FR2783915A1 - Ultrasonic transducer producing high quality acoustic performance uses sensitive element with two electrodes and a protective membrane within an injection molded plastic body - Google Patents

Abstract

A body (20) made from a plastic such as phenyl polysulfide is extruded into a jar shaped box (12) and locked into position by a ridge and groove (20e,22). A piezoceramic disk (26) fitted into the body (20) has metallic electrodes (26a,26b) on rear and forward faces with electricity supply by conductors (34,36) from a cable (32). Chemical and physical protection is provided by membrane (38) coupled acoustically to forward electrode (26b).

Description

The present invention relates to an ultrasonic transducer

  comprising, a housing in which is housed a sensitive element which is capable of transforming an electrical signal into ultrasonic waves and which has two opposite faces, said sensitive element being provided with two electrodes arranged, one on the so-called front face facing the medium for propagation of the ultrasonic waves and the other on the opposite so-called rear face directed towards the bottom of said housing, a protective membrane arranged against the housing and the front face of said sensitive element, secured to said housing and acoustically coupled to

  the front face of the sensitive element.

  1 0 In the field of liquid or gas metering by ultrasound it is known to use this type of ultrasonic transducers which is for example described in the document FR 2

462,837.

  In this document, the housing comprises, on the side opposite the bottom, a housing for the sensitive element (for example a piezoelectric ceramic disc) which is produced

  1 5 in the form of a concentric annular recess.

  The protective membrane is produced in the form of a thin sheet of flexible plastic material which projects radially beyond the sensitive element and which is

  example bonded to the housing and to the front face of said sensitive element.

  This sheet of flexible plastic material serves to maintain elastically sensitive element

in axial and radial directions.

  As mentioned in this document, the diameter of the annular recess is slightly larger than that of the sensitive element so that it remains between the two

  a clearance necessary to absorb the phenomena of thermal expansion.

  However, it is not easy to control the dimensions of the annular recess in which the sensitive element is installed in order to ensure a very radial positioning.

  precise of it in the annular recess.

  It therefore happens that during the manufacturing phase, the positioning of the sensitive element in its housing is not ensured in a sufficiently precise manner because the manufacturing tolerances of the housing are too large, which can subsequently lead to

  poor acoustic performance of the ultrasonic transducer thus manufactured.

  The present invention provides for overcoming this drawback by proposing an ultrasonic transducer comprising, a housing in which is housed a sensitive element capable of transforming an electrical signal into ultrasonic waves and which has two opposite faces, a protective membrane arranged against the housing and the front face of said sensitive element, secured to said housing and acoustically coupled to the front face of the sensitive element, characterized in that said ultrasonic transducer further comprises a body made of plastic material and which is injection molded between the housing

  and the sensitive element so as to serve as a housing for receiving said sensitive element.

  As the plastic body is molded directly inside the housing, the dimensions of the housing defined by the body are adjusted more precisely than on a machined part because the molded plastic body catches the imperfections of the housing. The positioning of the sensitive element in its housing no longer depends on the

  control of the bonding operation of said sensitive element with the membrane and the housing.

  The mounting clearance existing between the housing defined by the body and the sensitive element is therefore better controlled and, thus, the sensitive element is better positioned in its

housing than before.

  According to one characteristic, the ultrasonic transducer also comprises an electric cable which passes through an orifice made in the bottom of the case and which is electrically connected to the two electrodes, the plastic body forming a stop

  axial for said electrical cable in the direction of the sensitive element.

  This avoids that an axial thrust on the electric cable does not affect the sensitive element and leads to a deformation of the protective membrane, or even a

  separation of the latter from the housing.

  In order for the plastic body to be able to serve as an axial stop, it can have

  at least one extension forming an anchorage in the housing.

  To ensure the mechanical strength of the electric cable relative to the box, it is mounted crimped inside a metal sleeve which is engaged in the opening made

in the bottom of the case.

  According to an advantageous characteristic, the metal sleeve has a radial shoulder in the direction of the housing in order to prevent an axial withdrawal of said sleeve in a

  direction opposite to that of the sensitive element.

  In addition, the shoulder of the metal sleeve bears on the material body.

  plastic which acts as an axial stop towards the sensitive element.

  Thus, the housing-sleeve-body assembly is mechanically integral and can withstand axial forces of thrust and withdrawal, deliberate or not, exerted on the sleeve

  metal without affecting the operation of the transducer.

  According to a first embodiment of the anchoring of the plastic body in the housing, said body comprises a radial projection which extends in an annular groove formed in the housing from a peripheral zone of said body which is in contact

with said housing.

  According to a second embodiment of the anchoring of the plastic body in the housing, said body extends inside at least one channel formed in the bottom of the housing and opens out on the other side of said bottom in a flared shape that prevents any

  axial thrust towards sensitive element.

  More specifically, the plastic body extends inside three axial channels which open on the other side of the bottom of the housing in an annular channel arranged perpendicular to said channels. For example, the three axial channels and the channel are arranged coaxially with the orifice

practiced in the bottom of the case.

  The electric cable is for example connected to the electrodes by means of two connection wires, the electrode arranged on the front face of the sensitive element extending

  1 0 to the rear side thereof.

  The protective membrane is for example metallic.

  The electrical cable is for example connected to the electrodes via, on the one hand, a connection wire and on the other hand, the metal protective membrane, the housing

  which is metallic and of the metallic sleeve in which said electric cable is crimped.

  The sensitive element is for example a piezoceramic disc.

  The invention also relates to a method of manufacturing an ultrasonic transducer consisting in forming a housing, in securing a sensitive element capable of transforming an electrical signal into ultrasonic waves to a protective membrane, in installing said sensitive element in a housing provided for this purpose and for fixing said protective membrane to the housing, characterized in that said method consists in injecting under pressure a plastic body between the housing and the sensitive element of

  so that said body serves as a housing for receiving said sensitive element.

  Advantageously, to avoid any subsequent condensation phenomenon, the method consists in injecting a neutral gas inside the housing before the installation of

  the sensitive element in its housing.

  In order to make the transducer hermetic, the method consists in welding the membrane

  metal on the metal housing.

  Other characteristics and advantages will appear during the description which follows.

  follow, given only by way of example, and made with reference to the accompanying drawings in which: - Figure 1 is an axial sectional view of an ultrasonic transducer according to a first embodiment, - Figure 2 is a first variant of the ultrasonic transducer of Figure 1, - Figure 3 is a second alternative embodiment of the ultrasonic transducer of Figure 1, - Figure 4 is a third alternative embodiment of the ultrasonic transducer of Figure 1, - Figure 5 is a fourth embodiment of the ultrasonic transducer of Figure 1, - Figure 6 is an alternative embodiment of the ultrasonic transducer of Figure 5, - Figures 7 and 8 are views of a second embodiment of ultrasonic transducer according to the invention respectively in axial section and in internal perspective, - Figure 9 is an alternative embodiment of the ultrasonic transducer of Figures 7 and 8, - Figure 10 is an axial section view of an ultrasonic transducer during the

  realization of the plastic body.

  As represented in FIG. 1 and designated by the general reference noted 10, an ultrasonic transducer comprises a housing 12 in the form of a pot which can be made of metal or plastic and has a symmetry of revolution about the Z axis. orifice 14 having a direction aligned along the axis Z is arranged in a part

of the housing called bottom 12a.

  The part of the housing 12b located at the end of said housing opposite the bottom 12a has an opening 16 which extends over almost the entire width of the housing and which communicates with a central recess 18 formed in the housing 12 around the axis Z The transducer 10 comprises a body 20 made of a plastic material such as, for example, phenylene polysulphide and which is placed in the recess 18. The body 20 has a bottom wall 20a perpendicular to the axis Z, in contact with the bottom 12a of the housing and which has in its center a hole 20b disposed opposite

orifice 14.

  The body 20 also has side walls 20c which are arranged in contact

  with the interior walls 12c of said housing delimiting the recess 18.

  The side walls 20c are configured so as to form a radial shoulder 20d

  opposite the central recess 18.

  The body 20 has an annular radial projection 20e which extends from a peripheral zone of the side walls 20c in an annular groove 22 of shape

  additional provided in the housing 12.

  The projection has for example a triangular shape seen in longitudinal section in the plane

of figure 1.

  This extension of the body 20 allows it to be anchored in the housing 12 so that

  any axial movement of said body is prevented.

  The body 20 is injection molded into the central recess 18 through the opening 16

provided in the housing 12.

  The injection molding operation will be described later in more detail for a

  body of different shape as shown in Figures 7 to 10.

  However, the principle of the operation which will be described therein allows a person skilled in the art to carry out injection molding of the body 20 of FIG. 1 and of the other bodies.

  shown in Figures 2 to 9 without undue difficulty.

  The radial shoulder 20d formed by the side walls 20c of the body 20 defines a concentric annular housing 24 intended to receive a sensitive element produced by

  example in the form of a piezoceramic disc 26.

  This sensitive element has two opposite faces 26a and 26b each provided with a

metal electrode 28, 30.

  The so-called rear face 26a is turned towards the central recess 18 and the bottom of the

  housing while the front face 26b said is oriented towards the outside of the housing.

  The electrode 30 of the front face 26b extends to the rear face 26a so as to form what is called an "electrode return" which will allow an electrical connection

  direct by an electrical connection wire.

  An electric cable 32 passes through the hole 14 in the bottom of the case and is electrically connected

  to the two electrodes 28, 30 respectively by two electrical connection wires 34, 36.

  When an electrical signal is transmitted to the sensitive element 26, this emits ultrasonic vibrations which are directed towards the outside of the housing 12 from the front face 26b and which propagate in the propagation medium (water, gas, fuels ,

  industrial liquids ...) in which the transducer is placed 10.

  The transducer 10 also comprises a protective membrane 38 which is arranged simultaneously against the housing, more precisely against an annular surface 12d of the

  housing, and against the front face 26b of the sensitive element 26.

  The membrane 38 has the function of protecting the sensitive element 26 from attack

  chemical and physical from the outside environment.

  This membrane is acoustically coupled to the front face 26b of the sensitive element.

  by bonding and is secured to the housing for example by bonding.

  The protective membrane can be made of plastic or be metallic and by

example in stainless steel.

  However, when the protective membrane is metallic, it can be secured to the housing by welding, which ensures better resistance.

  mechanical in case of depression in the propagation medium.

  Welding can for example be carried out by means of a laser.

  In the absence of a body 20, the internal dimensions of the housing 12 and the dimensions of the sensitive element 26 are defined with manufacturing tolerances which, when added up, can lead to imprecise positioning of the sensitive element

  in its housing inside the housing.

  Having an injection molded body inside the housing makes it possible to obtain internal dimensions of the housing defined by said body more precise than were

  internal dimensions of the housing defined by the housing itself.

  This results in a more precise positioning of the sensitive element in its housing.

  According to a first alternative embodiment, the transducer shown in FIG. 2 differs from that of FIG. 1 only in that the plastic body 40 has a bottom wall 40a whose dimensions are greater than those of the bottom wall a of FIG. 1, thus leaving a reduced space for the hole 40b made opposite

orifice 14.

  In FIG. 2, the bottom wall 40a penetrates into the fictitious cylindrical space defined by the walls of the orifice 14 so that the cable 32 comes into axial abutment against said wall of

background 40a.

  This arrangement makes it possible to prevent the cable from coming into contact with the sensitive element under the action of an external force and thus exerts on the membrane 38 mechanical deformations. This also avoids breaking the connection wires

34, 36 which are fragile.

  According to a second alternative embodiment, the transducer shown in Figure 3 differs from that of Figure 1 in that the electrical cable 32 is engaged inside

  of a metal sleeve 42 which is itself forcibly engaged in the orifice 14.

  Two crimps 44, 46 are made at a certain distance from the housing 12 so as to

  ensuring sufficient mechanical retention of the electric cable 32 in the sleeve 42.

  The cable thus stiffened is protected and can better withstand the operations of transporting the transducer. The fact that the cable is stiffened also makes it possible to better protect the wires from

  electrical connections 34, 36 which are generally fine wires and therefore brittle.

  It is in fact understood that the cable which is not crimped in a metal sleeve can be subjected, during the transport of the transducer, to deformations which can induce shocks on the connection wires and cause them to break, thereby rendering any

  subsequent use of the transducer not possible.

  According to a third alternative embodiment, the transducer shown in FIG. 4 differs from that of FIG. 3 only in that the plastic body 48 has a bottom wall 48a whose dimensions are greater than those of the wall of bottom a of figure 3, thus leaving a reduced place for the hole 48b practiced opposite

orifice 14.

  In FIG. 4, the bottom wall 48a enters the fictitious cylindrical space defined by the walls of the orifice 14 so that the cable 32 comes into axial abutment against said bottom wall 48a, this for the same reasons as those expressed above in relation to the

figure 2.

  FIG. 5 illustrates a fourth alternative embodiment in which the metal case 12, the body 20 and the protective membrane 38 are identical to those of the

  Figure 3 and keep the same references.

  1 0 The sensitive element 50 carries two electrodes 52, 54 each disposed on two opposite faces 50a, 50b of said sensitive element, the electrode 52 is arranged on the so-called rear face 50a facing the bottom of the housing 12 while the electrode 54 is arranged

  only on the so-called front face 50b facing the propagation medium.

  This sensitive element is capable of emitting ultrasonic vibrations at a

  frequency for example equal to 1 MHz.

  The protective membrane 38, for example made of stainless steel, is bonded to the electrode 54 by means of a non-conductive adhesive which is more advantageous than a conductive adhesive. Indeed, a conductive adhesive is, on the one hand, more expensive and,

  on the other hand, more difficult to implement than a non-conductive adhesive.

  In addition, there are relatively few conductive adhesives on the market.

  The thickness of the adhesive joint is relatively small and for example is equal to 1 0.m.

  The protective membrane 38 is fixed to the annular surface 12d of the housing by welding, for example laser, which seals against the environment external to the transducer. An electric cable 56 is mounted in a metal sleeve 58 and held at

  the interior thereof by two crimps 60, 62.

  The sleeve 58 is forcibly engaged in the orifice 14 of the housing.

  The insulating sheath of the cable 56 has for example been locally removed prior to mounting said cable in the sleeve and a third crimping 64 is carried out at the place where the cable is stripped in order to ensure electrical contact between the sleeve and the

cable shield.

  It is also possible to make the electrical contact by stripping part of the sheath located at the end of the cable which is turned towards the transducer and by rolling up the shielding of said cable on the sheath in a direction opposite to the transducer, as

shown in Figure 7.

  The cable core is connected to the electrode 52 by a connection wire 66 or by a contact

electric spring.

  To make the electrical contact between the membrane assembly 38 and the electrode 54, one can for example use a conductive adhesive or for example a non-conductive adhesive whose thickness obtained by pressing is sufficiently small (a few microns) to guarantee at least an electrical contact point between the membrane and the electrode. The transducer thus obtained can be considered as a Faraday cage which perfectly isolates the sensitive element from the outside, which therefore makes it insensitive.

  external electromagnetic interference.

  FIG. 6 illustrates an alternative embodiment of FIG. 5 but which could also apply to the transducers shown in FIGS. 1 to 4 o the electrical contact between the cable and the electrodes is effected by means of two connection wires

34, 36.

  The transducer of Figure 6 differs from that of Figure 5 in that the metal sleeve 70 carries at its end engaged in the orifice 74 formed in the bottom of the

  housing 76, a radial shoulder 72 directed towards the housing.

  The housing 76 and the plastic body 78 are locally modified in order to accommodate

shoulder 72.

  The shoulder 72 comes to bear on an annular zone 78b of the bottom wall 78a of the

  body which also forms a shoulder.

  The annular zone of the bottom wall 78a of the body acts as an axial stop in the direction of the sensitive element 50 for the metal sleeve 70, this in order to prevent deformation of the protective membrane 38 and a break in the connection wire. 66

  under the action of an external force transmitted to the sleeve.

  Furthermore, the shoulder 72 projecting radially into the housing 76 prevents the sleeve from being removed axially in a direction opposite to that of the element.

sensitive 50.

  The configuration of the transducer thus obtained offers great mechanical resistance to external actions, deliberate or not, and therefore provides better protection.

  mechanics of the sensitive element and of the connection wire than in the prior art.

  In this configuration, the body 78 provides mechanical maintenance of the housing 76 and the

sleeve 70.

  FIGS. 7 and 8 illustrate another embodiment of the invention which reproduces the characteristics of the transducer of FIG. 6 and their references with regard to the sensitive element 50 and its electrodes 52, 54, the metal sleeve 70 and the

crimps 60 to 64.

  However, in this embodiment, part of the sheath of the cable 80 situated at the end oriented towards the sensitive element has been removed over part of its length and the shield 82 of said cable has been rolled up from this end by going up. in the opposite direction to said sensitive element so as to be superimposed on the sheath remaining in place on the non-stripped part of said cable. Thus, the crimping 64 of the metal sleeve on the upturned shield 82 of the cable

  ensures electrical continuity with electrode 54.

  The plastic body 84 differs from that of FIGS. 1 to 6 by its extension which forms an anchorage in the housing 86 but it retains the general shape shown on

  Figures 1 to 6 except that the projection 20 is no longer useful.

  Three axial channels of direction parallel to the Z axis (cable axis 80) are arranged in the housing 86 at the periphery of the sleeve 70 and two 88, 90 of these channels are

  shown in axial section in Figures 7 and 8.

  These three channels partially cross the bottom 86a of the housing and open on the side of said bottom opposite to the sensitive element 50 in a flared shape in one direction.

opposite to said sensitive element.

  The three channels open more precisely into an annular channel 92 which surrounds the sleeve 70, is arranged perpendicular to the axial channels and is arranged at

  wall flower on the outer wall of the bottom 86a of the housing.

  The body 84 extends from the general shape shown in Figures 1 to 6 (without the projection 20e) and disposed in the central recess 18 inside the axial channels and

laugh 92.

  Thus, the bottom wall 84a and the side walls 84b are integral with the crown 84c filling the channel 92 by means of the three cylindrical parts occupying the axial channels (only two cylindrical parts 84d and 84e are shown in FIGS. 7 and 8 ) so as to form a single mechanical part. In this way, any axial force exerted by the sleeve 70 on the part 84a of the body and directed towards the sensitive element is compensated by the part 84c of said body which forms an axial retention.

  The body 84 thus formed securely holds the housing 86 and the sleeve 70 together.

  It should be noted that this embodiment can also be applied to an ultrasonic transducer such as that shown in FIG. 9, where the electrical contact between the cable 94 and the electrodes 28, 30 is made by means of two wires 34, 36 analogous to those of FIGS. 1 to 4. Here, only two crimps are

  made for the mechanical retention of the cable 94 in the metal sleeve 96.

  In this configuration, the mechanical protection offered by the housing assembly 86 -

  body 84 - sleeve 96 is also very useful because of the presence of the two connection wires and therefore of the statistically higher risk of breakage than for

  single wire transducers.

  FIG. 10 illustrates the manufacture of the body 84 of the ultrasonic transducer shown in FIGS. 7 and 8. During the manufacture of the transducer, the housing 86 and the sleeve 70 are produced separately: the housing made of stainless steel is obtained by machining as is the muff. The housing is held between two upper half-shells 100 and

lower 102.

  1 0 The sleeve is then engaged by its end opposite to that carrying the shoulder 72 in the central recess 18 through the opening 16 so that said shoulder comes into contact with the housing 86 in a housing 104 of annular shape planned to

this effect.

  A core 106 comes into contact with an annular surface 86b of the housing coaxial with

  1 5 the central recess 18 and is partially engaged therein.

  Another core 108 passes through the core 106 in its central part and comes against

  the shoulder 72 of the sleeve 70 to maintain the latter in the housing 104.

  Plastic material is then injected under pressure through a channel 110 disposed in the upper half-shell 100 as indicated by the arrow in order to fill the space

  left empty between the housing 86 and the different parts of the mold.

  After the plastic material has solidified, the different parts of the mold are removed (demoulding) and the body 84 then has the final shape which is shown on the

Figures 7 and 8.

  In parallel with this operation, the protective membrane 38 is bonded to the element

  sensitive 50 equipped with its two electrodes 52, 54 (Figs 7 and 8).

  The partially stripped electrical cable 80 and of which a part of the shield has been turned over on the sheath of the remaining part of said cable is engaged in the sleeve 70 and

  the three crimps 60, 62 and 64 are carried out.

  The cable core is connected to the electrode 52 of the sensitive element 50 by the wire

connection 66 which is soldered.

  A neutral gas (for example nitrogen) is then injected inside the housing 86 of

  so as to fill the central recess 18.

  This has the function of avoiding trapping air in the recess 18 during the

  subsequent closure of the transducer.

  In fact, air having a non-negligible humidity level could, if it were trapped in the housing, give rise subsequently to condensation phenomena capable of damaging the electrical contact and degrading the performance of the sensitive element 50.

  The sensitive element equipped with the protective membrane 38 is then positioned in its annular housing 112 (FIG. 7) trapping a volume of nitrogen in the central recess 18 o now reigns in a dry atmosphere and laser welding under nitrogen is

  then carried out to fix the protective membrane 38 to the annular surface 86b.

  Directly injecting the body into the housing and mounting the electrical cable in a metal sleeve fixed to the housing are simple operations to perform and which

  lead to a mechanically sound and reliable transducer.

  It should be noted that the outside air cannot penetrate into the central recess 18 contrary to what is provided in the transducer described in the document FR 2 462 837, due, on the one hand, to the laser welding of the membrane 38 on the housing and, on the other hand, the plastic body which fills the entire space between the housing 86 and

the central recess 18.

Claims (17)

  1. Ultrasonic transducer (10) comprising, a housing (12; 76; 86) in which is housed a sensitive element (26; 50) capable of transforming an electrical signal into ultrasonic waves and which has two opposite faces (26a, 26b; 50a, 50b), said sensitive element being provided with two electrodes (28, 30; 52, 54) arranged, one on the so-called front face (26b; 50b) facing the medium for propagation of the ultrasonic waves and the other on the opposite so-called rear face (26a; 50a) directed towards the bottom of said case, a protective membrane (38) arranged against the case and the front face of said sensitive element, secured to said case and acoustically coupled to the front face of the sensitive element, characterized in that said ultrasonic transducer further comprises a body (20; 40; 48; 78; 84) made of plastic material and which is injection molded between the housing and the sensitive element so as to serve as a housing (24) to receive said sensitive element.   2. Ultrasonic transducer according to claim 1, also comprising an electric cable (32; 56; 80; 94) which passes through an orifice (14) formed in the bottom (12a 86a) of the housing and which is electrically connected to the two electrodes (28 , 30; 52, 54), the plastic body forming an axial stop for said electric cable in   direction of the sensitive element (26; 50).   3. Ultrasonic transducer according to claim 1 or 2, wherein the body (20;; 48; 78; 84) of plastic material has at least one extension forming an anchorage in said housing (12; 76; 86).   4. Ultrasonic transducer according to claim 2, in which the electric cable (32; 56; 80; 94) is mounted crimped inside a metal sleeve (42; 58; 70; 96)   which is engaged in the orifice (14) formed in the bottom of the housing.   5. Ultrasonic transducer according to claim 4, in which the metal sleeve (70; 96) has a radial shoulder (72) in the direction of the housing in order to prevent axial withdrawal of said sleeve in a direction opposite to that of the sensitive element.   6. Ultrasonic transducer according to claim 5, in which the shoulder (72) of the metal sleeve (70; 96) bears on the body (78; 84) of material.   plastic which serves as an axial stop towards the sensitive element.   7. Ultrasonic transducer according to claim 3, in which the plastic body (20) has a radial projection (20e) which extends in an annular groove (22) formed in the housing (12) from an area peripheral of said body which is in contact with said housing.   8. Ultrasonic transducer according to claim 3, in which the plastic body (84) extends inside at least one channel (88, 90) formed in the bottom (86a) of the housing (86) and opens on the other side of said bottom into a flared shape which   prevents any axial thrust towards the sensitive element.   9. Ultrasonic transducer according to claim 8, in which the plastic body (84) extends inside three axial channels which open on the other side of the bottom of the housing in an annular channel (92) arranged perpendicularly. channel audits.   10. Ultrasonic transducer according to claim 9, in which the three axial channels and the channel (92) are arranged coaxially with the orifice (14) formed in the bottom. of the housing.   11. Ultrasonic transducer according to claim 2, in which the electric cable (32; 94) is connected to the electrodes (28, 30) by means of two connection wires (34, 36), the electrode arranged on the face front of the sensitive element (26) extending   up to the rear face (26a) thereof.   12. Ultrasonic transducer according to one of claims 1 to 10, in which the   protective membrane (38) is metallic.   13. Ultrasonic transducer according to claims 2, 4 and 12, wherein the cable   electrical (56; 80; 94) is connected to the electrodes (28, 30; 52, 54) via, on the one hand, a connection wire (66) and on the other hand, the housing (12 ; 76; 86) which is   wire (58; 70) and the metal sleeve in which said electrical cable is crimped.   14. Ultrasonic transducer according to one of claims 1 to 13, in which   the sensitive element (26; 50) is a piezoceramic disc.   15. Method for manufacturing an ultrasonic transducer consisting in forming a housing (12; 76; 86), in securing a sensitive element (26; 50) capable of transforming an electrical signal into ultrasonic waves to a protective membrane (38) , installing said sensitive element in a housing provided for this purpose and fixing said protective membrane to the housing, characterized in that said method consists in injecting under pressure a body (20; 40; 48; 78; 84) of plastic material between the housing and the sensitive element so that said body serves as a housing for receiving said sensitive element. 16. The method of claim 15, comprising injecting a neutral gas inside   of the housing prior to the installation of the sensitive element in its housing.   17. The method of claim 15 or 16, comprising welding the membrane   metal (3 8) on the metal housing.