EP0385889A1 - Piezoelectric unit with a flat supporting surface and elastic holding elements - Google Patents

Abstract

The capsule according to the invention comprises a hollow metal cover (8) fitting onto a base body (7) made up of a printed circuit, and a piezoelectric membrane (1) fitting to rest against an anterior step (17 ) of the cover (8). An elastic holding member (22), fixed to the base body (7), is supported on the central zone of the rear face (5) of membrane (1). The elastic member (22) simultaneously ensures the mechanical retention of the membrane (1) and the peripheral seal, by the fact that the corresponding surfaces of anterior step (17) and of membrane (1) are perfectly flat.

Description

The present invention relates to piezoelectric transducers, the vibrating element of which consists of a membrane with a layer of piezoelectric material provided with electrodes and maintained around its periphery in a capsule housing.

This type of capsule has been known for many years, and is described, for example, in documents GB-A-2,046,554, JP-A-58,202,699, or JP-A-60 199298.

In these known capsules, the membrane is formed of a thin disc on which a layer of piezoelectric ceramic is secured, covering the central zone of the disc, the two faces of the layer of piezoelectric ceramic being metallized to respectively constitute an internal electrode applied against the thin disk and an external electrode. The membrane is housed in a housing formed by a base body and a cover which adapts to one another along a closed periphery, generally circular. The basic body forms a first wall substantially parallel to the disc and delimiting with the disc a posterior acoustic cavity. The cover forms a second wall substantially parallel to the disc and delimiting with the disc an anterior acoustic cavity. The housing includes a surface, for example in an inner peripheral step, against which the periphery of the disc is kept applied by holding members. Electrical connection means provide the respective electrical connections of one and the other of the electrodes with a first and a second connection terminal outside the housing.

In known capsules, the mechanical retention of the membrane relative to the housing is ensured by a rigid or elastic element in abutment around the entire periphery of the membrane disc.

Thus, in document GB-A-2 046 554, the periphery of the disc bears on a peripheral surface of the base body, against which it is held in abutment by a peripheral O-ring inserted between the cover and the disc. In documents JP-A-58 202699 or JP-A-60 199298, the disc is also held in abutment against a peripheral surface of the case, against which it is pushed by a peripheral O-ring inserted between the disc and the case.

The inventors have found that the presence of such O-rings in a capsule has drawbacks when mass production; in fact, the known seals which can be used in large series are generally of irregular dimensions, which induces flatness defects in the holding surfaces of the piezoelectric membrane, and dispersions in the acoustic qualities of the capsules thus obtained. Defects linked to the presence of O-rings also result from thickness dispersions of the other stacked parts constituting the capsule, in particular the base body and the cover, creating a gripping force that is difficult to control around the periphery of the disc. The mechanical holding means according to the invention make it possible to avoid these dispersions, and ensure the holding of the membrane without using an O-ring seal.

According to another object of the invention, the new capsule structure results in a substantial increase in the possibilities of deformation of the peripheral zone of the disc, so that the electroacoustic qualities of the capsule are significantly improved.

When it is sought to overcome the presence of an O-ring seal or a peripheral embedding of a membrane, the difficulty comes from the fact that the seal or embedding, in known capsules, not only ensure that the membrane remains in contact with the housing step, but also provides a seal between the anterior acoustic cavity disposed in front of the membrane and the posterior acoustic cavity disposed behind the membrane. For this, the particular means according to the invention, ensuring the mechanical retention of the membrane in the housing, make it possible to simultaneously produce a sufficient seal between the anterior and posterior acoustic cavities.

Such sealing is further obtained, according to the invention, in combination with a structure leading to substantially increase the possibilities of deformation of the peripheral zone of the disc. A priori, the two functions lead to contrary requirements, which the invention makes it possible to reconcile.

The present invention also provides specific embodiments leading to the general improvement of the electroacoustic qualities of the capsule. In particular, it is sought to distribute the natural frequencies of mechanical vibration of the various organs of the capsule, so as to obtain a most flat possible in the usual operating frequency range.

Another desired effect is to obtain a good compromise between the acoustic effect of the capsule and its dimensional size, in particular its thickness.

To achieve these and other objects, the capsule according to the invention incorporates the main elements of known capsules, namely a membrane formed by a thin disc carrying a layer of piezoelectric ceramic covering the central area of the disc, a housing formed a base body and a cover, a peripheral step of the case against which the periphery of the disc is held applied by elastic retaining members, first and second electrical connection terminals connected to the electrodes of the piezoelectric membrane by first and second electrical connection members; according to the invention:
the thin disc has a peripheral part of its front face having a substantially planar shape, forming an area of contact with the front peripheral step,
the anterior peripheral step has a precise circular reach defining a continuous flat surface of contact with the disc,
- The elastic holding members are supported on the intermediate zone of the rear face of the membrane.

The precise circular surface defining a flat continuous surface of contact with the disc has a surface quality such that the flatness is ensured with a tolerance less than or equal to 0.03 mm.

Such precision can be obtained by usual methods of machining a blank, for example by turning. The usual stamping methods, commonly used for the production of aluminum elements, do not allow sufficient precision to be obtained for mass production.

According to the invention, it is advantageously possible to produce the cover by a fluo-turning method, which consists in placing a blank in a mandrel having the desired external shape of the cover, and in causing the material to flow by penetration into the blank. a punch having the internal shape of the cover to be obtained. It turns out that such a method is entirely appropriate for obtaining the precision in surface quality sought.

According to the invention, the step of the housing, produced in the cover, advantageously has, in radial section, a trapezoid profile whose small base forms the precise circular bearing surface of the membrane, said circular bearing being separated from the wall. lateral cover by a peripheral groove allowing free oscillation of the disc edge forming the membrane.

The inventors have found that it is particularly advantageous, in particular to obtain the widest and flattest possible frequency response curve of the capsule, to provide that the elastic retaining members are supported on the membrane in the zone located in the vicinity of the nodal circle of the membrane, that is to say the area formed by the location of the nodes of the third mode of circular vibration of the membrane, said elastic holding members being supported on said membrane at at least two points distant from each other.

According to a first embodiment, the elastic holding members may comprise several elastic holding elements which are distinct from each other, each bearing on the one hand on the rear face of the membrane and on the other hand on the base body of the housing .

According to another embodiment, said elastic retaining members can be assembled into a single element with several branches, comprising a single support area on the base body, the ends of the branches being supported on the rear face of the membrane.

Preferably, to each fulcrum of the elastic holding members on the external electrode corresponds an elastic connection with different mechanical characteristics, thus having different own vibratory modes.

A good compromise between the acoustic efficiency of the capsule and its dimensional size is obtained according to the invention by combining a membrane whose natural frequency is approximately 1 kHz with an acoustic filter comprising an anterior acoustic cavity and a posterior acoustic cavity whose respective heights are approximately 1 mm, for a diameter of approximately 30 mm. The heights of approximately 1 mm allow the membrane to oscillate freely, and allow the connection members to be accommodated.

• - la figure 1 représente une vue de dessous de la surface extérieure du corps de base de capsule selon l'invention ;
• - la figure 2 est une vue de face d'une capsule selon l'invention en coupe selon le plan A-A de la figure 1 ;
• - la figure 3 est une vue de côté de la capsule selon l'invention en coupe selon le plan B-B de la figure 1 ;
• - la figure 4 représente en vue de dessous une membrane de capsule selon l'invention ;
• - la figure 5 représente en vue de dessus un mode de réalisation particulier des organes élastiques de maintien selon l'invention ; et
• - la figure 6 illustre les résultats obtenus par une capsule selon la présente invention relativement à la réponse en fréquence.

Other objects, characteristics and advantages of the present invention will emerge from the following description of a particular embodiment, given in relation to the attached figures, among which:

• - Figure 1 shows a bottom view of the outer surface of the capsule base body according to the invention;
• - Figure 2 is a front view of a capsule according to the invention in section along the plane AA of Figure 1;
• - Figure 3 is a side view of the capsule according to the invention in section along the plane BB of Figure 1;
• - Figure 4 shows a bottom view of a capsule membrane according to the invention;
• - Figure 5 shows a top view of a particular embodiment of the elastic holding members according to the invention; and
• - Figure 6 illustrates the results obtained by a capsule according to the present invention relative to the frequency response.

As shown in the figures, a piezoelectric capsule according to the invention comprises a membrane 1 formed by a thin disc 2, with circular outline, on which is fixed a layer 3 of piezoelectric ceramic covering the central zone of the disc 2. In the mode embodiment shown, the disc 2 is made of an electrically conductive material, for example brass, and the piezoelectric ceramic layer 3 has a concentric circular contour with respect to the disc 2, covering the central area of the disc and leaving a visible border device of the disk face 2 on which it is applied. The piezoelectric ceramic layer is metallized on its two faces, its internal face 4 forming an internal electrode and being pressed against the disc 2, its external face 5 forming an external electrode and being visible.

The membrane 1 is housed in a housing 6 formed by a base body 7 and a hollow cover 8, secured to one another along a closed periphery 9, for example circular as shown. The base body 7, in the embodiment shown, consists of a flat plate of insulating material, for example a material generally used in the technique of printed circuits, the outer face 10 of which carries conductive tracks 11 and 12 shown in Figure 1. The cover 8 is made of a conductive material electricity, for example aluminum or aluminum alloy, and has a circular central wall 13 connecting to a peripheral side wall 14. The peripheral wall 14 includes a rear step 15, forming a narrowing of the interior space of the housing 6 as shown in Figure 3 or Figure 2, against which rests the periphery of the base body 7. The edge 16 of the peripheral wall 14 is folded inwardly to come to bear on the outer face 10 of the body base 7, and crimp said base body 7 on the cover 8.

The peripheral wall 14 of the cover 8 further comprises an anterior step 17, forming a second narrowing of the interior space of the housing 6 as shown in FIGS. 2 and 3, arranged approximately halfway between the central wall 13 and the step posterior 15. The membrane 1 is held pressed against the front step 17 of the cover 8, by elastic members which will be described below.

When the capsule is assembled, as shown in Figures 1 to 3, the base body 7 constitutes a first wall substantially parallel to the disc 2, with which it forms a posterior acoustic cavity 70, and the central wall 13 of the cover 8 is substantially parallel to the disc 2 and forms with it an anterior acoustic cavity 80.

In a known manner, the central wall 13 of the cover 8 has holes 130 for the passage of sound from the anterior acoustic cavity 80 to the external atmosphere, and the base body 7 comprises a passage 71 provided with an acoustic filter connecting the posterior acoustic cavity 70 and the external atmosphere.

According to the invention, the respective heights H1 and H2 of the anterior 80 and posterior 70 acoustic cavities are advantageously equal to approximately 1 mm, for example between 0.9 and 1.1 mm.

A first electrical connection terminal 18 is fitted on the base body 7 and protrudes from the external face 10 of said base body 7 to be electrically connected to an external circuit. Likewise, a second electrical connection terminal 19 is fitted to the base body 7 and protrudes from the outer face 10 of said base body 7 to be connected to an external electrical circuit. The first electrical connection terminal 18 is connected by welding to the first conductive track 11 of the base body 7, said conductive track 11 comprising a peripheral portion 20 in contact with the cover 8. Thus, the first electrical connection terminal 18 is electrically connected to the internal electrode 4 via the conductive disc 2, the conductive cover 8 and the conductive track 11.

The second electrical connection terminal 19 is electrically connected to the external electrode 5 of the membrane. This connection can for example be provided by traditional means such as those described in the previous patents, for example an electrical conductor in the form of a wire welded on the one hand to the connection terminal and on the other hand to the external electrode 5 .

An elastic holding member 22 keeps the membrane 1 in abutment against the front step 17 of the cover 8, while freeing the periphery of the membrane. For this, the elastic retaining member 22, in support or fixed to the center of the base body 7, for example fixed by the rivet 21, has its own elasticity and a shape such that, when the capsule is assembled as shown by Figures, with the membrane 1 inserted between the cover 8 and the base body 7, and with the elastic holding member 22 inserted between said membrane 1 and the base body 7, the elastic holding member is in abutment against the intermediate zone of the posterior face of the membrane and elastically pushes the membrane 1 in the direction of the cover 8 to press its periphery in abutment against the front step 17.

The resulting bearing force exerted by the elastic member 22 on the membrane 1 is advantageously between 0.3 Newton and 0.5 Newton, for a membrane whose diameter is equal to approximately 30 mm.

In the embodiment shown, the front step 17 has, in radial section, a trapezoid profile, the small base 170 of which forms a circular crown and constitutes the bearing surface of the disc 2, the said small base being separated from the peripheral wall 14 by a peripheral groove 171.

This embodiment of anterior step 17 with small base 170 and groove 171, associated with the maintenance of the membrane by the elastic holding member 22, ensures an improvement in the frequency response curve of the capsule. Thus, FIG. 6 represents two comparative frequency response curves relating to capsules of identical dimensions, curve 100 corresponding to a capsule in which the disc is held, by an O-ring, in abutment on a housing step, the curve 101 corresponding to a capsule according to the invention.

According to the invention, it is sought to obtain a good seal between the periphery of the membrane 1 and the cover 8. Sufficient sealing is obtained by providing for the use of a very flat disc 2, having in particular on the peripheral part of its front surface a sufficient surface condition and flatness. Simultaneously, the circular bearing formed by the small base 170 has sufficient flatness, so that the two corresponding surfaces formed by the periphery of the disc 2 and the small base in circular crown 170 are pressed against each other very narrow and achieve sealing themselves, by the action of the elastic retaining members holding them in abutment against one another.

Good results have been obtained by providing a flatness of the small base 170 produced with a tolerance of less than or equal to 0.03 mm.

Simultaneously, it is necessary to find a compromise between the seal obtained and the freedom that one wants to keep in the movements of the peripheral edge of membrane 1. It appears that freedom of movement, allowing oscillations of the membrane 1, requires to provide a width of small base 170 relatively small. On the other hand, a good seal is easier to obtain with a small relatively large base 170. A good compromise is obtained, according to the invention, by providing a sufficient surface condition as specified above, and a width of small base 170 equal to approximately 0.5 mm.

In the advantageous embodiment shown in the figures, the production of the electrical connections between the external electrode 5 and the connection terminal 19 is further simplified, by making the elastic retaining members 22 participate in this connection. For this, the piezoelectric ceramic layer 3 is placed on the side of the base body 7, as shown in the figures, so that the external electrode 5 faces the base body 7.

The second electrical connection terminal 19 is electrically connected by soldering to the second conductive track 12, itself in contact with a central metal rivet 21. The central metal rivet 21, engaged in a central hole in the base body 7, is integral elastic means 22 which conduct electricity in contact with the external electrode 5. Thus, the second electrical connection terminal 19 is electrically connected to the external electrode 5 via the conductive track 12, the central rivet 21 and the elastic electrically conductive means 22. Naturally, the conductive tracks 11 and 12 are electrically isolated from one another. Thus, the elastic retaining member 22 simultaneously performs the function of electrical connection between the external electrode 5 and the corresponding electrical connection terminal 19.

In the embodiment shown in the figures, the elastic electrically conductive means 22 is a metal part, made for example of a material sold under the brand Duriflex, comprising a central part 23 provided with a hole for the passage of the fastening rivet 21, and to which three branches 24, 25 and 26 are connected. The three branches 24, 25 and 26 each develop substantially in the form of a spiral starting from the central part 23. Their respective ends 27, 28 and 29 are equidistant from the center 30 of the central part 23 or center of the hole through which the fixing rivet 21 passes, and are equidistant from each other, distributed regularly at 120 ° around the center 30. The respective lengths of the three branches are unequal, and their widths are also uneven. In the embodiment shown, the first branch 24 develops at an angle B of approximately 92 °, while the second branch 25 develops at an angle C of approximately 120 °, and the third branch 26 develops at an angle A of about 86 °. The respective widths of the branches are chosen as a function of the respective lengths so that the bearing forces exerted by each of the ends 27, 28 and 29 of the branches against the external electrode 5 are substantially equal. Under these conditions, each of the branches has its own vibratory characteristic different from that of each of the other two branches.

The distance between the center 30 and each of the ends 27, 28 and 29 of the branches is chosen to be substantially equal to the radius of the nodal circle of the membrane 1. By nodal circle is meant the location of the vibration nodes of the third circular mode of vibration of the membrane 1. This nodal circle can be determined empirically, by requesting a vibrating membrane according to the third mode of vibration, and by locating the position of the vibration nodes forming the nodal circle. The detection of the vibration nodes can be carried out by any means well known in the art, for example by observing the movement of a powder distributed on the surface of a membrane stressed in vibration.

The part 22 can be produced by cutting and forming a plate of suitable thickness from an elastic and electrically conductive material.

As an alternative, provision may be made for using two or three separate parts, one end of which is fixed to the base body 7, and the other end of which comes to bear on an appropriate zone of the external electrode 5.

In order to reduce the dispersions in the electroacoustic properties of the capsules, it is advantageously possible, during assembly, to place a bead of silicone between the front step 17 of the housing and the peripheral portion of the front surface of the membrane.

The present invention is not limited to the embodiments which have been explicitly described, but it includes the various variants and generalizations thereof contained in the field of claims below.

Claims (15)

1 - Electroacoustic capsule with piezoelectric membrane, in which:
- the membrane (1) is formed of a thin disc (2) on which is fixed a layer of piezoelectric ceramic (3), covering the central area of the disc (2), the two faces of which are metallized and respectively constitute an electrode internal (4) and an external electrode (5),
- the membrane (1) is housed in a housing (6) formed by a base body (7) and a cover (8) adapting to one another along a closed periphery (9), the base body (7) having a first wall substantially parallel to the disc (2) with which it forms a posterior acoustic cavity (70), the cover (8) comprising a second wall (13) substantially parallel to the disc (2) with which it forms an anterior acoustic cavity (80),
the housing (6) comprises an anterior peripheral step (17) against which the periphery of the disc (2) is kept applied by elastic retaining members,
- a first (18) and a second (19) electrical connection terminals are electrically insulated and mounted on the base body (7),
the first electrical connection terminal (18) is electrically connected to the internal electrode (4) by first electrical connection members,
the second electrical connection terminal (19) is electrically connected to the external electrode (5) by second electrical connection members,
characterized in that:
the thin disc (2) has a peripheral part of its front face having a substantially planar shape, forming a zone of contact with the front peripheral step (17),
the anterior peripheral step (17) has a precise circular bearing surface defining a continuous flat surface of contact with the disc (2), the flatness being produced according to a tolerance less than or equal to 0.03 mm,
- the elastic retaining members (22) are supported on the intermediate zone of the rear face (5) of membrane (1), so that the flatness of the contact surfaces of the peripheral step anterior (17) and the thin disc (2) pressed against each other provides sealing between the anterior (80) and posterior (70) acoustic cavities. 2 - Electroacoustic capsule according to claim 1, characterized in that the circular bearing of the anterior peripheral step (17) is produced by fluo-turning. 3 - Electroacoustic capsule according to one of claims 1 or 2, characterized in that, in radial section, the front step (17) has a trapezoidal profile whose small base (170) constitutes the circular bearing surface of the membrane (1), said small base being separated from the peripheral wall (14) of the housing by a peripheral groove (171). 4 - Electroacoustic capsule according to claim 3, characterized in that said small base (170) forming a circular surface has a width equal to approximately 0.5 mm. 5 - Electroacoustic capsule according to any one of claims 1 to 4, characterized in that a silicone bead is inserted between said front step (17) of the housing and the peripheral portion of the front face of membrane (1). 6 - electroacoustic capsule according to any one of claims 1 to 5, characterized in that said elastic holding members (22) are supported on the membrane (1) in the vicinity of the nodal circle of the membrane (1), according to au minus two contact points distant from each other. 7 - Electroacoustic capsule according to claim 6, characterized in that at each fulcrum of said elastic holding members (22) on the membrane (1) corresponds an elastic connection with different mechanical characteristics, having different own vibratory modes. 8 - Electroacoustic capsule according to one of claims 6 or 7, characterized in that said elastic holding members comprise several distinct elastic elements, each of the elements being fixed to the base body (7) and being supported on the membrane (1 ). 9 - Electroacoustic capsule according to one of claims 6 or 7, characterized in that said elastic holding members (22) comprise a single element comprising several branches, a central part of the single element being fixed to the base body (7 ), the ends of the branches being supported on the membrane (1). 10 - Electroacoustic capsule according to claim 9, characterized in that the branches have different widths and lengths relative to each other, while producing substantially equal bearing forces on the membrane (1). 11 - Electroacoustic capsule according to one of claims 9 or 10, characterized in that the single element forming an elastic holding member (22) comprises three branches whose points of support on the external electrode (5) are distributed at 120 ° around the center of said electrode, the support forces being equal. 12 - Electroacoustic capsule according to any one of claims 9 to 11, characterized in that the branches of the single element forming elastic holding member (22) each have a spiral shape. 13 - Electroacoustic capsule according to any one of claims 1 to 12, characterized in that the bearing force exerted by the elastic holding members on the membrane (1) is between 0.3 Newton and 0.5 Newton. 14 - Electroacoustic capsule according to any one of claims 1 to 13, characterized in that:
the piezoelectric ceramic layer (3) forms the rear face of the disc (2), facing the base body (7), so that the external electrode (5) faces the base body (7),
the front face of the periphery of the disc (2) is opposite said housing step (17) itself formed in the cover (8),
- the elastic retaining members (22) are made of an electrically conductive material and are inserted between the external electrode (5) and the internal face of the base body (7), bearing on the one hand on the 'external electrode (5) and on the other hand on a portion of the base body (21) electrically connected to the second electrical connection terminal (19), to ensure the electrical connection between the second electrical connection terminal (19) and the external electrode (5). 15 - Electroacoustic capsule according to any one of claims 1 to 14, characterized in that the respective heights (H1, H2) of the anterior (80) and posterior (70) acoustic cavities of the capsule are equal to approximately 1 mm.

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