EP1848989A1

EP1848989A1 - Method for producing a high-dynamics and high-spatial resolution eddy current testing head

Info

Publication number: EP1848989A1
Application number: EP06709453A
Authority: EP
Inventors: Jean-Marc Decitre; Thierry Sollier
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2005-02-04
Filing date: 2006-01-27
Publication date: 2007-10-31
Also published as: WO2006082334A1

Abstract

The invention concerns a method for producing an assembly of at least one transmission coil (B1) and one reception coil (B2) for eddy current testing, the reception coil receiving in the absence of fault a complex amplitude signal VR, subjected to a variation δVR in the presence of a characteristic fault to be detected. The method consists in selecting the distance ΔER between the axes of the transmission coil and the reception coil so as to maximize the ratio I δVR/VR I.

Description

METHOD FOR PRODUCING A CONTROL HEAD A

CURRENT FOUCAULT OF GREAT DYNAMICS AND

HIGH SPACE RESOLUTION

The present invention relates to an eddy current control method with separate emission / reception functions, with a high dynamic range of operation. It also makes it possible to carry out checks with a set of coils of emission and reception very compact. Therefore, it is particularly advantageous for the detection of small defects, in particular for the non-destructive testing (NDT) of conductive mechanical parts.

Definition: according to widespread use, when the transmit or receive coils are printed on their geometrical shape is called in the following "pattern"; this term is also used to designate associations of some coils designed to work together (eg a transmitter and two receivers located on both sides).

Furthermore, the term fem (abbreviated electromotive force) will be used to designate a voltage induced in a coil by a variation of the electromagnetic field passing through it.

It is known that a defect can significantly alter the mechanical strength of a thin part, even if it is small. It is recalled that the principle of defect detection by Eddy currents in a conductive part consists in emitting in the vicinity of this part, using a transmitting coil, an electromagnetic field of frequency adapted to the conductivity of this material and to the depth of the desired defects. On the one hand, an electromotive force is measured at the terminals of the receiver coil due to the direct coupling of the magnetic field lines between the transmitting coil and the receiver coil in the presence of the conductive part and, secondly, a small electromotive force variation. which is superimposed on it when there is a defect in the material. The invention is limited to processes using at least one coil assigned to the emission of the electromagnetic signal capable of generating eddy currents in the material to be tested, and at least one coil assigned to the reception of the signals induced by the eddy currents (configuration called with separate functions). The electromotive force V _R induced at the terminals of each receiving coil is obviously at the same frequency as the current I _E sent into the associated transmitting coil, and requires demodulation to obtain the wanted signal. In addition, this induced electromotive force V _R varies very strongly when approaching a faultless part at the working distance. In the presence of a fault, the complex induced electromotive force V _R becomes V _R ± δV _R , and only the variation δV _R , very small in front of V _R , carries information. In practice, the ratio | δV _R / V _R | determines the quality of the measurement and the sensitivity of the process. The detection electronics must therefore have a very great dynamic of operation. Given the technological possibilities, this constraint very strongly limits, in the prior art, the possibility of detecting defects of small dimensions, especially when they are badly oriented. Any defects in the material to be tested are all the easier to detect as they further modify the flow of eddy currents. These circulate in the thickness of the test material along a path comparable to the current lines of the inductor coil, but in the opposite direction. It is easier to detect the defects extending in the plane formed by the axes of the transmitting coil and the receiver coil, and placed between them (type I defects). Conversely, it is difficult to detect defects extending in the mediating plane to the plane formed by the axes of the two coils (so-called type II defects).

To limit this incidence of the random orientation of the defects on their detection, numerous methods and devices have been realized, either by carrying out several successive measurements by sweeps of the measuring probe in orthogonal directions, or by multiplexing several transducers. These are generally matrices of transmitters and receivers assembled on the same measurement head, so that the same defect sees in a first direction the field lines going from a first transmitter to a first receiver, and in a second direction, different from the first, the field lines from the same transmitter to a second receiver or a second transmitter to a second receiver. In the latter case, however, the improvement of the detection is generally obtained by the accumulation of a large number of coils on a small surface, but this arrangement creates problems of crosstalk. We then resort to a relatively large spacing of the patterns between them (which limits the density of the patterns) or to a multiplexing of the emitters and receivers of the patterns (which leads to a maximum density of patterns but a slowing of the measure ). The resolution is in any case limited by the distance between the coils of the same pattern, itself limited by their outer radius. To summarize, the very principle of

Foucault leads to the detection of very small complex variations δV _R of V _R , around the much larger value of V _R , which varies in large proportions with the distance from the coils to the part to be controlled, as well as their relative orientation; it also varies, to a lesser extent, with the conductivity of this part, a high conductivity being more favorable. This difficulty, aggravated by the small size of the defects to be detected, is also due to their orientation.

Another characteristic of small-size fault detection is the small size of the transducer coils (small patterns), which leads to detected signals of very low amplitude.

To overcome these difficulties, the devices according to the prior art play on associations of a larger number of coils. Thus, US Patent 6,310,476 "Eddy current flaw detector" filed by Mitsubishi discloses a device with an even number of receive coils, arranged symmetrically with respect to the transmitting coil and differentially connected. In addition to the heaviness and the cost of this device, it also has the defects of the differential assemblies: the undesirable signals are eliminated only if they appear at the same time on the two coils connected in opposition, with the same amplitude and the same phase. In addition, it is imperative that these two coils, and their respective measurement channels, have identical characteristics. To adapt to pieces of curved geometry, and to scan a large area without passing faults of a certain size, General Electric discloses in its patent EP 0 512 796 A2 "Eddy current probe arrays", a device using matrices or eddy-current sensor arrays, whose coils are made on flexible support to remain as close as possible to the defects when the test piece is of curved geometry, and are further spatially correlated. The inductors consist of two rectangular coils very elongated in the direction orthogonal to the scan. Faced with these two inductors, two rows of receiver coils are arranged staggered vis-à-vis the first row of coils. Any fault located between two receiving coils of the first row is located above the coils of the second row in the preferred direction of scanning, and vice versa. Nevertheless, despite the cost and heaviness of such a device, the defects difficult to detect because of their orientation relative to the first row of coils keep this same orientation, and therefore this same difficulty of detection, compared to the second row of receiver coils. Finally this document specifies several embodiments of the coils and the methods of adding a shield. The receiver coils all receive on their entire surface the field lines immediately emerging from the corresponding transmitter coil (large mutual inductance).

A second patent of General Electric USP 5,262,722

"Apparatus for near surface nondestructive eddy current scanning of a conductive part using a multi-layer eddy current probe array", discloses a device, similar to the previous one, but implemented in a multiplexed manner.

In conclusion, the current state of the eddy current control methods and devices does not make it possible to be certain of the detection of dimensional defects detrimental to the strength of certain mechanical parts, in particular when the size of the defects or their orientation. is unfavorable to their detection.

DISCLOSURE OF THE INVENTION: The invention relates to a method of producing an assembly of at least one transmitting coil and a receiving coil for an eddy current control, the receiving coil receiving in the absence of defect a complex amplitude signal V _R , subjected to a variation δV _R in the presence of a standard defect to be detected, characterized in that one chooses the distance Δ _ER between the axes of the coils of emission and reception of to maximize the ratio | δV _R / V _R | .

Each coil is composed of turns, generally of several dimensions. The widest turn describes a loop which is called R the greatest distance from its center of gravity. For coils whose axial section has an elongated shape, it is advantageous to introduce two values of R: R = R _M along the axis of maximum dimension, and R _m along the minimum dimension axis.

The method according to the invention aims to perform the eddy current control by placing and maintaining between the respective axes Ai and A ₂ of two transmitting flat coils Bi and B ₂ receiver, a distance comparable or equal to Δ _ERO which optimizes the ratio | δV _R / V _R | characterizing the quality of the measurement. This optimization is then performed intrinsically for each pair of transmitting / receiving coils positioned according to the method detailed below.

The process according to the invention is involved in the design and the realization of the transducer head, after the characteristics of the material to be controlled, the value of the air gaps e and e 'between this piece and each of the coils, made it possible to determine the working frequency, the shape and the size of the coils whose largest radius is R. It comprises, according to a preferred embodiment, in which the greatest distance between the center of gravity of the largest dimension coil and the point farthest from the largest turn of this coil is called R, the following steps: a- determining a value of the frequency of the excitation current flowing through the transmission coil, this frequency being determined in a conventional manner; b- determining a standard defect to be detected, characterized by its size and average depth relative to the surface of the part to be controlled; c- from the values obtained in steps a and b to determine, either by modeling or experimentally, the following three variables: i) the complex V _R emf induced in the receiver coil when the distance Δ _E R between the coil axes of transmission and reception varies at least in the interval {0; 3R}, ii) change .DELTA.V _R, the complex V _R emf induced in a receiver coil for the same change in distance Δ _E R in the range of at least {0; 3R}, iii) the variation | δV _R / V _R | report module of the two quantities defined in ii) and i), for the same change in distance Δ _E R in the range of at least {0; 3R} d- draw the curve of the ratio | δV _R / V _R | as a function of the variation of the distance Δ _ER in the interval of at least {0; 3R}, then determine the maximum ordinate | δV _R / V _R | _{0 /} and the abscissa Δ _ER0 of this maximum, this value constituting the optimal operating point; e- optionally and suboptimally, determine a range of distances {Δmin; ΔMax} on both sides of Δ _ER0 , for which the ratio | δV _R / V _R | worth one third of its maximum value, this range corresponding to what will subsequently be referred to as "distance comparable or equal to Δ _E RO"; f- if the Δ _E values of R is found in step d when searching the optimum criterion, or in step e when the search criterion suboptimal, are not acceptable, modify the steps of the parameters a and b; g- producing the coils or coil assemblies, such that the operating point of each receiving coil satisfies either the optimum criterion of step d or the suboptimal criterion of step e.

According to a first preferred embodiment, the distance between the respective axes of each pair of transmit / receive coils is fixed and the frequency of the excitation current is adjusted according to the minimum dimension and the mean depth of the standard defects to be detected. . In this case, it is necessary for the excitation electronics to be capable of modifying the working frequency of the electronic means associated with the coils, for example whenever the size of the standard defects to be detected is modified, or else if changes the material of the part to be controlled. By electronic associated with the coils is meant electronic means for supplying current to the transmission coil (s), for demodulating and processing the signals of the reception coil (s), and possibly for adding, subtracting or multiplex signals from several coils. According to a second preferred embodiment, the excitation frequency is constant, and the method according to the invention makes it possible to determine, as a function of the minimum dimension and the mean depth of the standard defects to be detected, an optimum value (step d). or suboptimal (step e) of the distance _e Δ R between the respective axes of each pair of transmitting and receiving coils.

According to a first variant of the first or second preferred embodiment, the distance Δ _E R between the respective axes Al and _A2 of the two coils Bi respectively transmitting and receiving B ₂ varies in the range {0; 3R}. This first variant is particularly advantageous for the detection of small defects. According to a second variant of the first or second preferred embodiment, the distance Δ _ER between the respective axes Ai and A ₂ of two coils respectively transmitting Bi and B ₂ receiver varies in a range greater than {0; 3R}, and preferably equal to {0; 9R}. In this case, there is generally, for abscissa greater than 3R, a second maximum of the ratio | δV _R / V _R | when Δ _ER varies. This second maximum is all the higher as the standard defect to be detected is greater. At this second maximum of abscissa Δ _ERO2 and ordinate | δV _R / V _R | _{2 /} corresponds to a second range of values {Δmin; ΔMax} ₂ on either side of Δ _E R _O 2 _/ for which the ratio | .DELTA.V _R / V _R | is one third of its maximum value | δV _R / V _R | ₂ . This variant is advantageous for the search for large defects while obscuring small defects and decreasing the risk of artifacts. According to a third variant of this second preferred embodiment, the distance Δ _ER between the respective axes Ai and A ₂ of two coils respectively transmitting Bi and B ₂ receiver is carried out in step g adjustably. In this case, the transmitting coils are on a first support, for example a first printed circuit, and the receiving coils are on a second support, for example a second printed circuit, and the two supports can move relative to each other. the other through sliding means. These means allow not only an adjustable sliding of the relative position of the two supports, but also its maintenance during the measurement phases, once the control head is made. They can be passive of the mechanical type (for example with screw and / or slide), or active comprising at least one micro-actuator (for example piezoelectric).

When the coils have an axial section of elongate shape, the widest turn having a large radius R _M and a small radius Rm, it is possible to apply to each of the embodiments described above the variant below. In the plane comprising the axis of the coil and the large radius R _M , the application of the invention makes it possible to define a first optimum distance (Δ _E Ro) Maχ and an associated range of values {Δmin; ΔMax} _Ma χ _/ whereas in the plane comprising the axis of the coil and the small radius R _m . , the invention makes it possible to define a second optimal distance (Δ _ER0 ) associated min and a second range of values {Δmin; ΔMax} _min .. The existence of several optimal distances along several axes thus makes it possible to produce patterns where the coils are not necessarily at the vertices of squares or lozenges. Such a variant will be further illustrated by FIGS. 7b, 7c and 8b.

Step c of the process can be determined either by modeling or experimentally using a device similar to that described below by way of example. The two flat coils Bi for the emission and B ₂ for the reception are each made on a thin kapton printed circuit, of identical dimensions: 6 spiral plane spirals, the widest of which has a radius R of 0.5 mm and the smaller one has the diameter of the central metallized hole used to pass the connection. They can slide against each other. The respective axes A ₁ and A ₂ of Bi and B ₂ can thus have a variable distance Δ _ER between them. A standard defect is fixed corresponding to either the type of defect the smallest that one wishes to detect, or to a defect considered representative. In the example, we choose a parallelepipedal defect elongated in a main direction, and measuring in this direction 0.4 mm, and in other directions 0.1 mm wide (parallel to the surface) and 0.2 mm deep . It is flush with the surface of the room to be controlled. The orientation used for the modelizations or the experiments is that which maximizes the probability of detection: the length of 0.4 mm is centered on the axis connecting the centers of the coils. The part to be tested has a conductivity σ of 1 MS / m and a thickness of 3 mm. The working frequency is chosen equal to 2 MHz.

The pair of transmitting / receiving coils is then placed in front of the part to be inspected, at a distance e from the nearest coil (for example the receiving coil) and e 'of the other coil, the distance e. e being the sum of the axial length of the nearest coil and the thickness of the support. This distance e is chosen with the help of the usual practice of eddy current checks. The transmitter coil is excited by a current I _E at a frequency adequate to induce eddy currents. The receiver coil is then the seat of an electromotive force V _R induced at its terminals.

The phase φ that presents V _R with respect to I _E changes between the position where the axes Ai and A ₂ are almost identical, and the position where these axes are separated by about two times R or slightly more. Between these two positions, there is an intermediate position where the phase φ varies very suddenly and where the module of | V _R | goes through a minimum. When the distance between the axes A ₁ and A _{2 is further increased} , the modulus of the electromotive force increases again without reaching a value as high as that of the first maximum, then decreases, but with a phase which varies more slowly. Figure la shows the module | V _R | the complex value of the electromotive force V _R induced at the terminals of the receiver coil when the distance Δ _ER is varied between the axes of the transmitting and receiving coils. It is plotted for Δ _ER varying in the interval {0; 3R}, the gap value is e = 0.1 mm and e '= 0.15 mm, and the room conductivity is σ = lMS / m. For a zero Δ _ER distance, the coils have the same axis and the complex amplitude V _R of the electromotive force collected at the terminals of the receiver coil is obviously maximum. When Δ _ER increases, this amplitude falls very rapidly to pass through a first minimum indicated in the figure by an arrow, and the phase varies very rapidly by about 180 °. In the absence of a conductive part in front of the transducer coils, the electromotive force V _R vanishes to change direction, and the phase φ switches almost instantaneously 180 °.

In front of a room of conductivity σ located at distances e of the transducing coils (which are in planes extremely close to each other), the phase variation is less abrupt, according to a profile depending on the values of σ and e, and the modulus of the complex amplitude of V _R passes through a minimum which is no longer equal to zero, and whose position and value depend in particular on the values of σ, gaps e and frequency. In the presence of a piece to be controlled, it is the appearance of an imaginary component, not canceling for the same abscissa as the real component, which has the effect of shifting the position of this minimum and making its non-zero value.

This minimum of the modulus of the complex amplitude of V _R makes it much easier to detect a small variation δV _R around V _R. However, it is not directly the minimum of V _R that determines the quality of the measurement and the dynamics, but the ratio | δV _R / V _R | . It is this relationship that must be as great as possible, and that it is important to maximize when σ and e vary.

An elongated standard defect is then chosen in a main direction where it measures for example 0.4 mm, its other dimensions being negligible. After the figure, and for the same parameters gap, and the same range of variation Δ _E R in the range {0; 3R}, the curve Ib of the variations of the electromotive force δV _R induced by the standard defect of 0.4 mm is noted experimentally when Δ _E R varies (Δ _E R being on the abscissa and δV _R on the ordinate). The quality of the measurement is then represented by a third curve Ic representing the variation of the ratio | δV _R / V _R | (in ordinates) according to the variations of Δ _ER (in abscissas). On this curve, the ordinate | δV _R / V _R | ₀ of the first maximum, which corresponds to the optimum operating point with the greatest possible measurement dynamics. The corresponding abscissa is then the optimal value Δ _ER0 of the distance Δ _ER .

It should be noted that this optimum operating point does not quite match the minimum value of | V _R | , which itself does not quite correspond to the value that cancels the mutual inductance in the absence of a piece to be controlled vis-a-vis the transducer coils.

On both sides of the optimum operating point of ordinate | δV _R / V _R | ₀ , we find the points whose ordinate is one-third of | δV _R / V _R | ₀ , the first of abscissa Δmin corresponds to the minimum distance for which the operation is close to the optimum, and the second of abscissa ΔMax corresponds to the maximum distance for which the operation is close to the optimum. The interval {Δmin, ΔMax} constitutes the range of distances for which the operation is either optimal or suboptimal, close to the optimum. The method according to the invention for producing a set of transducer coils, comprising at least one transmitting coil Bi and a receiver coil B ₂ , allows numerous variants and combinations, provided that for each pair of transmission coils / reception, their axes Ai and A ₂ are separated by a distance Δ _ER within the range of distances {Δmin, ΔMax}. Preferably, the coils are flat, that is to say that their axial length is more weak than the largest radius of their largest turn.

With this same device, it is easy to highlight the second variant of the second preferred embodiment, according to which the variation range of Δ _ER is wider than 3R. As shown in FIGS. 2a, 2b and 2c, there exists for abscissa greater than 3R a second maximum of | δV _R / V _R | . Its amplitude increases with the size of the defects that one seeks to detect, as shown by the succession of FIGS. 2, 3 and 4

Figures 2a, 2b and 2c are similar to Figures la, Ib and Ic, but they are plotted for Δ _ER variant in the interval {0; 10R}, the gap value (e = 0.1 mm and e '= 0.15 mm) and the conductivity of the workpiece (σ = lMS / m) remain the same, but the main length of the defect is here 1 mm, against 0.4 mm in Figure 1. Figures 3a, 3b and 3c are similar to Figures Ic Ic, but the main length of the defect here is 2 mm; and Figures 4a, 4b and 4c are similar to the previous ones with a main defect length of 3 mm. These three figures show, on the one hand, the existence of a second maximum whose amplitude increases with the size of the standard defect to be detected, and, on the other hand, that it is not useful to increase Δ _ER to- beyond 9R.

In addition to these precisions, it is recalled that for other characteristics such as the excitation frequency of the transmitting coils, the dimensions of the coils and their type (for example with wire, with or without core, or in printed technology), the choices are made taking into account the material to be controlled, the size and depth of the desired defects, using the known experience of eddy current control processes.

Although the method inherently allows each pair of transmit / receive coils a virtual cancellation of the electromotive force V _R induced at the terminals of the receiving coil, it is possible to choose to use, according to a first variant, a transmission coil and two reception coils located symmetrically and connected in opposition, or two emission coils located symmetrically with respect to the same receiving coil and connected in opposition. This produces a differential device, but free from the main defect of the differential devices not implement the process.

Indeed, these defects result from the need for perfect symmetry both from the point of view of the signals to be eliminated and from the point of view of the hardware means corresponding to the two channels connected differentially. According to the invention, the electromotive force V _R induced at the terminals of each receiver coil is intrinsically very close to zero and the slope of the curve representing | V _R | according to Δ _E R vanishes at Δ _E RO and is therefore very low in the vicinity of the optimal point, thus greatly reducing the effects of any asymmetry of the signals to eliminate or characteristics of the coils and measurement channels connected in differential.

According to a simplified variant of the method of the invention combinable with any other variants, it is considered that for Δ _E R near distances Δ _E RO variations | V _R | as a function of Δ _ER are sufficiently weak to be neglected. In this case, the maximum of the ratio | δV _R / V _R | when Δ _ER varies can be approximated by the maximum of | δV _R | when Δ _ER varies. In this case, the steps c, d and e of the method can be written in an equivalent manner: - c'- from the values obtained in steps a and b to determine, either by modeling or experimentally, the variation δV _R , of the a complex emf V _R induced in a receiver coil when the distance Δ _ER between the axes of the transmitting and receiving coils varies at least in the interval {0; 3R}

-d'- draw the curve of | δV _R | as a function of the variation of the distance Δ _ER in the interval of at least {0; 3R}, then determine the maximum ordinate | δV _R | ₀ , and the abscissa Δ _ER0 of this maximum, this value constituting the optimum operating point;

-e'- optionally, determine a range of distances {Δmin; ΔMax} on both sides of Δ _ER0 , for which the ratio | δV _R | is worth one third of its maximum value. A preferred application of this variant is to determine as previously the optimal value Δ _ER0 of Δ _ER or a suboptimal value in the range {Δmin; ΔMax} and vary when certain use conditions such as the nature of the part to be inspected or the size of defects, using the present embodiment for adjusting the distance Δ _E R. More elaborate combinations will be discussed later in the detailed description.

DESCRIPTION OF THE FIGURES:

- Figures la, Ib and Ic show two flat coils Bi and B ₂ of the same radius which move in parallel planes so as to grow their spacing Δ _E R, how to vary the modulus | V _R | the complex voltage induced across the reception coil (Fig la), the signal | δV _R | induced by a 0.4 mm length defect (Fig Ib) and the | δV _R / V _R | between these two quantities, which determines the sensitivity of the process.

FIGS. 2a, 2b and 2c are similar to FIGS. 1a, 1b and 1c, but for a defect of 1 mm in length

FIGS. 3a, 3b and 3c are similar to FIGS. 1a, 1b and 1c, but for a defect 2 mm long, FIGS. 4a, 4b and 4c are similar to FIGS. 1a, 1b and 1c, but for a defect 3mm long

- Figures 5a and 5b show schematically the two coils of a basic system for measuring eddy currents according to the invention, - Figure 6 shows the variations required for the excitation frequency to keep the same optimum distance Δ _ER o when the main dimension of the defect varies,

FIGS. 7a, 7b and 7c schematize the three coils of an alternative embodiment of an eddy current measuring system according to the invention,

FIGS. 8a and 8b schematize the five coils of an alternative embodiment of an eddy current measuring system according to the invention,

FIG. 9 schematizes a variant of the system presented in FIG. 5a including the addition of a ferromagnetic strip which channels the magnetic field lines between the transmitter and the receiver.

FIGS. 10a and 10b schematize two embodiments in which the distance Δ _ER between the coils is adjustable mechanically (FIG. 10a) or by piezoelectric micro-actuator (FIG. 10b)

FIG. 11 schematizes an example of matrix association according to the method of the invention of several couples of transmit / receive coils.

DETAILED DESCRIPTION OF PREFERENTIAL EMBODIMENTS:

According to the embodiment preferably implemented of the invention, the main parameters are those exposed in the explanation of step c. The part to be tested has a conductivity σ of 1 MS / m and a thickness of 3 mm. The coils are on air, directly etched on both sides of the same kapton flexible printed circuit 50 μm thick. A 100 μm thick Teflon protective film is applied to the kapton face in contact with the target, providing electrical insulation and mechanical protection. The gap e is thus 0.1 mm for the receiver coil, and e '= 0.15 mm for the transmitter coil.

In order to ensure the detection of these defects, the same pattern is chosen for the transmit and receive coils, composed of a planar spiral whose largest diameter is 1 mm, and comprising 6 turns etched in copper of thickness 5 μm, the smallest turn having the diameter of the metallized hole providing the electrical connection, ie approximately 0.25 mm.

The corresponding working frequency is around 2 MHz.

It is sought to detect a standard defect elongated in a main direction, and measuring in this direction 0.4 mm (the other dimensions of the defect being 0.1 mm and 0.2 mm as above). It is flush with the surface of the room to be controlled. The latter has a conductivity σ of 1 MS / m and a thickness of 3 mm.

Is considered in the transmitter coil an inductor current I _E taken equal to 20mA; the module | V _R | the complex voltage induced in the receiver coil is then of the order of microvolts when Δ _E R is in the range {Δmin; ΔMax}.

Modeling in the first variant of the second preferred embodiment leads to the drawn curves, Ib and Ic for a distance Δ R _E between the axes of the transmitting and receiving coils varying in the range {0; 3R}, i.e. {0; 1.5 mm}.

By a simple quotient of the values according to the curves Ia and Ib, we obtain the curve of the figure Ic which represents the variations of the ratio I ÔVR / VR I when Δ _E R varies in the interval {0; 1.5 mm}. We thus note the ordinate | δV _R / V _R | ₀ of the maximum equal to 0.19 (dimensionless), the corresponding abscissa of 0.6 mm is called Δ _E RO _/ this value constituting the optimal operating point of step d. If we aim at the suboptimal operation of step e, we look for the points whose ordinates I ÔVR / VRI are equal to 0.063 (the third of the maximum | δV _R / V _R | ₀ ). Then we find the corresponding abscissa Δmin = 0.53 mm, and ΔMax = 0.69 mm, these values being located on either side of Δ _E RO- In practice, we choose the optimal operation, which leads to fix Δ _ER = Δ _ER0 = 0.6 mm, according to the embodiment shown in Figures 5a and 5b. where the coils in the form of a planar spiral whose copper track has a width of 20 microns have been schematized by a succession of concentric circles for the convenience of the drawing. The coils both have an outer diameter of 1 mm, an inner diameter of 0.5 mm and a number of turns of 6, and the working frequency is 2 MHz.

The transmitter coil B1 is shown at the top, and the receiver coil B2 is shown at the bottom, but the invention would not be changed by inverting them. The insulating support 1 is a soft kapton film 50 μm thick. The coil connections 4 use metallized holes whenever necessary to pass from one side to the other of the dielectric support. Thus all the connections are brought to a single face of the printed circuit. According to the first preferred embodiment, the working frequency can be adjusted when the characteristics of the defects or the nature of the piece to be controlled vary. Thus, when the main dimension of the standard defect to be detected increases from 0.4 mm to 3 mm, the initial frequency of 2 MHz must increase to approximately 2.45 MHz to maintain the same optimum distance Δ _ER0 . According to the second preferred embodiment, the operating frequency is fixed, and the optimum Δ _ER0 distance Δ R _E between the axes of the transmitter and receiver coils is determined as described above in the case of the first variant in which the distance Δ _E R between the respective axes Al and _A2 of the two coils Bi respectively transmitting and receiving B ₂ varies in the range {0; 3R}.

In the case of the second variant in which this distance Δ _E R varies in a range {0; 9R}, the procedure is analogous: the upper value Δmax is simply pushed back from 1.5 mm to 4.5 mm, as shown in FIGS. 2, 3 and 4. The interval chosen during step e, referred to as {Δmin ₂ ; ΔMax ₂ }, gives a value of I ÔVR / VRI twice that of the interval {Δmin; ΔMax} corresponding to the previous variant.

At this stage, it is easier to expose the first preferred embodiment in which the distance between the respective axes of each pair of transmit / receive coils is fixed and the frequency of the excitation current is adjusted according to the dimension. minimum and the average depth of the standard defects to be detected. Suppose, as before, that the part to be tested has a conductivity σ of 1 MS / m and a thickness of 3 mm, the coils are on air, directly etched on both sides of the same flexible kapton circuit of 50 μm. thickness. The gap e is thus 0.1 mm for the receiver coil, and e '= 0.15 mm for the transmitter coil. By modeling, FIG. 6 shows the variations in the frequency that the excitation of the inductive coils must have as a function of the main dimension of the defect, when this varies from 0.4 mm (then f = 2 MHz) to 1 mm, 2 mm and then 3 mm (then f = 2.45 MHz), in order to keep Δ _{ER at} optimum Δ _ER0 = 0.6 mm. As the fault length increases, it is necessary to slightly increase the excitation frequency. A pair of transmit / receive coils thus defined significantly improves the detection of type I defects. recalls that the most frequent defects being generally very long elongated in a main direction, they are classified into different types according to the inclination of their largest axis with the plane formed by the axes Al and A2 of a pair of coils of transmission / reception. Thus they are said to be of type I, when their largest axis is in the plane defined by the axes Al and A2, or at the parallel rigor and situated at a short distance in front of the radius R. They are said of type II, when their The largest axis lies in the plane orthogonal to that defined by the axes A1 and A2, while being orthogonal to the plane constituted by the surface to be tested.

Preferably, for the detection of type I defects, there is provided a receiver coil arranged at a location such that the electromagnetic field produced by the two transmitting coils is canceled at this receiver coil, the distance between the receiver coil and each transmitting coils being around the range of distances {Δ min; ΔMax} located around Δ _ERO - On the other hand, this configuration makes it difficult to detect so-called type II defects.

FIG. 7a represents an implementation variant, mainly intended to overcome the edge effects of the parts to be controlled, and possibly to improve slightly the detection of type II defects. It comprises a pattern of three coils, a priori a central transmitting coil B1 and two receiving coils B2 and B3 situated on either side and distant from that of transmitting a value lying in the range { Δmin; ΔMax}. But we would not change the performance by assigning a reception function to the central coil, and a transmission function to the two side coils. We thus obtain a basic pattern consisting of three coils, one of reception located in the center and two of emission, located on both sides, and distant from that of emission of a value included in the range {Δmin; ΔMax}. The windings are in the same direction. For the sake of clarity, the electrical connections have not been shown. The method is carried out as above, and the value found for the distance between the axes A1 and A2 of the transmitting coils B1 and B2 receiving, is reported identically between the axes A1 and A3 of the B transmitting coils and B3 receiver. Advantageously, axes AI, A2 and A3 are in the same plane.

However, this pattern makes it possible to overcome the edge effects only in the case, preferential, where the direction of scanning is in a direction passing through the axes A2 and A3. In this case, in fact, the dissymmetry introduced by the edge of the part affects in the same manner the two receiver coils B2 and B3.

Nevertheless, according to another variant, the scanning direction is orthogonal to a direction passing through the axes A2 and A3, and the asymmetry of the mutual introduced by the edge of the part does not affect in the same manner the two receiving coils B2 and B3. In this case, it is important to correct it by introducing, for example at the level of the demodulation means of the signals coming from the coils B2 and B3, an offset (offset) of amplitude and / or phase. A lack of symmetry between these coils (systematic error) can also be corrected in this way.

FIG. 7b represents another variant, similar to that of FIG. 7a, but in the particular case where the coils have an axial section of elongate shape, the widest turn having a large radius R _M and a small radius R _m . In the plane comprising the axis of the coil and the small radius R _m, the application of the invention allows to define a first maximum distance (Δ _E Ro) mm and a range of values associated {Δmin; ΔMax} _min . In the case of FIG. 7c, the plane comprising the axis of the coil and the large radius R _{M /} the invention makes it possible to define a second optimum distance (Δ _E Ro) maχ associated and a second range of values {Δmin; ΔMax} _max .

Since the transmitting and receiving functions can be inverted, the operation is equivalent by choosing the receiver coil B1 and the two emitter coils B2 and B3. In the latter case, the proximity of a coin edge inducing a mutual difference between the pairs of coils B1 / B2 and B1 / B3 can be compensated at the supply means of the coils B2 and B3 in alternating current, in generating signals of different amplitudes and / or phases, able to compensate for this asymmetry. A lack of symmetry between these coils (systematic error) can also be corrected in this way.

This second embodiment with a pattern with three coils makes it possible to increase the density of the patterns when scanning in a direction orthogonal to the plane containing the three axes, and thus improves the detection of type I defects. On the other hand, it does not improve the detection of the defects of type II. For the control in the vicinity of the edges of the parts to be inspected, the plane comprising the three axes of the coils is placed parallel to the edge of the part. It is then possible to wire the two receiving coils in differential mode, which makes it possible to eliminate the disturbance induced by the proximity of the edge of the part. More generally, such a differential configuration minimizes the effects of distance variation between the transmitting coil and the receiver, which allows less sensitivity to distance differences when producing coils.

The detection of Type II defects is improved by a refinement of the pattern comprising a receiving central coil B1 and the two coils B2 and B3 emitting on either side. This improvement is characterized in that two additional receiver coils B4 and B5 (FIG. 8a) associated respectively with each transmitting coil B2 and B3 are added at a distance in the range of distances {Δ min; ΔMax} located around Δ _{ERO /} and in a direction substantially perpendicular to the plane passing through the axes of these transmitting coils. These second and third receiver coils make it possible to detect elongated defects in this perpendicular direction (of type II), the distance between each transmitting coil and the receiver coil being optimized for the detection of these defects in this direction. In this way, a five-coil configuration is obtained, schematized in FIG. 8a, which represents a third embodiment. The coils B2 (20) and B3 (30) of axes A2 and A3, located on a first face of a printed circuit, are emitter coils, receiving electronic excitation means 34 an alternating current at the frequency of chosen excitation. The detection, on the other hand, is ensured by a set of three receiving coils B1 (10), B4 (40) and B5 (50) of axes A1, A4 and A5, situated on the second face of the printed circuit and connected to preamplification and detection means 35. The axes A2, A3, Al, A4, A5 are parallel to each other. The coil B1, has its axis Al located preferentially in the plane defined by A2 and A3 and necessarily equidistant from these two axes by a distance d belonging to the range of distances Δd. The signal it delivers is applied to the input of a first measurement channel. The coil B4 has its axis A4 located at a distance d, belonging to the range of distances Δd, of A2 and defining with A2 a plane orthogonal to the plane defined by A2 and A3. The coil B5 has its axis A5 located at a distance d from A3, d belonging to the range of distances Δd, and defining with A3 a plane orthogonal to the plane defined by A2 and A3. The two coils B4 and B5 are connected in series or in opposition, to form the same signal source, applied to a second measurement channel of an amplifier 40.

FIG. 8b shows a variant of the coils of FIG. 8a in the particular case where the coils have an axial section of elongate shape, the widest turn having a large radius R _M and a small radius Rm. In the plane comprising the axis of the coil and the large radius R _{M /} the application of the invention makes it possible to define a first optimum distance (Δ _E Ro) Maχ and an associated range of values {Δmin; ΔMax} _Ma χ _/ whereas in the plane comprising the axis of the coil and the small radius R _m . , The invention allows defining a second maximum distance (Δ _E Ro) mm and associated a second range of values {Δmin; ΔMax} _min ..

More generally, this variant, which can be cumulated with all the preceding and following embodiments, is characterized in that the emission and reception coils have an elongate axial section, the widest coil having on the one hand a small radius Rm, determining in its orientation a range of distance {Δmin; ΔMax} equal to {Δmin; ΔMax} _min in which is chosen the distance between the axes of turns in this orientation, and secondly a large radius R _M determining a range of distance {Δmin; ΔMax} equal to {Δmin; ΔMax} _Ma χ- in which is chosen the distance between the axes of the turns according to this orientation.

The preamplification and detection means (35) comprise a first preamplifier receiving the signals from the measuring channel coming from the coil B1, a first preamplifier receiving the signals from the measuring channel coming from the coils B4 and B5, and electronic means of demodulation and processing. They are designed to determine the difference between the peak values of the emf. existing between the two measurement channels. The resulting information, possibly improved by corrections or filtering known to those skilled in the art, provides good detection of all defects, regardless of their orientation. If the test area is close to the edge of the test piece, playing on the intensity of the currents and their phase eliminates the mutual for each line of movement along the edge of the room. Providing two transmitting coils B2 and B3 also makes it possible to compensate, by a control electronics, the symmetry defects between the coils B2 and B3. For this purpose, this compensation is performed by phase and / or amplitude variations.

In these two embodiments associated with FIGS. 7 and 8, a large dynamic of the disturbance signal due to the defect is obtained, and furthermore, the optimum spacing between the transmitting coil and the receiving coil depends little on the size of the defect. It should be noted that the optimum operating point of the device can be adjusted by adjusting the supply frequency of the transmitter coil as shown in FIG.

In one variant, which can be combined with all the other variants, the magnetic field lines are channeled and improves the signal-to-noise ratio of the device by implanting, in the vicinity of the coils and on the side opposite the part to be controlled, a soft magnetic material (weak hysteresis), for example ferromagnetic. In this way, the reluctance of the magnetic circuit of each pair of transmit / receive coils is significantly reduced. Thus the coupling between the transmitter and the receiver is greater and the induced fem is greater (both the fem with the direct coupling of the coils and the fem variation due to the defect). The magnetic material may for example consist of ferrite plates. Nevertheless, in the case of a flexible kapton insulating support, it will be advantageous to favor a flexible magnetic material. It can notably use flexible ferrites, permalloy ferromagnetic ribbons or nanocrystalline materials (taking care to electrically isolate this ribbon from the turns of the coils if the ribbon is driver). It is also possible to deposit this soft magnetic material by electrolysis.

Figure 9 shows a variant of the system shown in Figure 5a including the addition of a ferromagnetic tape (9) as described above. It channels the magnetic field lines between the transmitter and the receiver. In the case where the ribbon is conductive, an insulating layer 8 is inserted between the ribbon and the printed circuit engraved on the kapton 1.

Figures IQa and IQb schematically a variant combinable with all other variants. The transmission coils are located on a first insulating support 1, the receiver coils (not shown) are located on a second insulating support 11 in contact with the first, and at least one device to change the distance Δ _E between R coils or pairs of transmitting / receiving coils.

According to the variant of Figure 10a, this device is mechanical. The ends of the first insulating support 1 are glued to an end support 2 having two threaded holes, while the ends of the second insulating support 11 are glued to an end support 12 having two holes of diameter greater than the threads of the support 2. . between the parts 2 and 12 are interposed elastic strips 13 made of elastomer, more or less compressed by the screw 14 so as to change the distance Δ R _E between the axes of each pair of transmitting / receiving coils. According to the variant of Figure 10b, the device for changing the distance Δ _E R between the pairs of coils transmitting / receiving comprises at least one active mechanical device (micro actuator) that allows to change the distance Δ _E between R coils of at least one transmitting torque. FIG. 10b is therefore identical to FIG. 10a, except the elastic bands 13 which are here replaced by piezoelectric micro-actuators 14 fed by electrical conductors 15.

Finally, the invention can be combined with the known technique of associating a plurality of transmit and receive coils into a matrix of transducer coils. Each of the above embodiments, or a combination of these modes, may thus be reproduced a plurality of times in one or two dimensions, each representing a step of measurement. For these matrix combinations, it is particularly advantageous to produce the coils by etching the two faces of a printed circuit. Since these embodiments constitute simple transpositions of the examples described, it is not necessary to represent them by a specific figure.

Advantageously, the two receiver coils mounted to detect the second type of fault, are arranged in series or in opposition. According to a preferred embodiment that can be combined with all the other embodiments except those of FIGS. 5a and 5b, the method according to the invention is implemented by coils on air, etched on a flexible double-sided printed circuit, one of which the faces carrying the transmitting coils, and the other the receiving coils. With such a measurement method, a spatial resolution of the defects substantially greater than the resolution obtained with the devices operating in separate functions as they exist according to the prior state of the art is obtained. In addition, the gain is noticeable on the parameter | δV _R / V _R | , which determines the detection sensitivity of the process. Finally, there is a variant for detecting faults even more easily, while reducing defects smaller than a predefined critical size, as well as artifacts.

It is specified only that the choice of a printed circuit technology for producing the coils is advantageous for the implementation of the invention. This printed circuit is advantageously flexible, like those that can be achieved with kapton, for certain types of parts to control, given the importance of maintaining a constant very small air gap. It is advantageous to use a double-sided printed circuit, one of the faces carrying the coils assigned to the transmission, and the other side carrying the coils assigned to the reception. If the number of turns of each coil is too high to be etched on a single face of a circuit, it is advantageous to use multilayer printed circuits, some layers carrying the coils affected to remission, and other layers bearing the coils assigned to the reception.

Finally, the present invention may be advantageously combined with a matrix structure for the transmitting and / or receiving coils. In this case, the eddy current measuring device comprises a pattern of at least two flat coils Bi, B ₂ according to the invention, this pattern being repeated a plurality of times so as to constitute a detection matrix, and The associated electronics include means for multiplexing the transmitting coils and demultiplexing the receiver coils. By way of example, an embodiment of this type (matrix or bar configuration) is shown schematically in FIG. 11. The succession of transmission / reception coil pairs is organized on two lines, orthogonal to the direction of displacement, and offset laterally. half a step to improve the probability of detection.

The electronic processing means of the signals from the receiver coils can be constituted by any type of circuit for measuring the fem of a coil at its terminals. This circuit optionally includes one (or more) amplification stage (s) followed by a demodulation stage intended to suppress the excitation frequency of the inductor coil or coils.

If the electronics connected to these coils allow it, we can also reverse the role of the transmit and receive coils for each measurement. This arrangement is particularly advantageous in the case of a combination of several emitter and receiving coils matrices. The role reversal of the transmit and receive coils must then be managed by the multiplexing / demultiplexing system.

The invention is compatible with all existing electronic supply of the transmitting coils and signal processing from the receiving coils. It is simply necessary to ensure, if one wishes to invert the transmission and reception functions of certain coils, that the used electronics accept coils of the same impedance for these two functions.

Claims

1 Method of producing an assembly of at least one transmitting coil and a receiving coil for eddy current monitoring, the receiving coil receiving, in the absence of a fault, a complex amplitude signal V _{R /} subject at a variation δV _R in the presence of a standard defect to be detected, characterized in that the distance Δ _ER is selected between the axes of the transmit and receive coils so as to maximize the ratio | δV _R / V _R | .

The method of claim 1, wherein the greatest distance between the centroid of the larger coil and the furthest point of the largest turn of this coil is called R, which process is characterized by the following steps: a- determining a value of the frequency of the excitation current flowing through the transmitting coil; b- determining a standard defect to be detected, characterized by its average size and depth relative to the surface to be controlled; c- from the values determined in steps a and b, determination, either by modeling or experimentally, of the following three quantities: i) the complex emf V _R induced in the receiver coil when the distance Δ _ER between the axes of the coils of transmission and reception varies at least in the interval {0; 3R}, ii) the variation δV _R , of the complex femur V _R induced in a receiver coil for the same variation of the distance Δ _ER in the interval of at least {0; 3R}, iii) the variation | δV _R / V _R | the modulus of the ratio of the values obtained in ii) and i), for the same variation of the distance Δ _ER in the interval of at least {0; 3R} d- plot of the report curve | δV _R / V _R | as a function of the variation of the distance Δ _ER in the interval of at least {0; 3R}, then determination of the maximum ordinate | δV _R / V _R | ₀ , and the abscissa Δ _ER0 of this maximum, this value constituting the optimal operating point; e- optionally and suboptimally, determining a range of distances {Δmin; ΔMax} on both sides of Δ _E RO _/ for which the ratio | δV _R / V _R | worth one third of its maximum value; f- if the values of Δ _E R found at optimal step d or step suboptimal e are not acceptable, changing parameters obtained in steps a and b; g- realization of the coils or coil assemblies, such that the operating point of each receiving coil satisfies either the optimum criterion of step d or the suboptimal criterion of step e.

3. Method according to claim 2, characterized in that the distance between the respective axes of each pair of transmit / receive coils is fixed and the frequency of the excitation current is adjusted according to the minimum dimension and the average depth of the coils. Typical defects to detect.

4 Process according to claim 2, characterized in that the excitation frequency is constant, and it is determined, as a function of the minimum dimension and the average depth of the standard defects to be detected, an optimum value (step d) or suboptimal (step e) of the distance _e Δ R between the respective axes of each pair of transmitting and receiving coils. 5 A method according to one of claims 2 to 4, characterized in that the distance Δ _E R between the respective axes (Al and A2) respectively of the two coils (B) and emitter (B2) receiver varies in the range {0; 3R}.

6 Method according to one of claims 2 to 4 characterized in that the distance Δ _ER between the respective axes (A1 and A2) of two coils respectively (Bi) and transmitting transmitter (B ₂ ) receiver varies in a range greater than {0 ; 3R}, and preferably equal to {0; 9R}, and in that for a given abscissa greater than 3R a second maximum of the ratio | δV _R / V _R | obtained in step d, this second maximum being associated with a second range of values {Δmin; ΔMax} ₂ on either side of Δ _ER0 2 _/ obtained in step e, for which the ratio | δV _R / V _R | is one third of its maximum value | δV _R / V _R | ₂ .

7 Method according to claim 2 characterized in that adjusts the distance Δ _ER between the respective axes (A1 and A2) of two coils respectively (B1) transmitter and (B2) receiver.

8 Process according to claim 7 characterized in that the distance Δ _ER is adjusted using mechanical and passive means.

Process according to Claim 7, characterized in that the distance Δ _ER is adjusted using at least one micro-actuator. Process according to any one of claims 2 to

9, characterized in that the coils have an elongate axial section, the widest coil having a large radius R _M and a small radius Rm, and that in the plane comprising the axis of the coil and the large radius R _{M /} we define a first optimal distance (Δ _ER o) Maχ and an associated range of values {Δmin; ΔMax} _Ma χ _/ while in the plane comprising the axis of the coil and the small radius R _{m /} defines a second optimum distance (Δ _ER0 ) min associated and a second range of values {Δmin; ΔMax} _min ..

Method according to any one of claims 2 to 10, characterized in that the assembly of at least one transmitting coil and a receiving coil comprises an elementary pattern consisting of three coils, a coil (B1) of emission located in the center and two coils (B2) and (B3) receiving, located on both sides, and distant from the emission of a value in the range {Δmin; ΔMax}.

Process according to any one of claims 2 to

10, characterized in that the assembly of at least one transmitting coil and a receiving coil comprises an elementary pattern consisting of three coils, a receiving central coil (B1) and two coils (B2) and (B3). transmission, located on both sides and distant from that of emission of a value in the range {Δmin; ΔMax}.

Process according to any one of Claims 2 to 10, characterized in that the assembly of at least one transmitting coil and a receiving coil comprises an elementary pattern consisting of three coils, a receiving central coil (B1). and two coils (B2) and (B3) emitter located on both sides. Method according to claim 13, characterized in that the elementary pattern further comprises two further receiver coils (B4 and B5) associated respectively with each transmitting coil (B2 and B3) at a distance in the range of distances {Δmin ; ΔMax} located around Δ _E RO _/ and in a direction substantially perpendicular to the plane passing through the axes of these emitting coils.

Method according to one of claims 2 to 9 and using step e, characterized in that the transmit and receive coils have an elongate axial section, the widest turn having a from a small radius Rm, determining according to its orientation a range of distance {Δmin; ΔMax} equal to {Δmin; ΔMax} _min in which is chosen the distance between the axes of turns in this orientation, and secondly a large radius R _M determining a range of distance {Δmin; ΔMax} equal to {Δmin; ΔMax} _Max - in which the distance between the axes of the turns is chosen according to this orientation.

Method according to one of the preceding claims, characterized in that the transmit and receive coils form a repetitive pattern a plurality of times to form a detection matrix, and the associated electronic excitation and processing means signals comprising means for multiplexing the signals at the transmitting coils and demultiplexing the signals of the receiver coils. Method according to any one of the preceding claims, characterized in that, in the vicinity of the coils and on the side opposite the part to be inspected, a soft magnetic material is arranged so as to reduce the reluctance of the magnetic circuit of each pair of emission coils. /reception. 18 A method according to claim 2 characterized in that after obtaining a first maximum value or under optimum distance Δ _E R is readjusted to this value every small change in a parameter by replacing steps c, d and e respectively by the following steps c ', d' and e ': - c'- from the values obtained in steps a and b to determine, either by modeling or experimentally, the variation δV _R , of the complex emf V _R induced in a receiver coil when the distance Δ _ER between the axes of the transmitting and receiving coils varies at least in the interval {0; 3R} -drawing the value curve | δV _R | as a function of the variation of the distance Δ _ER in the interval of at least {0; 3R}, then determine the maximum ordinate | δV _R | _{0 /} and the abscissa Δ _ERO of this maximum, this value constituting the optimum operating point; -e'- optionally, determine a range of distances {Δmin; ΔMax} on either side of Δ _E R _{O /} for which the ratio | .DELTA.V _R | is worth one third of its maximum value.