FR2566907A1 - Pressure sensor. - Google Patents

Abstract

Modulated differential pressure sensor in particular for vortex gas meter. The sensor comprises a deformable enclosure 10 having a lateral wall 12. The two pressures whose difference is to be measured are introduced via pipes 30, 44 on either side of the enclosure 10. The variations in the radii of curvature of the lateral wall 12 are detected by an optical fibre 50, 52, which is wound and adhesively bonded on the lateral wall 12. The variations in the radius of curvature of the fibre 50 which are identical to those of the wall 12 cause variations in the optical transmission coefficient of this fibre. These variations are detected in order to calculate the frequency of the differential pressure modulations to be measured.

Description

PRESSURE SENSOR
The present invention relates to a pressure sensor.

More precisely, the present invention relates to a pressure sensor making it possible to detect the modulations of the pressure applied to it. Preferably, this pressure is a differential pressure.

 There are a number of cases where it is desired to measure the frequency of modulation of a pressure or more precisely of a differential pressure. without the measurement of the value of the differential pressure itself being necessary.

This is particularly the case with vortex meters for gas.

This type of counter based on the Karman effect is well known. Reference may be made to French patent 2,506,933 for a detailed description. It suffices to recall that such a meter includes an obstacle placed across the pipe through which the fluid flows, the flow of which is to be measured. This obstacle generates vortices which detach alternately from each of the faces of the obstacle arranged substantially parallel to the flow of the fluid. The frequency of formation of these vortices is proportional to the speed of the fluid.

To measure this frequency, the variations in the difference in fluid pressures in the vicinity of the two faces of the obstacle are measured. For this, the obstacle comprises two conduits which open respectively into the faces of the obstacle. It is understood that in this case it is the frequency of modulation of the differential pressure which is the characteristic quantity and not the value at each instant, of the differential pressure.

To measure the modulation frequency of the differential pressure, it has been proposed to use a sensor comprising a membrane on the faces of which the two pressures are respectively applied, the difference of which is to be measured. The displacements of the membrane are measured by a detector with variable inductors.

Such a detector which involves circulation. of electric current is unsatisfactory in the case where the vortex meter is used to measure the flow of an explosive gas, since it requires the use of an explosion-proof enclosure.

In addition, frequently, there is a significant distance between the place where the meter is installed with its differential pressure sensor and the control area where you want to have the display of the flow measurement made by the meter . In the case of the inductive sensor, this information is transmitted in the form of an electrical signal which can be disturbed by electromagnetic interference.

To remedy these drawbacks. an object of the invention is to provide a pressure sensor for measuring the frequency of modulation of the pressure which does not require an electrical or electronic circuit in the immediate vicinity of the meter and which, moreover, allows a remote transmission of the signal of measurement which is not likely to be disturbed by electromagnetic interference.

To achieve this goal, the pressure sensor according to the invention comprises a deformable sealed enclosure and a conduit for applying in said enclosure a variable pressure over time, and it is characterized in that said enclosure comprises a portion whose curvature is modified under the effect of variations of said pressure and of a pressure external to the enclosure, and in that it further comprises an optical fiber partially fixed on the external face of said enclosure so that part of said fiber has at each instant a curvature substantially identical to that of said portion of the enclosure, means for applying to a first end of the fiber a light signal of determined intensity, and. means; for my; ti.r.er. the.

variations in attenuation of said light signal over time. said attenuation being representative. at each instant, of the value of said pressure by means of said variations in curvature.

In other words, variations in the difference in pressures applied on either side of the enclosure cause variations in the curvature of the enclosure and therefore in the portion of optical fiber integral with the enclosure. Variations in the curvature of the fiber modify the transmission coefficient of the fiber.

As will be explained later the transmission coefficient decreases when the radius of curvature decreases, due to losses. By measuring the variations of the signal collected at the second end of the optical fiber, it is possible to deduce the modulation frequency from the difference of the pressures applied on either side of the sealed enclosure.

Other characteristics and advantages of the invention will appear more clearly on reading the description which follows of several embodiments of the invention given by way of nonlimiting examples. The description refers to the appended drawing in which: - Figure 1 is a partially cutaway perspective view of a first embodiment of a pressure sensor according to the invention; - Figure 2 is a longitudinal sectional view of the sensor of Figure 1; - Figure 3 is a simplified view of the optical part of the sensor according to a first improved embodiment of the sensor; - Figure 4 is a simplified view of the optical part of the sensor according to a second improved embodiment of the sensor; and - Figures 5a to Sc are simplified cross-sectional views of three embodiments of the deformable enclosure of the sensor.

Referring to Figures 1, 2 and Sa, we will describe a first embodiment of a sensor according to the invention.

The sensor comprises a deformable sealed enclosure 10 formed by a side wall 12 and by two end walls 14 and 16. The side wall 12 is a cylindrical surface of axis XX 'whose cross section has the shape shown in FIG. 5a . The side wall is formed by two half-cylinders 18 and 20 connected by two plane portions parallel to each other 22 and 24.

 The side wall 12 is, for example, made with a copper sheet 50 ssm thick which is covered with a layer of adhesive as will be explained later. The radius R of the half-cylinders is 5 mm and the distance between the two half-cylinders is 20 mm. The length of the enclosure along the axis XX '.

is 50 mm.

A first pipe 30 opens inside the enclosure 10 by a nozzle 32 mounted in the end wall 14. The pipe 30 serves to introduce into the enclosure 10 a gas at a first pressure P1 dependent on time.

Inside the enclosure 10 the sensor comprises a filling part 34 which has a shape similar to that of the enclosure 10. The part 34 is fixed by its two ends to the end walls 14 and 16. The piece 34 defines inside the enclosure 10 a free volume 36 of substantially constant thickness h.

The end piece 32 is extended by two internal pipes 38 and 39 formed in the filling part which open into the free space 36.

The enclosure 10 is enclosed in a rigid watertight bolt case 40. The space 42, better visible in FIG. 2 between the enclosure 10 and the housing 40 has a thickness h ′ substantially equal to h the thicknesses h and h ′ are equal for example 0.5 mm. A pipe 44 opening into the space 42 allows a gas b to be introduced at a second pressure P2. Thus, the side wall 12 of the enclosure 10 is subjected to the pressure difference P1-P2; for pressure
P1 = P2 the enclosure 10 a, in cross section. the shape shown in FIG. Sa. When P1 and P2 are different, the side wall 12 of the enclosure is formed under the effect of the differential pressure; 1.

because of its very thin thickness. In particular, the wall portions 18 and 20 which are semi-cylindrical for Plan2 see their mean radius of curvature vary, these portions no longer being semi-cylindrical.

More precisely. if P1 is greater than P2, the mean radius of curvature increases, and in the opposite case it decreases. Typically, when the pressure sensor is used with a vortex counter, the differential pressure varies from 0.1 mbar to 1 bar.

To detect variations in the mean radius of curvature of the wall portions 18 and 20, which are representative of variations in the differential pressure P1-P2, the pressure sensor comprises an optical fiber 5Q, a portion 52 of which is wound on the middle portion of the side wall 12 of the enclosure 10. Indeed, it is in this region furthest from the end walls 14 and 16 that the deformation is greatest for a given pressure difference.

In FIGS. 1 and 2 the coiled part 52 of the optical fiber comprises three turns, approximately contiguous, the mean plane of each turn being substantially perpendicular to the axis XX 'of the enclosure.

The wound portion 52 of the fiber comprises: at least one half-turn which is bonded to one of the two semi-cylindrical portions. The wound part 52 is glued to the side wall 12 of the enclosure using an adhesive such as that which is marketed under the brand name zAraldite ". Preferably, the turns of the reel are embedded in a layer of adhesive 53 of thickness - for example equal - 300 - Ba - and in any case '' greater than the diameter of the fiber, which covers the entire side wall 12. Optionally, the layer of adhesive is covered by a second metal sheet to avoid contact between the adhesive and the gas. Thus, the deformations of the wall 12 are fully transmitted to the turns of the optical fiber 50. In particular, changes in the mean radius of curvature of the portions 18 and 20 of the enclosure cause identical modifications in the radius of curvature of the portions of the wound part 52 of the fiber which cover the portions 18 and 20 of the enclosure 10. To concentrate the deformations of the side wall in the zones having the smallest radius of curvature (half -cyl indres 18 and 20 in the case of FIG. 1), it is advantageous to stiffen the portions having the greatest radius of curvature (flat portions 22 and 24 in the case of FIG. 1).

This result can be obtained by depositing a layer of adhesive 53 only on the flat portions 22 and 24, or by depositing on the surface portions 22 and 24 a layer of adhesive thicker than on the semi-cylindrical portions 18 and 20. It is also possible to give the flat portions 22 and 24 themselves a thickness greater than that of the semi-cylindrical portions 18 and 20. The wound portion 52 of the fiber is extended by two portions 54 and 56 of the same fiber having a radius of curvature as large as possible and which pass through the external housing 40 by watertight bushings 58 and 58 '. Likewise, the pipe 30 crosses the external bolt case 40 by a sealed crossing 59.

In a control zone ZC the sensor comprises a light source 60 which applies to the end 54a of the fiber 50 a light signal of predetermined intensity. The source 60 can be an incandescent lamp, a light emitting diode or a Helium-Neon gas laser delivering a monochromatic beam
In the control zone ZC, the sensor also includes an opto-electric converter 62, which delivers an electric signal IR proportional to the intensity of the light beam received by the converter 62 at the end 56a of the fiber 50. The converter 62 can be a photodiode associated with an amplifier. The IR signal can also be compared in circuit 64 to the electrical signal IE controlling the light intensity applied to the input of the optical fiber 50. The ratio a = IR / IE is proportional to the coefficient of transmission of the entire optical fiber 50.

It is well known that the transmission coefficient depends in particular on the value of the radius of curvature of the fiber (see for example the article by D. Gloge in the journal Applied Optics, Vol 11, Moly, November 1972, pages 2506 a 2513.

 Thus the frequency of variations of the differential pressure P1-P2 is detected as a first approximation by measuring the frequency of variation of the light signal collected by the converter 62 if the intensity of the light signal applied to the fiber is Coeffective Indeed, the frequency of modulation of this signal is representative of the frequency of variations of the radius of curvature R of the enclosure which is itself representative of the frequency of variation of the differential pressure P1-P2.

After having described the principle of this first embodiment of the sensor, it is important to return to certain specific points.

The purpose of the filling piece 34 is to avoid the creation of a large dead volume inside the enclosure 10. The "thicknesses" h and h 'of the spaces 36 and 42 respectively located inside the enclosure 10 and between the wall 12 of the enclosure 10 and the external bolt case 40 are substantially equal. This value corresponds to the maximum of the amplitude of deformation of the flat portions 22 and 24 of the wall 12 corresponding to the maximum. normally allowable differential pressure. Thus the bolt bowl 40 and the filling part 34 constitute stops for the side wall 12.

In addition, it is necessary that the natural frequency of the enclosure 10 is significantly greater than the maximum of the pressure modulation frequency which it is desired to measure. For a sensor associated with a vortex counter, the maximum frequency is of the order of one kHz.

FIGS. 5b and 5c show other possible forms of the enclosure 10. In the case of FIG. 5b, the cross section of the side wall 12 ′ is el-liptigue,
The filling part 34 'and the external bolt case 40' have corresponding shapes. In the case of FIG. 5c, the side wall 12 of the enclosure is constituted by two portions of a circle 80 and 82 corresponding to an angle at the center greater than 180 *.

These two circular portions are connected by curved portions to rectilinear portions 84 and 86 parallel to each other. The filling piece 34 "and the housing 40" have corresponding shapes.

In all three cases, the side wall of the enclosure is a cylindrical surface whose section through planes perpendicular to the direction of its generatrices is a closed curve having two portions. symmetrical with respect to a line which have a reduced radius of curvature, and two equally symmetrical portions with respect to a line perpendicular to the first line. which have a larger radius of curvature.

As shown in FIG. B, the connection between the sensor itself (enclosure 10) and the measurement assembly (light source 60, opto-electric converter 62) is carried out by the fiber portions 54 and 56. This transmission can be carried out over a relatively large distance and without risk of electromagnetic interference. However, regardless of the precautions taken, the portions of optical fiber 54 and 56 may be subjected to localized deformations the frequency of which may interfere with the frequency of variation of the radius of curvature of the enclosure 10.

To remedy this drawback, a first solution shown in FIG. 4. consists in making the optical circuit as follows
A first optical fiber element 100 connects the light source 60 to a light-splitter connector 102.

The separator 102 divides the beam emitted by the source 60 into two light beams. One of the beams is sent into a second optical fiber element 104 which includes a part wound on the enclosure 10.

The other beam is sent in a third element
of optical fiber 110 not wound on the enclosure 10. Des
connectors 106 and 108 connect fibers 104 and 110 to
binding fibers 112 and 114. Fibers 112 and 114
are mechanically linked together so that any
deformation undergone by one of the fibers is undergone so
substantially identical by the other fiber element.

The light beams at the exit of the
fiber 112 and 114 are converted into electrical signals
by opto-electronic converters 116 and 118.

The electrical signals delivered by the converters
116 and 118 are amplified by amplifiers 120 and
122 having different gains respectively equal to
G1 and G2 with G2 = Gl 1). The difference of
gain of the two amplifiers is intended to compensate
the two optical fiber elements 104 and 110 which have
different transmission coefficients, so
that the continuous part of the signals delivered by the
converters 120 and 122 is equal. This may be
obtained by enslaving the two gains G1 and G2 with
a time constant much greater than the range of
frequency of modulation of the pressures to be measured.The - signals delivered - by the amplifiers 120; eut 122 are
applied to the inputs of a subtractor 124 which delivers
the measurement signal 5. We understand that by performing the
difference of the signals delivered by the converters
116 and 118, the parasitic modulations of the
measurement signal resulting from localized deformations
that could have been applied to the fiber element
optics 100 or at portions 108 and 114 of the second and
third elements of fiber, these two portions being
mechanically linked.

In Figure 4 there is shown an alternative embodiment of the optical circuit of Figure 3. This circuit comprises a first optical fiber element 130 having two cores 132 and 134 in the same sheath 136. The light source 60 sends via of the optical splitter 138 two substantially identical light beams respectively in the hearts 132 and 134. The hearts 132 and 134 are connected by a connector 140 respectively to a second optical fiber element 142 has a single core which is wound on the enclosure 10, and has a third optical fiber element 144 which has a fixed configuration and which is not wound on the enclosure 10. Finally, the optical circuit comprises a fourth optical fiber element 146 comprising two hearts 148 and 150 embedded in the same sheath 152.

The hearts 150 and 148 are respectively connected to the second optical fiber element 142 and to the third optical fiber element 144 by a connector 154. At the end of the hearts 148 and 150 of the fourth optical fiber element there is an electronic processing circuit identical to 9elut of FIG. 3 and which includes the opto-electronic converters 116 and 118, the amplifiers 120 and 122 and the subtractor 124. In this case again any deformation applied to the optical fiber elements 130 and 146 has no effect on the signal 5. Indeed, any deformation undergone by one of these optical fiber elements is applied in substantially the same way to the two hearts constituting the fiber. By carrying out the weighted difference of the two optical signals the identical attenuations undergone by the two hearts are eliminated.

In the embodiments of FIGS. 3 and 4, it can be seen that the optical circuit comprises on the one hand an optical measurement path similar to that of FIG. 1 and which is constituted either by the optical fiber elements 100, 104 and 114, either by the optical fiber elements 132, 142, and 150, and an optical compensation path consisting either of the optical fiber elements 100, 110 and 112 or by the optical fiber elements 134, 144 and 148.

 It should be added that the optical fibers can be of the multimode type. To obtain the best attenuation effect as a function of the variations in the radius of curvature, it is preferable to inject the light ray under an incidence such that it is the modes of fiber transmission of the highest levels which are transmitted. In the case of FIG. 3, this can be achieved by a judicious choice of the characteristics of the connector-splitter 102.

 In the case of Figure 4 this incidence is obtained by the connector 140. In addition, it appears that the attenuation effect in function of the radius of curvature. the optical fiber is further improved if the fiber sheath at the curvature is made of an absorbent material.

Finally, the sensor has a completely symmetrical structure which makes it insensitive to the effects of mechanical shock.

Claims (6)

1. Pressure sensor comprising a waterproof deformable enclosure and a pipe for applying in said enclosure a variable pressure over time. characterized in that said enclosure (10) comprises a portion (18,20) whose curvature is modifiable under the effect of said pressure and of a pressure external to said enclosure, an optical fiber (50) fixed on said enclosure ( 10) so that part of said fiber (52) has at each instant a curvature substantially identical to that of said portion of the enclosure, means (60) for applying to one end of said fiber a light signal of intensity determined and means (62, 64) for measuring the variations in attenuation of said light signal over time, said attenuation being representative, over time, of variations in said pressure over time via said variations in curvature 2. Sensor according to claim 1, characterized in that said enclosure (10) is placed in a sealed housing (40) non-deformable to receive said external pressure whereby said attenuation is representative over time of the difference between said pressure and said external pressure. 3 sensor according to claim 2, characterized in that said enclosure (10) comprises a side wall (12) cylindiadric of which a part of the cross section has said curvature (18,20), and two end walls (14,16 ), perpendicular to the generatrices of said side wall and in that said fiber (50) comprises at least one half-turn (52) surrounding said enclosure (10) and is fixed on it, said half-turn being arranged in a mean plane perpendicular to the generatrices of said side wall (12). 4. Sensor according to claim 3, characterized in that said fiber (50) comprises a wound portion (52) forming at least one half-turn fixed on said side wall - (12) a first portion (54) of connection connecting said light source (60) to said wound portion (52) and a second connecting portion (56) connecting said wound portion (52) to said attenuation measuring means (62,64). 5. Sensor according to claim 4. characterized in that it further comprises a second fiber comprising a first connecting portion (134) integral with said connecting portion (132) of the first fiber. a second connecting portion (148) integral with said second connecting portion (150) of the first fiber. and a third portion (144) for connecting said first (134) and second (148) connecting portions, in that said means for measuring attenuation comprises means (116 to 124) for determining a weighted difference of the light intensity collected at the end of the second portions (148-150) of said first and second fibers. and in that the means (60) for applying said light signal comprises means (138) for applying fractions of said light signal respectively to the end of the first portions of said first and second fibers.  6. Sensor according to any one of claims 1 to 6, characterized in that it further comprises a filling part (34. 34 ', 34 ") fixed inside said enclosure (10) and leaving between itself and the wall of said enclosure a free space (42) for applying said pressure.