FR3138209A1 - Optical liquid level measurement system - Google Patents

Abstract

Optical liquid level measurement system Such a system (3) comprising one or more light sources (21) each capable of delivering light pulses, one or more light detectors (31), an opto-mechanical head ( 160), and a connection device (350) by optical fibers connecting the light source(s) to the opto-mechanical head, and connecting the detector(s) to the opto-mechanical head, system in which the opto-mechanical head comprises at least three distinct collimators optically associated with three optical fibers (51, 52, 53) of the connecting device, the three fibers being connected to the light source or sources; or being connected to the light detector or detectors. Figure to be published with the abstract: Figure 4

Description

Optical liquid level measurement system

The technical field is that of measuring a liquid level, in particular in a vehicle tank.

TECHNOLOGICAL BACKGROUND

We know different types of liquid level sensors, based for example on a measurement by radar, by ultrasound, by capacitive effect, or even based on a pressure measurement or on the movement of a float.

But a sensor using radar measurement or a capacitive effect, for example, is poorly suited to harsh electromagnetic environments, likely to be subject to electromagnetic disturbances. And a float sensor or pressure measurement sensor may prove unsuitable in a vehicle subject to strong acceleration, such as an aircraft, for example.

More generally, the operation of the sensors mentioned above is disrupted when the tank containing the liquid is located in a harsh environment, subject to high temperatures, electromagnetic disturbances and/or mechanical vibrations.

Finally, the types of sensors mentioned above generally lead to distorted indications in the case of vehicle tanks whose inclination varies. It is well known, for example, that most car fuel level sensors provide an erroneous result when the car in question climbs a steep slope.

SUMMARY

In this context, the present technology relates to a system for optically measuring a liquid level, by time of flight of light pulses, the system comprising one or more light sources, one or more light detectors, as well as a opto-mechanical head, which serves as a measuring head and which has the particularity of being offset, distant from the source or sources, and from the detector or detectors, the head being connected to these by a connection device by optical fiber.

The measuring head, to be mounted on the liquid reservoir, can therefore be passive, without electronics (the electronics, in particular light emission and detection, being remote elsewhere), which makes it possible to carry out reliable measurements even if the The tank environment is severe (high temperature, vibrations, electromagnetic disturbances). Furthermore, as the connection between the remote head and the rest of the system is an optical fiber connection, the head can be freely moved away from the rest of the system without introducing additional noise or electronic interference.

This measuring system also has the particularity of being configured to allow a distance measurement between: the head on the one hand, and a given point (or zone) of the free surface of the liquid on the other hand, and this for at least three different points of this free surface. These three distance measurements make it possible to determine both a position (typically an altitude) of this surface, and an inclination of this surface (this inclination being typically characterized by two angles of inclination, for example an angle of inclination before rear, and a left-right tilt angle).

This distance measurement at at least three different points allows a characterization of the liquid level both in terms of position and inclination, whereas the level sensors of the prior art mentioned above only provide an indication of average position of this free surface. Determining both the position and the inclination of the free surface of the liquid is particularly useful for an aircraft tank (fuel tank, or lubricant collection tank, for example), whose inclination is likely to vary greatly, both in terms of roll and pitch.

These different characteristics make the system particularly well suited to measuring the liquid level in a vehicle tank, for example an aircraft tank, subject to both a harsh environment and variations in inclination.

The opto-mechanical head of this measuring system comprises at least three distinct collimators optically associated with three distinct optical fibers of the connecting device, the three fibers being connected to the source or sources of light; or being connected to the light detector or detectors.

• d’émettre de la lumière (provenant des trois fibres en question) depuis trois positions différentes (i.e. : depuis les trois collimateurs, distincts), et optionnellement dans trois directions différentes, permettant ainsi de réaliser les trois mesures de distance en question, ou
• de collecter (et coupler dans les trois fibres en question) de la lumière, en trois positions différentes (i.e. : au niveau des trois collimateurs en question, distincts) et, optionnellement, provenant de trois directions différentes, permettant ainsi de réaliser les trois mesures de distance en question.

This three collimator arrangement allows:

• to emit light (coming from the three fibers in question) from three different positions (ie: from the three collimators, distinct), and optionally in three different directions, thus making it possible to carry out the three distance measurements in question, or
• to collect (and couple into the three fibers in question) light, in three different positions (ie: at the level of the three collimators in question, distinct) and, optionally, coming from three different directions, thus making it possible to carry out the three measurements distance in question.

By collimator is meant an optical system which comprises at least one lens, and which is globally convergent (positive overall vergence, for this optical system). Each collimator thus reduces the divergence of the light beam passing through it, or, in other words, makes this beam more convergent. In particular, each collimator can be configured to produce a collimated light beam (that is to say a beam of parallel rays), or partially collimated, from the divergent light beam which exits the corresponding optical fiber. Conversely, each collimator can be configured (in terms of vergence and distance to the fiber) to couple, in the fiber with which it is associated, an incident light beam, for example a parallel or substantially parallel incident beam.

• les trois collimateurs sont agencées de manière à émettre la lumière provenant des trois fibres dans trois directions distinctes ; ou pour collecter et coupler dans les trois fibres de la lumière provenant respectivement de trois directions distinctes ;
• le dispositif de liaison comprend un dispositif de couplage optique ayant, d’un côté, au moins trois ports de connexion, connectés respectivement aux trois fibres optiques, et, d’un autre côté, un port commun, chaque port de connexion étant couplé optiquement à ce même port commun, le système comportant une seule source de lumière reliée à ce port commun, ou comportant un seul détecteur de lumière relié à ce port commun ;
• le dispositif de couplage optique comprend un coupleur fibré 1 vers N avec N supérieur ou égal à 3, et les trois fibres optiques ont des longueurs différentes, une différence de temps entre les temps de parcours respectifs de la lumière dans deux quelconques de ces trois fibres étant supérieure ou égale à une durée desdites impulsions lumineuses ;
• le dispositif de couplage optique comprend un commutateur électro-optique configuré pour coupler sélectivement ledit port commun à l’un desdits ports de connexion, sélectionné parmi lesdits ports de connexion en fonction d’un signal électrique de commande dudit commutateur ;
• les trois fibres optiques sont des fibres aller, reliées chacune à la source ou à l’une des sources de lumière, et le dispositif de liaison comprend au moins trois fibres optiques supplémentaires, retour, associées respectivement aux trois fibres aller et reliées chacune au détecteur ou à l’un des détecteurs ;
• chacun desdits collimateurs, associé optiquement à l’une des fibres aller, est aussi associé optiquement à l’une des fibres retour, le collimateur étant commun à ladite fibre aller et à ladite fibre retour ;
• les trois fibres optiques sont appelées fibres optiques de pointage, et le dispositif de liaison comprend une fibre optique supplémentaire, dite fibre commune, un champ de collection ou d’émission de la fibre commune couvrant, pour chaque fibre optique de pointage, une partie au moins d’un champ d’émission ou de collection de la fibre de pointage considérée ;
• la tête opto-mécanique comprend, pour l’un au moins des collimateurs, un conduit situé dans le prolongement du collimateur, à l’opposé de la fibre optique associée au collimateur, le conduit étant au moins aussi profond que large ;
• le système comprend une unité électronique, reliée au détecteur ou aux détecteurs de lumière, et configurée pour déterminer :
  • une position et une inclinaison d’un niveau de liquide, ou une donnée représentative d’un volume de liquide présent dans un réservoir,
  • à partir de signaux électriques délivrés par le ou les détecteurs de lumière lors de la réception d’impulsions lumineuses, reçues par le ou les détecteur en réponse à l’émission d’une ou plusieurs impulsions lumineuses par la ou les sources de lumière.

In addition to the characteristics mentioned above, the system which has just been presented may include one or more of the following optional characteristics, considered individually or in any technically feasible combination:

• the three collimators are arranged so as to emit the light coming from the three fibers in three distinct directions; or to collect and couple into the three fibers light coming respectively from three distinct directions;
• the connection device comprises an optical coupling device having, on one side, at least three connection ports, respectively connected to the three optical fibers, and, on the other side, a common port, each connection port being optically coupled to this same common port, the system comprising a single light source connected to this common port, or comprising a single light detector connected to this common port;
• the optical coupling device comprises a 1 to N fiber coupler with N greater than or equal to 3, and the three optical fibers have different lengths, a time difference between the respective travel times of the light in any two of these three fibers being greater than or equal to a duration of said light pulses;
• the optical coupling device comprises an electro-optical switch configured to selectively couple said common port to one of said connection ports, selected from said connection ports as a function of an electrical control signal from said switch;
• the three optical fibers are forward fibers, each connected to the source or to one of the light sources, and the connection device comprises at least three additional return optical fibers, respectively associated with the three forward fibers and each connected to the detector or to one of the detectors;
• each of said collimators, optically associated with one of the forward fibers, is also optically associated with one of the return fibers, the collimator being common to said forward fiber and to said return fiber;
• the three optical fibers are called pointing optical fibers, and the connection device comprises an additional optical fiber, called a common fiber, a collection or emission field of the common fiber covering, for each pointing optical fiber, a part at less than one emission or collection field of the pointing fiber considered;
• the opto-mechanical head comprises, for at least one of the collimators, a conduit located in the extension of the collimator, opposite the optical fiber associated with the collimator, the conduit being at least as deep as it is wide;
• the system comprises an electronic unit, connected to the detector or light detectors, and configured to determine:
  • a position and inclination of a liquid level, or data representative of a volume of liquid present in a tank,
  • from electrical signals delivered by the light detector(s) upon reception of light pulses, received by the detector(s) in response to the emission of one or more light pulses by the light source(s).

The present technology also relates to a vehicle, for example of the aircraft type, comprising a reservoir for liquid and comprising a liquid level measuring system as described above, the opto-mechanical head of the measuring system being mounted on a wall of the tank.

The present technology also relates to a method of measuring a liquid level, implemented by means of a measuring system as described above, the method comprising at least three time-of-flight measurements, for three light pulses , emitted or collected by said three collimators, the three pulses reflecting on a free surface of the liquid, at three distinct points of this free surface.

This technology and its various applications will be better understood on reading the following description and examining the accompanying figures.

BRIEF DESCRIPTION OF THE FIGURES

The figures are presented for informational purposes only and are in no way limiting.

schematically represents, in a generic manner, a system for optically measuring a liquid level.

schematically represents a first embodiment of such a measurement system.

schematically represents a second embodiment of such a measuring system.

schematically represents a third embodiment of such a measuring system.

schematically represents an opto-mechanical head of the system of the , perspective view.

is a partial schematic view showing a pair of fibers mounted on the opto-mechanical head of the .

schematically and partially represents a variant of the opto-mechanical head of the .

schematically represents another variant of the opto-mechanical head of the .

is a partial schematic view showing how to arrange a double-clad composite optical fiber on the opto-mechanical head of the .

schematically represents a bundle of composite optical fibers that can equip the system with .

schematically represents a multi-fiber bundle that can equip the system with .

schematically represents a fourth embodiment of a liquid level measuring system.

schematically represents a fiber optic coupler that can equip the system with .

schematically represents an electro-optical switch that can equip the system with .

schematically represents a fifth embodiment of a liquid level measuring system.

DETAILED DESCRIPTION

• une ou plusieurs sources de lumière, 20,
• un ou plusieurs détecteurs de lumière, 30
• une tête opto-mécanique 60, qui sert de tête de mesure et qui a la particularité d’être déportée, distante de la source ou des sources et du détecteur ou des détecteurs, la tête étant reliée à ceux-ci par un dispositif de liaison par fibres optiques, 50.

There represents in a generic, synoptic manner, a system 10 for optical measurement of a liquid level conforming to the present technology. Such a system 10 makes it possible to measure the level of a liquid 8 by time of flight of light pulses. As indicated in the preamble, it includes:

• one or more light sources, 20,
• one or more light detectors, 30
• an opto-mechanical head 60, which serves as a measuring head and which has the particularity of being offset, distant from the source or sources and the detector or detectors, the head being connected to these by a connecting device by optical fibers, 50.

This system also has the particularity of allowing a distance measurement between: the head 60 on the one hand, and a given point (or zone) of the free surface 9 of the liquid 8 on the other hand, and this for at least three different points of this free surface 9. These three distance measurements (or more) make it possible to determine both a position (typically an altitude) of this surface 9, and an inclination of this surface, this inclination typically being characterized by two tilt angles (e.g. a front-back tilt angle, and a left-right tilt angle).

As explained in the “summary” section, such a system is particularly well suited to measuring the liquid level in a vehicle tank (fuel tank, or lubricant collection tank, for example), for example an aircraft tank. , subjected to both a severe environment and substantial tilt variations.

In order to carry out this distance measurement for at least three distinct points (or three zones) of the free surface of the liquid, the opto-mechanical head 60 is provided with at least three distinct collimators, optically associated with three distinct optical fibers of the device connection, these three fibers being connected to the light source or sources 20, or being connected to the light detector or detectors 30.

• d’émettre de la lumière (provenant des trois fibres en question) depuis trois positions différentes (i.e. : depuis les trois collimateurs, distincts), et optionnellement dans trois directions différentes, permettant ainsi de réaliser les trois mesures de distance en question, ou
• de collecter (et coupler dans les trois fibres en question) de la lumière, en trois positions différentes (i.e. : au niveau des trois collimateurs en question, distincts) et, optionnellement, provenant de trois directions différentes, permettant ainsi de réaliser ces trois mesures distinctes de distance.

This three collimator arrangement allows:

• to emit light (coming from the three fibers in question) from three different positions (ie: from the three collimators, distinct), and optionally in three different directions, thus making it possible to carry out the three distance measurements in question, or
• to collect (and couple into the three fibers in question) light, in three different positions (ie: at the level of the three collimators in question, distinct) and, optionally, coming from three different directions, thus making it possible to carry out these three measurements distinct in distance.

The three collimators in question can thus be arranged, optionally, so as to emit the light coming from the three optical fibers in three distinct directions; or to collect and couple into the three fibers the light coming respectively from three distinct directions (as in the case of the For example). The three collimators can, as here, be integral with each other. The head here includes a mechanical support, on which the collimators are mounted. The head may comprise one or more members for fixing the head to a wall of a tank, such as a collar, a flange or a fixing groove, or tapped or smooth holes.

Five different embodiments of such a measuring system, numbered respectively 1, 2, 3, 4 and 5, are represented schematically, respectively in Figures 2, 3, 4, 12 and 15.

For embodiments 1 to 3 ( to 4), each of the three optical fibers mentioned above is coupled to another optical fiber, one serving to bring the light to the opto-mechanical head (go fiber), and the other serving to recover the light retroreflected by the free surface 9 of the liquid (return fiber). The connection device 150; 250; 350 thus comprises three forward fibers, 51, 52, 53 and three return fibers 54, 55, 56, in these three embodiments.

Note that, for each pair of forward and return fibers (for example the pair of fibers 51 and 54), the two fibers, forward and back, do not necessarily form two distinct cords. The forward fiber and the return fiber can for example be intertwined with each other and together form the same cord (multi-fiber). The forward fiber and the return fiber can also be nested directly into each other (Figures 8 to 10), one forming the core, and the other forming a cladding (typically multimode) of a kind of fiber double-clad composite optics. More generally, the expression "go fiber", and "return fiber" here designates an optical guide (possibly itself composite), that is to say a structure capable of guiding light to transmit it to a distant point , the guide corresponding to the forward fiber, and the guide corresponding to the return fiber being generally distinct from each other (although possibly intertwined, or nested, or simply parallel to each other, within of the same cord). In certain variants, this outward guide and this return guide could, however, be confused, and take the form of a single wide-core optical fiber.

Figures 5 and 6 show in more detail the opto-mechanical head, 160, of system 1; 2; 3 according to these first three embodiments. Figures 7 to 8 show variants, 160’; 160” of this optomechanical head, which can be used in place of the 160 head.

• les trois fibres optiques mentionnées plus haut, associées optiquement aux trois collimateurs en question, appelées aussi fibres optiques de pointage,
• ainsi qu’une fibre optique supplémentaire, appelée fibre commune (il s’agit de la fibre 51, sur la ; et de la fibre 54, sur la ), la lumière servant à réaliser les trois mesures de distance en question étant soit amenée par cette même fibre optique commune, et cela pour les trois mesures (cas de la ), soit collectée dans cette même fibre commune, et cela pour les trois mesures (cas de la ).

As for embodiments 4 and 5 (Figures 12 and 15), the connecting device 450; 550 of system 4; 5 includes:

• the three optical fibers mentioned above, optically associated with the three collimators in question, also called optical pointing fibers,
• as well as an additional optical fiber, called common fiber (this is fiber 51, on the ; and fiber 54, on the ), the light used to carry out the three distance measurements in question being either brought by this same common optical fiber, and this for the three measurements (case of the ), or collected in this same common fiber, and this for the three measurements (case of the ).

Thus, in embodiments 4 and 5, the optical fiber connection device, 450; 550, includes: a single forward fiber, 51, and three return fibers 54, 55, 56, (case of the ) ; or three forward fibers 51, 52, 53, pointing, and a single return fiber, 54 (case of ).

Despite these differences, these five embodiments have many common elements and characteristics. Also, from one embodiment to another, identical or corresponding elements will be identified as much as possible by the same reference numbers, and will not necessarily be described each time. These different embodiments are now described in more detail, one after the other.

In the first embodiment , the system 1 comprises three distinct interrogators, 111, 112, 113, one for each pair of forward and return optical fibers (pair of fibers 51 and 54, pair of fibers 52, 55, and pair of fibers 53, 56). Each interrogator, 111, 112, 113, comprises a light source, 21, 22, 23, and a light detector (for example a photodiode), 31, 32, 33.

Each source 21, 22, 23 is capable of emitting a light pulse, for example in the visible or near infrared region. The duration of the pulse in question is for example less than 300 ns, or even less than 50 ns or even 20 ns. The source can be a laser, typically a semiconductor laser, or a light-emitting diode.

For the pair of fibers 51 and 54, the forward fiber 51 routes the pulse produced by the source 21 to the collimator 161 ( ), to emit it towards the free surface 9 of the liquid 8, while the associated return fiber, 54, collects at least part of this pulse, after it has been retroreflected by the free surface 9, to bring it up to the detector 31, thus allowing a distance measurement at a first point of the free surface 9. In the same way, the forward fiber 52 connects the source 22 to the head 160 while the associated return fiber, 55, connects the head 160 to the detector 32. And the forward fiber 53 connects the source 23 to the head 160 while the associated return fiber, 56, connects the head 160 to the detector 33.

The forward fiber 51 and the return fiber 54 are associated with each other, in the sense that the majority of the light collected by the fiber 54 comes from the corresponding forward fiber, 51 (and not from the other forward fibers 52 , 53). It is the same for the other pairs of forward and reverse fibers. This selective coupling between the forward fiber and the return fiber of each pair is due in particular to the distribution of the optical fibers on the head 160. In fact, the two fibers, forward and return, of each pair of fibers are located at each both close to each other, on the opto-mechanical head, and at a distance from the other optical fibers (the two fibers of the same pair being for example separated by a distance less than a third, or even less than a tenth of the distance which separates the different pairs of fibers, on the head). Furthermore, in the space located between the head and the liquid, the light emitted by the forward fiber and collected by the corresponding return fiber both propagate in the same average direction, or, at least, substantially in the same direction. average direction (i.e. for example with an angular deviation less than 30 degrees). An emission field of the forward fiber (area covered by the light coming from this fiber) and a collection field of the corresponding return fiber are superimposed, for the most part with each other (but not with the fields of 'emission of other fibers go).

In this first embodiment, for each pair of forward and return fibers (for example for the pair of fibers 51 and 54), the forward fiber and the return fiber form two distinct cords (see Figures 5 and 6). The connection device 150 thus comprises six cords (each integrating one of these six optical fibers). Furthermore, in this embodiment each optical fiber, 51, 52, 53, 54, 55, 56, is optically associated with a collimator of its own, 161, 162, 163, 164, 165, 166 (Figures 5 and 6).

The opto-mechanical head, 160, comprises a mechanical support 170, produced for example in the form of a single piece (in one piece), to which the collimators are fixed. The optical fibers 51 to 56 are also integral with the mechanical support 170, either because they are fixed to the collimators (for example thanks to a ferrule for fixing optical fiber, with which the collimator is provided), or because they are fixed directly to the mechanical support 170.

The three pairs of back and forth fibers can, as here, be arranged respectively at the three vertices of a triangle, in this case equilateral. Other arrangements are nevertheless possible (for example at the vertices of a right triangle, or in a line but with different emission directions).

Each collimator 161 – 166 is located at the bottom of a conduit which is located in the extension of the collimator considered, opposite the optical fiber associated with the collimator. This conduit promotes angularly selective emission, or, respectively, collection of light. For example, this prevents light from the forward fibers 52 and 53 from penetrating the return fiber 54 of the pair 51-54, after reflection on the surface of the liquid (or, at least, this limits this effect, which would correspond to a sort of “crosstalk” between the different round-trip channels).

Here, for each pair of forward and return fibers, the two corresponding collimators (for example collimators 161 and 164) are located at the bottom of the same conduit (duct 167, for the two collimators 161 and 164). The opto-mechanical head thus comprises three conduits, 167, 168, 169, here. Alternatively, each collimator could however be located at the bottom of its own conduit (with one conduit per collimator). As shown on the , the conduits in question, 167, 168, 169, correspond here to the interior conduits of three elements which extend projecting relative to the body of the mechanical support 170. These three projecting elements are portions of tube, here. Alternatively, the head could however be devoid of such projecting elements, the conduits in question then being holes made directly in the body of the mechanical support. In any case, each of these conduits 167, 168, 169 is at least as deep as it is wide. By depth, we designate the distance, in this conduit, between, on the one hand, an external surface of a front lens of the collimator considered (surface located opposite the fiber), and, on the other hand, a conduit outlet opening. By width we mean the width of the outlet opening. The deeper rather than wider character of each of these conduits contributes to the directivity of the emission and collection of light, for each of the measurement channels.

As already indicated, Figures 7 and 8 show two variants, 160' and 160'' of the opto-mechanical head of system 1 of the .

For these two variants, for each pair of forward and return fibers, the forward fiber and the return fiber are both optically associated with the same collimator.

In the case of the , for example, the same collimator 161' shapes the beam coming from the forward fiber 51, and couples into the corresponding return fiber, 54, the light received in return, after reflection on the free surface of the liquid. In the 160' variant of the , the two fibers going and returning from each pair form two distinct cords, mounted side by side on the mechanical support 170' of the head (these two cords being for example both engaged in the same orifice of the support).

In the case of the , the outward and return fibers of each pair are on the other hand intertwined with each other, one forming the core, and the other forming a sheath (typically multimode) of this bundle of two coaxial fibers (bundle which forms a “double clad” composite optical fiber). The three collimators 161'', 162'' and 163'' of the head 160'' are optically coupled, respectively to the bundle of fibers grouping the fibers 51 and 54, to the bundle grouping the fibers 52 and 55, and to the beam grouping the fibers 53 and 56. For each of these fiber bundles, the fiber goes 51, 52, 53.

The three collimators 161'', 162'', 163'' are each centered on an axis, X1, The three collimators (as well as the three fiber bundles) are mounted on the 170” mechanical support of the head with their three axes X1, X2, X3 inclined relative to each other. The light coming from the three forward fibers 51, 52 and 53 is emitted by these three collimators 161'', 162'', 163'' in three distinct directions.

There schematically represents an example of a composite optical cord, including an and R on the opposite, these four ports being connected together by the coupler in X. This composite cord comprises, on these different sections (on the four branches of the X) an optical guide, wide (typically several tens or even hundreds of microns , or more), multimode. In the part of the cord which extends from port A to port S (via the X coupler), an optical fiber of smaller diameter, for example single mode, is integrated into the cord, at the heart of it. The port S can be mounted opposite the collimator 161'', for example near the object focus of the collimator, while the port A is mounted opposite the source 21, so that the The pulse emitted by the source 21 is coupled into the core fiber 51 of this composite cord, and thus routed to the port S, to then be emitted by the collimator 161''. As for the light collected by the collimator 161'', it is coupled, at port S, into the sheath which surrounds the core fiber 51 (sheath which forms the return fiber 54, and which is also surrounded itself even by another sheath, external, making it possible to keep the light confined in the sheath 54, hence the name "double clad" usually used to designate this structure). The multimode nature of the "fiber" formed by this sheath also facilitates this coupling, and makes it possible to recover, in the fiber 54, a significant fraction of the light collected by the collimator 161''. At the X-shaped coupler, the return light, which circulates in the return fiber 54, is distributed, partly towards port A, and partly towards port B. Part of the return light, corresponding for example to 50% or more of the light power coupled in the return fiber 54, is thus directed towards port B, and therefore towards detector 31. As for port R, it is unused here.

The composite cord of the makes it possible to conveniently separate the forward light and the return light, for the measurement channel considered, because the separator, in this case the X-shaped coupler, is fibered, integrated directly into the cord.

Alternatively, the separation between the forward light and the return light could however be carried out outside the bead itself, for example by means of a semi-reflecting blade, or a beam splitter cube or prism. In this case, the back and forth fibers 51 and 54, nested one inside the other, form the same composite fiber of the “double-clad” type (double sheath), which extends from the opto-mechanical head to 'to the interrogator, the separation between the forward path and the return path being carried out in the interrogator, by means of the blade or cube or prism in question, interposed between the source and an entry face of the composite fiber bringing together the back and forth fibers 51,54.

As a further variant, the “double-clad” type composite optical fibers of the could each be replaced by a conventional optical fiber, with a single cladding, comprising a core of large diameter (a few tens or hundreds of microns, or more) surrounded by a cladding making it possible to keep the light confined in the core. In this case, the forward light and the return light circulate in the same optical fiber, and are separated, at the level of the interrogator, by means of a separator element such as mentioned above (coupler, blade, cube or prism For example).

• plusieurs fibres individuelles pour la voie aller (l’ensemblede ces fibres individuelles, qui forment une fibre optique composite, étant à nouveau appelé fibre optique aller 51, ici), et
• plusieurs fibres individuelles pour la voie retour (l’ensemblede ces fibres individuelles étant à nouveau appelé fibre optique retour 54, ici).

According to another variant, each pair of back and forth fibers could take the form of a multifiber bundle, such as that represented on the , with :

• several individual fibers for the forward path ( all of these individual fibers, which form a composite optical fiber, being again called forward optical fiber 51, here), and
• several individual fibers for the return path ( all of these individual fibers being again called return optical fiber 54, here).

On the side of the collimator 161'', the individual fibers of the forward path and those of the return path are intertwined with each other, intertwined with each other in the same bundle. On the other hand, on the other side (the interrogator's side), the individual fibers of the forward path (nine in number, in the example of the ), and the individual fibers of the return path (ten in number, in this example) are separated into two distinct bundles (two separate cords), one connected to the source 21, and the other connected to the detector 31. The individual fibers mentioned above each include a core, and a sheath to confine light within the core. These can be single-mode or multi-mode fibers.

In the different variants of forward and reverse fiber pairs presented above, the optical fibers generally have a high optical core diameter and numerical aperture (except possibly for the single-mode core of the ), for example 100 microns in diameter or more, and 0.2 numerical aperture or more, to maximize the power emitted and collected by the opto-mechanical head. This also concerns the optical fibers of other embodiments of the measuring system, 2; 3; 4 and 5.

The different variants of forward and reverse fiber pairs presented above can be used in place of the pairs 51-54, 52-55 and 53-56 represented on the , to connect the 160'' head to the interrogators of the measuring system. These different variants have in common that they are well adapted to the configuration of the head 160'', in which the same collimator 161'', 162'', 163'' is optically associated with both the forward fiber and the fiber corresponding return, grouped into the same beam.

This configuration, with a single collimator per measurement channel, makes it possible to reduce the number of optical components of the system (compared to the head 160 of the which includes six collimators, for example). Furthermore, thanks to this configuration, the emission field and the collection field corresponding respectively to the forward fiber and the return fiber are largely superimposed on one another, by construction. Indeed, at the exit face of the outward and return fibers, facing the collimator, these fibers (or these bundles of individual fibers) have a light emission zone and a light collection zone. which are extremely close to each other, even intertwined or even intertwined with each other.

The overall structure of the measurement system 2; 3; 4 and 5 is now described for the other embodiments of this system.

As already indicated, measurement system 2; 3 according to the second and third embodiment comprises three pairs of back and forth fibers, 51-54, 52-55 and 53-56, as for the first embodiment. The different variants of the opto-mechanical head 160; 160’; 160'', and the different types of fibers or optical fiber bundles presented above, during the description of the first embodiment can also be used, in the same way, in these two embodiments.

The measurement system 2 according to the second embodiment differs from the first embodiment in that it comprises a single light source 21 (instead of three), connected to the three forward fibers 51, 52, 53 by a device for optical coupling 251. For the rest, the measuring devices 1 and 2 are identical, or at least similar.

The coupling device 251 has, on one side, a common port 252, and, on the other, three connection ports 253, 254, 255 connected respectively to the three forward optical fibers 51, 52, 53. In the coupling device coupling, each connection port 253, 254, 255 is optically coupled (but not necessarily permanently) to this same common port 252.

The coupling device 251 can for example be a one-to-three fiber coupler (such as the coupler 351 of the ). Such a passive device is robust and inexpensive.

The coupling device 251 could also be an electro-optical switch (a “switch”) configured to selectively couple the common port 252 to one of the connection ports, this port being selected from the connection ports 253, 254, 255, depending on an electrical signal for controlling the switch. Such a device is more expensive than a passive fiber coupler. On the other hand, it allows the three distance measurements mentioned above to be carried out at three different times, which considerably limits any possible cross-talk between these measurements.

The measurement system 2 includes a single interrogator, 210 (instead of three individual interrogators), which integrates the source 21, the three detectors 31, 32, 33 and the coupling device 251.

The measuring system 3 according to the third embodiment differs from the first embodiment in that it comprises a single light source 21 (instead of three), and a single detector 31 (instead of three). For the rest, measuring devices 1 and 3 are identical, or at least similar.

The source 21 is connected to the three forward fibers 51, 52 and 53 via a one-to-three fiber coupler, 351, having a common port 352 connected to the source 21 and three connection ports 353, 354, 355 connected respectively to fibers 51, 52 and 53.

The detector 31 is also connected to the three return fibers 54, 55 and 56 by a one-to-three fiber coupler 356 (for example identical to the coupler 351), having a common port 357 connected to the detector 31 and three connection ports 358, 359, 360 connected respectively to fibers 54, 55 and 56. The measurement system 3 comprises a single interrogator 310 (instead of three), this interrogator integrating the source 21, the detector 31 and the two fiber couplers 351 and 352.

As shown schematically on the , the three return fibers 54, 55 and 56 have different lengths (while the forward fibers have the same length), a time difference Δt _propag between the respective travel times of the light in any two of these three fibers 54, 55, 56 being greater than or equal to a duration Δt _pulse of the light _pulses emitted by the source 21 (or even greater than three times _{Δt pulse )} . The duration Δt _pulse is for example the total duration of the pulse, taken halfway up the maximum power of _this pulse.

Thanks to this difference in lengths, the three light pulses, which make it possible to carry out the three distance measurements mentioned above, and which are routed respectively by the fiber 54, by the fiber 55 and by the fiber 56, reach the detector 31 at different moments, without temporally superimposing one on the other. In the electrical signal delivered by the detector 31, these three pulses can therefore be distinguished from each other, and the three distances in question can thus be measured independently of each other thanks to this form of time multiplexing. . This arrangement has the advantage of being passive, inexpensive, and requiring only a single source and a single detector.

Alternatively, the three return fibers 54, 55 and 56 could have the same length, the travel time differences in question being obtained thanks to the forward fibers 51, 52, 53 of different lengths. More generally, it is the total travel time, to travel both the forward fiber and the return fiber of the pair considered, which varies from one pair of fibers to another (i.e.: from one measurement channel to the other). The initial light pulse emitted by the source 21 is divided into three pulses by the coupler 351. These three pulses then have different total travel times, to reach the common detector 31 (due to the differences in total length between couples fibers), which makes it possible to distinguish them from one another, in the electrical signal delivered by this detector.

In a variant (not shown) of this third embodiment, the fiber coupler 351 could be replaced by an electro-optical switch (a “switch”) as described above. In this case, the different forward fibers 51, 52, 53 can have the same length, as well as the different return fibers 54, 55, 56. The measuring system is then configured so that the source 21 emits three successive light pulses, and the “switch” is controlled jointly to direct the first pulse towards fiber 51, then to direct the second pulse towards fiber 52, and finally to direct the third pulse towards fiber 53. These three pulses, initially emitted on different dates , therefore reach the common detector 31 at different times even if the travel times of the three measurement channels are identical. This variant could also be implemented by retaining the fiber coupler 351, and replacing the fiber coupler 356 with such a switch (instead of the reverse). The joint control of the source and the switch is for example carried out by an electronic unit of the measuring system, configured for this purpose.

As already indicated, the measurement system 4; 5 according to the fourth and fifth embodiment differs from the first three embodiments in that it comprises three return fibers 54, 55, 56 and a single forward fiber 51 ( ), or a single return fiber 54 and three forward fibers 51, 52, 53 ( ), instead of three pairs of fibers going back and forth.

The measurement system 4 according to the fourth embodiment comprises an interrogator 410 which includes a light source, 21, and a light detector, 31. The forward fiber 51, common to the three measurement channels, is connected on the one hand to the source 21, and on the other hand to the opto-mechanical head 460. On this head, the three return fibers 54, 55, 56 (pointing fibers) are for example arranged at the three vertices of a triangle, for example equilateral. The common forward fiber 51 can for example be placed in the center of this triangle. The emission field of the forward fiber 51 (after possible crossing of a collimator associated with this fiber) is sufficiently wide on the angular plane for a part of the light emitted by this fiber to reach the three collimators coupled to the return fibers 54 , 55, 56, after reflection on the free surface of liquid 8.

The three return fibers 54, 55, 56 are each connected to the same detector 31 via a coupling device 451. The coupling device 451 has, on one side, a common port 452 connected to the detector 31, and , on the other, three connection ports 453, 454, 455 connected respectively to the three return optical fibers 54, 55, 56. In the coupling device 451, each connection port is optically coupled (but not necessarily permanently) to the common port 452.

In this embodiment, the connection device 450 is arranged so that light pulses, collected respectively by the three return fibers 54, 55, 56, reach the common detector 31, at different times.

For this, the coupling device 451 can for example be produced in the form of a three-to-one fiber coupler 451A ( ), the three return fibers 54, 55, 56 then having different lengths, so that the time difference Δt _propag between the respective travel times of the light in any two of these three fibers is greater than or equal to the duration Δt _pulses light pulses emitted by source 21 (even greater than three times Δt _pulse ).

An initial pulse, emitted by the source, is then routed by the forward fiber 51 to the head 460. Part of this pulse is reflected on a first point (or a first zone) of the free surface 9 then is collected by the return fiber 54, forming a first return pulse. In the same way, the return fibers 55 and 56 collect two other return pulses, corresponding to reflections at two other points (or two other zones) of the free surface 9. These three return pulses are routed to the detector 31, and reach the detector at different times, thanks to the differences in length between fibers mentioned above.

The coupling device 451 can also be produced in the form of a controllable electro-optical switch, 451B ( ). In this case, the different return fibers 54, 55, 56 can have the same length. The measuring system is then configured so that the source 21 emits three successive light pulses. Each of these pulses gives rise, each time, to three return pulses transported respectively in the three return fibers. The switch 451B is then controlled to first connect the fiber 53 to the detector 31 in order to transmit the return pulse from the fiber 54 to the detector (while the other two return pulses, from the fibers 55 and 56, are unused), then then connect the fiber 54 to the detector 31, and finally to connect the fiber 55 to the detector. This makes it possible to successively carry out a distance measurement for measurement channel 1 (fiber 54), then for measurement channel 2 (fiber 55), and finally for measurement channel 3 (fiber 56). Here too, the joint control of the source and the switch can be carried out by an electronic unit of the measurement system, configured for this purpose.

The measurement system 5 of the fifth embodiment is comparable to that of the fourth embodiment, except that it comprises a single return fiber 54 and three forward fibers 51, 52, 53 instead of the reverse. The three forward fibers 51, 52, 53 are connected to the same source, 21, by the same coupling device 451 as that presented above.

The three forward fibers 51, 52, 53 are arranged on the head 560 at the three vertices of a triangle, for example equilateral. The return fiber 54, common to the three measurement channels, is for example located in the center of this triangle. The collection field of the return fiber 54 (generally provided with a collimator) is sufficiently wide on the angular plane to collect light from both the forward fiber 51, the forward fiber 52 and the forward fiber 53.

The connection device 550 is arranged in such a way that the light pulses, emitted respectively by the three forward fibers 51, 52, 53 to carry out the three distance measurements, reach the detector 31 at different times, without overlapping temporally. 'one to the other.

For this, the coupling device 451 can, as previously, be produced in the form of the three-to-one fiber coupler 451A, the three forward fibers 51, 52, 53 then having different lengths, so that the time difference Δt _propagate between the respective travel times of the light in any two of these three fibers is greater than or equal to the duration Δt _pulse of the light pulses emitted by the source 21 (or even greater than three times Δt _pulse ). Thus, an initial pulse emitted by the source 21 is divided into three pulses by the fiber coupler 451A, these three pulses traveling respectively in the fiber 51, 52 and 53, to reach the head 560 at three different successive instants (due to this difference in length), which makes it possible to carry out the three corresponding distance measurements independently of each other, even if the system includes a single return fiber and a single detector.

The coupling device 451 can, here too, be produced in the form of the controllable electro-optical switch 451B. In this case, the light source emits three successive light pulses, directed by the switch respectively towards fiber 51, then towards fiber 52, and finally towards fiber 53.

• reliée au détecteur 31 ou détecteurs 31, 32, 33 de lumière, et
• configurée pour déterminer:
  • une position et une inclinaison de la surface libre 9 du liquide 8, ou pour déterminer une donnée représentative d’un volume occupé par ce liquide 8 dans le réservoir qui le contient,
  • à partir de signaux électriques, d’écho, produits par le ou les détecteurs de lumière lors de la réception des impulsions lumineuses retour.

In the different embodiments described above, the measuring system 1; 2; 3; 4; 5 may include at least one electronic unit (not shown),

• connected to the light detector 31 or detectors 31, 32, 33, and
• configured to determine:
  • a position and an inclination of the free surface 9 of the liquid 8, or to determine a data representative of a volume occupied by this liquid 8 in the tank which contains it,
  • from electrical, echo signals produced by the light detector(s) upon reception of the return light pulses.

As already indicated, the inclination of the free surface of the liquid is for example characterized by two angles of inclination.

The electronic unit is for example configured to determine these quantities from time shifts between the echo signals, and a reference signal which identifies for example the instant of emission of an initial pulse. The link between the quantities to be determined (position/incline and/or remaining volume) on the one hand, and these time shifts on the other hand, can for example be determined during a prior phase of calibration of the measuring system (or even of the measuring system and tank assembly). This link can be stored in the electronic unit, for example in the form of a correspondence table, or in the form of a conversion formula parameterized by coefficients, the values of which are determined during calibration (the values coefficients then being stored in a memory of the electronic unit).

The electronic unit comprises an electronic circuit including an interface for receiving the electrical signals mentioned above, as well as an electronic computer (microprocessor, or programmable circuit on the fly, for example, etc.).

As indicated in the “summary” section, this measurement system can in particular equip an aircraft, the opto-mechanical head then being mounted on a wall of a tank of this aircraft.

Different variants can be made to the measurement system which has just been described, in addition to those already mentioned.

Thus, the measurement system could for example include more than three measurement channels, for example four or five different measurement channels. The system could thus include five pairs of back and forth fibers, and five associated collimators, to measure the distance between the head and the surface of the liquid at five different points, in order to obtain a form of redundancy between measurements to determine the position and tilting the liquid surface (or liquid volume) even more robustly and precisely.

Claims (12)

System (1; 2; 3; 4; 5) for optical measurement of a liquid level comprising:

• one or more light sources (21, 22, 23) each capable of delivering light pulses,
• one or more light detectors (31, 32, 33),
• an opto-mechanical head (160; 160';160'';460; 560), and
• a connection device (150; 250; 350; 450; 550) by optical fibers connecting the light source(s) (21, 22, 23) to the opto-mechanical head, and connecting the detector(s) (31, 32, 33) to the opto-mechanical head,
• in which the opto-mechanical head (160; 160';160'';460; 560) comprises at least three associated distinct collimators (161, 162, 163; 161';161'',162'',163'') optically to three optical fibers (51, 52, 53; 54, 55, 56) distinct from the connecting device (150; 250; 350; 450; 550), the three fibers being connected to the source or sources of light; or being connected to the light detector or detectors.

System (1; 2; 3; 4; 5) according to claim 1, in which the three collimators (161, 162, 163; 161'; 161'', 162'', 163'') are arranged so as to emit the light coming from the three fibers (51, 52, 53) in three distinct directions; or to collect and couple into the three fibers (54, 55, 56) light coming from three distinct directions respectively. System (2; 3; 4; 5) according to claim 1 or 2, in which the connecting device (250; 350; 450; 550) comprises an optical coupling device (251; 351, 356; 451) having, d 'one side, at least three connection ports (253, 254, 255; 353, 354, 355; 453, 454, 455), connected respectively to the three optical fibers (51, 52, 53; 54, 55, 56), and, on the other hand, a common port (252; 352; 452), each connection port (253, 254, 255; 353, 354, 355; 453, 454, 455) being optically coupled to this same common port , the system comprising a single light source (21) connected to this common port (252; 352; 452), or comprising a single light detector (31) connected to this common port (452). System (3; 4; 5) according to claim 3, in which the optical coupling device (351, 356; 451) comprises a fiber coupler (351, 356; 451A) 1 to N with N greater than or equal to 3, and in which the three optical fibers (51, 52, 53; 54, 55, 56) have different lengths, a time difference between the respective travel times of the light in any two of these three fibers being greater than or equal to one duration of said light pulses. System (2; 3; 4; 5) according to claim 3, wherein the optical coupling device (251; 451) comprises an electro-optical switch (251; 451B) configured to selectively couple said common port (252; 452) to one of said connection ports (253, 254, 255; 453, 454, 455), selected from said connection ports as a function of an electrical control signal from said switch. System (1; 2; 3) according to one of the preceding claims, in which the three optical fibers (51, 52, 53) are forward fibers, each connected to the source (21) or to one of the sources of light (21, 22, 23), and in which the connection device (150; 250; 350) comprises at least three additional optical fibers (54, 55, 56), return, associated respectively with the three forward fibers and each connected to the detector (31) or to one of the detectors (31, 32, 33). System (1; 2; 3) according to the preceding claim in which each of said collimators (161'; 161'', 162'', 163''), optically associated with one of the forward fibers (51, 52, 53) , is also optically associated with one of the return fibers (54, 55, 56), the collimator being common to said forward fiber and to said return fiber. System (4; 5) according to one of claims 1 to 6, in which said three optical fibers (54, 55, 56; 51, 52, 53) are called optical pointing fibers, in which the connecting device (450 ; 550) comprises an additional optical fiber (51; 54) called a common fiber, and in which a collection or emission field of this same common fiber covers, for each pointing optical fiber, at least part of a field emission or collection of the pointing fiber considered. System (1; 2; 3; 4; 5) according to one of the preceding claims, in which the opto-mechanical head (160) comprises, for at least one of the collimators (161, 162, 163, 164, 165 , 166), a conduit (167, 168, 169), located in the extension of the collimator, opposite the optical fiber (51, 52, 53, 54, 55, 56) associated with the collimator, the conduit being at less as deep as wide. System (1; 2; 3; 4; 5) according to one of the preceding claims, comprising an electronic unit, connected to the detector or light detectors, and configured to determine:

• a position and inclination of a liquid level, or data representative of a volume of liquid present in a tank,
• from electrical signals delivered by the light detector(s) upon reception of light pulses, received by the detector(s) in response to the emission of one or more light pulses by the light source(s).

Vehicle, for example of the aircraft type, comprising a tank for liquid and comprising a system (1; 2; 3; 4; 5) for measuring the liquid level according to one of the preceding claims, the opto-mechanical head (160; 160'; 160''; 460; 560) of the measuring system being mounted on a wall of the tank. Method for measuring a liquid level, implemented by means of a measuring system (1; 2; 3; 4; 5) according to one of claims 1 to 10, the method comprising at least three measurements of time of flight, for three light pulses emitted, or collected by said three collimators, the three pulses reflecting on a free surface (9) of the liquid (8), at three distinct points of this free surface.