DE102021129229B9 - Piezoelectric transducer - Google Patents

Abstract

A piezoelectric transducer (1) for exciting mechanical waves in a structure (11) to be examined is described comprising a piezoelectric actuator (2), the piezoelectric actuator (2) having a plurality of piezoelectric elements (3) which are arranged next to one another along a longitudinal axis (X) of the piezoelectric transducer (1), - a carrier layer (4), the piezoelectric actuator (2) being arranged on an upper side (4a) of the carrier layer (4), - a protective element (5, 15, 25 ), wherein the piezoelectric actuator (2) is completely introduced into the protective element (5), - at least one contact element (6) for the electrical connection of the piezoelectric converter (1), wherein the contact element (6) is at least partially inserted into the protective element (5, 15, 25) is integrated, the respective piezoelectric element (3) having a width (w1), a length (l) and a thickness (t), where w1>>t and l>>w1, and wherein the piezoelectric transducer (1 ) is operated in a longitudinal mode of the respective piezoelectric element (3).

Description

The present invention relates to a piezoelectric transducer. The present invention further relates to a method for monitoring a structure using a piezoelectric transducer.

The use of ultrasonic waves is a common technique for inspecting and monitoring the condition of a structure. Exemplary areas where this is used to detect cracks and defects in structure, as well as to measure mechanical properties and geometric changes, are the aerospace industry, bridge construction, the automotive industry and the oil industry.

• - Anregung reiner Longitudinal- oder Scherwellen, die üblicherweise zum Aufspüren von Rissen und Inhomogenität etc. von Grundmaterialien/Substraten verwendet werden;
• - Anregung von Lamb-Wellen, welche zur Prüfung von platten Strukturen verwendet werden.

Electromagnetic or piezoelectric transducers are used to excite ultrasonic waves, with the latter being the most widespread. Simplistically, the piezoelectric types can be divided into two main categories based on the waves they excite:

• - Excitation of pure longitudinal or shear waves commonly used to detect cracks and inhomogeneity etc. of base materials/substrates;
• - Excitation of Lamb waves, which are used to test flat structures.

Piezoelectric converters and underlying piezoelectric actuators are, for example, in the documents WO 2013/011036 A1 , WO 2008/147891 A1 and EP 2 267 807 A2 described.

The document DE 10 2006 040 316 A1 describes a multilayer board made of a ceramic monolithic body in which positive and negative electrodes are sintered with ceramic plates into a ceramic monolithic body. The surface actuator uses the d33 effect and can generate high levels of strain at low operating voltages.

The document DE 10 2019 206 997 A1 describes ultrasonic transducers in which an electrode is provided on each of the two opposite surfaces of the piezoelectric ceramic plate-shaped elements. The two ultrasonic transducers that form a pair can be laminated into an elastically deformable material.

The document DE 100 51 784 C1 describes a functional module in which a piezoceramic is glued into a shell made of upper and lower carrier foil. Conductor tracks for electrical contacting of the piezoceramic are glued to the inside of the carrier film. The carrier foils each have a connecting lug for connecting external electrical circuits, which is led out of the fiber composite structure.

The document U.S. 2015/0 163 598 A1 describes a piezoelectric vibration module having a piezoelectric element, a flexible wiring board (FPC) as a wiring element connected to one surface of the piezoelectric element, an elastic plate connected to the other surface of the piezoelectric element by an adhesive, and a case member covers the piezoelectric element together with the elastic plate.

The document WO 2017/ 223 219 A1 describes a method for monitoring battery condition, comprising: providing a battery cell, generating a guided wave that propagates in the plane of the battery cell, receiving an incoming wave corresponding to the guided wave, and determining a condition of the battery cell based on the incoming wave .

The present disclosure focuses on applications for examining the condition and/or quality of plate structures in which Lamb waves are excited (e.g. for application in Li-ion automotive traction batteries (isotropic or multilayer structures)).

The aim of the present invention is to describe a piezoelectric transducer which solves the problems prevailing in the prior art.

This object is solved by a piezoelectric transducer according to the independent claims.

According to one aspect, a piezoelectric transducer is described. The piezoelectric transducer is designed to excite mechanical waves in a structure to be examined. The piezoelectric transducer is designed to monitor a condition of the structure or its materials. As a result, a temperature change in the structure can subsequently be inferred. In this case, the temperature cannot be measured directly by the piezoelectric converter. Rather, in this context, changes in material properties and changes in geometry are measured and, from this, temperature/humidity/barometric pressure changes are determined (indirect determination of temperature/humidity/barometric pressure changes). If reference is made below to a temperature measurement, this means the indirect determination of the temperature.

The piezoelectric converter (in the following the term “converter” can also be used for “piezoelectric converter”) has a piezoelectric actuator. The piezoelectric actuator is a composite actuator. A composite actuator has a composition of a piezoelectric material (piezoceramic) and a polymer.

The piezoelectric actuator has a multiplicity of piezoelectric elements. The piezoelectric elements are rectangular. The piezoelectric elements are preferably designed in the form of strips. The piezoelectric elements are preferably piezo strips. The piezoelectric elements are arranged next to one another, in particular one after the other, along a longitudinal axis of the piezoelectric converter or the piezoelectric actuator. In this case, a main direction of expansion (length) of the respective piezoelectric element runs perpendicular to the longitudinal axis of the transducer.

The transducer also has a backing layer. The carrier layer has a thickness of between 1 μm and 200 μm. The carrier layer can be designed to be electrically conductive. The backing may comprise steel, glass, a hard polymer (e.g. epoxy) or a soft polymer (e.g. polyurethane). The piezoelectric actuator is arranged on an upper side of the carrier layer, preferably glued with an adhesive. A thickness of the adhesive layer is as thin as possible.

The carrier layer protects and stabilizes the piezoelectric actuator without excessively increasing the thickness of the transducer (i.e. the extent of the transducer perpendicular to the longitudinal axis). Furthermore, the carrier layer has only a minor influence on the vibration properties of the piezoelectric actuator. A particularly compact, robust and reliable converter can thus be provided, which can even be embedded in small structures consisting of different layers.

The converter also has a protection element. The piezoelectric actuator is fully incorporated into the protective element. The converter is protected from external influences by the protective element. The protective element is designed to seal the converter. This increases the robustness of the converter.

The converter also has at least one contact element for the electrical connection of the piezoelectric converter. The contact element forms an interface to other electronics to which the converter is connected. The contact element is at least partially integrated into the protective element. As a result, the converter is very compact and—owing to the integration of the contact element in the protective element—also less susceptible to external influences. A converter that can be used very flexibly is thus provided.

The converter is very compact. An outer dimension of the transducer is approximately 25mm x 10mm. The respective piezoelectric element has a width w1, a length l and a thickness t. The width of each piezoelectric element extends along the longitudinal axis of the transducer. Length l and thickness t each extend perpendicularly to the longitudinal axis of the transducer. The respective piezoelectric element is dimensioned in such a way that w1>>t and l>>w1. l ≥ 3 x w1 is preferred. w1 ≥ 3*t is preferred.

Due to the special configuration, vibrations of the respective piezoelectric element in undesired directions, and thus interference modes, can be effectively reduced. This enables the greatest possible energy transfer from the piezoelectric element to the structure to be examined and the desired direction.

Furthermore, the converter is designed in such a way that it is operated in a longitudinal mode of the respective piezoelectric element, depending on the polarization. The piezoelectric elements are polarized in the thickness direction.

A main direction of deformation of the respective piezoelectric element consequently extends along the length l of the piezoelectric element and thus perpendicularly to a length of the transducer. The length of the converter is understood to mean an extension of the converter along the longitudinal axis and in particular a main direction of extension of the converter. From this it can be seen that the longitudinal mode of the respective piezoelectric element corresponds to a width mode of the entire piezoelectric transducer. In addition, the piezoelectric element can also be excited in thickness mode.

Due to its special design, the transducer achieves directional radiation of mechanical waves for optimized condition monitoring in a structure while avoiding unwanted reflections from surrounding obstacles/features. Due to the specific design of the piezoelectric actuator, the transducer also has optimized piezoelectric properties for maximum wave excitation and detection. It still has a clear vibration mode in the target area even if the piezoelectric actuator has a small aspect ratio. It becomes independent A clean resonance is achieved depending on the number of piezoelectric elements and thus independently of a length L of the piezoelectric actuator. All in all, a highly efficient, compact, robust, durable, and inexpensive converter is provided.

According to one embodiment, the piezoelectric elements are excited simultaneously. In an alternative embodiment, the piezoelectric elements can also be excited individually. In particular, the piezoelectric elements can be excited with a relative time delay to one another. This allows the excited vibrations to be guided and focused, which makes the converter particularly effective. For this configuration, the piezoelectric elements should have single or grouped electrodes.

Depending on the design of the contact, the piezoelectric elements can be controlled individually (e.g. contacting each piezoelectric element individually) or all together (e.g. contacting via a conductive adhesive or a conductive element). When all piezoelectric elements are driven together, they share a common electrode. If the piezoelectric elements are controlled with a time delay or independently of one another, they have their own signal electrode. However, the ground electrode is shared.

According to one embodiment, the protective element has a polymer. The piezoelectric actuator is embedded in the polymer or encased by the polymer. The converter can thus be protected from external influences in an effective, simple and cost-effective manner. The energy is transferred from the carrier layer into the structure to be monitored. The polymer has dampening properties.

According to one embodiment, the protective element has a housing. The housing surrounds the piezoelectric actuator. The housing can be metal or plastic. The housing reliably protects the piezoelectric actuator from external forces.

According to one embodiment, an interior of the housing has a clearance (cavity) between an outer surface of the piezoelectric actuator and an inner surface of the housing. In particular, the area between the outer surface of the piezoelectric actuator and the inner surface of the housing is free of material. This allows damping effects to be avoided.

As an alternative to this, the free area can also be filled with a foam or a polymer, for example polyurethane, silicone or epoxy. This increases the mechanical strength of the converter. Furthermore, the filling increases the resistance of the transducer to moisture and dust.

According to one exemplary embodiment, a width B of the piezoelectric actuator 2 corresponds to the length l of the respective piezoelectric element, in other words B=l. The width B defines an excitation frequency of the piezoelectric actuator. Preferably, 4mm≦B≦20mm.

A thickness t of the piezoelectric actuator corresponds to the thickness t of the respective piezoelectric element. Preferably t < 2mm. Furthermore, a distance A between two adjacent piezoelectric elements is preferably ≦1/10 of the length l of the respective piezoelectric element 3 (A≦1/10). This provides a very compact converter that can be used flexibly and that is even suitable for very cramped installation situations.

According to one embodiment, the contact element is flexible. The contact element has flexible electronics (so-called “flex circuit” or flex cable or flex printed circuit board) or a UHF (Ultra High Frequency) coaxial cable. As an alternative to this, the contact element can also have two wires.

The piezoelectric actuator has at least one upper electrode (signal electrode) and at least one lower electrode (ground electrode). The actuator can also have a plurality of top electrodes. In this case, each piezoelectric element of the actuator has a top electrode.

The contact element is electrically conductively connected to the upper electrode. The contact element is non-detachably connected to the converter. The contact element is detachably connected to external electronics for the electrical connection of the converter.

When all piezoelectric elements are driven together, they share a common electrode (i.e. the piezoelectric actuator has exactly one top electrode and exactly one bottom electrode). If the piezoelectric elements are driven with a time delay or independently of one another, they have their own signal electrode/upper electrode. That is, the piezoelectric actuator has a plurality of upper electrodes. However, the bottom electrode (ground electrode) is shared.

The contact element is designed to be very space-saving. If a flex cable is used, its thickness is preferably less than 200 μm. With a UHF coaxial cable, a diameter of ≤ 2 mm, for example 1 mm, can be expected. An integration of the converter is thus possible even in very tight spaces.

According to a second aspect, a piezoelectric transducer is described. The piezoelectric transducer is designed to excite mechanical waves in a structure to be examined. The piezoelectric transducer is designed to monitor a condition of the structure and/or to monitor temperature effects across the structure or its materials. All properties and advantageous embodiments that are disclosed in relation to the piezoelectric transducer according to the first aspect are also disclosed accordingly for the transducer according to the second aspect, even if the respective property is not explicitly mentioned in the context of the second aspect.

The piezoelectric converter has a piezoelectric actuator. The piezoelectric actuator is a composite actuator. The piezoelectric actuator has a piezo plate. The piezo plate is preferably rectangular. A width B of the piezo plate is consequently smaller than a length L of the piezo plate. The width of the piezo plate extends perpendicular to the longitudinal axis X of the transducer.

The piezoelectric converter also has a carrier layer. The carrier layer has a thickness of between 1 μm and 200 μm. The carrier layer can be electrically conductive or electrically insulating. The backing may comprise steel, glass, a hard polymer (e.g. epoxy) or a soft polymer (e.g. polyurethane). The piezoelectric actuator is arranged on an upper side of the carrier layer, preferably glued with an adhesive. A thickness of the adhesive layer is designed to be as thin as possible.

The piezoelectric converter also has at least one protective element. The piezoelectric actuator is fully incorporated into the protective element. The converter is protected from external influences by the protective element. The protective element is designed to seal the converter. This increases the robustness of the converter.

The piezoelectric converter also has at least one contact element for the electrical connection of the piezoelectric converter. The contact element forms an interface to other electronics to which the converter is connected. The contact element is at least partially integrated into the protective element. As a result, the converter is very compact and—owing to the integration of the contact element in the protective element—also less susceptible to external influences. A converter that can be used very flexibly is thus provided.

The piezoelectric converter is very compact. An outer dimension of the transducer is approximately 25mm x 10mm. The piezoelectric transducer is operated in a width mode of the piezo plate. A main direction of deformation of the piezo plate consequently extends along the width of the piezo plate and thus perpendicular to a length of the transducer.

Consequently, the width mode of the piezo plate corresponds to a width mode of the entire piezoelectric transducer. The width mode can be influenced by harmonics of the length mode of the piezo plate. In addition, the piezo plate can also be excited in thickness mode. The piezo plate is preferably polarized in the direction of the thickness.

Due to its special design, the transducer achieves directional radiation of mechanical waves for optimized condition monitoring in a structure while avoiding unwanted reflections from surrounding obstacles/features. Due to the specific design of the piezoelectric actuator, the transducer also has optimized piezoelectric properties for maximum wave excitation and detection. It still has a clear vibration mode in the target area even if the piezoelectric actuator has a small aspect ratio. A clean resonance is achieved regardless of the length L of the piezoelectric actuator. All in all, a highly efficient, compact, durable, and inexpensive converter is provided.

The drawings described below are not to be taken as true to scale. Rather, individual dimensions can be enlarged, reduced or also distorted for better representation.

Elements that are the same or that perform the same function are denoted by the same reference symbols.

• 1 eine Seitenansicht eines piezoelektrischen Wandlers gemäß einem ersten Ausführungsbeispiel,
• 2 eine Seitenansicht eines Teils des piezoelektrischen Wandlers gemäß 1,
• 3 eine Seitenansicht eines piezoelektrischen Wandlers gemäß einem zweiten Ausführungsbeispiel,
• 4 eine perspektivische Ansicht eines Teils des piezoelektrischen Wandlers,
• 5 eine Draufsicht auf einen Teil des piezoelektrischen Wandlers,
• 6 eine Draufsicht auf einen Teil des piezoelektrischen Wandlers inklusive elektrischer Kontaktierung,
• 7 eine Draufsicht auf ein System zur Überwachung einer Struktur,
• 8a eine Seitenansicht des Systems gemäß 7,
• 8b eine Draufsicht auf einen Teilbereich des Systems gemäß 7
• 9 eine Seitenansicht eines piezoelektrischen Wandlers gemäß einem weiteren Ausführungsbeispiel,
• 10 eine perspektivische Ansicht eines Teils des piezoelektrischen Wandlers gemäß 9.

Show it:

• 1 a side view of a piezoelectric transducer according to a first embodiment,
• 2 a side view of a part of the piezoelectric transducer according to FIG 1 ,
• 3 a side view of a piezoelectric transducer according to a second embodiment,
• 4 a perspective view of a part of the piezoelectric transducer,
• 5 a plan view of part of the piezoelectric transducer,
• 6 a top view of part of the piezoelectric transducer including electrical contacting,
• 7 a plan view of a system for monitoring a structure,
• 8a a side view of the system according to FIG 7 ,
• 8b a plan view of a portion of the system according to FIG 7
• 9 a side view of a piezoelectric transducer according to a further embodiment,
• 10 a perspective view of a part of the piezoelectric transducer according to FIG 9 .

the 1 shows a side view of a piezoelectric transducer 1 according to a first embodiment. The piezoelectric converter 1 is designed to excite mechanical waves (Lamb waves) in a structure. In particular, the piezoelectric transducer 1 (hereinafter transducer 1) is designed to be integrated into applications for investigating the condition, temperature effects and/or the quality of plate or plate-like structures. It should be noted that the temperature is not measured directly. Rather, changes in material properties and changes in geometry are measured and from these the temperature/humidity/air pressure changes can be determined. In addition, the converter 1 enables the investigation of SOX from lithium-ion cells. For example, the converter 1 is suitable for use in a Li-ion car traction battery.

The converter 1 has a piezoelectric actuator 2 (see also 4 ). The piezoelectric actuator 2 is rectangular. In other words, the piezoelectric actuator 2 has a length L that is larger than a width B of the piezoelectric actuator 2 ( 4 ). The width B defines the optimal excitation frequency of the transducer 1.

The piezoelectric actuator 2 is a composite actuator. The piezoelectric actuator 2 has a multiplicity of piezoelectric elements 3 which are arranged along a longitudinal axis X of the piezoelectric actuator 2 .

The length L of the piezoelectric actuator 2 depends on the number of piezoelectric elements 3.

The piezoelectric elements 3 are in the form of strips (piezo strips). The piezoelectric elements 3 are rectangular. A length l of the respective piezoelectric element 3 is consequently greater than a width w1 of the respective piezoelectric element 3 ( 4 ). Preferably l ≥ 3 x w1.

The length l of the respective piezoelectric element 3 corresponds to the width B of the piezoelectric actuator 2 (l=B), like that 4 can be removed. A distance A between two consecutive piezoelectric elements 3 (see 4 ) is a maximum of 1/10 of the length l of the respective piezoelectric element 3.

The respective piezoelectric element 3 also has a thickness t. The thickness t of the piezoelectric elements 3 corresponds to the thickness of the complete piezoelectric actuator 2 and is <2 mm. The thickness t of the respective piezoelectric element 3 is preferably at most one third of its width w1, in other words w1≧3×t. Overall, l>>w1 and w1>>t applies to the converter 1.

The converter 1 is operated in the so-called longitudinal mode of the respective piezoelectric element 3 . This means a main direction of deformation D (see 4 ) or a targeted vibration direction V of the piezoelectric actuator 2 extends along the longitudinal extension (along the length l) of the piezoelectric elements 3, such as 5 and 6 can be removed. In relation to the converter 1, the converter 1 is thus operated in a width mode of the converter 1.

In summary, due to its specific design (composite structure, l >> w1 and w1 >> t), the piezoelectric actuator 2 enables the emission of energy essentially in a direction transverse to the longitudinal axis X of the transducer 1, i.e. along the longitudinal extent of the piezoelectric elements 3. A clean resonance is achieved regardless of the number of piezoelectric elements 3 . In other words, regardless of the length L of the piezoelectric actuator 2, clean resonance is obtained.

The converter 1 also has a carrier layer 4 . The carrier layer 4 has an upper side 4a and an underside 4b. The underside 4b is that outer surface of the carrier layer 4 which faces the structure to be examined or is attached to it.

The carrier layer 4 serves as protection and/or carrier for the piezoelectric actuator 2. The piezoelectric actuator 2 is arranged on the upper side 4a of the carrier layer 4 and is fastened by means of an adhesive 7 (adhesive layer 7).

The adhesive 7 can either only be arranged between the carrier layer 4 and the piezoelectric actuator 2 (see 2 ), Or (alternatively or additionally) between the individual piezoelectric elements 3, as from the 1 emerges. The adhesive can be arranged between the carrier layer 4 and the piezoelectric actuator 2 and additionally also between the individual piezoelectric elements 3 .

A thickness of the carrier layer 4 is in the range from 20 μm to 200 μm, the limits included. The thickness of the backing layer 4 depends on a material of the backing layer. The carrier layer 4 can have hard materials such as steel and glass. Alternatively, the backing may comprise a hard polymer (epoxy) or a soft polymer (polyurethane). The adhesive layer 7 between the piezoelectric actuator 2 and the support layer 4 is as rigid and thin as possible.

The converter 1 also has a protective element 5 . The protective element 5 serves to protect the piezoelectric actuator 1 from moisture, dust or other external influences. The protective element 5 stabilizes and seals the transducer 1. The piezoelectric actuator 2 is completely embedded in the protective element 5,15. Furthermore, the protective element also covers the entire upper side 4a of the carrier layer 4.

Electrodes 8, 9 are formed on a top side 3a and on a bottom side 3b of the piezoelectric elements 3 for the electrical connection of the piezoelectric actuator 2 (see in particular the 4 and 5 ). The electrical contacting of the electrodes 9 on the underside 3b of the piezoelectric elements 3 (ground electrodes 9) can take place either via a winding electrode (so-called wrap-up electrode) either on the long or short sides of the piezoelectric elements 3. As an alternative to this, the ground electrode 8 can also be contacted via the carrier layer 4 . In this case, the carrier layer 4 is designed to be electrically conductive.

The converter 1 also has a contact element 6 for the electrical connection of the converter 1 . The contact element 6 is at least partially embedded in the protective element 5, as shown in FIG 1 is evident. The contact element 6 is designed and arranged to contact the electrodes 8 on the upper side 3a of the piezoelectric elements 3, such as from FIGS 1 , 3 and 6 emerges.

All the piezoelectric elements 3 can be connected in parallel by means of the electrodes 8 on the upper side 3a of the piezoelectric elements 3 (signal electrode 8), so that the piezoelectric elements 3 are excited simultaneously. The electrodes 8 can be connected by means of a conductive element 13 (see 6 ) are electrically contacted together by the contact element 6 and thus excited at the same time.

As an alternative to this, the piezoelectric elements 3 can also be electrically connected individually if an individual excitation of each piezoelectric element 3 is desired. In this way, the individual piezoelectric elements 3 can also be excited with a time delay in relation to one another. In this way, the excited waves can be guided and focused, which makes the converter 1 particularly effective. In this case the in 6 illustrated conductive element 13 omitted.

The contact element 6 can, for example, be flexible electronics/a flex cable (so-called flex circuit), as in 6 shown, a UHF coaxial cable, or two wires. The dimensions of the contact element 6 vary depending on the type used. If a flex cable is used, its thickness is preferably less than 200 µm. With a UHF coaxial cable, a diameter of ≤ 2 mm, for example 1 mm, can be expected. Alternative solutions have different dimensions.

The contact element 6 forms an interface to further electronics. The contact element 6 is fixed to the converter 1 and cannot be detached from the converter 1 . The contact element 6 can be detached or unplugged from the additional electronics that are located outside of the converter 1 . The transducer 1 can also be operated at the thickness resonance frequency (above 1 MHz depending on the thickness t of the piezoelectric elements 3) in order to generate predominantly longitudinal waves at the surface of the structure to be examined.

the 3 shows a side view of a piezoelectric transducer 1, transducer 1 for short, according to a second embodiment. In contrast to the converter 1 according to 1 indicates the converter 1 in 3 a housing 25 as a protective element 5. With regard to all other features of the transducer 1, in particular the piezoelectric actuator 2 with the piezoelectric elements 3, the carrier layer 4 and the contact element 6, reference is made to the above description.

The housing 25 is particularly useful in order to protect the piezoelectric actuator 2 from external influences. In particular, the housing 25 is designed and arranged to avoid mechanical loading of the piezoelectric elements 3 and a short circuit of the electrodes 8, 9 of the transducer 1.

The housing 25 may comprise metal or plastic, for example. The housing 25 is arranged on the upper side 4a of the carrier layer 4. FIG. The housing 25 has an interior area in which the piezoelectric actuator 2 is arranged. The housing 25 completely encloses the piezoelectric actuator. Furthermore, at least a partial area of the contact element 6 is enclosed by the housing 25 . This means that the contact element 6 is at least partially integrated into the housing 25 .

The inner area of the housing 25 has a free area 10 between an outer surface 2a of the piezoelectric actuator 2 and an inner surface 25a of the housing 25 . In other words, the top and side surfaces of the piezoelectric actuator 2 are spaced from the inner surface 25a of the housing. Preferably, the free area 10 is empty, in other words it is free of any material. In this way, damping effects caused by a material located in the free area 10 can be prevented.

As an alternative to this, the free area 10 can be filled with a polymer or a foam (not explicitly shown). This increases the mechanical robustness of the converter 1 and serves as additional protection against moisture and/or dust. The polymer may comprise a soft material such as silicone or polyurethane. As an alternative to this, the polymer can also have an epoxide.

A thickness of the converter 1 including the carrier layer 4 and the protective element 5 is less than or equal to 3 mm. The converter 1 is thus designed to be particularly compact. This predestines the converter 1 for integration in particularly small spaces, in particular also in structures that consist of different layers.

the 7 , 8a and 8b show a system for monitoring a structure 11 and an associated monitoring method. The system has two converters 1, E, R, as described above, with the contact elements 6, which are used for the electrical connection of the converters 1, E, R. A first converter 1, E is used to convert electrical energy into mechanical energy. This means that the piezoelectric elements 3 of the first transducer 1, E are excited to oscillate, as a result of which mechanical waves are generated in the structure 11. A second converter 1, R is used to convert mechanical energy into electrical energy. This means that the mechanical waves generated in the structure 11 in turn excite the piezoelectric elements 3 of the second electrical converter 1, R to oscillate.

A surface to which the transducer 1 is attached is flat. This flat or even surface can belong directly to the structure 11 to be monitored. Alternatively, and if the structure 11 to be monitored is curved, an adapter can be added between the transducer 1 and the structure 11 to bridge the geometry deviation.

The length/width/thickness ratio of the structure 11 should be chosen such that Lamb waves are excited. The structure 11 has a top 11a and a bottom 11b. To monitor the structure 11, the first transducer 1, E and the second transducer 1, R are attached to a surface (top side 11a here) using an attachment means 12 ( 8a) . The fastener 12 is, for example, a hard epoxy to ensure good strain transmission from the piezoelectric elements 3 to the structure 11 and vice versa. If the converters 1, E, R are to be removed again, a (for example with acetone) removable superglue can also be used as the fastening means 12, for example.

The transducers 1, E, R are mounted on the surface at a specific distance from each other. The distance between the two transducers 1, E, R depends on an excitation frequency of the respective transducer 1, E, R and on a thickness and/or material properties of the structure 11 to be monitored.

If the transducers 1, E, R are attached, the piezoelectric elements 3 of the first transducer 1, E are vibrated in the direction of vibration V ( 8b) stimulated. As already described above, the main direction of deformation of the piezoelectric elements 3 extends along their length. As described above, the transducer 1, E generates mechanical waves and in particular a directed acoustic field F in the structure 11 ( 7 ).

The mechanical waves running through the structure 11 are received by the second piezoelectric transducer 1, R, as a result of which it is also excited to oscillate/vibrate. The nature of the mechanical waves received by the second transducer 1, R is dependent on the properties of the structure 11. The signals received by the second transducer 1, R are then evaluated, to monitor the condition, quality and/or temperature of the structure 11.

the 9 shows a side view of a piezoelectric transducer 1 according to a further embodiment. As already described in connection with the previous figures, the piezoelectric converter 1 is designed to excite mechanical waves (Lamb waves) in a structure. In particular, the piezoelectric transducer 1 (hereinafter transducer 1) is designed to be integrated into applications for investigating the condition, temperature effects and/or the quality of plate or plate-like structures.

The converter 1 has a piezoelectric actuator 2 , a carrier layer 4 , a protective element 5 , 15 , 25 , an upper electrode 8 , a lower electrode 9 and a contact element 6 . With regard to the properties and functions of the corresponding components, in particular of carrier layer 4, contact element 6, electrodes 8, 9 and protective element 5, 15, 25, reference is made to the above statements.

In contrast to the converter 1 according to the above explanations, the piezoelectric actuator 2 does not have a large number of piezoelectric strips, but rather a piezo plate 20 (see also 10 ). The piezoelectric actuator 2 or the piezo plate 20 is rectangular. In other words, the piezo plate 20 has a length L that is greater than a width B of the piezo plate 20. The width B defines the optimal excitation frequency of the transducer 1. The piezo plate 20 also has a thickness t. The thickness t is preferably <2 mm.

The converter 1 is operated in the width mode of the piezo plate 20 . This means a main direction of deformation D (see 10 ) of the piezoelectric actuator 2 extends along the width B of the actuator 2. Referring to the transducer 1, the transducer 1 is consequently also operated in a width mode.

The transducer 1 is designed to be fixed, preferably glued, to a flat surface for monitoring a structure 11, as already discussed in connection with FIGS 1 until 8b was described.

The description of the objects given here is not limited to each specific embodiment. Rather, the features of the individual embodiments can be combined with one another as desired--insofar as this is technically reasonable.

reference list

1

Piezoelectric transducer

2

Piezoelectric Actuator

2a

Outer surface of the piezoelectric actuator

3

piezoelectric element

3a

Top of the piezoelectric element

3b

Bottom of the piezoelectric element

4

backing layer

4a

top of the backing layer

4b

underside of the backing

5

protective element

6

contact element

7

Adhesive / adhesive layer

8th

electrode / signal electrode

9

electrode / ground electrode

10

free area

11

structure

11a

top

11b

bottom

12

fasteners

13

conductive element

15

polymer

20

piezo plate

25

Housing

25a

inner surface of the housing

X

longitudinal axis

D

principal direction of deformation

l

Length of the piezoelectric element

L

Length of the piezoelectric actuator

w1

Width of the piezoelectric element

B

Width of the piezoelectric actuator

t

Thickness of the piezoelectric element/actuator

A

Distance between two adjacent piezoelectric elements

f

acoustic field

E

First piezoelectric transducer

R

Second piezoelectric transducer

V

Target vibration direction

Claims (20)

Piezoelectric transducer (1) for exciting mechanical waves in a structure (11) to be examined, having: - A piezoelectric actuator (2), the piezoelectric actuator (2) having a plurality of piezoelectric elements (3) arranged side by side along a longitudinal axis (X) of the piezoelectric transducer (1), the piezoelectric elements (3) having a relative time delay to each other are excited, - a carrier layer (4), the carrier layer (4) being designed to be electrically conductive and the piezoelectric actuator (2) being arranged on an upper side (4a) of the carrier layer (4), - a protective element (5, 15, 25), the piezoelectric actuator (2) being incorporated completely in the protective element (5), - at least one contact element (6) for the electrical connection of the piezoelectric transducer (1), the contact element (6) being at least partially integrated into the protective element (5, 15, 25), the contact element (6) being flexible and at least partially protrudes from the protective element (5, 15, 25), the respective piezoelectric element (3) having a width (w1), a length (l) and a thickness (t), where w1 >> t and l >> w1, and wherein the piezoelectric transducer (1) is operated in a longitudinal mode of the respective piezoelectric element (3). Piezoelectric converter (1) according to claim 1 , wherein the piezoelectric elements (3) are formed in strips. Piezoelectric converter (1) according to claim 1 or 2 , wherein a main deformation direction (D) of the respective piezoelectric element (3) extends along the length (l) of the piezoelectric element (3). Piezoelectric transducer (1) according to one of the preceding claims, in which the protective element (5, 15) comprises a polymer (15), and in which the piezoelectric actuator (2) is completely embedded in the polymer (15). Piezoelectric transducer (1) according to one of Claims 1 until 3 , wherein the protective element (5, 25) has a housing (25), the housing (25) completely surrounding the piezoelectric actuator (2). Piezoelectric converter (1) according to claim 5 wherein an inner area of the housing (25) has a free area (10) between an outer surface (2a) of the piezoelectric actuator (2) and an inner surface (25a) of the housing (25). Piezoelectric converter (1) according to claim 6 , wherein the free area (10) is filled with a polymer or a foam. Piezoelectric transducer (1) according to one of the preceding claims, in which the piezoelectric actuator (2) is fixed to the upper side (4a) of the carrier layer (4) with an adhesive (7). Piezoelectric transducer (1) according to one of the preceding claims, in which the carrier layer (4) has a thickness of between 1 µm and 200 µm. Piezoelectric transducer (1) according to one of the preceding claims, wherein a thickness (t) of the piezoelectric actuator (2) corresponds to the thickness (t) of the respective piezoelectric element (3), and wherein t < 1.5 mm. Piezoelectric transducer (1) according to one of the preceding claims, wherein a distance (A) between two adjacent piezoelectric elements (3) is ≤ 1/10 of the length (l) of the respective piezoelectric element (3). Piezoelectric transducer (1) according to one of the preceding claims, wherein the contact element (6) has at least one wire, a flex circuit board or a UHF coaxial cable. Piezoelectric transducer (1) according to one of the preceding claims, wherein the piezoelectric actuator (2) has at least one upper electrode (8) and at least one lower electrode (9) and wherein the contact element (6) with the at least one upper electrode (8) is electrically conductively connected. Piezoelectric transducer (1) for exciting mechanical waves in a structure (11) to be examined, having: - a piezoelectric actuator (2), the piezoelectric actuator (2) having a rectangular piezo plate (20), - a carrier layer (4), the carrier layer (4) being designed to be electrically conductive and the piezoelectric actuator (2) being arranged on an upper side (4a) of the carrier layer (4), - a protective element (5, 15, 25), the piezoelectric actuator (2) being completely introduced into the protective element (5), - at least one contact element (6) for the electrical connection of the piezoelectric transducer (1), the contact element (6) being at least partially integrated into the protective element (5, 15, 25), the contact element (6) being flexible and at least partially protrudes from the protective element (5, 15, 25), the piezoelectric converter (1) being operated in a width mode of the piezoelectric plate (20). Piezoelectric converter (1) according to Claim 14 , wherein the piezoelectric actuator (2) is arranged on the upper side (4a) of the carrier layer (4) and is fastened by means of an adhesive (7). Method for monitoring a structure, comprising the following steps: A) providing a flat structure (11); B) providing a first piezoelectric transducer (1, E) and a second piezoelectric transducer (1, R) according to any one of the preceding claims; C) arranging the piezoelectric transducers (1, E, R) on a surface of the structure (11) and fastening the piezoelectric transducers (1, E, R) on the surface by a fastening means (12), D) excitation of mechanical waves in the structure (11) by the first piezoelectric transducer (1, E) to generate a directed acoustic field (F); E) reception of the mechanical waves running through the structure (11) by the second piezoelectric converter (1, R); F) Evaluation of the waves received by the second piezoelectric transducer (1, R). procedure after Claim 16 , wherein a distance between the two piezoelectric transducers (1, E, R) on the surface depends on an excitation frequency of the respective piezoelectric transducer (1, E, R) and on a thickness and/or material properties of the structure (11) to be monitored . procedure after Claim 16 or 17 wherein the attachment means (12) comprises a permanent or a removable adhesive. Use of the piezoelectric transducer (1) according to one of Claims 1 until 15 for monitoring a condition of the structure (11) and/or for monitoring temperature effects on the structure (11) or its materials. Use of the piezoelectric transducer (1) according to one of Claims 1 until 15 for monitoring a condition and/or quality of Li-ion traction batteries.

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