WO1991001814A1

WO1991001814A1 - Manufacture of electromechanical resonators which are electrically excited and piloted by the magnetostrictive effect

Info

Publication number: WO1991001814A1
Application number: PCT/FR1990/000609
Authority: WO
Inventors: André BILLMANN
Original assignee: Billmann Andre
Priority date: 1989-08-10
Filing date: 1990-08-10
Publication date: 1991-02-21
Also published as: FR2650766A1; FR2650766B1

Abstract

The invention relates to the manufacture of composite electromechanical resonators with a very high vibratory amplitude. These transducers comprise an electric excitation unit (3) (piezoelectric or magnetostrictive). The metal masses (30 and 40) consist in whole or in part of ferromagnetic alloys selected for their low internal coefficient of friction. A double servo system makes it possible: to power the electric excitation at the resonant frequency of the resonator, whatever the latter's drift; to re-establish the maximum amplitudes of the mechanical vibrations, whatever the acoustic impedance and damping of the noiseless medium. Its operation is based on the magnetostrictive effect. In fact, the vibrations of the ferromagnetic mass (30) produce variations in the magnetisation in this material which are reflected by flux variations across the coil (60). The resultant induced current (20) correctly represents the state of vibration of the material. This current is subjected to a simple electronic treatment in the filter-dephaser (1) before being amplified in amplifier (2) powering the electric excitation unit (3). In addition, the electronic comparator (4) maintains the mechanical vibrations at their maximum amplitude through the action of the controlling device (5) which continuously rebalances the resonator at the level of the free end (31) of the damping mass (30) and may maintain the fixation plane in harmony with a nodal plane (6). These resonators constitute powerful ultrasonic generators and also facilitate the study of: high-amplitude vibrations and their propagation in various materials; the properties of these materials in the non-linear field and especially their fatigue and ageing.

Description

REALIZATION OF ELECTROMECHANICAL RESONATORS, ELECTRICALLY EXCITED AND SERVOED BY MAGNETOSTRICTIVE EFFECT.

The present invention relates to the production of very efficient electromechanical resonators intended to generate an intense flow of power ultrasound.

In the current technique, the conversion of electrical energy into mechanical acoustic energy is done by means of devices called transducers (or translators).

In the case of power ultrasound, composite electromechanical resonators are often used as transducers, constituted by a set of metallic masses, generally of different nature and / or shape, set in vibration at frequencies between 10 and 90 kHz, by means of an effect linked to electric current such as piezoelectricity (or magnetostriction, etc.). Therefore, this type of acousto-mechanical resonator, also called sonotrode, can be considered as the association of an electrical excitation dipole _n interaction with a mechanical resonator made up of metallic masses, whether or not symmetrical.

The most effective devices are generally based on piezoelectric excitation and vibrate in a longitudinal mode (compression waves); the length of the resonator must correspond to an integer of quarter wavelength (two or more).

As a general rule, a resonator intended to supply power ultrasound has an asymmetrical structure: the mass emitting ultrasound is called the front mass; the other mass is called rear mass or shock absorber.

However, the vibrational quality of such a resonator, that is to say its aptitude to be the seat of a system of on s of stationary resonance, requires that very strict boundary conditions be satisfied, mainly at the interfaces. in contact with the electrical excitation as well as at the free ends, but also at the level of the plane of attachment of the resonator on its support or in its protective case, plane which must coincide strictly with a nodal plane c vibrations. When these different conditions are met and the resonator is powered by a sinusoidal current at its resonant frequency, it is the site of very large amplitude vibrations (a few tens of micrometers) with very high acousto-mechanical energy flows ( a few tens of watts / cm ² in the resonator). It is even possible to increase the amplitude of the vibrations by using displacement amplifiers (for example conical or exponential). Unfortunately, this type of ultrasonic generator has several faults.

First of all, their vibratory ability can be greatly modified according to the mechanical properties (acoustic impedance, damping, etc.) of the insonified media which interact on the vibrating mass via the emitting interface: a displacement of the bellies is observed and vibration nodes as if the apparent length of the vibrating mass in contact with the soundproof medium were increased.

Then when such a resonator is at the maximum of its power, it is the seat of a significant heating due mainly to visco-elastic internal friction, in the metal parts where the particuiaire displacement is greatest, that is to say -to say in the vicinity of the bellies displacement vibrations (which correspond to constraint nodes). Not only does this loss of acousto-mechanical energy, in the form of ^" heat, limit the ultrasonic energy that can be used, but also heating leads to a noticeable shift in the resonance frequency (several tens of hertz). In addition, it is found that the vibrations of the metallic masses are no longer sinusoidal and their study by Fourier analysis shows the appearance of harmonics in considerable proportion, sometimes beyond the order of 10. Then the dipole of electric excitation has a nonlinear response. the current state of the art, there is no servo-control device making it possible to search for the maximum amplitude of the vibrations as a function of the sound-insulated medium, while most frequency servo-controls rely on the search for a maximum of the amplitude of the electric supply current and malfunction at high powers, because if there then appears a noticeable difference between the fundamental frequency ntale of mechanical resonance and the frequency corresponding to a maximum of the electric current.

The present invention relates to the production of electromechanical power resonators, of the type described above * - Its purpose is to considerably increase the vibratory amplitude of the resonator as well as the power of this useful ultrasonic delivered, whatever the conditions of use:

- thanks to a double automatic control acting on. the frequency of the electrical supply current so that it follows any possible drift in the resonant frequency of the resonator,. the apparent length of one or both vibrating masses in function of the variation of the mechanical properties of the insonified media, - by the use of devices for fixing the resonator on its support, devices which can be reduced to three types:. either a floating fastening with low mechanical coupling so as to minimize losses by energy transfer to the support,. either a slaved mobile attachment, with pneumatic or oleo-pneumatic locking, making it possible to constantly search for perfect coincidence with a nodal plane, in particular during changes in the soundproof environment,. or an immutable fixing placed between the piezoelectric plates, in a position which is a vibration node when the system is properly adjusted. - By the choice of suitable materials, having very low damping coefficients and internal friction, so that the very large vibrational amplitudes obtained do not cause unacceptable energy losses in the resonator, in the form of heat. (With regard to metallic materials, a certain number of alloys containing elements like nickel, cobalt, chromium, iron, etc ... meet these conditions.)

The automatic control systems used by the invention are based on the phenomenon of magnetostriction of which the ferromagnetic materials are the seat, so that these automatic control systems are based on the only veritable parameter, namely the amplitude of the mechanical vibrations whose masses metal of the resonator are the seat. Indeed it is well known that a bar of a ferromagnetic material is magnetized when it undergoes a longitudinal tensile force; the magnetization reverses for compression. It is the effect V i lari.

Under these conditions, if a solenoid surrounds the bar, the electromotive force e = - dΦ / dt, induced by the variation of magnetic flux due to the variation of magnetization, represents the derivative

(with respect to time) of the deformation of the bar, with very satisfactory fidelity. So lén oïd plays the role of

"mechanical vibration sensor".

To apply this property to electromechanical resonators, we can use:

- either a massive magnetostrictive material,

- either a non-ferromagnetic material, jacketed by a tube of ferromagnetic material 2 to 3 mm thick, or incrusted- tee on its surface by one or more small ferromagnetic rules. A sensor, consisting of a small coil of several hundred turns placed around the vibrating ferromagnetic mass, makes it possible to obtain an electromotive force of a few tenths of volts.

Then the frequency control is obtained automatically because the free vibration of the resonator induces, in the coil, a current which is amplified before returning to supply the electrical excitation of the resonator, at the frequency which gives the greatest amplitude. In reality, it is necessary to preamplify this current, filter it to select the driving frequency (in general the fundamental frequency but possibly one of its harmonics), integrate it and put it back in phase before sending it to the amplifier power. This type of frequency control, already known, is used in all of the embodiments of resonators concerned by the invention.

The enslavement of the apparent length of the vibrating masses is made necessary: - firstly because the mechanical properties of the materials, constituting these masses, vary with the temperature, and in particular the speed of propagation of the elastic waves, which involves a change in boundary conditions,

- then because the insonified medium interacts with the mass emitting ultrasound; indeed, when the emitting interface is in contact with a solid or liquid medium, it does not remain a constraint node (belly of vibrations): everything happens as if the length of this vibrating mass had been increased: here again there is modification of the boundary conditions.

This apparent elongation depends on the mechanical properties of the insonified medium (acoustic impedance and damping) and it is all the greater the denser or more absorbent the medium. In addition, it causes a small displacement of the ventral and nodal planes accompanied by a slight frequency drift.

It is possible to compensate for these apparent elongations by modifying the repo rt it i o n of the vibrating m a s s, by displacing m asses on the surface and / or inside the vibrating masses constituting the mechanical part of the resonator.

According to the invention, these masses are made up:

- Either by a set of two metal rings, internally threaded and externally toothed, which are screwed on a outer lettering of the cylindrical vibrating masses: an electric motor, the axis of which carries a pinion meshing with the external teeth of the two rings, makes it possible to control the rotation of the assembly of the two rings and therefore their position; a micro-piston or a deformable tube, with oleo-pneumatic control, makes it possible to block the two rings on the thread and make them integral with the vibrating masses,

- either by a tube, closed at one end, screwed onto the vibrating mass called shock absorber and blocking on its thread using a micro-piston with oleo-pneumatic control,

- either by small diameter tubes, drilled in the mass according to a diameter or wound in a spiral and welded to the periphery of the vibrating mass: mercury or gallium can be introduced into it in greater or lesser quantity by an oleo device pneumatic,

Instead of using a mass displacement to modify the boundary conditions at the level of the shock absorber, one can act on the free face of the latter by means of a powerful adjustable magnetic coupling, possibly reinforced by the use finely divided magnetic materials, in the form of powders or liquids (ferro-fluids).

The search for the maximum amplitude of vibrations is controlled by an electronic device hereinafter referred to as a comparator which analyzes the signals coming from one or more sensor-coils and detects any reduction in amplitude.

Below a certain adjustable value V _{seu j |} of the amplitude of vibration, the comparator successively controls the different devices fitted to the system and control:

- moving the weights, - changing the magnetic coupling,

- the displacement of the fixing plane, in order to restore the maximum amplitude of vibration. The control of each of the servos can be either suspended or activated manually. In all cases, a fundamental problem arises: to use the resonator, it is necessary to fix it on a support or inside a protection box or to suspend it. However, the fixation must be made at the level of a nodal plane of vibrations, otherwise the vibration amplitude will be considerably reduced.

Three fixing methods can fulfill these conditions.

- The preferred fixing method is the simplest but only applies to a piezoelectric excitation unlike two others. It consists of interposing, between the ceramics, a metal plate a few millimeters thick which will henceforth be designated by the name central plate. It allows easy fixing on any support. This plate exactly coincides with a vibration node, when its thickness is small, that the resonator is symmetrical or that it is properly adjusted, according to the invention.

- The second type of fixation consists in fixing the resonator by one of the metallic masses at the level of a nodal vibration plane. To this end, the mass concerned has a thread which is screwed into a tapped hole made in a fixing plate. In order to limit the contact in the immediate vicinity of the nodal plane, without reducing the rigidity of the connection, the plate is thinned on the edges * of the hole. Thus, a slight rotation produces a displacement of the resonator along its axis of revolution and allows the nodal plane to be brought back into the plane of the support. The search for this coincidence is one of the control functions devolved to the comparator which then blocks the resonator in the optimal position using a ring fitted with an oleopneumatic micro-piston.

- The third type of connection is reserved for simpler embodiments of the invention, limited to only frequency control and more specifically intended to produce physicochemical actions in fluids, by ultrasonic cavitation. It consists in using a suspension box, consisting of two parallel metal plates placed between metallic masses and electrical excitation, and which completely surrounds the latter. This housing has a flexible suspension device, by an axis or a ring for example. It has two tubes, serving as a passage for the various electrical conductors and allowing the circulation of a gaseous current. The latter creates an overpressure preventing any infiltration of liquid and ensures effective cooling.

The invention and its various embodiments will be better understood by referring to Figures 1 to 12.

Figures 1 and __ 2 show the principle of servos¬ ment by magnetostriction, using coil (s) -sensor (s) and a microprogrammed comparator. Figures 3 and 4 show the use of liners and superficial inlays of ferromagnetic materials.

Figures 5 to 8 show how it is possible to change the apparent length by moving rings (Figures 5 and 6) or less dense liquids (Figures 7 and 8).

REPLACEMENT SHEET Figure 9 shows schematically an embodiment with frequency servo and magnetic adjustment with the preferential fixing of the central plate, on a shoulder inside the protective housing. Figures 10 and 11 show schematically the principle of fixing by carrier plate, subject to remain in coincidence with a nodal plane.

FIG. 12 represents an embodiment of the invention, limited to only frequency control and provided with a suspension box ensuring electrical isolation.

FIG. 13 shows a focusing tip calculated to obtain a maximum concentration 15 mm in front of the sonotrode. In the production of a simple resonator model, the mass 30 (damper) is made of a dense material, while the mass 40 (emitter) is made of a material of low density, so as to be driven by displacements particles of greater amplitude, at the plane 41 which may possibly be a free end, emitting ultra-sounds. (For example, for a half-wave resonator vibrating at 20 kHz, the length between the interfaces 31 and 41 is of the order of a deci¬ meter). However, nothing prevents the use of identical masses to constitute a symmetrical resonator.

In a more elaborate embodiment, the mass 40 is extended beyond the plane 41, by a speed transformer in order to obtain displacements of the order of a millimeter. So,

420 shows schematically an exponential speed transformer in which 41 and 44 represent the traces of two ventral planes of vibration while 43 corresponds to a nodal plane. The coils

60, 61 and 62 allow phase comparisons to be made. The electrical excitation is represented in 3. It can be magnetostrictive or piezoelectric. In the latter case, the assembly of the piezoelectric ceramics 3, prestressed by clamping between the masses 30 and 40, is carried out according to one of the techniques known in the current art (central screw, peripheral screws, etc.).

The electrical contacts are provided by the washer 8 (in beryllium cui¬ v) e '^' _ plate 9 which, in the preferred embodiment, serves to fi r the resonator on the shoulder 76 secured to the protective housing 75. In addition, the plate 9 ensures efficient cooling of the ceramics 3.

The control, on which the invention is based, consists in using a ferromagnetic material to constitute the two masses 30 and 40 or simply one of them, for example the

REPLACEMENT SHEET mass 30. If the massive use of a ferromagnetic material has a drawback, one can simply encrust ferromagnetic bars 310 according to some of the generators of mass 30 and / or mass 40 or line with a ferromagnetic tube 311.

The coil 60 comprises a few tens to a few hundred turns and captures the variations in the magnetic flux due to the change in the magnetostrictive magnetization of the mass 30 according to its vibrational state. The current 20, induced in the coil 60, has a phase shift relative to the current 27 supplying the electrical excitation 3.

This is why it is preamplified, filtered, integrated then put back in phase in the amplifier-phase shifter 1 before being sent in the power amplifier 2 which feeds the electrical excitation 3. The chain 60, 1, 2 and 3 allows the resonator to be maintained indefinitely on its resonant frequency, instantly upon power-up, without prior adjustment or control.

On the other hand, part of the current induced in the coil 60 is sent to the electronic comparator 4, which comprises a micro-programming and controls the various mechanisms 5 for adjusting the apparent length and / or the displacement of the fixing plane, in order to reduce the vibrations to their maximum amplitude.

In one of the possible embodiments, the adjusting device 5 is constituted by an electric motor (not shown) whose axis 51 carries a toothed wheel 511 which rotates the twin rings 320 and 321, screwed onto the external threads of the masses 30 and or 40. The ring 321 carries a tube of elliptical section 322 and / or micro-pistons 323. Under the action of an oil pressure, controlled by an oleo-pneumatic pump belonging to the device 5, the elliptical tube (and / or the piston) deforms, exerts pressure on the ring 320 and causes the blocking of the two rings 320 and 321 on the threads of the thread 301.

In a second embodiment, the masses 30 and / or 40 comprise capillary tubes, either diametrical 340, or welded at their periphery, connected by the flexible tubes 342 to an oleo-pneumatic control device. Under the action of an oil displacement, a certain quantity of a dense liquid (mercury, gallium, etc.), contained in the reservoir 343, is pushed into the capillary or vice versa brought back into the reservoir so as to adjust the apparent length of the vibrating mass.

In another embodiment, the mass 30 constitutes the armature of a magnetic circuit constituted by the parts 549,

REPLACEMENT SHEET 550 and 551, with high magnetic permeability. This magnetic circuit, fixed to the protective housing 75 via a block of hard rubber 54, is magnetized by the coil 552 which receives from the comparator a servo current 554. This device makes it possible to exert a very energetic action. on the ground

30. The effect can be further enhanced by the use of finely divided magnetic materials 555, placed in the air gap 556; 557 is a second, annular air gap, while 553 represents an induction line. It will be noted, in this arrangement, the use of the preferred method of fixing by central plate 9

The second type of attachment consists of the carrier plate 70, 10 to 12 millimeters thick, pierced with a tapped hole 72. The plate 70 is thinned around the hole 71, to a thickness of 4 to 5 millimeters at the thread 72. The tapped hole 72 is screwed onto the thread 301 of the cylindrical mass 40 or that of the mass 30. In this case, the mass 30 and / or the mass 40 are optionally extended by a metal slice corresponding to half a wavelength. The comparator 4 controls, through the adjusting device 5, the rotation of the resonator in order to search, by small transverse displacements, for the coincidence between the nodal plane of the mass 30 or 40 and the median plane 700 of the carrier plate 70. When this is carried out, the resonator is locked. Locking is provided by a threaded ring 321, provided with a deformable elliptical ring 322 and / or micro-piston 323.

In another embodiment, the fixing is done by means of a housing 90 comprising a suspension device 92 fixed to two parallel plates 91 which bear on the shoulders 94 which the masses 30 and 40 have in this type of embodiment. The assembly and the prestressing are ensured by the tightening screw 95 and the support washer 96. 98 represents an electrical isolation ring. Two pipes 93, only one of which is shown, allow the passage of electrical conductors and pressurization by circulation of the gas stream 97.

To facilitate the drawing, only the excitation by piezoelectric ceramic discs has been shown, but all of the above is valid, in principle, for magnetostrictive excitation.

In certain particular embodiments, it may be useful to use several sensor coils 60, 61, 62, 63 and 64 whose respective induced currents 20, 21, 22, 23 and 24 are

REPLACEMENT SHEET analyzed by an elaborate version of comparator 4 in order to characterize the vibrations of masses 30 and 40 as well as their propagation,

Finally, in certain embodiments, it may be useful to use a focusing tip whose surface 304 is calculated to concentrate the acoustic energy at a point F located a few centimeters from the plane 305.

In conclusion, the use of ferromagnetic materials, associated with the use of appropriate sensors and an analysis device (comparator), allows:

- to obtain currents characterizing the vibratory state of the system at its different points,

- to follow any drift of the resonant frequency Fo,

- to compensate for the excursions of the nodal and ventral planes, so as to preserve the conditions required for optimum resonance.

The use of metallic materials with a very low coefficient of internal friction considerably improves the mechanical efficiency. Thus, the invention makes it possible to produce very efficient resonators which constitute sources of very intense ultra¬ sounds, stable in frequency and in power.

It should be noted that, in addition to the usual uses of sonotrodes, these power resonators, frequency-controlled and driven by strong mechanical vibrations, make it possible to study - compression waves of very large amplitude (closer to the wave shock than sinusoidal vibration), - properties of materials under the effect of such a stress (damping, non-linear propagation, viscoelasticity, internal friction, fatigue, etc.). Thus, the invention also allows the study of certain materials, their fatigue and their aging under constraints of the order of a hundred megapascals, at frequencies varying between 10 kHz and 100 kHz.

REPLACEMENT SHEET

Claims

1. Method, applicable to the production of various types of electromechanical composite resonators, capable of numerous applications in different fields, using:

- the creation of induced electric currents, thanks to the magnostrictive effect, by the mechanical vibrations animating these resonators

- and frequency control by these currents, characterized in that it allows:

- obtaining, after integration, electrical signals proportional to mechanical vibrations and representing them with satisfactory fidelity,

- visualization, recording and analysis (in particular by Fourier transformation) of the electrical signals generated by mechanical vibrations, - adaptation to changes in boundary conditions linked to changes in the acoustic impedance of soundproofed media,

- the reduction of energy losses, in the form of heat, by the use of suitable materials, having a very low coefficient of internal friction, - the forced resonance of a mechanical system constituted by a set of metallic masses in particular masses 30 and 40,

- the production of vibrations of large amplitude (of the order of a millimeter), - the implementation of very strong constraints, of the order of a hundred megapascals,

- the use of a frequency range from 10 kHz to 100 kHz, said process making it possible: - to provide efficient power ultrasonic generators,

- to produce sources of intense cavitation in liquids,

- to study, in various materials: - the shape of large amplitude mechanical vibrations as well as their spectral distributions, from the sine wave to the shock wave, - the fatigue and aging of these materials at frequencies varying between 10 kHz and 100 kHz.

2. Control device for implementing the method, according to claim 1, characterized in that it comprises:

REPLACEMENT SHEET - ferromagnetic materials to constitute the resonator, totally (mass 30 and / or mass 40) or partially (inlays 300 or jacket 301),

- a sensor coil 60 (or several coils 61, 62, etc.), surrounding the mass 30 (and / or the mass 40), coil (s) intended ^) to be supplied, thanks to the magnetostrictive effect, a electrical voltage 20 (and or electrical voltages 21, 22, etc.) induced by variations in magnetic flux due to variations in magnetization produced by mechanical vibrations in ferromagnetic masses.

3. Control device according to claim 2, characterized in that it comprises:

- a comparator 4 allowing:

. to obtain an electrical voltage (s) representative of the vibratory state of the system (at one and / or several points).

. and / or compare the current value of this voltage with a previous value and / or with a set value,. and or to compare the voltages induced in the coil 60 with each other, with the voltage 21 induced in the coil 61,. compare and / or take into account the voltages induced in all of the cap coils 60, 61, 62, etc.

- A set of adjustment apparatus 5, controlled by the compa¬ rator 4 and making it possible to modify the apparent length of any one or vibrating masses 30 and / or 40 so as to obtain or recover the maximum amplitude of vibration.

4. Apparatus for modifying the apparent length of the threaded vibrating masses, by displacement and blocking of monkfish on this thread, according to claims 2 and 3, characterized in that it consists of two twin rings, provided with a file - internal stage and external toothing, driven together in rotation by means of a pinion, one of the rings being capable of exerting on the other a high pressure by means of a deformable part, under the action of an oleo-pneumatic control.

5. Apparatus for modifying the apparent length by displacement of mass on the surface or inside the resonator according to claims 2 and 3, characterized in that it comprises tubes, of very small internal diameter, drilled in the mass of the resonator or wound and welded to its surface, capable of being more or less filled with a dense liquid by an oleo-pneumatic means.

6. Apparatus for modifying the apparent length according to claims 2 and 3, characterized in that it comprises a magnetic circuit 549, 550 and 551 allowing to exert an action very energetic on mass 30, action reinforced if necessary by, magnetic materials 555 (powders or ferrofluids).

7. A fixing device for implementing the method according to claims 2 and 3, in a simplified version, characterized in that it comprises a fixing of the resonator on any support or inside a protective housing 70 , via the shoulder 76 by means of the plate 9

(called central).

8 Slave device (or triggered manually) according to claims 2, 3 and 4 characterized in that it comprises, in certain embodiments:

- a single or multiple thread thread 301 on one of the masses 30 and or 40 of the resonator,

a tapped hole 72, with thinned edges 71, made on the plate 70 and intended to receive the screw 301, in order to fix the resonator on a frame or in a protective case,

a rotary system allowing the resonator to be displaced along its axis, by simple screwing or unscrewing, so as to bring a nodal vibration plane in coincidence with the mean plane 700 of the plate 70 then its blocking,

a blocking system intended to immobilize the resonator, in the optimal position, on the thread 301, using a threaded ring 321, held close to the plate 70 by a flexible fixing and capable of exerting on it high pressure by oleo-pneumatic control 322.

9. Device according to claims 2, 3 and 4, characterized in that it comprises, in certain embodiments, a mechanical sound connection housing 90 comprising: two parallel plates 91 bearing on the shoulders 94, a fixing system 92 , two pipes 93 for the passage of electrical connections, the circulation of a gas stream 97 and the creation of an overpressure, housing constituting a suspension or fixing means and a sealed electrical isolation.

10. Device according to claims 2 and 3, characterized in that it comprises, screwed to the emitting end, a focusing tip whose shape is calculated to obtain a concentration of acoustic energy at a point on the axis of the resonator and a few centimeters from the transmitting end.

REPLACEMENT SHEET