Piezoelectric cell support for an ultrasonic transducer

A piezoelectric cell support for an ultrasonic transducer, the support including a front face having formed thereon: a first shoulder that defines a bearing surface and a bottom of a central cavity surrounded by the bearing surface, the bearing surface being suitable for having a piezoelectric cell placed thereon; retention device(s) for holding the piezoelectric cell on the bearing surface and for positioning it angularly; and a rear face having a sloping surface that gives a top portion of the support a first thickness that is less than a second thickness of a bottom portion of the support.

The invention relates to the field of ultrasonic transducers, and in particular to ultrasonic transducers included in ultrasonic fluid meters.

BACKGROUND OF THE INVENTION

When designing a fluid meter, it is naturally ensured that the performance of the metrological portion of the fluid meter, as obtained when the fluid meter is in operation “in the field”, is similar to the performance as obtained when the fluid meter is calibrated on a test bench in the factory.

It is therefore necessary to take account of numerous factors that might disturb measurement, and possibly even falsify it completely. These factors include in particular the conditions surrounding the fluid meter in operation, and in particular the temperature and the pressure of the fluid applied to the metrological portion.

For an ultrasonic fluid meter including ultrasonic transducers, each provided with a respective piezoelectric cell, mastering assembly of the piezoelectric cell is fundamental for ensuring robustness for the metrological portion.

With reference toFIG.1, a conventional ultrasonic transducer1of a fluid meter is assembled by so-called “horizontal” assembly. The ultrasonic transducer1includes a housing2having a bottom3and a vertical inner wall4that extends perpendicularly from the bottom3. Assembly consists in applying adhesive5to the bottom3of the housing2, in pressing the front face of the piezoelectric cell6against the bottom3of the housing2, and while the adhesive is setting in applying a vertical force on the support7(or “backing”) of the piezoelectric cell6in order to fasten the piezoelectric cell6against the bottom3of the housing2.

Horizontal assembly is nowadays well mastered.

With reference toFIG.2, it has nevertheless been envisaged that the front face of the piezoelectric cell9of an ultrasonic transducer10should be pressed, not against the bottom11of the housing12, but rather against a surface of the vertical inner wall13of the housing12. This is referred to as “vertical” assembly. It is then very difficult to apply a horizontal force against the support14while the adhesive is setting, particularly since the width l of the housing12may be very small (typically equal to 1 centimeter (cm)).

The quality of the coupling between the piezoelectric cell9and the vertical inner wall13is of great importance in such vertical assembly. Specifically, the pressure of the water acting against the vertical outer wall15tends to cause the piezoelectric cell9to become unstuck from the vertical inner wall13. Degraded coupling leads to a loss of signal level, or even to total loss of signal in the event of the piezoelectric cell9becoming completely unstuck from the vertical inner wall13.

Naturally, coupling optimization must not be done to the detriment of measurement accuracy. In order to guarantee measurement performance that is stable as a function of temperature, it is known that it is necessary to take care in selecting all of the materials of all of the elements making up the ultrasonic transducer10, and in particular the material(s) used for making the support14. The acoustic impedance of the support14must be as stable as possible as a function of temperature. The optimum material for providing this stability of acoustic impedance is air because, given the large impedance difference between the piezoelectric cell9and air, the amount of energy that is transmitted into air is almost zero, regardless of whether the temperature is equal to 5° C. or to 70° C. All of the energy produced by the piezoelectric cell9is thus transmitted into the water. Nevertheless, a support made only of air would not guarantee stable measurement performance as a function of the pressure of the fluid acting against the vertical outer wall15of the housing12of the ultrasonic transducer10.

Furthermore, it is appropriate to ensure that the elements making up the ultrasonic transducer can be assembled in a manner that is repeatable in order to guarantee good reproducibility of measurement performance between different transducers.

With reference toFIG.3, this problem is particularly acute when the piezoelectric cell16has a first electrode17and a second electrode18of the wraparound type. A first electric wire is connected to the first electrode17and a second electric wire is connected to the second electrode18. The shape of the acoustic field depends on the position of the connection, which must therefore be well-controlled and repeatable.

FIG.4shows the angular positions of the piezoelectric cells19of the two ultrasonic transducers of a first meter, and the angular positions of the piezoelectric cells20of the two ultrasonic transducers of a second meter. It can be seen that, when the angular positions are different, the acoustic fields are different and there is potential for measurement performance also to be different.

OBJECT OF THE INVENTION

An object of the invention is to provide a piezoelectric cell support that serves to make assembly of a piezoelectric cell in an ultrasonic transducer easy and repeatable, in particular for vertical assembly, and that serves to take measurements that are stable as a function of the temperature and the pressure that are applied by the fluid.

SUMMARY OF THE INVENTION

In order to achieve this object, there is provided a piezoelectric cell support for an ultrasonic transducer, the support including a front face having formed thereon:a first shoulder that defines a bearing surface and a bottom of a central cavity surrounded by the bearing surface, the bearing surface being suitable for having a piezoelectric cell placed thereon;retention means and angular positioning means for holding the piezoelectric cell on the bearing surface and for positioning it angularly;and a rear face having a sloping surface that gives a top portion of the support a first thickness that is less than a second thickness of a bottom portion of the support.

When the piezoelectric cell is mounted on the support and held in position by the retention means, the edge of the piezoelectric cell is positioned against the bearing surface, and the central cavity, which is full of air, is closed by a central portion of the piezoelectric cell. The measurements taken by an ultrasonic transducer including the piezoelectric cell and its support are thus stable as a function both of temperature and of pressure. The means for angularly positioning the piezoelectric cell on the bearing surface ensure that the connections are in the same positions in each assembly of a support and a piezoelectric cell, thereby making such assembly entirely repeatable.

The sloping surface and the difference in thickness between the top and bottom portions of the support greatly facilitate vertical assembly of the piezoelectric cell, in particular in a housing of small width.

There is also provided a support as described above, wherein the angular positioning means include a passage for electric wires that are connected to the electrodes of the piezoelectric cell.

There is also provided a support as described above, wherein the passage includes a hole passing through a top outer side wall of the top portion of the support.

There is also provided a support as described above, wherein the bottom portion of the support includes a bottom face of the support that is for placing on a bottom of a housing of a casing of an ultrasonic transducer.

There is also provided a support as described above, wherein the bearing surface and the front outer surface of the front face of the support form a second shoulder on the front face of the support, the retention means comprising at least one projection formed on an inner wall of the support that extends between the front outer surface and the bearing surface.

There is also provided a support as described above, wherein the bearing surface is arranged to receive a piezoelectric cell of section that is circular.

There is also provided a support as described above, wherein the bearing surface is arranged to receive a piezoelectric cell of section that is square or rectangular.

There is also provided a support as described above, the support being made of a plastics material.

There is also provided a support as described above, the support being made of a metal material.

There is also provided an assembly comprising a piezoelectric cell and a support as described above, the piezoelectric cell being mounted on the bearing surface and closing the central cavity, which is full of air.

There is also provided an ultrasonic transducer comprising a casing defining a housing, a piezoelectric cell, and a support as described above, the piezoelectric cell being mounted on the support, the piezoelectric cell and the support being positioned in the housing.

There is also provided an ultrasonic transducer as described above, wherein the housing includes a bottom and an inner wall that extends from the bottom of the housing, the piezoelectric cell and the support being arranged in the housing in such a manner that the piezoelectric cell is positioned against the inner wall of the housing.

There is also provided an ultrasonic transducer as described above, wherein the inner wall of the housing slopes at an angle lying in the range 45° to 90° relative to the bottom of the housing.

There is also provided an ultrasonic fluid meter including first and second ultrasonic transducers as described above.

There is also provided an ultrasonic fluid meter as described above, including a duct for passing a flow of fluid, a first housing of the first ultrasonic transducer and a second housing of the second ultrasonic transducer extending into the inside of the duct respectively from first and second regions of an inside wall of the duct, a first piezoelectric cell of the first ultrasonic transducer and a second piezoelectric cell of the second ultrasonic transducer being situated and oriented facing each other and in parallel.

There is also provided an ultrasonic fluid meter as described above, wherein a first longitudinal axis of the first housing and a second longitudinal axis of the second housing slope respectively relative to the first and second regions of the inside wall at an angle lying in the range 45° to 90°.

There is also provided an ultrasonic fluid meter as described above, wherein the first and second ultrasonic transducers are positioned so that a first position of the first ultrasonic transducer presents a first offset on one side of a plane containing a central longitudinal axis of the duct, and in such a manner that a second position of the second ultrasonic transducer presents a second offset on the other side of said plane.

There is also provided a method of assembling an ultrasonic transducer as described above, the method comprising the steps of:mounting the piezoelectric cell on the bearing surface of the support;applying an adhesive to a first surface of an inner wall of the housing and/or to a front face of the piezoelectric cell;inserting the piezoelectric cell and the support in the housing of the transducer, in such a manner that the bottom portion of the support rests on the bottom of the housing and in such a manner that the piezoelectric cell is pressed against the first surface of the inner wall of the housing;inserting a tool into the housing between the rear face of the support and a second surface of the inner wall of the housing that is situated facing the first surface, in such a manner that the tool slides on the sloping surface of the support from the top portion towards the bottom portion of the support, thereby pressing the piezoelectric cell against the first surface of the inner wall of the housing.

The invention can be better understood in the light of the following description of a particular, nonlimiting embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference toFIGS.5to9, a piezoelectric cell support20of the invention has a front face21and a rear face22.

The front face21has a first shoulder23and a second shoulder24.

The first shoulder23defines a bearing surface25, a bottom of a central cavity26, and a first inner wall27that extends perpendicularly from the bottom of the central cavity26to the bearing surface25. The central cavity26is defined by the bottom and by the first inner wall27.

The bearing surface25is annular in shape and it surrounds the central cavity26.

The bearing surface25, a front outer surface28, and a second inner wall29form the second shoulder24. The second inner wall29extends perpendicularly from the bearing surface25to the front outer surface28. The front outer surface28surrounds the bearing surface25.

The bottom of the central cavity26, the bearing surface25, and the front outer surface28are mutually parallel surfaces that are centered on a common axis, and they are arranged in succession in this order from the bottom of the central cavity26to the outside of the support20.

The support20also has two projections30, each of length extending radially from the second inner wall29on the bearing surface25. Each projection30has the same height as the second inner wall29and is of a length that is considerably shorter than the width of the bearing surface25.

The rear face22has a sloping surface32that gives a top portion33of the support20a first thickness E1that is less than a second thickness E2of a bottom portion34of the support20.

The bottom portion34of the support20has a bottom face35of the support20, which bottom face is a plane surface.

The bottom portion34of the support20also has a rear plane face36situated on the rear face22, and that is perpendicular to the bottom face35. The intersection between the bottom face35and the rear plane surface36forms a rear bottom edge37of the support20.

The sloping surface32of the rear face22extends from the rear plane surface36to a top outer side wall39of the top portion33of the support20. The top outer side wall39presents a shape that is rounded towards the outside of the support20. The top outer side wall39is thus situated opposite from the bottom face35. The slope of the sloping surface32, i.e. the angle between the sloping surface32and the rear plane surface36, lies in the range 5° to 85°, and advantageously in the range 35° to 55°.

A slot-shaped hole40is formed in a central portion of the top outer side wall39of the support20. The hole40extends from the bearing surface25to the front outer surface28(and is thus open to the outside), and it passes through the top outer side wall39in order to open into a groove41that extends radially in the bearing surface25between the two projections30.

It should be observed that the top portion33of the support20is hollowed out both in the top outer side wall39and also in the central portion where the hole40is made. These hollows have the effect of causing the thickness of the material that forms the support20to be substantially the same over a large portion of the support20. This is particularly advantageous during manufacture of the support20when the support20is made using an injection molding technique.

In this example, the support20is made of plastics material, e.g. polyphenylene sulfide (PPS), polyether ether ketone (PEEK), polyphenylene oxide (PPO), or polyamide (PA). The support20could also be made out of metal, e.g. out of stainless steel, brass, or bronze.

A piezoelectric cell50is mounted on the bearing surface25. More precisely, the edges of a rear face51of the piezoelectric cell50are pressed against the bearing surface25. The piezoelectric cell50is in the shape of a disk: it has a circular section and a thickness that is considerably smaller than its radius. When the piezoelectric cell50is mounted on the support20, the piezoelectric cell50and the central cavity26are coaxial.

The diameter of the piezoelectric cell50is slightly smaller than the outer diameter of the bearing surface25. The piezoelectric cell50has a diameter that lies typically in the range 2 mm to 50 mm.

The support20includes retention means for holding the piezoelectric cell50against the bearing surface25. In this example, the retention means comprise the two projections30. The sum of the diameter of the piezoelectric cell50plus the length of the projections30is very close to the outer diameter of the bearing surface25, such that when the piezoelectric cell50is mounted on the bearing surface25, it comes to bear radially against the projections30, which hold it stationary.

The support20also includes angular positioning means for angularly positioning the piezoelectric cell50on the bearing surface25. In this example, the angular positioning means include a passage for electric wires52that are connected to the electrodes of the piezoelectric cell50. The passage includes both the hole40passing through the top outer side wall39of the top portion33of the support20, and also the groove41. The angular positioning means also include the electric wires52themselves.

Thus, when the piezoelectric cell50is mounted on the support20, the electric wires52are inserted in the hole40and, because the hole40has a width that is small and substantially equal to the diameter of the wires52, the wires52and the hole40serve to center the piezoelectric cell50and to hold it in a fixed angular position. The orientation of the electric wires52and thus the positions of the connections are well controlled, thereby achieving better reproducibility among different products.

When the piezoelectric cell50is mounted on the support20, the edges of the piezoelectric cell50are placed on the support surface25, thereby ensuring very good stability of measurements as a function of water pressure.

Furthermore, when the piezoelectric cell50is mounted on the support20, it closes the central cavity26, which is full of air.

This central cavity26full of air allows the piezoelectric cell50to vibrate freely and guarantees that acoustic performance is stable between a low temperature (e.g. equal to 0° C.) and a high temperature (e.g. equal to 70° C.).

It is known that the distribution of the energy produced by the piezoelectric cell50complies with a Gaussian distribution, i.e. most of the energy is generated in a central portion of the piezoelectric cell50. Thus, placing the edges of the piezoelectric cell50on the bearing surfaces25does not penalize the performance of the piezoelectric cell50.

With reference toFIG.10, there follows a description of a method of assembling the ultrasonic transducer including a support20of the invention.

The electric wires52are initially connected to the electrodes of the piezoelectric cell50, e.g. by soldering, (step E1). Thereafter, the piezoelectric cell50is mounted on the support20, with the electric wires52being inserted in the hole40(step E2).

The ultrasonic transducer60includes a casing61having a longitudinal housing62.

The housing62has a bottom63and a vertical inner wall64that extends from the bottom63of the housing62, perpendicularly to the bottom63. A drop of adhesive65is applied on a first surface of the vertical inner wall64of the housing62(and/or on the front face of the piezoelectric cell50). The piezoelectric cell50and the support20are then inserted in the housing62in such a manner that the bottom face35of the bottom portion34of the support20rests on the bottom63of the housing62, and in such a manner that the front face of the piezoelectric cell50is pressed against the vertical inner wall64of the housing62(step E3).

Thereafter, a tool68is inserted in the housing62between the rear face22of the support and a second surface of the vertical inner wall64of the housing62, which surface is situated facing the first surface, the tool being inserted in such a manner that it slides over the sloping surface32from the top portion33of the support20towards the bottom portion34. Insertion of the tool68is thus made possible by the difference in thickness between the top portion33and the bottom portion34, which difference is due to the sloping surface32.

The tool68thus presses the piezoelectric cell50against the first surface of the vertical inner wall64of the housing62, generating a force F perpendicular to the sloping surface32of the rear face22of the support20(step E4).

Advantageously, the tool has its own sloping surface69sloping at an angle equal to or close to the angle of slope of the sloping surface32of the support20.

Thereafter, the casing61of the ultrasonic transducer60is filled with resin70. The tool68used for inserting and adhesively bonding the support20may optionally remain in place inside the casing61of the ultrasonic transducer60during (and thus after) the application of resin (step E5).

It is possible to add grooves in the surface of the vertical inner wall64of the housing62of the ultrasonic transducer60in order to improve adhesion of the resin on this inner wall, which wall is generally made of plastics material. This serves to improve sealing of the ultrasonic transducer60and its ability to withstand water pressure.

It is also possible to include holes in the tool68that is used for inserting and adhesively bonding the support, for the purpose of improving adhesion of the resin. This serves to reinforce the portion situated at the rear of the piezoelectric cell of the ultrasonic transducer, thereby improving its impact resistance and its ability to withstand water pressure.

With reference toFIGS.11and12, there follows a description of the way in which first and second ultrasonic transducers81and82are incorporated in the ultrasonic water meter80.

The first and second ultrasonic transducers81and82act in succession as emitters and as receivers of ultrasound measurement signals Su that travel along a path of defined length L in the duct83. The speed of the water flowing in the duct83of the ultrasonic water meter80is estimated on the basis of these ultrasound measurement signals Su.

The first and second ultrasonic transducers81and82are both mounted to project into the duct from an inside wall of the duct83.

InFIG.11, it can be seen that the first and second ultrasonic transducers81and82both extend from respective first and second regions of a top portion of the inside wall of the duct83, but other configurations would be possible. By way of example, the first ultrasonic transducer81could extend from a first region of the top portion of the inside wall of the duct83, and the second ultrasonic transducer could extend from a second region of the bottom portion of the inside wall of the duct83.

The first housing84of the first ultrasonic transducer81and the second housing85of the second ultrasonic transducer82thus both extend into the inside of the duct83.

In this example, it can be seen that the first and second housings84and85are positioned vertically, which explains why it is advantageous for assembly of the piezoelectric cell and the support to be vertical. Thus, a first longitudinal axis Y1of the first housing85and a second longitudinal axis Y2of the second housing86both slope relative to the first or second region respectively of the top portion of the inside wall of the duct83at an angle equal to 90°. This angle could be different, and for example it may lie in the range 45° to 90°.

InFIG.12, it can be seen that the first and second ultrasonic transducers81and82are not in alignment along a central longitudinal axis X of the duct83. In contrast, it can be seen that the first and second ultrasonic transducers81and82are positioned so that a first position of the first ultrasonic transducer81presents a first offset Δ1on one side of a plane containing the central longitudinal axis X of the duct83, and in such a manner that a second position of the second ultrasonic transducer82presents a second offset Δ2on the other side of the plane. In this example, the plane is a vertical plane, but it could be a plane that is horizontal or sloping at any angle of inclination.

It can be seen inFIG.12that the first piezoelectric cell87of the first ultrasonic transducer81and the second piezoelectric cell88of the second ultrasonic transducer82are situated so that they are oriented facing each other and in parallel, thereby optimizing the reception of the ultrasound measurement signals Su.

It may be observed at this point that the frequency of a piezoelectric cell advantageously lies in the range 1 megahertz (MHz) to 4 MHz when the fluid is water, and in the range 100 kilohertz (kHz) to 500 kHz when the fluid is a gas. In the above-described application, the fluid is water and the frequency of each of the first and second piezoelectric cells87and88is equal to 2 MHz.

Naturally, the invention is not limited to the embodiment described, but covers any variant coming within the ambit of the invention as defined by the claims.

It can be seen above that the support of the invention is very advantageous for “vertical” assembly, i.e. when the piezoelectric cell is mounted against an inner wall of a housing, which inner wall is perpendicular to the bottom of said housing. The inner wall might slope relative to the bottom of the housing at a different angle, e.g. lying in the range 45° to 90°.

The piezoelectric cell is not necessarily circular in section and it could have some other section, e.g. square or rectangular.

The retention means and the angular positioning means could be different from those described above. It is entirely possible to have common means serving both to hold the piezoelectric cell and also to position it angularly.

The fluid meter incorporating the ultrasonic transducers naturally need not necessarily be a water meter, but could be a meter for a different fluid, e.g. a gas meter or an oil meter.

The materials used for making the support could naturally be different from the materials mentioned.

In the water meter described, the path of defined length L is a straight line path. Nevertheless, it is possible to use any type of path that is of defined length, possibly including one or more reflectors, mirrors, etc.