WO2018189450A1

WO2018189450A1 - Method and device for ultrasonic probing by adaptive focusing by means of a reverberating solid object

Info

Publication number: WO2018189450A1
Application number: PCT/FR2018/050821
Authority: WO
Inventors: Guillemette RIBAY; Paul ZABBAL
Original assignee: Commissariat à l'énergie atomique et aux énergies alternatives
Priority date: 2017-04-10
Filing date: 2018-04-03
Publication date: 2018-10-18
Also published as: FR3065078A1; FR3065078B1

Abstract

This method for ultrasonically probing a medium, using a device including a reverberating solid object having a face to be placed in contact with the medium to be probed and at least one electroacoustic transducer placed against a wall of the reverberating solid object, includes: a learning phase (100) including the measurement (106), at at least one point on the contact face, of an ultrasonic response to an excitation signal emitted (104) by the electroacoustic transducer; a phase (200) of probing the medium, including the emission (206) by the electroacoustic transducer of a signal reconstructed on the basis of adaptive ultrasonic focusing (204) of the measured ultrasonic response. In the learning phase (100), the excitation signal includes a pseudo-random binary sequence of order higher than or equal to 2 and the measured ultrasonic response is convoluted (110) with the conjugated sequence of this pseudo-random binary sequence. In the probing phase (200), the reconstructed signal results from adaptive ultrasonic focusing of the convoluted measured ultrasonic response.

Description

METHOD AND DEVICE FOR ULTRASONIC SURVEY BY ADAPTIVE FOCUSING USING A REVERBERANT SOLID OBJECT

The present invention relates to a method and a device for probing a medium by emitting ultrasonic waves.

Many different ultrasonic sounding methods and devices are known. Some are designed to directly contact one or more electroacoustic transducers with the medium to be probed. Others provide a reverberant solid object interposed between the electroacoustic transducer (s) and the medium to be probed so as to take advantage of the invariance properties of the sound propagation equation in a non-dissipative solid medium by inversion of time. After a learning phase generally consisting in measuring in at least one target point of the solid object reverberating an ultrasonic response to an impulse excitation, they proceed to a temporal compression of this ultrasonic response by time reversal or inverse filtering. These two temporal compression methods are based on an adaptive ultrasound focusing as defined in part 2 of François Vignon's thesis, entitled "Ultrasound focusing by time reversal and inverse filter, application to transcranial ultrasound", September 30th. 2005. The reverberant solid object is often called a "chaotic cavity", even if it is not a cavity per se.

In some applications, particularly in harmonic imaging, in measurement of waves guided by thin structures or in hyperthermia, whether in non-destructive testing of materials or in the medical field, it may be desired to emit ultrasonic waves of strong amplitude in the medium to be probed to exploit the nonlinearities. In this case, known methods and devices designed to directly contact the electroacoustic transducers with the medium to be probed require power electronics incompatible with the dimensions (large transducers are then required), safety (voltages approaching 3000 V may be necessary), reliabilities (the nonlinearities of the inspected medium are noisy by the non-linearities of the transmission / reception chain) and the desired complexities (in particular in terms of component costs) of the devices. Methods and devices designed to enhance the waves emitted using intermediate solid reverberant objects seem to be more suitable, although the aforementioned problems may persist to a lesser extent. The invention thus applies more particularly to a method of ultrasonic sounding of a medium, using a device comprising a reverberant solid object having a contact face intended to be placed in contact with the medium to be probed and tested. minus an electroacoustic transducer disposed against a wall of the reverberant solid object, the method comprising:

a learning phase including measuring, at at least one point of the contact face, an ultrasonic response to an excitation signal emitted by said at least one electroacoustic transducer in the reverberant solid object, and

a phase of probing the medium, when this medium is in contact with said at least one point of the contact face, comprising the emission by said at least one electroacoustic transducer of a signal reconstituted on the basis of an ultrasonic focusing; adaptive of said measured ultrasonic response.

Such a method is thus proposed in the article by Montaldo et al (2001) entitled

"Generation of very high pressure with 1-bit time reversal in a solid waveguide", published in Journal of the Acoustical Society of America, Volume 1 10, No. 6, pages 2849 to 2857, December 2001. The reverberant solid object is implemented in the form of a cylindrical metal waveguide at one end of which a plurality of ultrasonic pulse emitting transducers are glued. Such a method is also proposed in the article by Montaldo et al (2005), entitled "Building three-dimensional images using a time-reversal chaotic cavity", published in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Volume 52, No. 9, pages 1489 to 1497, September 2005. The reverberant solid object is implemented in the form of a chaotic cavity of parallelepiped shape truncated by a spherical machining. The two devices described however require in practice a large number of emitting transducers and / or a high power electronics to ensure a sufficient amplitude of the ultrasonic waves emitted by the contact surface of the waveguide or the chaotic cavity. This may not be a problem for some medical applications. On the other hand, it is an obstacle to any application in non-destructive testing, given the generally accepted costs.

A similar method is proposed in the article by Bou Matar et al (2009), entitled "On the use of a chaotic cavity transducer in nonlinear elastic imaging", published in Applied Physics Letters, volume 95, pages 141913-1 to 141913- 3, October 2009. He proposes an application of crack control in a thin structure by nonlinear wave guidance and apparently only requires a single transducer glued against the chaotic cavity. It also exploits the reverberant character of the controlled structure, the reverberant properties of the chaotic cavity being obviously not sufficient. Thus, the application of such a device for inspecting nonlinearities of a medium that is not necessarily reverberant is impossible. In addition, the chaotic cavity must be bonded to the controlled structure for optimal coupling, which constitutes a strong constraint limiting the industrial outlets of such a process. An improvement is proposed in the article Bou Matar et al (2010), entitled "Optimization of chaotic cavity transducers to nonlinear elastic imaging," published on the occasion of the ^10th French Acoustics Congress, held from 12 to April 16, 2010 in Lyon (FR). It consists in proposing the emission of a "chirp" type excitation signal during the learning phase. But the recompression of a chirp is not ideal and does not allow either to do without the reverberant character of the controlled structure.

It may thus be desirable to provide an ultrasonic sounding method that makes it possible to overcome at least some of the aforementioned problems and constraints.

It is therefore proposed a method of ultrasonic sounding of a medium, using a device comprising a reverberant solid object having a contact face intended to be disposed in contact with the medium to be probed and at least one electroacoustic transducer disposed against a wall of the reverberant solid object, the method comprising:

a phase of probing the medium, when this medium is in contact with said at least one point of the contact face, comprising the emission by said at least one electroacoustic transducer of a signal reconstituted on the basis of an ultrasonic focusing; adaptive of said measured ultrasonic response,

and being such that:

during the learning phase, the excitation signal comprises a pseudo-random binary sequence of order greater than or equal to 2 and the measured ultrasonic response is convolved to the conjugate sequence of said pseudo-random bit sequence,

during the sounding phase, the reconstituted signal results from an adaptive ultrasonic focusing of the convolved measured ultrasonic response.

Learning is not done on a pulse but on a longer time signal and higher energy so that the measurement of the ultrasonic response is of better quality. Moreover, by convoluting the ultrasound response measured during the learning by the conjugate sequence of the pseudo-random binary sequence before proceeding with the adaptive ultrasonic focusing during the sampling phase, it is then sufficient to choose a pseudo-random binary sequence with good recompression properties by autocorrelation to obtain a significantly improved amplitude of the wave finally emitted by the contact face of the reverberant solid object. It is thus possible to reduce the number of transducers necessary to obtain the emission of a wave that is effectively sensitive to nonlinearities of the probed medium, even when the latter is not itself reverberant. The electronic transmission / reception chain can also be designed with lower power and lower costs. Finally, the coupling between the medium to be probed and the reverberant solid object can be done simply with the aid of a gel without requiring a bond given the gain in amplitude obtained, which makes its industrial use much less restrictive.

Optionally, the pseudo-random bit sequence is chosen so as to verify at least one of the following properties:

it is defined on the basis of a periodic series of values produced by a linear feedback shift register exploring all the values that can be produced by this shift register,

- its autocorrelation mathematically produces the Dirac distribution, and

it is of the MLS type, then also called M. sequence

Optionally also, the reconstituted signal results more precisely from a time reversal of the convolved measured ultrasonic response.

Optionally also:

during the learning phase, the ultrasonic response is measured at several points of the contact face, and

during the sounding phase, the reconstituted signal results from an inverse filtering performed using said measurement at several points of the ultrasonic response. Optionally also, the measurement of the ultrasonic response is done using an electroacoustic transducer disposed against said at least one point of the contact face or an optoelectronic transducer pointing towards said at least one point of the face. of contact.

Also optionally, the probing phase comprises a measurement step performed by at least one electroacoustic receiver disposed against the contact face of the reverberant solid object and operating at a central frequency that is significantly different from a central transmission frequency. at least one point of the contact face, in particular in a reception bandwidth located outside transmission bandwidth, for measuring at least one non-linear ultrasound response of the probed medium.

Optionally also:

the emission step of the sounding phase comprises the emission of the reconstituted signal and that of its opposite,

the sounding phase comprises a measurement step, carried out from the emissions of the emission step, during which ultrasonic responses of the probed medium to the opposite reconstituted signals emitted are respectively measured and then summed in order to suppress a component common linear they include.

Optionally also, the method is performed for a non-destructive testing application for guided wave measurements in a thin structure against which the contact face of the reverberant solid object is disposed, and the probing phase includes a step of measurement, performed by a single or multi-element electroacoustic receiver (s) also arranged against the thin structure, during which:

at least one ultrasonic response of the thin structure is measured and processed by time-frequency analysis when the electroacoustic receiver is mono-element, or

several ultrasonic responses of the thin structure are measured and processed by estimation of dispersion curves, in particular by double

Spatio-temporal FFT, multiple classification of signals, or decomposition into singular values when the electroacoustic receiver is multi-element,

for detection of propagation modes guided by the thin structure. Optionally also, the method is performed for a non-destructive testing application for guided wave measurements in a thin structure against which the contact face of the reverberant solid object is disposed, and:

during the learning phase, at least one ultrasound response is measured at each of several distinct points of the contact surface thus forming an array of several point emitters from which an ultrasonic excitation profile is emitted, and

the emission step of the sounding phase comprises the emission by said network of point emitters of several signals reconstituted by adaptive ultrasonic focusing of said measured ultrasonic responses, these measured ultrasonic responses being furthermore combined together so that the ultrasonic excitation profile thus emitted generates the selection of at least one predetermined guided wave propagation mode in the thin structure, this combination being performed as a function of at least one physical parameter mathematically related to a phase velocity, in a given frequency range, said at least one predetermined guided wave propagation mode to be selected and positions of said distinct points of the contact surface.

There is also provided an ultrasonic sounding device of a medium, comprising:

a reverberant solid object having a contact face intended to be placed in contact with the medium to be probed,

at least one electroacoustic transducer disposed against a wall of the reverberant solid object,

a control system for said at least one electroacoustic transducer, comprising:

A learning module configured to generate an excitation signal intended to be emitted in the reverberant solid object by means of said at least one electroacoustic transducer, and

A medium sounding module configured to generate a reconstituted signal based on an adaptive ultrasonic focusing of an ultrasound response to the excitation signal measured at at least one point of the contact face, this reconstituted signal being intended for to be emitted in the reverberant solid object using said at least one electroacoustic transducer,

in which :

the learning module is more precisely configured to generate the excitation signal in the form of a pseudo-random binary sequence of order greater than or equal to 2 and to convolute the measured ultrasonic response to the conjugate sequence of said pseudo binary sequence -random, and

the probing module is more precisely configured to generate the reconstituted signal on the basis of an adaptive ultrasonic focusing of the convolved measured ultrasonic response.

Optionally, said at least one electroacoustic transducer comprises at least one of the elements consisting of a piezoelectric transducer PZT, a micro-machined transducer of the type cMUT, pMUT or mMUT, a transducer PVDF film and an electromagnetic-acoustic transducer.

Optionally also, the reverberant solid object is made of a material at least as little acoustically attenuating as aluminum or an aluminum alloy or fused silica.

Also optionally, the reverberant solid object has at least one geometrical asymmetry, in particular in the form of at least one spherical machining dug in a parallelepipedal general shape.

The invention will be better understood with the aid of the description which follows, given solely by way of example and with reference to the appended drawings in which:

FIG. 1 schematically represents the general structure of an ultrasonic sounding device according to one embodiment of the invention,

FIGS. 2A and 2B illustrate in perspective two examples of solid reverberant objects that can be used for the device of FIG. 1,

FIG. 3 illustrates the successive steps of an ultrasonic sounding method carried out using the ultrasonic sounding device of FIG.

1, according to one embodiment of the invention, and

FIG. 4 schematically represents an experimental test installation of the ultrasonic sounding device of FIG. 1 when it comprises the reverberant solid object of FIG. 2A. The installation shown diagrammatically in FIG. 1 comprises an ultrasonic sounding device 10. This device comprises a reverberant solid object 12, at least one electroacoustic transducer 14 and a control system 16 of the electroacoustic transducer 14.

The reverberant solid object 12 is of any shape and has at least one contact face 18 intended to be placed in contact with a medium to be probed (not shown) and a wall 20, which may optionally be different from the contact face. 18, against which is disposed the electroacoustic transducer 14. In the example of Figure 1, the wall 20 is a face opposite to the contact face 18, but it could also be a side wall or the like. To multiply the reverberations within its volume from the emission of an excitation signal by the electroacoustic transducer 14, the reverberant solid object 12 advantageously consists of a material as little acoustically attenuating as possible in the field of ultrasound, for example aluminum, an aluminum alloy such as an aluminum-copper alloy, fused silica or any other material even less attenuating than these three examples. In particular, it may be advantageous to use a crystal of sufficient purity, this assessment being within the abilities of those skilled in the art, to obtain an interior volume at least as little acoustically attenuating as aluminum, which an alloy of aluminum or that fused silica. It can in particular be a quartz crystal or Nd: YAG (of the English "Neodymium-doped Yttrium Aluminum Garnet." The reverberant solid object 12 also preferably has at least one geometrical asymmetry, like this will be detailed with reference to FIGS. 2A and 2B, so as to behave acoustically as a chaotic cavity, that is to say a volume in which the trajectory followed by the excitation signal has a property of ergodicity by traversing it in the most homogeneous way possible.

The electroacoustic transducer 14 transforms an electrical excitation signal supplied by the control system 16 into an ultrasonic excitation signal capable of propagating in the interior volume of the reverberant solid object 12. It may for example be designed in the form a piezoelectric transducer PZT (for Titano-Lead Zirconate), a micro-machined transducer of the cMUT ("capacitive Micromachined Ultrasonic Transducer") type, pMUT ("piezoelectric Micromachined Ultrasonic Transducer") ) or mMUT (of the English "magnetostrictive Micromachined Ultrasonic Transducer"), a PVDF (PolyVinyliDene Fluoride) film transducer or a electromagnetic-acoustic transducer. It can be glued to the wall 20 using an epoxy glue. It can also be simply coupled to this wall 20 with the aid of an ultrasonic transmission gel. Advantageously, it has a bandwidth representing at least 50% of its central operating frequency.

The control system 16 as shown diagrammatically in FIG. 1 comprises a processing unit 22 conventionally associated with a memory 24 (for example a processing data storage RAM memory).

For example, the processing unit 22 and the memory 24 may be implemented at least partially in a computing device such as a computer comprising a processor associated with one or more memories for storing data files and programs. computers. The processing unit 22 can itself be considered to be formed at least in part of a processor associated with a storage memory of the instructions that it executes in the form of computer programs.

The processing unit 22 as illustrated in FIG. 1 thus comprises two functional modules at least partly made up of one or more computer programs. A first learning functional module 26 comprises an electric excitation signal generator 28, a module 30 for processing an ultrasonic response to the excitation signal generated by the generator 28 and an arithmetic operator 32 performing a convolution product. The ultrasonic response processed by the module 30 is measured at at least one point of the contact face 18 of the reverberant solid object 12. A second sounding function module 34 comprises a generator 86 of reconstituted electrical excitation signal and an operator 38 adaptive ultrasound focusing. It will also be noted that the aforementioned functional modules of the processing unit 22 and their possibly programmable constituent elements are presented as distinct, but this distinction is purely functional. They could as well be grouped according to all possible combinations into one or more software. Their functions could also be at least partly micro programmed or micro wired in dedicated integrated circuits. Thus, in a variant, the computing device implementing the processing unit 22 and the memory 24 could be replaced by an electronic device composed solely of digital circuits and memories or registers (without a computer program) for the realization of the same actions. Alternatively, it should be noted that the treatments carried out by the functional modules 26 and 34 may be at least partially analog so that they can be implemented in the form of analog electronic circuits.

At the output of the generators 28 and 36 of electrical excitation signals, an amplifier 40 may be provided in the processing unit 22 before providing any electrical signal to the electroacoustic transducer 14.

The memory 24 may notably store values of the excitation signal denoted e (t), or at least parameters making it possible to reconstitute it, intended to be supplied to the generator 28. It may furthermore store the ultrasonic response denoted u (t). to this excitation signal e (t) as measured and transmitted to the processing module 30. It can also store the result noted v (t) of the convolution product produced by the arithmetic operator 32.

Finally, the installation of FIG. 1 comprises a detector 42 of the ultrasonic response u (t) to the excitation signal e (t) generated by the generator 28, measured at at least one point P of the contact face 18 of the reverberant solid object 12. This detector 42 is for example an electroacoustic transducer performing the inverse function of the electroacoustic transducer 14, namely transforming the ultrasonic response at the point P into an electrical signal intended to be transmitted to the processing module 30 of the first module 26. It may for example also be designed in the form of a PZT piezoelectric transducer, a micro-machined transducer of the cMUT, pMUT or mMUT type, a PVDF film transducer or a electromagnetic-acoustic transducer. It can in particular be coupled to the contact face 18 with the aid of an ultrasonic transmission gel. As a variant, the detector 28 may also be an optoelectronic transducer performing the function of transforming the ultrasound response at the point P, detectable optically, into an electrical signal intended to be transmitted to the processing module 30 of the first learning functional module 26. can be designed as a laser interferometer or equivalent which then points precisely to the point P. This is the variant illustrated in Figure 1.

According to the general principles of the invention, the learning module 26 is more precisely configured to generate the excitation signal e (t) in the form of a pseudo-random binary sequence of order greater than or equal to 2, or including such a pseudo-random bit sequence, and to convolve, using the arithmetic operator 32, the corresponding measured ultrasonic response u (t) with the conjugate sequence of this binary sequence pseudorandom. A pseudo-random binary sequence of order m is a binary sequence that periodically, and randomly, searches within each period, L = 2 ^m -1 values. This gives the signal v (t) in the form:

v (t) = (t) * e (t),

where "*" is the symbol of the convolution product and "" is the symbol of the complex conjugate signal.

Also in accordance with the general principles of the invention, the probing module 34 is more precisely configured to generate the reconstructed signal based on adaptive ultrasonic focusing of the convected measured ultrasonic response v (t). The operator 38 indeed performs an adaptive ultrasonic focusing on the signal v (t). In a preferred embodiment, adaptive ultrasound focusing is a simple time reversal. The reconstituted signal e ^R (t) is thus obtained in the form:

e ^R (t) = v (-t).

This reconstituted signal can be encoded at any time on a number of bits between 1 and 64 to be processed by computer. This number can obviously evolve, especially increase, depending on performances and computer calculations.

Since the measured ultrasonic response u (t) results from a convolution between the excitation signal e (t) emitted by the electroacoustic transducer 14 and the impulse response of the interior volume of the reverberant solid object 12, the fact that then convoluting this measured ultrasonic response u (t) to the conjugate sequence e (t) mathematically produces an autocorrelation on e (t). Now choosing this signal e (t) in the form of a pseudo-random binary sequence of order greater than or equal to 2 has the consequence that its autocorrelation approaches substantially the Dirac distribution with respect to the autocorrelation of an impulse excitation or even a chirp. Such a pseudo-random bit sequence therefore has better recompression properties by autocorrelation, which ultimately makes it possible to increase the amplitude of the wave finally emitted at the point P when the reconstituted signal e ^R (t) is emitted to From the electroacoustic transducer 14. Since, moreover, the learning is based on a longer time signal and with a higher energy than a simple impulse, it is itself improved in terms of the quality of the electroacoustic transducer. temporally inverted signal. The same reasoning applies when the excitation signal e (t) is not simply constituted by a pseudorandom binary sequence of higher order or equal to 2 but has one: in this case the measured ultrasonic response u (t) is convolved, not with the conjugate sequence e (t), but with the conjugate sequence of the pseudo-random binary sequence that the signal of excitation e (t) has.

It is possible to improve learning by increasing the order of the pseudo-random bit sequence to get as close as possible to the Dirac distribution. In concrete terms, when the pseudo-random bit sequence is of the MLS type (of the "Maximum Length Sequence"), also called the M-sequence, very good results can be obtained. Such a sequence can in particular be defined and generated on the basis of a periodic series of values produced by a linear feedback shift register exploring all the values that can be produced by this shift register. When its autocorrelation actually mathematically produces the Dirac distribution, the result is optimal.

It is also possible to replace the time reversal operation carried out by the operator 38 by inverse filtering, especially if it is desired to optimize the spatial resolution of the ultrasonic wave that will be emitted at the point P, generally with a slight loss of 'amplitude. In this case, the ultrasonic response u (t) to the excitation signal e (t) is measured at several points of the contact face 18 in a predetermined neighborhood around the point P, to obtain several values of v ( t), and the sounding module 34, or more precisely its operator 38, reconstructs the signal e ^R (t) by means of inverse filtering on these values of v (t).

Finally, a simplified embodiment consists of coding the reconstituted signal e ^R (t) at each instant only on a single bit.

FIGS. 2A and 2B illustrate two embodiments of the reverberant solid object 12. According to the illustration of FIG. 2A, which shows in perspective the configuration illustrated in FIG. 1, it has a generally parallelepipedal shape, for example cubic, in which a spherical shape is machined 44. This machining 44 produces a geometrical asymmetry sufficient to produce the desired ergodicity property. In particular, the test illustrated in Figure 4 which will be detailed later was performed with an aluminum cube of 50 mm sides and a spherical machining of 25 mm radius reducing the size of the three edges from the top on which it is formed at respectively 14, 20 and 25 mm. The contact face 18 is for example the unmachined bottom face. Alternatively, it may have a semi general shape cylindrical as shown in Figure 2B. The contact face 18 is for example the lower face in the form of a half-disk. Other variants are obviously possible and will be retained by the skilled person for their good ergodicity property which can be verified experimentally.

Whatever the embodiment of the reverberant solid object 12, its contact face 18 is not necessarily flat. In particular, in non-destructive control by contact it can be worked in correspondence with the shape of a part to control. It can also be frosted, in particular by machining with more or less spaced grooves except for a predetermined neighborhood around the point P, to limit contact with the part to be controlled and to promote ergodic reverberations in the solid object. 12.

The successive steps of an ultrasonic sounding process that can be performed with the aid of the ultrasonic sounding device 10 of FIG. 1 will now be detailed with reference to FIG.

This method comprises a learning phase 100 during which the excitation signal e (t) is generated in the form of a pseudo-random binary sequence of order greater than or equal to 2 by the generator 28, using temporal values or parameters of this signal stored in memory 24, in a first step 102.

During a next step 104, the excitation signal e (t) is amplified by the amplifier 40 and / or converted into an electrical analog signal if necessary before being transmitted to the electroacoustic transducer 14 and then transmitted in the form of a signal. ultrasonic wave in the reverberant solid object 12.

Then, during a step 106, the ultrasonic response u (t) is measured at the point P of the contact face 18, or at several points located in a vicinity of the point P, using the detector 42.

The measurement (s) resulting from the step 106 is or is transmitted during a step 108 in the form of an electrical signal to the processing module 30, the main function of which is to store them in memory 24, or to transmit them directly to the arithmetic operator 32.

Finally, during a last step 1 10 of the learning phase 100, the arithmetic operator 32 convolutes the measurement (s) resulting from the step 106 with the conjugate sequence of the excitation signal e (t ) to form at least one signal v (t) which is then stored in memory 24. Following the successful execution of the learning phase 100, a probing phase 200 can be undertaken.

During a first step 202 of this probing phase 200, the sounding device 10 is arranged to probe a medium. In particular, the point P of its contact face 18 is disposed in contact with this medium for the emission of ultrasonic waves in the medium from this point P.

Then, during a step 204, the operator 38 realizes a time reversal of the signal v (t) or an inverse filtering of the plurality of signals v (t) recorded in memory 24. The result of this processing is then transmitted to the generator 36 which generates the reconstituted signal e ^R (t).

In the following step 206, the reconstituted signal e ^R (t) is amplified by the amplifier 40 and / or converted into an electrical analog signal if necessary before being transmitted to the electroacoustic transducer 14 and then transmitted in the form of Ultrasonic wave in the reverberant solid object 12. This results in the desired ultrasonic excitation at the point P and transmitted in the medium.

The steps 106 to 206 have previously been envisaged for the single-element emission of an ultrasound wave from a single point P. But by spatial linearity of the adaptive ultrasound focusing methods, they could be realized for a multi-stage emission. elements from several points Pi, P _N distinct from the contact surface 18 thus forming a network of N transmitters. A law of delay could even be applied to this multi-element emission. It is sufficient to measure at step 106 N ultrasonic responses denoted by Ui (t), ... u _N (t) respectively at points P _1; P _N distinct or in several points respectively located in neighborhoods of points P _1; P _N , using the detector 42. Then, the adaptive ultrasonic focusing performed by the operator 38 in step 204 can be combined for measurements from each of the points P _1; P _N so that the emission of the reconstituted signal e ^R (t) in step 206 generates the desired ultrasonic excitations at the points P _1; P _N.

Finally, during a last step 208 of the sounding phase 200, a measurement is made from the emission of the ultrasonic excitation or the ultrasonic excitations resulting from the step 206. This measurement differs according to the envisaged applications. . It can be done by any known means: laser interferometer, mono- or multi-element electroacoustic transducer), with or without a shoe, etc. For an application in harmonic imaging of the medium sampled by volume waves, the emission is done at the point P as previously taught and the measurement 208 in reception can be done using at least one electroacoustic receiver stuck on the face of contact 1 8 in another place, this receiver operating at a central frequency significantly different from that emitted at the point P, and in particular for example in a reception bandwidth located outside transmission bandwidth. A nonlinear ultrasound response of the probed medium is thus measured.

In accordance with this application but also for other applications exploiting the non-linearities of the probed medium, it is noted that the present invention also makes it possible to apply to non-destructive testing the method, which is otherwise well known in the medical field, of inversions of emitted pulses (ie successively emits a pulse and its opposite) by limiting the emitting voltages of the electroacoustic transducer 14 despite the need for high emission amplitude at the point P. In this case, during the step of emission 206, the reconstructed signal e ^R (t) and its opposite - e ^R (t) is transmitted. Then, during the measurement step 208, the ultrasound responses of the probed medium to the emission of signals e ^R (t) and - e ^R (t) are respectively measured and then summed in order to eliminate a common linear component that they include. .

For a non-destructive testing application in the measurement of guided waves in thin structures, the emission is at the point P as previously taught, or at several points Pi, P _N as also previously envisaged. "Thin structures" means structures whose thickness represents a fraction of the average wavelengths emitted. The measurement 208 in reception can be done using a sensor disposed in contact with the medium to be probed but at a different place than the ultrasonic sounding device 1 0. This sensor can be single element, for a signal processing received by time-frequency analysis, or multi-element analysis, for dual-FFT (time-domain Fast Fourier Tranform) signal processing or by multiple classification of signals when the transmission is single-element, or for signal processing by singular value decomposition when the emission is multi-element, for the purpose of detecting and estimating dispersion curves. It is thus possible to detect guided propagation modes that are often characteristic of the structural properties of the thin structure under consideration.

The skilled person will refer for example: the article by Alleyne et al, entitled "A two-dimensional Fourier transform method for the measurement of propagating multimode signais", published in Journal of the Acoustical Society of America, volume 89, No. 3, pages 11.59 to 1,168, March 1991, for spatiotemporal dual FFT processing when the signals are measured using a multi-element sensor and when the emission is mono-element,

- in the article by Ambrozinski et al, entitled "Identification of material properties - efficient modeling based on guided wave propagation and spatial multiple signal classification", published in Structural Control and Health Monitoring, volume 22, pages 969 to 983, 201 5 , for multi-classification processing of the signals when the signals are measured using a multi-element sensor and when the emission is mono-element,

- Patent FR 2 946 753 B1 or the article by Xu et al, entitled "Sparse SVD method for high-resolution extraction of the dispersion curves of ultrasonic guided waves", published in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control , volume 63, No. 10, pages 1514 to 1524, October 2016, for singular value decomposition processing when the signals are measured using a multi-element sensor and when the emission is multi-element .

For an application in hyperthermia therapy, the emission is made at several points Pi, P _N for high energy focusing at any predetermined target point of the medium. The measurement step 208 is optional or even unnecessary in this case.

Beyond the three aforementioned applications for which exploitation of the nonlinearities of the medium to be probed is desired, the multi-element transmission envisaged above also makes it possible to acquire a matrix of impulse responses during step 208 to using a multi-element sensor. This acquisition is generally referred to as FMC (Full Matrix Capture) acquisition.

Other applications such as beamforming, focusing in all points, selection of guided propagation modes, comparison of measurements with a digital model, etc. can also be envisaged.

More specifically, with regard to the selection of guided modes, it applies to non-destructive testing in the measurement of guided waves in a thin structure against which the contact face 18 of the reverberant solid object 12 is disposed. adaptation of the general principles of the present invention. During the learning phase 100, at least one ultrasonic response is measured at each of several distinct points of the contact surface 18 of the reverberant solid object 12 thus forming a network of several point emitters from which a profile is emitted. ultrasound excitation. Then, during the emission step 206 of the sounding phase 200, the signals reconstituted by adaptive ultrasonic focusing (step 204) of the measured ultrasonic responses are emitted by this point emitter network. The ultrasonic responses measured are furthermore combined with one another so that the ultrasonic excitation profile thus emitted generates the selection of one or more predetermined guided wave propagation mode (s) in the thin structure. This combination is performed as a function of at least one physical parameter mathematically related to a phase velocity, in a given frequency range, of the predetermined guided wave propagation mode (s) to be selected and positions said discrete points of the contact surface 18. This physical parameter is for example the phase velocity itself, the wave number or the group velocity, these three notions being, in a well-known manner, mathematically related to each other.

By way of nonlimiting example, the teaching of the following documents can be used to achieve the appropriate combination of the measured ultrasonic responses according to the desired predetermined waveguide propagation mode (s) (s). ):

- Alban Leleux's dissertation, entitled "Non-destructive testing of composite ultrasound waves guided, generated and detected by multielement", 19 November 2012, in particular Chapter 3 for the so-called "phased array" method,

- the article by Bai et al, entitled "Multichannel wideband mode-selective excitation of ultrasonic guided waves in long cortical bone", published in Proceedings of the 2016 IEEE International Ultrasonics Symposium held in Tours (FR) of September 18 to 21, 2016. Compared with these teachings of the state of the art, the advantage of the present invention is to allow the generation of a multi-element transmitter whose inter-element distances can be, by time reversal or reverse filtering in the reverberant solid object 12, much lower than that allows a conventional multi-element transmitter and whose emission amplitudes are higher. It is therefore possible, on the basis of these lessons and in combination with the the present invention, to obtain significantly improved results in terms of propagation mode selection (s).

A test shown in FIG. 4 was carried out to verify the technical effect of the invention on a prototype whose reverberant solid object 12 is made of aluminum and geometrically conforms to the embodiment of FIG. 2A.

According to the experimental installation chosen, the electroacoustic transducer 14 is of the PZT type with a center frequency at 1 MHz and a peak-to-peak transmission voltage of 60 V. It will be noted that these emission conditions are markedly lower in voltage at what is currently practiced in harmonic imaging for nondestructive testing. The electroacoustic transducer 14 is bonded with phenyl salicylate to the wall 20 of the reverberant solid object 12. The detector 42 is a laser interferometer measuring a normal particle displacement expressed in nm. A titanium plate 1, 6 mm thick is placed on the contact face 18 with the aid of an ultrasonic coupling gel 46. A polyethylene terephthalate film 48 is spread on the free face of the plate 44 to ensure a good normal reflection of the laser beam of the detector 42. The latter is placed at a distance from the plate 44 so as to emit its beam in the direction of the normal to the plane of the plate 44 by pointing towards the point P, or to several possible points of a vicinity of the point P by moving it parallel to the contact face 18. The probing device 10 is used in the probing phase and the measurement of the detector 42 is made in transmission through the thickness of the plate 44 at a point P 'of polyethylene terephthalate film 48. Under these conditions, the following comparative results were obtained:

The method described as "Reference" is the direct emission at point P, without a reverberant solid object, of an ultrasonic excitation using an electroacoustic transducer of the PZT type with a central frequency at 1 MHz and a peak voltage at peak in emission of 60 V. The method described as "Gaussian impulse RT" is the emission according to the installation of FIG. 4 by the electroacoustic transducer 14 in the reverberant solid object 12 of an ultrasonic wave learned by temporal reversal of the ultrasonic response at point P to a Gaussian impulse.

The method described as "GA Gaussian impulse" is the emission according to the installation of FIG. 4 by the electroacoustic transducer 14 in the reverberant solid object 12 of an ultrasonic wave learned by inverse filtering of the ultrasonic response in a neighborhood of the point P to a Gaussian pulse.

The method described as "Chirp RT" is the emission according to the installation of FIG. 4 by the electroacoustic transducer 14 in the reverberant solid object 12 of an ultrasonic wave learned by temporal reversal of the ultrasonic response at the point P at a chirp.

The method described as "Chirp FI" is the emission according to the installation of FIG. 4 by the electroacoustic transducer 14 in the reverberant solid object 12 of an ultrasonic wave learned by inverse filtering of the ultrasonic response in a neighborhood of the point P to a chirp.

Finally, the method described as "Sequence M RT" is the emission according to the installation of FIG. 4 by the electroacoustic transducer 14 in the reverberant solid object 12 of an ultrasound wave learned by temporal reversal of the ultrasonic response to the point P for a sequence M. For the purposes of the experimental test, a sequence M of order 18 or 18 bits, that is to say 2 ¹⁸ -1 = 262143 values scanned, sampled at 25.10 ⁶ values per second and therefore a duration of about 10.49 ms, was used.

Since the displacement measured at the point P 'is obviously directly related to the amplitude of the ultrasonic wave emitted from the point P, the above results show the rather considerable improvement provided by the use of a pseudo bit sequence. -random of order greater than or equal to 2, especially a sequence M, during the learning phase, whether by comparison with the reference method (amplitude in transmission multiplied by more than 50) or with any one other methods applying a time reversal or inverse filtering by learning (amplitude in transmission multiplied by more than 10). The displacement of 33.1 nm is in particular largely sufficient to exploit the nonlinearities of the medium to be probed, such as for example in harmonic imaging.

As a variant of the test of FIG. 4, a detector 42 of transducer type PZT can be directly glued to the point P. By lowering the peak-to-peak voltage in transmission at 40 V for the electroacoustic transducer 14, the following comparative results are obtained:

The voltage measured at the point P is obviously directly related to the amplitude of the ultrasonic wave emitted from the point P, the above results again show the quite considerable improvement provided by the use of a sequence M during the learning phase by comparison with the method applying a time reversal with the use of a Gaussian pulse during the learning phase (amplitude in transmission multiplied by more than 13).

By simulation, it is further shown that the M sequences have particularly advantageous recompression properties compared to other known pseudo-random binary sequences such as the Golay or Kasami sequences. For learning sequences of the same duration of approximately 10 με, the amplitude measurable at the point P is more than doubled with a sequence M with respect to what is measurable with a Golay or Kasami sequence.

It clearly appears that an ultrasonic sounding device such as that described above makes it possible to improve the amplitude of the waves emitted at at least one point P of contact with the medium to be probed, in particular to exploit the nonlinearities, without for all that need to increase the power of the electronic transmission chain. Moreover, the coupling constraints are less strong, whether in the device between the electroacoustic transducer (s) and the reverberant solid object or at the interface of the device and the medium to be probed.

Note also that the invention is not limited to the embodiments described above.

Thus, alternatively, the signal reconstituted at emission e _R (t) can also be convoluted with another predefined signal to modulate its shape according to the intended application, as is otherwise well known ultrasonic sounding.

In a variant also, it is not only one but several electroacoustic emission transducers that can be arranged against the wall 20 of the reverberant solid object 12, as for example taught in the article by Montaldo et al (2005) cited above. . It will be apparent more generally to those skilled in the art that various modifications may be made to the embodiments described above, in light of the teaching just disclosed. In the following claims, the terms used are not to be construed as limiting the claims to the embodiments set forth in this specification, but should be interpreted to include all the equivalents that the claims are intended to cover because of their formulation and whose prediction is within the reach of those skilled in the art by applying his general knowledge to the implementation of the teaching that has just been disclosed to him.

Claims

1. Method for ultrasound probing of a medium, using a device (10) comprising a reverberant solid object (12) having a contact face (18) intended to be placed in contact with the medium to be probed and at least one electroacoustic transducer (14) disposed against a wall (20) of the reverberant solid object (12), the method comprising:

a learning phase (100) including measuring (106), in at least one point (P) of the contact face (18), of an ultrasonic response to an excitation signal emitted by said at least one transducer electroacoustic (14) in the reverberant solid object (12), and

a phase (200) for probing the medium, when this medium is in contact with said at least one point (P) of the contact face (18), comprising emission (206) by said at least one electroacoustic transducer (14); ) a reconstructed signal based on adaptive ultrasonic focusing (204) of said measured ultrasonic response,

and being characterized in that:

during the learning phase (100), the excitation signal comprises a pseudo-random binary sequence of order greater than or equal to 2 and the measured ultrasound response is convolved (1 10) to the conjugate sequence of said sequence pseudo-random binary, during the probing phase (200), the reconstituted signal results from an adaptive ultrasonic focusing of the convolved measured ultrasonic response.

An ultrasonic sounding method according to claim 1, wherein the pseudo-random bit sequence is selected to verify at least one of the following properties:

its autocorrelation mathematically produces the Dirac distribution, and

it is of the MLS type, then also called M. sequence

An ultrasonic sounding method according to claim 1 or 2, wherein the reconstituted signal results more precisely from a time reversal (204) of the convolved measured ultrasonic response.

An ultrasonic sounding method according to claim 1 or 2, wherein:

during the learning phase (100), the ultrasonic response is measured (106) at several points of the contact face (18), and during the probing phase (200), the reconstituted signal results from a filtering inverse (204) performed using said measurement (106) at several points of the ultrasonic response.

An ultrasonic sounding method according to any one of claims 1 to 4, wherein the measurement (106) of the ultrasonic response is done using an electroacoustic transducer disposed against said at least one point (P) of the contact face (18) or an optoelectronic transducer (42) pointing to the at least one point (P) of the contact face (18).

6. An ultrasonic sounding method according to any one of claims 1 to 5, wherein the probing phase (200) comprises a measuring step (208) performed by at least one electroacoustic receiver disposed against the contact face (18). the reverberant solid object and operating at a central frequency significantly different from a central transmission frequency to said at least one point (P) of the contact face (18), in particular in a reception bandwidth located outside the bandwidth of emission, for the measurement of at least one nonlinear ultrasound response of the probed medium.

An ultrasonic sounding method according to any one of claims 1 to 6, wherein:

the emission step (206) of the sounding phase (200) comprises the emission of the reconstituted signal and that of its opposite, the sounding phase (200) comprises a measurement step (208), made from the transmissions of the transmitting step (206), in which ultrasonic responses of the probed medium to the emitted reconstructed signals are respectively measured and summed to remove a common linear component which they include.

8. Ultrasonic probing method according to any one of claims 1 to 7 for a nondestructive test application able to of guided waves in a thin structure against which is disposed the contact face (18) of the reverberant solid object (12), wherein the probing phase (200) comprises a measuring step (208), performed by a mono-or multi-element electroacoustic receiver (s) also arranged against the thin structure, during which:

at least one ultrasonic response of the thin structure is measured (208) and processed by time-frequency analysis when the electroacoustic receiver is mono-element, or

several ultrasonic responses of the thin structure are measured (208) and processed by estimation of dispersion curves, in particular by double space-time FFT, multiple classification of signals, or singular value decomposition when the electroacoustic receiver is multi-elements,

for detection of propagation modes guided by the thin structure.

9. An ultrasonic sounding method according to any one of claims 1 to 8 for a non-destructive testing application in guided wave measurement in a thin structure against which is disposed the contact face (18) of the reverberant solid object (12), wherein:

during the learning phase (100), at least one ultrasonic response is measured (106) at each of several distinct points of the contact surface (18) thus forming a network of several point emitters from which a profile is emitted ultrasound excitation, and

the emitting step (206) of the probing phase (200) comprises the emission by said network of point emitters of several signals reconstituted by adaptive ultrasonic focusing (204) of said measured ultrasonic responses, these ultrasonic responses being measured by elsewhere combined with each other such that the ultrasonic excitation profile thus emitted generates the selection of at least one predetermined guided wave propagation mode in the thin structure, this combination being performed as a function of at least one physical parameter mathematically related to a phase velocity, in a given frequency range, of said at least one predetermined guided wave propagation mode to be selected and positions of said discrete points of the contact surface (18).

10. Device (10) for ultrasound probing of a medium, comprising:

a reverberant solid object (12) having a contact face (18) intended to be placed in contact with the medium to be probed, at least one electroacoustic transducer (14) arranged against a wall (20) of the reverberant solid object (12) ,

a system (16) for controlling said at least one electroacoustic transducer (14), comprising:

A learning module (26) configured to generate an excitation signal to be emitted into the reverberant solid object (12) by means of said at least one electroacoustic transducer (14), and

A middle sounding module (34) configured to generate a reconstructed signal based on an adaptive ultrasonic focusing (38) of an ultrasonic response to the excitation signal measured at at least one point (P) of the face contact (18), this reconstituted signal being intended to be emitted into the reverberant solid object (12) by means of said at least one electroacoustic transducer (14),

characterized in that

the learning module (26) is more precisely configured to generate (28) the excitation signal in the form of a pseudo-random binary sequence of order greater than or equal to 2 and to convolute (32) the ultrasonic response measured at the conjugate sequence of said pseudo-random bit sequence, and

the probing module (34) is more precisely configured to generate (36) the reconstructed signal based on adaptive ultrasonic focusing of the convolved measured ultrasonic response.

1 1. An ultrasonic sounding device (10) according to claim 10, wherein said at least one electroacoustic transducer (14) comprises at least one of a PZT piezoelectric transducer, a micro-machined transducer of type cMUT, pMUT or mMUT, a PVDF film transducer and an electromagnetic-acoustic transducer.

An ultrasonic sounding device (10) according to claim 10 or 11, wherein the reverberant solid object (12) is made of at least one material. little acoustically attenuating than aluminum or aluminum alloy or fused silica.