CA1203008A - Eddy current flaw detector - Google Patents
Eddy current flaw detectorInfo
- Publication number
- CA1203008A CA1203008A CA000405028A CA405028A CA1203008A CA 1203008 A CA1203008 A CA 1203008A CA 000405028 A CA000405028 A CA 000405028A CA 405028 A CA405028 A CA 405028A CA 1203008 A CA1203008 A CA 1203008A
- Authority
- CA
- Canada
- Prior art keywords
- eddy current
- core
- flaw detector
- workpiece
- current flaw
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Abstract
ABSTRACT OF THE DISCLOSURE
An eddy current flaw detector particularly for use inside a steel pipeline, comprising an alternating or pulsating current drive coil having its axis essentially perpendicular to the pipe wall or other workpiece and surrounding at least one eddy current sensor. The sensor is formed by a C or E shaped core, which may be either ferrite or other ferromagnetic material such as laminated iron, having the free ends of the two or three legs thereof adjacent the workpiece, with a pair of sense coils wound approximately symmetrically on the core and connected, preferably in series reinforcement, to give a null signal in the absence of flaw. Where an array of sensors is used within a single drive coil to increase coverage, these are preferably arranged in a chevron or herringbone pattern with chevron axes oriented in the direction of scan in order to decrease directional sensitivity. Signal processing circuitry is provided which typically includes a narrow band pass synchronous amplifier tuned or gated by reference to the same signal that is used to drive the drive coil.
An eddy current flaw detector particularly for use inside a steel pipeline, comprising an alternating or pulsating current drive coil having its axis essentially perpendicular to the pipe wall or other workpiece and surrounding at least one eddy current sensor. The sensor is formed by a C or E shaped core, which may be either ferrite or other ferromagnetic material such as laminated iron, having the free ends of the two or three legs thereof adjacent the workpiece, with a pair of sense coils wound approximately symmetrically on the core and connected, preferably in series reinforcement, to give a null signal in the absence of flaw. Where an array of sensors is used within a single drive coil to increase coverage, these are preferably arranged in a chevron or herringbone pattern with chevron axes oriented in the direction of scan in order to decrease directional sensitivity. Signal processing circuitry is provided which typically includes a narrow band pass synchronous amplifier tuned or gated by reference to the same signal that is used to drive the drive coil.
Description
12(~3(~
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 lZ~30~3 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.
i2~30~
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 :~2~)3(~
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 1~30~3 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 ~o~
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.
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 lZ~30~3 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.
i2~30~
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;
~;2V30~
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 :~2~)3(~
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 1~30~3 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 ~o~
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.
Claims (10)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In an eddy current flaw detector, for detecting flaws in a workpiece, such as the steel walls of a pipe, as said detector is moved therealong, and comprising an alternating or pulsating current drive coil having a longitudinal axis and arranged such that said axis is substantially perpendi-cular to said workpiece when in operative relation thereto and circumscribing 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 said output signal; the improvement wherein said sensor comprises an array of cores arranged in said circum-scribing coil in a non co-linear array and connected so as to give a null signal in the absence of flaws in said work-piece in order to detect flaws, aligned in any direction in said workpiece surface, over a substantial width perpendicular to the direction of travel of said detector.
2. An eddy current flaw detector as claimed in claim 1 wherein each of said cores in each said array is selected from E cores and C-cores.
3. An eddy current flaw detector as claimed in claim 2 wherein said core is an E-shaped core and one of said sense coils is wound on said core between each respective pair of legs thereof.
4. An eddy current flaw detector as claimed in claim 2 wherein said core is an E-shaped core and one of said sense coils is wound on said core on each respective outer leg thereof.
5. An eddy current flaw detector as claimed in claim 2, wherein said core is a ferrite E-shaped core.
6. An eddy current flaw detector as claimed in claim 5, wherein said sense coils are wound on said core in series reinforcement so as to give a null signal in the absence of flaws in said workpiece.
7. An eddy current flaw detector as claimed in claim 1, 2 or 3 wherein said coil and sensor is arranged for insertion into a pipeline and adapted to scan an internal surface of said pipeline.
8. An eddy current flaw detector as claimed in claim 1, 2 or 3 wherein said drive coil is located between the poles of a permanent magnet so as to facilitate attachment to said workpiece.
9. An eddy current flaw detector as claimed in claim 1, 2 or 3 wherein said array of sensors is arranged in a single said drive coil in a chevron pattern the axis of which is oriented in the direction of travel of said detector, so as to provide a tightly magnetically coupled flaw detector substantially devoid of directional sensitivity.
10. An eddy current flaw detector as claimed in claim 2, wherein said core is a laminated iron core.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US27689281A | 1981-06-24 | 1981-06-24 | |
| US276,892 | 1981-06-24 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1203008A true CA1203008A (en) | 1986-04-08 |
Family
ID=23058515
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000405028A Expired CA1203008A (en) | 1981-06-24 | 1982-06-11 | Eddy current flaw detector |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1203008A (en) |
-
1982
- 1982-06-11 CA CA000405028A patent/CA1203008A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |