FI59302B - FOERFARANDE FOER ATT UTFOERA ICKE-DESTRUCTORING CONTROL MED TILLHJAELP AV VIRVELSTROEMMAR OCH HAERFOER LAEMPAD ANORDNING DAER MAN ANVAENDER EN FLERFREKVENT MAGNETISERING OCH MED VARS HJAELP MAN KAN ELIMINAR VISSA PAR - Google Patents

Description

· 4Μ ~ »1 [Β] (11) ADVERTISEMENT 59302 WA lJ 1; UTLÄCGN INGSSKAIFT 3 7 ° U ^ C Patent: ny - ;: inr1, ty 10 07 1931 * Patent mumsel ^ ^ (SI) Kv.lk-Wci.3 G 01 N 27/90 FINLAND — FINLAND OD Patent application - Pttentaneeknlnf 762581 ( 22) HaJcemlapilvI — AiwAkninfsdif 08.09.76 '' (23) AlkiiplM —- GlMghetedag 08.09.76 (41) Tullut lulkiukil - Bllvlt offuntllg 10.03.77 PMMtU- | a rakittwitallKu · WI - »» -. I.taMd—

Patent and registration authorities Aneekan utlagd och. »Krtften puUicurad 31.03.81 (32) (33) (31) Pyy *** utuolkuM-B ^ lnl priority 09-09-75 France-France (FR) EN 7527615 (71) Commissariat a l'Energie Atomique, 29 »rue de la Federation, Paris 156, France-Frankrike (FR) (72) Michel Pigeon, Bures sur Yvette, France-Frankfurt (FR) (Ik) Oy Kolster Ab (5I +) to carry out the inspection by means of eddy currents, and a suitable device for this purpose, which uses multi-frequency magnetization and by means of which certain parameters can be removed - Förfaran-de för att utföra icke-destructive control med tillhjälp av virvel-strömmar, och härför warmad anordning, där man använder In the case of high frequency magnetization, and with the same power can be eliminated by all parameters

The invention relates to a method for performing a non-destructive inspection by means of eddy currents, and to an apparatus for carrying out this method. The invention can be applied to the inspection of metal bundles, in particular bundles of pipes for heat exchangers, condensers or steam generators or steam generators.

As is known, control by eddy currents is based on the study of variations in the currents induced in a metal body by the effect of a magnetic field generated by a coil flowing through an alternating current that magnetizes. Such induced currents, in turn, generate a field which acts against the inducing field and thus changes the impedance of the magnetizing coil. This coil is placed in a probe that moves along the body to be inspected.

2 59302

Any fault in the inspected body that occurs at the height of the probe (change in some dimension, variation in electrical conductivity, cracks, etc.) changes the flow or intensity of eddy currents and the impedance of the winding, respectively.

The probe used for the inspection is generally formed of two adjacent windings which are fed in opposite directions and which are placed in two adjacent branches. As the fault passes the probe field, the bridge becomes unbalanced twice, first in one direction and then in the other. The voltage generated by the probe is amplified and, after analysis, can be represented on the image surface of the cathode ray tube. This representation is made by expressing the resistive (i.e., real) component x and reactive (i.e., imaginary) component y of the measured voltage, the complex voltage generated by the probe is thus described as a point determined by the coordinates X, Y. When the fault passes the probe field, this descriptive point plots Each fault can then be identified by the phase of the block of eight (the slope with respect to the reference axis) and its amplitude.

Known methods based on a single excitation frequency for performing eddy current inspections are ill-suited for poorly defined tasks where it is desired to inspect a body with known and acceptable deformations, as well as discontinuities due to e.g. the presence of massive metal bodies in the vicinity of the body to be inspected.

This is the case, for example, with pipes intended for heat or heat and which are connected to a tubular plate, to the rods or to anti-vibration rods. These discontinuities manifest in the inspection device as very strong signals which can obscure any signals corresponding to the faults sought.

In order to remove parameters considered harmful, methods are already known which retain in the curve showing the variations of the signal generated by the probe only those parts which are representative of the defects sought. In these types of methods, multi-frequency excitation signals are used and the representative curve is gradually rotated so that the signals corresponding to these harmful parameters disappear. In this regard, reference is made, for example, to U.S. Patent 3,706,029, issued December 12, 1972.

The invention relates to a method and a device in which the magnetization 59302 is also performed by means of several frequencies and in which the contribution of one or more parameters to the measurement signal is eliminated. The novelty of the invention is based on the way in which this elimination is carried out.

According to the invention, it is taken advantage of the fact that the behavior of the curves representing the magnetin-iscin-is1n is dependent on the accuracy. In this case, it is possible to eliminate the contribution of the parameter by appropriately combining the curves obtained at different frequencies so that the contribution of the harmful parameter can be compensated by the contribution of the curve obtained at another frequency of this same parameter.

The invention is not limited to the elimination of a single parameter, but it is generally within the scope of the invention to eliminate the n-1 parameter by means of a magnetizing signal of n different frequencies.

More specifically, the invention relates to a method for performing non-destructive inspection by eddy currents, which method is of the type: - causing a probe to move in the vicinity of the body to be inspected, - supplying said probe with a magnetizing current of n different frequencies, - analyzing components in each probe. , characterized by: - assigning to each component a resistive part X in phase with a magnetizing current of the same frequency and its reactive part Y in a square, - converting the phase and amplitude of the parts X1 and Y1 of the first frequency signal so that the the signal component caused by the parameter coincides in phase and amplitude with the parts of the second frequency signal caused by the same parameter and Y2 ^ »- subtracted from the thus converted parts and Yj parts Ja Y2» ηϋη to obtain a new set of resistive and reactive parts X 'and YT so as to obtain a representative curve from which the contribution of the harmful parameter has been eliminated, - the signal represented by components X' and Y 'in the plane is shown.

The elimination of several parameters can be performed in the same way as described above by suitably combining the parts X and Y characteristic of the components obtained at two different frequencies, or by combining the parts X'and Y 'obtained as a result of the first elimination with the parts characteristic of the component obtained at the second frequency. .lie.

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The invention also relates to an apparatus for carrying out the method described above, which apparatus is of the type having: - a sonci arranged in the vicinity of the body to be inspected, the probe and the body moving relative to each other during the inspection, - means for supplying this probe with a magnetizing current obtained by summing n excitation currents with n different frequencies, - means for obtaining components corresponding to each of said n frequencies from the measurement signal generated by the probe, the means being. n analysis circuits which, for each frequency, give a resistive part X in phase with a magnetizing current of the same frequency and a reactive part Y in a square, - means for displaying the measurement signal in a plane marked by two axes perpendicular to each other, one of which has a measurement signal the parts X of the components and the parts Y to the other such that at each frequency a point of said plane whose coordinates are X and Y moves during the inspection of the part along a generally block-like curve, each block corresponding to the defect of the yank parameter of the inspected part, and that it further comprises means which, from the description of the measurement signal, eliminates those parts of the curve which correspond to faults related to certain harmful parameters, which means comprise as many e-minimization circuits as there are parameters to be eliminated, each elimination circuit having; - means for converting the resistive and reactive parts X 1 and Y 2 of the component obtained at one first frequency, which parts at this frequency coincide in the zone corresponding to the fault described by the parameter to be eliminated, with parts X2 and Y2 of the component obtained at the second frequency, to reduce parts X2 and Y2 from the modified parts X1 and V-, so as to obtain a new batch of resistive and reactive parts X 'and Ί which are then connected to means describing the measurement signal and resulting in a curve from which the block corresponding to said eliminated parameter is removed .

The features and advantages of the invention will be explained in more detail below with reference to the accompanying drawings, which illustrate some of your embodiments.

Figure 1 illustrates the known principle of showing a component of a measuring voltage by indicating on the image surface a point whose coordinates correspond to the part of said component which is in phase with phytagnetization and the part which is square in phase.

5 59302

Figure 2 shows the different shapes that a representative curve can achieve with the same body when the excitation frequency is changed.

Figure 3 illustrates the parameter elimination principle according to the invention by compensating for the effects which the same parameter develops at two different frequencies.

Figure 4 schematically shows a dual frequency circuit by means of which the parts X and Y of both components of the measurement signal can be obtained.

Figure 5 schematically shows a circuit by means of which, according to the invention, a parameter can be eliminated by means of these two components of different frequencies.

Figure 6 schematically shows a circuit that can be used to eliminate two parameters between signals occurring at three different frequencies.

Figure 1 illustrates a known principle for showing the measurement voltage generated by a probe under eddy currents. The plane is marked by two axes Ox - Oy perpendicular to each other, and on these axes the part X of the measurement signal, which is in phase with the excitation signal n, and the part Y, which is square-phase with respect to the magnet oi n ti signal, are marked. In such a complex representation, the point determined by the coordinates X and Y thus represents the measurement voltage at some moment at some excitation frequency 11a, and the curve drawn by this point depicts the variations of the component occurring at this frequency as the probe and the object to be moved move relative to each other. The curve drawn by the point M is, as is known, generally octagonal. This curve is indicated by the number 10 and is shown only in this Figure 1.

The amplitude and slope of the obtained curve depend, as is known, on the inspection frequency. Figure 2 illustrates this dependence in the case of a piece of "Incone1" tube with diameters of 22.2 to 20.7 mm, in which the probe moves. For the sake of presentation, it is assumed that there is a defect in the inner surface of this tube, denoted D1, a defect in the outer surface, denoted D, and a metal spacer is attached to this tube so as to create a discontinuity P. Figure 2 shows the curves shown in obtained due to these three deviations at three different frequencies, 20 1Hz, 100 kHz, and 240 kHz, respectively.

At 20 kHz, the internal and external faults are in phase and their amplitude is small compared to the amplitude caused by the spacer P. 59302. This is explained by the fact that at this low frequency the penetration of the magnetic field is large so that the field reaches the plate.

At the interval frequency of 100 kHz, the ratio of the amplitudes is less significant, and it is observed in particular that phase shift occurs between the inner and ul hard drives.

The frequency of 240 kHz corresponds to the case where the phase shift o between internal and external faults is 90 °. It is already known that there is generally a frequency at which the phase shift between the faults inside the pipe and the faults on the outside of this pipe acquires this advantageous value. In this regard, reference is made, for example, to report no.

R 4073, published in October 1970 as "Contribution a 'letude des courants de Foucault et application au controle mu 11iparametre des tubes", Michel Pigeon,

In the selected example, at this frequency, the penetration depth of the 240 kHz eddy currents is of the same order of magnitude as the wall length of the tube, so there is only a weak field outside this tube, which explains the decrease in amplitude of the signal corresponding to the spacer. The ratio of the amplitudes of the internal and external faults is of the order of 0.4.

In order to completely eliminate the fault corresponding to the plate P, the frequency should thus be continued to be increased, but this would inevitably lead to a limiting e 1 imi of the faults on the outer surface.

This approach would therefore not be advantageous. Thanks to the invention, this disadvantage is avoided and the elimination can be specifically carried out by applying the method, the principle of which is schematically illustrated in the diagram of Figure 3.

In this figure, the blocks which form the curve drawn by the point M during the inspection of the pipe are shown schematically and for simplicity by vectors whose direction is the middle direction of the block and whose amplitude is the same as the maximum amplitude of the block. Figure 3 further shows three faults corresponding to the three faults shown in Figure 2, namely an internal fault, an external fault and a spacer.

In Fig. 3a, these faults correspond to the first frequency A, and in Fig. 3b to the frequency B. The frequency A is e.g. 100 kHz and the frequency B 240 kHz. Based on the frequency, an index A or B is assigned to each representation of the fault. Thus, the notation DgA> corresponds to an external fault representation at frequency A.

Fig. 3c shows the defects obtained from this Fig. 3a by the transformation a 11a to Fig. 3a homothetically with the ratio k, where k is chosen 7 59302 so that the amplitude of the vector kPA at frequency A corresponding to the plate P becomes equal to the amplitude of the vector PB obtained at frequency B.

It may happen that the amplitudes of the vectors D A and D.A at frequency A change in the same way and become both = kD A and kD.A. Only

6 X

for the sake of simplicity of the description, it is assumed that the coordinates X and Y have been converted in the same ratio k, but without departing from the scope of the invention, the coordinates X can be multiplied by the first coefficient and the coordinates Y by the second coefficient k.

Fig. 3d shows the transformation directed to Fig. 3c when this figure is rotated at such an angle that the vector kPA becomes parallel to the vector PB representing the same fault at frequency B. Of course, when rotated in this way, the vectors kD A and kD.A rotate equally

s X

much and become vectors kD'A and kDTA.

Figure 3e shows the result obtained by subtracting the diagram of Figure 3d from the diagram of Figure 3b. Since the vectors depicting the discontinuity due to the presence of the plate are in phase and their amolitudes are equal in Figures 3b and 3d, these vectors disappear in the subtraction concept, so only the vectors depicting internal and external defects, namely the vectors, remain in the diagram of Figure 3e. - * - »and -> - * DB-kDTA D B-kD'A.

ii e e

It may be that the simplified representation applied in this Figure 3, which removes a block corresponding to one vector, does not necessarily mean that this block only rotates homothetically, because when multiplying by k and k, the block can deform if x J y u k = k. x y These variations, which are characteristic of the method according to the invention, have thus eliminated the effect caused by the undesired parameter P on the curve representing the variations of the measurement signal. This method can be used if the block corresponding to the undesired parameter is clearly separated from the other blocks, as is the case in Figure 2, but all the more so when the fault to be detected is located close to the object corresponding to the undesired parameter. parameter. This is the case, for example, if the fault in the pipe to be inspected is located close to the plate. In this case, the two blocks corresponding to the disk (the harmful parameter to be eliminated) and the fault (to be expressed) are mixed together, and the second may be overwhelmed by the first in case the latter is small. When the adjustment of the measuring device has been performed in such a way that the block corresponding to the plate is eliminated by the method according to the invention, the block describing the fault clearly appears and the fault can be analyzed and identified.

The transformations characteristic of the method according to the invention are suitably performed by the resistive part X and the reactive part Y of each component of the measurement signal. These parts can be obtained by analyzing the signal given by the measuring probe using methods known per se. In this connection, reference may be made, for example, to the aforementioned report and to U.S. Patent 3,229,198, issued January 11, 1966. Fig. 4 shows, for the sake of clarity, a circuit operating in two frequencies, by means of which these parts X and Y of both components of the measurement signal can be obtained.

In Fig. 4, the measuring probe indicated by reference 12 is formed of a first winding 14 connected to the second winding 16 in a balanced bridge connection with two resistances 18 and two inductances 20. This probe has an input 22 and an output 24. Such a probe is magnetized by a first oscillator 26, which generates a current at frequency A, and a second oscillator 28 which generates a current at frequency B. The currents generated by these two oscillators are summed in an adder circuit 30, possibly followed by an amplifier 32. The output of this amplifier is connected to the probe s terminal 22.

The measurement signal generated by the probe is obtained from output 24. This signal can be pre-amplified by a preamplifier stage 34, e.g. 10 decibels, so that the measurement signals have a sufficient level to perform balancing treatments. These treatments are based on compensating for the imbalance in the probe structure and are performed in a circuit not indicated by reference 36 and connected to oscillators 26 and 26 by connections 38 and 40, respectively, which conduct signals in the same or square phase to the currents generated by the oscillators. After equilibration, the measurement signal is amplified in amplifier circuit 42, e.g. 30 decibels, which gain is such that the signal is not saturated in any way.

The measurement signal provided by amplifier 42 includes signals at shackled frequencies A and B, which are separately filtered by a first bandpass filter 44 centered at frequency A and a bandpass filter 46 centered at frequency B. These filters are suitably quite steep, e.g. de ci be liä / octave. Thus, the filtered signals are amplified by amplifier circuits 48 and 50, and then analyzed by memory sampling circuits 59302 52 and 54. These circuits receive, via terminals 56 and 56, two reference signals at frequencies A and B, respectively, in phase and square phase with respect to the currents generated by the oscillators. These memory samplers provide 60-62, respectively, at both their output terminals. 64-66 with respect to the excitation currents, phases X and square phases Y. These sampling devices may be followed by low-emission filters 6Θ and 70, which can be used to eliminate the remaining sampling noise. The circuit as a whole thus finally gives the resistive parts and the reactive parts Y 1 of the components having frequency A, and the parts Xg and Yg of the components having frequency B. These signals X and Y are equal voltages that vary with the displacement of the probe.

The above circuit has been described by way of example only, so that any other previously known device can be used to determine said resistive and reactive parts at each frequency, and which can be connected to the simulation circuit e1 described below with reference to Fig. 5.

The circuit shown in Fig. 5 is connected to the output of the analysis circuit shown in Fig. 4 and thus receives at its inputs the resistive and reactive parts X 1 and Y 2 corresponding to frequency A and Xg and Yg corresponding to frequency B. This elimination circuit has a balancing circuit 80 in a channel corresponding to frequency A and formed of two balancing devices 80x and BOy. This circuit multiplies the parts ΧΔ and Y., by the coefficients k and k, respectively, which are possibly equal to MM xy, and yields the parts X 'and Y' ,, which are such that the amplitude of the signal corresponding to the parameter to be eliminated \ Γ ~ 7λ 7 “2 ΚΧΔ + Y λ is equal to this same parameter at the frequency MA | j 2 2 B amplitude of the corresponding signal YXg + Yg

That is, the balancing circuit 80 performs the task corresponding to the processing shown in Fig. 3c in the case that k = k. This balancing processing could otherwise be performed simultaneously along channel E using a second balancing circuit 82 in this channel formed of two balancers 82x and 82y. After the balancing circuit 80, a phase shift circuit 84 follows, which converts the parts ΧΔ and Y "and gives the new parts X" and Y "so that at the fault corresponding to the parameter to be eliminated these new parts are equal to the corresponding parts Xg and Yg at frequency B. That is, the phase shift circuit 84 performs this deinterleaving as illustrated in Figure 3d. The subtraction circuits Θ6 and 8Θ finally form the difference between the transistive parts Xg and X "^ and Yg and Y" ^.

Two new resistive and reactive parts X 'are available at the outputs of circuits 86 and 88. and Y ', from which the undesired parameter is eliminated. These parts are fed to the display device 90, possibly after being passed through a phase shift circuit 92, by means of which the curves obtained on the image surface of the display device 90 can be directed.

The display device 90 is suitably connected to an electronic commutator 94, by means of which a lime curve corresponding to the component A of frequency A can be obtained on the image surface after the balancing and rotation processing, which is obtained by supplying to these devices 90 the parts X "^. With the aid of these commutator devices 94, a curve corresponding to frequency B can also be made to appear on the image surface. If the commutator 94 has an electronic cutter, these two curves can be made to alternate and thus the weighting circuit 80 and the phase shift circuit 84 can be adjusted so that after the subtraction processing the undesired parameter is appropriately eliminated. that a curve is displayed which is obtained after the unwanted parameter has been eliminated. One skilled in the art is already familiar with the circuit 84, by means of which the parts X and Y can be moved from the phase to each other so that the representing curve rotates around the starting point. In this regard, reference may be made to said U.S. Patent 3,706,029, in which such circuits are described.

The commutator devices 94 are illustrated only schematically in Figure 5. They could be partially incorporated into the display devices, especially in the case where they have a two-dimensional cathode ray tube. In addition, these commutator devices can be connected to the device to dim the light spot of the image surface when moving from one curve to another. These prior art devices are not described in more detail herein and may be included in the display device system 90.

The elimination of several parameters can be performed sequentially, as shown in Fig. 6.

This figure 6 shows a circuit by which two parameters between three signals occurring at three different frequencies can be eliminated. The circuit 100 generates the parts X. and Y of the component having the frequency A, the parts Xn and YD of the component having the frequency B,

n A □ e D

the components Xg and Yg of the component having a strength C. These components depend on the three parameters' X-./S, ^.

n 59302

The first elimination circuit 102, similar to that shown in Fig. 5, removes the components having frequencies A and B as a parameter and generates components X'and Ί ', which now depend only on both parameters oC and / 3. The second elimination circuit 104, which is similar to the circuit 102, allows the same parameter // to be removed between signals having frequencies B and C. This second circuit 104 develops the parts X "and Y", which also now only depend on the parameters cL and / 3. The third elimination circuit 106 eliminates the parameter fi by using the parts X'and Y'on the one hand and the parts X "and Y" on the other hand. This elimination circuit 106 generates the parts X ^ and Τ '”which depend only on the parameter oi. The parameter / & and X. is thus obtained. eliminated.

Claims (4)

1. A method for non-destructive control by means of eddy currents, of the type according to which: a probe (12) moves in the vicinity of the object to be controlled, feeds said probe with a magnetizing current at n different frequencies , - in the measurement signals emitted by the probe, analyze the components at each of the n frequencies, characterized in that: the phase and amplitude of the parts and Y Y of a signal having a first frequency such that the component of the signal indicated by the parameter to be eliminated coincides with its phase and amplitude with the parts X2 and Y2 indicated by the same parameters of a signal with a second frequency, subtracts from the well-modified portions X 1 and Y 1 portions Y 2 and X 2, giving a new set of resistive and reactive portions X 'and Y', allowing v a representative curve, where the participation of the unwanted parameter is eliminated, produces the signal with components X 'and Y' in a piano. A method according to claim 1, characterized in that after eliminating a first parameter according to the method according to claim 1, a second undesired parameter is eliminated by performing the operations according to claim 1 between parts X 'and Y', obtained after the elimination of the first parameter, and the parts X X and Y Y of a third component having a third frequency. Apparatus for non-destructive checking with eddy currents for carrying out the method of claim 1, which is of the type comprising: a probe (12) disposed in the vicinity of the piece to be checked, the probe and the piece displacing in relation to each other during the control, - means (26, 28) for supplying said probe with a magnetizing current obtained by superposition of n magnetizing currents at n different frequencies, means for emitting from the probe the measuring signal obtaining the components with each of said n frequencies, which are made up of n-analyzing circuits (52, 54) which for each frequency provide the resistive part X in phase with the magnetizing current of the same frequency, and the reactive the part Y in square, means (90) for representing the measurement signal in a plane formed by two perpendicular axes, one of which is provided with the parts X and the other with the parts Y of the components of the measurement signal, that, for each frequency, the point in said plane, whose coordinates are X and Y, describes during the control of the paragraph a general lobe-shaped curve, each lobe corresponding to an error in a parameter in the paragraph to be checked, characterized in that it in addition, means for eliminating in the representation of the measurement signal include portions of the curve corresponding to errors associated with certain undesired parameters, which means include as many elimination circuits as parameters to be eliminated, each elimination circuit comprising: means (80, 84) for modifying the resistive and reactive parts Xj and Yj of a component of a first frequency, for which they coincide in a zone corresponding to an error of a parameter to be eliminated, of the parts X2 and Y2 of a component of a second frequency, and means (86, 88) to subtract from the modified parts X1 and Y2 parts X2 and Y2, giving a new set of resistive and reactive parts X and h Y ', which is then applied to the means to represent the measurement signal and which results in a curve which lacks the loh corresponding to said eliminated parameter. 4. Device according to claim 3, characterized in that each elimination circuit comprises: an equalization circuit (80) connected to the analysis circuit corresponding to the first frequency, which equalization circuit multiplies the parts X X and Y ^ provided by said analysis circuit. coefficients and delivers new coordinates X1, and Y '_, so that 1/2 2 ~' the amplitude Vx'-] + Y'-j for the signal with the first frequency and the corresponding parameter to be eliminated is equal to the amplitude Vx2 + Y2 for the solar signal for the second frequency parameter, - a phase shift circuit device (84) arranged at the output of the equalizer to modify the parts X 1 and Y