CA1158314A - Eddy current surface probe - Google Patents
Eddy current surface probeInfo
- Publication number
- CA1158314A CA1158314A CA000359392A CA359392A CA1158314A CA 1158314 A CA1158314 A CA 1158314A CA 000359392 A CA000359392 A CA 000359392A CA 359392 A CA359392 A CA 359392A CA 1158314 A CA1158314 A CA 1158314A
- Authority
- CA
- Canada
- Prior art keywords
- coil
- eddy current
- coils
- lift
- probe
- 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
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/72—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables
- G01N27/82—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws
- G01N27/90—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws using eddy currents
- G01N27/9046—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws using eddy currents by analysing electrical signals
- G01N27/9053—Compensating for probe to workpiece spacing
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/72—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables
- G01N27/82—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws
- G01N27/90—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws using eddy currents
- G01N27/9006—Details, e.g. in the structure or functioning of sensors
Landscapes
- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- Physics & Mathematics (AREA)
- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Investigating Or Analyzing Materials By The Use Of Magnetic Means (AREA)
Abstract
TITLE
EDDY CURRENT SURFACE PROBE
INVENTORS
Hugh W. Ghent Valentino S. Cecco ABSTRACT OF THE DISCLOSURE
The eddy current probe consists of two coils mounted coaxially with each coil capable of separate axial adjustment. The coils are connected to the adjacent arms of an AC bridge circuit which can be balanced at the desired test frequency. The voltage output from the AC bridge due to lift-off or defects is the difference between the voltage change of each coil. The coils are adjusted axially to provide a voltage output due to lift-off which is in the order of 90° with regard to the voltage output due to shallow defects making the latter simple to detect. A thin conducting shim may be positioned between one of the coils and the test surface in order to further rotate the lift-off signal of that coil.
EDDY CURRENT SURFACE PROBE
INVENTORS
Hugh W. Ghent Valentino S. Cecco ABSTRACT OF THE DISCLOSURE
The eddy current probe consists of two coils mounted coaxially with each coil capable of separate axial adjustment. The coils are connected to the adjacent arms of an AC bridge circuit which can be balanced at the desired test frequency. The voltage output from the AC bridge due to lift-off or defects is the difference between the voltage change of each coil. The coils are adjusted axially to provide a voltage output due to lift-off which is in the order of 90° with regard to the voltage output due to shallow defects making the latter simple to detect. A thin conducting shim may be positioned between one of the coils and the test surface in order to further rotate the lift-off signal of that coil.
Description
3 ~ ~
Back~round of the Invention This invention is directed to an eddy current surface probe and in par~icular -to a probe for the detec~ion or shallow surface defec-ts.
Eddy current nondestructive testing techniques in use today do not readily detect shallow surface defects in the order of ~.1 mm deep. This is because the phase of the voltage change in the sensing coil due to a shallow defec~
is at nearly the same anyle as the voltage change due to a small variation in the distance between the coil and test surface which is known as lift~off. The ~roltage change from deeper defects, i.e. iIl the order of Q.5 mm has significant phase rotation from that Qf lift-off signals to make detection of deeper defects much easier.
United Sta-tes Patent 3,197,693 which issued to H.L. Libby on July 27, 1965 and United States Patent 3,753,096 which issued to W.C. Wi.ers on ~ugust 14, 1973 are examples of eddy current probes in which attempts are made to compensa-te for lift-off.
The Li~by patent descri.bes an eddy cuxrent prob~
which has two or more Goi.ls that are co-axial, co-planar and wound on separate magnetic core.s. The output is ta~en ~rom the inner coil~ while the phase and amplit.ude of the outer coil i~ va.ried by electronic mearls to alter the phase of the l.ift off signal sensed hy the inner coil.
The Wiers patent describes an eddy current coil design invol~in~ an inner and outer coil wound on separate inller and outer, coaxial magneti.c cores. The cores are able to move axially such that they exh.~i-t substantially th~ same amount of lift-off ~ariation in the output of each coil. The inner coil is more sensiti.ve to defects than the outer coil.
Each coil is connected to the adjacen-t arms of an AC bridge 3 ~ ~
so that the bridge remains "substantially" balanced over normal variations in lift-off, yet is unbalanced when the coil passes over surface defects. However, this patent does no-t indicate what it considers to be a normal variation in lift-off or a substantially balanced bridge.
Summary of the Inven-tion It is therefore an object of this invention to provide an eddy current probe in which the lift-off signal is rotated away from the signal due to shallow defects.
This and other objects of the invention can be achieved in an eddy current probe for connection to an AC
bridge detec-tor circuit, which includes a coil arrangem~nt that has at least two dissimilar coils fixed to non-magnetio co axial cores. The cores arP mounted on a non-magnetic base section which controls the posi~ion of ~he probe wi-th respect to the surface to be tested. The core mounting arrangemen-t allows for the adjustment of the cores in an axial direction within the base section. The coils in the probe may differ in radius, length and/or inductance. In addition, the eddy current probe ma~ include a conduc-tive shim positioned adjacent to one of the coils, which is preferably the inner coil in a two-coi~ probe.
Many other objects and aspec~s of the invention will be clear from the detailed description of the drawings.
Brief Description of the Drawings ~ . .. .. . _ . , . ~
In the drawings:
Figure 1 illustrateq a typical AC bridge circuit used with the present invention;
E~igures 2, 3 and 4 include lift-off curves on the non-dimensionalized impedance planes. The slope of the lift-off curve represents the value ¦dZX/dZRl;
1 1 5~3 ~ ~
Figure~ 5, 6 and 7 include curves showing how the rate of change in coil impedance with liEt-off (dZ/dQ) varles with li~t-of~;
Figure 8 illustrates the outputs of the probe coils for various lift-off distancesi Figure 9 illustrates in cross-section, a surface probe in accordance wi~h the present invention;
Figure 10 illustrates a probe's output signals for lift-offs and defects; and Figure 11 illus-trates a shimmed proba's output signals for lift-ofs and defects.
Deta l _ De~ æ___n of_the Embodiments The eddy current surace probe includes two co-axial coils Cl and C2 mounted in a housing such tha~ when the probe is being utilized, ~he coils may be positioned substantially parallel to the surface being tested whether tha~ surface is flat or curved such as the inter~or of a large tube. The coils Cl~and C2 axe connected as arms of an AC bridge circuit 1 as shown in figure 1. The ~C hridge 1 is ~n~rgized by an AC
source 2 which normally woulcl have an adjustable frequency.
The other two arms of the bridge consists of variable impedanc~s 3 ancl 4 so that the briclg~ ~ carl he balancecl at any clesirecl test frequency~ Detect.or;S is u~f~d to balance the bridge 5 and to detect the signals due to lift-off and/or defects in ~he test surface.
The impeclance of a coil i.5 denot~d by the sym~ol Z. It is com~osed of a reactivfe cc~mponent (ZX) and a resistive component (ZR) When the distance between the coil and test surface, commonly called lift-off (Q), changes by a small amount, thexe is a corresponding change in reactive impedance (~ZX) and in resistive impedance (~ZR)~ It has been :~15~3~4 determined that the value ¦dZx/d~Rl for a small change in lift-off increases with coil diameter, with increasing test frequency, ~lith increasing lift-off and to a lesser ex-tent with increasing coil len~th. In figures 2 to 4, lift of-E
eurves are shown which illustrate this behaviour. The slope oE the lift~off eurve represents the value ¦dZX/dZRl. The vertical axis ZX has been normalized by dividing by 2~fLo where f is the operating frequency (Hertz) and I~o is the inductanee of the coil in air (Henry). ~he horizon-tal axis has been normallzed by irst subtracting the eoils DC
reSiStanCe (Rcoil) and then dividing hy 2~fL .
Figure 2 which shows lift-off eurves A, B and C for three eoils with clifferent diame~ers, illustrates that ¦dZx/dZRl increases with increasiny diameter of the coil.
Figure 3 whieh shows lift-off eurves A, B, C and D for a coil operating at four different fre-quencies, illustrates that ldæx/dzRl increases with an inereasing operating source frequerlcy.
~igure 4 which shows li~t-of~ curves A, B, and C for the coils ha~ing three dif~erent lengths~
illustrates that ¦dZx/dZ~l incxeases ~ith an increasing len~h o~ the coil.
It has also been dete~nined that the coupling o the magnetic field ereated by the electric curren-t in the coil to the test material increases as eoil diameter increases, as eoil length decreases and as lift-off (Q) deereases. With inereasing test frec~uency, the induced ecldy current density near the test surface lncreases. Therefore, the coupling of the magnetie field generated by eddy currents to the eoil inereases as test frequency increases. The rate of change ~f 8 3 ~ ~
coil impedzrlce wlth lift-off ¦'dZ/dQ¦ is a complicated function of these param~ters - coil length, coil diameter, lift-o~
ard test frequency. Figures 5 to 7 illustrate these affects.
Figure 5 which shows Ln¦dZ/dQl vs. li~t-off curves A, B and C for three coils with dif~erent diameters, illus-trates -that with increasing coil diameter, ~dZ/dQ~ increases.
Figure 6 which shows Ln¦;dZ/dQ¦ vs. lift-ofE
curves A, B, C and D for a coil operating at four different frequencies, illus-trates that with increasing operating source frequency, ¦~dZ/dQ¦ increases.
Figure 7 which shows Ln¦dZ/dQ¦ vs. lift-of cur~es A, B and C for three coils having diferent lengths, illustrates that with increasing coil len~th, ¦dZ/dQ¦ decreases.
The information contained in figures 2 ~o 7 shows that the lif-t-o~f signals from coils o~ diferent shapes and si2es can be very different~ and it can be inferred that the curves resulting from ~he signal oE either coil taken separately can also be ver~ dif~ererlt: from a curve re~ultin~
from the diferences in the lift-off si~nals from two coils.
~Q To confirm ~his, the series of lif~-off curve~ illustrated i~
figure 8, where ~btairled using a ~ro~e havilly two coaxi~l c~c)il~
connected to an AC ~ridge operatin~ at 500 kH~ wlth a sam~le surface macle o~ ~ircaloy--2 (trademark). 'rhe AC hridge was balanced with inner and oul:er coils set at the distances ~rom the surace listed in the table below:
Tah]e 1 Inner Coil Ou~er Coi]
Dist. (~m ~ _ Dist tmm) a) 0.32 0.0 b) 0032 0.32 c) 0.32 0.64 d) 0.32 0.80 e) 0.32 0.95 ~) ~.32 1.11 _5_ 3 ~ 4 With the bridge in balance, a state represented by the X in each set of the series oE curves a-f, each set of lift-off curves were obtained by:
I) raising only the ou-ter coil giving the horizontal line to the right, i.e. curves I~-If;
II) raising only the inner coil giving the line dipping to the left, i.e. curves IIa-IIf; and finally III) raising both coils simultaneously in intervals o~ 0.086 mm producing the do-tted differential lift-off curves 1~ IIIa-IIIf.
It can be seen from figure 8 that the initial airection of the differential lift-off curve III can be rotated relative to the lift-off curves of coil I or II by simply adjusting the initial axial distance of the outer coil from the tes-t surface. This effect may be enhanced by selec-ting differen-t coil diameters and coil lengths, by having differing coil inductances and by operating at different frequencies.
Figure 9 illustrates an eddy current probe in accordance with the present inventlon. The probe 10 shown in cross-section, includes a base sectlon 11 which, a~ shown, is cylindrically shaped with an open end 1~ and a closed end 13. The closed end 13 is very thin and contacts th~ surIace to be tested. Two coaxial coils, the inner coil 14 and the outer coil 15 are mounted on cores 16 and 17 respectively such tha-t they ma~ be positioned within the base section 11 at a predetermi~ed desired distance from end 13 of the base section 11. The outer core 17 has a hollow cylindrical body 1~ with a fl2nge 19, the body 18 slides into base section 11 A cap 20 fits over and is secured to flange 19. Screw 21 fixes the core 17 to the body section 11 and springs 22 over the screws 21 maintain the outer coil 15 at the desired distance from end --6~
$ 3 ~
13. Core 16 is cylindric~l in shape and moves freely within core body 18. It is secured wi-thin -the body 18 by a bolt 23.
The position of core 1~ relative to body 18 may be adjus-ted by a nut 24 and is maintained by a spring 25. All of the ~lements in the probe 10, including the screws, are made from non-magnetic material such as Delrln~ The parameters of the copper wire coils 14 and 15 for one particular probe are listed below in Table II.
Table II
-~_ I.D. O.D. Length L ~H Turns (nun) (rmn) (mm) Coil 14 O.S 2.5 0~5 13.5 128 Coil 15 4.6 S.l 1.3 16 ~2 In operation, it is desirable to adjust the two coils 14 and 15 relative to one another in the probe as well as to the end 13 oE the probe and thus the surface to be testecl such that- the defect signal fxom a shallow defect is in the order of 90 out of phas~ with the differential lifk-off curve. In a probe of the type described above, the inner coil 14 was adjusted tC3 be 0.32 ~1'1 frorn a test surface arld the outer coi1 15 was adju~ted to be 1~0 mm from th~ test surface. The AC bridge operatin~ at a frequerlcy o~ 500 kHz was then balanced, represented by the point X in figure 10.
The differences between the resulkillg outpuk signals due to lift-ofE and defects are clearly shown. The lif~oEf curve is substantially perpendicular to the defect signals producecl by the three notches in the test surface which were 0.13 mm, 0~25 mm and 0.50 ~n deep; 0.13 ~n wide; and 6.3 ~n long.
~L ~5~3 ~ 4 In order to increase the ratio of defect signal to llft-off signal, a thin condllctin~ shim 26 may be inserted between the inner coil 14 and test surface as shown in figuxe 9~ The shim 26 changes the lift-off characteristic of coil 14 by increasing the ~alue of the slope of the lift-o~f curve ¦dZx/dZRl and decreasing the value ¦dZ/dQ¦. In selecting the conductivity and thickness of the shim 26, it is possible to have nearly identical lift-oEf curves of the inner and outer coils 14 and 15, respectively, over a significant lift-off rang2 and at a specific tes~ ~requency. The ef~ect of a 0.1 mm ~ircaloy-~ shim inserted between the inner coil 14 and a test surface is shown in figure 11. The lift-off signal is rotated about 180 and compared to the 0.13 mm deep defect signal, there is very little lift-off signal for about 0.17 mm of lift-off. ~s seen in figure 10 for a no-shim probe, a 0.17 mm change in lift-off produces a large signal compared to the 0.13 mm deep defec~ signal ~lany modifications in ~he above dcscribed embodiments of the invention can he carried Ou t withou~
departing from the scope thereof, and therefore, the scope of the pres~nt inventic-n is interl~ed to be limited only ~y the apperlded claims.
~0 8-
Back~round of the Invention This invention is directed to an eddy current surface probe and in par~icular -to a probe for the detec~ion or shallow surface defec-ts.
Eddy current nondestructive testing techniques in use today do not readily detect shallow surface defects in the order of ~.1 mm deep. This is because the phase of the voltage change in the sensing coil due to a shallow defec~
is at nearly the same anyle as the voltage change due to a small variation in the distance between the coil and test surface which is known as lift~off. The ~roltage change from deeper defects, i.e. iIl the order of Q.5 mm has significant phase rotation from that Qf lift-off signals to make detection of deeper defects much easier.
United Sta-tes Patent 3,197,693 which issued to H.L. Libby on July 27, 1965 and United States Patent 3,753,096 which issued to W.C. Wi.ers on ~ugust 14, 1973 are examples of eddy current probes in which attempts are made to compensa-te for lift-off.
The Li~by patent descri.bes an eddy cuxrent prob~
which has two or more Goi.ls that are co-axial, co-planar and wound on separate magnetic core.s. The output is ta~en ~rom the inner coil~ while the phase and amplit.ude of the outer coil i~ va.ried by electronic mearls to alter the phase of the l.ift off signal sensed hy the inner coil.
The Wiers patent describes an eddy current coil design invol~in~ an inner and outer coil wound on separate inller and outer, coaxial magneti.c cores. The cores are able to move axially such that they exh.~i-t substantially th~ same amount of lift-off ~ariation in the output of each coil. The inner coil is more sensiti.ve to defects than the outer coil.
Each coil is connected to the adjacen-t arms of an AC bridge 3 ~ ~
so that the bridge remains "substantially" balanced over normal variations in lift-off, yet is unbalanced when the coil passes over surface defects. However, this patent does no-t indicate what it considers to be a normal variation in lift-off or a substantially balanced bridge.
Summary of the Inven-tion It is therefore an object of this invention to provide an eddy current probe in which the lift-off signal is rotated away from the signal due to shallow defects.
This and other objects of the invention can be achieved in an eddy current probe for connection to an AC
bridge detec-tor circuit, which includes a coil arrangem~nt that has at least two dissimilar coils fixed to non-magnetio co axial cores. The cores arP mounted on a non-magnetic base section which controls the posi~ion of ~he probe wi-th respect to the surface to be tested. The core mounting arrangemen-t allows for the adjustment of the cores in an axial direction within the base section. The coils in the probe may differ in radius, length and/or inductance. In addition, the eddy current probe ma~ include a conduc-tive shim positioned adjacent to one of the coils, which is preferably the inner coil in a two-coi~ probe.
Many other objects and aspec~s of the invention will be clear from the detailed description of the drawings.
Brief Description of the Drawings ~ . .. .. . _ . , . ~
In the drawings:
Figure 1 illustrateq a typical AC bridge circuit used with the present invention;
E~igures 2, 3 and 4 include lift-off curves on the non-dimensionalized impedance planes. The slope of the lift-off curve represents the value ¦dZX/dZRl;
1 1 5~3 ~ ~
Figure~ 5, 6 and 7 include curves showing how the rate of change in coil impedance with liEt-off (dZ/dQ) varles with li~t-of~;
Figure 8 illustrates the outputs of the probe coils for various lift-off distancesi Figure 9 illustrates in cross-section, a surface probe in accordance wi~h the present invention;
Figure 10 illustrates a probe's output signals for lift-offs and defects; and Figure 11 illus-trates a shimmed proba's output signals for lift-ofs and defects.
Deta l _ De~ æ___n of_the Embodiments The eddy current surace probe includes two co-axial coils Cl and C2 mounted in a housing such tha~ when the probe is being utilized, ~he coils may be positioned substantially parallel to the surface being tested whether tha~ surface is flat or curved such as the inter~or of a large tube. The coils Cl~and C2 axe connected as arms of an AC bridge circuit 1 as shown in figure 1. The ~C hridge 1 is ~n~rgized by an AC
source 2 which normally woulcl have an adjustable frequency.
The other two arms of the bridge consists of variable impedanc~s 3 ancl 4 so that the briclg~ ~ carl he balancecl at any clesirecl test frequency~ Detect.or;S is u~f~d to balance the bridge 5 and to detect the signals due to lift-off and/or defects in ~he test surface.
The impeclance of a coil i.5 denot~d by the sym~ol Z. It is com~osed of a reactivfe cc~mponent (ZX) and a resistive component (ZR) When the distance between the coil and test surface, commonly called lift-off (Q), changes by a small amount, thexe is a corresponding change in reactive impedance (~ZX) and in resistive impedance (~ZR)~ It has been :~15~3~4 determined that the value ¦dZx/d~Rl for a small change in lift-off increases with coil diameter, with increasing test frequency, ~lith increasing lift-off and to a lesser ex-tent with increasing coil len~th. In figures 2 to 4, lift of-E
eurves are shown which illustrate this behaviour. The slope oE the lift~off eurve represents the value ¦dZX/dZRl. The vertical axis ZX has been normalized by dividing by 2~fLo where f is the operating frequency (Hertz) and I~o is the inductanee of the coil in air (Henry). ~he horizon-tal axis has been normallzed by irst subtracting the eoils DC
reSiStanCe (Rcoil) and then dividing hy 2~fL .
Figure 2 which shows lift-off eurves A, B and C for three eoils with clifferent diame~ers, illustrates that ¦dZx/dZRl increases with increasiny diameter of the coil.
Figure 3 whieh shows lift-off eurves A, B, C and D for a coil operating at four different fre-quencies, illustrates that ldæx/dzRl increases with an inereasing operating source frequerlcy.
~igure 4 which shows li~t-of~ curves A, B, and C for the coils ha~ing three dif~erent lengths~
illustrates that ¦dZx/dZ~l incxeases ~ith an increasing len~h o~ the coil.
It has also been dete~nined that the coupling o the magnetic field ereated by the electric curren-t in the coil to the test material increases as eoil diameter increases, as eoil length decreases and as lift-off (Q) deereases. With inereasing test frec~uency, the induced ecldy current density near the test surface lncreases. Therefore, the coupling of the magnetie field generated by eddy currents to the eoil inereases as test frequency increases. The rate of change ~f 8 3 ~ ~
coil impedzrlce wlth lift-off ¦'dZ/dQ¦ is a complicated function of these param~ters - coil length, coil diameter, lift-o~
ard test frequency. Figures 5 to 7 illustrate these affects.
Figure 5 which shows Ln¦dZ/dQl vs. li~t-off curves A, B and C for three coils with dif~erent diameters, illus-trates -that with increasing coil diameter, ~dZ/dQ~ increases.
Figure 6 which shows Ln¦;dZ/dQ¦ vs. lift-ofE
curves A, B, C and D for a coil operating at four different frequencies, illus-trates that with increasing operating source frequency, ¦~dZ/dQ¦ increases.
Figure 7 which shows Ln¦dZ/dQ¦ vs. lift-of cur~es A, B and C for three coils having diferent lengths, illustrates that with increasing coil len~th, ¦dZ/dQ¦ decreases.
The information contained in figures 2 ~o 7 shows that the lif-t-o~f signals from coils o~ diferent shapes and si2es can be very different~ and it can be inferred that the curves resulting from ~he signal oE either coil taken separately can also be ver~ dif~ererlt: from a curve re~ultin~
from the diferences in the lift-off si~nals from two coils.
~Q To confirm ~his, the series of lif~-off curve~ illustrated i~
figure 8, where ~btairled using a ~ro~e havilly two coaxi~l c~c)il~
connected to an AC ~ridge operatin~ at 500 kH~ wlth a sam~le surface macle o~ ~ircaloy--2 (trademark). 'rhe AC hridge was balanced with inner and oul:er coils set at the distances ~rom the surace listed in the table below:
Tah]e 1 Inner Coil Ou~er Coi]
Dist. (~m ~ _ Dist tmm) a) 0.32 0.0 b) 0032 0.32 c) 0.32 0.64 d) 0.32 0.80 e) 0.32 0.95 ~) ~.32 1.11 _5_ 3 ~ 4 With the bridge in balance, a state represented by the X in each set of the series oE curves a-f, each set of lift-off curves were obtained by:
I) raising only the ou-ter coil giving the horizontal line to the right, i.e. curves I~-If;
II) raising only the inner coil giving the line dipping to the left, i.e. curves IIa-IIf; and finally III) raising both coils simultaneously in intervals o~ 0.086 mm producing the do-tted differential lift-off curves 1~ IIIa-IIIf.
It can be seen from figure 8 that the initial airection of the differential lift-off curve III can be rotated relative to the lift-off curves of coil I or II by simply adjusting the initial axial distance of the outer coil from the tes-t surface. This effect may be enhanced by selec-ting differen-t coil diameters and coil lengths, by having differing coil inductances and by operating at different frequencies.
Figure 9 illustrates an eddy current probe in accordance with the present inventlon. The probe 10 shown in cross-section, includes a base sectlon 11 which, a~ shown, is cylindrically shaped with an open end 1~ and a closed end 13. The closed end 13 is very thin and contacts th~ surIace to be tested. Two coaxial coils, the inner coil 14 and the outer coil 15 are mounted on cores 16 and 17 respectively such tha-t they ma~ be positioned within the base section 11 at a predetermi~ed desired distance from end 13 of the base section 11. The outer core 17 has a hollow cylindrical body 1~ with a fl2nge 19, the body 18 slides into base section 11 A cap 20 fits over and is secured to flange 19. Screw 21 fixes the core 17 to the body section 11 and springs 22 over the screws 21 maintain the outer coil 15 at the desired distance from end --6~
$ 3 ~
13. Core 16 is cylindric~l in shape and moves freely within core body 18. It is secured wi-thin -the body 18 by a bolt 23.
The position of core 1~ relative to body 18 may be adjus-ted by a nut 24 and is maintained by a spring 25. All of the ~lements in the probe 10, including the screws, are made from non-magnetic material such as Delrln~ The parameters of the copper wire coils 14 and 15 for one particular probe are listed below in Table II.
Table II
-~_ I.D. O.D. Length L ~H Turns (nun) (rmn) (mm) Coil 14 O.S 2.5 0~5 13.5 128 Coil 15 4.6 S.l 1.3 16 ~2 In operation, it is desirable to adjust the two coils 14 and 15 relative to one another in the probe as well as to the end 13 oE the probe and thus the surface to be testecl such that- the defect signal fxom a shallow defect is in the order of 90 out of phas~ with the differential lifk-off curve. In a probe of the type described above, the inner coil 14 was adjusted tC3 be 0.32 ~1'1 frorn a test surface arld the outer coi1 15 was adju~ted to be 1~0 mm from th~ test surface. The AC bridge operatin~ at a frequerlcy o~ 500 kHz was then balanced, represented by the point X in figure 10.
The differences between the resulkillg outpuk signals due to lift-ofE and defects are clearly shown. The lif~oEf curve is substantially perpendicular to the defect signals producecl by the three notches in the test surface which were 0.13 mm, 0~25 mm and 0.50 ~n deep; 0.13 ~n wide; and 6.3 ~n long.
~L ~5~3 ~ 4 In order to increase the ratio of defect signal to llft-off signal, a thin condllctin~ shim 26 may be inserted between the inner coil 14 and test surface as shown in figuxe 9~ The shim 26 changes the lift-off characteristic of coil 14 by increasing the ~alue of the slope of the lift-o~f curve ¦dZx/dZRl and decreasing the value ¦dZ/dQ¦. In selecting the conductivity and thickness of the shim 26, it is possible to have nearly identical lift-oEf curves of the inner and outer coils 14 and 15, respectively, over a significant lift-off rang2 and at a specific tes~ ~requency. The ef~ect of a 0.1 mm ~ircaloy-~ shim inserted between the inner coil 14 and a test surface is shown in figure 11. The lift-off signal is rotated about 180 and compared to the 0.13 mm deep defect signal, there is very little lift-off signal for about 0.17 mm of lift-off. ~s seen in figure 10 for a no-shim probe, a 0.17 mm change in lift-off produces a large signal compared to the 0.13 mm deep defec~ signal ~lany modifications in ~he above dcscribed embodiments of the invention can he carried Ou t withou~
departing from the scope thereof, and therefore, the scope of the pres~nt inventic-n is interl~ed to be limited only ~y the apperlded claims.
~0 8-
Claims (9)
1. An eddy current probe for connection to an AC
bridge detector circuit to detect shallow defects in surfaces, comprising:
- coil means having at least two dissimilar coils fixed to non-magnetic co-axial cores;
- a non-magnetic base section for positioning the probe near the surface to be tested; and - adjusting means for mounting the cores to the base section such that each coil may be adjusted in an axial direction within the base section.
bridge detector circuit to detect shallow defects in surfaces, comprising:
- coil means having at least two dissimilar coils fixed to non-magnetic co-axial cores;
- a non-magnetic base section for positioning the probe near the surface to be tested; and - adjusting means for mounting the cores to the base section such that each coil may be adjusted in an axial direction within the base section.
2. An eddy current probe as claimed in claim 1 wherein the coils differ in radius.
3. An eddy current probe as claimed in claim 1 wherein the coils differ in length.
4. An eddy current probe as claimed in claim 1 wherein the coils differ in inductance.
5. An eddy current probe as claimed in claim 1, 2 or 3, which further includes conductive shim means positioned adjacent one of the coils.
6. An eddy current probe is claimed in claim 1 wherein the coil means consists of an inner coil and an outer coil, the outer coil having an inner diameter greater than the outer diameter of the inner coil.
7. An eddy current probe as claimed in claim 6 wherein the inner and outer coils differ in length.
8. An eddy current probe as claimed in claim 6 wherein the inner and outer coils differ in inductance.
CLAIMS (cont.)
CLAIMS (cont.)
9. An eddy current probe as claimed in claim 6, 7 or 8, which further includes conductive shim means positioned adjacent the inner coil.
Priority Applications (6)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000359392A CA1158314A (en) | 1980-08-18 | 1980-08-18 | Eddy current surface probe |
| IT68073/81A IT1144610B (en) | 1980-08-18 | 1981-07-31 | SURFACE PARASITE CURRENT PROBE |
| DE19813130685 DE3130685A1 (en) | 1980-08-18 | 1981-08-03 | Eddy current surface probe |
| GB8123782A GB2082330B (en) | 1980-08-18 | 1981-08-04 | Eddy current surface probe |
| FR8115475A FR2488693A1 (en) | 1980-08-18 | 1981-08-10 | CURRENT SURFACE PROBE |
| JP56128655A JPS5772055A (en) | 1980-08-18 | 1981-08-17 | Eddy current type probe |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000359392A CA1158314A (en) | 1980-08-18 | 1980-08-18 | Eddy current surface probe |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1158314A true CA1158314A (en) | 1983-12-06 |
Family
ID=4117782
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000359392A Expired CA1158314A (en) | 1980-08-18 | 1980-08-18 | Eddy current surface probe |
Country Status (6)
| Country | Link |
|---|---|
| JP (1) | JPS5772055A (en) |
| CA (1) | CA1158314A (en) |
| DE (1) | DE3130685A1 (en) |
| FR (1) | FR2488693A1 (en) |
| GB (1) | GB2082330B (en) |
| IT (1) | IT1144610B (en) |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CA1201481A (en) * | 1982-10-22 | 1986-03-04 | Majesty (Her) In Right Of Canada As Represented By Atomic Energy Of Canada Limited/L'energie Atomique Du Canada Limitee | Eddy current probe with defect-noise discrimination |
| SE457117B (en) * | 1984-02-17 | 1988-11-28 | Bengt Hjalmar Toernblom | SET AND DEVICE FOR TESTING ELECTRICALLY CONDUCTIVE SAMPLES |
| FR2574938B1 (en) * | 1984-12-19 | 1986-12-26 | Snecma | METHOD OF CONTROLLING BY CONTACTLESS EDGE CURRENT AND DEVICE FOR IMPLEMENTING IT |
| GB8521266D0 (en) * | 1985-08-24 | 1985-10-02 | Thorburn Technics Int | Electromagnetic inspection probe |
| DE69233065D1 (en) * | 1991-06-11 | 2003-06-26 | Newt Holdings Ltd | probe |
| CA2310467A1 (en) * | 1997-11-21 | 1999-06-03 | Volvo Aero Corporation | A testing method for machined workpieces |
| US7289913B2 (en) * | 2004-10-08 | 2007-10-30 | Jentek Sensors, Inc. | Local feature characterization using quasistatic electromagnetic sensors |
| CN107907587B (en) * | 2017-11-10 | 2021-05-11 | 南昌航空大学 | Under-damping state pulse eddy current detection system |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3197693A (en) * | 1960-10-04 | 1965-07-27 | Hugo L Libby | Nondestructive eddy current subsurface testing device providing compensation for variation in probe-to-specimen spacing and surface irregularities |
| US3449664A (en) * | 1966-05-16 | 1969-06-10 | Bell Inc F W | Magnetic reaction testing apparatus and method of testing utilizing semiconductor means for magnetic field sensing of an eddy-current-reaction magnetic field |
| US3753096A (en) * | 1971-02-04 | 1973-08-14 | Automation Ind Inc | Eddy current flaw detection system with left off compensation |
| SU595671A2 (en) * | 1976-01-28 | 1978-02-28 | Иркутский Филиал Всесоюзного Научно-Исследовательского И Проектного Института Алюминиевой Магниевой И Электродной Промышленности | Structure scope |
| DE2746568A1 (en) * | 1977-10-17 | 1979-04-26 | Kraftwerk Union Ag | EDDY CURRENT PROBE FOR TESTING FERROMAGNETIC MATERIALS |
-
1980
- 1980-08-18 CA CA000359392A patent/CA1158314A/en not_active Expired
-
1981
- 1981-07-31 IT IT68073/81A patent/IT1144610B/en active
- 1981-08-03 DE DE19813130685 patent/DE3130685A1/en not_active Withdrawn
- 1981-08-04 GB GB8123782A patent/GB2082330B/en not_active Expired
- 1981-08-10 FR FR8115475A patent/FR2488693A1/en active Granted
- 1981-08-17 JP JP56128655A patent/JPS5772055A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| JPS5772055A (en) | 1982-05-06 |
| GB2082330A (en) | 1982-03-03 |
| IT1144610B (en) | 1986-10-29 |
| GB2082330B (en) | 1984-05-31 |
| FR2488693B1 (en) | 1984-01-06 |
| DE3130685A1 (en) | 1982-04-15 |
| FR2488693A1 (en) | 1982-02-19 |
| IT8168073A0 (en) | 1981-07-31 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US5512821A (en) | Method and apparatus for magnetically detecting defects in an object with compensation for magnetic field shift by means of a compensating coil | |
| US5541510A (en) | Multi-Parameter eddy current measuring system with parameter compensation technical field | |
| CA1158314A (en) | Eddy current surface probe | |
| US5689183A (en) | Electromagnetic-induction type inspection device employing two induction coils connected in opposite phase relation | |
| US3358225A (en) | Lift-off compensation for eddy current testers | |
| EP0543648A1 (en) | Inspection device using electromagnetic induction and method therefor | |
| JPH1078336A (en) | Eddy current measurement system using multiple parameters with parameter compensation | |
| Nonaka | A double coil method for simultaneously measuring the resistivity, permeability, and thickness of a moving metal sheet | |
| US4182986A (en) | Test, control and gauging method using locked oscillators | |
| GB2240184A (en) | Measuring device for determining an angle of rotation | |
| EP0076617B1 (en) | Process and apparatus for identifying coins | |
| US3464002A (en) | Low q test coils for magnetic field sensing | |
| CA1254265A (en) | Lift-off compensation of eddy current probes | |
| US4703265A (en) | Method and apparatus for compensating for different depths of sun currents induced in the object tested for imperfections | |
| US5744954A (en) | Magnetic field generation in ferromagnetic metal objects | |
| CA1330829C (en) | Metal measuring method and apparatus | |
| US3050678A (en) | Magnetic testing system for metallic bodies | |
| Kakimoto | Detection of surface defects on steel ball bearings in production process using a capacitive sensor | |
| EP0544911B1 (en) | Device for detecting magnetic flux | |
| US2574311A (en) | Magnetic testing of elongated material | |
| US5006801A (en) | Eddy current inspection probe with probe coil resonace reduction | |
| SU1711105A1 (en) | Method of determining permeability of the sample | |
| SU1375943A1 (en) | Thickekss gauge | |
| Capobianco et al. | Mapping of eddy current probe fields | |
| SU1368925A1 (en) | Method of determining eccentricity of conducting core of cable relative to circumference of its insulation |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |