FR3044413A1 - FIBER OPTIC MEASURING PROBE - Google Patents

Abstract

A measuring probe (1) having a plurality of internal ducts (C1 ..) extending in a body (3), a plurality of measurement acquisition means (5) disposed in said internal ducts, is provided. the end, and comprising pressure transducers and a plurality of pressure sensors adjacent to the pressure transducers. The pressure transducers comprise optical fibers (9).

Description

Fiber optic measurement probe

The present invention relates to the field of measurement probes.

In particular, pressure measurements are considered, in particular for gas flow velocity measurements, on a turbomachine, in particular in an aircraft.

The measurement of gas flow velocity is a delicate task. When this measurement has to be carried out in so-called "severe" environments (high temperatures, high gas velocities, polluted / particle-laden flow, high pressures, etc.) such as those encountered in turbomachine-specific flows, this task can become very complicated. .

More generally, probe pressure measurements are problematic. Thus, on Pitot-type meters, which measure at a point the total pressure (Pt) and the static pressure (Ps) in order to determine the dynamic component of the pressure, directly related to the Mach number of the flow, the measurement pressure is carried out by means of capillaries running in the Pitot tube, connected to a manometer or a transducer. Conventional Pitot tubes must be perfectly aligned with the flow to be able to measure the total pressure, which is a constraint in flows where the direction of the velocity vector can change. To work around this problem, multi-hole (multi-duct) type meters are used. These meters, more cumbersome than conventional pitot tubes, measure the pressure at several points (typically at least three) of a given probe profile. Several profile shapes can be used depending on the operating conditions of the probe. These measurements are then converted to Mach number and angle of incidence vis-à-vis the probe head through pre-established calibration tables. The pressure measurement on conventional multi-hole (multi-pipe) probes is performed in the same way as on Pitot tubes, via capillaries connected to transducers. The disadvantage of pitot tube meters, whether conventional or with multi-hole probes, is that the actual pressure measurement is done remotely, which generates a very low frequency resolution, typically of a few Hz at most. To circumvent this constraint, probes with miniature transducers (for example of the Kulite ™ type, so-called "fast response" probes) inside the multi-hole probe body are used. It is in this context that the invention proposes a compact and reliable measurement probe solution that makes it possible to carry out measurements in a so-called severe environment.

Potentialities of use for a measurement of speed of flow of gas at high frequency (higher than 10kHz), and / or in severe turbomachine environment: high temperature (> 400 ° C), medium loaded with particles, are favorably targeted. In this regard, a major disadvantage in the use of existing high frequency pressure sensors is that they only withstand moderate temperatures of the order of a few hundred degrees (<250 ° C), penalizing the robust character associated with conventional multi-hole probes.

Thus, on the basis of a known measuring probe comprising: a body provided with a measurement taking end, a plurality of internal conduits extending in the body up to said end, a plurality of acquisition means of measurements disposed in said inner ducts, near the end, and comprising: - pressure transducers, - a plurality of pressure sensors adjacent to the pressure transducers, it is proposed that the pressure transducers comprise optical fibers.

Such a probe is expected to be more robust and less cumbersome than a piezoelectric transducer probe. Its mode of operation must be able to measure with a high frequency resolution and / or at higher temperatures in bodies of smaller diameter probes, or for the same diameter (or section) of probe body, more sensors can be integrated , or a larger measuring cone.

Structurally, it is recommended that, on probes, pressure transducers and pressure sensors together define optical interferometers.

This must be favorable for the measurement of very small deformations (a few nm) of the membrane of the pressure sensor in question, due to the ambient pressure, all the more so if a reflective or semi-reflecting membrane separated from (1 ') is used. core of) the optical fiber transducer by a medium of a given optical index (air in general) as shown below in Figures 10-13, these devices constituting interferometers Fabry-Perot type.

In the context of multi-hole probes mentioned above, it is recommended that the measurement end of the probe body is tapered and has at least three internal coplanar ducts for measurements in the plane, and five ducts. non-coplanar for three-dimensional measurements.

On this point, it should be noted that the optical transducers can be made using optical fibers of 0.125 mm or 0.065 mm in diameter. It is thus possible to house in an optical multi-hole probe at least twice as many transducers as in a standard probe and five or even ten times more than in a fast probe.

Moreover, to preserve the fibers mechanically and in the quality of their information transmissions and avoid generating excessive losses in the signal that passes through them, the bodies of these fibers and at least some of the internal ducts and fibers The optics contained therein may be curved, but with one or more radii of curvature, always greater than 400 times the cladding radius.

And to limit the size of the probe and the offset of the measuring point relative to the general axis of the probe, we can use a "cobra" probe respecting the minimum radius of curvature R, but limiting the length L of the nozzle probe, the body and at least some of the inner ducts and contained optical fibers being then curved, with two successive opposite inflections giving a general shape S.

Another solution can be used to limit the stresses due to the integration of optical fibers and their small permissible radius of curvature. The sensors can indeed be located in the rectilinear portion of the probe body, set back, and connected to the outside by the ducts, which are opening. This solution strikes very slightly the frequency resolution achievable with the probe, but has the advantage of significantly limiting congestion.

Thus, it is provided in this case that the body and at least some of the inner ducts have at least one elbow having a radius R of curvature less than 400 times the cladding radius, and that the optical fibers contained in these internal ducts are interrupted. before the elbow.

An alternative also envisages that at least some of said inner conduits follow twisted patterns, which may be helical.

This must enable precise, fast and distinctly distributed multipoint data acquisitions with good spatial resolution, making it possible, if necessary, to make several acquisitions on several heights with the same device if the measurements are spatially scaled.

In terms of embodiment, several mounts can be used.

Among these, the following are in particular mentioned: an embodiment in which at least some of the optical fibers have a constitution comprising two sections of the same optical fiber, a short section of which is disposed at the end of the other longer section, the short section carrying the pressure sensor and containing a pressure sensitive medium and having a predetermined optical index, said medium being intermediate between the pressure sensor and said other longer section, - an embodiment in which at least some optical fibers have a constitution comprising two sections of optical fibers, including a short section disposed at the end of the other longer section, the short section carrying the pressure sensor and enclosing a pressure-sensitive medium and having a predetermined optical index, said medium being intermediate between the pressure sensor and said other longer section, whose e will have a section smaller than that of said intermediate medium, - an embodiment in which at least some of the optical fibers have a constitution comprising an optical fiber section provided externally, at one end, with a capillary carrying the pressure sensor and enclosing a pressure-sensitive medium having a predetermined optical index, said medium being intermediate between the pressure sensor and said fiber.

In all cases, it is also provided that the optical fiber or the optical fiber section may have a Bragg grating for acquiring data related to a physical parameter other than the pressure such as temperature, thus making it possible to compensate the elongation of the cavity due to thermal expansion.

The above achievements have the advantage of implemented well mastered, simple and powerful.

The first is more economical. The second retains the advantage of being able to use only fibers of the same outside diameter (see FIG. 11), but with a pressure sensor (deformable membrane) and a said intermediate medium of larger diameter (section), thus still better sensitivity especially at high frequencies (above 10kHz).

As already mentioned, an application aims at a probe that can work in these high frequencies and in a severe turbomachine type environment: high temperature (> 400 ° C), medium loaded with particles. This is why it is proposed that at least some of the optical fibers, as well as the membranes of which the pressure sensors are provided, are based on a material having a temperature of at least 1100 ° C., such as silica. or sapphire, said constitution of the optical fibers further ensuring high-frequency pressure measurements, above 10kHz. The invention will, if necessary, be better understood and other details, characteristics and advantages of the invention will become apparent on reading the following description given by way of nonlimiting example with reference to the appended drawings, in which: FIG. 1 is a side view of a probe according to the invention; FIG. 2 is a longitudinal section of this probe, likewise for FIGS. 3, 4, which show probe alternatives, FIG. alternative probe, in perspective and Figure 6, a local internal view, - Figure 7 shows detail VII and Figures 8-9 are views according to VIII, - Figures 10-13 are views in longitudinal section (axial ) of four possible embodiments of the probe of the invention, with in particular different pressure sensors implemented.

Figures 1-6 and in conjunction with the enlargement of Figure 7, we see four preferred embodiments of a probe 1 for measuring a physical parameter, in particular pressure, or temperature.

In a common way to the embodiments, the probe 1 comprises a body 3 provided with a measuring end 3a, a plurality of internal conduits (C1 to C9, depending on the versions) extending in the body 3 to said end 3a and a plurality of measurement acquisition means disposed in said inner ducts near the end 3a. At or towards the end 3a, the internal ducts, such as those C1 to C3 in FIG. 7, open outwards (EXT).

Each measurement acquisition means 5 comprises a pressure transducer 7 and (at least) a pressure sensor 8 adjacent to said pressure transducer.

And each pressure transducer 7 comprises (at least) an optical fiber 9 (FIGS. 10, 12, 13) or 9a, 9b (FIG. 11).

Thus, the solution proposed here is based on the concept of the multi-duct probe, with pressure measurements that are no longer carried out, especially via integrated miniature remote or piezoelectric transducers, but by optical fiber transducers.

It is recommended that these transducers are, as illustrated, individually formed with at least one optical fiber 9; 9a, 9b and a deformable, reflective or semi-reflective membrane 11a-11 of FIGS. 8-11 belonging to the pressure sensor 8.

The main part 90; 90a, 90b (the longest along the general axis 10 of the probe) of this optical fiber is that which will guide in the transducer, to a processing unit 13 located further back, the data from the pressure sensor 8. Between the membrane 11 a-11 d and this main part 90; 90a, 90b of the optical fiber is interposed a medium 15 of a given optical index (which may in particular be air). This device thus constitutes an optical interferometer. It can be considered in particular that it is a Fabry-Perot type interferometer capable of measuring very small membrane deformations due to the ambient pressure (EXT).

As already mentioned in part, probes 1 thus produced will be more robust and less cumbersome than piezoelectric transducer sensors. Flow velocities at higher temperatures can be measured in probe bodies of smaller diameter than on these prior sensors. Alternatively, for the same diameter of the probe body, more measuring acquisition means can be inserted, with a correspondingly larger measuring cone if, as advised and illustrated in FIG. measurement of the body 3 of the probe is tapered and comprises at least three internal conduits (C1 ..., see the views Figures 8-9 with respectively five and nine internal through ducts). There is interest in multiplying the number of these internal conduits because this number acts on the value of the angle of incidence of the fluid to be analyzed that the probe can capture. As an indication, the standard multi-conductor probes typically use capillaries of 0.3 to 0.5 mm in diameter, while the piezoelectric sensors used in the "fast response" probes have diameters typically of the order of 1.5 to 2 mm ( or more).

Optical transducers, on the other hand, can be made using fibers 9; 9a, 9b between 0.125 and 0.300mm, see 0.065mm, in diameter. It is thus possible to house in an optical multi-duct probe at least twice as many optical fiber transducers than in a standard probe and ten times more than in a fast probe.

The means for acquiring measurements, and in particular the fibers 9; 9a, 9b, may be inserted into the conduits C1 ... preformed (for example drilled) in the body 3 of the probe and thus walk inside this body.

The probe bodies 3 may be made of metallic materials (low and / or medium temperature applications <700 ° C.), or made of ceramics, for higher temperature applications.

With regard to the means for acquiring measurements, the fibers may be single or multimode, may be made from a single fiber (external diameters D1 and inside d1 common, as in FIG. 10) or several fibers of the same outside diameter D2, but with sections (typically diameters) inner d2 / d3 different (Figure 11), or a fiber and a capillary 21 joining the fiber 9 to the membrane 11 c, 11 d (Figures 12-13). The characteristics of the membrane (thickness, diameter, Young's modulus, Poisson's modulus, density, maximum permissible stress, in particular) and its mode of fixing to fiber or main part thereof will make it possible to define the measuring range and the sensor sensitivity. The size of the cavity or medium 15 must be adapted to the measurement range. It is by varying, in this cavity or medium 15, the axial distance L between the membrane considered and the adjacent end of the (main part, if any, of the) fiber that will be solicited optical transducer , by pressure variation.

In the solution shown diagrammatically in FIG. 10, the optical fiber has a constitution comprising two sections 90a, 90b of the same fiber 9. The short section 90b is disposed axially at the end of the longer section 9a. The short section carries the pressure sensor 8 and encloses the pressure sensitive medium. The long section 90a is a complete fiber, with a central core 17 and its cladding 18 (in particular optical). The two sections 90a, 90b can be glued together; reference 19.

In the solution shown schematically in FIG. 11, the differences are that the optical fiber comprises two fiber sections 9a, 9b, again in an axially end-to-end arrangement. The long section 9a is a complete fiber, with a central core 17. The short section again carries the pressure sensor 8 and encloses the intermediate medium 15. The core of the long section 9a has a section d2 smaller than that d3 of said medium intermediate 15. The outer diameters D1 fiber sections 9a, 9b, are the same. In both cases (FIGS. 10, 11), the membranes 11a, 11b are in the central axial channel of the short section and close the free end thereof.

In the solution shown schematically in Figure 12, there is always a long section of optical fiber 9 having a core 17. This section is provided externally and axially, at one end, the capillary 21 carrying the pressure sensor 8 (membrane 11b). The capillary 21 radially surrounds the intermediate medium 15 and again the pressure sensor 8 closes this intermediate medium, at the axial end, being applied against the annular capillary 21.

In the solution shown diagrammatically in FIG. 13, there is the same arrangement as FIG. 12, except that the section of optical fiber 9 has a Bragg grating 23 for acquiring data related to a physical parameter other than the pressure. In particular, it may be temperature data at the probe end. This solution or at least some of the optical fibers individually have a Bragg grating is applicable to all the aforementioned cases.

The aforementioned fibers can be made from various materials, such as silica, which can be used for temperatures up to 1100 ° C, or sapphire which can be used for temperatures up to 1700 ° C. And the transducers can allow high frequency pressure measurements beyond 10kHz, up to 200kHz. Since the probes concerned here use optical fibers, the radius of curvature R must be controlled in order to avoid generating excessive losses in the signal. This constraint could lead to significant offsets of the measuring point vis-à-vis the axis 10 of the probe (Figure 1). To avoid this and the size of the probe, we can use a "cobra" probe (Figure 3), with two successive opposite inflections giving the probe head a general shape in S. This will respect the radius of curvature minimum R while limiting the offset L of the probe nose.

In fact, if the measurement acquisition means 5 must extend almost to the free end 3a of the body, it is generally advised that the body 3 and its conduits (C1 ...), as well as the fibers contained therein are curved, with one or more radii of curvature, preferably always greater than 400 times the radius Rg of sheath (with Rg = D1 / 2 or D2 / 2, according to the diameter indicated in FIG. 10 or 11) .

An alternative is that the radius of curvature R is smaller, as for example in FIG. 4, with a body having a bend 25 then having a radius of curvature of less than 400 times the said radius Rg of sheath and optical fibers, here identified 9, contained in the internal ducts C1-C3 and interrupting before the elbow. It would therefore be possible to have a probe body with a right angle, likewise for its internal conduits, and means 5 for acquiring rectilinear measurements, as illustrated. With measurement acquisition means thus withdrawn and connected to the outside by the ducts C1-C3 emerging, this solution maintains the advantage of limiting the space requirement, even if it strikes very slightly the frequency resolution attained by the probe.

Another solution can also be envisaged with C1-C9 conduits according to twisted traces, which can be helical, as shown in FIGS.

This is a solution presented in FR-A-2960596 where a machined probe 27, which can be used for gyration measurements of a turbomachine compressor, comprises a first cylindrical portion 29, of small diameter, extended by a second cylindrical portion 31 of larger diameter. This second portion 31 is connected to a base 33 having a machined base 35 for fixing the probe. The first part 29 defines a zone 37 of pressure acquisition which has orifices 01 to 09, the arrows 5 showing an arrival of air flow to be probed. These orifices, aligned on detection planes of different heights, are connected to the internal conduits C1-C9, respectively. The ducts form parallel helical curves between said planes, around axis 10. Optical fiber acquisition means as mentioned above could be placed in ducts C1-C9.

Claims (12)

Measuring probe comprising: a body (3) provided with a measuring end (3a), a plurality of internal conduits (C1 to C9) extending in the body to said end, a plurality of means Measurement acquisition device (5) arranged in said inner ducts, near the end, and comprising: - pressure transducers (7), - a plurality of pressure sensors (8) adjacent to the pressure transducers, characterized in the pressure transducers comprise optical fibers (9; 9a, 9b). 2. Measuring probe according to claim 1, wherein the pressure transducers (12) and the pressure sensors together define optical interferometers. 3. Measuring probe according to one of claims 1 or 2, wherein the measuring end of the probe body (3) is tapered and comprises at least three internal conduits (C1-C3). 4. Measuring probe according to any one of the preceding claims, wherein the body (1) and at least some of the internal ducts (C1-C9) and optical fibers (9; 9a, 9b) contained are curved, with a or several radii of curvature, always greater than 400 times the radius (D1 / 2, D2 / 2) of sheath of the fiber. 5. Measuring probe according to any one of claims 1 to 3, wherein the body (1), and at least some internal conduits (C1 to C9) and optical fibers (9; 9a, 9b) contained are curved , with two successive opposite inflections giving a general shape in S. 6. Measuring probe according to any one of claims 1 to 3, wherein the body (1) and at least some of the inner ducts (C1-C9) have at least one elbow (25) having a radius of curvature less than 400 times the cladding radius (D1 / 2, D2 / 2) of the optical fiber, and the optical fibers (9; 9a, 9b) contained in these internal ducts stop before the elbow. 7. Measuring probe according to any one of the preceding claims, in which at least some of the optical fibers (9; 9a, 9b) have a constitution comprising two sections of the same optical fiber (9), including a short section ( 9b, 90b) disposed at the end of the other longer section, the short section carrying the pressure sensor (11a) and enclosing a pressure sensitive medium and having a predetermined optical index, said medium being intermediate between the pressure sensor and said other longer section. 8. Measuring probe according to any one of claims 1 to 6, wherein at least some of the optical fibers have a constitution comprising two sections of optical fibers (9a, 9b), a short section (9b) disposed after the other longer section, which has a core (17) the short section carrying the pressure sensor (11b) and enclosing a pressure sensitive medium and having a predetermined optical index, said medium (15) being intermediate between the sensor pressure and said other longer section, whose core has a lower section than said intermediate medium (15). 9. Measuring probe according to any one of claims 1 to 6, wherein at least some of the optical fibers have a constitution comprising a section of optical fiber (9) having a core (17), the section being provided externally with a end of a capillary (21) carrying the pressure sensor (11c, 11d) and enclosing a pressure sensitive medium and having a predetermined optical index, said medium being intermediate between the pressure sensor and said fiber. 10. Measuring probe according to any one of the preceding claims, wherein at least some of the optical fibers (9) individually have a Bragg grating (23) for acquiring data related to a physical parameter other than the pressure. 11. Measuring probe according to any one of claims 7 to 10, wherein at least some of the optical fibers (9; 9a, 9b), and the membranes (11a-11d) that comprise the pressure sensors, are based on a material having a temperature of up to at least 1100 ° C., such as silica or sapphire, said constitution of the optical fibers providing high-frequency pressure measurements, above 10 kHz. 12. Measuring probe (27) according to any preceding claim, wherein at least some conduits (C1-C9) follow twisted traces.