GB2516538A

GB2516538A - Method for operating an electromagnetic ultrasonic transducer

Info

Publication number: GB2516538A
Application number: GB1408777.9A
Authority: GB
Inventors: Patrick Jackel; Frank Niese
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2013-05-17
Filing date: 2014-05-19
Publication date: 2015-01-28
Anticipated expiration: 2034-05-19
Also published as: GB201408777D0; DE102013008570A1; GB2516538B; DE102013008570B4

Abstract

A method is disclosed for operating an electromagnetic ultrasonic transducer 1, having a plurality of HF coil segments 4, which are uniformly spaced along at least one spatial direction x and are controlled individually by electric current for the induced coupling of ultrasonic waves having at least one first trace wavelength Î» within a test specimen 3. The coil segments 4 for the excitation without coupling means of the ultrasonic waves having the first trace wavelength are divided into individual first groups, of which each first group comprises a first number of adjacent coil segments loaded with uniformly orientated electric currents, and the coil segments of directly adjacent first groups are loaded with oppositely orientated electric currents, so that ultrasonic waves with the first trace wavelength emanate from two adjacent first groups in each case. To generate ultrasonic waves with a second trace wavelength, the coil segments are divided in a corresponding manner into respectively second groups G2, in which a second number of HF coil segments adjacent along the spatial direction are contained.

Description

Method for operating an electromagnetic ultrasonic transducer

Technical Field

The invention relates to a method of operating an electromagnetic ultrasonic transducer, commonly referred to as an EMUS transducer, having a number of HF coil segments for the induced excitation of ultrasonic waves having at least one first trace wavelength within a test specimen having electrically conductive material, which coil segments are arranged along at least one spatial direction with a uniform spacing and are controlled separately using electric currents.

Prior art

Such guided ultrasonic wave modes can be used for non-destructive material testing.

These guided wave modes have the property that they can propagate over relatively large distances with low damping or attenuation.

Guided wave modes are always formed when it is no longer possible to speak of an infinitely extended voluminous body, owing to the spatial delimitation of the test specimen. This includes e.g. an exposed surface, on which surface waves, also known as Rayleigh waves, can propagate.

If the test specimen volume is further reduced to a plate or a rod with a thickness or a diameter in the order of magnitude of the ultrasonic wavelengths, then plate wave modes or rod wave modes in the event of a suitable choice of the working point, i.e. in the event of mutually matched choice of excitation frequency and trace wavelength of the ultrasonic waves and also test specimen geometry. In plates, one speaks of lambda wave modes and guided SH wave modes and in rods, one then speaks of longitudinal rod wave modes, torsional wave modes and bending wave modes. In pipes, one obtains modes very similar to the modes that occur in plates, as the pipe wall can be imagined as a rolled plate.

All guided wave modes, with the exception of the lowest guided SH wave mode SSO and the Rayleigh wave, are dispersive. This means that both the phase and the group velocity are dependent on the product of the working frequency (f) of the ultrasonic waves and plate thickness or rod diameter (d) of the respective test specimen.

Electromagnetic ultrasonic probes, (EMUS probes), can advantageously be used for generating and for coupling with guided ultrasonic wave modes. This type of ultrasonic transducer is generally characterised in that it is possible to work without physical coupling means, that is to say contactlessly, and thus the type of surface and the surface characteristics have either no effect or a negligible effect on the results of any measurement.

EMUS probes, which are constructed in the form of phased array probes, consist of a number of individually controllable ultrasonic wave transducer segments, which generate ultrasonic waves and wherein each ultrasonic wave transducer segment provides at least one HF coil segment, which can be provided with electric currents to induce eddy currents within the test specimen. Depending on the construction of the EMUS probe, either an electromagnet or a permanent magnet unit is attached per ultrasonic transducer segment or a correspondingly constructed magnet unit is provided for all ultrasonic transducer segments, by means of which the eddy currents, which are induced within a magnetic field in the test specimen. As a result, depending on the magnetic properties of the electrically conductive test specimen, forces acting in an elastically deforming manner on the atomic structure of the test specimen are created, which appear in the form of the Lorentz force and/or on the basis of the magnetostrictive effect and are able to generate the ultrasonic waves in the test specimen directly.

The HF coil segments are typically operated with a tone frequency burst with a changing current and a burst duration adjusted in accordance with the size of the HF coil segments. In addition to the choice of the so-called working frequency, which can be predetermined by means of a corresponding setting on the frequency burst generator, the ultrasonic wavelength can be predetermined by means of the geometric arrangement and construction of the HF coil segments, namely by means of the spacing between two coil conductor sections of two adjacent HF coil segments, through which current flows with the same current direction. This spacing originating from the HF coil segment arrangement and construction is also termed trace wavelength As.

By separately predetermining a working frequency and a trace wavelength of ultrasonic waves that can be generated within the test specimen: an option is provided to set a well-defined working point in the dispersion diagram and thus selectively excite a particular ultrasonic wave mode. Thus, the working frequency and the trace wavelength can be chosen in such a manner that an ultrasonic wave mode is generated, which has a particularly low dispersion characteristic for a given test specimen geometry with specific material properties.

The trace wavelength can be predetermined in terms of design by the structural design of the ultrasonic transducer and is usually adapted to the geometry of the test specimen to be investigated, for example to a diameter thereof in the case of a rod-shaped specimen. Furthermore, the degree of dispersion of an ultrasonic wave in a specimen depends on the geometry of the specimen, particularly on a diameter in the case of rod-shaped specimens or on the thickness in the case of planar specimens.

This means that for first and second test specimen with respective first and second diameters, first and second combinations of operational frequencies and a trace wavelengths lead to ultrasonic waves having particularly low-dispersion characteristics.

Should a plurality of different trace wavelengths be created for a test configuration, then an exchange of the EMUS probes, having a different arrangement and construction of HF coil segments in each case, is usually undertaken. Additional set-up work arises as a result, which necessitates a renewed exact positioning of the EMUS probe with respect to the test specimen to be investigated, particularly in those cases, in which exact runtime measurements are undertaken.

To avoid the storing of differently constructed EMUS probes, which are able to generate ultrasonic waves with a different trace wavelength in each case, it has been suggested in the article by Jens Prager and Karsten Hoever "Untersuchung zur Anregung gefUhrter Wellen in Platten mittels Gruppenstrahlertechnik" [Investigation for exciting guided waves in plates by means of phased array technology], Poster 41, DGZfP-Jahrestagung 2010, to place a phased array probe onto the surface of a test specimen by means of an attachment wedge, wherein the individual ultrasonic wave transducer segments of the phased array probe are activated with corresponding delay times, in order to generate a changing angle of incidence, at which angle the ultrasonic wave front impinges onto the test specimen surface through the stock wedge. Different trace wavelengths can thus be brought about by a different choice of the delay times for controlling the individual ultrasonic transducer segments and a change of the angle of incidence caused thereby.

A method and a device for testing a rod-or strip-shaped electrically conductive specimen is taught in patent document DE 102008 061 849 Al. Here, at least two generator coils are arranged one behind the other around a specimen to be tested, which extends linearly in the propagation direction of the ultrasonic waves, which coils are operated under electronic control in such a manner, that due to constructive interference, only those ultrasonic waves for which the frequency and wavelength are specifically predetermined are superimposed. The design and the control for the generation of ultrasonic waves capable of propagating appears to be complex and is solely limited to the investigation of rod-or strip-shaped test specimens.

An electrodynamic ultrasonic transducer for the testing of metal specimens is known from DE 26 55 804 Al, in which sender and receiver transducers are tuned to resonate in the case of sound frequency, wherein the reactance L of the sender transducer and the reactance LE of the receiver transducer are compensated by connecting capacitances.

DE 33 31 727 Al shows an electromagnetic transducer for the testing by means of induced test signal within metallic work-pieces in the form of one or a plurality of segments with a plurality of conductor tracks running parallel to one another, in which transducer, for a wavelength spectroscopy of the received ultrasonic waves for the segment(s), frequencies and wavelengths can be predetermined in a matrix link with shod cycle sequence, wherein the conductor tracks have winding parts that can be connected in such a manner by control by means of switching elements, that for each frequency in a plurality of switching states, wavelengths of proportionally small, whole and co-prime numbers result.

Statement of the Invention

The invention is based on the object of developing a method for operating an electromagnetic ultrasonic transducer, having a plurality of HF coil segments, which are uniformly spaced apart along at least one direction and are controlled separately using electric currents, for the excitation without coupling means of ultrasonic waves having at least one first trace wavelength in a test specimen having electrically conductive material, has a multiplicity of HF coil segments, which are arranged along at least one spatial direction with a uniform spacing and are controlled separately using electric currents, in such a manner that without expensive outlay in terms of apparatus and also process technology, the possibility is created of generating ultrasonic waves of at least one second trace wavelength, preferably as many trace wavelengths as are desired, using the same EMUS transducer.

The solution of the object on which the invention is based is specified in Claim 1.

Features which advantageously develop the method according to the solution are to be drawn from the sub-claims and also the further description, particularly with reference to the exemplary embodiments.

Suitable for realising the method according to the solution is an EMUS transducer, which is known per se, for example in the form of a phased array probe, which has a plurality of uniformly spaced apart HF coil segments arranged along at least one spatial direction, which can be individually activated by electric currents. One example of a known EMUS transducer of this type is described in DE 35 11 768 Al.

The known EMUS transducer has a shaped array of individual base elements, of which some are arranged along a spatial direction with uniform spacing with respect to one another. Each of the base elements consists of a magnetic or magnetisable rod element, around which the at least one HF coil is wound in each case, which coil can be separately provided with electric current with the aid of a burst generator in each case.

For induced coupling of ultrasonic waves within a test specimen having an electrically conductive material with variable trace wavelength, the method according to the solution then provides a segmentation or division of the HF coil segments arranged along a spatial direction into individual groups, of which each group comprises a certain number, which number is equal in each case, to the number of adjacent HF coil segments along the spatial direction. Should a certain first trace wavelength be generated with the aid of the electromagnetic ultrasonic transducer, for example, then the HF coil segments arranged along the spatial direction are divided into respectively first groups, wherein each first group comprises a first number of directly adjacent HF coil segments along the spatial direction. To generate ultrasonic waves with the first trace wavelength, the HF coil segments combined in a first group are in each case loaded with uniformly orientated electric currents, wherein the HF coil segments of directly adjacent first groups along the spatial direction in each case are loaded with mutually oppositely orientated electric currents, so that ultrasonic waves with the first trace wavelength emanate from two adjacent first groups in each case.

Without changing the geometric arrangement of the HF coil segments, for the induced coupling of ultrasonic waves having a second trace wavelength within the test specimen, the same HF coil segments are divided into individual second groups, of which each second group comprises a second number of adjacent HF coil segments, which second number is not equal to the first number, along the spatial direction. All HF coil segments combined in a second group are likewise, as described in the previously described case, in each case provided with uniformly orientated electric currents, wherein the HF coil segments of directly adjacent second groups along the spatial direction in each case are provided with mutually oppositely orientated electric currents, so that in this case, ultrasonic waves with the second trace wavelength emanate from two adjacent second groups. It is important that the first and second trace wavelengths and also the first and second number of HF coil segments differ from one another.

The method according to the invention allows the generation of ultrasonic waves with different trace wavelengths, the length variance of which is fundamentally determined by the total number of HF coil segments provided along a spatial direction and also the uniform spacing thereof between two adjacently arranged HF coil segments in each case. Ultrasonic waves with the smallest possible trace wavelength can be generated with the aid of the EMIJS transducer, in that the HF coil segments arranged adjacent to one another along the spatial direction are each consecutively provided with mutually oppositely orientated electric currents. In this case, each individual group of HF coil segments divided into respective first groups comprise a single HF coil segment.

To generate ultrasonic waves with a second trace wavelength, which differs as little as possible from the first trace wavelength, the HF coil segments arranged along the spatial direction are each combined in pairs in the spatial sequence thereof, so that two HF coil segments, which are adjacent in direct sequence, are provided with uniformly orientated electric currents, wherein HF coil segment pairs adjacent along the spatial direction are provided with mutually oppositely orientated electric currents.

In this case, ultrasonic waves with a second trace wavelength are generated, which wavelength corresponds to the spacing of two HF coil segment pairs, which are each provided with uniformly orientated electric currents. The trace wavelength in this case corresponds to the spacing between the geometric centre of an HF coil segment pair and the geometric centre of the next-but-one following HF coil segment pair in the spatial direction.

The electrical wiring according to the solution of all HF coil segments for generating ultrasonic waves with respectively different trace wavelengths therefore conforms to the number n of HF coil segments combined in a group in each case. Due to the geometric reality of HF coil segments arranged along the single spatial direction uniformly spaced apart, trace wavelengths A3 can be created in accordance with the method, with a length raster which complies with the following relationship: X3 = 2 n Ax, where Ax corresponds to the spacing between adjacent HF coil segments in the spatial direction and n is the number of HF coil segments assigned per group.

The method according to the invention differs markedly from the mode of operation that is known per se from generic EMUS transducers, which is based on so-called phased-array technology, in which all HF coil segments are activated with phase-delayed amplitude allocation for the purposes of spatial pivoting of the induced field direction of an ultrasonic wave field shut down in the test specimen. By contrast, the basic idea according to the invention for operating an EMUS transducer with variable trace wavelength consists in segmenting the HF coil segments and merging the same via simple electrical wiring in such a manner that a spatial exertion of force in the test specimen with differently dimensioned trace wavelengths becomes possible.

To this end, in a preferred operating mode, all available HF coil segments are provided with electric currents at the same time, wherein the criterion according to the invention of simultaneous current feed of all HF coil segments in the spatial distribution of the electric current directions is to be seen.

Of course, it is however also possible to apply the principle of phased array technology to the operating mode according to the solution of an EMUS transducer.

Here, the HF coil segments divided into equal groups in each case are operated by means of phased array technology in such a manner that all HF coil segments in each case combined to form a group are loaded simultaneously and the HF coil segments in each case, which belong to different groups, are loaded in a phase-delayed manner in each case with electric currents. In this manner, ultrasonic wave fields with different induced field directions can be emitted into the test specimen, wherein different trace wavelengths can be set, depending on the grouping of the HF coil segments, without the geometric arrangement of the EMUS transducer having to be changed.

Brief Description of the Figures

The invention is described by way of example in the following without limitation of the general inventive idea on the basis of exemplary embodiments with reference to the drawings. In the figures: Figs la, b, c show schematic EMUS transducer structures with permanent magnets and coil carriers on a ferromagnetic comb structure, illustrated with different electrical wiring in each case.

Methods of realising the invention, industrial applicability The Figures la to c each show an EMUS transducer 1 in a schematic illustration, which has a permanent magnet 2, which is physically connected to a ferromagnetic comb structure 21, which assumes a magnetisation predetermined by means of the magnetisation of the permanent magnets 2. The individual comb elements 22 are constructed in the manner of webs or columns and arranged along the spatial direction termed the x-direction with an equidistant spacing Ax with respect to one another. Here, Ax is measured as the spacing between two comb element centres following one another directly in the spatial direction x, as can be drawn from Figure 1 a.

The end faces of the comb elements 22, which end freely on one side, form a common plane E, which corresponds to the bearing plane of the EMUS transducer 1, on which the EMUS transducer 1 is placed on the surface 3' of a test specimen 3. By means of the direct contact of the pre-magnetised comb structure 21 with the surface of the test specimen 3 via the end-face ends of the comb elements 22, magnetic field lines, which are not illustrated, perpendicularly penetrate into and emerge from the test specimen 3 consisting of electrically conductive material at the end-face contact surfaces.

In addition, a HF coil segment 4, which is separately connected to a current source (not illustrated) in each case, is wound around each comb element 22.

By means of the separate wiring of each individual HF coil segment 4 to a current source, preferably in the form of a burst generator, it is possible to separately supply each individual HF coil segment 4 with current. In the case of the EMLJS transducer 1 illustrated in Figure la, the HF coil segments 4 of two comb elements 22 directly adjacent to one another in the spatial direction x are loaded with respectively oppositely orientated current directions, see arrow directions along the coil wires of the HF coil segments 4. Thus, the first HF coil segment 4 is loaded from the left in Figure 1 a with an anti-clockwise-orientated current direction and the second HF coil segment is loaded from the left with a clockwise-orientated current direction, etc. In this manner, along the spatial sequence x of the comb elements 22 resting on the test specimen surface 3', the test specimen 4 experiences an alternating exertion of force F, which is illustrated by the arrow illustration within the test specimen 1 and by means of which the trace wavelength X5 of the ultrasonic waves generated within the test specimen 1 is determined.

In the case of the operating mode of the EMUS transducer illustrated in Figure la, the HF coil segments 4 are each divided into first groups, wherein each first group comprises a single HF coil segment 4. By contrast, Figure lb shows an operating mode of the otherwise unchanged EMUS transducer, in which the HF coil segments 4 are divided into second groups G2, which each comprise two HF coil segments 4.

The HF coil segments 4 within the second group G2 are each provided with uniformly orientated electric currents, whereas the HF coil segments 4 in two groups G2, which are directly adjacent in the x direction, are each provided with oppositely orientated electric current. This leads to the generation of induced ultrasonic waves into the test specimen 1 with a trace wavelength)., which as twice as large as the trace wavelength X, which has been generated with the aid of the operating mode according to Figure la.

Finally, in Figure ic, a division of the HF coil segments into third groups G3 is illustrated, of which, each third group G3 comprises three HF coil segments 4 in each case, which are directly adjacent in the x direction and are each provided with uniformly orientated electric current. The ultrasonic waves that can be generated using this operating mode within the test specimen 1 have a trace wavelength X, which has three times the length compared to the operating mode according to Figure la.

In accordance with the invention, it is possible to generate ultrasonic waves with trace wavelengths X3, the size of which is scalable with a factor n, corresponding to the number of HF coil segments contained per group. Fundamentally, the trace wavelength X8 can be scaled as desired in the preceding manner and this is enabled by means of simple electrical wiring of the HF coil segments with an appropriate current source. The smaller the spacing Ax is chosen to be, then the trace wavelength can be set more easily and finely.

A change of the trace wavelength X3 at the same test specimen 1 is also always connected with a change of the working frequency. For reasons of a beneficial electrical impedance adaptation of the individual HF coil segments to a current source supplying the HF coil segments with electric current, it can moreover be advantageous to connect the individual HF coil segments together to some extent in series or to some extent in parallel. Cases are also conceivable, in which individual coil segments are switched off completely.

Reference List 1 EMLJS transducer 2 Permanent magnet 21 Comb structure 22 Comb element 3 Test specimen 3' Test specimen surface 4 HF coil segment

Claims

Patent claims 1. A method for operating an electromagnetic ultrasonic transducer (1), which, having a plurality of HF coil segments, which are uniformly spaced apart along at least one direction (x) and are controlled separately using electric currents, for the induced coupling of ultrasonic waves having at least one first trace wavelength (Xs) within a test specimen (3) of electrically conductive material, wherein that the HF coil segments (4) for the induced coupling of the ultrasonic waves having the first trace wavelength (X5) are divided into individual first groups, of which each first group comprises a first number of adjacent HF coil segments (4) along the spatial direction (x), wherein the HF coil segments (4) combined in a first group are each provided with uniformly orientated electric currents, wherein the HF coil segments (4) of directly adjacent first groups along the spatial direction (x) in each case are provided with mutually oppositely orientated electric currents, so that ultrasonic waves with the first trace wavelength (X5) emanate from two adjacent first groups in each case, and wherein the HF coil segments (4) for the induced coupling of ultrasonic waves having a second trace wavelength (X5) within the test specimen (3) are divided into individual second groups, of which each second group comprises a second number of adjacent HF coil segments (4), along the spatial direction (x), wherein the HF coil segments (4) combined in a second group are each loaded with uniformly orientated electric currents, wherein the HF coil segments (4) of directly adjacent second groups along the spatial direction (x) in each case are loaded with mutually oppositely orientated electric currents, so that ultrasonic waves with the second trace wavelength (Xs) emanate from two adjacent second groups in each case, and wherein the first and second trace wavelengths (X5) together with the first and second number of HF coil segments (4) differ from one another.
2. The method according to Claim 1, wherein the number of HF coil segments (4) is at least four, and wherein the HF coil segments (4) are identical.
3. The method according to Claim 1 or 2, wherein the choice of the number of first and second of HF coil segments (4) combined in first and second groups is undertaken exclusively by means of electrical wiring of the HF coil segments (4) to a current source.
4. The method according to one of Claims 1 to 3, wherein the HF coil segments (4) divided into first and second groups are in each case operated by means of phased array technology, in that all HF coil segments (4) in each case combined to form a group are provided with electric current simultaneously and the HF coil segments (4), in different first or second groups in each case, are provided with electric current in a phase-delayed manner.
5. The method according to one of Claims 1 to 4, wherein the ultrasonic waves are generated with trace wavelengths As, for which the following applies: As = 2 n x, with n »= 1 and n:= number of HF coil segments belonging to one group in each case, and Ax:= centre spacing between two HF coil segments (4) adjacent along the spatial direction.
6. A method substantially as described herein with reference to any one or more of the Figures as shown in the accompanying drawing sheet.
7. An apparatus comprising an electromagnetic ultrasonic transducer (1), having a plurality of HF coil segments, which are uniformly spaced apart along at least one direction (x) and are controlled separately using electric currents, for the induced coupling of ultrasonic waves having at least one first trace wavelength (X5) within a test specimen (3) of electrically conductive material, adapted to perform any one or more of the methods in accordance with claims 1 -5.