DE102007037088A1 - Ultrasound transducer for use in intake and exhaust system of automobile internal combustion engine, has intermediate layer between piezoelectric transducer unit and adjustment layer for reducing thermal stresses between layer and unit - Google Patents

Abstract

The transducer (112) has a piezoelectric transducer unit (216), and an adapter (218) with an adjustment layer (220) for facilitating a vibration coupling between the unit (216) and an ambient fluid medium, where the layer is made up of epoxy resin, polyester, phenolic resin, thermoplastic resin, elastomer e.g. silicon, polymer blend, or polyamide e.g. kapton(RTM: Polyimide film). An intermediate layer (310) is provided between the unit (216) and the layer (220) for reducing thermal stresses between the layer (220) and the unit (216), where the layer (310) includes a fiber material (312). Independent claims are also included for the following: (1) a flow measurement system including ultrasound transducer (2) a method for manufacturing an ultrasound transducer.

Description

State of the art

The invention is based on known ultrasonic transducers which are used, for example, in ultrasonic flowmeters in process technology or in the automotive sector, in particular in the intake and / or exhaust tract of internal combustion engines, for volume flow or mass flow measurement. In this case, ultrasound transducers are typically used which can both emit ultrasonic waves into a fluid medium (a gas and / or a liquid) and can receive ultrasonic waves. Usually, ultrasonic signals are transmitted by the flowing fluid medium from an emitter to a receiver, and the transit time, transit time differences or phases of the ultrasonic signals or combinations of these measured variables are measured. These signals are influenced by the flow of the fluid. From the degree of influence of the running time can be concluded on the flow velocity of the fluid. An example of such ultrasonic flowmeters that can be used in various measuring arrangements is in DE 10 2004 060 064 A1 described.

at the ultrasonic flow measurement is one non-contact, fast and at least partially over the flow cross-section integrating measuring method. About that In addition, in contrast to the thermal measuring method no filigree and sensitive heating structures within the flow needed. These basic benefits can Depending on the conditions of use, this results in improved measuring accuracy.

One Disadvantage of many known ultrasonic flowmeter is however at least for gaseous media in the small signal swing, d. H. z. B. the low runtime changes that the Process at low flow rates often very Drift-prone - a mistake that often happens is called "zero flow error".

To make matters worse, the vibration energy generated by a conventional ultrasound generator (eg a piezoceramic) has to overcome a high acoustic impedance difference (approximately a factor of 6 × 10 ⁵ ) when coupled into the medium to be measured. As a result, about 99.9995% of the sound energy is usually reflected back on the way from a piezoceramic in air at the corresponding interface and can not be used for the measurement. The same reflection loss occurs again at the second receiving transducer (which may also be identical to the first transducer).

Around the acoustic coupling between the piezo element and the to be measured To improve fluid, are usually measures used for impedance matching. For example, can be between the piezoelectric element and the fluid one or more matching layers to introduce impedance matching, which have acoustic impedances which are between those of the piezoelectric material and the fluid. However, this limits the possible approaches strong for the construction of the ultrasonic transducer. For example, membranes or λ / 4 layers can be used for impedance matching insert, glued to the usually thin piezoelectric element is. At least in the case of a membrane, however, the resonance frequency of the transducer rather determined by the membrane geometry than by the piezo element alone.

Also other types of matching layers are known which affect the piezoelectric element be applied. A large-scale production of air ultrasonic transducers according to the well-known principles, however, involves elaborate and expensive process steps. In particular, in many cases a machining of the matching layer required what ever according to material type leads to rough surfaces, the an acoustically advantageous thin and reproducible bond difficult. Even with a production of the matching layer without cutting Processing (eg in a casting / injection process) remains the need for a separate bonding process.

Further Restrictions on the ultrasonic frequency to be used and the transducer design arise for known ultrasonic transducers physical effects such as sound field formation in the fluid, absorption and dispersion.

additionally These design restrictions may be out of the ordinary Target tolerances for a flow measurement in the intake area to be derived from car engines the requirement that the oscillating Transient response of the ultrasonic transducer, at least in the sense of Differential transit time measurement purely mechanically always approximately exactly remains the same, with an accuracy of for example about 1/1000 of a single mechanical vibration. Go in this mechanical transient course in addition to transducer basic design and transducer geometry also critical the internal material properties and compounds of the materials with each other. This makes it difficult the usual in a thermal flow measurement To achieve or even improve tolerances.

The matching layer is usually glued as a prefabricated part with an adhesive on the piezoelectric transducer or on the electrode or a matching layer material is in un crosslinked state brought into contact with the piezoelectric transducer, so that a liability between the piezoelectric element and matching layer is not adjusted by a separate adhesive, but after curing by the molecules of the matching layer itself.

Around a favorable acoustic coupling between the piezoelectric element and the fluid medium, in particular air, the acoustic impedance of the matching layer (or the acoustic impedances the matching layers) are in a range that is common to many fabrics can only be achieved by a density reduction. As a material for the matching layer is, for example, epoxy resins, unsaturated Polyester or BMC compounds (BMC: bulk molding compound, a fiber-matrix semifinished product, which mostly short glass fibers and a polyester or vinyl ester resin or other fiber / resin systems) with glass hollow sphere admixtures or foamed, sintered or otherwise voided Plastics in question. Here, thermal stresses should be at the transition between piezo material and matching layer can be avoided. Especially at high thermal loads and temperature shock changes, which occur in the automotive industry, can in this transition area high mechanical stresses occur, causing aging effects or even failure of the ultrasonic transducer could.

adversely in known systems, therefore, it is not only subject to severe design restrictions but that too the signal quality and the function tolerances in many cases potential for further improvements exhibit. The production of such elements is usually expensive and expensive. In addition, in particular, the thermal load capacity and the aging behavior room for further improvement.

Disclosure of the invention

It Therefore, an ultrasonic transducer, a flow measuring system, which comprises this ultrasonic transducer and a method for Production of an ultrasonic transducer proposed which the Disadvantages of known systems and methods at least largely avoids. In particular, the ultrasonic transducer is more stable thermal loads, which makes it suitable for automotive applications, especially under engine mounting conditions, is particularly suitable. In addition, the proposed ultrasonic transducer has low functional tolerances regarding ultrasonic flow measurements. Such an ultrasonic transducer and such a flow measuring system Thus, in particular, as a more robust replacement of today's usual thermal Be used air mass meter system, or even at engine control technically more favorable Locations are used, to which the usual thermal Measuring principle reaches its limits (for example after a charge air cooler or in a cooled Exhaust gas recirculation line).

Of the proposed ultrasonic transducer comprises a piezoelectric Transducer element and at least one matching body with a Adaptation layer to favor a vibration coupling between the piezoelectric transducer element and a surrounding one fluid medium. The term of the piezoelectric transducer element is to be understood and includes, for example, electric-acoustic Transducers, which after ferroelectric, electrostatic, magnetostrictive, magnetoelectric effects or combinations of these effects work can.

Between the piezoelectric transducer element and the matching layer is at least an intermediate layer for reducing thermal stresses between the matching layer and the piezoelectric transducer element. According to the invention, this intermediate layer comprises at least one fiber material. Under an intermediate layer is here to understand a layer which is at least partially not identical is identical to that with the matching layer or layers of the piezoelectric transducer element, so a distinguishable layer.

there the intermediate layer may in particular have a modulus of elasticity (E-modulus), which basically in the entire Area between the modulus of elasticity of the piezoelectric transducer element and the modulus of elasticity of the matching layer can be settled. Also about it Exceeding E-modules are possible. This is due to that the intermediate layer usually much thinner is considered the acoustic wavelength, so the acoustics through the choice of modulus of elasticity is not adversely affected becomes. Only a higher flexibility of the intermediate layer compared to the matching layer can have a negative impact, because in this case the acoustic coupling would be unfavorable.

More important than a suitable choice of the modulus of elasticity of the intermediate layer may be the selection of the thermal expansion coefficient (often referred to as α) of the interlayer material. The thermal expansion coefficient, which is often reported in ppm / ° K, should preferably be closer to the corresponding coefficient of thermal expansion of the piezoelectric transducer material (which is typically about 7 ppm / ° K) than that of the matching layer (which typically is in the range of> 30 ppm / ° K). For example, materials which have a coefficient of thermal expansion of less can be used for the intermediate layer than 20 ppm / ° K, and more preferably less than 15 ppm / ° K.

The Fiber material may be loose fibers, a fiber braid, a fiber composite material or similar fiber materials or fiber material combinations. The fiber material may be an organic fiber material, a carbon fiber material, a boron-containing fiber material, in particular boron fibers, a metal-containing Fiber material, in particular metal fibers, an inorganic fiber material, in particular in Polkristalliner or amorphous form, in particular a glass fiber material or a ceramic fiber material, or combinations this or other materials. Other materials or Material combinations are conceivable.

Next the fiber material, the intermediate layer may continue at least have a matrix material. The matrix material can in particular one or more of the following materials: a resin, in particular an epoxy resin; a polyester; a BMC material (BMC: bulk molding compound); an adhesive; a thermoplastic polymer; a thermosetting polymer. In this case, this matrix material preferably also be at least partially identical with at least one material of the matching body, in particular the matching layer. So can The matrix material itself, for example, also a boundary layer of the matching body, in particular the matching layer, which is adjacent to the piezoelectric transducer element. In This boundary layer can embed the fiber material described above become.

The Matching layer can basically also those described above Materials of the matrix material include. In particular, it is preferred if the matching layer is a thermally, chemically or photochemically curable or crosslinkable polymeric material. This polymer material can for example, a syntactic material, a thermoset material (In particular, an epoxy resin, a polyester, a phenolic resin or a cyanate ester), a thermoplastic, an elastomer (especially Silicone), a polymer blend, a polyimide (especially Kapton), or a mixture of said and / or other materials. Besides can, as described above, this polymer material continue have at least one filler, in particular at least one of the following fillers: cavities (for Example, cavities created by foaming), fibers, Hollow glass balls, hollow plastic balls, a volatile Filler (especially one during or after crosslinking of the polymer escaping or removable filler) or combinations of said and / or other filler materials include.

In terms of The geometric design of the intermediate layer are numerous advantageous options. Especially preferred it is when the intermediate layer is the piezoelectric transducer element at least partially encloses. For example, the Intermediate layer next to an end face of the piezoelectric Transducer element also side surfaces of the same or partially contact or enclose.

Farther The intermediate layer may also, in addition to the properties of reduction thermal stresses by using the fiber material, others have advantageous properties. So it is especially preferable when the intermediate layer is at least partially electrically conductive Features. In this case, in particular the intermediate layer, which for generating the conductivity, for example, a Conductive adhesive, metal particles, carbon particles, a conductive Plastic and / or other materials or combinations of said May include materials, in particular for contacting a or multiple electrodes of the piezoelectric transducer element used become.

to Connection of the intermediate layer with the piezoelectric transducer element can be, in addition to cohesive compounds (for example, by Gluing) and / or positive connections (for example by the enclosing of the piezoelectric described above Wanderelements through the intermediate layer) advantageous, alternatively or additionally also positive-locking clamp connections Insert with the piezoelectric transducer element. This training The invention is particularly preferred in connection with a conductive intermediate layer and the possibility described above the contacting of at least one electrode of the piezoelectric Transducer element through the intermediate layer. In addition to a clip connection can also be an adhesive bond used become.

Next the described ultrasonic transducer is still a flow measuring system proposed, which at least one ultrasonic transducer after includes one of the embodiments described above. Farther the flow measuring system has one with the ultrasonic transducer Connected control and evaluation to determine at least a flow variable of a flowing fluid medium.

The proposed ultrasonic transducer and the proposed flow measuring system have numerous significant advantages over known ultrasonic transducers and known flow measuring systems. Thus, the matching body efficiently ensures that the acoustic impedance and / or the resonant properties are fused favor coupling between the transducer element and the surrounding fluid.

Theoretically, for good ultrasonic coupling, the impedance of the matching layer should at least approximately be the geometric mean between the impedances of the air (or fluid) and the piezoelectric material. However, in most cases this is virtually impossible to achieve. As a particularly preferred range for the impedance of the matching layer, therefore, a range between 0.1 × 10 ⁶ kg / (m ² s) and 2.5 × 10 ⁶ kg / (m ² s) was determined, particularly preferably a range between 0, 5 x 10 ⁶ kg / (m ² s) and 1.6 x 10 ⁶ kg / (m ² s) or even 1.5 x 10 ⁶ kg / (m ² s). For example, a value of 0.8 × 10 ⁶ kg / (m ² s) may be preferably used. These preferred values apply to a single layer matching layer and air or other gases at normal pressure as the fluid medium. With liquids higher impedances can be used, whereby density reductions (eg by fillings with glass hollow spheres) can be dispensed with. Still other impedances can be used with multilayer matching layer constructions. In particular, it may be attempted to adjust the impedances stepwise from the impedance of the piezoelectric material to the impedance of the fluid medium.

in the Boundary region of the matching body to the piezoelectric transducer element in contrast, the fibers in the intermediate layer (which, as above described, can also be part of the matching body) for a favorable coefficient of thermal expansion and thereby ensure that mechanical and / or thermomechanical Significantly reduced tension in this transition area can be. This is particularly advantageous when piezoceramics can be used, which compared to conventional plastics the matching body significantly lower thermal expansion exhibit. Here are in particular materials with the above preferred thermal expansion coefficient of advantage. So can be especially at temperature shock changes aging effects of Avoid internal ceramic properties (for example polarization). Also, aging effects of the electrodes of the piezoelectric transducer elements and / or the adhesion of the piezoelectric transducer elements themselves or their Electrodes on the matching layer (or an intermediate layer Adhesive) can be significantly reduced in this way.

Farther The flexibility in terms of design and / or the choice of materials for the fitting body or the matching layer significantly increased. Would for example, to reduce thermo-mechanical stresses at the transition between piezoelectric transducer element and matching layer flexible adhesive used, it would usually be the disadvantage that such flexible adhesives usually a low Glass transition temperature (Tg) have. exceeds the ambient temperature or operating temperature this glass transition temperature, so the flexibility of the adhesives increases strongly, and the modulus decreases. In this case, the adhesive layer can not do the ultrasound more effectively couple into the matching layer. This reduces the upper limit temperature of the maximum usable temperature range the ultrasonic transducer and / or the flow measuring system. In addition, with the flexibility of the plastic mostly increases also the coefficient of thermal expansion too, so the glue even the tension partly despite the higher flexibility may even increase.

One The goal is usually the coefficient of thermal expansion of the plastic, which is connected to the piezoelectric transducer element adjacent (for example, an adhesive or the matching layer) as possible to be as small as those of the piezoceramic itself. This can generally only be harder or filled Plastics are realized, as filling example Ceramic particles or glass or mineral fibers come into question. such harder or filled plastics are suitable still relatively good for transmitting the ultrasound from the piezo, but because of their high acoustic impedance for the Introduction into the fluid medium, especially in air, bad suitable. The technical possibility, the adaptation layer from a suitably filled plastic with the usual mixing of filler and matrix molecules to realize So leads in particular in an air-ultrasonic transducer not to the desired result.

used on the other hand, an adhesive alone, whose thermal expansion coefficient would be adapted to the piezoelectric transducer element, then would have the adhesive thickness can be chosen so that it tensions can break down or shifted from the piezoelectric element in the bond become. The tension between the glue and the stronger but then there will still be an expanding adaptive layer so that the adhesion of the adhesive to the matching layer is problematic becomes.

According to the invention, however, the adhesion of the matching layer or the matching body on the intermediate layer or the piezoelectric transducer element according to the invention embedding of the fiber material (for example, the fiber braid) based not only on an adhesion, but also on a positive connection within the spaces between the fibers. The tension is thus no longer primarily at the expense of piezoe lektrischen transducer element and the adhesion, but is mainly intercepted by the fibers themselves. The resulting increase in the acoustic impedance occurs where it least disturbs, namely close to the piezoelectric transducer element. Depending on the type of fiber and the volume fraction within a matrix material, in particular within a plastic matrix, the intermediate layer, a suitable gradient can be achieved both with regard to the acoustic impedance and with respect to the coefficient of thermal expansion.

Brief description of the drawings

embodiments The invention is illustrated in the drawings and in the following Description explained in more detail. It shows:

1 an embodiment of a known flow measuring system in which the ultrasonic transducer according to the invention can be used;

2 an ultrasonic transducer known in the art;

3 a first embodiment of an ultrasonic transducer according to the invention with a separate intermediate layer;

4 An embodiment of an ultrasonic transducer according to the invention with integrated in a matching layer intermediate layer;

5 an embodiment of an ultrasonic transducer according to the invention with partially circumferential fiber braid without matching body;

6 the ultrasonic transducer according to 5 after attaching the electrode contacts and the matching body; and

7A to 7D An embodiment of an ultrasonic transducer according to the invention with an intermediate layer fixed by staples.

embodiments the invention

In the 1 is an embodiment of a flow measuring system 110 represented, in which inventive ultrasonic transducer 112 can be used. Such flow measuring systems can be used, for example, in process engineering for measuring flow rates, or in automotive technology for measuring intake and / or exhaust air masses for an internal combustion engine. For example, can be by means of the flow measuring system 110 to the following quantities of gas (or, more generally, the flowing fluid medium): to a flow rate of the medium, to a mass flow and / or (by means of additional information such as a pressure) to a temperature of the medium. Also other embodiments than those in 1 illustrated form and other types of application are conceivable. Not shown in 1 a generally existing control and evaluation, which the ultrasonic transducer 112 controls and receives received signals accordingly.

The two ultrasonic transducers 112 are in a flow tube 114 embedded, which in a main flow direction 116 (or in the opposite direction) is flowed through by a fluid medium, such as an intake air mass of an internal combustion engine, at a speed v. For fluids containing little or no entrained particles, there is usually no way to measure with the commonly used Doppler method. Therefore, the sound propagation times, transit time differences or phase differences of ultrasonic signals are usually measured between two ultrasonic transducers 112 with a directional proportion in or against the main flow direction 116 be sent back and forth.

From the geometric arrangement (ultrasonic path of length L, tilt angle α) and the flow velocity v to be measured, the two ultrasonic transit times result:

The resolution according to the flow velocity v yields: v = L (1 / t 1 - 1 / t 2 ) / 2cosα

there can the flow rate in many cases be considered as a measure of the flow rate. This approximation applies in the first order, d. H. at the same constant velocity profile over the pipe cross-section essentially results in a proportional relation between flow velocity and flow rate, or linear characteristic between these two sizes. A multiplication with the density of the fluid medium (for gases ~ p / T) then gives the particle flow or mass flow.

Notwithstanding the in 1 For example, measuring arrangements are also known, in which both ultrasonic transducers 112 are arranged on the same side of the tube with respect to a reflection surface.

In 2 is an example of a possible realization of an ultrasonic transducer 112 shown. In addition to the ultrasonic transducer 112 is schematically the coefficient of thermal expansion α (in 2 with the reference number 210 referred to), the normally occurring during operation thermo-mechanical stresses σ (in 2 with the reference number 212 and the acoustic impedances Z of the individual materials (in 2 with reference number 214 respectively) as a function of a coordinate parallel to the longitudinal extent perpendicular to the layer structure of the ultrasonic transducer 112 shown.

The ultrasonic transducer 112 includes a piezoelectric transducer element 216 (Also referred to as "piezo" or "piezoelectric element" here and below) and a matching body 218 which is in the in 2 illustrated embodiment, only a matching layer 220 includes. Also other layer structures of the matching body 218 or the matching layer 220 (For example, multi-layered structures) are conceivable.

The piezoelectric transducer element 216 points at the in 2 illustrated embodiment on its end faces electrodes 222 which, for example, can extend over these end surfaces over the entire surface or almost over the entire surface. And which by electrode contacts 226 . 228 and electrode leads 230 . 232 can be contacted.

Between the matching layer 220 and the piezoelectric transducer element 216 is at the in 2 illustrated, the prior art corresponding ultrasonic transducer 112 an adhesive layer 234 brought in. For applications in non-high temperatures, for example, either a flexibilized plastic can be used, which is designed so that it can even compensate for tension to some extent, but still hard enough to ensure sufficient acoustic coupling. However, due to their flexibility, corresponding adhesives often have a glass transition temperature (Tg) which is not too high above which the modulus of elasticity decreases so much that no or only a slight coupling is possible. On the other hand, higher Tg adhesives are usually harder and advantageous for good acoustic coupling. Examples of such plastics are epoxy adhesives.

For such a typical, hard adhesive, such as an epoxy resin is in 2 schematically the course 210 the thermal expansion coefficient α along the transducer axis of symmetry 236 (However, of course, non-rotationally symmetric transducer geometries are used) shown, wherein the coordinate along this axis of symmetry 236 is denoted by x.

It can be seen that due to the jump in the thermal expansion coefficient α at the transition between piezoelectric transducer element 216 and matching layer 220 a mismatch arises, which leads to high mechanical stresses in the ceramic, especially during thermal shock changes. These tensions are in 2 denoted by σ (curve 212 ) and have a strong maximum in the area of the adhesive layer 234 at the transition between piezoelectric transducer element 216 and matching layer 220 on. These voltages can cause the piezo 216 age (where the polarization decreases) and possibly even the adhesive layer 234 may have or cracks in the material of the matching layer 220 and / or the piezoelectric transducer 216 can form. The acoustic impedance curve Z (curve 214 ), however, is usually by the use of the hard adhesive for the adhesive layer 234 not disturbed, but takes advantage of the piezo 216 towards the surrounding fluid medium (in particular air, below the matching layer 220 in 2 ).

In 3 In contrast, an embodiment of an ultrasonic transducer according to the invention 112 schematically in analogous representation to 2 displayed. In contrast to the prior art ultrasonic transducer 112 according to 2 has the ultrasonic transducer 112 in the embodiment according to 3 an intermediate layer 310 on which between the matching layer 220 and the piezoelectric transducer element 216 is embedded. This intermediate layer 310 includes besides a fiber material 312 a matrix material 314 , in which the (in this illustration exemplified as a woven fiber mat) fiber material 312 is embedded. For the production of in 3 illustrated ultrasonic transducer 112 offers, for example, a method in which first the matching layer 220 is manufactured in a substantially cylindrical shape. However, other forms are conceivable. The matching layer 220 For example, may have a thickness of about λ / 4 with respect to the desired ultrasonic wavelength and be prefabricated accordingly. The actual thickness to be chosen with regard to the transducer efficiency, the signal shape and the bandwidth generally results depending on the material properties and can be determined, for example, by FEM simulations (FEM: finite element method) and / or by experimental iteration. Special geometries for the fine optimization of the vibration behavior and the radiation or in terms of construction, manufacturing and handling aspects can bring further deviations from the cylindrical shape with it.

In a further process step, the matrix material 314 on the matching layer 220 and / or the piezo 216 applied. For example, as stated above, this matrix material may be 314 be an adhesive that is curable (for example, chemically, thermally, by UV irradiation or by similar methods or a combination of these methods). This application can be done, for example, by dispensing, stamping, doctoring or similar techniques. Alternatively or additionally, the matrix material 314 also in or on the fiber material 312 be introduced or introduced. For example, it may be in the fiber material 312 to trade a fiberglass braid.

The fiber material 312 is then between the piezo 216 and the matching layer 220 is placed, and the overall composite thus created is compressed. Subsequently, a curing, for example, as described above, by thermal or chemical treatment and / or by irradiation with UV light.

It should be noted that the in 3 illustrated ultrasonic transducer 112 can also be prepared in other ways than by the method described. In the described method, in addition to the illustrated method steps, additional steps can be performed, which are not listed. In addition, the time sequence can vary with respect to the sequence listed, and individual process steps can be carried out in parallel and / or repeatedly over time.

Again, in 3 also schematically the thermal expansion coefficient α (curve 210 ), the thermal stresses σ (curve 212 ) and the impedance Z (curve 214 ) symbolically applied. The thermal expansion coefficient α of the adhesive / fiber combination decreases sharply by the fibers and approaches the ceramic or the electrode layer 224 of the piezoelectric transducer 216 (the electrode layer usually comprises a silver thick-film electrode, but may also comprise other electrode materials). As a result, the mechanical stress decreases overall and at the same time shifts from the piezoceramic into the connection points between the adhesive 314 and fibers 312 , Since at this point, in which now high voltages occur, but not only a flat connection prevails (adhesion), but within the mesh is a form-fitting, also affects the voltage less strongly on the strength of the junction between the piezo 216 and matching layer 220 out. Since the acoustic impedance Z of the adhesive / fiber combination is still between the values of the piezo 216 and the matching layer 220 is the acoustic coupling continues to be good, as long as the fiber braid 312 not too thick, which in turn would cause an attenuation of the ultrasound. On the other hand, the strength can be increased over a sufficiently thick fiber structure, at the same time increasing the range over which the voltages can be distributed.

In 4 is another embodiment of an ultrasonic transducer according to the invention 112 shown. Unlike the in 3 illustrated embodiment is in the in 4 shown example no separate layer as a matrix material 314 for the intermediate layer 310 but a boundary layer of the matching layer 220 , For this purpose, the fiber material 312 in which, for example, once again a fiber braid, in particular a glass fiber braid, may be involved, into a layer of the matching layer which has not yet been completely or not yet fully cured 220 embedded, for example, pressed.

As a manufacturing method for this embodiment of the invention, for example, offers a method in which first a (not shown in the figures) mold with a not yet crosslinked base material for the matching layer 220 is filled. Subsequently, an already electrically contacted piezo 216 with a single-sided fiber braid 312 previously impregnated with the material contained in the matching layer material (for example, a clear resin) is easily embedded in the matching layer material in the mold. This structure is then cured (for example, once again thermally, photochemically or chemically) to the in 4 to obtain the structure shown.

In the 5 and 6 an embodiment is shown, which can also be combined with the embodiments described above, but which is essentially a modification of the in 4 illustrated embodiment shows. In this embodiment, the piezo is 216 wholly or partly of the fiber braid 312 enclosed. This has the advantage that the fiber braid 312 even during embedding in the matching layer 220 can also be used as a fixation. This is in 5 symbolically by the arrows 510 shown which the on the piezo 216 or the fiber braid 312 show acting forces.

Both electrodes can be used 222 . 224 already, as in the embodiment according to 3 , through appropriate electrode contacts 226 . 228 and electrode leads 230 . 232 be contacted. Alternatively, what's in 6 is shown, also the fiber braid 312 itself be used for contacting, in particular to the lower Elektro de 224 to contact electrically. For this purpose, for example, a self-conductive or provided with a conductive coating / filling fiber braid, which can be soaked in addition, for example, with conductive adhesive, are used and for matching layer-side contacting of the electrode 224 serve. The remaining process steps, ie in particular the embedding in the not yet cured matching layer 220 , take place analogously to the above description. Alternatively, however, this method can also be applied to a design according to 3 be applied.

After or before the curing of the matching layer 220 can then other layers / components of the matching body 218 be introduced as in 6 is shown. For example, it may again be a potting compound, which, for example, the attachment of the electrode lead 230 mechanically stabilized.

The in the 5 and 6 dargstellte variant simplifies handling in a production of ultrasonic transducers 112 considerably. Especially when using larger and heavier piezos 216 offers the illustrated variant, in which the piezo 216 wholly or partly over the fiber braid 312 can be held, advantages, since in these cases usually simple suction grippers, as they are usually used for the production, the heavy and large Piezos 216 can hold only bad.

In the 7A to 7B are further embodiments of an ultrasonic transducer according to the invention 112 shown, which on the idea of fixing the piezos 216 and the fiber braid 212 through brackets 710 based. In each case, for reasons of clarity in these illustrations, the matching layer is used 220 which, for example, when executed in 7A on the underside of the fiber braid 312 would join, omitted or there are intermediates shown, which then still (for example, analogous to the embodiment in 3 or the embodiment in 4 ) with such a matching layer 220 must be connected or in a matching body 218 must be embedded.

So show the 7A and 7B a sectional view ( 7A ) or a partial perspective view ( 7B ) of a first such variant, in which the piezo 216 over a bracket 710 will be contacted. This clip 710 is configured in the illustrated embodiment as a stamped and bent part, but other embodiments are possible.

The brackets 710 In the present example, both serve to fix the fiber braid 312 as well as the electrical contact. However, it is also conceivable only a fixation of the fiber braid 312 to effect and / or only an electrical contacting of the piezo 216 ,

In the in the 7A and 7B illustrated embodiment, the electrodes 222 respectively. 224 each with recesses 712 on, as for example from the perspective view in 7B can be seen. There are several parentheses 710 provided, wherein in each case at least one clip, the upper electrode 222 and at least one more clip 710 the lower electrode 224 contacted. For example, in the illustration according to 7A the left of the two brackets 710 for contacting the upper electrode 222 provided so that the lower electrode 224 in the area of the lower arm of this left bracket 710 a recess 712 having. Analog is the right of the brackets shown 710 for contacting the lower electrode 224 provided so that the upper electrode 222 in the area of the upper arm of this right bracket 710 a corresponding recess 712 has (see also 7B ). However, other embodiments are also conceivable which short-circuit the two electrodes 222 . 224 through the brackets 710 prevent, for example, corresponding embodiments of the geometry of the fibers 312 , At the in 7A and 7B illustrated embodiment, the brackets 710 an upwardly protruding connection contact 714 on, by which a contact is easily possible, for example by clamping and / or soldering corresponding electrode leads (in the 7A and 7B not shown).

In the 7C and 7D is a modification of the in the 7A and 7B shown embodiment, which in turn the idea of using brackets 710 realized. It shows 7C a top view of the ultrasonic transducer 112 so that the upper electrode 222 is visible. According to shows 7D a plan view from below of the ultrasonic transducer 112 (Again, the fitting layer 220 not yet attached), leaving the bottom electrode 224 and the overlying fiber braid 312 are recognizable.

In this embodiment, three brackets 710 intended. Of these, two parentheses (in 7C the clamps in 11 o'clock position and 7 o'clock position) for contacting the upper electrode 222 and at the same time for clamping the fiber braid 312 used. The third bracket 710 (in 7C on the 3 o'clock position), on the other hand, is exclusively for contacting the lower electrode 224 used. Accordingly, in turn, the upper electrode 222 a recess 712 on, so that in 7C in 3 o'clock position 710 this electrode 222 not contacted. The fiber network 312 however, which in this embodiment has approximately a round shape, also has a recess 316 on it the third bracket (in 7C in 3 o'clock position) allows the lower electrode 224 to contact.

The brackets described above 710 can additionally with adhesive and / or conductive adhesive on the fiber braid 312 and / or at the electrodes 222 respectively. 224 be fixed or contact them.

In the in the 7A to 7D used embodiment, as in the preceding embodiments, as possible fiber materials of the fiber material 312 organic materials such as carbon fiber (a material having a negative coefficient of expansion α, which also usually has conductive properties), boron fibers, metals, inorganic materials such as glass fibers and / or ceramic fibers (polycrystalline or amorphous) or other fiber materials. These fibers can additionally be silanized to ensure adhesion to the matrix material 314 (in particular the adhesive) and / or the matching layer 220 to improve.

Again, as in the other embodiments, also in the embodiments in the 7A to 7D the ultrasonic transducer 112 be made so that the fibers of the fiber material 312 in a separate matrix material 314 (For example, an adhesive and / or a conductive adhesive) come to rest or in a material of the matching layer 220 , For example, a clear resin content of this matching layer material.

Instead of one, like in the 7A to 7D shown fiber braid as fiber material 312 or another prefabricated structure, in this as well as in the preceding embodiments, a plastic, an adhesive and / or a polymer material can be rather disordered filled with fibers, this composite then on the piezo 216 are applied, which in turn the Anpassschichtkörper 220 is pressed or its base material in the not or not fully networked state "wet-on-wet" is brought into contact with the transducer and the fiber composite and the entire part is then cured together.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• - DE 102004060064 A1 [0001]

Claims (20)

Ultrasonic transducer ( 112 ) comprising a piezoelectric transducer element ( 216 ) and a matching body ( 218 ) with a matching layer ( 220 ) to promote a vibration coupling between the piezoelectric transducer element ( 216 ) and a surrounding fluid medium, characterized in that between the piezoelectric transducer element ( 216 ) and the matching layer ( 220 ) at least one intermediate layer ( 310 ) for reducing thermal stresses between the matching layer ( 220 ) and the piezoelectric transducer element ( 216 ), the intermediate layer ( 310 ) a fiber material ( 312 ). Ultrasonic transducer ( 112 ) according to the preceding claim, wherein the fiber material ( 312 ) comprises at least one of the following materials: loose fibers; a fiber braid; a fiber composite material. Ultrasonic transducer ( 112 ) according to one of the two preceding claims, wherein the fiber material ( 312 ) comprises at least one of the following materials: an organic fiber material; a carbon fiber material; a boron-containing fiber material, in particular boron fibers; a metal-containing fiber material, in particular metal fibers; an inorganic fiber material, in particular in polycrystalline or amorphous form, in particular a glass fiber material or a ceramic fiber material. Ultrasonic transducer ( 112 ) according to one of the preceding claims, wherein the intermediate layer ( 310 ) next to the fiber material ( 312 ) a matrix material ( 314 ), wherein the matrix material ( 314 ) comprises at least one of the following materials: a resin, in particular an epoxy resin; a polyester; a BMC material; an adhesive; a thermoplastic polymer; a thermosetting polymer. Ultrasonic transducer ( 112 ) according to the preceding claim, wherein the matrix material ( 314 ) is at least partially identical with a material of the matching body ( 218 ), in particular the matching layer ( 220 ). Ultrasonic transducer ( 112 ) according to any one of the preceding claims, wherein the matching layer ( 220 ) comprises a polymer material, in particular a thermally, chemically or photochemically curable or crosslinkable polymer material. Ultrasonic transducer ( 112 ) according to the preceding claim, wherein the polymer material comprises at least one of the following materials: a syntactic material; a thermosetting material, in particular an epoxy resin, a polyester, a phenolic resin or a cyanate ester; a thermoplastic; an elastomer, in particular silicone; a polymer blend; a polyimide, especially Kapton. Ultrasonic transducer ( 112 ) according to one of the two preceding claims, wherein the polymer material has at least one filler, in particular at least one of the following fillers: cavities; fibers; Hollow glass spheres; Hollow plastic beads; a volatile filler, especially a filler which escapes or is removable during or after crosslinking of the polymer. Ultrasonic transducer ( 112 ) according to one of the preceding claims, wherein the intermediate layer ( 310 ) the piezoelectric transducer element ( 216 ) at least partially encloses. Ultrasonic transducer ( 112 ) according to one of the preceding claims, wherein the intermediate layer ( 310 ) has at least partially electrically conductive properties. Ultrasonic transducer ( 112 ) according to the preceding claim, wherein the intermediate layer ( 310 ) comprises at least one of the following materials: a conductive adhesive, metal particles, carbon particles, a conductive plastic. Ultrasonic transducer ( 112 ) according to one of the preceding claims, wherein the intermediate layer ( 310 ) by a frictional connection, in particular by a clamp connection, with the piezoelectric transducer element ( 216 ) connected is. Ultrasonic transducer ( 112 ) according to the preceding claim, wherein the frictional connection at least one electrode ( 222 . 224 ) of the piezoelectric transducer element ( 216 ) contacted electrically. Ultrasonic transducer ( 112 ) according to one of the two preceding claims, wherein the clip connection comprises a clip ( 710 ), wherein the bracket ( 710 ) additionally by an adhesive bond and / or a conductive adhesive with the fiber material ( 312 ) and / or the piezoelectric transducer element ( 216 ), in particular an electrode ( 222 . 224 ) of the piezoelectric transducer element ( 216 ), connected is. Ultrasonic transducer ( 112 ) according to one of the preceding claims, wherein the piezoelectric transducer element ( 216 ) wholly or partly into the matching body ( 218 ) is embedded. Flow measuring system ( 110 ) comprising at least one ultrasonic transducer ( 112 ) according to one of the preceding claims and mindes least one with the ultrasonic transducer ( 112 ) connected control and evaluation for determining at least one flow variable of a flowing fluid medium. Method for producing an ultrasonic transducer ( 112 ), in particular an ultrasonic transducer ( 112 ) according to one of the preceding, to an ultrasonic transducer ( 112 ), wherein the ultrasonic transducer ( 112 ) a piezoelectric transducer element ( 216 ) and a matching body ( 218 ) with a matching layer ( 220 ) to promote a vibration coupling between the piezoelectric transducer element ( 216 ) and a surrounding fluid medium, wherein between the piezoelectric transducer element ( 216 ) and the matching layer ( 220 ) at least one intermediate layer ( 310 ) for reducing thermal stresses between the matching layer ( 220 ) and the matching layer ( 220 ), whereby the intermediate layer ( 310 ) a fiber material ( 312 ), the method comprising the steps of: - the matching body ( 218 ) with the matching layer ( 220 ) is generated; - the intermediate layer ( 310 ) is generated; and - the matching body ( 218 ) is connected to the piezoelectric transducer element ( 216 ), the intermediate layer ( 310 ) between the matching layer ( 220 ) and the piezoelectric transducer element ( 216 ) is embedded. Method according to the preceding claim, wherein an adhesive and / or a curable plastic on the piezoelectric transducer element ( 216 ) and / or the matching body ( 218 ) is applied, wherein the fiber material ( 312 ) is embedded in the adhesive and / or the curable plastic, wherein the piezoelectric transducer element ( 216 ) and the matching body ( 218 ) are joined together by pressing and wherein the adhesive and / or the curable plastic are at least partially cured. Method according to one of the two preceding claims, wherein at least one material of the matching body ( 218 ) and at least one material of the intermediate layer ( 310 ) are cured simultaneously. Method according to one of the three preceding claims, wherein the matching body ( 218 ) has at least one curable material, wherein in forming the intermediate layer ( 310 ) the fiber material ( 312 ) in an uppermost layer of the curable material of the matching body ( 218 ) and wherein the curable material of the matching body ( 218 ) is cured.