Ultrasonic transducers for non-invasive measurements

Abstract

The present invention relates to an ultrasonic Lamb wave transducer (100, 102) for non-invasive measurement. The transducer (100, 102) is configured to transmit or receive an ultrasonic Lamb wave beam (110, 112) for measurement. The transmission and reception of the ultrasonic Lamb wave (110, 112) each have a direction, and the pulse is defined by at least one parameter. The ultrasonic Lamb transducer (100, 102) includes a piezoelectric composite actuator configured to control the transmission direction of the ultrasonic Lamb wave by transmitting acoustic radiation at a frequency suitable for the generation of the ultrasonic Lamb wave.

Description

Technical Field

[0001] The present invention relates to an ultrasonic transducer for non-invasive measurement, a controller and a method for manufacturing the ultrasonic transducer for non-invasive measurement. Background Art

[0002] Ultrasonic transceivers are used in medical applications to scan, for example, organs, or in mechanical applications to investigate potentially damaged materials, which can be performed in a non-destructive manner by ultrasonic techniques. In addition, they can be applied to level measurements of liquids or tissues in containers, for example. In these cases, ultrasound waves are emitted by an ultrasonic device, usually at the top or bottom of the container, and an echo of the same frequency (or a higher harmonic) is reflected from the level interface back to the device, which receives the echo. That is, a straight acoustic beam parallel to the tank wall is used.

[0003] Since the bottom of the container is usually not accessible, the measurement can be performed by attaching an ultrasonic device to the side wall of the container. Non-invasive level measurement of liquids in tanks, for example, can therefore be performed by a transmitter and a receiver. The transmitter is placed at a first position below the liquid level outside the container wall and at the liquid surface. The transmitter transmits the ultrasonic wave as a primary wave into the container wall, causing a portion of the primary Lamb wave to leak from the container wall into the liquid in an oblique and upward direction toward the liquid surface in the form of a pressure wave.

[0004] The receiver is placed at a second position outside the container wall and below the level of the liquid surface in such a way that the receiver can receive the secondary wave generated by the pressure wave hitting the container wall after being reflected by the liquid surface.

[0005] Therefore, for non-invasive measurements (such as time-of-flight measurements), it is desirable to have the possibility to control the direction of the acoustic beam adapted according to certain conditions. The term "time-of-flight" relates to the time elapsed between the emission and reception of a signal. For example, the measurement of the liquid level from the side of a tank requires that the acoustic beam angle be adjusted so that the beam reflected from the liquid surface hits the receiver. Since the liquid level to be measured is not constant, the direction of the transmitted wave must be changed. A type of transmitter is desired that is able to steer acoustic waves in a wide angular range and with sufficient amplitude. Since the angular control of Lamb waves is directly related to the wavelength of the acoustic field, large angular tunability requires a wide bandwidth of the electroacoustic transducer element in the frequency domain. Summary of the invention

[0006] This problem is solved by the subject matter of the independent claims. Embodiments are provided by the dependent claims, the following description and the drawings.

[0007] The embodiments similarly relate to an ultrasonic Lamb wave transducer method for manufacturing an ultrasonic transducer for non-invasive measurement, use of an ultrasonic Lamb wave transducer, and a controller for controlling the radiation angle of an acoustic beam in a medium. Synergistic effects may arise from different combinations of the embodiments, but they may not be described in detail.

[0008] Further, it should be noted that all embodiments of the present invention relating to methods may be performed using the order of steps as described, however this is not necessarily the only and required order of the method steps. The methods provided herein can be performed using another order of the disclosed steps without departing from the corresponding method embodiments, unless otherwise explicitly mentioned below to the contrary.

[0009] Technical terms are used according to their common sense. If specific meanings are expressed to certain terms, the definitions of the terms will be given below in the context in which the terms are used.

[0010] A transmitter that converts electrical energy into acoustic energy and a receiver that converts acoustic energy into electrical energy are referred to in this disclosure as a transducer.

[0011] According to a first aspect, an ultrasonic Lamb wave transducer for non-invasive measurement is provided. The transducer is configured to transmit or receive ultrasonic Lamb wave pulses for measurement. The transmission and reception of ultrasonic Lamb waves (110, 112) each have a direction, and the pulse is defined by at least one parameter. The measurement is based on the transmission direction or the reception direction of the ultrasonic Lamb waves and at least one parameter describing the pulse. The ultrasonic Lamb transducer includes a piezocomposite actuator configured to control the transmission direction of the ultrasonic Lamb waves by emitting acoustic radiation at a frequency suitable for the generation of the ultrasonic Lamb waves. The term "Lamb wave pulse" refers to a short emission of a wave relative to time.

[0012] According to an embodiment, the at least one pulse parameter is one of: carrier frequency shift; flight time; pulse width; attenuation; pulse amplitude; or a combination thereof.

[0013] That is, for example, in time-of-flight measurement applications (e.g., liquid level measurement), where the propagation path of the wave is not straight, but travels from the emission point to the reflection point and from the reflection point to the receiving point, the reflection point must be hit so that the reflected beam hits the receiver. For example, in the case of liquid level measurement, in order to find the exact reflection point where the receiver detects the maximum number of energies of the received wave, the beam of the ultrasonic wave must be directed to different angles until the maximum number of energies is found. With the known positions of the transmitter and the receiver, it is possible to calculate, for example, the level of the liquid based on the knowledge of the emission angle and the length of the propagation path. The maximum received energy at a certain frequency is found, and the current emission angle is known by the predetermined relationship between frequency and angle. It must be noted that the present invention is not limited to this arrangement. Depending on the geometry of the emission and receiving transducer positions, the size of the container and the liquid level, a wide range of beam emission angles may be required. Since the angle variability is achieved by the generation of Lamb waves, this requirement translates into a need for an electroacoustic transducer element with a high frequency bandwidth. This need can be met by using a piezoelectric composite actuator in the transducer. Piezoelectric composite actuators allow electroacoustic conversion with high conversion efficiency in a wide frequency range. The frequency is controlled within a frequency range that results from the properties of the piezoelectric composite.

[0014] Piezoelectric composite actuators can be adapted by adjusting their properties, such as the quality factor, which is responsible for the achievable bandwidth, which affects the angular variability of the emitted acoustic beam when using Lamb waves. The quality factor (Q factor) is affected by the elastic stiffness and viscosity of the piezoelectric composite, which is also related to the acoustic losses. Typically, piezoelectric composite actuators have a lower quality factor than the commonly used piezoelectric ceramic actuators and therefore have a wider bandwidth. The wide bandwidth is advantageous not only for the generation of short pulses (which may be desirable for some applications) but also for exciting Lamb waves. The variation of the excitation frequency within a large frequency range causes a large range of radiation angles of the emitted Lamb waves. The variation of the center frequency can be achieved with a sufficiently high acoustic amplitude.

[0015] A suitable type of wave is a Lamb wave, also known as a plate wave. Lamb waves are mechanical waves generated in a plate, in which the wave propagation is affected by the reflection of the wave at the wall side and the propagation space thus limited. Therefore, they show properties similar to those of waves propagating in a waveguide. In a plate in a gaseous medium or in a vacuum, the wave is completely reflected at the plate side and is therefore kept inside the plate. In the case where the plate is in contact with a liquid, the reflectivity of the interface is reduced, and the Lamb waves are able to emit acoustic energy into the surrounding liquid medium. Due to the emission of acoustic energy into the surrounding medium, they are also known as leaky Lamb waves. For leaky Lamb waves, the angle of the emitted sound wave entering the liquid when measured relative to the tank wall depends on the frequency of the sound wave. Therefore, by coupling to Lamb waves with different frequencies, it is possible to change the direction of the emitted beam.

[0016] In some applications, other pulse parameters are measured (eg, carrier frequency shift, pulse duration or pulse width, or pulse amplitude of a received pulse) and the difference in value between the transmitted and received pulses is determined.

[0017] According to an embodiment, the base material of the piezoelectric composite actuator is a piezoelectric ceramic material. Variants of the piezoelectric ceramic material may be lead zirconate titanate (PZT), PZT4 and PZT8. Alternatively, ceramics based on (Bi,Na)TiO3-(BNT-) or BT-BaTiO3-(BT-) may be used. Typically, standard PZT material is used for actuation. PZT is a low-cost and commercially available material. It has a high electromechanical coupling factor, which allows efficient generation of acoustic waves. Since piezoelectric ceramic elements typically have a narrow bandwidth, they can be used with reasonable efficiency only near their natural resonance.

[0018] According to a further embodiment, the piezoelectric composite actuator comprises a polymer as composite material, so that the piezoelectric composite actuator comprises a combination of polymer and piezoelectric ceramic material. Examples of polymer materials are spurr resin or Poss EP3512 resin.

[0019] According to an embodiment, the piezoelectric composite element is mounted on, for example, a nylon wedge, which has optimal material properties for coupling acoustic waves (including shear waves) in the tank wall to generate Lamb waves.

[0020] Originating from a pure piezoelectric ceramic, which, as described above, has a high mechanical quality factor that limits the bandwidth of the system's resonance to a narrow frequency range, a polymer material is added to reduce the quality factor, so that a broadband behavior with a higher bandwidth is obtained. At the same time, due to the piezoelectric ceramic content, the high electromechanical coupling factor is almost applicable or only slightly reduced. Due to the polymer content, the acoustic impedance of the piezoelectric composite is closer to the acoustic impedance of the host material (which can be, for example, nylon for the transducer wedge), which leads to a significantly enhanced acoustic coupling between the actuator and the transducer substrate. Both effects (i.e., low quality factor and improved acoustic coupling) allow the creation of an actuator that is able to excite vibrations very effectively with large amplitudes over a wide frequency range, which in turn leads to enhanced angular variability.

[0021] As an example, a PZT material may have a mechanical quality factor of Qe = 950. If a polymer with a quality factor of Qp = 50 (e.g. PZT8-Epotek combined with Duralco 128) is added at a volume fraction v = 0.5, the quality factor of the material combination is approximately In this example, Q=95 is obtained. Since the bandwidth of the transducer is proportional to 1 / Q, by changing the material from pure piezoelectric ceramic to composite, the bandwidth is increased by a factor of 10. At the same time, the piezoelectric property (coupling to voltage) decreases only linearly with v (i.e., by half). This explains the gain by using a composite material.

[0022] According to a further embodiment, the transducer is wedge-shaped, wherein the wedge is adapted to host a piezoelectric composite actuator. Different acoustic velocities in the metal and the plate give rise to different wavelengths. The resonance condition of the different wavelengths is thus satisfied at a certain angle, which depends on the different wavelengths and therefore on the velocities in the wedge and the metal plate of the container. A high amplitude of the Lamb wave is thus obtained, thereby satisfying the condition Angle Introducing a wave from a transducer, where is the speed of sound in the wedge, and is the frequency-dependent acoustic velocity in the plate. Similarly, the radiation angle from the plate into the liquid is ,in is the angle between the wall and the pressure wave, is the speed of sound in the liquid, and is the frequency-dependent phase velocity of the Lamb wave (also called the primary Lamb wave) in the wall. Frequency dependence (dispersion), changing the frequency causes changes.

[0023] According to an embodiment, the transducer comprises a backing layer and at least one matching layer, wherein at least one side of the backing layer and / or the matching layer has a curved shape.

[0024] Composite structures can be used in wedge-like arrangements (like PZT) also combined with appropriate backing and matching layers. In addition, they can be produced in arbitrary shapes (e.g. curved shapes) and with predefined anisotropy. This can be used, for example, to focus the acoustic beam onto a portion of a metal plate (using a "lens" effect) or by adjusting the shape of the eigenmodes so that the transmitted wave pattern suppresses unintended side lobes (which can occur, for example, due to shear and longitudinal waves excited by the actuator). Some of these waves can be suppressed or their temporal / spatial distribution changed by appropriate shape design.

[0025] Therefore, piezoelectric composites can be used as a simple replacement for typical piezoelectric ceramic elements. This takes advantage of the better acoustic properties as described above. In addition, numerous manufacturing parameters of piezoelectric composites (such as fiber distribution, convex or concave shape for focusing the acoustic beam, etc.) can be exploited in order to optimize Lamb wave excitation.

[0026] According to a further embodiment, the transducer is either a transmitter or a receiver. Since for reception the same properties apply in the opposite direction, the transducer is also deployable as a receiver. Therefore, a symmetrical geometric arrangement is advantageous. Alternatively, as a receiver, a typical PZT ultrasound sensor may be used.

[0027] According to an aspect, a controller is provided, which is configured to control a non-invasive measurement system including an ultrasonic Lamb wave transducer as described above. The controller can be configured to control a transmitter and a receiver. In an example of liquid level measurement, it thus controls a) the transmitter so as to transmit primary Lamb waves at different frequencies, b) for each of the different frequencies, the flight time of the pressure wave and / or the amplitude of the received secondary Lamb wave are determined, c) the frequency of the transmitter is changed until the determined flight time reaches a minimum number or until the determined amplitude reaches a maximum number, and d) the liquid level is determined based on the relationship between the measured flight time, the pulse propagation path and the speed of sound in the medium involved. In this liquid level measurement example, the radiation angle between the pressure wave and the horizontal axis is between 0 and 90 degrees.

[0028] In addition, the controller may be connected to other devices to which it sends the measurements. Such devices may be, for example, displays, near field communication devices including client-server architectures, servers, or other peer-to-peer or cloud devices, which may be wirelessly connected using, for example, Bluetooth or mobile communication connections, or may be connected by wires.

[0029] Thus, the controller may comprise a control unit having, for example, a microcontroller, a storage medium, a communication unit, a power circuit, etc. The storage medium may provide memory for measurement data and for a program (eg a computer program or an embedded program) run by the microcontroller.

[0030] According to a further aspect, the ultrasonic Lamb wave transducer can be used for time of flight measurements in a level measurement arrangement such as described above.Further applications include additional non-invasive measurements including, for example, acoustic tomography, where control of the beam angle is beneficial.

[0031] According to another aspect, a method for manufacturing an ultrasonic transducer for non-invasive measurement is provided. The method includes the following steps: providing a piezoelectric ceramic material (e.g., lead zirconate titanate (PZT)) as a base for an ultrasonic actuator for emitting ultrasonic waves; adding a composite material to the piezoelectric ceramic material to form a piezoelectric composite ultrasonic actuator; and embedding the ultrasonic actuator in the main material or attaching it thereto to form an ultrasonic transducer.

[0032] According to an embodiment, the step of adding the composite material to the piezoelectric ceramic material is performed using one of the following techniques:

[0033] Rod placement techniques, dice-filling techniques, ultrasonic cutting, injection molding, lost mold, laser processing, coextrusion, tape lamination, and fiber insertion methods are known to those skilled in the art.

[0034] These and other features, aspects and advantages of the present invention will become better understood with reference to the drawings and following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 A sketch showing non-invasive level measurement of a liquid in a container,

[0036] Figure 2 An example of a piezoelectric composite based on a cube and fill manufacturing technique is shown.

[0037] Figure 3 A flow chart showing a method for manufacturing an ultrasound transducer for non-invasive measurements. DETAILED DESCRIPTION

[0038] Figure 1 An example of non-invasive level measurement of a liquid in a container 104 is shown, performed by a transmitting element 100 mounted on a wedge 116 and a receiving element 102 mounted on a wedge 118, each representing a transducer as described above. The transmitter 100 is placed at a first position outside the container wall and below the level of the liquid surface 106 or 108, respectively. The transmitter 100 transmits an ultrasonic wave as a primary wave into the container wall, so that a portion of the primary Lamb wave leaks from the container wall into the liquid 114 in an oblique and upward direction toward the liquid surface 106, 108, respectively, in the form of a pressure wave 110 or 112. The receiver 102 is also placed at a second position opposite to the transmitter 100, outside the container wall and below the level of the liquid surface 106, 108. The pressure wave is reflected at the surface 106, 108 at a certain reflection point in such a way that the receiver can receive a secondary wave, which is generated by the pressure wave hitting the container wall after being reflected by the liquid surface 106, 108. Figure 1 , the generation of Lamb waves leaking into the liquid at two different angles is illustrated. A first beam 110 is reflected at the surface 106 at a lower liquid level, while a second beam 112 is reflected at the surface 108 at a second liquid level. The liquid level interface is perpendicular to the tank wall, so that the beams are received at the same angle at which they were emitted.

[0039] The controller 120 is connected to the transmitter 100 and the receiver 102. It may further provide an interface to a server or display ( Figure 1(not shown). The interface to the transmitter, receiver, server and / or display may be achieved via a wired or wireless connection.

[0040] Figure 2 An example of a piezoelectric composite based on a cube and fill manufacturing technique is shown, where epoxy resin 202 as the composite material fills the relaxor - Diced pillars of PT (lead titanate) material 206. The PT substrate 204 is removed after filling.

[0041] Figure 3 A flow chart of a method for manufacturing an ultrasonic transducer for non-invasive measurement is shown. In a first step 302, a piezoelectric ceramic material as a substrate for an ultrasonic actuator for emitting ultrasonic waves is provided. In a second step 304, a composite material is added to the piezoelectric ceramic material to form a piezoelectric composite ultrasonic actuator, and in a further step 306, the ultrasonic actuator is embedded in a host material or attached to a host material to form an ultrasonic transducer.

[0042] Other variations of the disclosed embodiments may be understood and implemented by those skilled in the art in implementing the claimed invention by studying the drawings, the present disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may implement the functions of several items or steps described in the claims. The mere fact that certain measures are stated in mutually different dependent claims does not indicate that a combination of these measures cannot be used advantageously. The computer program may be stored / distributed on a suitable medium (e.g., an optical storage medium or solid-state medium supplied together with other hardware or as an integral part of other hardware), but may also be distributed in other forms (e.g., via the Internet or other wired or wireless telecommunications systems). Any reference symbols in the claims should not be understood as limiting the scope of the claims.

Claims

1. An ultrasonic Lamb wave transducer (100, 102) for non-invasive measurement, the ultrasonic Lamb wave transducer (100, 102) being configured to transmit or receive ultrasonic Lamb wave (110, 112) pulses, wherein The emission and reception of the ultrasonic Lamb waves (110, 112) each have a direction, and the pulses are defined by at least one parameter; The ultrasonic Lamb wave transducer (100, 102) includes at least one piezoelectric composite actuator; in, The piezoelectric composite actuator is formed by adding a polymer material to a piezoelectric material for reducing the quality factor and increasing the bandwidth of the transducer; and The piezoelectric composite actuator is configured to be controlled to vary a center frequency within a frequency range provided by the increased bandwidth and to transmit or receive the Lamb wave in different directions according to the varied center frequency.

2. The ultrasonic Lamb wave transducer (100, 102) of claim 1, wherein the at least one pulse parameter is one of: carrier frequency shift; flight time; pulse width; attenuation; pulse amplitude; or a combination thereof.

3. The ultrasonic Lamb wave transducer (100, 102) of claim 1 or 2, wherein the frequency is controlled within a frequency range resulting from properties of the composition of the piezoelectric composite.

4. The ultrasonic Lamb wave transducer (100, 102) according to claim 1 or 2, wherein the piezoelectric composite actuator comprises a polymer as a composite material.

5. The ultrasonic Lamb wave transducer (100, 102) according to claim 1 or 2, wherein a base material of the piezoelectric composite actuator is a piezoelectric ceramic material including lead zirconate titanate (PZT).

6. The ultrasonic Lamb wave transducer (100, 102) of claim 1 or 2, wherein the transducer (100, 102) is wedge-shaped; and wherein the wedge is adapted to hold the piezoelectric composite actuator.

7. The ultrasonic Lamb wave transducer (100, 102) according to claim 1 or 2, wherein the transducer (100, 102) comprises a backing layer and at least one matching layer, wherein at least one side of the backing layer and / or the matching layer has a flat or curved shape.

8. The ultrasonic Lamb wave transducer (100, 102) of claim 1 or 2, wherein the transducer (100, 102) is a transmitter (100) or a receiver (102).

9. A controller (120), the controller (120) being configured to control a non-invasive measurement system, the non-invasive measurement system comprising the ultrasonic Lamb wave transducer (100, 102) for non-invasive measurement according to any one of claims 1 to 8; The controlling includes changing the center frequency within the frequency range provided by the increased bandwidth and transmitting or receiving the Lamb wave in different directions according to the changed center frequency.

10. Use of the ultrasonic Lamb wave transducer (100, 102) according to any one of claims 1 to 8 for liquid level measurement.

11. A method (300) for manufacturing an ultrasonic Lamb wave transducer (100, 102) for non-invasive measurement according to claim 1, the method (300) comprising the following steps: providing (302) a piezoelectric ceramic material as a substrate for an ultrasonic actuator for emitting ultrasonic waves; adding (304) a polymer material to the piezoelectric ceramic material to form the piezoelectric composite ultrasonic actuator, wherein the polymer material is added to reduce a quality factor and increase a bandwidth of the transducer; and The ultrasonic actuator is embedded in or attached (306) to a host material to form the ultrasonic Lamb wave transducer (100, 102).

12. The method of claim 11, wherein the step of adding a composite material to the piezoelectric ceramic material is performed using one of the following techniques: rod placement technique, cube-filling technique, ultrasonic cutting, injection molding, demolding, laser processing, co-extrusion, tape lamination, and fiber insertion method.

Images