EP0567551B1 - Device for causing an untuned structure to vibrate ultrasonically - Google Patents

Abstract

PCT No. PCT/FR92/00033 Sec. 371 Date Jul. 19, 1993 Sec. 102(e) Date Jul. 19, 1993 PCT Filed Jan. 16, 1992 PCT Pub. No. WO92/12807 PCT Pub. Date Aug. 6, 1992.The present invention relates to the vibration of an untuned structure by means of an ultrasound converter. The main but not unique purpose is to cause a filtering cloth, a perforated metal sheet or a screening mesh to vibrate in order to improve the flow rate in the filtering or sieving process, with equal surface energy, without clogging the mesh or harming the product being processed. The device according to the invention is characterized in that it comprises at least one electro-acoustic converter (12) ridigly fixed to said structure (10) by means of metallic securing elements (14) tuned to the frequency of the converter, the links with the structure being in a maximum region of amplitude (V)j of said securing elements and resonance of the assembly being provided by a nut or any metallic unit tuned as an integral multiple of a half wave length.

Description

The present invention relates to a device for ultrasonic vibration of one or more non-tuned structure (s). These structures can be of very different types, however within the framework of the present description, one will be limited essentially to a sieving structure, without this having to be interpreted as a limitation of the object of the present invention.

For many years, it has been well known that the ultrasonic vibration of sieves improves the qualitative and quantitative yields of sieving fine powders or granules known to be difficult to process.

It should be recalled, for example, the problems encountered with micron and submicron, mineral, metallic and ceramic powders, or even spheroid granules which tend to clog the sieves. Ultrasonic assistance techniques have already been described in the specialized literature and in particular in the work entitled Crowford Engineering by Frederick and Co.

The state of the art can also be illustrated by FR-A-2 233 108 which describes sieving equipment provided with an ultrasonic transducer placed in direct contact with the sieving fabric.

It will be observed that such a device is limited to circular sieves. The emission of ultrasound being done from a single anchor point, the mesh is also subjected to non-negligible stresses at this level, which can cause its premature wear and, consequently, pollution of the passing product. .

In the case of the screen using several stages of fabric, it is necessary to multiply the number of transducers by as much with the complexity of assembly that this imposes. This device of the prior art has another drawback insofar as, in all cases, the converter will be exposed directly to the treated product and will therefore most of the time have to be mounted in leaktight manner.

Document DE-A-38 13 178 discloses a device for vibrating a structure at an ultrasonic frequency, comprising at least one electro-acoustic converter able to vibrate in a direction of vibration and rigidly fixed to said structure by means of metal fasteners tuned to the frequency of the converter, the links with the structure being located in a belly area of amplitude of said fasteners.

The object of the present invention is precisely to develop an ultrasonic vibration device which makes it possible to eliminate all the drawbacks of the devices of the prior art.

The main object of the present invention is to make a support structure resonant, for example a support for canvas, membrane or any other plate which, by construction, has no resonance at the frequency used by the ultrasonic transmitter.

The ultrasonic device according to the invention also aims to optimize the ultrasonic efficiency, whatever the shapes and dimensions of the structure to be vibrated. Of course, one of the main objectives of the present invention has been to prevent the ultrasonic transducer from coming into direct contact with the screen fabric or the working membrane. In addition, in the case of the sieve, the device according to the invention makes it possible to excite the mesh from ultrasonic energy previously distributed in its support frame, thus limiting as much as possible the stresses at the anchor points. Finally, as will be explained in more detail in the remainder of this description, the ultrasonic device, according to the present invention, must also be able to adapt to existing structures by modifying the geometric shape of the connecting elements tuned in integer multiples. half wavelength of the delivered frequency.

In accordance with the present invention, these objectives have been achieved by the development of a device for vibrating at an ultrasonic frequency of a non-tuned structure, which is characterized by the fact that it comprises at least one electroacoustic converter rigidly fixed to said structure by means of metallic fasteners tuned to the frequency of the converter, the structural links being located in a belly area of amplitude of said fasteners and the resonance of the assembly being ensured by fixing a nut or any metallic assembly tuned as an integer multiple of half-wavelength.

• les figures 1 et 2 représentent deux types de profilage différents de barreaux de couplage, de manière à s'adapter aux contraintes dimensionnelles d'une structure à faire vibrer, tout en respectant la fréquence de résonance;
• les figures 3 à 8 illustrent différentes variantes de modes de réalisation destinés à mettre en vibration ultrasonique des structures simples ou multiples affectant des formes différentes.

Other characteristics and advantages of the subject of the present invention will appear on reading the detailed description given below, in particular with reference to particular embodiments illustrated in the accompanying drawings, drawings in which:

• Figures 1 and 2 show two different types of profiling of coupling rods, so as to adapt to the dimensional constraints of a structure to be vibrated, while respecting the resonance frequency;
• Figures 3 to 8 illustrate different variants of embodiments intended to set into ultrasonic vibration of single or multiple structures affecting different shapes.

It will be recalled that one of the essential aims of the ultrasonic device, according to the invention, is to be able to easily adapt to different types of pre-existing structures as well as to structures having shapes and dimensions imposed according to their destination. The ultrasonic device according to the invention achieves this objective, for example using the two embodiments shown diagrammatically in FIGS. 1 and 2.

In these figures, the structure has been shown diagrammatically by the representation bearing the reference 10. It is a structure of any kind which has not been granted, for example a structure supporting any working member, in particular a screen cloth .

The ultrasonic device according to the invention firstly comprises an electroacoustic converter 12 which must therefore be rigidly fixed to said structure 10. In all the embodiments described, the connections with the structure 10 are located in an area belly of amplitude V.

This fixing is carried out by means of metal elements which are tuned to the frequency of the converter, it being understood that the length of said elements includes the thickness of the connecting lugs with the structure 10. On the embodiments illustrated in FIGS. 1 and 2, the metal fastening elements consist of coupling bars 14. The coupling bars 14 are interposed between two walls facing the structure 10.

Of course, these metal fasteners must be tuned to the frequency of the converter. In this case, they were chosen at half the wavelength of the frequency provided by the ultrasonic converter, the resonance of the assembly implying the presence of a nut or any metallic element tuned to the frequency of work in extension of the connecting elements. It will be specified that the ultrasonic converter can be constituted by one or more transmitters of any kind, it can indeed be electrostrictive transmitters. magnetostrictive, electrocapacitive, or even piezoelectric.

It will be observed that the coupling bars have an outer surface which has been profiled so as to adapt to the particular dimensional configuration of the structure and so as to respect the resonance.

Thus in the ultrasonic device of FIG. 1, there is illustrated a way of intervening over the length of a half-wave at the frequency of the converter 12. In the embodiment of FIG. 1, the surface outside of the coupling bars 14 has, in the vicinity of their nodal zone, a radial narrowing 16 having a symmetry of revolution about the axis of said bars 14.

This particular embodiment in which the coupling bars have a narrowed shape in the vicinity of their nodal zone due to the presence of a groove or the like 16, makes it possible to reduce the coupling bars 14, while retaining the resonance.

Conversely, the embodiment illustrated in FIG. 2 involves coupling bars 14 whose external surface has, still in the vicinity of their nodal zone, a radial bulge 18 also having a symmetry of revolution about the axis bars 14.

This particular profiling adopted in the embodiment of FIG. 2 makes it possible, on the contrary, to lengthen the length of the coupling bars 14, still respecting the natural frequency of the converter 12.

The embodiment illustrated in Figure 3 shows the vibration of a support frame 10 on which is maintained a filter cloth 20. The attachment of the vibration device is carried out on two opposite sides 22, 24 of the support frame 10. The electroacoustic device used consists in this configuration of a unidirectional converter 12, of a coupling bar 26 which is tuned as an integer multiple of half wavelength, as well as of a nut 28 also tuned in half length wave. Note that the attachment of this set of vibration on the support frame 10 is also carried out in a belly area of amplitude. The connection between the different elements present, namely, the converter 12, the coupling bar 26 and the tuned nut 28, is advantageously obtained by screwing through orifices made in the opposite parts of the support 10. However, note that the rigid connection of the setting device in vibration with the structure 10 can be achieved by any other suitable means, the main thing being to obtain a perfectly rigid connection between the structure and the vibrational device. This is how recourse may be had to forcibly fitting the various bodies present, or else to bonding and / or welding.

In the embodiment of FIG. 3, it is also possible to replace the through holes, receiving the connecting threads between the converter 12, the coupling bar 26 and the tuned nut 28, by slots opening out at the upper edge of the elements 22 of the support frame. The narrowed areas of connection between the different elements 12, 26 and 24 can thus force-engage in said slots to ensure the rigid connection with the support frame 10.

FIG. 4 illustrates another embodiment making it possible to resonate a circular structure 30 which is not tuned, between fixing lugs 32 and 34 secured to the structure 30. In this particular embodiment, two end bars 36 and 38 ensure the tightening of the electroacoustic assembly on the aforementioned fixing lugs 32 and 34. The coupling bars 36 and 38 are tuned in an integer multiple of half wavelength of the frequency emitted by the converter 12.

Of course, if necessary, the external surface of the coupling bars 36 and 38 can be profiled to adapt to the particular configuration of the frame 30 while respecting the resonant frequency of the converter 12.

The embodiment illustrated in Figure 5 relates to a variant of the device shown in Figure 3. However in this particular case, the electroacoustic assembly constituted by the converter 12 and the two associated coupling bars 40 and 42 have been arranged outside the work area of the support frame 10 carrying the filter cloth 20.

In this design, the coupling bars 40, 42 can also be welded or preferably screwed to the converter, the connection with the extension of the parts of the support frame can also be advantageously carried out by screwing a nut granted on the threaded end 44 of each coupling rod 40, 42.

Again, any suitable rigid attachment means can be used, the main thing being to achieve a good mechanical connection between the support frame 10 and the ultrasonic device of the invention. This particular embodiment in which the ultrasonic device is arranged outside the working area of the structure 10, has the advantage of being able to be used for example in a humid atmosphere. It will also be noted that this design makes it possible to implant the structure, or even the entire structure and the electroacoustic device, within a sealed chamber where a controlled atmosphere of gas prevails.

FIG. 6 illustrates an embodiment in which a converter 12 produces the excitation of a stack of three identical structures 10 constituted by support frames of filter cloth. In this type of configuration, each of the structures 10 vibrates in its own mode and in resonance with the ultrasonic emission device. The connection of the various structures 10 with the frame of the entire ultrasonic device is non-rigid. For this purpose, use will advantageously be made of an elastomer type material which makes it possible on the one hand to secure the structure and on the other hand to seal it between each stage. In addition, each of the structures will be connected to the neighboring structure according to the same diagram, in order to preserve the phenomenon of acoustic resonance on each floor. The flexible connection produced by the elastomer-based material also ensures sealing while not restricting the various stages of the structure and thus allowing them to enter into resonance. As in the previously defined embodiments, the converter 12 is made integral with fixing lugs 46 by using a plurality of coupling bars 48, preferably associated with one another by screwing. Of course, in accordance with the present invention, the fixing members constituted by the coupling bars will be tuned to the frequency of the converter 12 produced in half wavelength.

FIG. 7 illustrates another variant of a device for exciting a stack of several non-tuned circular structures 10. The device shown is in all respects similar to that shown in FIG. 6, with the exception, however, that the structures here have circular shapes. Finally, Figure 8 illustrates a final embodiment in which the vibrating device comprises an ultrasonic converter 12 coupled by screwing to a coupling rod 50, produced at half wavelength and coming to enclose a circular plate 52 constituting the upper disc of a cylindrical drum 54 can for example play the role of support member of a mesh or a filter.

In all of the embodiments described above, it may be advantageous to adapt the nature of the ultrasonic transmission elements to the materials constituting the structure to be put into acoustic vibration. This can be achieved by adapting the acoustic impedances. This avoids any heating phenomenon at the level of the connections, and this therefore also makes it possible to increase the electroacoustic efficiency of the assembly set in vibration.

Still in order to improve the performance of the ultrasonic device according to the invention, it may be advantageous to use two non-quadrature emission frequencies simultaneously, so as to eliminate the nodal zones on the work surfaces, for example on the canvas. vibrant. By using for example two ultrasonic elements of frequency not in quadrature, such as 20 and 30 kHz, the presence of nodal zones, at the level of the filter cloth, zones which are precisely inactive regions, is limited as much as possible.

Finally, it should be noted that the ultrasonic vibration device according to the invention can be used interchangeably and in superposition with any other low frequency vibrating device commonly encountered on the market. The emission of ultrasounds communicated to the non-tuned structure can be carried out continuously or in a pulsed mode. As indicated above, it can be produced by superimposing low frequency vibrations in a range of 100 to 3000 vibrations / min. for amplitudes of the order of 1 to 30 mm and preferably in a range of 300 to 1,500 vibrations / min. for amplitudes of the order of 5 to 20 mm.

Depending on the particular application envisaged, recourse will be had, to ensure the emission of ultrasound, either to bidirectional symmetrical electroacoustic converters or to unidirectional asymmetrical converters. In addition, the amplitude of the ultrasonic vibrations is adapted to the product treated in said structure and is advantageously between 2 and 30 microns peak to peak and preferably between 5 to 20 microns peak to peak.

Claims (14)

1. Device for causing an untuned structure to vibrate at an ultrasonic frequency comprising at least one electro-acoustic converter (12) adapted to vibrate in a vibration direction and fixed rigidly to said structure (10, 30) by metal fixing members (14, 26, 28, 36, 38, 40, 42, 48, 50) tuned to the frequency of the converter, the couplings with the structure being disposed in a region of maximum amplitude (V) of said fixing members, characterised in that the metal fixing members are disposed in said vibration direction of the converter and in that, said structure being untuned at said ultrasonic frequency, the whole is caused to resonate by the presence of a nut or any metal assembly tuned to an integer multiple of half the wavelength.
2. Ultrasonic device according to claim 1 characterised in that said metal fixing members comprise at least one bar (14) coupling to said structure (10) the exterior surface of which may be shaped to match any dimensional configuration of said structure whilst remaining compatible with the resonant frequency.
3. Ultrasonic device according to claim 2 characterised in that the exterior surface of the coupling bar (14) has in the vicinity of its nodal zone a radial contraction (16) which has symmetry of revolution about the axis of said bar (14).
4. Ultrasonic device according to claim 2 characterised in that the exterior surface of said bar (14) has in the vicinity of its nodal zone a radial enlargement (18) which has symmetry of revolution about the axis of said bar.
5. Ultrasonic device according to any one of claims 1 to 4 characterised in that the operating frequency is between 10 kHz and 100 kHz and preferably between 20 kHz and 40 kHz.
6. Ultrasonic device according to any one of claims 1 to 5 characterised in that the ultrasound emission mode is pulsed or continuous.
7. Ultrasonic device according to any one of claims 1 to 6 characterised in that the amplitude of the ultrasonic vibrations is adapted to the product treated in said structure and is advantageously between 2 microns and 30 microns peak-to-peak and preferably between 5 microns and 20 microns peak-to-peak.
8. Ultrasonic device according to any one of claims 1 to 6 characterised in that two emission frequencies not in quadrature are used simultaneously to eliminate nodal zones on the operative surface of said structure.
9. Ultrasonic device according to any one of claims 1 to 8 characterised in that an ultrasonic converter (12) causes to vibrate and resonate a stack of untuned structures (10) fastened together at the points of connection to said electro-acoustic device.
10. Ultrasonic device according to claim 9 characterised in that said untuned structures vibrating and resonating are disposed in an enclosure adapted to contain a moist atmosphere.
11. Ultrasonic device according to any one of claims 1 to 10 characterised in that the electro-acoustic/structure assembly is disposed in a sealed chamber.
12. Ultrasonic device according to any one of claims 1 to 11 characterised in that the mode of emission of ultrasound to the untuned structure is continuous or pulsed and superimposed on low-frequency vibration in the range of frequencies from 100 vibrations/minute to 3 000 vibrations/minute for amplitudes in the order of 1 mm to 30 mm and preferably in the range of frequencies from 300 vibrations/minute to 1 500 vibrations/minute for amplitudes of 5 mm to 20 mm.
13. Ultrasonic device according to any one of claims 1 to 12 characterised in that the ultrasound is emitted by symmetrical bidirectional or asymmetrical unidirectional electro-acoustic converters.
14. Ultrasonic device according to any one of claims 1 to 13 characterised in that said structure is a screening structure.

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