WO2015067483A1

WO2015067483A1 - Eddy current probe and eddy current testing device

Info

Publication number: WO2015067483A1
Application number: PCT/EP2014/072830
Authority: WO
Inventors: Timur KUDYAKOV
Original assignee: Magnetische Prüfanlagen Gmbh
Priority date: 2013-11-06
Filing date: 2014-10-24
Publication date: 2015-05-14
Also published as: DE102013222523A1; EP3066460A1

Abstract

An eddy current probe comprises an excitation system (110) comprising at least one electrical conductor, which is provided to connect to an AC voltage source, and a receiving system (130) having at least one receiver coil (140) which is separate from the excitation system, which receiver coil has one or more windings. The excitation system has an excitation section (115) in which one conductor section or multiple conductor sections runs or run in such a way that a primary magnetic alternating field (PF) having magnetic field lines (FL) extending around the excitation section is generated by the conductor section(s) of the excitation section during connection to an AC voltage source. The receiver coil (140) is arranged in relation to the excitation section (115) in such a way that the receiver coil is symmetrically penetrated by the primary magnetic alternating field of the excitation section in such a way that a temporal change dϕ/dt of the magnetic flux ϕ of the primary magnetic alternating field substantially disappears through the receiver coil.

Description

Eddy current probe and eddy current tester

APPLICATION AND PRIOR ART K

The invention relates to an eddy current probe according to the preamble of claim 1 and to an eddy current testing device having at least one such eddy current probe. In non-destructive testing (NDT) electroconductive materials, eddy current testing has been proven in many applications, such as automated nondestructive testing of semi-finished products for the metalworking and metalworking industries, for testing safety-critical and mission-critical components for land - and aircraft or in plant construction. The sensor systems used for the eddy current testing are commonly referred to as "eddy current probes".

In the eddy current test, an eddy current probe is arranged at a small distance (test distance) to a surface of a test specimen to be tested, which consists at least in the region of the surface of an electrically conductive material. As a rule, a relative movement between the test specimen and the eddy current probe is generated for the test, for example by the eddy current probe passing over the area of the surface to be tested.

A generic eddy current probe has a excitation arrangement with at least one electrical conductor, which is provided for connection to an AC voltage source, and a receiver arrangement with at least one separate from the exciter assembly receiver coil having one or more turns. Typically, the electrical conductor of the exciter assembly forms at least one coil, which is referred to as exciter coil or field coil. A receiver coil is often referred to as a measuring coil.

The excitation coil is connected to an AC voltage source for the purpose of the test and can then generate a primary electromagnetic alternating field (primary magnetic field) which penetrates into the test material during the test and essentially generates eddy currents in a near-surface layer of the test material by mutual induction the receiver coil (s) of the eddy current probe act back. The secondary magnetic field (secondary magnetic field) caused by the eddy currents is opposite to the primary magnetic field according to Lenz's rule. A defect in the tested area, such as a crack, contamination or other material inhomogeneity, disturbs the Spreading of the eddy currents in the test material and thus changes the eddy current intensity and thereby also the intensity of the magnetic field applied to the receiver coil secondary. The resulting changes in the electrical properties of a receiver coil, for example, in particular the impedance of the receiver coil, lead to electrical measurement signals that can be evaluated by means of an evaluation device to identify and characterize defects.

Eddy current probes in which the excitation coil and the receiver coil are formed by the same coil are referred to as parametric eddy current probes. Eddy current probes having an excitation coil and a receiver coil separate therefrom, with coupling between the exciter coil and the receiver coil mediated over the specimen material, are referred to as transformer eddy current probes. The present application relates to a transformer eddy current probe.

The exciter coil and the receiver coil normally have coil axes parallel to each other, so that the primary alternating magnetic field of the exciter coil induces an AC voltage in the receiver coil. The voltage or voltage curve in the receiver coil changes when the eddy current probe is brought into the vicinity of an electrically conductive specimen. These changes are evaluated for measurement purposes.

Depending on the type of construction of the receiver coil and the associated type of measured value detection, a distinction is usually made between so-called absolute coils (receiver coils in absolute arrangement) and so-called differential coils (receiver coils in differential arrangement), which each have specific advantages and disadvantages.

An absolute coil has one or more coils running in the same direction, which are penetrated by the primary and the secondary magnetic field. Among other things, the strength of the measuring signal depends strongly on the electrical conductivity of the specimen material, on the distance between the receiver coil and the specimen surface ("distance effect" or "lift-off effect"), on the inclination of the receiver coil with respect to the specimen surface ("tilt effect" or "inclination effect"). The distance effect is expressed by the fact that the signal amplitude of the measuring signal drops sharply as the distance to the test object surface increases.When the eddy current probe is slightly tilted or tilted in relation to the specimen surface during measured value recording, then this has an effect The edge of a test object, ie the transition to air or a material with a different electrical conductivity, affects the measurement signal even at a relatively large edge distance (lateral distance between the measuring position and the edge) - about that absolute coils can be made with small dimensions, so that high spatial resolution can be achieved.

A typical variant of a differential coil has two identically dimensioned, subtractively connected partial coils, wherein each partial coil has one or more windings running in the same direction and the turns of the partial coils run in opposite directions to one another. If the partial coils are penetrated by the primary and the secondary magnetic field, the measuring signal represents the difference between the voltages induced in the partial coils. In contrast to absolute coils, in each case no signal voltage is generated in air and via "healthy", ie homogeneous, defect-free material, since identical conditions and thus identical induced voltages are present below the partial coils, so that the differential voltage is zero In contrast to the absolute coil, a differential coil generally has a higher dynamic range and can therefore detect defects such as cracks very sensitively In addition, the sensitivity of a differential coil decreases with increasing test distance, despite the smaller distance dependence of the signal amplitude.The edge effect is usually greater with differential coils than with absolute coils nzipbedingt larger sizes, so that the achievable spatial resolution is lower than absolute coils.

TASK AND SOLUTION

It is an object of the invention to provide an eddy current probe which combines some advantages of absolute coils and differential coils without having their disadvantages to the same extent. In particular, the eddy current probe should allow material testing with high spatial resolution and sensitivity and be relatively insensitive to variations in the test distance. Furthermore, a good localization of defects should be achieved.

To solve these and other objects, the invention provides an eddy current probe having the features of claim 1. Advantageous developments are specified in the dependent claims. The wording of all claims is incorporated herein by reference.

In the eddy current probe according to the claimed invention, a particular arrangement of conductor sections of the excitation section (one or more conductor sections) with respect to the (at least one) receiver coil ensures that the receiver coil in relation to the excitation section or on those conductor sections which form the excitation section is arranged in such a way "magnetically symmetrical" that the primary magnetic field generated by the excitation section passes though the receiver coil, but in the receiver coil, no total voltage is induced when If the receiver coil is in air or at the correct test distance without tilting or the like against a healthy, defect-free specimen material, there are components of the field lines of the primary field running obliquely to the coil plane, but they pass through the receiver coil in different directions and with different signs Defects in the specimen material disturb this symmetry and then lead to measuring voltages which can be evaluated If no measurement voltage is generated, the evaluation can be carried out with high sensitivity in order to be able to evaluate deviations from the reference state (no electrical voltage at the output of the receiver arrangement) precisely with respect to amplitude and phase with respect to the exciter current of the AC voltage source.

For the function of the eddy current probe, the described magnetic symmetry of the receiver coil with respect to the primary alternating magnetic field generated by the exciter arrangement is to be aimed for. This can be achieved particularly advantageously by arranging the excitation arrangement and the receiver coil (s) adjacent to one another after certain symmetry considerations. In some embodiments, a receiver coil defines a receiver coil axis and a receiver coil plane oriented perpendicular to the receiver coil axis, and a plane of symmetry oriented perpendicular to the receiver coil plane, and the conductor section or sections of the exciter section are symmetrical about the plane of symmetry parallel to the receiver coil plane. The plane of symmetry can divide the coil surface of a receiver coil into two partial surfaces, which are chosen so that a magnetic flux change of the primary field through one of the partial surfaces is equal to the flux change of the primary field through the other of the partial surfaces, resulting in the mentioned magnetic flux compensation. If the conductor sections are then flowed through by the exciter current in the same way, the desired magnetic symmetry automatically results due to the geometric symmetry.

In some embodiments, it is provided that the excitation arrangement has a rectilinear excitation section, in which run a straight line section or a plurality of rectilinear conductor sections parallel to one another. As a result, the desired symmetry with respect to the magnetic flux of the primary alternating magnetic field can be achieved in a particularly simple manner. The rectilinear conductor sections can, for example, by straight Be formed sections of rectangular coils. However, it is also possible that the excitation section has one or more curved conductor sections, which are formed for example by sections of rounded coil turns.

A receiver coil may e.g. have an approximately rectangular, possibly square coil shape with straight conductor sections or a rounded coil shape without straight conductor sections, e.g. a circular coil shape.

Particularly advantageous embodiments are those in which the excitation section runs perpendicular to the receiver coil axis centrally diagonally across the receiver coil. It can thereby be achieved that the magnetic field strength generated by the exciter section is particularly high at the location of the receiver coil, so that strong eddy currents can be generated in the test object when the eddy current probe is brought into the vicinity of the test object surface. However, it is also possible that some or all of the conductor sections of the excitation section lie outside the coil surface defined by the windings of the receiver coil. For example, conductor sections of the excitation section may be arranged symmetrically to a plane of symmetry of the receiver coil on diametrically opposite sides each outside the receiver coil in such a way that the components of the primary alternating magnetic field generated by the outer conductor sections in the region of the receiver coil in the manner described above compensate magnetically. The coil surface itself may be free of conductor sections of the exciter arrangement. Although it is possible for the eddy current probe to be constructed at least in part by means of electrical conductors which are not in the form of coils, in preferred embodiments it is provided that the excitation arrangement comprises a first exciter coil with one or more first windings and a second exciter coil with one or a plurality of second windings, that the turns of the first and the second excitation coil are arranged mirror-symmetrically to the plane of symmetry of the receiver coil and that when connected to the AC voltage source, these windings are electrically mirror-symmetrical.

In particular, the arrangement may be made such that the first exciting coil has an inner conductor portion facing the second exciting coil and an outer conductor portion spaced away from the second exciting coil, and the second exciting coil has an inner conductor portion facing the first exciting coil and one remote from the first exciting coil outer conductor section, wherein the inner conductor sections form the excitation section and when connecting the excitation arrangement to the AC Selspannungsquelle be traversed in the same direction of current and the outer conductor sections in the same direction with each other and in opposite directions to the inner conductor sections of current to be traversed. Because the inner conductor sections are traversed in the same direction, the portions of the primary magnetic alternating field produced by these conductor sections add constructively and thus reinforce one another, so that high magnetic field strengths can be achieved in the region of the inner conductor sections. On the other hand, the outer conductor sections can, with a corresponding arrangement, serve to concentrate the magnetic primary field better in the region of the inner conductor sections, by increasing the density of the magnetic field line in this area. An eddy current probe may have a single receiver coil. However, it is also possible to design an eddy current probe with an array arrangement comprising a plurality of receiver coils in a suitable arrangement, for example two, three, four, five, six or more receiver coils. For each of the receiver coils, the condition of the magnetic field compensation can be fulfilled, so that advantages of the claimed invention can also be realized with array probes.

In one embodiment, the receiver assembly has at least one rectilinear receiver coil row having two or more receiver coils arranged to provide the described magnetic field compensation. With the aid of such a line array, it is possible to test a test specimen width corresponding to the length of the line array, whereby a high spatial resolution of the eddy current probe can be achieved simultaneously in each test track swept by a single receiver coil. It is also possible to provide a plurality of line arrays connected in series in one direction of movement, the receiver coils of which are arranged, for example, in "gap" with receiver coils of another line array in such a way that seamless testing over a large test width is possible two or more receiver coils, the signals of each of the receiver coils can be evaluated independently of the signals of the other receiver coils It is also possible to provide a common evaluation of the signals from some or all of the receiver coils In some embodiments, receiver coils of the receiver coil series are electrically in series so that the receiver coil row as a whole generates a summation signal resulting from the addition of voltages or voltage components induced in the individual receiver coils, thereby making it possible, among other things, to test larger areas with higher error resolution , The invention also relates to an eddy current testing device having at least one eddy current probe according to the claimed invention. The eddy current testing device has a supply and evaluation unit, which has an AC voltage source and an evaluation device. The exciter arrangement of an eddy current probe is connected to the AC voltage source in order to be able to generate a primary alternating magnetic field during test operation. The receiver arrangement of the eddy current probe is connected to the evaluation device, which is configured to process electrical measurement signals occurring at the receiver arrangement.

In some embodiments, the exciter assembly is associated with two or more receiver coils that form a coil array, such as a rectilinear receiver coil row. Preferably, each of the receiver coils is connected to a channel of a receiver coil connection means configured to receive output signals of the individual receiver coils in separate channels and to supply them to the evaluation means. Preferably, the evaluation device is configured so that it can be operated in an evaluation mode, which determines a sum signal by adding output signals from at least two receiver coils by means of an adding operation. Although it is possible for output signals to be added only from a selected subset of receiver coils, it is preferable to synchronously add the output signals of all the receiver coils of a receiver coil array in the add mode. As a result, among other things, relatively wide test tracks with high spatial resolution corresponding spatial resolution of a single receiver coil can be tested. It is also possible to divide the receiver coils into two or more subgroups, each connected in an array, and then to add signals from receiver coils within the subgroups. This can u.a. be achieved that even with a limited number of evaluation channels larger areas can be checked with high spatial resolution.

Preferably, the evaluation device is alternatively or additionally configured to process the output signals of each one of the receiver coils separately in an evaluation mode and to assign them to the position of the respective receiver coil in a receiver coil array. This makes it possible to locate defects over the test width of a sensor array. BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and aspects of the invention will become apparent from the claims and from the following description of preferred embodiments of the invention, which are explained below with reference to the figures. 1 shows schematically an eddy current testing device according to an embodiment of the invention;

Fig. 2 shows a schematic vertical section through the eddy current probe of the eddy current testing apparatus of Fig. 1;

3 shows an exciter coil of a conventional eddy current probe in operation at a distance to the surface of a test object;

Fig. 4 shows the exciter coil of Fig. 3 in Fig. 4A in combination with an absolute coil and in Fig. 4C in combination with a differential coil arrangement, each with measurement signals in Figs. 4B and 4D, respectively;

Fig. 5 shows schematically an eddy current probe according to an embodiment of the invention in the vicinity of the surface of a test specimen;

Fig. 6 schematically illustrates the magnetic flux compensation of the primary field in the receiver coil of Fig. 5;

FIG. 7 shows an eddy-current probe with a variant of the arrangement from FIG. 5, wherein conductor sections through which flow in opposite directions are arranged on both sides of the exciter section; FIG. 8 shows the eddy current probe from FIG. 7 when passing over a defect as well as a part of an associated measurement signal; FIG.

Fig. 9 shows an embodiment of an array eddy current probe having four receiver coils arranged in a straight row;

10 to 12 show various test scenarios and possibilities of signal evaluation of signals of the receiver coils of an array eddy current probe without and with signal addition. DETAILED DESCRIPTION OF THE EMBODIMENT

In Fig. 1, an eddy current tester 200 according to an embodiment of the invention is shown schematically. The eddy current testing device has an eddy current probe 100, which is electrically connected via electrical connection lines to a supply and evaluation unit 190. The eddy current probe 100 includes an exciter assembly 1 10 having a plurality of electrical conductors, which are connected via connecting elements to an AC voltage source 120 of the supply and evaluation unit. Furthermore, the eddy current probe has a receiver arrangement 130 with a receiver coil 140, which is electrically isolated from the exciter arrangement and has one or more windings, which are connected via connection lines to an evaluation device 180 of the supply and evaluation unit 190. FIG. 2 shows a schematic vertical section through the eddy current probe of the eddy current testing device from FIG. 1.

The eddy current probe 100 is manufactured as a multilayer eddy current probe using printed circuit board technology. The receiver coil 140 is a pancake having windings in a common plane (receiver coil plane 142) defining a receiver coil axis 144 that is perpendicular to the receiver coil plane. The windings of the receiver coil are applied to a surface of an electrically insulating carrier layer 170. The electrical conductors of the exciter arrangement are also applied to a surface of an electrically insulating carrier layer (the same carrier layer 170 or another carrier layer) in such a way that at least one electrically insulating carrier layer lies between these electrical conductors and the windings of the receiver coil, so that these are electrically isolated from each other.

The electrical conductors of the exciter assembly 1 10 form a first exciter coil 1 10-1 and a second exciter coil 1 10-2, each of which may have one or more turns and are each formed as flat coils with coil planes parallel to the receiver coil plane. The turns of the first and second excitation coils are arranged mirror-symmetrically to a plane of symmetry 145 which runs centrally through the receiver coil and contains the receiver coil axis 144.

The first exciter coil 1 10-1 has a straight inner conductor section 1A facing the second excitation coil 1 10-2 and an outer conductor section 1 10-1 B arranged at a greater distance from the symmetry plane away from the second exciter coil and parallel to the inner conductor section 1 10-1 A is going. Accordingly, the second exciting coil 1 has 10-2 a straight inner conductor portion 1 10-2A in the vicinity of the first exciting coil, and a away from the first exciter coil arranged straight outer conductor portion 1 10-2B, which is parallel to the inner conductor portion 1 10-2A.

When connected to the AC voltage source 120, the two arranged symmetrically to the plane of symmetry 145, located close to each other and centrally extending transversely over the receiver coil away inner conductor sections 1 10-1 A and 1 10-2B in the same direction from the excitation current through, while the farther outside outer conductor sections 1 10- 1 B and 1 10-2B respectively in opposite directions to the inner conductor sections, but in the same direction with each other and that is in the same direction, be traversed by the stream. The rectangular excitation coils thus lie mirror-symmetrically to the plane of symmetry 145 and are also electrically operated mirror-symmetrically to this plane of symmetry.

The inner conductor sections 1 10-1 A and 1 10-2A together form the rectilinear exciter section 15 of the exciter arrangement. The exciter section 15 runs perpendicularly to the receiver coil axis in the middle diagonally across the receiver coil 140. The inner conductor sections are traversed by alternating current when connected to the AC voltage source 120 and together generate a primary alternating magnetic field PF whose field lines FL extend around the exciter section 15 (see Fig. 2).

The receiver coil 140 is arranged "magnetically symmetrical" with respect to the excitation section 15 in such a way that the receiver coil is passed through by the primary alternating magnetic field PF of the exciter section, but the receiver coil is penetrated symmetrically by the primary alternating magnetic field such that a temporal As a result, although the primary magnetic field passes through the receiver coil, it does not induce any electrical voltage in this receiver coil, as a result of which the change in the magnetic field Φ of the primary alternating magnetic field through the receiver coil as a whole completely disappears This is achieved by the fact that the plane of symmetry 145 of the receiver coil 140 is such that the receiver coil is divided into two magnetically equivalent sub-areas 146-1 and 146-2, the magnetis The flow change through one of the partial surfaces is the same as the change in flux through the other partial surface, so that the changes in the magnetic fluxes are compensated. If Φ-ι is the magnetic flux through the first partial area 146-1 and Φ _{2 is} the magnetic flux through the second partial area 146-2, then the condition applies to the induced voltage V:

V = - (d <Pi + d <P ₂ ) / dt * 0 For a better understanding of the function and advantages of this arrangement over conventional eddy current probes, some characteristics of conventional eddy current probes in absolute and differential arrangement will be explained with reference to Figs.

FIG. 3 shows an excitation coil 310 of a conventional eddy current probe in operation at a distance from the surface 382 of an electrically conductive specimen 380. The exciter coil is typically oriented towards the specimen surface such that the exciter coil axis 312 is oriented as perpendicular as possible to the specimen surface. The excitation coil generates a primary alternating magnetic field PF, which induces eddy currents EC in the conductive specimen material. Due to the frequency-dependent skin effect, the magnetic field strength decreases exponentially with the penetration depth of the primary field. With this arrangement, the density of magnetic field lines in the test volume V in the vicinity of the coil axis 312 sharply decreases with increasing penetration depth.

Such an exciting coil may be combined with different types of receiver coil arrangements. FIG. 4A shows the excitation coil 310 in combination with an absolute coil ABS. The alternating magnetic field generated by the exciting coil induces a measuring voltage in the absolute coil. The voltage changes in the presence of an electrically conductive material and defects in the material can be analyzed. In such an absolute arrangement, the strength of the measurement signal depends strongly on the test distance A between the receiver coil and the sample surface and the conductivity of the sample material. Furthermore, the measuring voltages are sensitive to tilting of the eddy current probe (tilting effect). Reliable readings can only be obtained from areas that have a sufficient lateral distance B (edge distance) from the edge of the sample, otherwise a disadvantageous edge effect occurs. Advantages of absolute coils are i.a. The fact that very small sizes or small coil surfaces are possible, so that a high spatial resolution can be achieved. With absolute probes it is also possible to determine the defect lengths to a certain extent. In addition, the measurement signals are relatively independent of the direction in which a probe is guided over a defective area.

4B schematically shows a typical measurement signal of a conventional absolute coil when driving over an electrically conductive test object. In air, a constant finite measurement voltage V1 is induced by the primary field in the absorption coil. This measuring voltage changes in the vicinity of the test object and shows characteristic changes when passing over defects.

In Fig. 4C, the exciter coil 310 is shown in combination with a receiver coil assembly DIFF in differential circuit. The receiver coil assembly has a first receiver coil S1 and an opposite direction to this wound second receiver coil S2 with otherwise identical electrical properties. The coils are arranged symmetrically with respect to the excitation coil 310, so that the primary alternating field of the excitation coil induces a voltage in both the first coil S1 and in the second coil S2. However, these voltages compensate each other due to the difference circuit to a measuring voltage V = 0, as long as the differential probe is operated in air or against a defect-free material. A defect in the specimen disturbs the symmetry and leads to a measuring signal different from 0. To achieve the desired complete compensation of the measuring coils tight manufacturing tolerances must be complied with. Even with differential coils, the sensitivity depends on the test distance. A disadvantage of differential coil arrangements is, inter alia, their minimum size due to the design, so that the spatial resolution is limited. The determination of defect lengths is more difficult than with absolute probes. In addition, the edge effect affects the measurement results with differential coils more than with absolute coils.

4D shows a typical measurement signal of a differential coil arrangement when driving over an electrically conductive test object. Corresponding signal forms also result when passing over a defect.

Important differences between eddy current probes according to embodiments of the present invention and conventional eddy current probes will now be explained with reference to FIGS. 5 shows schematically an eddy current probe 500 according to an embodiment of the invention in the vicinity of the surface 582 of an electrically conductive specimen 580. The exciter assembly consists in this simple embodiment of a single rectilinear electrical conductor 510, which forms the exciter section 515 and for the Test mode is connected to an AC voltage source. The planar receiver coil 540 defines a receiver coil plane 542, which should be aligned for operation as parallel as possible to the specimen surface. A symmetry plane 545 of the receiver coil runs centrally diagonally across the receiver coil perpendicular to the receiver coil plane. The exciter section 515 extends in a plane parallel to the coil plane conductor plane 512 along the line of intersection between the plane of symmetry 545 and the conductor plane.

If the exciter section is traversed by alternating current, a primary magnetic field PF is generated, whose field lines FL run around the exciter section in planes lying perpendicular to the course of the exciter section. A substantial portion of the magnetic field lines in the vicinity of the excitation section passes through the receiver coil and penetrates into near-surface regions of the test specimen, where eddy currents EC are induced. A peculiarity is that with this arrangement, no voltage is induced in the receiver coil 540 as a whole, although an alternating magnetic field passes through the coil. Although the receiver coil 540 is an absolute coil in the sense that no signal compensation with another receiver coil. Nevertheless, in this arrangement, the receiver coil also has characteristics of a differential coil arrangement. In this arrangement, namely, a compensation of the induced voltages is achieved in that the receiver coil is permeated magnetically symmetrically by the primary alternating magnetic field of the exciter section in such a way that the time change d <J> / dt of the magnetic flux of the primary alternating magnetic field by the receiver coil disappeared altogether. In this case, each component K1 of the magnetic flux which does not run parallel to the receiver coil plane generates an induced voltage as it passes through the coil plane. Due to the magnetic symmetry, however, there is a corresponding oppositely directed component K2, which would induce a corresponding voltage with opposite polarity, so that overall no output signal (measurement voltage = 0 V) results (FIG. 6).

The signal compensation thus takes place by direct compensation of magnetic field fluxes of the primary field in the receiver coil. This magnetic field compensation or primary field compensation means that when this eddy current probe is placed in or near a defect-free material, the total flux changes of the primary alternating magnetic field generated by the exciting section and the secondary magnetic field generated by the eddy currents of the device under test disappear altogether, so that in the receiver coil in FIG In this case no voltage is induced. The electrical voltage measured between the outputs of the receiver coil 540 is therefore equal to 0 in this case. It is important to note that this magnetic flux compensation is only necessary for the region within the receiver coil. By contrast, the conditions outside the detection range of the receiver coil have no influence on the output signal of the receiver coil.

Even with this simple arrangement, it can be seen that the density of the magnetic field lines FL in the test volume V immediately below the exciter section does not diverge in the manner that is the case with the conventional exciter arrangement in FIG. 3. The effect of the primary alternating field is therefore better spatially concentrated or focused than in the case of conventional eddy current probes. This results, inter alia, in a significant reduction of the edge effect explained above, because the effect of the edge of the test object on the measurement signal is only recognizable at a smaller edge distance in the measurement signal. An improved magnetic field concentration in the central region of the test volume centrally under the receiver coil can be achieved by providing conductor sections of the excitation arrangement on both sides next to the excitation section, which sections are also flowed through by the exciter current, but in the opposite direction to the exciter section. As a result, alternating magnetic fields are generated on both sides of the exciter section, which are opposite to the alternating magnetic field of the exciter section and limit its lateral extent, so that a stronger focusing of the primary magnetic field of the exciter section results in the test volume 7 shows an eddy current probe 700 with a variant of the arrangement from FIG. 5, wherein on both sides of the exciter section 715, two conductor sections 710-1 A or 710-1 B are arranged at a defined lateral distance to the latter which are traversed by current at any given time in the opposite direction as the excitation section 715 and therefore produce oppositely directed primary AC fields which weaken the primary AC field of the exciter section outside the test area, so that the primary AC field more concentrated in the test volume below the pathogen section. The bold line in the test specimen indicates the course of the eddy current density.

As can be seen, for example, from FIG. 2, these measures for primary field focusing are also present in the embodiment of FIG. 1, since there the respectively further outer conductor sections 1 10-1 B, 1 10-2B of the excitation coils in the opposite direction to Passage section of primary current. In contrast to the variant of FIG. 7, however, the excitation section 1 15 has not only a single electrical conductor, but two mutually parallel, linear conductor sections 1 10-1 A and 1 10-2A, which together generate the primary alternating field of the exciter arrangement. Due to this local concentration of the effective range of the primary magnetic field PF on the area centrally below the receiver coil, a particularly efficient defect inspection with high spatial resolution becomes possible. This will be explained with reference to the schematic FIG. 8A, which shows the eddy current probe 700 from FIG. 7 during the test of a test piece 780 which has a defect D, for example in the form of a crack or voids, below the specimen surface 782. To carry out the test, the eddy current probe is arranged at a small distance from the test object surface and a relative movement between eddy current probe and test specimen is carried out, for example, in such a way that the eddy current probe is moved relative to the stationary test specimen substantially parallel to its surface at a constant test distance. If the defect moves into the test area below the receiver coil 740, this leads to a change of the induced secondary magnetic field emanating from the test object. field, which overlaps with the primary field of the eddy current probe. The bold line in the test specimen indicates the course of the eddy current density. As a result, the magnetic symmetry of the magnetic field within the receiver coil 740 is disturbed, so that full magnetic flux compensation is no longer present. As a result, the voltage at the output of the receiver coil increases from the value 0 (for homogeneous, defect-free test object material) to a finite value. The resulting measurement signal, which is generated by a partial surface of the receiver coil (FIG. 8B), is similar in shape to the measurement signal of a conventional absolute coil. The other sub-surface generates a similar signal when overflowing the defect, but with amplitude in the opposite direction. Since the signals have absolute signal character, signal addition is possible, which will be explained in connection with FIGS. 9 to 12.

Since the undisturbed specimen material does not generate a measuring voltage, the evaluation of the measuring signal can be carried out with much higher sensitivity, so that even relatively small absolute signals can be reliably distinguished from minor interference signals that do not originate from defects. This results in a significant increase in the sensitivity of the receiver coil while reducing the noise level compared to conventional absolute coils same coil size and number of turns. The spatial resolution can correspond to that of conventional absolute coils of the same size and number of turns.

An eddy current probe can have a single receiver coil (cf., for example, FIG. However, it is also possible for the excitation arrangement to be associated with two or more receiver coils forming a coil array. By way of example, FIG. 9 shows, as an exemplary embodiment, an eddy current probe 900 with an excitation arrangement 910, which is associated with four identical receiver coils 940A to 940D arranged in a rectilinear row. In principle, the excitation arrangement 910 has the same structure as the excitation arrangement 110 of FIG. 1, for which reason identical or corresponding elements bear the same reference numbers, increased by 800. The two exciter coils 910-1 and 910-2, with their mutually facing inner conductor sections 910-1 A and 910-2A form a rectilinear excitation section 915 with two mutually parallel conductor sections which, when connected to an AC voltage source 920, are in phase in the same direction from the excitation current be flown through. The receiver coils 940A to 940D are arranged in a straight receiver coil row 940 symmetrical to the excitation section so that in relation to the primary alternating field generated by the excitation section in each of the measuring coils in air, the magnetic flux compensation described above and thus in principle a voltage of 0 volts as a measurement signal results. Each of the receiver coils is connected to a channel of a receiver coil connection device 985, which makes it possible to receive the output signals of the individual receiver coils in separate channels and to supply them to a downstream evaluation device 980. The evaluation device can be operated in different evaluation modes. In a first evaluation mode ("single signal"), the output signals of each of the receiver coils are processed separately and assigned to the position of the respective receiver coil in the row, thereby allowing localization of defects across the width of the sensor array, and another evaluation mode determining a "sum signal "by adding the output signals from at least two receiver coils of the receiver coil row by means of an adding operation. Preferably, in this adding mode, the output signals of all receiver coils 940A-940D of the receiver coil row are synchronously added. The adding operation corresponds in electrical terms to a series connection of the respective receiver coils. Several different evaluation modes can be executed at the same time or with a time delay.

An array arrangement with a plurality of receiver coils makes it possible, inter alia, to examine larger test widths with high spatial resolution in a scanning examination of a test object surface by the eddy current probe and the test object in a relative direction of movement R perpendicular to the longitudinal direction of the receiver coil row or the coil array relative be moved together. Various test situations and typical measurement signals will be explained with reference to FIGS. 10 to 12.

The test specimen P in FIG. 10 has a line-shaped first defect D1 extending transversely to the direction of relative movement R and a smaller, rather punctiform second defect D2. The eddy current probe W has a row with four identical receiver coils E1 to E4, which are arranged symmetrically to the exciter section EA of the excitation device in a straight line. The small defect D2 lies only in the test track run over by the first receiver coil E1, while the longer defect D1 extends over the entire width of the receiver coil row, ie over all four receiver coils. In the right part of the figure, the corresponding measurement signals are shown schematically. The longer defect D1 generates in the first receiver coil E1 a first signal S1 of a certain signal amplitude. Since this defect also passes under the other receiver coils and there causes corresponding signals S2, S3 and S4, resulting from the Addition operation is compared to the single signal S1 larger sum signal ZSi = S1 + S2 + S3 + S4.

The second defect D2, which lies only on the track of the first receiver coil E1, generates only in this first signal S1. The other receiver coils are at the same time on homogeneous DUT material and therefore provide no output voltage, so that the simultaneously detected sum signal ZSi consists only of the signal S1 of the first receiver coil.

In Fig. 1 1 for comparison, a situation is shown in which only on the track of the second receiver coil E2 is a relatively small defect D2. In this situation, a second signal S2 results in the channel of the second receiver coil E2. Since the other receiver coils simultaneously drive over defect-free material, they do not supply any output voltage, so that the sum signal ZSi corresponds to the second signal S2.

By assigning the individual signals to the channel of the corresponding receiver coil, a localization of the defect in the width direction, that is perpendicular to the direction of relative movement, is thus possible. From Fig. 12 it can be seen that such an array eddy current probe is relatively insensitive to the edge effect, when the eddy current probe is guided substantially parallel to the course of the edge and this can also overlap. In the example case, the first receiver coil E1 is moved over the edge K in such a way that a part of the receiver coil still covers the test piece material while another part runs in air. Since the magnetic flux compensation described above takes place here in the first receiver coil both in the region of the test specimen and in air, no measuring voltage is present at the output of the first receiver coil. Even with the third and fourth receiver coil E3, E4, which run over defect-free material, no measurement voltage occurs. Only the second receiver coil E2 generates a measurement signal when passing over the defect D2, because it destroys the symmetry condition of the magnetic flux compensation when passing over, so that a finite measurement voltage or a finite output signal S2 results. Since the other receiver coils do not supply any output voltage, the sum signal ZSi corresponds to this signal S2.

In the exemplary embodiments, the conductor sections of the excitation arrangement and the windings of the receiver coil are each formed by conductor tracks which are produced by means of printed circuit board technology, for example by printing or another coating technique. It is also possible to construct the eddy current probes with wire wound coils or conductor assemblies. It may be a relatively rigid or rigid arrangement of Act ladders. It is also possible to construct the eddy current probe as a flexible or flexible eddy current probe, which can adapt to different surface contours of test specimens.

Claims

claims

1 . Eddy current probe with:

a field device (110) having at least one electrical conductor which is provided for connection to an AC voltage source (120); and

a receiver arrangement having at least one receiver coil (140) which is separate from the excitation arrangement and has one or more windings,

characterized in that

the excitation arrangement has an exciter section (1 15) in which a conductor section or a plurality of conductor sections (1 10-1 A, 1 10-2A) run in such a way that a primary one or more conductor sections of the exciter section are connected to an AC voltage source alternating magnetic field (PF) is generated with around the exciter section extending magnetic field lines (FL); and

the receiver coil (140) is arranged with respect to the excitation section (15) such that the receiver coil is symmetrically penetrated by the primary alternating magnetic field of the excitation section so that a temporal change of the magnetic flux φ of the primary alternating magnetic field by the receiver coil substantially disappears ,

2. Eddy current probe according to claim 1, characterized in that a receiver coil (140) defines a receiver coil axis (144) and a receiver coil plane (142) oriented perpendicular to the receiver coil axis and a plane of symmetry (145) oriented perpendicular to the receiver coil plane, and the conductor section or the Conductor portions of the excitation section (1 15) symmetrically to the plane of symmetry parallel to the receiver coil plane.

3. eddy current probe according to claim 1 or 2, characterized in that the exciter assembly (1 10) has a rectilinear exciter section (1 15), in which a straight conductor section or a plurality of parallel straight line conductor sections (1 10-1 A, 1 10-2A ), wherein preferably the excitation section (1 15) runs perpendicular to the receiver coil axis in the middle diagonally across the receiver coil (130).

4. Eddy current probe according to one of the preceding claims, characterized in that the exciter arrangement (1 10) comprises a first excitation coil (1 10-1) with one or more first turns and a second excitation coil (1 10-2) with one or more second turns in that the turns of the first and the second excitation coil are arranged mirror-symmetrically to a plane of symmetry (145) of the receiver coil and that, when conclusion to the AC voltage source these turns are electrically mirror-symmetrical connected.

5. eddy current probe according to claim 4, characterized in that the first excitation coil (1 10-1) one of the second excitation coil (1 10-2) facing inner conductor portion (1 10-1 A) and a remote from the second excitation coil outer conductor portion (1 10-1 B) and in that the second exciter coil (1 10-2) has an inner conductor section (1 10-2A) facing the first exciter coil (1 10-1) and an outer conductor section (1) located at a distance from the first exciter coil 10-2B), wherein the inner conductor sections (1 10- 1 A, 1 10-2A) form the excitation section (1 15) and when connecting the Erre- geranordnung to the AC voltage source (120) are passed in the same direction of current and the outer Conductor sections are passed in the same direction with each other and in opposite directions to the inner conductor sections of electricity.

6. Eddy current probe according to one of the preceding claims, characterized in that the receiver arrangement (910) has a receiver coil row (940) with two or more receiver coils (940A - 954D), which are arranged such that each of the receiver coils with respect is arranged on the exciter section (915) such that the receiver coil is penetrated symmetrically by the primary alternating magnetic field of the excitation section such that a temporal change of the magnetic flux φ of the primary alternating magnetic field by the receiver coil substantially disappears.

7. Eddy current probe according to claim 6, characterized in that the receiver coils of the receiver coil row (940) are electrically connected in series, so that a sum signal can be generated by the receiver coil row, which results from an addition of voltages or voltage components, the be induced in the individual receiver coils.

8. eddy current testing device (200) with a supply and evaluation unit (190) having an AC voltage source (120) and an evaluation device (180), and with at least one eddy current probe (100) according to one of the preceding claims, wherein the Excitation arrangement (1 10) of the eddy current probe to the AC voltage source (120) and the receiver arrangement (130) of the eddy current probe to the evaluation device (180) is connected.

9. eddy current testing device according to claim 8, characterized in that the exciter assembly (910) are associated with two or more receiver coils (940A - 940D), which are an Emp- form fängererspulen array, in particular in the form of a rectilinear receiver coil row (940).

10. Eddy current tester according to claim 9, characterized in that each of the receiver coils (940A - 940D) to a channel of a receiver coil connection means

(985) which is configured to receive output signals of the individual receiver coils in separate channels and to supply them to the evaluation device (980).

1 1. Eddy current testing apparatus according to claim 9 or 10, characterized in that the evaluation device is configured so that it is operable in an evaluation mode, which detects a sum signal by output signals from at least two receiver coils are added by means of an addition operation, preferably the output signals of all receiver coils of a receiver coil array are added synchronously.

12. eddy current testing device according to claim 9, 10 or 1 1, characterized in that the evaluation device is configured to process the output signals of each of the receiver coils (940A - 940D) separately in an evaluation mode and the position of the respective receiver coil in the receiver coil Assign array.