EP0390959A2 - Ultrasonic transducer - Google Patents

Abstract

The invention relates to an ultrasonic transducer with a piezoelectric transducer element which can be excited to radiate ultrasonic oscillations. Whereas previously the approach has been to overdimension the adapter body in the direction of the diameter, the approach in the invention is to achieve an effective, smallest-possible diameter of the adaptor body by adapting the lateral circumferential area of the adaptor body. <IMAGE>

Description

The invention relates to an ultrasonic transducer according to the preamble of claim 1.

Such an ultrasonic transducer is known for example from DE-PS 25 41 492 or DE-PS 34 01 979.
The aforementioned known ultrasonic transducers have already shown solutions to the fundamental problem existing in the design of ultrasonic transducers, namely to achieve the highest possible sound pressure and a large sound amplitude with the same energy.

Thus, the impedance matching problem that exists between the piezo-ceramic transducer element excited to vibrate and the air has been solved relatively satisfactorily by applying a λ / 4 matching body to the piezoceramic transducer element. The acoustic impedance of the matching body was chosen between the impedance of the piezoceramic and that of the air.

As is shown in FIGS. 1 to 4, which will be discussed below, for a better comparison with the invention, the alternative solutions that have been used so far must be regarded as unsatisfactory with regard to a good radiation characteristic of an ultrasonic transducer with the smallest possible size.

The original concept of an ultrasonic transducer with a platelet-shaped, piezoceramic transducer element and a λ / 4 adaptation layer bonded thereon is a step in the right direction the main radiation, this λ / 4 adaptation layer is exactly adapted to the piezoceramic transducer element and has, for example, a circular cylindrical shape, a very wide central main lobe in the sound lobe profile with a large number of relatively equally strong side lobes.

The solution of the ultrasonic transducer proposed in DE-PS 25 41 492 does provide a λ / 4 layer as the adaptation body. However, the adaptation body itself has a much larger surface area with the area protruding beyond the edge of the piezoceramic transducer element. In addition, a weighting ring spaced from it is bonded to the underside of this protruding area, so to speak at the level of the piezoceramic transducer element.
On the basis of the same excitation frequency and energy, the resulting sound lobe profile shows a relatively narrow central main lobe, which, however, is accompanied by two extremely strong, closely adjacent first side lobes.

The solution alternative proposed in DE-PS 34 01 979 provides in a quite comparable manner a λ / 4 adaptation body which is substantially larger in area and which is designed as a λ / 2 oscillator in the area which projects radially beyond the piezoceramic transducer, so to speak.
The resulting sound lobe profile is characterized by a relatively strong, central main lobe and several, weaker side lobes.

The comparison of the three prior art alternatives mentioned above shows that, with small ultrasonic transducers in terms of area, a favorable sound beam profile with a strong central main beam and strongly suppressed side beams cannot be achieved. On the other hand, there are also large ultrasonic transducers in the rear with regard to dimensioning, control energy and sound beam profile have not yet shown a satisfactory solution.

Taking this state of the art into account, the object of the invention is therefore to design an ultrasonic transducer of the generic type with small dimensions with good radiation characteristics and also to enable simple adaptation options to changed parameters, in particular the frequency.

The object is achieved by the features of the characterizing part of claim 1.
A fundamental idea of the invention can therefore be seen in the fact that it was not based on an oversizing of the adaptation body on the piezoceramic transducer element, but rather a reorientation towards an λ / 4 adaptation body which was almost identical in diameter as the piezoceramic transducer element. However, it was surprisingly found that the effective main radiation area or end face of the λ / 4 adaptation body deviates slightly from the main area of the piezoceramic transducer element.
In the following, the effective radiation area is to be understood as the area up to which there is no node in the amplitude diagram of the vertical or thickness vibration of the λ / 4 adaptation body or a phase inversion of the amplitude does not occur.

As a result, the knowledge that the main surface of the λ / 4 adaptation body facing away from the piezoceramic transducer element is slightly larger or smaller than that bonded onto the piezoceramic transducer element and usually matching surface of the adaptation body. There is therefore a deliberate deviation from a strictly circular cylindrical shape of the λ / 4 adaptation body, which according to the invention has a larger or smaller main surface oriented in the radiation direction, the diameter or the size of the main surface as well as the peripheral surface and its side line is determined by the frequency, the material parameters of the adaptation body and the thickness.

The invention therefore makes use of the knowledge that the actually desired thickness vibration or vertical vibration perpendicular to the main surface of the ultrasonic transducer is superimposed by radial vibrations. These radial vibrations are present above all if the thickness of the λ / 4 adapter body is relatively small in relation to the diameter of the adapter body. This superposition of vertical vibration and radial vibration modes can result in node points or node rings, which result in a smaller effective radiation area.

Although these nodes have so far been influenced and minimized by a protruding ring structure of the λ / 4 adapter body, the invention has recognized that the radial vibration modes in the adapter body are very sensitive to the reflection boundary conditions on the lateral peripheral edge of the adapter body. This has the corresponding effect that the vibrations of the ultrasonic transducer can be significantly influenced by small changes in the diameter of the main surface of the adapter body, based on the diameter of the piezoceramic transducer element, and small changes in shape of the peripheral wall of the adapter body.
Frequency, modulus of elasticity and Poisson transverse number therefore influence the precise shape of the lateral peripheral wall, the configuration being such that knots form in the Amplitude diagram of the vertical vibration can be avoided. In the simplest configuration, the side line of the peripheral surface of the adapter body is a straight line that diverges or converges, so that the diameter of the main surface of the adapter body deviates slightly from the main surface of the piezoceramic transducer element. For example, with a diameter of 30 mm of the transducer element, the diameter of the main surface of the adapter body facing away from the transducer element can be 32 mm. Depending on the ratio of the thickness of the adapter body to the diameter of the transducer element, slightly positive or slightly negative curved side lines are also advantageous in order to achieve a relatively centered high sound pressure.
The change in the diameter of the main surface of the adapter body compared to the transducer surface, which is assumed to be circular, is suitably in the range from 4% to 15%, especially around 6% to 10%. Although coaxial circular surfaces between the transducer element and the adapter body are common, the main surface of the adapter body facing away from it may also have a different contour, especially if the diameter is larger than that of the transducer element.
Another possible variation is to make the diameter of the surface of the adapter body bonded to the transducer surface slightly smaller or larger than the piezoceramic transducer surface. For certain frequencies and transducer diameters, this, in combination with the above-mentioned shaping, has a favorable influence on the sound beam profile.

In order to be able to influence the side lobes even better in the ultrasonic transducer according to the invention, it seems to be extremely advantageous to assign an external ring transducer of the same thickness to a truncated cone-shaped ultrasonic transducer according to the invention with a minimal air gap or spacing. Here, the ring transducer consists of a ring-like, piezoceramic transducer element with an inner diameter slightly larger than the largest diameter of the inner adapter body. The ring converter is on the converter element a ring-cylindrical λ / 4 adapter body is bonded, which is usually strictly cylindrical.

Although such a ring transducer offers advantages in itself compared to transducers according to the prior art, it is particularly suitable for better matching the radiation characteristics of the entire ultrasonic transducer. For example, with the same main area, an ultrasound transducer combined with a ring transducer, in comparison to the main area of a single truncated cone-shaped ultrasound transducer, can have a very specific influence on the side lobes by means of a different phase and magnitude ratio between the two transducers. This makes it possible to push back the problematic side lobes even more than is possible with pure truncated cone-shaped transducers and with the known solutions mentioned above. On the other hand, even embodiments open up, e.g. on one side of the sound lobe profile wanted to reach a wide sound field with intense side lobes, while on the other side the side lobes are almost non-existent.

• Fig. 1 einen Axialschnitt durch ein schematisch dargestell­tes Ultraschallwandlerelement nach dem Stand der Technik mit zylindrischem Aufbau;
• Fig. 2a ein entsprechendes Schallkeulenprofil des Wandlers nach Fig. 1, wobei für den nachfolgenden Vergleich von gleicher Frequenz, Leistung und gleichem Durch­messer eines piezokeramischen Wandlerelementes aus­gegangen wird;
• Fig. 2b ein Amplitudendiagramm des Wandlers nach Fig. 1 über dessen Radius;
• Fig. 3a einen Axialschnitt durch einen Ultraschallwandler mit Beschwerungsring;
• Fig. 3b ein Schallkeulenprofil des Wandlers nach Fig. 3a;
• Fig. 3c ein entsprechendes Amplitudendiagramm des Wandlers nach Fig. 3a;
• Fig. 4a einen Axialschnitt durch einen Ultraschallwandler nach dem Stand der Technik mit λ/2-Anpassungskör­per;
• Fig. 4b ein Schallkeulenprofil des Wandlers nach Fig. 4a;
• Fig. 4c ein Amplitudendiagramm des Wandlers nach Fig. 4a über seinen Radius;
• Fig. 5a einen Axialschnitt durch einen erfindungsgemäßen Ultraschallwandler in schematischer Darstellung;
• Fig. 5b das Schallkeulenprofil des Wandlers nach Fig. 5a bei gleicher Frequenz und Leistung wie in den voraus­gehenden Beispielen;
• Fig. 5c ein Amplitudendiagramm der Vertikal- bzw. Dicken­schwingung des Wandlers nach Fig. 5a über den Radius des Anpassungskörpers;
• Fig. 6 ein weiteres Beispiel eines erfindungsgemäßen Ultra­schallwandlers im Axialschnitt;
• Fig. 6a das Amplitudendiagramm des Wandlers nach Fig. 6;
• Fig. 7a einen Axialschnitt durch ein Beispiel einer Kombina­tion eines kegelstumpfförmigen Wandlers mit einem Ringwandler;
• Fig. 7b die Draufsicht auf den Wandler nach Fig. 7a;
• Fig. 7c das Schallkeulenprofil des Wandlers nach Fig. 7a, 7b und
• Fig. 7d das Amplitudendiagramm dieses Wandlers.

The invention is explained in more detail below in comparison with the prior art using a few exemplary embodiments. Show it:

• Figure 1 is an axial section through a schematically illustrated ultrasonic transducer element according to the prior art with a cylindrical structure.
• FIG. 2a shows a corresponding sound lobe profile of the transducer according to FIG. 1, the same frequency, power and the same diameter of a piezoceramic transducer element being assumed for the subsequent comparison;
• 2b shows an amplitude diagram of the transducer according to FIG. 1 over its radius;
• 3a shows an axial section through an ultrasound transducer with a weighting ring;
• 3b shows a sound lobe profile of the transducer according to FIG. 3a;
• 3c shows a corresponding amplitude diagram of the transducer according to FIG. 3a;
• 4a shows an axial section through an ultrasonic transducer according to the prior art with a λ / 2 adaptation body;
• 4b shows a sound lobe profile of the transducer according to FIG. 4a;
• 4c shows an amplitude diagram of the transducer according to FIG. 4a over its radius;
• 5a shows an axial section through an ultrasonic transducer according to the invention in a schematic representation;
• 5b shows the sound lobe profile of the transducer according to FIG. 5a with the same frequency and power as in the preceding examples;
• 5c shows an amplitude diagram of the vertical or thickness vibration of the transducer according to FIG. 5a over the radius of the adaptation body;
• 6 shows a further example of an ultrasonic transducer according to the invention in axial section;
• 6a shows the amplitude diagram of the transducer according to FIG. 6;
• 7a shows an axial section through an example of a combination of a truncated cone-shaped converter with a ring converter;
• 7b shows the top view of the converter according to FIG. 7a;
• 7c, the sound lobe profile of the transducer according to FIGS. 7a, 7b and
• 7d shows the amplitude diagram of this transducer.

The ultrasound transducer shown schematically in axial section in FIG. 1 has a circular cylindrical, plate-shaped piezoceramic transducer element 2, on which a λ / 4 adaptation body 3 is applied. The main radiation direction of this transducer 1 is perpendicular to the upper main surface of the adaptation body. The adapter body 3 is precisely adapted to the transducer element 2 in diameter.

In Fig. 2a this is at a certain frequency and excitation energy and a certain diameter of the transducer 1 of e.g. 30mm obtained sound beam profile shown in the horizontal plane. This sound lobe profile has a relatively wide, central main lobe 5, which is accompanied on both sides by several strong side lobes 6.

FIG. 2 b shows the amplitude diagram of the transducer 1 according to FIG. 1, the amplitude A being plotted on the ordinate as a function of the radius R of the transducer 1. Amplitude A is understood to mean the vertical or thickness vibration of the transducer 1. In this amplitude diagram of FIG. 2b, the problem can be recognized very blatantly, which is to be expected in the known transducer. This problem consists in that there is a phase inversion of the amplitude with the node towards the outer radius R, so that after the point of intersection with the abscissa R there is a negative amplitude up to the maximum radius. This phase reversal of the amplitude, which occurs in terms of area as a node ring in the transducer 1, therefore does not produce any sound radiation in the direction of the main radiation direction, but rather represents a loss of energy which can be avoided in any case.

A further ultrasonic transducer 8 according to the prior art is shown in Fig. 3a. This transducer 8, constructed in accordance with DE-PS 25 51 492, has a λ / 4 adapter body 9, which projects significantly beyond the diameter of the piezoelectric transducer element 2. A weighting ring 10 is present on the underside of the adaptation body 9.
The amplitude profile of the transducer 8 according to FIG. 3a is shown in FIG. 3c. The maximum, standardized amplitude A can be found in a direction perpendicular to the center of the transducer 8. Towards the outside of the radius of this transducer 8, a phase inversion with a negative amplitude occurs approximately after two thirds of the radius R, this negative amplitude again having a positive amplitude to the peripheral edge transforms. In the diagram, the vertical line on the right outer area indicates the maximum radius or the circumferential surface of the transducer 8.
In this converter 8 according to FIG. 3a there are therefore inevitable energy losses, as can be seen from the amplitude diagram.

The sound lobe profile according to FIG. 3b is characterized by a relatively narrow central lobe and two pronounced side lobes.

4a also shows schematically in axial section the further ultrasonic transducer according to DE-PS 34 01 979. This transducer 12 also comprises a λ / 4 adapter body 13, which acts as a λ / 2 oscillator in the region 14 projecting radially beyond the transducer element 2 is trained. The sound lobe profile according to FIG. 4b shows a main lobe filling the 10 ° range and several, but smaller, side lobes, the difference from FIG. 3b being due to the amplitude distribution (FIG. 4c).

The amplitude diagram of the transducer 12 according to FIG. 4a shown in FIG. 4c has, in addition to the main maximum, a further secondary maximum in the region of two thirds of the maximum radius. Between these maximas there is a minimum with almost no amplitude oscillation. In the edge area, however, a phase reversal, albeit slight, has also been found here.

5a, an ultrasonic transducer 20 according to the invention is shown in basic section in axial section. A λ / 4 adaptation body 22 is bonded onto the piezoceramic transducer element 21, which is circular, for example. The surface 23 of the transducer element 21 corresponds to the lower surface 24 of the adapter body. In the main radiation direction S, the lateral peripheral surface is designed to diverge. This way he there is a horn-like or frustoconical shape for the adaptation body, the main surface 25 facing away from the transducer element 21 has a somewhat larger diameter than the transducer element 21. In the example according to FIG. 5a, the side line 26 is therefore a straight line which, for. B. connects a main surface 25 with a diameter of about 32 mm with the main surface 24 of about 30 mm.

Matched to a corresponding frequency, a material constant and the transducer diameter, an amplitude diagram for the thickness or vertical vibration is obtained, taking into account possible superimpositions by the radial vibration, as shown in FIG. 5c. The ordinate A shows the amplitude, based on the maximum amplitude 1.0, as a function of the radius R. The right discontinuity value on the abscissa R therefore characterizes the amplitude at the outer edge of the front main surface 25 of the adaptation body 22 lying in the radiation direction.

According to the amplitude diagram according to FIG. 5c, therefore, the maximum of the vertical amplitude lies somewhat radially spaced from the axis of the transducer or the main radiation direction, but avoiding a node or a phase reversal of the amplitude due to the configuration of the lateral peripheral surface in coordination with the diameter of the front main surface becomes. The amplitude distribution therefore approaches that of a rigid piston vibration.

In the sound lobe profile according to FIG. 5b, this configuration of the λ / 4 adaptation body 22 according to FIG. 5a achieves a central lobe 28 in the region of a 10 ° radiation angle from the main radiation direction 0 with two smaller, but nevertheless noticeable side lobes 29.

The advantage of the transducer according to the invention can therefore be seen above all in comparison with the transducer according to the prior art according to FIG. 1 and from the comparison of the corresponding sound diagrams. The minimally changed design configuration of the transducer 20 according to FIG. 5a, which, however, largely excludes the formation of a node for the vertical oscillation, results in a much more intensive, more concentrated main lobe 28 in comparison with the central main lobe 5 according to FIG. 2 of the prior art.

In comparison to the other transducers 8 and 12 of FIGS. 3a and 4a, an extremely efficient ultrasound transducer is therefore achieved with a very small transducer diameter, which is very remarkable in terms of efficiency and cost.

6 shows another ultrasound transducer 40 according to the invention. This transducer 40 is characterized by a piezoceramic transducer element 21, on which a λ / 4 adapter body is applied, which has a convex side line of the peripheral surface in axial section.
The same thickness in the radiation direction for the transducer and adapter body and also the same resonance frequency as assumed in the example according to FIG. 5a, this positively curved, convex profile of the side surface is expedient if the transducer element has a smaller diameter, for example, of about 20 mm instead of 30 mm Has. The course of the vertical amplitude is shown in Fig. 6a. Here the amplitude maximum moves further to the edge. However, an amplitude reversal is also avoided here.

With regard to a further improvement in the sense of pushing back the side lobes, the combination of a converter according to FIG. 5a with a ring converter 52 shown in FIG. 7a has proven. The entire converter 50 therefore exists from an inner compact transducer 51 with transducer element 21 and λ / 4 adapter body 22. Minimally spaced apart by an air gap 56 or another material gap, this transducer 51 is surrounded by a ring transducer 52. This ring transducer 52 itself has an annular, piezoelectric transducer element 54, on which a λ / 4 adapter body, for example in the form of a ring cylinder, is applied with the same thickness. Since these two transducers 51 and 52 can be controlled equally in terms of phase and amount, but can also be controlled differently, this opens up possibilities for influencing the sound beam profile in an excellent manner. One form of this influence is shown in the sound lobe profile shown in FIG. 7c, in which, for example, the side lobes 29, which are still relatively noticeable in FIG. 5b, are pushed back into the now very small side lobes 59.

FIG. 7d shows the amplitude diagram of the vertical vibration of the transducer combination 50 according to FIG. 7a. In comparison with the amplitude diagram according to FIG. 5c, the maximum therefore shifts somewhat closer to the ordinate. On the other hand, one also obtains a positive amplitude in the outer region of the radius, which has a Z-shaped transition point with a subsequent minimum of the amplitude oscillation approximately in the region of half the radius. The amplitude oscillation then increases in the positive range towards the maximum radius.
This example according to FIG. 7d shows that in the combination of a compact transducer 51 with a ring transducer 52 according to FIG. 7a, a broad spectrum of desired amplitude profiles for the vertical vibration and thus also for the sound beam profile can be set.

The invention therefore opens up a new way of being able to achieve extremely effective power or range with partially controllable side lobes even with very small ultrasonic transducers.

Claims (9)

1. Ultrasonic transducer
with a piezoelectric transducer element which can be excited to emit ultrasonic vibrations and which has a λ / 4 adaptation body on its main surface essentially in the direction of the main radiation,
characterized by
that the surface (24) of the λ / 4 adapter body (22, 40) facing the transducer element (21) essentially corresponds to the main surface (23) of the transducer element, and that the surface (25) of the λ / 4 facing away from the transducer element (21) 4-adaptation body has a slightly different surface from the main surface of the transducer element, in particular a slightly different diameter. 2. Ultrasonic transducer according to claim 1,
characterized by
that the side line (26) connecting the surfaces (24, 25) of the adaptation body (22) is provided diverging in section. 3. Ultrasonic transducer according to claim 1,
characterized by
that the side line connecting the surfaces (24, 25) of the adaptation body (22) is provided in a converging manner. 4. Ultrasonic transducer according to one of claims 1 to 3,
characterized by
that the side line connecting the surfaces (24, 25) of the adaptation body (22) is a straight line or has a concave or convex shape. 5. Ultrasonic transducer according to one of claims 1 to 4,
characterized by
that the diameters of the surfaces (24, 25) of the adaptation body (22) differ by 4% to 15%, in particular by 6% to 10%. 6. Ultrasonic transducer according to one of claims 1 to 5,
characterized by
that the facing surface (25) of the adaptation body (22) from the main surface (23) of the transducer element (21) has a different contour. 7. Ultrasonic transducer according to one of claims 1 to 6,
characterized by
that an outer ring is provided radially adjacent to the front end face of the adapter body (22), which itself is constructed as a transducer (52) with a piezoelectric transducer body (54) and a ring adapter body (55) of the same thickness as the central λ / 4 adapter body. 8. Ultrasonic transducer according to claim 7,
characterized by
that the piezoelectric transducer elements (21, 54) are separated from one another and, in particular, have different controls in terms of phase and amount. 9. Ultrasonic transducer according to claim 1,
characterized by
that the surface (24) of the λ / 4 adaptation body (22, 40) facing the transducer element is slightly smaller or larger than the main surface (23) of the transducer element.

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