ a pipe and
comprising an alternating or pulsating current drive coil
arranged to have its axis substantially perpendicular to a
surface of said workpiece when in operative ~elation ~hexeto
and surrounding at least one eddy current sensor arranged to
provide an output signal in the presence of a flaw in said
workpiece, and signal processing means to receive and process
saidoutputsignal; the improvement where~n said sensor comprises
a ferromagnetic C or E-shaped core arranged in said coil to en-
able the legs thereof to abut said surface of said workpiece and
having a pair of sense coils wound on said core so as to provide
a tightly magnetically coupled flaw detector.
Description of the Drawings
Figure 1 is a side elevational view of one embodiment
of the device of the present invention mounted inside a permanent
magnet assembly;
Figure 2 is a si.de elevational view of the device of
Fi~ure 1, on a larger scale, with the permanent magnet removed;
Figure 3 is a side elevational view of a sense coil used
in the device of Figures 1 and 2;
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Figure 4 is a plan view of the device of Figure 2,
Figure 5 is a sketch showing flux linkage through
a device of Figure 1 when no flaws are present;
Figure 6 is a sketch showing flux linkage through a
device of Figure 1 in the presence of a flaw; and
Figure 7 is a signal processing circuit block diagram.
Detailed Description of the Invention
Figure 1 shows an eddy current flaw detector 1 of
the present invention and which comprises an alternating or
pulsating current drive coil 2 having one or more sensors 3
mounted therein. Each sensor comprises two or more sense
coils 4, 5 wound on ferrite E cores 6 and mounted inside the
drive coil 2. The sense coils 4, 5 are coupled to an electronic
signal processing device 7. The detector 1 may be mounted
between the pole pieces of a permanent magnet assembly 8 which
serves to hold the detector 1 in close proximity with the iron
or steel pipline or workpiece 9. The entire detector device
is designed so that numbers of them may be mounted on an
electronic pig which may be forced through a pipeline in known
manner so as to scan in the direction shown by the arrow in
Figure 1.
Figures 2 and 3 show the sense coils 4, 5 and E cores
6 in more detail. Preferably the E cores 6 are fabricated of
ferrite but other ferromagnetic materials such as laminated iron
may also be employed. The use of ferromagnetic cores enables
the sense coils to be closely coupled magnetically to the
steel of the workpiece without needing to be close geometrically.
The sense coils 4, S wound on a core 6 are preferably
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two approximately symme-trical parts and wound around the body
of the E cores 6 between adjacent legs thereof ~as shown in
Figure 3) or on the outer legs thereof. The coils 4, 5 may
~ven.be wound one on an outer leg and the other between the
o~her two legs. The two parts 4, 5 comprising the sense coil
are preferably wound in series reinforcement to give a null
signal in the absence of flaws. In effect an E core wound
in this manner acts as a leakage flux concentrator or collector
as the actual flux is determi.ned principally by the current
in the drive coil and in the workpiece material. Figure 5
shows the flux linkages to be expected from this configuration
when no flaws are present and Figure 6 shows the flux linkages
to be expected in the presence of a flaw 10.
As seen most clearly in Figure 4 the drive coil 2
i9 arranged with its axis essentially perpendicular to the work-
piece 9 and surrounding at least one sensor 3. As also shown in
Figure 4 a plurality or array of sensors 3, each comprising
sense coils 4, 5 wound on cores 6, may be mounted within a
single drive coil 2 and connect~d either in series or parallel.
Preferably, when an array of coils is used they are arranged
in chevron fashion, as shown in Figure 4, or in herringbone
array that is at angles of approximately +5 and -5 to the
direction of scan of the drive coil, in order to minimize
signal variations due to defect axis alignment.
The signal processing circuit is shown in Figure 7.
When external drive coil 2, in contact with a workpiece 9, is
energized wi.th alternating or pulsating current supplied by
pulse generator 70, an ou-tput signal is generated by sense
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coils 4, 5, mounted within the coil 2, usually of the order
oE a few microvolts, in response to differences in the mag-
netic flux in the sense coils ~, 5 in the presence of a flaw,
as described with reference to Figure 6. It will be appreciated
that in the absence of a flaw the flux in the sense coils 4, 5
is the same and no output signal or a null is generated.
An output (200~s on,2.4 ms period) from pulse gener-
ator 70, is fed through attenuator and phase controller, 90,
to a multiplier 80. The output from sense coils 4, 5 provides
a second input to multiplier 80. The attenuator and phase
controller 90, i5 adjusted 50 that, when there is a signal
from sense coils 4, 5 due to a flaw adjacent to cor0, 6,
the output from pulse generator 70, which it supplies to
multiplier 80, is in phase with the si~nal and of suitable
amplitude for multiplier 80.
Multiplier ~0 acts as a synchronous detection system
since it is tuned to the eddv current frequency by the
reference input through attenuator and phase controller 90
from pulse generator 70, since pulse generator 70 also energizes
drive coil 2. Synchronous detection i5 used in order to
reduce induced or electrical noise by effectively using narrow
bandwidth tuning. An output from multiplier 80, which is
proportional to the product of the reference signal derived
from the pulse generator 70, and the defect induced signal from
sense coils 4, 5 is fed to amplifier 100 which typically
uses feedback gain stabilisation as known to those skilled in
the art. Synchronous amplified product-signal from amplifier 100
is fed through low pass active ~ilter 101 the purpose of which
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is to reduce unwanted noise and thereby enhance signal/noise
ratio further. Low pass Eilter 101 may comprise, for example,
a fourth order low pass Butterworth circuit which is well
known to those skilled in this art. The cut-off frequency of
filter 101 is selected by reference to the fundamental frequency
of pulse generator 70 which is in turn determined in part by the
required detector scanning velocity.
An oscillator may be used in place of pulse generator
70 if more power is available.
The output from low pass active filter 101 is an alter-
nating signal whose amplitude is determined by the magnitude of
the corrosion or flaw. It may be recorded, displayed on an
oscilloscope, converted to an audible tone or otherwise suitably
processed.
Suitable grounds and bus line voltages are supplied to
all circuit blocks. The gain of amplifier 100 may be suitably
adjusted for sensitivity without overloading the detector or
electronics by scanning the detector over a workpiece with a
suitable simulated flaw or corrosion, such as a hole milled
partly through the workpiece.
It will be appreciated that while this invention has
been described with particular reference to E-shaped cores this
is only a matter of practical convenience as the centre leg
thereof is not essential to the operation of the device. C-shaped
cores may equally well be used and are considered part of the
present invention.
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