Vibration measurement system including a Bragg grating optical fiber(s)

Abstract

Vibration measurement system comprising an optical fiber (501) with Bragg grating(s). The optical fiber has one or more measurement lines (5012; 5015), each measurement line having one or more vibration measurement segments (5013), each incorporating a Bragg grating (503). For each measurement line, the system includes a rigid capillary (502; 602) through which the optical fiber (501) is threaded. The optical fiber is tensioned inside the rigid capillary over each of its vibration measurement segments, the ends of said vibration measurement segment being fixed to said rigid capillary by adhesive dots. The vibration measurement system is compact, lightweight, easy to integrate into a constrained environment, and performs well even with a basic interrogator. It also allows for multiplexing of measurements. Figure for the abstract: Fig. 5

Description

Title of the invention: Vibration measurement system comprising a Bragg grating optical fiber

[0001] This application relates to the field of structure health monitoring, particularly in constrained environments such as aircraft. More specifically, the invention relates to the field of vibration measurement and the determination of vibration or acoustic signatures enabling the detection of abnormal behavior. Previous art

[0002] Optical fiber sensors incorporating a Bragg grating are known. Such a sensor incorporates an optical fiber equipped with a Bragg grating, the Bragg grating being a series of periodic refractive modifications on a short segment of the fiber. The Bragg grating acts as an optical filter, reflecting a specific wavelength of light passing through the fiber, called the Bragg wavelength, while allowing other wavelengths to pass through. The Bragg wavelength depends on the grating period and the refractive index of the fiber.

[0003] Bragg grating sensors are widely used for detecting stress, temperature, and vibration. When a vibration acts on an optical fiber equipped with a Bragg grating, it causes mechanical variations in the fiber's structure, thus modifying the Bragg grating period or refractive index, which in turn changes the Bragg wavelength. These changes result in a variation in the wavelength of light reflected by the grating. By analyzing these variations, the frequency and amplitude of the vibrations to which the fiber is subjected can be deduced.

[0004] Known sensors take the form of housings measuring a few tens of millimeters on each side or in diameter, with a volume of up to one or more cubic centimeters, and weighing a few hundred grams. These sensors must be connected to an interrogator capable of processing the signal generated by the sensor, via at least one connecting cable. The housing is screwed onto the structure to be monitored, allowing for a point measurement of the vibrations to which the structure is subjected at the location where the sensor is attached.

[0005] To monitor the health of an aeronautical structure, it is necessary to carry out measurements at a plurality of points using as many Bragg grating sensors as there are desired vibration measurement points.

[0006] A first option is to mount the sensors in parallel, each sensor being connected directly to the interrogator. The major drawback of this solution is that it This multiplies the number of cables (each sensor requires at least one cable for its connection to the interrogator). Furthermore, as previously mentioned, each sensor is relatively bulky and, while not heavy, its weight is not negligible. Similarly, the increased number of connecting cables significantly increases the overall system's mass and size, a major drawback in a constrained environment such as an aircraft, where size and weight are critical factors. The increased number of cables also complicates the system's integration within the structure being monitored.

[0007] A second option is to mount the sensors in series. This option reduces the number and length of connecting cables used, but the overall volume of the system remains significant, as each sensor can measure several cm3.

[0008] Distributed Acoustic Sensing (DAS) measurement systems are also known. These systems use optical fibers to monitor acoustic signals in real time along a length of fiber. Acoustic or vibrational waves interacting with the fiber induce very slight deformations that modify the fiber's refractive index and, consequently, the properties of the light signal passing through it. The DAS system detects these variations by analyzing the interference created in the backscattered signal by the Rayleigh effect.

[0009] Unlike Bragg grating point sensors, the DAS system uses the entire length of the fiber as a continuous sensor array, enabling distributed monitoring over long distances. However, it has the disadvantage of low spatial resolution (on the order of one meter for pulse durations of 10 ns). Another drawback is the need to use an expensive (several tens of thousands of euros) and bulky (shoebox-sized) interrogator, which immediately precludes its integration into constrained environments. Description of the invention

[0010] The invention aims to overcome at least one of the aforementioned drawbacks by providing a vibration measurement system that is particularly well-suited to monitoring a structure in a constrained environment, such as an aircraft structure (e.g., a propulsion system). The system must, in particular, be less bulky and lighter than known point sensor systems, while remaining efficient, reliable, and possessing good spatial resolution; it must achieve these objectives with reduced manufacturing and integration costs. Another objective of the invention is to provide a vibration measurement system that allows for multiplexing measurements, that is, multiplying the vibration measurement points with the same interrogator, and this without prohibitively increasing the volume and mass of the system or the cost of the associated interrogator.

[0011] To this end, the invention relates to a vibration measurement system enabling the measurement of vibrations at at least one vibration measurement point over at least one measurement area, the system comprising a Bragg grating optical fiber.

[0012] The measurement system according to the invention is characterized in that:

[0013] - the optical fiber comprises, for each measurement zone, a measurement linear of length corresponding to said measurement zone, said measurement linear comprising one or more optical fiber segments, called vibration measurement segments, each corresponding to one of the vibration measurement points of said measurement zone, each of said vibration measurement segments incorporating a Bragg grating,

[0014] - for each measurement linear (or for each measurement zone, which amounts to (at the same time), the measurement system includes a rigid capillary in which the optical fiber is threaded, the rigid capillary having a length, along an axial direction of said capillary, which corresponds to said measurement zone (and consequently also to the corresponding linear measurement length of the optical fiber),

[0015] - for each of its vibration measurement segments, the optical fiber (and by consequently also the Bragg network) is stretched inside the rigid capillary on said section, the ends of said vibration measurement section being fixed to said rigid capillary by dots of glue.

[0016] Each vibration measurement point corresponds to a Bragg grating inscribed within a section of optical fiber stretched in a capillary tube. The vibrations experienced by the capillary are transmitted to the Bragg grating, which can then measure them. Since the Bragg grating is inside the capillary tube, it is inherently protected, precisely in areas of the structure being monitored where vibrations can be significant.

[0017] The measurement system according to the invention is thus free of bulky point sensors. The capillary is slightly wider than the optical fiber it encases; it typically has an external diameter on the order of a millimeter and an internal diameter of, for example, 600 µm. The internal diameter of the capillary must be sufficient to allow the fiber to be bonded to the ends of its measurement section and remain suspended in a taut state within the capillary along the length of said measurement section (i.e., in the Bragg grating region).Furthermore, the rigid capillary can be easily fixed to the structure to be monitored by any suitable and compact means capable of establishing a rigid connection between the structure and the capillary so that the latter vibrates in the same way as the structure; this fixing is achieved for example by gluing or by using point fixings such as brackets screwed onto the structure or by welding onto the structure or by bridge (electric welding) with retaining clips, this list not being exhaustive.

[0018] Since optical fiber can be directly connected to an interrogator, no other connecting cable is necessary, and a single link (namely the optical fiber itself) allows all vibration measurement points to be connected to said interrogator.

[0019] The particularly simple architecture of the measurement system according to the invention makes it lightweight, compact, and inexpensive. It also considerably facilitates the integration of the measurement system into a constrained environment.

[0020] The glue points keep the fiber in a taut state inside the capillary and the use of a rigid capillary ensures both the transmission of vibrations from the structure to the optical fiber (and therefore to the various Bragg gratings) and the holding of the optical fiber in position.

[0021] According to particular embodiments of the invention, the measurement system further meets the following characteristics, implemented individually or in any technically possible and operational combination.

[0022] Advantageously and according to the invention, the optical fiber has at least one end provided with an optical connector, for its connection to an interrogator.

[0023] The measurement system can be configured to measure vibrations at a single vibration measurement point (and therefore over a single measurement area). In this configuration, the optical fiber comprises a single vibration measurement segment equipped with a single Bragg grating, and the system includes a single rigid capillary, reduced to the length of the aforementioned vibration measurement segment. The fiber is then attached to the capillary only by means of two adhesive points, one at each end of the rigid capillary.

[0024] That being said, a major advantage of the system according to the invention is that it can be configured to measure vibrations at numerous points along a single optical fiber. Thus, it can be configured, in particular, to measure vibrations at several vibration measurement points, preferably close to one another, defining a single measurement zone.In this case, as defined above, the measurement system comprises one (and only one) rigid capillary as previously defined, i.e., enveloping the optical fiber and having a length corresponding to the measurement area; furthermore, the optical fiber comprises several vibration measurement sections, each of said vibration measurement sections incorporating a Bragg grating and corresponding to one of the vibration measurement points, the ends of each of the vibration measurement sections being fixed to the rigid capillary by points of glue, the optical fiber being stretched inside the rigid capillary over each of said vibration measurement sections.

[0025] In certain embodiments, the measurement system is configured to measure vibrations at several vibration measurement points defining several measurement zones, which may be distant from each other, each measurement zone comprising one or more vibration measurement points, preferably relatively close together. In this case, the above definition of the measurement system according to the invention translates to the fact that:

[0026] - the measuring system comprises several rigid capillaries (as many as there are zones of measurement), each of said rigid capillaries corresponding to one of said measurement zones,

[0027] - the optical fiber has several measurement lines (as many as there are zones of measurement), each linear measurement of the fiber corresponding to one of the measurement zones and extending inside one of the rigid capillaries, each linear measurement comprising one or more vibration measurement segments as previously defined, i.e. one or more fiber segments which each include a Bragg grating, are fixed to the rigid capillary by glue points at each of the ends of said segments, and along which the optical fiber is stretched, which vibration measurement segment(s) correspond to the measurement point(s) of said measurement zone.

[0028] In this preferred case, the optical fiber is free of capillary or other rigid protective device outside of its measurement lines. Alternatively, outside of the measurement lines, it may be protected by a simple protective sheath made of plastic or corrugated metal tubing.

[0029] In certain embodiments, the measurement system according to the invention is configured to also measure a temperature at at least one point, referred to as the temperature measurement point, on at least one of the measurement zones. In this case, in addition to its vibration measurement section(s) defined above, the optical fiber measurement section corresponding to said measurement zone includes a temperature measurement section incorporating an additional Bragg grating and whose ends are fixed to the rigid capillary by adhesive dots, the optical fiber being slack (left loose) inside the rigid capillary on said temperature measurement section.

[0030] In certain embodiments, the adhesive has a coefficient of thermal expansion sufficiently close to that of the rigid capillary so that the adhesive does not crack and does not cause break points in the fiber when expanding. For example, the adhesive is chosen from among those having a coefficient of expansion such that the ratio between the coefficient of expansion of the adhesive and that of the optical fiber is between 70% and 130%, preferably between 90% and 110%.

[0031] In certain embodiments, each of the rigid capillaries is made of a material selected from metallic alloys, in particular steels, such as austenitic stainless steels like the stainless steel known under the reference AISI 304L, and nickel-chromium superalloys such as those marketed under the brand Inconel®, plastics and in particular fiber-reinforced plastics.

[0032] The invention extends to a method for manufacturing a vibration measurement system as previously described. The invention therefore extends to a method for manufacturing a vibration measurement system enabling the measurement of vibrations at at least one vibration measurement point over at least one measurement area, the system comprising a Bragg grating optical fiber.

[0033] The manufacturing process according to the invention is characterized in that:

[0034] - the optical fiber used comprises, for each measurement zone, a linear of length measurement corresponding to said measurement zone, said measurement linear comprising sections of optical fiber, referred to as vibration measurement sections, each corresponding to one of the vibration measurement points of said measurement zone, each of said vibration measurement sections incorporating a Bragg grating,

[0035] - the measurement system used comprises, for each measurement linear of the fiber optical, a rigid capillary having a length, along an axial direction of said capillary, which corresponds to said measurement line,

[0036] - the optical fiber is threaded into said rigid capillary until the linear The measurement of the optical fiber corresponding to the rigid capillary is found entirely inside the rigid capillary.

[0037] - one of the first ends of said measuring line is fixed to one of the first ends the ends of the corresponding rigid capillary, using a first dot of glue,

[0038] - after this first dot of glue has set, the free end of the fiber is pulled located on the side of the second end of said rigid capillary so as to tension the optical fiber inside the rigid capillary, and the second end of the vibration measurement linear is fixed to the second end of the rigid capillary using a second dot of glue,

[0039] - if the measurement linear comprises several vibration measurement sections (i.e., if the measurement area in question includes several vibration measurement points),

[0040] — a rigid capillary is used, provided with slits, called partition slits, extending along a circumferential direction of the capillary, said partition slits being spaced from each other along the axial direction of the capillary so that the rigid capillary is thus divided into partition segments extending either between two consecutive partition slits, or between one end of the capillary and the nearest partition slit, each of said partition segments of the rigid capillary being intended to correspond to one of the vibration measurement segments of the optical fiber,

[0041] — Intermediate dots of glue are deposited in each of the partition slots This allows the optical fiber to be fixed to the rigid capillary at each end of the vibration measurement sections of the fiber, taking care to keep the optical fiber taut on each of said vibration measurement sections by applying traction if necessary on the free end of the optical fiber (from the second end of the rigid capillary).

[0042] Preferably, the intermediate adhesive dots are deposited before attaching the optical fiber to the second end of the rigid capillary by means of the second adhesive dot, preferably starting from the first end of the rigid capillary and waiting for each adhesive dot deposited in a partition slot to harden before depositing the adhesive dot in the next slot, in order to ensure that the fiber is properly tensioned on each of the vibration measurement sections. However, it remains consistent with the invention to apply the second adhesive dot before the intermediate adhesive dots and / or to apply the intermediate adhesive dots in any order.

[0043] In certain embodiments, for at least one of the measurement zones, a rigid capillary is used which has more partition segments than vibration measurement points in said zone, one of the partition segments corresponding to a fiber segment without a Bragg grating. This makes it possible to define vibration measurement segments of reduced length and to ensure adequate tension in the optical fiber, and therefore accurate vibration measurement, on each of the vibration measurement segments, particularly in the case where the measurement zone has two consecutive measurement points that are somewhat too far apart with respect to the maximum desired length for the vibration measurement segments.

[0044] The invention also relates to a method for monitoring an aircraft structure in which vibration measurement points are defined on the structure, distributed over one or more measurement zones, and a vibration measurement system as previously defined is implemented on the structure, which system comprises:

[0045] — on the one hand, an optical fiber comprising, for each measurement zone of the a previously defined structure, a measurement linear comprising as many vibration measurement sections, each equipped with a Bragg grating, as there are vibration measurement points defined for said measurement zone,

[0046] — on the other hand, as many rigid capillaries as there are measurement zones defined for the structure, each rigid capillary thus corresponding to a measurement linear of the fiber which is inserted into said rigid capillary, the optical fiber being fixed to the rigid capillaries by dots of glue at the ends of each of its vibration measurement sections and at the ends of the rigid capillaries, the optical fiber being stretched along each of said vibration measurement sections.

[0047] Optionally, one or each of the optical fiber measurement linears used in the monitoring process further includes a temperature measurement section provided with an additional Bragg grating, the optical fiber being fixed to the corresponding rigid capillary by glue points provided at the ends of said temperature measurement section, the optical fiber being slack along said temperature measurement section.

[0048] The invention, according to an exemplary embodiment, will be better understood and its advantages will become more apparent upon reading the following detailed description, given by way of example and in no way limiting, with reference to the accompanying drawings in which: • Fig. 1 illustrates the manufacturing steps of a first example of a vibration measurement system according to the invention, configured to measure vibrations at a single measurement point; the system is shown schematically in longitudinal section; • Fig. 2 illustrates the manufacturing steps of a second example of a vibration measurement system according to the invention, configured to measure vibrations at a plurality of measurement points on a single measurement area; the system is shown schematically in longitudinal section; • [Fig.3] is a longitudinal cross-sectional view of a third example of a vibration measurement system according to the invention, configured to measure vibrations at a plurality of measurement points on a single measurement area; • [Fig.4] is a longitudinal cross-sectional view of a fourth example of a vibration measurement system according to the invention, configured to measure vibrations at a plurality of measurement points on a single measurement area; • [Fig.5] is a longitudinal cross-sectional view of a fifth example of a vibration measurement system according to the invention, configured to measure vibrations at a plurality of measurement points distributed over two measurement zones;

[0049] Identical elements represented in the aforementioned figures are identified by identical numerical references.

[0050] Figure [1] relates to a first example of a vibration measurement system according to the invention, configured to measure vibrations at a single measurement point; this figure shows the manufacturing steps of this example of a measurement system, which can be seen fully assembled in the drawing at the bottom of Figure [1].

[0051] The vibration measurement system of [Fig. 1] comprises an optical fiber 101 and a rigid capillary 102 through which the optical fiber is threaded. Portion 1012 of the fibre, called linear measurement, which is surrounded by the rigid capillary 102, here comprises a single section 1013, called vibration measurement section, provided with a Bragg grating 103.

[0052] The system is obtained by performing the various steps illustrated respectively by the four drawings in [Fig. 1] from top to bottom of the figure. The fiber 101 is threaded into the capillary 102 (first drawing at the top of the figure) until the measuring section 1012 of the fiber, i.e., here the vibration measuring section 1013, extends completely into the rigid capillary, that is, until the Bragg grating 103 is located substantially at the center of the rigid capillary. The optical fiber 101 is then fixed (second drawing) to a first end 1020 of the rigid capillary by a first dot of adhesive 104. With the fiber now held at its first end 1010 by the first dot of adhesive 104, the free end 1011 of the optical fiber is pulled to maintain it in a taut state inside the rigid capillary (third drawing).A second drop of glue 105 is then deposited at the other end of the rigid capillary to fix the fiber in this stretched state within the capillary.

[0053] The capillary typically has an external diameter on the order of a millimeter and an internal diameter on the order of 0000 µm. The distance between the two adhesive points 104, 105, i.e., the length of the vibration measurement section 1013 of the fiber, which here also corresponds to the length of the rigid capillary 102 and the measurement section 1012 of the fiber, is on the order of a centimeter, the Bragg grating 103 typically extending over a few millimeters. This distance can be increased if required.

[0054] Figure 2 shows a second example of a vibration measurement system according to the invention, configured to measure vibrations at a plurality of measurement points on a single measurement area; this figure shows the manufacturing steps of this second example of a measurement system, which can be seen fully assembled on the last drawing (bottom drawing) of Figure 2.

[0055] The vibration measurement system of [Fig. 2] comprises an optical fiber 201 and a rigid capillary 202 through which the optical fiber is threaded. The portion 2012 of the fiber, referred to as the linear measurement section, which is surrounded by the rigid capillary 202, comprises three vibration measurement sections 2013, each equipped with a Bragg grating 203. The rigid capillary 202 has two partition slots 2022 extending over a portion of the capillary's circumference at two sections of the capillary located at a distance from each other and from the ends 2020 and 2021 of the capillary. The partition slots 2022 thus delimit three partition sections 2023 of the capillary, which correspond to the three vibration measurement sections 2013 of the fiber.

[0056] The system is obtained by performing the different steps illustrated respectively by the three drawings in [Fig. 2] from top to bottom of the figure. Fiber 201 is The optical fiber 201 is threaded through the capillary 202 until its measuring linear 2012, i.e., its three vibration measuring segments 2013, extends entirely within the rigid capillary. Each Bragg grating 203 is then located approximately at the center of one of the partition segments 2023 of the capillary. The optical fiber 201 is then attached to a first end 2020 of the rigid capillary by a first dot of adhesive 204 (first drawing at the top of the figure). With the fiber now held at its first end 2010 by the first dot of adhesive 204, the free end 2011 of the optical fiber is pulled to maintain it in a taut state inside the rigid capillary (second drawing).A second adhesive dot 205 and two intermediate adhesive dots 206 are deposited respectively at the second end 2021 of the rigid capillary and in the partition slots 2022, while maintaining the fiber in this taut state within the capillary (third drawing, at the bottom of the figure). Each of the three vibration measurement sections 2013 is thus fixed to the capillary at both ends. Note that the order in which the second adhesive dot 205 and the intermediate adhesive dots 206 are deposited can be arbitrary.

[0057] A third example of a vibration measurement system according to the invention, configured to measure vibrations at a plurality of measurement points over a single measurement area, can be seen in [Fig. 3]. Like the previous example, this system comprises a rigid capillary 302 which includes two partition slots 3022 delimiting three partition segments 3023. This third example differs, however, from the second example in [Fig. 2] in that the optical fiber measurement linear 301 comprises only two Bragg gratings 303, defining two vibration measurement segments 3013 corresponding to the capillary partition segments located at its ends. The vibration measurement segments 3013 of the fiber are thus separated by a central fiber segment 3014 devoid of a Bragg grating, which is not used for vibration measurement.

[0058] The length of this central segment can be arbitrary; it is derived from the distance between the two measurement points. It should be noted that the length of a vibration measurement segment (i.e., the distance separating the two adhesive points on either side of a Bragg grating) is important. This length is typically on the order of a centimeter. It can be increased but must be limited because if it becomes too long, there is a risk that the natural or imposed curvature of the capillary will constrain the optical fiber. Distances on the order of ten centimeters can be considered without any problem a priori.

[0059] This third example of a vibration measurement system according to the invention is assembled as explained for the second example with reference to [Fig.2], using glue dots 304, 305, 306, while keeping the optical fiber in a taut state by applying traction to its free end 3011.

[0060] Figure 4 illustrates a fourth example of a vibration measurement system according to the invention, configured to measure vibrations at a plurality of measurement points on a single measurement area and to also measure the temperature of the monitored structure at a temperature measurement point located on said measurement area.

[0061] Similar to the example in [Fig. 3], this system comprises a rigid capillary 402 which includes two partition slots 4022 delimiting three partition segments 4023. It further comprises an optical fiber whose measurement linear comprises three segments, including two vibration measurement segments 4013, each equipped with a Bragg grating and corresponding to the partition segments 4023 of the capillary located at its ends. The central fiber segment 4014, which separates the two vibration measurement segments 4013, is a temperature measurement segment. Like the vibration measurement segments, it is equipped with a Bragg grating 407, which is referred to as an "additional" Bragg grating because it is in addition to the number of gratings corresponding to the vibration measurement points in the measurement area.In contrast to vibration measurement sections, along which the optical fiber is taut to experience the same vibrations as the rigid capillary, the optical fiber is slack along temperature measurement section 4014. Because the fiber is slack, it only experiences the effects of temperature, unlike taut sections which are subject to both temperature and deformation.

[0062] Those skilled in the art will readily understand that this fourth example of a measuring system can be assembled as follows. The fiber 401 is inserted into the rigid capillary 402 as previously explained until one of its vibration measuring sections 4013 (for example, the one on the right in the figure) is aligned with the corresponding partition section of the capillary. One end of the first vibration measuring section 4013 is then fixed to the capillary with a dab of adhesive. This can be an end dab, in this case the adhesive dab 404, or an intermediate adhesive dab 406. The other end of said first vibration measuring section is then fixed to the capillary while the fiber is kept taut inside the capillary by applying tension to its free end 4011. At the end of this step, one end of the central temperature measuring section is thus fixed to the capillary.The second end of the central section can then be fixed using an intermediate dot of adhesive 406 placed in the corresponding partition slot 4022, taking care to keep the fiber slack on this temperature measurement section by pushing the fiber into the capillary from its free end, if necessary. At the end of this step, the right end of the second vibration measurement section 4013 of the fiber is thus fixed to the capillary. Finally, the optical fiber is fixed to the second end of the capillary using a dot of adhesive. glue 405 while pulling on the fiber from its free end so that it is taut on said second vibration measurement section.

[0063] Figure 5 represents a fifth example of a vibration measurement system According to the invention, this system is configured to measure vibrations over two measurement zones, each comprising three vibration measurement points. It includes an optical fiber 501 with two measurement sections 5012 and 5015 corresponding to the two measurement zones, as well as two rigid capillaries 502 and 602, also corresponding to the two measurement zones. In this example, the measurement section 5012 is identical to that of the system in [Fig. 3], with two vibration measurement segments 5013 at the ends of the capillary and a central segment 5014 without a Bragg grating, while the measurement section 5015 is identical to that of the system in [Fig. 2], with three vibration measurement segments 5013. It therefore seems unnecessary to describe them again in detail and explain their manufacturing process.

[0064] Between its two measurement lines, i.e. between the two rigid capillaries 502, 602, the optical fiber 501 is here left free, i.e. without capillary or other rigid protection, which facilitates the handling and integration of the vibration measurement system, in particular when the environment is very constrained; alternatively, it may be provided to equip the fiber with a protective sheath made of plastic or corrugated metal.

[0065] As previously stated, the length of a vibration measurement section is limited, on the order of a centimeter and can extend up to about ten centimeters. However, there are no particular constraints regarding the distance between two measurement sections / zones. A distance of one meter between the second end of the first capillary 502 and the first end of the following capillary 602 is perfectly acceptable.

[0066] The ends 5010 and 5011 of the optical fiber are equipped with optical connectors (not shown), one of which allows the optical fiber to be connected to an interrogator (not shown), which interrogator is part of the vibration measurement system. It should be noted that the length of the fiber between its end 5010 or 5011 and the rigid capillary 502 or 602, respectively, may, in reality, be much longer than illustrated. In general, the accompanying drawings are schematic and purely illustrative, and no scale is to be observed.

[0067] The example of a vibration measurement system in [Fig.5] could also be obtained from two optical fibers, one for each measurement zone, which would be connected either directly to each other or via a third optical fiber, through optical connectors outside of said measurement zones.

[0068] Of course, the measurement system according to the invention is not limited to the illustrated examples, in particular in terms of the number of rigid capillaries (or measurement lines), the number of vibration or temperature measurement sections per measurement line, etc.

[0069] The vibration measurement system according to the invention finds applications in various fields such as monitoring the health of a structure, vibration analysis of mechanical parts, impact detection, and generally any application requiring the measurement of vibrations and / or the determination of the acoustic signature of phenomena.

Claims

Demands

1. A vibration measurement system enabling the measurement of vibrations at at least one vibration measurement point on at least one measurement zone, the system comprising an optical fiber (101; 201; 301; 401; 501) with Bragg grating(s), the measurement system being characterized in that: - the optical fiber comprises, for each measurement zone, a measurement linear (1012; 2012; 5012; 5015), of a length corresponding to said measurement zone, said measurement linear comprising one or more optical fiber segments (1013; 2013; 3013; 4013; 5013), referred to as vibration measurement segments, each corresponding to one of the vibration measurement points of said measurement zone, each of said vibration measurement segments incorporating a Bragg grating (103; 203; 303; 403; 503), - for each linear of measurement, the measurement system includes a rigid capillary (102; 202; 302; 302; 402; 502; 602) in which the optical fiber (101; 201; 301;401) is threaded, the rigid capillary having a length, along an axial direction of said capillary, which corresponds to said measurement linear, - for each of its vibration measurement sections, the optical fiber is stretched inside the rigid capillary on said section, the ends of said vibration measurement section being fixed to said rigid capillary by dots of glue (104, 105; 204, 205, 206; 304, 305, 306; 404, 405, 406).;

2. Vibration measurement system according to claim 1, characterized in that the optical fiber (101; 201; 301; 401; 501) has at least one end provided with an optical connector.

3. Vibration measurement system according to any one of claims 1 or 2, enabling the measurement of vibrations at several vibration measurement points defining several measurement zones, each measurement zone comprising one or more vibration measurement points, characterized in that the optical fiber (501) is devoid of capillary or other rigid protective device outside its measurement linears (5012; 5015).

4. Vibration measurement system according to any one of claims 1 to 3, also enabling the measurement of temperature at a point in one of the measurement zones, characterized in that, in addition to its vibration measurement sections (4013), the optical fiber measurement linear (401) which corresponds to said measurement area includes a temperature measurement section (4014) which incorporates an additional Bragg grating (407) and whose ends are fixed to the rigid capillary by glue points (406), the optical fiber being relaxed inside the rigid capillary on said temperature measurement section.

5. Vibration measurement system according to any one of claims 1 to 4, characterized in that the glue has a coefficient of thermal expansion such that the ratio between the coefficient of thermal expansion of the glue and the coefficient of thermal expansion of the optical fiber is between 70% and 130%, preferably between 90% and 110%.

6. Vibration measurement system according to any one of claims 1 to 5, characterized in that each rigid capillary (102; 202; 302; 302; 402; 502; 602) is made of a material selected from metallic alloys, in particular steels, such as austenitic stainless steels like the stainless steel known under the reference AISI 304L, and nickel-chromium superalloys such as those marketed under the Inconel® brand, plastics and in particular fiber-reinforced plastics.

7. A method for manufacturing a vibration measurement system enabling the measurement of vibrations at at least one vibration measurement point on at least one measurement zone, the system comprising an optical fiber (101; 201; 301; 401; 501) with Bragg grating(s), the manufacturing method according to the invention is characterized in that: - the optical fiber (101; 201; 301; 401) used comprises, for each measurement zone, a measurement linear (1012; 2012; 5012; 5015) of length corresponding to said measurement zone, said measurement linear comprising one or more optical fiber segments (1013; 2013; 3013; 4013; 5013), referred to as vibration measurement segments, each corresponding to one of the vibration measurement points of said measurement zone, each of said vibration measurement segments incorporating a Bragg network (103; 203; 303; 403;503), - the measurement system used includes, for each linear measurement of the optical fiber, a rigid capillary (102; 202; 302;

8. 302; 402; 502; 602) having a length, along an axial direction of said capillary, which corresponds to said measuring linear, - the optical fiber (101; 201; 301; 401) is threaded into said rigid capillary (102; 202; 302; 302; 402; 502; 602) until the measuring linear of the optical fiber which corresponds to said rigid capillary is entirely inside the rigid capillary, - one of the first ends of said measuring linear is fixed to one of the first ends (1020; 2020) of the corresponding rigid capillary, using a first dot of glue (104; 204; 304; 404), - after applying this first dot of glue, the free end of the fiber located on the side of the second end (1021; 2021) of said rigid capillary is pulled so as to tension the optical fiber inside the rigid capillary, and the second end of the vibration measurement linear is fixed to the second end of the rigid capillary using a second dot of glue (105; 205; 305; 405), - if the measurement line includes several vibration measurement sections, — a rigid capillary is used, provided with slots (2022; 3022; 4022), called partition slots, extending in a circumferential direction of the capillary, said partition slots being spaced from each other in an axial direction of the capillary so that the rigid capillary is thus divided into partition segments (2023; 3023; 4023) each extending either between two consecutive partition slots, or between one end of the capillary and the nearest partition slot, each of said partition segments of the rigid capillary being intended to correspond to one of the vibration measurement segments (2013; 3013; 4013) of the optical fiber, — Intermediate dots of glue (206; 306; 406) are deposited in each of the partition slots to fix the optical fiber to the rigid capillary at each end of the vibration measurement sections of the fiber, taking care to keep the optical fiber taut on each of said vibration measurement sections by applying traction if necessary on the free end (2011; 3011; 4011) of the optical fiber. A manufacturing method according to claim 7, characterized in that, for at least one of the measurement zones, a capillary is used

9. rigid (302) which has more partition segments (3023) than vibration measurement points of said area, one of the partition segments corresponding to a fiber segment (3014) devoid of a Bragg grating. A method for monitoring an aircraft structure in which vibration measurement points are defined on the structure, distributed over one or more measurement zones, and characterized in that a vibration measurement system according to any one of claims 1 to 6 is implemented on the structure, which system comprises: - on the one hand, an optical fiber (101; 201; 301; 401) comprising, for each measurement zone of the structure, a measurement linear (1012; 2012; 5012; 5015) having as many vibration measurement sections (1013; 2013; 3013; 4013; 5013), each provided with a Bragg grating (103; 203; 303; 403; 503), as there are measurement points defined for said measurement zone, - on the other hand, as many rigid capillaries (102; 202; 302; 302; 402; 502; 602) as there are measurement zones defined for the structure, each rigid capillary thus corresponding to a measurement linear of the fiber which is inserted into said rigid capillary, the optical fiber being fixed to the rigid capillaries by points of glue (104, 105; 204, 205, 206; 304, 305, 306; 404, 405, 406) at the ends of each of its vibration measurement sections and at the ends of the rigid capillaries, the optical fiber being stretched along each of said vibration measurement sections.