FR3006448A1 - ULTRASONIC IMPEDANCEMETRIC PROBE WITH SOLID WAVE OR SOLID-LIQUID PROJECTED GUIDES - Google Patents

Abstract

The invention relates to an ultrasonic device and method for measuring, inspecting and classifying the mechanical and / or thermal properties of a medium (60, 61, 62, 63, 64, 65, 100) by means of at least one broadband source (20, 21, 22, 250, 260, 270, 280, 290, 540) of acoustic waves coupled to at least one wave elastic solid waveguide (10, 17, 18, 150) impedance and effusivity, housed and damped (43, 44, 55) in an insulating structure (41, 42, 52) acoustically and thermally and coupled with the medium on a given surface (11, 14, 15, 16) with a controlled bearing force (F) (80,81), said coupling being able to result from a first coupling (92) of the solid guide to a liquid guide projected continuously with a constant or variable hydrostatic pressure, said liquid being able to It is also intermittently heated and sprayed in the form of a collimated jet (93) striking the middle surface. Applications include non-destructive testing, medical diagnosis, food grade classification, iron and steel, and directional velocimetry.

Description

DESCRIPTION The invention relates to non-destructive non-destructive testing (NDT) at high speed of projected liquid coupling scanning, ultrasonic elastography of elastic and viscoelastic samples and the identification or classification of the state of materials via their acoustic properties. or thermal. It is applicable to directional velocimetry. It consists of a device and a method for measuring longitudinal and transverse characteristic acoustic impedances in organic materials or fabrics by simple contact, immersion or projection of a known impedance guide. The materials may be solids or fluids and in particular viscoelastic liquids in solidification phase or high temperature phase change. It is composed of contact probes, solid or liquid wave or guided, contacted with the materials to be characterized. The contact surface is an essential component of the device since it defines the limit between fundamental speeds and propagation speeds in dispersed guided medium. It also defines the apparent thermal effusivity of the probe when there is also a heat transfer process between the probe and the sample. The method comprises generating longitudinal and transverse waves, or dispersive waves, at the desired frequencies, in a known impedance waveguide and profile, contacted with the test material. The material may have a high temperature because the piezoelectric transducers used to transmit and receive the ultrasonic waves are not directly in contact with the sample, a relatively long waveguide acting as a thermal buffer. Conversely the waveguide may be heated and yield some of its heat to the sample, while the sample is at room temperature prior to contact. It can also be cooled and have a lower temperature than the sample to absorb some of its heat. The objective is then a simultaneous measurement of the mechanical and thermal properties of the sample. An echo processing makes it possible to deduce the characteristic and radiation acoustic impedances of the sample, in the case where it is not plane, as well as the propagation velocities and the spatial and temporal variations of the propagation velocities in the sample. . A variant of the device makes it possible to determine local impedances by echographic method or by direct or differential coupling between several conical bending waveguides. In the field of time variations, the device can determine a flow rate of a fluid by contact with the solid or in the surrounding fluid. It can also be used to characterize a process of solidification by thermal cooling, with liquid-solid phase change, for example in the case of a metal plate several centimeters thick coming out of a blast furnace, the core of the plate being still melting at 1200 ° C (one thousand two hundred degrees Celsius), while the outer surface of the plate begins to solidify. In this case, depending on the raw materials used, mixing proportions, we seek to increase the expertise on the cooling rate and the temperature profile in the thickness of the material to better control the appearance of crystalline phases and in fine the mechanical properties of the material, for example cast irons or glass plates, or even steel plates, or other materials resulting from high temperature manufacturing processes by rolling or float bath.

The problem to be solved therefore consists in locally accessing the mechanical properties of elastic or viscoelastic heterogeneous materials or fabrics in space and / or time, by a local contact method, or even quasi-point, possibly accepting high temperatures. The parameters that are to be determined preferably simultaneously with controlled coupling conditions are in particular the longitudinal and transverse impedances of the materials. These parameters are used in many fields, in particular in the field of tactile acoustic learning interfaces described in patents FR0703651 of May 23, 2007, FR0955065 of July 21, 2009, FR1059657 of November 23, 2010, FR1156892 of July 28, 2011, FR1258664 of 14 September 2012, FR1258664 of September 14, 2012, exploiting the damping of resonance modes or the diffraction of ultrasonic waves based on the acoustic properties of the finger pulp which, when it comes into contact with the tactile surface, dampens, removes and diffracts the underlying vector vibration typically from the position of the touch or gesture to be recognized. The part of each of the phenomena is not yet known, and it is one of the advantages of the present invention to contribute to improving the knowledge of this problem. However, there are disparities from one person to another, as well as the age and activity of individuals, particularly in terms of callosity, contact area and the nature of the dermis. Another advantage of the present invention is to evaluate the acoustic properties of the finger pulp for a population of individuals, so that the locating method can be made functional for the entire population.

Moreover, the industrial production of tactile surfaces for learning, for example in the form of tablets or tactile pads, requires the availability of artificial fingers made of silicone and having the same mechanical properties as human fingers, particularly from the point of view of the longitudinal and transverse impedance to reproduce.

Indeed, it has been found for example that a silicone polymer material could be very close to a human finger with respect to the longitudinal characteristic impedance, but far away for the transverse characteristic impedance (or shear) so that with identical profile and contact area, the acoustic signatures of an artificial finger and a human finger could be quite different. By extension, the longitudinal and transversal impedances of materials subjected to high temperatures and in crystalline liquid-solid phase change situations are also variable in time and space, in particular in depth, and their monitoring over time (monitoring ) can provide a means of characterizing the thermal manufacturing process.

This same remark is valid for the control of aging or degradation of the mechanical properties of the dermis or of the underlying viscoelastic tissues, for example in the breast, because of a pathology or a cosmetic treatment aimed at improving the mechanical flexibility. skin with creams. On the other hand, in the food field, the ripening of a fruit or a vegetable often results in a variation of the mechanical properties of the superficial layer and the deeper layers. In this case, the simultaneous knowledge of the longitudinal and transversal impedances as well as thermal, combined with an indexation or a classification of these properties in an ordered database or associated with a scale of maturation, or even identification of the nature when it is for example to recognize animal meat, is a method for diagnosing the state of conservation or maturation of these foods. In addition, there are few elastographic and thermal measuring instruments that are affordable for the industry. Most of those who are developed exploit high-number of elements of transducers, typically a hundred, associated with significant electronic means in memory size, in analog-to-digital and digital-analog fast conversion, typically 50 megahertz and in need fast processing and transfer of data, making these tools unaffordable for small businesses: independent doctors, farmers, farmers or even conservators. To return to the tactile surfaces, such as those mentioned in the above patents, using plastic, metal or glass materials, the contact with a human finger involves an interaction with a mechanical vibration which comprises a longitudinal component and a component of shear. With respect to the coupling with the pulp, there is an out-of-plane vibration penetrating perpendicular to the pulp and a shearing component (i.e. located in the plane of the interface) from shearing the pulp.

A "good" artificial finger is a finger that simultaneously has good similarity with a human finger on both the longitudinal and transverse impedances as well as on the viscous damping generated at the interface and backscattered by the composition of its volume. back as well as on the thermal properties including the effusivity that the artificial finger must reproduce in a manner comparable to that of the human finger which gives up part of its heat in a situation of non-thermal neutrality. In this way, the acoustic signature of an artificial finger is the same as the acoustic signature of a human finger and it is then possible to precisely calibrate or study the reliability using a robotic arm, a tactile surface comprising several hundred reference coordinates or various thermodynamic conditions. Thus, when one seeks to develop an artificial finger and / or a tactile surface, one is finally interested in the same problems as those of medical pulse elastography using low frequency shear waves and high longitudinal acoustic illumination. frequency, for which it is necessary to have information such as density and Young's modulus, shear modulus and Poisson's ratio, or propagation velocities, or characteristic impedances, in order to predict the velocities of waves in the material and their sensitivity, for example in temperature. However, these parameters are rarely explicitly provided by the manufacturers of the materials, in particular for plastics and soft materials such as elastomers 20 which propagate weakly the shear waves. This type of information is even less available in manufacturing processes involving the coexistence of different crystalline phases. It is certainly possible to determine them from other information such as flexural and shear modulus or density, but this information is often approximate. In addition, the acoustic impedance parameter combines two values, velocity and density, which, when available, are often flawed or vary from one manufacturer to another, which reduces the accuracy available on the acoustic impedances. In addition, many materials undergo alteration of their physical properties depending on their aging or external agents, such as alteration by a chemical or mechanical wear. It may be necessary to periodically or continuously monitor the evolution of these properties over time. In practice, it is therefore desirable firstly to be able to precisely and locally measure the characteristic acoustic impedances associated with the longitudinal and transverse modes during a measurement campaign on a population of individuals in order to study the dispersion of the mechanical properties. living tissues between individuals or on tactile materials whether they are plastic or flexible in order to control their similarity to human skin, or in food control, cases of high-temperature manufacturing processes, in medical diagnostics or in Non Destructive Testing (NDT). It is the object of the present invention to provide a simple device, a local guided wave probe, associated with a process able to quickly characterize the acoustic impedance of the materials without damaging them, by simple contact on a flat face. of the order of one millimeter to one square centimeter and possibly a variation of viscoelastic properties by scanning.

By extension, the invention relates to the field of medical imaging by ultrasonic ultrasound or pulse elastography exploiting the combined use of longitudinal and transverse waves by means of one or more ultrasonic waveguides, optionally heated or cooled, propagating these two waves and put in contact with the medium of interest in order to determine its mechanical and thermal properties. In the case of shear wave impulse elastography, the use of shear waves is limited to low frequencies, typically between 50 Hz and 5 kHz due to the strong damping of shear waves in diffusing viscoelastic media which propagate in living tissue at a speed close to 3 m / s. A presentation of this method is for example given in the article by S. Catheline et al., "Diffraction Field of a Low Frequency Vibrator in Soft Tissues Using Transient Elastography," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 46, no. 4 (July 1999): 1013-1019. Developments in this process are also published by L. Sandrin et al., "Shear Modulus Imaging with 2-D Transient Elastography," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 49, no. 4 (Apiil 2002): 426-435, or by Mr.

Tanter et al., "Ultrafast Compound Imaging for 2-D Motion Vector Estimation: Application to Transient Elastography," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 49, no. 10 (October 2002): 1363-1374. Pulse elastography with low frequency shear waves has been the subject of numerous patent applications. One can quote the publication number 2 791 136 corresponding to the request FR9903157 of March 15, 1999 or the number 2 844 178 corresponding to the application FR0211074 of September 6, 2002. This type of elastography aims to detect cancerous nodules in the tissues humans, especially in the breast and liver and thus seeks to imaged these tissues to a depth of the order of 5 to 10 centimeters. The strong damping of shear waves in human tissues does not make it possible to operate at ultrasonic frequencies. Nevertheless, if one restricts himself to short distances of the order of a millimeter as indicated in the present invention, it is possible to exploit the shear waves at higher frequencies than those suggested in the state of the art. art, which are between 50 Hz and 5 kHz and achieve with the present invention ultrasonic frequencies. The objective is not only to exploit longitudinal waves to illuminate at an ultrafast rate of the order of 10 kHz the tissue being deformed under the effect of the passage of a pulse wave shear of high amplitude, tens of microns, generated by a vibratory pot, but also to exploit the shear wave as an additional wave, of different speed and polarization, generated by tapered waveguide transducers and exploited in traditional sonography for get a fine measurement. In addition, the present invention provides an additional means of characterizing the material by a thermal transfer process generating a thermal diffusion wave in the material. Finally, the present invention aims to propose a probe making it possible to generate the two types of waves at a distance and over a very limited lateral extent, by means of a projected collimated liquid coupler. In the low-frequency shear wave impulse elastography, we seek to know the shear wave velocity in the tissue and in particular the variations of this velocity and the wavefront in the presence of a more rigid cancer nodule, which has the effect, on the one hand, of reducing the amplitude of the deformation of the fabric and, on the other hand, of accelerating the speed of propagation of the shear wave. The medical diagnosis then consists in solving the inverse problem to access the shear modulus, in particular its increase and its spatial extent in the tissue. The resolution of the method depends on the image acquisition frequency, that is to say the distance traveled by the bending wave between two longitudinal measurements, which is in practice of the order of a millimeter. Note that the isostatic compression modulus in human tissues is of the order of 1 GPa while the shear modulus is a thousand times smaller, in connection with the very low shear wave speeds observed.

The object of the invention is also to be able to analyze or even image over an area of the order of one square centimeter by displacement and measurement of the position of the probe, the shear impedance in the vicinity of the dermis, or in general of the test surface, by means of a conical waveguide with dispersive bending waves, a quasi-point contact surface and therefore a lateral dimension comparable to or less than the working wavelength and therefore having a low radiation impedance, typically 4 to 10 times smaller than the transverse impedance of the same volume material and therefore more suitable for measuring small values of shear impedance or radiation. It is thus possible to image by differential technique small variations of impedance on a surface of the order of one square centimeter. In addition, the tapered profile of the tip reduces the apparent thermal effusivity of the waveguide, generally metallic, which is then close to that of the skin. Alternatively, the same device can access by simple contact to the flow velocity of a fluid in a vein by ultrasonic differential transit time or by continuous or pulsed Doppler measurement. From a physical point of view, the acoustic impedance Z is the ratio between the generated pressure P and the vibration velocity of the particles y of the material in which the wave propagates: When the lateral dimensions are sufficient to reason on the waves planes, this term is the product of the density p by the propagation velocity c in the propagation medium: Z pc The relation expressing the transmissions / reflections of the waves at the interfaces of different media can be expressed by the ratio of the impedances acoustic. A first approach to determine the impedance of a material is to evaluate separately the two parameters, density and propagation speed, to deduce the acoustic impedance of the material and to predict the transmission coefficient of a wave propagating from a first material to a second material sharing with the first a large lateral extent interface in front of the wavelength. When the lateral dimension of the interface becomes comparable to the wavelength, we will no longer speak of characteristic impedance, but of radiation impedance. The radiation impedance involves the evolution of the contact section a little before and a little after the interface, over a distance at least equal to the half-wavelength and corresponding to the volume area of matter. coming in reaction to the vibration at the interface.

The density can be known from the technical data of the considered material or evaluated by the method of Archimedes, which exist standardized methods of measurement by immersion, ISO 1183, DIN 53 479, ASTM D792. The propagation velocity can be measured according to the transit time of the wave through a known dimension of the material. Such a method is for example described in the article by R. Selfi-Idge, "Approximate Material Properties in Isotropic Materials", Sonics and Ultrasonics, IEEE Transactions on, Vol. 32, no. 3, p. 381 -394, May 1985. When the damping is low, the speed can also be determined precisely by measuring the transit time of several echoes on the faces of the material. Several techniques for determining the propagation velocities of the two fundamental modes and for characterizing the properties of the materials from these data are thus presented in the chapter DK Pandey and S. Pandrey, "Acoustic Waves - Chapter 2: Ultrasonics: A technique of Another approach to accessing characteristic impedances is from the Z = P / v relationship. . Numerous measurement methods based on different configurations of microphones and loudspeakers exploiting acoustic waveguides are found in the literature. The technique consists in emitting a wave, in monochromatic form or in a broad spectral band, emitted continuously or impulse which is reflected or transmitted by the test material. The wave is characterized in amplitude and / or in phase and the signal can undergo a mathematical treatment to go back to the intrinsic mechanical properties of the material such as the characteristic impedance. For example, there is a method described in the patent application FR2547055B1 exploiting sound waves in the air to evaluate the absorbent properties of materials. It operates a cylindrical waveguide, closed on one side by a sound source emitting a continuous wave and, on the other, by the test material whose impedance is to be determined. The waveguide is hermetically applied against a flat wall of the object to be analyzed. Two microphones are inserted into the waveguide and disposed at known distances from the interface so that the acoustic distribution of the resulting stationary wave can be measured and the impedance of the material can be deduced, with reference to the A stationary wave distribution for a standard material, the mechanical impedance of which is also known by another method. The applications of this method mainly concern civil engineering for building soundproofing.

Another method described in FR2806162B1 is to emit a longitudinal wave beam through a fluid, for example water, to a plate of a material to be analyzed and for which it is desired to evaluate its homogeneity. A layer of impedance adapter varnish between the plate and the water allows maximum penetration of the wave into the material. Then, it is reflected on the opposite side until the transmitting transducer that has switched, meanwhile, in receiver mode. It is then possible by analysis of the different echoes to deduce the impedance of the material as well as its homogeneity, especially when the material is a plate which one seeks to control the absence of defects by scanning over its entire area. . The use of a fluid as a medium ensures the transmission of a perfectly longitudinal wave. The method is applied to the non-destructive testing of aircraft wing and aims to detect inhomogeneities and the possible presence of defects in the material. US6298726 discloses an acoustic impedance measuring device by analyzing reflected waves on several elements located at depth of the material. Particular parameters of the frequency response of the echo signal are extracted and compared with calibrated values to identify particular materials. An algorithm based on a polynomial equation, calibrated for the transducer, delay and sample set, enables the identification of the material. This device can adopt different configurations with a waveguide, focusing or not the longitudinal wave on a small surface. The excitation signal is a trapezoidal burst with rise and fall times adapted to cancel the echo in the material. Alternatively, the trapezoidal pulse may consist of two pulses shifted in time, the second pulse being shifted by an interval creating a wave in phase opposition with the wave from the first pulse and an amplitude to cancel the echo from the test material.

The patents just cited are all intended to be used in a single mode of longitudinal propagation of the echo. There are also publications dealing with acoustic impedance measurement in living materials or tissues. The publication "A contact method for the assessment of ultrasonic velocity and broadband attenuation in cortical and cancellous bone", Clin Phys Physiol Meas, vol. 11, no. 3, p. 243-249, August 1990, CM Langton et al, exploits a principle of emission of an acoustic wave in a living part of an animal or a human, here the horse paws and makes it possible to determine the speeds of propagation in different tissues and bone types, by analyzing transmitted waves or echoes of ultrasonic waves through the sample to be analyzed. A temporal representation makes it possible to determine the propagation speeds in the different materials. The goal is to diagnose fracture risks. But, it is not question of modes of propagation other than longitudinal, nor of characteristic impedance analysis. The publication "Direct measurement of index finger mechanical impedance at low force", in Eurohaptics Conference, 2005 and Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems, 2005. World Haptics 2005. First Joint, 2005, p. 657 - 659, C.-Y. Fu and M. Oliver, allows to evaluate the mechanical impedance of the whole finger at low frequency (<500 Hz) by the use of an electromagnetic device of excitation (vibrating pot) and measurement of the movements with a laser vibrometer for different ranges of forces applied and measured using a force sensor. The publication "An ultrasonic indentation system for biomechanical properties assessment of soft tissues in vivo," Biomedical Engineering, IEEE Transactions on, Vol. 43, no. 9, p. 912-918, Sept. 1996, Y.-P. Zheng and A. F. T. Mak, shows a device consisting of a cylindrical waveguide coupled to a force sensor. The purpose of this system is to apply the end of the waveguide to a biological tissue under the condition of a bearing force. The indentation of the probe in the tissue can be measured and an ultrasound wave emission system makes it possible to measure the thicknesses of the different layers of tissues such as the epidermis, dermis, muscle and bone. A burst of waves is sent, then the echoes on the different tissues are recorded. By knowing the speed of propagation and by measuring the flight time between the different echoes, it is possible to determine the thickness of each layer. Knowing the support force and the thickness of each layer of tissue, the device makes it possible to trace and determine a viscoelastic behavior profile of the organic element and the various layers constituting it. In this publication, the device exploits an ultrasonic emission through a waveguide in organic materials and approaches the device of the present invention, but is not intended to measure the characteristic impedance of the materials and does not distinguish the type of waves propagating in the tissue (longitudinal or transverse).

With regard to ultrasonic waveguides with bending wave cone guides, the state of the art mentions the work of JP Nikolovski and D. Royer (1997) "Local and selective detection of acoustic waves on the surface of a material, 699-703 IEEE Ultrasonics Symposium Proceedings, Toronto, 1997, and Nikolovski's patent application, FR2755225 of 1996 concerning a non-contacting touch-sensitive tapered-wave ultrasonic probe, and a flowmeter with conical points with ultrasonic waves for gas flow measurement, as well as the patent FR0759603 of December 6, 2007 concerning the measurement of thermal effusivity of a sample at room temperature using a heated tip. These applications mention many means of transmitting / receiving bending waves in a solid or a fluid from a conical tip, possibly flattened at its end to reduce the radiation impedance. The device is known, but its use is controlled only in particular coupling situations where the coupling section does not need to be known. Nothing is disclosed concerning the measurement of the transverse impedance of the test material, nor the refraction angle and wave detection in the test material when the velocity of the waves in the material is comparable to the velocity in spikes, nor on the method of use of spikes when they are brought into contact with very hot materials, the temperature of which exceeds several hundred degrees Celsius the Curie temperature of the piezoelectric ceramics used to emit and receive the waves in the tips, or the coupling of three or more points for the simultaneous measurement of longitudinal and transverse velocities in the sample, nor on the mechanical integration of the points in a granular carrier structure simultaneously performing damping, ultrasound, the acoustic and thermal insulation, the heating or the measurement of the support force and the control of the impedance of radius truncation of the conical tip to a position where the phase velocity reaches a given value. In patents FR2547055B1 and FR2806162B1, the use of a fluid medium (air and water) only allows a longitudinal propagation mode. However, in all the fields of application of the invention mentioned above, whether in metallurgy, cosmetics, dermatology, power supply or tactile interfaces, the mechanical coupling implements shear waves with the test interface. In addition, in non-destructive testing, or in medical ultrasound, the use of slow-spreading transverse waves can improve spatial resolution, and reveal small variations in transverse impedances (associated with malignant tissue). Sensitivity to small variations in transverse impedance is also useful in CND, in areas of cracks, delamination or crystalline phase change.

In addition, the applicant has not found in the state of the art, particularly in the field of high-frequency elastography method of measuring shear impedance at the scale of one cubic millimeter. Finally, the Applicant has also not found in the literature information relating to the coupling between a dispersive waveguide and a collimated liquid guide carrying the ultrasound and projected at high speed on a sample to be inspected, said sample being in very fast relative displacement relative to the liquid guide. However, the simultaneous use of several conical waveguides integrated bending wave in a barrel as proposed in the invention or a collimated liquid guide carrying ultrasound would increase the measurement dynamics, the lateral spatial resolution and in-depth and to obtain a better diagnosis, especially when scanning a multi-tip probe for two-dimensional imaging (curvilinear and deep) of cancerous nodules or plaque of atheromas, by combining several types of measurements in a minimum volume, each type of measurement providing for example a B-scan image in a given color. Preferred but nonlimiting aspects of the device according to the invention are the following: the knowledge of the characteristic impedances or propagation velocities of the two propagation modes, as well as the knowledge of the radiation impedance at the point of contact when the lateral extent of the guide is small in front of the wavelength, gives access to several mechanical properties of the material such as the modulus of traction / compression (Young's modulus), the modulus of flexion / shear, the coefficients of Lamé, the Poisson's ratio, the radiation impedance of the material involving a limited coupling zone, as well as the thermal properties of the material in contact with the tip, when it is heated or subjected to a temperature different from that of the sample. - The radiation impedance is defined here as the product of the phase velocity by the density of the material constituting the guide. A flat-end large diameter guide, for example a cylindrical guide with a diameter of 13 mm (0.5 inches), generates waves considered quasi-planar and the impedance of the guide is equal to the characteristic impedance of the material. component. The volume guides are thus adapted to flat, soft and isotropic materials, while the tips are adapted to local measurements on curved interfaces and for materials with rapid spatial variation of mechanical properties or high surface temperatures. Unlike the above-mentioned methods, the invention makes it possible to identify in the same place, the longitudinal and transversal characteristic impedances and of radiation as well as the local variations of impedance and effusivity simultaneously by simple contact, without deterioration of the medium. to analyze. The system exploits the reflective properties at the interface of the waveguides and the material to be analyzed. The waveguides have dimensions adapted to the transit time specific to each type of wave so that a longitudinal wave, propagating faster than a transverse wave, does not disturb the transverse wave arriving earlier than this. last. The mechanical properties of the materials are directly calculated from a comparison between a reference measurement, obtained without contact or at a given point of the reference surface of the sample, and a measurement on the sample, without approximation by a calibrated model beforehand. In cases where the test medium has finite dimensions, whether laterally or in depth (stratified plate, dermis, food, ...), the waveguide has a tapered geometry and is chosen so as to propagate dispersive bending waves, the section of the waveguide at the interface being selected to work with a certain phase velocity of 10% to 100% of the transverse wave velocity and therefore some radiation impedance value for optimum energy transfer in the test surface. - The waveguides are preferably made of materials having a characteristic impedance greater than or equal to that of the test material, for example low density metals such as aluminum or titanium when one wants to characterize the dermis, because the waveguides with spikes are very tapered with opening angles typical of 5 ° which provides on the one hand a bandwidth sufficient to work at a high frequency of the order of Megahertz and other A radiating impedance can be small, in practice close to the longitudinal characteristic impedance of the skin, which allows for contrasting measurements on the local impedance variation of the dermis; In particular, the tapered profile of a tip makes it possible to reduce its phase speed of the bending mode as it approaches its end, down to below the speed of propagation of the waves in the water . When the diameter of an aluminum tip typically reaches 150 kun, the phase velocity at 600 kHz falls below 780 m / s (4 times lower than the transverse velocity in a large block of aluminum which is 3100 m / s) and twice as low as in water. In this configuration, the wave can no longer be refracted in water because the refraction angle is greater than 90 °. The tip must then be pushed deeper into the water until it reaches a depth where the phase velocity exceeds 1500 m / s and refraction is possible. The amplitude of the echo corresponding to the bending wave reflected inside the tip, at its end, then decreases very quickly.

Examples: Aluminum block volume: VT = 3100 m / s (transverse velocity) VL = 6400 m / s, (longitudinal velocity) p = 2700 kg / m3 (density) ZL = 17.3 MRayl (longitudinal impedance) ZT = 8.4 MRayl (cross impedance) Tapered waveguide aluminum tip (vertex angle = 4.8 °) truncated when the section diameter reaches 0.15 mm: Vt (d = 0.15 mm) = 780 put Zt (d = 0.15 mm) = 2.1 MRayl Water: VL = 1500 m / sp = 1000 Kg / m3 ZL = 1.5 MRayl - The use of a dispersive point whose end is partly or partially immersed immersed in a fluid and whose phase velocity is comparable to the speed of propagation of the waves in the fluid makes it possible to obtain a pressure wave firing angle, substantially aligned with the central axis of the emitting tip with a phase inversion on either side of the axis of the tip. - When the sample is the dermis or food, the tip is preferably aluminum alloy or titanium. The radiation impedance of the tip is lowered by refining its end by polishing so that an incident wave is fully transmitted (or the reflection coefficient is lowered to a threshold value) in the skin or food when the end of the tip is in contact with the dermis or food. - The tip section is very small, around 0.1 mm2 (tenth of a square millimeter), the impedance measurements can be carried out finely and on the scale of millimeter square which is of interest in cosmetics and dermatology. In particular, the speed of shear waves in the viscoelastic media such as the dermis can decrease very strongly to reach values of the order of one meter per second. In this case, the velocities measured will be those of Rayleigh waves or horizontal transversals close to each other within 5%. - Where possible, and that the radiation impedance of the tip is comparable to that of the medium, one can drive the tip into the test medium (silicone, viscous liquids, vegetables, fruits, etc.), in order to determine the necessary height to cancel any echo inside the tip. This height is characteristic of the longitudinal impedance of the medium. In practice, rather than canceling the echo, a depth of penetration will be determined where the echo is decreased by 50% or 75%, which is easier to measure. - A bipod consists of two pointed waveguides in contact with the test surface. The axes of the points are in the same plane perpendicular to the plane tangent to the surface at one of the points of contact. Deep drilling is carried out in the osculating plane with a lateral resolution determined in particular by the contact diameter of the tips, which is in practice less than one millimeter. The lateral extent of the beam is also conditioned by pulsed or continuous operation. In pulsed mode, the configuration is suitable for measurements by differential transit time or pulsed Doppler velocimetry. A bipod with superalloy metal conical guides, for example Inconel 600 or 625, is used to probe materials carried at high temperatures of several hundred degrees Celsius. The contact surface between the test material and the conical guide is reduced, which slows down the heat transfer mechanism and creates an apparent thermal effusivity well below the thermal effusivity of the same volume material. The height and the opening of the conical guide in superalloy, is at least 25 mm and is preferably chosen between 50 mm and 150 mm, with a good compromise between sensitivity and bandwidth obtained with a tip height of 100 mm for an angle at top of 4.8 °. The waveguide then has an acoustic transfer function between its base and its end which is similar to a low-pass filtering of cutoff frequency greater than 700 kHz. These dimensions make it possible to work at a center frequency between 600 and 700 kHz for an aluminum alloy. The height and the angle at the apex also make it possible to lower the temperature between the zone of contact, which can be carried for example at a temperature greater than 1000 ° C. and the base of the cone where the piezoelectric elements are bonded, generally ferroelectric, for which it is not necessary to exceed the temperature of Curie. It is also the advantage of the present invention to provide an effective insertion device for longitudinal acoustic waves and shear by having a radiation impedance adapted to the test medium. Finally, the base of the cone where the PZT transducer is bonded is exposed to a temperature below the Curie temperature of the piezoelectric element used to generate and receive the ultrasonic waves. The base can also be actively cooled via a heat transfer circuit, so that its temperature does not exceed 300 to 500 ° C, while the end is heated to a temperature exceeding 1000 ° C. Reducing the section of the waveguide here brings a considerable benefit since it amounts to lowering the apparent thermal effusivity of the material constituting the tip. As a reminder, the thermal effusivity is the square root of the product of the density of the material by the specific heat and its thermal conductivity. It characterizes the process of heat transfer between two materials brought to different temperatures. It makes it possible to define the interface temperature as the centroid of temperatures weighted by the effusivity of the materials. But this definition is valid only under conditions where the interface is flat. When the lateral extent of one of the materials is very small in front of the other, as in the case of a point in contact with a plane, the apparent thermal effusivity of the tip is weak and it is therefore the plane which imposes its temperature on the tip. Nevertheless, as one goes up towards the base of the point, the thermal inertia increases as much as the apparent thermal effusivity which approaches that of the voluminal material, so that it is the base of the cone that imposes the temperature. The conical guide therefore functions as a thermal buffer. This configuration makes it possible to mechanically couple the guide to a medium heated to a temperature well above the Curie temperature of the piezoelectric element. In addition, the piezoelectric element is generally bonded to the guide with an epoxy adhesive. The latter can hardly withstand temperatures above 300 ° C. Beyond that, it is necessary to consider, for example, a solid couplant, such as a gold layer. Therefore, in order to operate an epoxy adhesive with a sample heated to 1000 ° C, it is desirable, on the one hand, that the section or volume of the guide exposed to a high temperature sample represents a smaller surface area or volume, less than 100 times smaller, than the section or the volume of the guide coupled to the ultrasonic source and exposed to the colder temperature of the heat transfer fluid, without limiting the amplitude of the transmitted signal. This is the advantage of conical bending waveguides which simultaneously have a high bandwidth of between 0.3 and 3 MHz, depending on the shape of the waveguide, a reduced apparent thermal effusivity at the end and a radiation impedance. typically 4 times weaker than transverse impedance. - A bipod, comprises a transmitter and a receiver, distant from each other by a value adapted to the laws of refraction in the sample and according to the depth probed. The ends of the tips are carefully truncated so that the phase velocity at the ends is just greater than or equal to the velocity of the longitudinal waves generated in the medium. A preferred arrangement in so-called tripod transmission consists in arranging two receiving points at equal distance from and in alignment with a bending wave emitter tip. The receiving tips are arranged angularly with respect to their central axis so that the received signals are in phase opposition. The excitation is impulse or continuous. The summation of these signals provides a spatial differential measurement allowing small variations in the local mechanical properties to be observed between the receiving points. A tripod arrangement can advantageously be exploited by reversing the transceiver roles, with the transmitters continuously vibrating in phase opposition, resulting from a phase inversion between the two electrical excitation signals, either because of the orientation angular spikes. The tips are typically spaced 1 mm apart. More generally, spike transducers can operate in bending mode at frequencies between 20 kHz and 2 MHz. But they may preferentially be excited by a sinusoidal pulse at the center frequency of the tip or modulated in frequency by performing a digital frequency scan for example between 300 kHz and 900 kHz (called "chirp" in English). The frequency range is for example divided into 200 frequencies in the 3 kHz step, uniformly distributed between 300 kHz and 900 kHz. The burst consists of a complete period of each of the 200 frequencies distributed. In this case, the signal is preferably a square signal. There is then only one receiver associated with a single variable gain electronic amplifier, followed by a bandpass filter between 300 kHz and 900 kHz, followed by a single analog-to-digital converter, digitizing the signal on 12 bits, at a preferred rate of 5 to 12 MHz (or mega samples per second) and a single random access memory (RAM) of typical size 2 megabytes. The rate of fire is between 20 and 1000 (thousand) shots per second. Each shot is digitized over typically 5,000 to 20,000 samples, representing an acquisition time of 1 to 4 ms and 10 to 40 kbytes of memory. RAM can store 50 to 200 successive shots. The reception signal is digitized at the same time as the transmission signal. This configuration makes it possible to probe the space underlying the central receiver by masking the direct signal between transmitters and central receiver to measure disturbances backscattered by the medium. The received signal can then be further amplified without saturating the electronics. The relative variations of the received signal are calculated by slowly sliding the tripod along a curve on the surface of the sample, the relative variation is calculated at the end of the sweep or when the memory is full by taking as reference the one of the acquisitions made during the contact. Depending on the rate of fire, the acquisition lasts between 0.05 seconds and 10 seconds. The relative variation of the signal with respect to a reference signal is performed in the time domain or in the frequency space (in module and in phase) after calculation of a Fast Fourier Transform (FFT). The relative variations of the signal are representative of variations in the mechanical properties of the backscattering medium during displacement of the tripod. In other words, the configuration with two spike transducers generating a phase-opposite acoustic field at the receiver, makes it possible to perform a relative multifrequency perturbation measurement. When working in the frequency space, the frequency components whose amplitude does not exceed 3% of the maximum amplitude observed are eliminated. Then the Euclidean norm of the relative perturbation vector in modulus is calculated. This vector is representative of the diffusing power of the medium over a set of frequency components. Any finding of a significant variation in the standard of this signal during manual or automatic mechanical scanning may result in a new scan possibly changing the scan rate or measurement rate to better visualize a change of ownership zone. backscattering. - The slow displacement of a tripod assembly on the surface of the medium, in the direction perpendicular to the segment connecting the three points, provides information on the spatial variations of impedance. It gives information on inhomogeneities of the environment. The temporal variations of a motionless tripod provide information on variations in the environment of the tripod, for example linked to mechanical deformations caused by the passage of blood or by swelling of the medium associated with breathing or muscle contraction or by thermal diffusion. - When it comes to the skin, the tip is truncated so as to avoid damaging the dermis, for example by keeping a diameter of at least 0.4 mm. The tip is brought into contact with the skin (or any other flexible material) with a constant force (for example equal to its weight when no dynamometer is available) or slightly greater by a few grams than its practical weight. at 15 grams) using a precision digital balance (having a resolution of at least 0.1 g and in practice from 0.01 to 0.001 g and a 4-20 mA or digital output allowing to enslave the position from the peak to a target force). Alternatively, the force exerted on the tip is directly measured by a capacitive method based on the deformation of the bearing structure of the tip, as will be described later (see Figure 22). - The use of two tripodes oriented 90 ° from each other, and sharing the same central point, is a pentapode, that is to say a system with 5 points. The pentapode is the most complex configuration, but also the most versatile of the invention. In such a system, it is advantageous for the peripheral peaks to operate as a transmitter so that there is only one electronic amplifier connected to the single central receiver. The two additional points can thus operate at the same frequencies or at different frequencies of the two transmitting points of the first tripod. In particular, they can vibrate in polarization oriented differently from the other two. - Each of the tips of a pentapode has a four-quadrant ceramic functioning as a linear combination of two dipoles oriented at 90 ° to each other and can be combined in amplitude and / or phase. This gives a total mastery of the acoustic vibration in the contact plane. The tips may in particular be programmed to emit shear waves or longitudinal waves towards the central tip depending on whether their direction of vibration is collinear or parallel to the main sensitivity axis of the central tip. When the central tip is operating in reception, the pentapode is in echographic configuration. In this configuration, it is sought to bring the points as close as possible so as to be able to detect rapid spatial variations and to exploit high frequency shear waves, preferably with a frequency greater than 20 kHz. - When the central tip operates in transmission and the four peripheral points in reception, the pentapode is in directional velocimetric configuration. In this case, the central tip operates in precession mode, while the 4 peripheral points operate in pairs of receivers diametrically opposed to the transmitter and detect the direct signal emitted by the central transmitter. The measurement is then a measure of differential transit time per pair of receivers. It provides information on temporal variations, for example a flow in the plane of the ends. In this configuration we try to move the peaks as far as possible in order to increase the resolution on the speed measurement. This configuration is more expensive in electronic amplification and digitization / storage components.

The invention thus makes it possible to evaluate several parameters of a medium without damaging it, in particular: The speed of the longitudinal waves propagating in a sample (and thus indirectly the longitudinal impedance by knowledge of the density).  On this point, many devices exist in the literature, so it is not this part alone that is the most innovative; nevertheless, this measurement is achievable simultaneously in combination with the following.  - The velocity of transverse horizontal or Rayleigh waves propagating in a sample (and thus indirectly transversal impedance by knowledge of the density).  Again, the generation and detection of only horizontal transverse waves or Rayleigh or Lamb from conical tip transducers is known in the literature, but already at this stage, there is no indication of the combination of several spike transducers in the same compact barrel device.  - Variations in the homogeneity of the medium by differential echography of the interface medium, and the associated two-dimensional imaging (B-Scan) in the time space or frequencies, that is to say a grayscale or color (curvilinear position, amplitude of the temporal signal) or (curvilinear position, modulus or phase of a frequency component obtained by Fourier transform).  - The speed and direction of a directional flow in the plane or in a plane underlying the tip plane by directional velocimetry.  - The simultaneous measurement in a volume less than 1 cm3 of characteristic properties o a longitudinal wave o a transverse wave o a thermal effusivity o a support force The waveguides are chosen according to the desired working frequency which can vary for elastic materials from 300 kHz to 900 kHz and for viscoelastic media from 20 kHz to 4 MHz. We will now go into the description of particular embodiments of the invention.  In its simplest expression, exploiting wave-wave guides, and therefore of lateral extent at least equal to three wavelengths, the velocity determination method exploits a comparative measurement between a reference measurement, for example without contact and a measurement during a contact on the sample to be analyzed.  The formulas giving the reflection coefficients at the interfaces are then exploited as a function of the impedances of the media.  It is a deterministic process.  Among the applications envisaged are the tactile hulls with calibration by artificial finger.  The prior knowledge of the impedance of the human finger from the device described according to the invention makes it possible subsequently to produce artificial fingers for different user profiles (children, adults), to optimize the thicknesses and coatings of the tactile hulls. choosing materials whose longitudinal and transverse radiation impedances optimize energy transfer with the finger.  - The device also finds applications in the analysis and monitoring of the change in viscoelastic properties of materials, particularly when the tip transducers are oriented to transmit and detect horizontal shear waves.  A tripod or a pentapode in echographic mode in shear configuration also makes it possible to check a possible angular anisotropy in the viscosity of a fluid, this thanks to its ability to perform the same measurement in two perpendicular directions.  The device according to the invention constitutes a powerful means for measuring spatial and temporal viscosity, particularly in the field of magnetorheological fluids or elastomers, which can rapidly change state under the action of a magnetic field with a lower response time. at the millisecond.  Indeed, even in pulsed mode, the tip transducers can be excited at a high rate of up to 1 to 2 kiloHertz (or PRF for Pulse Repetition Frequency in English) and thus likely to display rapid changes in state.  - In contrast, the device can find applications in slow processes of change of state, for example in the control of the maturity of food held at constant temperature in a cold room, the hardening of concrete, or lava, the polymerization of glues, sintering of powders, changes of crystalline phase, manufacture of gels.  - The device can give an estimate of the spatial variations of longitudinal or transverse acoustic impedance by a mechanical scanning in the plane of the surface of the sample (C-scan) and thus allow a mapping of a human tissue or a any material.  - The device according to the present invention can learn to recognize the successive contact positions with the medium during a mechanical scanning phase of a bipod, tripod or pentapode consisting in a first step to store the echoes associated with each position of the probe then to recognize the positions by distance calculation or inter-correlation during a second pass, following the first constituting a learning phase.  The present invention may allow a classification of materials by characterizing the impedances of the two fundamental modes and according to two directions of propagation, thus providing information on the anisotropy of the material as well as on the efficiency of the heat transfer of a heated tip. or cooled which is measured lowering or raising temperature after a fixed contact time with the sample at room temperature.  In conjunction with the use of a database of references that may be constituted, the device can be a "classifier" of materials, applicable to elastomers, human tissues, food and meat in particular for the control of their state of conservation or ripening, as well as various metal and plastic materials.  The present invention may finally allow the inspection of moving surfaces by combining a solid guide coupled to a liquid guide projected towards the sample.  This type of probe finds applications in nondestructive, high speed control.  The probe is constituted by a double wall bearing structure possibly heated to improve the ultrasonic damping and the viscosity of the fluid in circulation and comprising two waveguides arranged in series, the first being solid and consisting of a guide to conical dispersive tip for lowering the radiation impedance and optimizing the transfer to the second liquid guide, coupled to the first via a connection chamber and opening onto an ejection nozzle of the liquid coupling in the form of a collimated jet directional high pressure.  The liquid jet is protected from drafts by a skirt, for example in the form of a cylindrical multi-strand brush.  The jet may be intermittent to regularly strike the surface of the sample and generate low-frequency shear waves.  The latter point is of interest in pulse elastography and is an alternative to percussion methods vibrating pot or other electromagnetic or static contact usually juxtaposing the ultrasound transducer.  In the case of the deformation of the skin during a medical examination, the intermittent jet must allow a significant deflection of the skin, typically of the order of 1 mm.  The viscosity and the surface tension of the projected fluid make it possible to create a waveguide having a spatial continuity and for which a laminar flow can be imposed if the Reynolds number associated with the diameter of the jet, the density of the liquid, its speed ejection and its viscosity remains below 2300.  Thus, the fluid may advantageously be more viscous than water in order to maintain a laminar flow and therefore a collimated projection over a greater distance or to limit the spreading of the fluid on the inspection surface, for example for the sake of cleanliness .  By way of example, a water jet with a diameter of 0.8 mm and a viscosity of 1 mPa. s, of density 1000 kg / m3, expelled at a speed of 1 m / s is associated with a Reynolds number of value 800.  The turbulent regime is then reached for an expulsion speed of about 3 m / s.  Other aspects, objects and advantages of the present invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of nonlimiting example and with reference to the appended drawings, in which: FIG. 1 is a schematic view of a transduction assembly with a waveguide 150 forming an elementary probe for a longitudinal or transverse propagation mode, coupled to a material to be analyzed 100; FIG. schematic view of the complete device 400 allowing the characterization of the two fundamental modes of propagation simultaneously, thanks to two separate probes 125 and 225 in contact with the material 100.  FIG. 3 is a schematic view defining the incident waves 600, reflected 620 and transmitted 610 at the interface with the sample 100.  - Figure 4 defines the incident waves 600, reflected (620, 640 and 650) and transmitted 610 and 630 in the case where the sample is composed of two layers 100 and 101.  - Figure 5 shows the interface echo in the case of a guide in plexiglass longitudinal wave, bathing in the air.  - Figure 6 shows the interface echo in the case of a transverse wave plexiglass guide, bathed in the air.  FIG. 7 shows an example of peak-to-peak amplitude variation of the longitudinal interface echo, an attenuation of about 50% between the ambient air and a sample of human skin on a subject of study, highlight the effect of a large variation in load longitudinal impedance and to determine the longitudinal impedance of this sample (about 1 MRayl) which is about 32% that of the Plexiglas (ZL = 3.1 MRayl).  FIG. 8 shows an example of a peak-to-peak amplitude variation of the interface transverse echo, ie 9%, between the ambient air (high value) and the same human skin sample (low value) setting evidence a transverse load impedance (0.06 MRayl) much lower than that of Plexiglas (Z1 = 1.26 MRayl).  FIG. 9 is a schematic view of two variants of the device associating transducers 260, 270, 280 and 290 for the two fundamental modes of propagation on a single waveguide 150 in a concentric configuration (FIG. 9A) or series (FIG. 9B).  FIG. 10 is a block diagram of the mounting of the transducer 540 with the transmission and reception electronics using a limiter 510 avoiding saturating the amplification elements 520.  FIG. 11 is a block diagram of the mounting of the transducer 540 with the transmission and reception electronics using a fast switching function 515 between the transmitted signal and the measured echoes making it possible to avoid saturation of the amplification electronics 520 .  Fig. 12- (a) shows the end 11 of a polished truncated end splittable flexural waveguide.  In the far field, the vibration at the end of the cone is a dipolar source (Figure 12- (b)).  The conical profile and the polishing of the end reduce the mechanical impedance of radiation by a factor of 4 to 10 and bring it closer to the longitudinal impedance of the finger pulp, which allows the transfer of most of the the energy in the medium and therefore subsequently to have a better signal-to-noise ratio for the fine analysis of the impedance variations of the medium.  Figure 12- (c) schematically shows a tip in sectional view with a damping material filling a side tank 13.  FIG. 12 (d) diagrammatically shows two electrical connection methods making it possible to generate bending waves, one using a PZT disk with uniform polarization and connecting in series two piezoelectric capacitors formed by the two upper electrodes in half. and the lower common electrode, the other operating an alternating polarization by half-disk and an electric excitation E in parallel with the two associated piezoelectric capacitors.  FIG. 13 is a modeling of the dispersive propagation of a flexion wave in a duralumin cone at 600 kHz.  The modeling assumes the emission of a reflection wave packet at the end 11 of the cone and interference between the incident wave packet and the reflected wave packet with (a) experimental phase velocity deduced from the distances between maxima = half-wavelength.  (b) The theoretical phase velocity of the Lamb Ao mode in a Duralumin plate whose section decreases linearly at the same angle O as the cone.  (c) exemplary embodiment of a broadband flexural wave transducer incorporating a conical waveguide 10 and constructed from a PZT alternating polarized disk 20.  The impulse response of the disk is damped via an annular tank 13 filled with saturated tungsten powder saturated polymer 30.  (d) Time signals of the vibration velocities Vib1 and Vib2 measured perpendicular to the flat end of the tip 11 and 4.5 mm upstream of the end, for an electrical pulse of 20 Vdc consisting of a burst of 5 square periods at 700 kHz.  (e) Peak-to-peak amplitudes of vibration velocities by scan using a laser vibrometer along the tip, perpendicular to the flat end under the same conditions as before.  (f) Modeling of the radiation in the air, very close to the generator (at 0.1 mm), without reflected wave (dashed line) and with reflected wave (solid line).  FIG. 14 is a calculation of the refraction angle of the longitudinal waves in the sample as a function of the phase velocity in the tip.  The medium is here directly the air 67 that is to say the weakest impedance.  The angle of refraction is then closest to the perpendicular to the generator Ng of the cone, it is 14 ° relative to the generatrix of the cone for a duralumin cone of half-angle at the top 2. 4 ° having a theoretical dispersion speed according to graph (b) of Figure 13 which gives a phase velocity close to 800 m / s at its end 11.  FIG. 15 illustrates a bending-wave bipod 89 integrated in a sintered plastic support structure providing, on the one hand, an acoustic insulation between the guides, on the other hand, making it possible to manually grasp the assembly without disturbing the structure. propagation in the guides, finally to simply manipulate all as a large pen that is brought into contact with the sample, here water 66.  The metal waveguides are fixed (43, 44) on the very damping carrier structure for bending waves.  The structure is made by rapid prototyping sintered polymer type PolyAmide PA12.  The conical tips have a T-shaped geometry (10, 12) and are held around their periphery by force insertion into the support structure providing a double hold on the periphery of the tip at the level of the base of the cone and typically at 20 mm. upstream of the end 11, so as not to dampen the vibration by holding the tips too close to their end.  The acoustic path in the carrier structure requires a transit time greater than that of passing through the conical guides and the sample.  The distance e between the ends of the tips is adjusted according to the material to be probed.  It is typically 5 mm for a pentapode in mixed use (ultrasound or velocimeter) and for an elastic material having a transverse velocity of between 500 m / s and 3000 m / s, but much less, about 0.5 mm for materials. thin or diffusing viscoelastic, small localized variations of impedance to be detected.  The distance can be increased to 10 mm, if one tries to analyze relatively elastic materials, homogeneous on the surface, but having variations of impedance in depth or also if one wishes to have a good resolution in velocimeter mode .  The phase velocity of the bending waves near their end is close to 800 m / s for aluminum, which allows a very good insertion of the waves into the dermis and into the silicones and a radiation towards the front of a longitudinal beam, close to 90 ° (i. e.  almost parallel to the axis of the tip).  The spacing and inclination of the tips is therefore optimal for a certain sample thickness as shown in Figures 15 (a) and (c).  The tips open out from the supporting structure, projecting only a very small amount, approximately 0.5 mm, so that the supporting structure also serves as support and avoids damaging or injuring the probed medium, particularly if is the dermis.  The probe can then be dragged or moved point by point (pixel by pixel) on the surface of the test material.  In this configuration, the latter must be dry and clean so as not to pollute the tips, in particular fill the gaps between the tips and the carrier structure which would lead to measurement errors.  The images thus constituted are C-Scans, that is to say that it is possible to look deeper and deeper in the study material according to whether one looks at the head or more inside the wave packet.  The tips have a typical length of 85 mm, for a base diameter of 6.6 mm (half-angle at the top of 2. 4 °) which corresponds to a working frequency of 600 kHz.  The images can be amplitude or phase images (in particular, a cartography is taken by taking a particular zero crossing of the wave packet and we look at how the zero crossing changes according to the medium sampled during a displacement. lateral bipod).  There is therefore interest in bringing the points closer to each other when doing a phase mapping.  FIG. 16 shows probe variants for impedance measurement: (a) Penetration into a soft, molten sample 63, of the tip having a very fine end 16 to reduction of the Ar echo of 75 % and possibly for the plates, refraction and adjustment of the position of the truncated receiving tip 14 until detection of the maximum signal.  Then, deduction of the angle of maximum refraction and / or depth of penetration into the middle of the longitudinal impedance of the medium.  (b) Process based on the modification of the reflection conditions at the end 14 of a probe 88 with a truncated conical point, not immersed but in contact and loaded by the longitudinal impedance of the hard sample 60 and thus on the displacement interference nodes according to the load impedance Z2.  It can implement a laser vibrometer generator scan to measure the position of the nodes or a measurement of the displacement of the zero crossing of an echo inside the emitting tip.  (c) Fine-end probe 87 for mapping based on constant support force (F) echo analysis on a 3D hard surface 61.  (d) Longitudinal velocity measurement method VU or Rayleigh (VR) in a viscolelastic medium 61 and mapping based on longitudinal or Rayleigh radiation disturbance.  (e) Method for measuring transverse velocity of a sample 62 able to propagate them, from the generation and detection of horizontal transverse waves.  FIG. 17 illustrates a pentapode in ultrasound mode.  Depending on the medium, the distance between the central tip and the peripheral tips varies from 0.5 mm to 5 mm.  This configuration is particularly interesting in practice because, the excitation of the emitting points (Ex-, Ex +, Ey-, Ey +) are excited so as to cancel the signal at the center for the longitudinal 1 and transverse 2 configurations, or to double the signal in the center for longitudinal 3 and transverse configurations 4.  A pentapode can be closed and sealed by a thick silicone membrane with a thickness of 0.5 mm to 1 mm, with total wave transfer from the tips to the membrane and then from the membrane to the sample (not shown).  The emitting tips immersed in the silicone membrane refract in the medium at a very penetrating angle.  In this configuration, the pentapod is not likely to be contaminated and can be brought into contact with the skin, covered with a gel that provides sliding and a good acoustic coupling of the probe.  The four transmitting probes all operate a piezoelectric disc with four quadrants 22; they can switch electronically from a shear configuration to a longitudinal configuration.  A mixed configuration (a) can also be envisaged combining longitudinal 3 and shear 4 emissions simultaneously.  FIG. 18 illustrates an application of the bipod impedance meter in velocity-measurement probe 89 with alternating measurement: The two waveguides 10 function alternately as transmitters then as receivers.  The transit time to raise the blood flow is greater than the transit time to lower the blood flow.  The differential transit time is proportional to the speed of the blood.  Contact measurement requires a very precise time measurement, with a resolution less than one-tenth of a nanosecond.  The use of high frequency point probes (2 to 3 MHz) consisting of conical tips with a base diameter of 2 mm and a height of 25 mm in combination with time-digital components (or TDC: Time to Digital Converter) is here advantageous for performing local controls, especially on the venous network.  In addition, the measurement rate must be high around 1000 measurements per second in order to be able to average the measurements and also to avoid that the dimensions change between an upstream measurement and a downstream measurement.  It may be advantageous to operate a tripod, the three probes being aligned with the vein and the emitter probe disposed in the center.  The vibration axes are collinear and aligned with the direction of the vein.  The upstream-downstream differential measurement is thus simultaneous, which makes it possible to avoid errors related to dimensional variations generated by local variations in pressure and swelling of the vein.  FIG. 19 illustrates the use of a pulse transducer with pulsed Doppler velocimetry.  The end 14 of the conical Duralumin tip is truncated at the diameter of 4 mm and gently pressed against the dermis 65 so that the dermis surrounds the tip to a depth of at least 1 millimeter.  The dispersive wave refracts the fields 6 and 7 in the dermis with a maximum amplitude according to the angles ai and a2 with respect to the axis of the tip which disperses the Doppler angle between the extremes 01 and 02.  By choosing a phase velocity equal to that of the longitudinal waves of the medium, the dispersion of the Doppler angle is reduced.  FIG. 20 illustrates the use of a directional velocimeter pentapode in a configuration with a central emitter tip E vibrating in precession mode and four receiving points Rx-, Rx +, Ry-, Ry + devices sensitive to the flexion modes .  The precession tip has a round end 16, the finest possible (diameter close to 0. 1 mm polished) and propagates a combination of two bending modes at 90 °.  The vibration is obtained using a uniformly polarized PZT ceramic 22, divided on its upper face into 4 electrodes each covering 1 quadrant and an electrode forming a solid disc on its underside.  The upper electrodes operate in pairs opposite from the center.  Each electrode of a pair receives an electrical phase consisting of a burst called Burst (cot) for one electrode and its logical inverse for the other.  The other pair is shifted by n / 2 and includes an electrode raised to Burst voltage (cot-Tt / 2) where co is the angular pulse and another electrode brought to its logical inverse.  The burst function is a burst a number of slots, 5 in a privileged way.  The Burst switch between two voltages 0-100 VDC and are characterized by an extremely stable rise and fall time of the order of a few nanoseconds.  The ceramic typically has a diameter of 6. 6 mm, a thickness of 0.2 mm, a center frequency of 600 kHz and is made for example PZ27 or PZ29 (FeiToperm, Denmark).  The tips are made of a low density metal material (for example aluminum alloy).  The central tip has a length of 85 mm, a base diameter of 6.6 mm, a tank ring 13 with thin walls, thickness 0.4 mm, height 5 mm and diameter 20 mm, filled with epoxy resin saturated tungsten powder 13 to a density of at least 5 g / cm3.  The ends 11 of the peripheral tips are flattened by polishing, the normal to the polishing plane being directed towards the central tip.  This has the effect of increasing the air coupling between the emitter tip and the receiving tips and the directionality of the receiving tips.  The receiving tips do not need to be damped like the emitting tip, they are simply inserted into force in the sintered plastic carrier structure 40 which partially damps their resonance and this especially as it is achieved by sintering of plastic powder and that it is heated for example at a temperature of 50 ° C.  They have a simple T-shaped geometry.  Their ceramic 21, type PZT, has an upper electrode divided into two half-disks and a differential amplification of 90 dB is performed between these two half-electrodes, around the center frequency of 600 kHz.  The lower electrode is solid and constitutes a virtual mass.  Alternatively, with a ceramic 20, the upper electrodes are alternating polarization and then electrically connected, while the lower electrode is the electrical mass.  This increases the available electrical charges and therefore the signal-to-noise ratio in the case of a transimpedance amplification.  A zero crossing detection using an ultrafast comparator (the switching time 10% -90% is of the order of one nanosecond) is performed on the amplified electrical signal.  For the most stable time measurement, we choose the zero crossing corresponding to the maximum amplitude.  This configuration makes it possible to reach a measurement rate of 1000 shots per second and a temporal resolution of the order of one nanosecond (in the air) which sets the detection threshold at approximately 1 cm / s per axis in the air at 340 m / s (25 ° C).  The maximum speed exceeds several tens of meters / second.  FIG. 21 is a cross-sectional view of a longitudinal waveguide and shear sensor 17 comprising a solid cylindrical guide 18 propagating the longitudinal waves and a concentric tube 17 propagating the bending waves.  The figure illustrates the location and the resonance mode of the transducers as well as the location envisaged for the dampers 31 and 32.  FIG. 22 is a sectional view of a Joule-effect tapered tip probe 97 integrated into an acoustically and thermally insulating bearing structure with integrated measurement of the bearing force.  The double U-shaped wall of the supporting structure simultaneously distributes the heat in the inner wall in contact with the waveguide, the thermal insulation of the cone vis-à-vis the air currents by means of the outer wall separated from the inner wall by a thermal insulator, for example a layer of air.  The outer wall also performs the acoustic isolation of the cone vis-à-vis the manipulations, while the inner wall carries the maintenance and also the ultrasonic damping of the pulse bending mode, partially dissipated in the sintered granular structure.  FIG. 23 shows a probe 98 comprising 5 waveguides 10 with conical tips, and in particular a bayonet locking cap 55, elastically deformable, and serving to hold the guides in their housing.  The holding structure 55 is made of porous or polymer material loaded for example with alumina and also dampens the echoes to avoid resonances in the waveguides.  The hood is thus locked by a system of lugs 53 on the supporting structure.  Elastic strips 54 are furthermore created in the deformable cover in order to impose a bearing force perpendicular to the base of the guides.  Acoustic paths 56 between the support zone on the base of the guides and the periphery of the cowling are complex, for example S-shaped or zigzag-shaped and maximized in length in order to reduce acoustic cross-talk or short-circuits between the guides ( also called "cross-talk" in English).  - Figure 24 is a schematic sectional view of an ultrasonic probe 99 for non-destructive testing in a situation where the sample and the probe are in relative motion (Mov).  The carrier structure comprises two guides 10 and 93 in series, the first guide 10 is solid and consists of a truncated conical dispersive tip guide 14, for lowering the radiation impedance and optimizing the transfer to the second guide 93, liquid, coupled to the first via a connection chamber 92 and opening onto an ejection nozzle of the liquid coupling in the form of a collimated jet 93 directional and high pressure.  The liquid jet is protected from drafts by a skirt 95 in the form of a cylindrical multi-strand brush.  AT.  VOLUMIC WAVE PROBES FIG. 1 represents a first device using volume waveguides for determining the characteristic impedances of the skin by a panel of individuals, as well as identifying the characteristics of different materials.  Depending on the types of transducers used, it is possible to modify the transverse or longitudinal propagation mode and the frequency at which the impedance is measured.  The device allows by simple contact to evaluate the acoustic impedance parameter of a homogeneous material, by a non-destructive method and for the two fundamental modes of propagation, transverse and longitudinal.  By contacting two cylindrical probes for each fundamental mode with the sample, the two longitudinal and transverse characteristic impedances are determined.

The system (Figure 2) consists of a structure on which are fixed the two probes forming waveguides for sending and receiving acoustic pulses to the surface or in the material. The maintenance of the probes on the structure is done in such a way that there is an acoustic insulation, by an impedance break, between the structure and the waveguides. Insulation is necessary to prevent the interface echo from being disturbed by an acoustic short circuit via the waveguide holding structure. A transducer is attached to each waveguide to transmit / receive the acoustic pulses. This transduction system, which exploits elements that can be piezoelectric, can generate pulses under the best conditions. In the case of a piezoelectric active element, an impedance matching with the waveguide allows a better insertion of the modes at the desired frequency by the addition of a so-called "quarter wave plate" of which the Acoustic impedance is preferably intermediate between that of the active element Za and that of the waveguide Zg: Finally, a "backing" wave damping system arranged on the rear face or on the side of the piezoelectric element allows a better temporal resolution of the pulse by limiting the intrinsic resonances of the piezoelectric element. The system is simultaneously connected to a high voltage signal generator for transmission and an amplifier for reception. The receiving electronics tolerate the high excitation voltage of the transducer. Typically, a 20 volt pulse is applied to the transducer, and then the interface echo is amplified by a factor of 200 to produce an output signal of a few hundred millivolts. The constraint is that the amplitude excitation signal 20 Volts is also amplified by a factor of 200, which saturates the reception electronics, which can not deliver a signal of amplitude greater than +/- 10 V.

To overcome this case, we will use a clipper to limit the input voltage of the amplifier to a maximum allowable not causing too much latencies in the measurement or a fast switching between transmission and reception on the same transducer so that according to the desired action on the active element, the system operates sometimes in transmission and sometimes in reception. The output of the amplifier allows the measurement of the echo (s) of the acoustic pulse, coming from the interface and having passed through the waveguide. In Figure 1 is schematically illustrated a non-dispersive planar waveguide measurement probe. It comprises the waveguide 150 also serving as a probe, which is kept in contact with the sample 100. This waveguide is made of a material whose known acoustic impedance Zg is close to and greater than that of the material that one seeks to determine. Typically it may consist of low density metals such as aluminum or titanium, or more preferably plastics such as polycarbonate or plexiglass. The transducing system 300 comprises a piezoelectric component 250, having a sufficient bandwidth, for example 100% around the central frequency which is equivalent to say that its impulse response is short. The component works both as a transmitter and a receiver. An impedance matching blade 200 is fixed on the active face of the element 250, ideally sized to have an impedance Z, = ea.Zg and a thickness corresponding to a quarter of the wavelength of interest c / L = t / 4, and a damping block 350 on the opposite face (referenced by the term "backing" in English) so as to improve the temporal resolution by limiting the wave reflections on this face of the transducer, which limits the impulse response of the active element 250 to a bipolar pulse. The transducer assembly 300 may be a ready-to-use commercial component, for example the Panametrics (Waltham, Massachussetts, USA) M110 longitudinal wave transducer and the cross-wave Panametrics V153 transducer (Waltham, Massachussetts, USA). . These transducers have a respective center frequency of 5 MHz and 1 MHz. To optimize the acoustic transmission between the probe 150 and the sample 100, a viscous paste serving as a coupler can be used. A coupling adapted to the propagation mode of the wave is essential in the case of the transverse mode. Coupling referenced SWC (shear wave coupling in English) Panametrics company allows the transmission of the transverse mode. Its thickness is reduced as much as possible. Its use is not necessary with dispersive transducers spikes or when the nature of the material studied allows a satisfactory coupling, such as with fluids, gels, elastomers. FIG. 2 schematically illustrates a device 400 with two non-dispersive probes, a longitudinal wave probe 125 with its non-dispersive plane guide of length 110, the other transverse wave probe 225 with its non-dispersive plane guide of length 120. The guides are integrated in a support, typically made of sintered plastic, providing good acoustic insulation between the waveguides. The probes are in contact with the sample 100 having a lateral extent and a depth of at least five wavelengths and preferably at least ten wavelengths or longer. Means for regulating the bearing force may be envisaged such as springs at the rear of the probes. Fasteners to isolate "acoustically" probes from the structure. They can be foam, having a very low acoustic impedance compared to waveguides. The lengths of the guides 110 and 120 are dimensioned so that the pulses do not interfere with each other. For this, it suffices that the acoustic pulses traveling along the length 110 at the longitudinal speed and the length 120 at the transversal speed reach the sample at the same time. In addition, the lengths are calculated so that the incident wave and the reflected wave are well separated temporally. Concerning the last condition, the lengths 110 and 120 must have a minimum value satisfying the equation T = -n <--- 2. /, where T represents the duration of the pulse, which is the ratio of the number n of desired periods by the central frequency f of the pulse. This period must be less than one round trip of the wave, ie twice the distance 1, reported on the speed of propagation of the wave c generated by the transducer. We will therefore have the following dimensional condition for each probe: n-c 1>. With regard to the simultaneous arrival of the two waves on the material, it is necessary to check the equality - / 110 = 129, in which the lengths of the probes 110 and 120 depend on the longitudinal CL CL and transverse CT CT speeds. It is also possible to include a delay of excitation of the fastest wave equal to the difference of the transit times to reach the interface with respect to the slow wave, which makes it possible to preserve equal lengths of guides 110 and 120. Considering the fast propagation speed in the guide 110 and the slow propagation speed in the guide 120, the equation of the time delay of arrival of the wave fronts on the sample is obtained. in both probes: delay = 1120/110 CT CL Figure 4 shows the evolution of the waves at the time of measurement. An incident wave 600 propagates in the guide 150 towards the sample 100. At the interface, a portion of the energy is reflected, leading to a reflected wave 620 and another portion 610 is transmitted in the 'sample. The precise knowledge of the material constituting the impedance guide 150 Zg, intervenes in the reflection coefficients r and transmission t with the impedance Ze of the sample 100 according to the following formulas: r = Ar = Ze-Zg and t = A, = 2-Z (1) A, Z e + Z g A, Z g + Z e In which, A ,. denotes the amplitude of the reflected wave 620 and A ,, the amplitude of the incident wave 600, while 4 designates the amplitude of the transmitted wave 610. The relations (1) constitute a system of two equations to 2 unknowns relating the ratio of the 5 amplitudes of the reflected and transmitted waves to the characteristic impedances of the materials. Its resolution provides the impedance of the Ze sample. Moreover, since the amplitude of the incident wave 4 is likely to change according to the temperature conditions and since it is not desired to use additional equipment for calibrating the probe, a measurement relating to a reference makes it possible to deduce the impedance Ze by overcoming the variations of attenuation properties, temperature and size of the waveguides. The measurement used as a reference is the empty measurement, that is to say without contact. Indeed, for known propagation conditions in the probe under given temperature and humidity conditions, the impedance of the air Za is known and very low. In this case, the probe 15 provides a reflected wave of given amplitude 4. The guide 150 is then placed in contact with the sample 100 and a new measurement is made. An amplitude 42 corresponding to the reflected wave 620 on the unknown impedance material Z 1 is obtained. The two equations are obtained: Zair Zg Zair Zg Ze -Zg Ze-1-Zg This equation system leads to the relation: Z = Z 1 + a with a = 42 Zair -zg eg 1-a A 'Za, + Zg 141 A. 42 A, (2) Thus knowing the impedance Zg of the guide, the impedance Zan. from the air and from the two successive amplitude measurements of the received signal, the characteristic contact impedance of the material to be analyzed Ze is determined. For this method to work, it is necessary that the material be at least locally homogeneous and isotropic. The use of a system such as that of Figure 2 makes it possible to perform the acquisition of the two fundamental modes simultaneously. The use of these probes associated with a mechanical scanning on the surface, of the C-scan type, makes it possible to map the impedance and determine its homogeneity. The isotropy of a material can be deduced if the sample has a suitable geometry, for example cubic and has 3 orthogonal faces and for each of them, 3 impedance measurements are made: longitudinal mode impedance and the transverse mode in two directions, typically orthogonal. FIG. 3 shows the propagation of the waves at the interface between the waveguide 150 and the sample to be analyzed 100. If the impedance of the air Za is known, the impedance of the waveguide 15 wave Zg, the amplitude of the first echo empty, and the contact echo 620 is deduced the impedance of the sample Ze. On the other hand, one can establish the relation describing the transmitted wave part progressing in the sample 610: 41 ° = 2 Ze from that which is reflected: 420 = Ze zg, which leads to A600 Ze Zg A600 Zg Ze l expression of the transmitted waveband on the reflected waveband = 4 2 .Ze - (Ze + Zg) A610 A600 In the case of a stratified sample (see Figure 4), where there would be a second Impedance layer 101 Ze, the relationship of the reflected wave on the second interface A640 Ze '- Z, and the relationship of the transmitted wave of the first layer to the guide A610 Ze, Ze of Wave 4 "= 2 Ze 440 Zg + Ze. The impedance of the second layer of material is deduced from the established relationships: 420 A600 A620 - Ze2 Ze, - Z, A650 = A650 A640 A610 2 Z, Ze, - ze 2 Z, (Z, Zg 420 420 410 420 Zg Ze Ze Z + 4 = Z + 4 Z. + Ze 420 4 = Ze, it comes the relation: By putting fi = 450 Ze2 Zg2 Ze, - Ze fi +1 (3) ZZ 20 4. Z e2 e = e ig -1 with fi = A6 450 Ze 2 Zg2 Figure 5 shows an example of a curve recorded for a longitudinal propagation mode in a plexiglass guide in the open air. The signal in the vicinity of the instant t = 15 las, corresponds to the guide-air interface echo. The incident wave 600 passes through the probe to the interface where a portion corresponding to the characteristic impedance ratio is reflected under the identity 620 to the transducer. The wave is reflected again in the guide and makes a second round trip, visible with a strong attenuation at time t = 30 gs. Figure 6 shows an echo acquisition curve 620 for the transverse propagation mode, always with a plexiglass guide in the air. The amplitude peak at t = 31 ps corresponding to the transverse mode guide / air interface echo. It is temporally more spread because of a lower working frequency, 1 MHz, and comes late compared to the previous assembly because of a lower propagation speed than the longitudinal mode. The curve also shows the interference by the longitudinal mode with the arrival of a part of the energy of the longitudinal wave at t = 15 ps and a longitudinal conversion / shear at the guide / air interface arriving at t = 23 ps. Figures 7 and 8 show the variations of the respectively longitudinal (48%) and transverse (9%) reflection coefficient, with respect to air, when the sample (100) is an exposed part of human skin. Slight amplitude variations are observed during successive free / contact measurements. They may be caused by temperature changes during contact or variations in coupling thickness due to crushing or thermal equilibrium if the temperatures are not exactly the same.

It is for these reasons that the coupler will preferably have a very small thickness, typically less than 50 g, and the temperature equal between the probe and the test material in order to avoid any phenomenon of thermal diffusion or disturbance. the transverse load impedance by the impedance of the couplant.

In addition, by neglecting the thermal expansion in the case of small temperature variations, i.e. constant density and dimensions and known guide, the arrival of the first echoes is the round trip in the guide whose length is known. Therefore, the precise knowledge of the propagation speed c for a known density p guide, allows to refine its impedance Zg = p-c The impedance of the material to be analyzed will be characterized for the ambient temperature. We thus find longitudinal impedances of 1 MRayl and transverse impedances of 0.06 MRayl for this measurement on human skin. Variations of this method are possible, such as that of establishing a reference table of the reference echo in the open air as a function of the temperature. The measurements can also be made in a climatic chamber in order to impose the temperature and relative humidity, homogeneous between the probe and the material to be analyzed. Figure 9 shows a variant of the assembly for using the same waveguide 150 and therefore the same probe to generate the two basic modes of propagation. FIG. 9A shows a first concentric mounting of the transducers 260 and 270, disposed on impedance adapter blades 210 and 220, themselves bonded to the waveguide 150. A backing 350 is disposed on the rear face of the transducers 260 and 270 to improve the temporal resolution of the pulse. The impedance measurements according to each mode will be carried out successively so as not to interfere with each other. Given the high propagation speeds compared to a manual movement by a user, both acquisitions can be considered simultaneous. The transducer 260 will preferably be a longitudinal wave transducer and the horizontal transverse wave transducer 270. The variant of FIG. 9B illustrates a serial assembly of the transducers for each fundamental mode. The backing 350 is fixed to the back of a first transducer 280 resting on an impedance matching blade 230 which rests on a second transducer 290 and on a second impedance matching blade 240 and finally on the guide of The two impedances of the transducers being equivalent, the impedance matching blade 230 can be neglected. For a shear mode transducer 290, the adapter layer 240 will be optimal for transmitting a transverse wave in the waveguide 150. The transducer 280 will be in extension mode for transmitting a longitudinal wave in the waveguide. wave through the transducer 290 and the adapter layer 240. In the case of the longitudinal wave, the transfer will not be optimal, but the assembly allows to have a probe to emit the two fundamental modes in the 100.The impedance matching blade 240 is provided for the transducer 290 and a transverse mode Z "° = v290, but whose impedance of the longitudinal mode will be similar to that desired 2 ?, 4 ° jziL280.450 since the transducers 280 and 290 have identical properties and the ratio of longitudinal to transverse impedances of the adapter layer 240 is equivalent to that of the waveguide z240 z150, 7,240 -, 7150. The size of the The thickness of the adapter blade is adjusted to match LL to a thickness of a quarter of the transverse mode wavelength d24 ° = λ / 4. In solids the propagation velocity of the longitudinal mode is greater and at least Nr times the propagation velocity of the transverse mode, which implies a smaller wavelength. By using a higher excitation frequency for the longitudinal wave emitting transducer 280, a frequency can be chosen so that 27280 ° and so the impedance matching pad 240 will allow correct transmission of both transverse waves as well as longitudinal waves of the transducer 280. FIG. 10 shows a block diagram of the acquisition chain. This device is implemented for each transducer, each propagating one of the two basic modes. A signal generator 500 sends an electrical pulse into a transducer 540. This signal is also present at the input of the limiter 510 whose role is to limit the maximum voltage in order to protect the amplification electronics which carries out a measurement under high 20 impedance. The clipper 510 passes it to an amplifier 520 providing a signal in the time domain and the extraction of the useful parameters 530. The transmitted wave is followed by an echo of very low amplitude. Figure 11 is an alternative configuration of the measurement chain. In this configuration, a switch 515 is used to firstly connect the transducer 540 to the pulse generator 500 and in a second step to the amplification circuit 520. In this way, the high amplitude excitation signal does not disturb the amplification electronics and the low amplitude reception signal. As before, a measurement circuit makes it possible to extract the useful parameters 530 from the evaluation of the characteristic impedance.

The method thus makes it possible to measure with a good resolution, in a short time, the characteristic impedances of any material, by simple contact on its flat surface and without degrading it. B. DISPERSIVE GUIDED WAVE PROBES The volumetric waveguide method provides a fairly simple and intuitive approach to measurement of longitudinal and transverse impedance. But, it is limited to locally homogeneous and isotropic surfaces with a sufficient measurement volume. However, there are in practice many situations where the impedance to be characterized is poorly suited to the use of plane waves and can also be very low compared to the impedance of the waveguide. This is already the case with the human skin of Figure 8 which represents only 4.7% of the transverse impedance of Plexiglas. As can be seen, the acoustic load drops the signal around 140 mVcc, with an uncertainty of 2 mV. The measurement impedance is thus between 0.06 MRayl and 0.07 MRayl. The effect of the couplant is difficult to evaluate. With a plexiglass guide of 12 mm diameter, the same process applied to the pulp of the index on a panel of 10 people provides an average longitudinal impedance of 767 kRayl with a dispersion of between 480 and 1100 kRayl and a mean transverse impedance of 100 kRayl with a dispersion of between 52 and 220 kRayl. The waveguide waveguides may be used for the impedance measurement in particular so as to reduce the impedance of the waveguide to bring it closer to that of the medium to be analyzed. Guides can be made of metals or plastics, for example aluminum, titanium, Plexiglass (Poly Methyl Methacrylate) or Polycarbonate. By introducing a dispersive effect, the probe has a reduced impedance, smaller than its characteristic value by a factor of 4 to 10. The impedance of the guide is then determined by the phase velocity of the wave in the vicinity of the interface.

To understand the phenomena involved, we can model the decay of the phase velocity of a bending wave in a conical tip, such as that shown in Figure 12. Figure 12- (a) shows the truncated and polished machining of the tip 10 near its end 11 while Figure 12- (b) gives the field radiated by the far field tip. Near the base of the cone, the velocity of the wave is close to the velocity VT, transverse waves. Then, during its propagation, the speed decreases in first approximation like the speed of the first AoS asymmetric Lamb mode in an isotropic thin plate. The dispersion equation of this mode is used to describe the phase velocity of a dispersive wave A in a conical tip: if we call k the wave number, the working frequency, the radius of the section , VT the velocity of the transverse waves, VL the velocity of the longitudinal waves, and has the Poisson's ratio, and if kh << 1, it comes: V v2 1- 1 (1-) 2 a2 = VT + = V, \ For example: Let's take a conical point of half-angle at the top of the 2.4 ° apex and the end of which is polished. so as to further reduce its thickness to 0.1 mm, then with f = 600 kHz, VT = 3100 m / s, VL = 6420 m / s, a = 0.345, we obtain, Vp = 5355 m / s, Vd = 763 m / s and = 1.27 mm. This example and in particular formula (6) is illustrated in FIG. 13 (b). Having specified the law of variation of the phase velocity near the end of a conical tip, we must now specify its behavior as an acoustic antenna. 1. Field radiated by a conical waveguide Let u be a solution of the sinusoidal propagation equation in the medium in which the tip is immersed: Au + k2u = 0 (8) where k is fixed and denotes the wave vector: k = c "/Va.(Va = velocity of the air if the medium is for example a fluid and in this case air) Let G be the function of Green solution of the equation of propagation in the case of a point source localized in r ': AG (ro - r') + k2G (ro - r ') = -8 (ro - r') (9) In a spherical symmetry problem, and in the case where the source point r 'is coincident with the origin of the coordinates, the solution of the system of equations (8) and (9) is: G (r) = 4er r 1 eib. (10) a mathematical treatment of the system of equation (8) and (9) leads to the Kirchhoff-Sommerfeld formulation of the Huyghens-Fresnel principle stating that the diffracted field at ro = (x, y0) is obtained by summation of Spherical waves G (ro-r ') centered at r' = (x ', 0): u (xo, yo) = fu (x, O) DG (r) x ° dx X = 0 For simplicity, the mechanical displacement at the base of the cone is chosen unitary u (1.0) = 1. The progressive character of the wave is taken into account and therefore the sources are not (e + x) synchronous by introducing a phase shift e vd associated with the source of position (x, o). We also take into account the reduction of the cone section and therefore the increase of the amplitude of the mechanical vibration by conservation of the momentum by writing that the vibration undergoes a mechanical gain g (x, 0): g (x, 0) = uref (- .e ref, 0) + B (1 - Gc) xu (x, 0) Gee (12) Where u (x, 0) is the vibration at the end of the cone, uref the vibration at the position Ç, B a factor describing the progression of the gain near the end; 1 is the length of the cone. If in addition, we transform the three-dimensional problem into a two-dimensional problem (for example by taking a slice of the middle, the points being placed on the edge of the slice). Equations (8) to (11) then lead to the following diffraction integral: u (xo, y 0) = C + B (1- Gc) xv ke vd (x) -21 cbc x = - gee ## EQU1 ## where R = -'1 (x - xO) 2 + yO2 and C is a constant (C (1+ i)). In pulsed excitation, the integral (12) must be further transformed. It is first written that the time signal u (t) has a frequency spectrum U (co) = E (w) He (w) given by its Fourier transform. E (co) is the Fourier transform of the electrical excitation and He (w) is the Fourier transform of the electroacoustic impulse response of the piezoelectric element (assumed to be wider than E (w)). Conversely, when we know U (w) we can go back to the temporal response of the signal. Then we have: u (t) = 22r_ico Taking into account the diffraction, we get the general spatio-temporal shape of the radiated field: + c ° C xe GctX ilç, ru (xo, yo, t) = 2 Re -U (o) vd (x) e "dxdo 0 27r x = 0 + B (1- Gc) xrru (xo, yo, t) = 2 Re03 (xo, yo, t) + iQ (xo, yo, t )) where Re designates the real part of the double integral. In burst mode, the spectrum U (w) is very selective around the excitation frequency. It is approximated by a Dirac distribution centered at the coo excitation frequency. e- (rx v't-y0) x = -P Gc, e r- ev 'v, k (x) Y our (x0, yo, t) = Re .vk x = 0 + B (1 - G c ) xa -NT; 2. Consideration of the wave reflected at the end of the emitter cone An important phenomenon in the impedance measurement process is the taking into account of the reflection of the incident wave at the end of the guide. When the tip is free and bathes in the ambient air the mechanism is simple because the reflection takes place without phase change 20 and is practically +1. 2-int (13) (14) (15) The field refracted in the medium taking into account the wave reflected in the cone and the phase velocity Vde in the duralumin emitter cone is: Ge ur (xo, y0, t) = Re Vrc 0 + B (1- G) x (co-v coo va i (rx vat yo) I- r + x vat yo) e v'k (X) V "Vd (s) -I xl + 2.427r -1.21 f if Ixf 0.7, and vde (x) = 48 VPf 24 if lxi> 115 Yo dx (16) The phase velocity of formula (16) is modified with respect to formula (6) for take account of the fact that the cone is truncated and that the phase velocity is not zero when one is at the end of the cone (x = 0) Moreover, a cone can be locally flattened at its end. The opening angle is then locally modified in the polishing zone, which is only of interest for radiating energy in gaseous media characterized by a low wave propagation speed. 10 point so that the phase velocity gets closer In the case of the dermis 64 or the water 66, it may happen that the phase velocity in the tip is lower than that of the refracted waves. The bending wave A can then no longer radiate in the medium and it is necessary to drive the tip until its phase velocity again exceeds that of the medium. As shown in graph (b) of FIG. 13, the phase velocity in the tip is 1500 m / s for a section diameter of 0.4 mm which is reached at a distance of 4.8 mm. end of a perfect tapered tip (from vertex angle 4.8 °). It is therefore necessary to push a point of several millimeters in the water to totally cancel the echo inside the tip. The taking into account of the reflected wave is illustrated on the graph (f) of FIG. 13. On this same graph, the absence of a reflected wave gives the dashed curve. In addition, in the presence of reflected waves, there appears an interference phenomenon well made by the experimental measurement (graph (e) of Figure 13). According to formula (17), the distance between the minima 71- is given by the condition: co = + rut-. It corresponds to a half-length vde (x) 2 of local wave in the cone. 1) In conclusion, the angle of refraction is all the greater (ie directed towards the axis of the tip) that the impedance of the medium will be higher or that the section of the tip is smaller (Cf. Fig.14- (a)). Thus, for water, the emission angle can reach 90 ° when the phase velocity is less than or equal to the speed of wave propagation in water, while it is about 14 °, when a duralumin point has an angle at the top of 4.8 °, that the excitation frequency is 600 kHz, that its diameter at its truncated end is 0.4 mm and that it is immersed in the air. This is shown in Figure 14- (b). 2) the progressive wave in the cone loses its energy due to radiation. One measurement technique is to use a second tip disposed next to the first at the distance e as illustrated in Figure 15 (c). The sample is characterized by its thickness x and the wave refracted by the tip is reflected on the back side of the sample.

The thicker the sample, the longer the tips need to be spaced to find the maximum refracted signal as shown in Fig. 15 (a). FIG. 15- (b) shows the phase velocity in the vicinity of the end of an unpolished, duralumin conical tip for an apex angle of 4.8 ° and a truncated end 14 when the diameter is 0, 4 mm. When performing amplitude mapping, it may be advantageous to use a tripod or probe with three conical points, comprising a transmitting tip in the center and two receiving points aligned with the transmitting tip, at equal distances from it and all oriented so that their axes of vibration are collinear. This configuration is called longitudinal or Rayleigh because the emitting tip pushes the material towards the receiving points. If the medium is not homogeneous, variations in homogeneity can be measured via the two receiving tips which normally provide opposite signals in phase. It is then sufficient to add these signals to obtain a minimum reception signal whose amplitude will vary substantially with the local variations of impedance of the medium. The echo inside the transmitting tip provides information on the average load impedance, while the differential measurement provides information on longitudinal impedance fluctuations in the material (because the wave must propagate parallel to the surface of the material test). To measure a shear rate, simply turn the tips 90 °. Horizontal transverse vibrations are then detected (Fig. 16 (e)). These speeds are particularly low in diffusing viscoelastic materials such as organic tissues and transit times between two points spaced 1 mm apart can reach 0.3 milliseconds. The tips are therefore as close together as possible, for example at 0.5 mm from each other, and the working frequency is lowered to the minimum of the transducer bandwidth, a compromise can be found between 20 kHz and 600 kHz.

Different configurations of spike transducers are therefore possible depending on whether it is desired to measure a longitudinal or shear impedance or to carry out a surface or in-depth shear impedance mapping. FIG. 16 summarizes the various possible configurations: a) Radiation in the medium by depressing a transmitting tip 16, until cancellation or attenuation of the reflection coefficient Ar inside the transmitting tip at a set value, by example Ar = 25%. The tips are provided with PZT disk transistors 20 alternating polarization per half-disk, denoted (P, -P), glued on the base. The faces of a PZT disk are covered with a weldable paste. The base of the cone of typical height 80 mm and angle at the top 4.8 ° is surrounded by a tank 13 machined in the cone, with thin walls, typically 0.2 to 0.5 mm, filled with a damper 30, for example sintered metal powder. In the case where it would be necessary to work at high temperature, the powder is for example a refractory metal, for example tungsten, and the cone Inconel. For uses at room temperature and in contact with food or dermis, the cone is made of aluminum alloy or stainless steel or platinum and the powder is aggregated with a saturated polymer binder to a density of minus 5. The filling height of the tank 13 is typically 2 mm. PZT ceramics are typically PZ29 or PZ27 (references from the company Ferroperm located in Denmark). At high operating temperature PZ46 will be chosen for operation up to 550 ° C. The reservoir can also be traversed on its inner wall by a winding consisting of a conduit in which circulates a coolant 70 for example water having the effect of maintaining the ceramic PZT at a temperature below its Curie temperature. The end 16 of an Inconel tip can then be brought to a temperature of around 1000 ° C in contact with a cast iron plate during cooling, while the base of the cones is maintained at a temperature below 550 ° C. The receiving tip is remote from the transmitting tip, until reaching a spacing e (max) corresponding to a signal of maximum amplitude, refracted and reflected on the rear face of the semi-solid multiphase medium. The amplitude curve of the signal received as a function of the inter-peak spacing corresponds to a certain associated velocity profile and therefore of crystalline phase in the thickness of the material. After complete cooling and posterior crystallographic analysis of the material in its thickness, we obtain abacuses for connecting, for a given mixture, a phase profile or impedance to an inter-peak amplitude-spacing curve. b) At room temperature, the impedance of the sample can be determined from a given radiation impedance corresponding to a truncated tip 14 and the position of the interference nodes near the end. When the tip is free, the end vibrates along a belly of vibration, while when the tip is blocked, loaded by a much higher impedance, there is a vibration node. Between these two extremes, the tip can be charged first by a reference impedance Zref and produce a first vibration node at a distance from the end. When the reference impedance is replaced by an unknown impedance Z2, the node moves. This displacement can be physically observed using a laser vibrometer performing a scan of the generator of the cone. It can also be deduced from the phase shift of the electrical signal of the peak echo. A correspondence is then established between the impedance of the medium and an offset table of one or more minima or phase shift of one or more reference points of the echo electrical signal, for example of a shift to a zero crossing. c) When a sample has a very rugged curved profile, it can be associated with a lower radiation impedance that the surface is more convex (pointed). The use of a very tapered point 15 with low radiation impedance makes it possible to transfer the energy in the object more or less according to the local radius of curvature of the object (concave or convex). An amplitude mapping of the first echo carried out with constant force F provides a surface radiation impedance image of the material. In such a case, the impedance variations are low, control of the support force is critical. d) The previous setup 87 can be used for depth analysis with a bipod. If the vibration axis of the emitting tip 10 is collinear with the sensitivity axis of the receiving tip (Fig. 16-d), the device uses Rayleigh or longitudinal waves in the tip plane, and now the two points at a constant distance, for example 1 mm, and a constant support force F, for example 1 Newton, a scan of the surface is carried out with a spatial sampling period of less than or equal to half a length in the middle, typically 1 to 2 mm. Impedance variations of the medium at different depths can then be visualized by C-Scan (visualization of a map showing the amplitude of the signal at different time slots in the wave packet). The tips are oriented so that the vibration axes according to their bending mode are collinear. e) The previous assembly can be reproduced with one difference, the tips are turned 900 relative to their main axis, so that the direction of vibration / maximum sensitivity is perpendicular to the segment which connects the ends 14. The assembly is then capable of generating and detecting horizontal transverse waves if the medium 62 allows it. The coupling between the tips gives access to a horizontal transverse propagation velocity in the medium. The spacing between the tips is constant as well as the support forces F. Such an assembly is also interesting for detecting contiguous cracks. This configuration can furthermore be used to characterize diffusing viscoelastic media such as living organic tissues. The measurement consists of starting from contiguous points and moving them in small steps, typically of 10 tm. We then measure the phase shift and damping constant of the shear wave that can be compared to the decay curve of a shear wave in an elastic solid. When starting from contiguous points, the shear wave velocity can be calculated by phase shift measurement over a very short distance of the order of 100 g111. In such a case, the approximation or calibrated distance points can be achieved simply by a clamping screw through the carrier structure shown in Figure 15 making a quarter turn. Figure 17 shows the configuration of a 5-point device combining a central tip operating in reception mode R and whose directivity is programmable by means of the four-quadrant PZT 22 disk equipping its base. Two other pairs of points are aligned with the central point and are arranged in quadrature so as to occupy the vertices of a square. These tips operate as transmitters and are also equipped with four quadrant PZT 22 discs able to vibrate the tips in an electronically programmable main direction. There are three particular configurations. In the configuration (e), the vibrations of the emitting points are tangent to the circle passing through the vertices of the square and all oriented in the direct (or indirect) direction. The signals (Ex-, Ex +) are 90 ° out of phase with the signals (Ey-, Ey +). This configuration is optimized for transmitting / receiving shear waves. The central tip exploits its four electrodes to form two signals R1 and R2 corresponding to squares of sensitivity in quadrature. In the configuration (b), the vibrations of the transmitting tips are perpendicular to the circle and antisymmetric with respect to the center of the circle. The longitudinal or Rayleigh signals are emitted in the direction of the center and cancel in the center if they are emitted in opposition of phase 3. Otherwise they are added if they are in phase 1. The pairs are also excited in quadrature (one pair compared to the other). The central receiver remains in the same configuration with its signals R1 and R2 that we sum. The configuration (a) is called mixed. Two emitters of one pair vibrate in directions tangent to the circle symmetrically 4 with respect to the central point and generate a shear wave, while the other two emitters of another pair vibrate in a direction perpendicular to the circle, so that antisymmetric 3 with respect to the central point. This configuration approaches in his mind the low frequency (50 to 200 Hz) bending wave impulse elastography propagating in a viscoelastic tissue, revealed by high frequency longitudinal wave waves (3 to 5 MHz) illuminating the medium in which propagates the shear wave. Nevertheless, in the case of the present invention, the two peaks responsible for the generation of the shear wave operate at an ultrasonic frequency between 20 kHz and 600 kHz. The configuration (a) thus makes it possible to access longitudinal and transverse speeds and impedances as well as to illuminate the sample in different ways. This process can be followed in time, while the tips are preheated, for example at a temperature of 50 ° C and they are brought into contact with the medium. The diffusion of heat in the medium then changes the elasticity of the medium in a manner that differs according to the fundamental wave considered. C. LOCAL VELOCIMETRIC PROBE. 1. Differential transit time A bipod 89 can also operate as a local velocimetric probe as shown in Figure 18. The two transducers implement the principle of differential transit time measurement. They function alternately as transmitters and receivers. A longitudinal acoustic wave refracts in the dermis, enters a vein and descends or rises blood flow. It undergoes a positive or negative phase shift depending on the path taken, the amplitude of which will depend on the path traveled and the speed of the blood. Application: Let Vf be the speed of blood flow and Ve = the speed of propagation of ultrasonic waves in the blood (which is taken as equal to that of water).

Spacing of the tips and length of the path traveled in the vein: e = 5 mm Upstream transit time: Tup-Tderme-i-Tvup + Tderine. It consists of the transit time in the dermis to reach the Tderme vein, cumulated transit time in the vein Tviip in the upstream direction, cumulated transit time in the dermis Tdenne to go back to the receiving tip. Downstream transit time: Tdown-Tderme + Tvdown + Tdenne. It consists of the transit time in the dermis Tderme and in the vein Tvdovvn in the downstream direction. The upstream transit time in the vein occurs in the relation: e = (V-VOT, p The downstream transit time in the vein occurs in the relation: e = (V + Vf) Tvdown. gets: e ITve = ((Ve - Vf) 1 e Tvd () wil - "Vo ± Vf) AT = Tvup - Tvdovm = (\ ee (ve - v,) (ve + va, v2 (17) V; z 2 Numerical application: Ve = 1500 m / s, e = 5 mm, At = 0.1 ns, Vf = 2 cm / s For comparison, the mean blood velocity in the arteries is: Aorta 40 cm / s Arteries 10 - 40 cm / s Arterioles 0.1 - 10 cm / s Capillaries <0.1 cm / s 20 Veinules <0.3 cm / s Veins 0.3 - 5 cm / s Vena cava 5 -20 cm / s As can be seen, the differential transit time velocimetry does not allow to reach a resolution sufficient to detect the blood flow in the veinlets.In addition, the dimensional variations due to the deformation of the skin can taint the measurement of errors, especially when it is performed by upstream-downstream alternation and that the measurement rate is too low compared to the dimensional variations (a dimensional variation of 1 mm per second generates a transit time variation of the order of 0.6 ns between an upstream firing and a downstream firing (for a rate of 1000 shots / sec). It is then advantageous to use a tripod, ie three probes aligned with the vein, the emitting probe being in the center and equidistant from the receiving probes. The vibration axes are collinear and aligned with the direction of the vein. The upstream measurement is thus carried out simultaneously from the downstream measurement, which makes it possible to avoid errors related to dimensional variations related to local variations in pressure. Moreover, with a probe permanently operating in transmitter mode and two probes continuously operating in receiver mode, there is no longer any switching noise related to the control electronics. Finally, if the medium is diffusing, which is the case of red blood cells contained in the blood, the time signal can account for a velocity profile according to the zero crossing considered (speed near the surface for the head of the packet of waves and deeper and deeper for the tail of the wave packet). Figure 18 further provides an exemplary embodiment in a sectional view, the support conical tips. This support is preferably made of metal or sintered plastic. The interest is a partial damping of the waves in the sintered structure no longer requiring the use of a reservoir 13 lateral damping of the resonance of ceramics. This structure is also designed to avoid an acoustic short circuit between the transmitting tip and the receiving tip. Indeed, the acoustic path to go from the base of the emitter cone to the base of the receiving cone requires a much longer transit time through the structure that by the tips and the sample. Such a structure is feasible by 3D rapid prototyping, by stereo lithography of powder, for example PA12 polyamide material ("DuraForm ID PA Plastic," 2013, "Ensinger," 2013) having an average grain size of 58 μm with dispersion 25 gm-92 pm representing 90% of the particles. The tips are T-shaped and are held laterally in two places, one 43 on an extent z 1 located at the base on a length of 5 to 50 mm and the other 44 about 20 mm before the end. on a very short length of the order of a millimeter. They are inserted in force in the supporting structure. In the area of contact with the dermis 64, the supporting structure serves as a counter-support and normalizes the contact between the tips and the dermis. The supporting structure separating the two points has a withdrawal of 1 to 2 mm to avoid contact with the dermis. The tips thus protrude from a short end, about 0.5 mm which is called the support depth PA and are spaced about 5 mm. The base diameter of the tips is 6.6 mm and their height is 85 mm. The flanges 12 at the base have a small thickness, close to 0.5 mm. The collar has an outer diameter of about 10 mm. Its outer face does not come flush with the base of the cone, but has a shrinkage of about 1 mm, which allows the alternating polarization PZT ceramic 20 to build a main resonance radially at the frequency of 600 kHz.

Once the conical tips are inserted in force, the external faces of the ceramics are connected to the central part of the coaxial connectors 46 and 47 of subvisive or subclic type mounted on a cap 45, while the braid of the coaxial cables is connected via these connectors to the flanges 12 and constitutes the electric mass. The inner face of the cap and the carrier structure may optionally be metallized to form an electrical shield. 2. Pulsed Doppler Tapered Velocimeter Tapered waveguide probes 10 may also be used in continuous or pulsed Doppler velocimetry. The diagram of the pulsed Doppler velocimeter is given in FIG. 19. The formula giving the velocity Vf of the liquid is: Ve fd fj 2 cos (0,) f V fe d 2 2 cos (02) f For the same Doppler frequency fd, there appears a slight dispersion of the measured velocities lying between two extremes Vfl and V. The dispersion is greater as the refractive angles α1 and α2 of the acoustic fields 6 and 7 are higher. In principle, the angles ai and a2 should be identical, but the reflection at the epidermis / air interface increases ai (and thus decreases Gold). It is therefore appropriate to choose a tip whose phase velocity Va at the end is slightly greater and very close to the speed of propagation in the dermis and in general in the diffusing fluid inspected. The angles ar and a2 are then close and weak, which reduces the dispersion of the Doppler angle. A single tip 10 is used with (18) (18) its peripheral reservoir 13 and a PZT ceramic (PZ27 or PZ29) 20 or 21 with uniform polarization, but whose external electrode is divided into two half-disks respectively borne at a Electrical phase Burst (t) and are logical opposite. The burst burst consists of a burst of 5 slots of amplitude 0 to 100 V. The excitation E (t) with two opposite phases thus generates the bending wave. After transmission of the burst, the transducer is switched to receiver mode R (t). One of the outer electrodes shaped half-disk is then grounded while the other comes into input of a transimpedance amplifier. The remainder is in accordance with a conventional Doppler measurement (flat transducer without dispersive waveguide). In particular, in Doppler Velocimetry, the duration of a burst burst is rather long, especially when it comes to measuring the low speeds, which goes well with the length of the waveguide and its broadband response. . A 1 kHz PRF Pulse Repetition Frequency is a good compromise between resolution and measurement range. The lateral resolution is very good and comparable to the size of the tip, which is the main advantage of the device with its ability to operate at high interface temperature. 3. Angle Angular Probe The granular plastic carrier structure 89 of FIG. 18 may be modified to constitute a pentapode with 5 points arranged in accordance with FIG. 20. The points comprise a central point 16 vibrating according to a precession mode and 4 peripheral dipole points 11 to measure a component of the air velocity vector, in the plane of the ends. The tips are inserted into the U-shaped double wall U-shaped sintered structure 40 and the measurement is similar to that of the "wet finger" in which the finger is raised towards the sky, and feels cold. because of the calories absorbed by the part dried by the wind to determine its direction.

Here, the five points open slightly (about 2 mm) of the rounded carrier structure, to facilitate the flow of wind 67 on the generatrix of the carrier structure. The tips protrude sufficiently to reach the maximum coupling between them in the air, obtained for an adjacent area of about 2 mm.

D. VOLUMIC WAVE MIXED PROBES AND DISPERSIVE GUIDED WAVES Figure 21 shows a mixed configuration of the devices detailed in Parts A, "Wave Probes" and B, "Dispersed Guided Wave Probes". In this figure, a cylindrical waveguide 18 in the center is used to convey longitudinal waves of a longitudinal wave transducer 25 damped by a "backing" 31 and having a coaxial connector 48. The longitudinal waves are routed to the material to be analyzed with sufficient delay. Knowing the impedance of the waveguide makes it possible to deduce the longitudinal characteristic impedance of the sample 61. A tube 17 with a tapered section towards the sample makes it possible to guide a dispersive wave. This wave is generated by an annular transducer operating in radial mode and concentric with the rod 18. The tube is secured to the rod via the porous structure 49, but the two guides are considered to be acoustically isolated. Again, the annular transducer can be divided into two half-rings vibrating in phase opposition to change the radiation pattern of the transverse waves in line with the central guide 18. In the same way that the propagation in the tips to the party B, the wave propagating towards the sample 61 has a decrease in its phase velocity Vd. By adjusting the thickness of the hollow conical waveguide in contact with the sample, it is possible to lower its impedance of radiation in the sample at a value close to the transverse characteristic impedance of the latter. This optimizes the reflection coefficient on the sample for the annular transducer. The wave reaches the sample by shearing the material out of the waveguide tube and generates a shear wave in sample 7. As in Part A, the difference analysis echoes received for a reference measurement, for example in air, and when the probe is in contact with the sample, makes it possible to determine the longitudinal and transverse characteristic impedances of the studied material. E. U-DOUBLE-WALL GUIDED GUIDED WAVE PROBE Figure 22 is a cross-sectional view of a Joule-effect U-shaped double-wall tapered probe 97 with a capacitive force measuring device 81. F on the sample. In the absence of heating, the advantage of the double wall lies first of all in the possibility of avoiding thermal disturbances on the measurement of the transit time by thermally isolating the waveguide. In addition, the heating makes it possible to impose a temperature profile by means of a heating device 72 applied to the inner wall, as well as to increase the damping of the acoustic waves. Heating also makes it possible to simultaneously perform an effusiveness measurement, for example according to the transit time method explained in French patent FR0759603 of December 6, 2007, by Nikolovski et al. The double wall U, 41 and 42 can thus provide several features: - The heating of the inner wall for example Joule effect by means of an electric current flowing in a wire 82 wound around the inner wall. the damping of the ultrasounds when the successive echoes pass in the mechanical coupling zone of the waveguide to the sintered inner wall so that in the space of four to five successive echoes in the guide, the wave is damped at least 99% of its initial amplitude, which makes it possible to have a high measurement rate. - The thermal insulation of the inner wall by the outer wall, separated from the inner wall by a thermal insulator 73, for example a layer of air. The relative displacement of the inner wall with respect to the outer wall under the effect of a contact force of the tip on the sample causing a deformation of the supporting structure in its lower part 50. the measurement intermittently of the value of a capacitance of at least one capacitor 81 accounting for the relative position of the inner wall relative to the outer wall and one of whose electrodes is integral with the inner wall and the other electrode is integral with the outer wall, the value of the capacitance being, for example, derived from a resonant frequency or oscillations of a relaxation oscillator, or a capacitance measurement technique by means of a divider bridge from a reference capacitor and the law of variation of the capacitance being connected by a correspondence table, or a mathematical formula such as a polynomial interpolation law, to the force of support of the tip on the sample. Three capacitors can be used (but not shown), one to account for the sliding of the two inner and outer walls relative to each other under the effect of a force component located in the axis of the tip, while two other capacitors arranged between the inner and outer walls account for the force components perpendicular to the main axis of the conical guide and in two perpendicular directions having the effect of bending at least one of the walls under the effect of an oblique force, - Finally, the double wall provides a safety function for the tip and / or the sample, imposing a limit to the maximum penetration at zero support force of the tip in the sample by a maximum value corresponding to the support depth PA between 0 and 10 mm, and preferably between 0 and 1 mm. Thus, when the tip is supported on a hard surface, the relative sliding of the inner wall relative to the outer wall allows the tip to fully return to the outer wall. For this, the end of the U-shaped double wall is thin and deformable in the region 50 of the folding, under the action of a bearing force. Furthermore, the inner wall is heated by joule effect obtained by circulating a strong current, of the order of an ampere under 10 V, in a thin wire covered with an insulating varnish, wound around the inner wall 41 on a height corresponding to the contact area between the inner wall and the waveguide. The latter is cylindrical on a typical height of 5 to 10 mm at its base. The winding length produces a total resistance close to 5 to 10 ohms. The inner wall is thermally insulated from the outside air currents by means of the outer wall 42, separated from the preceding one by an air layer 73 having a thickness of 0.5 to 5 mm. The ohmic winding is housed between the two walls. The spacing is sufficient so that the winding does not touch the outer wall. The outer and inner walls are made of sintered plastic materials and covered with electrodes on at least a part of their surface and are electrically ground on at least another part of their surface. It is also possible to imagine a variant where the inner wall is made of sintered metal and the outer wall made of sintered plastic. The inner wall thus provides thermal conduction, while the outer wall provides thermal insulation. The heating circuit is also connected to an annular electrode fixed to a printed circuit 80, said printed circuit being mechanically secured to the base of the outer wall and forming with the upper part of the inner wall a capacitor 81 whose capacitance depends on the bearing force on the tip in the direction of the main axis of the tip. The distance between the top of the inner wall and the ring integral with the outer wall can thus vary as a function of the force exerted on the tip. For this purpose it is ensured that the double wall is deformable by thinning the wall in the zone 50 of folding of the U. Thus the coaxial connector connected to the heating circuit by the wire 82 can also be used to measure a resonance frequency or oscillations of a relaxation oscillator, determined by the average distance between the ring and the top of the inner wall via the wire 83. The winding of the ohmic heating circuit is also characterized by a specific inductance L and the Together, the heating circuit and the outer wall form an LC circuit whose resonance frequency can be measured and can be correlated to the support force. Furthermore, the outer wall provides acoustic isolation of the cone from handling, while the inner wall ensures the maintenance of the conical tip in two areas, one located near the base, the other 44 located closer to the end. The first zone is used for the ultrasonic damping of the pulsed bending mode, partially dissipated in the sintered granular structure, while the second zone extends over a very limited portion and serves only to guide the tip for mechanical retention. The bottom of the U-shaped double wall bears on the sample. It makes it possible to standardize the depth of support PA and to avoid damaging the sample or the tip. The bottom 50 of the double wall is deformable on its periphery thus allowing the tip to return into its housing under the effect of the bearing force F. The relative displacement of the tip relative to the ring attached to the base of the outer wall is thus measurable from the measurement of a capacitance. This law of variation is easily connected by a correspondence table or a mathematical formula translating the elasticity of the wall in the form of, for example, a polynomial interpolation law, to the support force. It is assumed of course that the driving stroke of the tip is limited, and at most equal to the support depth PA, preferably less than 0.5 mm and in all cases the maximum between 0 and 10 mm. The nominal value of the measuring capacity of the relative position of the two walls is of the order of 1 pF and the resolution of the order of 1 fF (femtoFarad). F. PROBE WAIST GUIDES SOLID DISPERSIVE AND LIQUID JET Referring to Figure 24, there is shown a probe 99 with dispersive waveguide 10, coupled to a liquid guide 93 consisting of a more or less viscous liquid coupler , providing a continuity of the liquid medium between the end 14 of the dispersive waveguide and the surface of the sample (60, 61, 62, 63, 64). It is recalled that a sample may have a hard 3D surface 60, constitute the surface of a viscoelastic medium 61, be able to propagate shear waves (62, be very hot and constitute a molten magma 63, constitute the dermis 64 of the skin, the wall of a vein 65, or the surface of a liquid 66. There is therefore provided a duct 91 for supplying the liquid coupler under constant hydrostatic pressure at the end 14 of the dispersive waveguide. This duct is for example made in the outer wall 42 of the double-walled U-shaped support 40. The duct opens into a chamber 92 into which also the end 14 of the conical tip opens, which is preferably truncated. extends in the chamber over a length of the order of a millimeter.The ultrasonic wave then refracts from the tip to the liquid in two main directions not aligned similarly to the directions 6 and 7 of Figure 19, but closer to the axis of the tip, with a high round-trip insertion rate given the lowering of the peak radiation impedance. The ultrasound-carrying liquid then exits via a nozzle whose inlet is oriented to receive the refracted waves in the liquid at one of the main angles and whose exit is of a straight or curved conical shape and is projected by a jet under pressure. 93 towards the surface of the sample. The liquid supply duct opens into the connection chamber 92 preferably at an angle to receive the other part of the refracted beam in the liquid and intended to be damped as it rises in the duct. brought to the inlet 90. The outer part of the ejection nozzle may have a thread 94 for screwing in an adapter piece of the diameter of the outlet jet. The variant of curved shape of the nozzle is not shown in FIG. 24, but its interest is obvious and this role can be provided by an adapter when it is desired to inspect the underside of pieces of complex shape, for example for crack detection under the mushroom of a railroad rail. The probe therefore comprises at least one broadband transverse wave transducer as previously described, coupled to the dispersive waveguide 10. The function of the dispersive waveguide is therefore to lower the peak radiative impedance and to optimize the ultrasonic transfer in the liquid. The coupling with the liquid is performed in the connection chamber 92 of the conduit and the tip and occurs over an extent of about 1 mm and a characteristic volume of about 1 mm3. The tip is truncated at the where its phase velocity is just greater than or equal to the velocity of compressional waves in the coupling fluid, usually water with antifreeze or oil. The method thus makes it possible to transfer most of the ultrasonic energy from the dispersive guide 10 to the liquid guide and vice versa during the return path. The liquid guide is a collimated jet. The liquid is fed through a hose connected to the probe for example at the outer wall via a hose connection 90. For a duralumin tip, the diameter of the truncated section is close to 0.4 mm when the angle at the top 0 is 4.8 °. The apex angle may be even smaller or the tip shorter if it is desired to have a working frequency greater than 1 MHz. The tip is itself guided at its base and its end and inserted into force in the connection chamber 92 and ultrasonic transfer. The chamber opens into the open by an outlet nozzle. The outlet nozzle may be a mechanical part in itself fixed on the support or a cast piece or sintered by 3D prototyping and constituting with the double-walled support U 40 a monolithic part. The wall of the duct, when manufactured by sintering, can be previously treated with a solvent, for example acetone to smooth its surface state. The liquid injected under pressure thus serves as a vehicle for the ultrasonic waves transmitted by the conical tip. It conducts the waves to the surface of the sample, over a typical distance of a few millimeters to a few centimeters. The width of the liquid jet is typically 0.5 mm to 2 mm. In Figure 24, the outlet nozzle is directed downward and its main axis and is not quite aligned with that of the conical tip due to the refraction angle in the liquid. On the other hand, it may be useful to probe the sample from below and the outlet nozzle may have a hook shape to project the liquid from the side or from below. These nozzle shapes have applications in the non-destructive testing of railway tracks, while the train is moving, or the inspection of complex parts, for example in aeronautics or wind power. As can be imagined, inspecting a part is less expensive if it can be done at high speed. In the case of a railway, it is sought that the inspection can be done while the train is moving at its nominal speed of 300 km / h But it would be interesting if the inspection could be carried out on trains regional express traveling at maximum speeds close to 120 km / h In this context, the rapid movement of the train and the associated air movement can lead to a dislocation of the inspection jet. It would then be useful, on the one hand, to increase the surface energy of the liquid so as to make it more difficult to fragment or nebulize, but the surface energy of the water, about 72 mN / m is already typically three times that of most common liquids, so it is difficult to do better at low cost and without impacting the environment. Nevertheless, one can play on the viscosity of the liquid which can be more easily increased by a factor of 10 to 100 by using oils, so as to have a laminar flow at higher ejection speeds, typically greater than 10 m / s and up to several tens of meters per second. By way of example, used sintered oil has a typical dynamic viscosity of 35 mPa.s, 35 times more viscous than water. The viscous liquid can itself be heated and thus first fluidized by circulating in a heated conduit. It is then expelled with a smaller diameter and cools very quickly in contact with the air. This treatment has the effect of improving the cohesion of the jet during its journey through the air. To avoid excessive curvature of the jet, the increase in the ejection speed of the liquid must be all the greater as the relative speed of movement of the nozzle relative to the surface of the sample is greater and therefore that the braking forces responsible for the curvature and dislocation of the jet become more important. In this, it is advisable to put the jet away from braking forces over the greater part of its path, to the surface of the sample, by means of a protective skirt 95, made for example in a flexible material of rubber type or made of metal strands forming a screw-shaped annular and cylindrical brush 98 on the outer wall of the probe 99. The protective skirt comprises a large number of strands so that if a number of strands breaks under the effect of friction on the rail, the protection function is always provided by other strands along the entire train path. The damaged skirts can then be easily replaced during a maintenance operation of the probe. This configuration of a liquid coupling probe has the advantage of consuming only a small amount of liquid coupling, of the order of 0.1 to 10 milliliters per second, ie 0.36 to 36 liters per hour and conical tip, and to have a good sensitivity considering the effective transfer of ultrasound to the solid-liquid interface 92. The couplant is projected at a distance of a few millimeters to a few tens of centimeters from the surface of the sample. By way of example, a pressure of the order of 2 to 3 bar makes it possible to create a water jet with a diameter of 0.8 mm at a speed of approximately 10 μm with a physical continuity of the jet reaching a few tens of centimeters. in a quiet room. This speed of expulsion is already in a turbulent regime and it would be better to use oils to stay in lamellar regime. In fact, the turbulence is characterized by the fact that concentric layers of liquid change their position inside the jet, which generates distortions in the acoustic signal. A speed of 10 m / s is sufficient to bend the skin safely to a depth of about one millimeter. Fig. 24 illustrates a probe having only one solid-liquid guide system. Nevertheless, it is obvious that this configuration can be extended to a bipod, tripod or pentapode. In these latter cases, the protective skirt 95 naturally surrounds all the points. It will be noted that the coupler under pressure is expelled by the nozzle towards the sample, but it will be understood that the fact that it is under pressure also allows it to completely fill the connection chamber 92. Moreover, in another field of applications concerning the medical diagnosis by pulse elastography and therefore corresponding to a case where the surface is deformable, as is the case with organic tissues, the hydrostatic ejection pressure can be variable and modulated over time in amplitude and / or phase. The modulation amplitude can reach an intermittent ejection regime with physical discontinuity of the jet and an ejection rate can vary between 1 Hz and 10 kHz. Knowing that the round-trip transit time of the ultrasonic wave propagating in the guide to the sample is of the order of 100 g for a throw of a few centimeters, it suffices that the spatial continuity of the jet between the end of the solid guide and the sample lasts for a period of at least 100 ps for an ultrasound measurement to be possible. Nevertheless, the duration of a jet may more advantageously be between 5 ms and 50 ms to allow the ultrasonic waves to penetrate deeply into the sample. This mode of operation generates a rhythmic percussion of the surface of the sample and approaches, in principle, the low frequency pulse elastography with here a major difference is that the low frequency shear wave is not generated by a vibratory pot or a bulky electromagnetic percussion device juxtaposed with the ultrasonic source, but results directly from the modulation of the ejection velocity or the intermittent impact of the liquid jet conveying the ultrasonic wave. And, given the speed of propagation of shear waves in human tissues, of the order of a few meters per second, it is therefore necessary to visualize the displacement of the shear wavefront generated by the impact that the Spatial continuity of the jet is ensured over a period of at least 10 to 100 ms. Moreover, while the jet or jets regularly strike the surface of the sample at a rate of eg 50 Hz (with a duty cycle of 50%), the ultrasonic pulses are generated at a much higher rate of the order from 0.5 to 10 kHz and the central ultrasound frequency is preferably between 300 kHz and 3 MHz. The diameter of the collimated jet 93 is typically 0.5 mm to 2 mm. The jet thus constitutes a quantity of movement displacing the surface of the skin to a depth that depends directly on its ejection speed and its duration of impact. The amplitude of the deformation is typically 1 mm. If water is used as coupling, its viscosity being of the order of 1 mPa.s, a Reynolds number close to 500 is obtained for a nozzle outlet speed of 1 m / s and a diameter of 0.5 mm nozzle. The flow is then laminar. It remains so up to a Reynolds number of 2300, which corresponds to approximately a nozzle exit velocity of 4.6 m / s. In order for the flow to remain laminar at a higher ejection speed, it is sufficient to increase the viscosity of the liquid coupling and the ejection pressure applied to the liquid. This type of probe and this mode of operation thus has the advantage of being able to be easily moved during a medical examination by pulse elastography. In addition, and it is the interest of the present invention, the support structure can be designed so that the jets are collinear or fan in order to be able to play on the lateral bulk of the ultrasonic beam by playing on the distance to the sample. The jet associated with each tip may also be associated with a modulation law amplitude and phase different from the neighboring jet. It is then possible, for example, in the case of a five-jet pentapode, or more generally of a multi-jet system, to control the radiation pattern of the shear wave generated in the biological tissue, according to the amplitude and the phase of the pressure applied to each jet. Finally, in the case of a medical check, it is desirable to recover the liquid coupling sprayed on the skin of the patient, even if it is in small quantities. The protective skirt here will not function to avoid a dislocation of the jet, which is not likely to occur here, but to confine the splash caused by the impact of the jet. In addition, since the movement of the probe during an examination is rather slow, of the order of a few millimeters per second, the skirt may be made of a less aggressive material than a wire brush, for example in the same plastic material sintered or injected as that of the guide support or in another smooth material having non-allergenic properties. An additional suction function of the liquid spray can be integrated on the periphery of the probe for example for the sake of cleanliness and comfort of the medical examination. For this, the skirt may consist of a double wall, the suction flow being carried out in the annular space between the two walls of the skirt. Since the suction flow itself is turbulent and generates low-frequency waves that can disturb the low-frequency shear waves generated by the jets, the suction is not carried out continuously, but only when the amount of couplant liquid is too important and requires vacuum cleaning. In this application, the viscosity of the liquid coupling may advantageously be greater than that of water to approach that of an oil to prevent the couplant from running too easily on the skin of the patient.

Moreover, as previously indicated, in order to maintain the collimated jet, and therefore in laminar flow, it is essential to control the value of the Reynolds number, defined according to the formula: Re = pvd P in which p denotes the density of the liquid the flow velocity of the liquid at the exit of the ejection nozzle, the outlet diameter of the ejection nozzle and the dynamic viscosity of the liquid. Since the Reynolds number must not exceed a defined value according to abacuses adapted to the shape of the preferably tapered flow conduit in order to preserve the collimated appearance of the jet, it appears that increasing the viscosity constitutes an effective solution for speeds ejection and to reach more distant inspection areas, or to avoid a dislocation of the jet for example by a strong side wind, or to facilitate the use of a liquid having a density greater than that water and thus be able to generate a more intense shock wave intermittent while retaining the collimated aspect of the jet. In the following claims, the terms used should not be construed as limiting the claims to the embodiments set forth in this specification, but should be interpreted to include all equivalents that the claims are intended to cover because of their formulation. and whose prediction is within the reach of the person skilled in the art by applying his general knowledge to the implementation of the teaching which has just been disclosed to him.

Claims (22)

REVENDICATIONS1. A device (8, 9, 87, 88, 89, 97, 98, 99, 400, 700, 710) for measuring and inspecting the mechanical and thermal properties of a medium (60, 61, 62, 63, 64, 65, 100, 101), characterized in that it comprises: at least one broadband source (20, 21, 22, 250, 260, 270, 280, 290, 540) of ultrasonic elastic waves, at least one elastic wave guide (10, 17, 18, 150), coupled to at least one ultrasonic broadband source, brought into contact with the sample, the at least one acoustic source generates at least one wave in the waveguide Transverse (S) or longitudinal (L) or dispersive (A) in continuous or pulsed mode, - the waveguide has a length greater than half the path traveled by the elastic wave during the duration of the pulse, the contact impedance Z1 of the guide is known and defined as the product of the phase velocity of the elastic wave at the point of contact with the sample by the density of the waveguide; waveguide is coupled with the medium on a known limited surface (11, 14, 15, 16), - the mechanical and / or thermal properties of the medium are deduced from the amplitude or phase variations of the reflected waves (Ar (x, y)) and / or transmitted (At (x, y)) in the medium. 2. A device (8, 9, 87, 88, 89, 97, 98, 99, 400, 700, 710) for measuring and inspecting the mechanical and thermal properties of a medium (60, 61, 62, 63, 64, 65, 100, 101) according to claim 1, characterized in that the waveguide comprises a cylindrical axis of symmetry and has a conical, solid (10) or hollow (17) profile and propagates flexural waves. (A) and is refined towards its end until the phase velocity at its end is between 10% and 100% of the velocity of the transverse wave and therefore its radiation impedance ( Zi (r)) is between 10% and 100% of the transverse impedance of the material constituting the guide. 30 3. A device (8, 9, 87, 88, 89, 97, 98, 99, 400, 700, 710) for measuring and inspecting the mechanical and thermal properties of a medium (60, 61, 62, 63, 64, 65, 100, 101) according to claim 1 or 2, characterized in that for a contact impedance guide Z1, the mechanical impedance Z2 of the sample is determined by the formula: Z2 = Z1 1+ ( X with 1-aa = 42 Za'-An Z ,,, r + Z1 where Zair denotes the mechanical impedance of air, Ar2, the reflection coefficient at the sample guide interface, An, the reflection coefficient at the air-guide interface 4. A device (8, 9, 87, 88, 89, 97, 98, 99, 400, 700, 710) for measuring and inspecting the mechanical and thermal properties of a medium (60, 61, 62, 63, 64, 65, 67, 100, 101) according to any one of claims 1 to 3, characterized in that the waveguide is preferably made of a material having a characteristic impedance greater than or equal to that of the test material for example low density metals such as aluminum or titanium, or hard plastics such as plexiglass or polycarbonate. 5. A device (8, 9, 87, 88, 89, 97, 98, 99, 400, 700, 710) for measuring and inspecting the mechanical and thermal properties of a medium (60, 61, 62, 63, 64, 65, 67, 100, 101) according to claim 1, characterized in that the waveguide is a solid cone and its end is truncated (14) at the point where the phase velocity at the end is equal to the speed of the longitudinal waves generated in the medium. 6. A method for measuring and inspecting the mechanical and thermal properties of a medium (63) comprising: - emission in the waveguide (10) of known impedance determined by the product of its density by its phase velocity, of at least one transverse (S) or longitudinal (L) or dispersive (A) wave in continuous or pulsed mode, - the depression of the waveguide (10) in the medium (63) such as silicones, viscous liquids, plants, or foods, of a characteristic depth (pp) for which the amplitudé of the internal echo (Ar) in the waveguide is decreased from 50% to 75% its value with respect to conditions where the end (16) of the waveguide is free. 7. A device (8, 9, 87, 88, 89, 97, 98, 99, 400, 700, 710) for measuring and inspecting the mechanical and thermal properties of a medium (60, 61, 62, 63, 64, 65, 67, 100, 101) according to any one of claims 1 to 5, characterized in that: - the base of the guide (10, 17, 18, 150) coupled to the ultrasonic wave source (20 , 21, 22, 250, 260, 270, 280, 290, 540) is exposed to a temperature T less than the Curie temperature Tc of the piezoelectric source (20, 21, 22, 250, 260, 270, 280, 290 , 540), the material constituting the waveguide (10, 17, 18, 150) is superalloyed, for example in inconel, the surface of the guide (10, 17, 18, 150) exposed to a temperature of higher sample has a smaller surface, at least 100 times smaller, than the surface of the guide exposed to the temperature T, the length of the guide (10, 17, 18, 150) ensures the maintenance of a difference of temperature between the base and the end of the guide, the time difference a temperature resulting from a convection phenomenon or free or forced conduction by a cooling fluid cooling system (70), and regulating the temperature of said ultrasonic source at the temperature T lower than its Curie temperature Tc. 8. Device according to claim 1, characterized in that it consists of a differential tripod 20 comprising two receiving points arranged at equal distances from a bending wave emitter tip, and aligned therewith. The receiving tips are arranged angularly with respect to their central axis so that the received signals are in phase opposition. 9. A method of visualizing the variations of mechanical properties of a material characterized in that it consists of a slow displacement of a differential tripod arrangement on the surface of the medium, in the direction perpendicular to the segment connecting the three points and providing amplitude and phase information on spatial variations of impedance. 10. Device according to claim 1 characterized in that it comprises a pentapode (98) comprising two tripods oriented at 90 ° relative to each other, and sharing the same central tip, forming a 5-pointed system . The peripheral tips operate as a transmitter (Ex-, Ex + ', Ey-, Ey +) so that there is only one electronic amplifier connected to the single central receiver (R). The two pairs of transmitter spikes can operate at the same or different frequencies. In particular, they can vibrate in longitudinal (1, 3) or shear (2, 4) polarization with respect to the main sensitivity direction of the central tip. 11. Device according to claim 10, characterized in that it comprises a pentapode (98) coupled to ceramic discs PZT four quadrants (22) operating in a linear combination of two dipoles oriented at 90 ° to each other and can be combined in amplitude and / or phase, thereby imposing an acoustic vibration direction in the contact plane. The tips are thus programmed to emit shear waves (2, 4) or longitudinal waves (1, 3) towards the central tip according to whether their direction of vibration is collinear or parallel to the main sensitivity axis of the central point. When the central tip is operating in reception, the pentapode is in echographic configuration. In this configuration, it is sought to bring the tips as close as possible, preferably to less than 0.5 mm, so as to be able to observe rapid spatial variations in the mechanical properties of the medium and to exploit the shear waves of frequencies between 20 kHz and 2. MHz. 12. Device according to claim 1, characterized in that it comprises a pentapode (98) with two differential tripods for the depth measurement of longitudinal velocity variations (1, 3) or transverse (2, 4). 13. Device according to claims 1 and 2, characterized in that it comprises a pentapode (98) and that the central tip operates in transmission and the four peripheral peaks in reception, the pentapode then being in directional velocimetric configuration. In this case, the central tip (10, 16) operates in precession mode, while the 4 peripheral points (10, 11) operate in pairs of receivers diametrically opposed to the transmitter and detect the direct signal emitted by the central transmitter. The measurement is then a measure of differential transit time per pair of receivers. It gives the directional flow in the plane of the ends. 14. A device (8, 9, 87, 88, 89, 97, 98, 99, 400, 700, 710) for measuring and inspecting the mechanical and thermal properties of a medium (60, 61, 62, 63, 64, 65, 100, 101) according to claim 1 or 2, characterized in that it comprises 5 guides (10) conical points forming a pentapode (98) in transit time measurement mode between the central transmitter and the spikes peripherals and a horizontal transverse wave (S) is emitted by a central tip in impulsive precession vibration and two pairs of peripheral tips oriented to detect the horizontal transverse wave (S) emitted by the central tip and giving a propagation velocity of this wave along two perpendicular directions and thus an angular anisotropy of the transit time. 15. A method for measuring and inspecting the mechanical and thermal properties of a medium comprising: a step of generating and detecting reference echoes in the medium by bipode (89), tripod or pentapode (98) type of probe successively contacting the medium at a set of reference positions along a curvilinear reference path, - a step of storing the reference echoes associated with each position of the probe on the medium, along the curvilinear path reference method - a step of recognizing the contact positions of the probe on the medium by calculating the minimum distance or by calculating the maximum of an inter-correlation function between a measurement echo obtained during a second pass on the curvilinear path and one of the echoes corresponding to the learned positions of reference. 16. Method for identifying a material and / or classifying a state from a reference database established from the measurement of at least one of its effusivity, impedance values longitudinal ZL and transverse ZT as well as angular anisotropy 20 said impedances in at least two different directions, for example orthogonal. 17. A device (8, 9, 87, 88, 89, 97, 98, 99, 400, 700, 710) for measuring and inspecting the mechanical and thermal properties of a medium (60, 61, 62, 63, 64, 65, 100, 101) according to claim 1 or 2, characterized in that in the case of contact with the epidermis (64), the conical tip transducers are truncated (14) and slightly pressed against the epidermis (64) so that the entire section of the guide is in contact with the epidermis. In the contact zone, the section of the guide (14) is such that its phase velocity (Vd) is equal to that of the longitudinal waves of the medium, ie about 1500 m / s. 18. A device (8, 9, 87, 88, 89, 97, 98, 99, 400, 700, 710) for measuring and inspecting the mechanical and thermal properties of a medium (60, 61, 62, 63 , 64, 65, 67, 100, 101) according to claim 1 or 2, characterized in that in the case of an air medium (67), it comprises a pentapode (98) with a central emitting tip (10) vibrating E in precession mode and 4 receiver points (Rx-, Rx +, Ry-, Ry +) sensitive to a bending mode. The central precession tip (10) has a round end (16), preferably of section diameter at its end less than 0.1 mm, and vibrating according to a combination of two bending modes oriented at 90 °, one of the other. 19. Device (9) for measuring and inspecting the mechanical and thermal properties of a medium (60, 61, 62, 63, 64, 65, 100, 101) according to claim 1, characterized in that it comprises at least one guide in the form of a rod (18) propagating longitudinal waves and a tube-shaped guide (17) concentric with the rod and propagating flexural waves. 20. A device (8, 9, 87, 88, 89, 97, 98, 99, 400, 700, 710) for measuring and inspecting the mechanical and thermal properties of a medium (60, 61, 62, 63, 64, 65, 67, 100, 101) according to claim 1 or 2, characterized in that its at least one guide (10) is held by a U-shaped double wall (40) providing at least one of the following functionalities: the control of a set temperature of the inner wall (41), for example by means of a resistive coil (72) intended to heat the conical tip (10) by Joule effect, - the damping of the successive echoes passing into the zone (43, 44) of mechanical coupling of the waveguide to the inner wall, 20 - the thermal insulation of the inner wall by the outer wall, separated from the inner wall by a thermal insulator (73), for example a layer of air, - the relative displacement of the inner wall relative to the outer wall under the effect of a force (F) of contact of the tip on the sample, - the intermittently measuring the value of a capacitance (81) of at least one capacitor accounting for the relative position of the inner wall with respect to the outer wall and one of the electrodes of which is integral with the inner wall and the other electrode is integral with the outer wall, the value of the capacitance being for example deduced from a resonance frequency or oscillations of a relaxation oscillator, or from a divider-bridge measurement of a capacitance of reference and the law of variation of the capacitance being connected by a correspondence table, or a mathematical formula such as a polynomial interpolation law, to the force of support of the tip on the sample, - a safety function for the tip and the sample, imposing a limit on the maximum penetration at zero bearing force of the tip in the sample by a maximum value (PA) of between 0 and 10 mm, and preferably less than 0.5 mm . 21. Device according to one of claims 8, 10, 19 or 20, characterized in that the carrier structure (40) is composed of a portion ensuring the positioning, the orientation (41, 44), and the insulation , the damping (42, 43, 73, 74) of the waveguides and a bayonet cowling (55) consisting of elastic deformable portions (54, 58) providing a holding force of the probes in the inner wall ( 41) and the locking (53, 57) of the flexible elements (54, 58) on the guide element (52). 22. Device (98) for measuring and inspecting the mechanical and thermal properties of a medium (60, 61, 62, 63, 64) according to one of claims 1 or 2, characterized in that it comprises, in in addition: a conduit (91) for supplying a liquid waveguide under pressure at the end (14) of a solid waveguide (10), a partial immersion of an end (14). ) of the solid waveguide in the liquid waveguide, in a connection chamber (92), on a limited extent, of the order of 1 mm and a volume of the order of 1 mm3, a nozzle of ejection of the liquid waveguide, of straight conical or curved hook-shaped shape for the inspection of external, lateral, or internal surfaces of a sample, such as a railroad rail, a protective skirt (95 ) of the liquid waveguide (93) against dislocations or fragmentation by a stream of air over at least a portion of its length to the surface of the sample or confinement and asp the splashing of the liquid jet projected onto the surface of the sample, a hydrostatic liquid ejection pressure that is constant or can vary over time, ensuring the generation of acoustic waves of low frequency shear, controlled in amplitude and / or in phase, in particular between the jets of a configuration with several liquid waveguidesune m'odulatioti of the hydrostatic pressure can reach an intermittent ejection regime with an ejection rate that can be between 1Hz and 10kHz.