Sensor, in particular capacitive sensor for aircraft tank

Abstract

The invention relates to a sensor (1) capable of allowing a measurement of a dielectric permittivity of a medium, in particular a fuel in a tank, comprising: - a support (3), - insulating fixing elements (17), - a plurality of plates (7) comprising at least a first plate (7a) and a second plate (7b) which are parallel and aligned along a stacking axis (X), - at least one first support column (11) connecting the first plate (7a) to one of the insulating fixing elements (17), - at least one second support column (13) connecting the second plate (7b) to one of the insulating fixing elements (17), - at least one additional plate (7c), and - at least one spacer column (15) connecting the additional plate to a plate which is the second closest neighbour. The support (3) and the plates (7) are made of a same first metal material and the spacer column (15) is made of a second metal material.

Description

[0001] DESCRIPTION

[0002] TITLE: Sensor, in particular capacitive sensor for aircraft tank

[0003] Technical field of the invention

[0004] The invention relates to a sensor, in particular a capacitive sensor, capable of measuring a dielectric permittivity of a medium, in particular fuel in an aircraft tank, as well as an assembly comprising such a capacitive sensor.

[0005] State of the prior art

[0006] Knowledge of the amount of fuel contained in an aircraft tank is of great importance during a flight. It is therefore necessary to be able to monitor a measurement of such quantity and density of the fuel in the tank in real time during the flight with a high degree of accuracy.

[0007] For this purpose, it is known to install suitable sensors, of the gauge type, in the fuel tanks. Such sensors are in particular of the capacitive type and measure a change in a capacitance between two plates extending into the tank and separated by an air gap into which the fuel penetrates. It is recalled that in the case of a planar capacitor comprising two parallel plates of the same area A, separated by a distance d and immersed in a medium of dielectric permittivity E, the measured capacitance C is given by C = E E0 , where z0 is the dielectric permittivity of vacuum.

[0008] The gauges are integrated into a multi-sensor system for measuring the quantity of fuel in the tank, for which redundancies and drift correction measures are provided, in order to improve the accuracy and reliability of the measurements.

[0009] In particular, a measure intended to improve the accuracy of such capacitive sensors is a measurement of the dielectric permittivity of the fuel, implemented by a capacitive compensation sensor placed near the bottom of the tank, also designated by the acronym "CIC" for "Capacitance Index Compensator" in English.

[0010] Such a sensor comprises flat metal plates arranged parallel to each other, fixed to a support and assembled together by means of columns of known length so as to form precise air gaps.

[0011] However, such sensors are susceptible to expansion when the temperature varies over a wide operating range, typically operating conditions extending for example from -55°C to 65°C, disturbing the parallelism of the plates and the regularity of the air gaps and leading to non-linearities and measurement errors. Such expansion is particularly prevalent when the plates and the support are made of materials with very different coefficients of thermal expansion, especially for a plastic support.

[0012] Presentation of the invention

[0013] The invention aims to overcome these drawbacks by enabling the measurement of the dielectric permittivity of the fuel in an aircraft tank with predictable and controlled drifts over a wide temperature range.

[0014] To this end, the invention relates to a sensor, in particular a capacitive sensor, capable of enabling measurement of a dielectric permittivity of a medium, in particular a fuel contained in an aircraft tank, comprising:

[0015] - a support,

[0016] - a plurality of insulating fixing elements fixed to the support,

[0017] - a plurality of plates comprising at least a first plate and a second plate extending substantially parallel to the support and aligned along a stacking axis perpendicular to the support,

[0018] - at least one first support column having a first length and connecting the first plate to one of the insulating fixing elements,

[0019] - at least one second support column having a second length greater than the first length and connecting the second plate to one of the insulating fixing elements,

[0020] - at least one additional plate, and

[0021] - at least one spacer column having a third length and connecting the additional plate to a plate of the plurality of plates which is the second closest neighbor of the additional plate in the direction of the support along the stacking axis, characterized in that the support and the plates are made of the same first metallic material, and in that the spacer column is made of a second metallic material.

[0022] The first metallic material may have a first coefficient of thermal expansion. The second metallic material may have a second coefficient of thermal expansion.

[0023] The coefficient of thermal expansion of a material, noted a, is the linear coefficient allowing the variation in length AL of the material in each dimension to be determined in relation to a reference length L oat a reference temperature, depending on the temperature variation AT with respect to said reference temperature, so that AL = a L AT. For an isotropic material, this coefficient is the same in all directions, which is the case for the materials considered here.

[0024] Thus, in the sensor according to the invention, the plates drilling the capacitive measuring electrodes and the support will be modified by the temperature according to an identical coefficient in all directions. Due to the low variation values, a linear approximation can be made for the variation of the surfaces of the facing plates, which varies with the temperature according to a coefficient 2o. The use of a second same material for the spacer columns will cause the thermal expansion, in a direction perpendicular to the plates, to maintain the parallelism of the plates with each other. The air gap then varies according to the same expansion law, with the expansion coefficient of the second material of the columns.

[0025] Maintaining the parallelism of the columns and plates during thermal expansion means that the calculation of the capacitance value can use the simple formula for parallel plate capacitances and only take into account the variation in surface area and air gap for the calculation of the capacitor value.

[0026] The first support column may be made of a third metallic material, in particular having a third coefficient of thermal expansion. The second support column may be made of a fourth metallic material, in particular having a fourth coefficient of thermal expansion.

[0027] Such a sensor makes it possible to avoid distortions caused by differences in thermal expansion between the support and the plates in directions transverse to the stacking axis. Such a sensor makes it possible to predict and control capacitance variations due to thermal expansion in the direction of the stacking axis from the first coefficient of thermal expansion and the second coefficient of thermal expansion, over the temperature range considered.

[0028] A second coefficient of thermal expansion of the second metallic material may be substantially equal to twice a first coefficient of thermal expansion of the first metallic material.

[0029] Such a characteristic allows for a substantially zero variation in the sensor capacity with temperature variations, over the temperature range considered.

[0030] The insulating fasteners may be made of a polymeric material, in particular polyetheretherketone, in particular comprising glass fibers, specifically at least 25% glass fibers.

[0031] Such a feature makes it possible to have a rigid fixing of the support columns on the support, with insulating fixing elements having low thermal expansions and substantially equal to the metallic materials used over the temperature range considered. The plurality of plates may comprise an even number of plates. In such a case, the second metallic material and the third metallic material, and in particular the fourth metallic material, may be identical.

[0032] The plurality of plates may comprise an odd number of plates, such as three or five plates. In such a case, the third metal material and the fourth metal material may be different.

[0033] Such a characteristic allows to have a mechanical assembly having good rigidity and sufficient total capacity to obtain good precision in measuring the dielectric permittivity.

[0034] Furthermore, the sensor may comprise an equal number of first support columns, second support columns and spacer columns.

[0035] Such a feature allows for a more rigid and distortion-resistant symmetrical mechanical assembly.

[0036] The invention also relates to an aircraft fuel tank comprising a capacitive sensor as above.

[0037] Brief description of the figures

[0038] The present invention will be better understood and other characteristics and advantages will become apparent upon reading the detailed description which follows, comprising embodiments given for illustrative purposes with reference to the appended figures, presented as non-limiting examples, which may serve to complete the understanding of the invention and the description of its implementation and, where appropriate, contribute to its definition, in which:

[0039] Figure 1 is an exploded perspective view of a support and plates of a capacitive sensor according to the invention;

[0040] - figure 2 is a sectional view of a capacitive sensor according to a first embodiment of the invention;

[0041] - Figure 3 is a schematic representation of the capacitive sensor of Figure 2; and

[0042] - figure 4 is a schematic representation of a capacitive sensor according to a second embodiment of the invention.

[0043] Detailed description of the invention

[0044] Figures 1 to 3 are respectively exploded perspective views, sectional views and a schematic representation of a sensor 1, in particular a capacitive sensor 1, according to a first embodiment of the invention. The sensor 1 is capable of allowing a measurement of a dielectric permittivity £ of a medium. In particular, the sensor 1 is intended to be installed in an aircraft fuel tank, in order to allow a measurement of a dielectric permittivity E of the fuel.

[0045] The sensor 1 is in particular intended to be placed near the bottom of the tank in order to be completely immersed in the fuel during the entire flight of the aircraft.

[0046] According to the invention, the sensor 1, in particular the capacitive sensor 1, comprises

[0047] - a support 3, and

[0048] - a plurality of plates 7, in particular a plurality of metal plates 7, shown in exploded perspective view in Figure 1.

[0049] The support 3 is notably made up of a flat plate made from a first metallic material having a first coefficient of thermal expansion eu.

[0050] For a metallic material, the coefficient of thermal expansion a varies little over a wide temperature range, and in particular over a temperature range considered to be operating for the sensor 1. In particular, the temperature range considered extends for example from -55°C to 65°C and corresponds to the operational conditions of the fuel in an aircraft tank.

[0051] According to an alternative embodiment, the sensor 1 may comprise a housing 5, in particular capable of containing measuring electronics of the sensor 1. In such an embodiment, the housing 5 is fixed to the support 3 and is capable, in particular, of containing the measuring electronics of the sensor 1 in an internal space isolated from the outside. For this purpose, the measuring electronics may comprise digital and / or analog components, as the case may be.

[0052] Under operating conditions, the measuring electronics are isolated from the fuel contained in the tank by the housing 5, in order to prevent its operation from generating heating and / or a spark in the vicinity of the fuel.

[0053] More particularly, the internal space of the housing 5 can be flooded with an insulating material, in order to improve the insulation of the measuring electronics.

[0054] The plates 7 may be substantially rectangular and flat. In addition, the plates 7 may be made of the same first metallic material as the support 3. Alternatively, the plates 7 may be of a different geometry, in particular circular or other likely to be compatible with a function of the plates 7.

[0055] The plates 7 are arranged substantially parallel to each other and aligned along a stacking axis X, extending in a direction perpendicular to the plates 7 and to the support 3. The plates 7 are arranged so as to form two sets of plates 7. More specifically, the plates 7 of the same set are electrically connected to each other. In addition, according to a particular embodiment, the plates 7 of two sets are alternated along the direction of the stacking axis X. Furthermore, the plates 7 of a set of plates may be identical to each other.

[0056] Such an arrangement allows the plates 7 to form a planar capacitor, the two sets of plates 7 being electrically connected to the measuring electronics placed in the housing 3, the connections not being shown in the figures.

[0057] The spaces separating the plates 7 facing each other are intended to receive the fuel contained in the tank, the capacity measured by the plane capacitor then depending linearly on the dielectric permittivity £ of the fuel.

[0058] In the example shown in Figure 1, the capacitive sensor comprises three plates 7, including a first plate 7a, a second plate 7b and a third plate 7c.

[0059] The third plate 7c constitutes an additional plate with respect to the first plate 7a and the second plate 7b of the plurality of plates 7.

[0060] According to the example shown in Figure 1,

[0061] - the first plate 7a is arranged between the support 3 and the second plate 7b, in the direction of the stacking axis X, and

[0062] - the second plate 7b is arranged between the first plate 7a and the third plate 7c.

[0063] The first plate 7a and the third plate 7c are identical to each other, electrically connected and form a first set. The second plate 7b forms the second set.

[0064] More particularly, the plates 7 have fixing orifices 8 and through-passage openings 9, in the direction of the stacking axis X.

[0065] In the example shown, the passage openings 9 are substantially rectangular and open onto at least one lateral edge 10 of the plates 7.

[0066] Alternatively, the passage openings 9 may have rounded, circular or oblong shapes and / or extend away from the lateral edges 10 of the plates 7.

[0067] The passage openings 9 of the plates 7 of each set of plates are located opposite the fixing holes 8 of the plates 7 of the other set of plates, in the direction of the stacking axis X.

[0068] Figure 2 is a sectional view in a horizontal plane parallel to the stacking axis X of the assembled capacitive sensor 1. The assembled capacitive sensor 1 is shown schematically in Figure 3.

[0069] As can be seen in Figures 2 and 3, the capacitive sensor 1 comprises a set of first support columns 11, a set of second support columns 13, and a set of spacer columns 15. In addition, the capacitive sensor 1 comprises insulating fixing elements 17.

[0070] In the example shown, - the set of first support columns 1 1 comprises four first support columns,

[0071] - the set of second support columns 13 comprises four second support columns, and

[0072] - the set of spacer columns 15 comprises four spacer columns 15. The first support columns 11, the second support columns 13 and the spacer columns 15 may have substantially tubular cylindrical shapes.

[0073] The first support columns 11 define first central conduits 12 extending between their ends and allowing a fixing member to be engaged.

[0074] The fasteners are, for example, bolts and are not shown in Figure 2 for the sake of clarity.

[0075] The first support columns 11 mechanically connect the first plate 7a to the insulating fixing elements 17 fixed to the support 3. More particularly, the first support columns 11 are fixed to the first plate 7a by their free end, for example by bolting, through the first central conduits 12. In addition, the first support columns 11 have identical lengths Li, so that the first plate 7a extends substantially parallel to the support 3.

[0076] The second support columns 13 mechanically connect the second plate 7b to the insulating fixing elements 17 fixed to the support 3. More particularly, the second support columns 13 are fixed to the second plate 7b by their free end, for example by bolting, through the first central conduits 12.

[0077] In particular, the second support columns 13 have shapes identical to the first support columns 11 except that the length L2 of the second support columns 13 is greater than the length Li of the first support columns 11.

[0078] The second support columns 13 define second central conduits 14 for engaging the fixing members.

[0079] The second support columns 13 are positioned so as to pass through the first plate 7a through the passage openings 9 of the first plate 7a.

[0080] In addition, the second support columns 13 have identical lengths L2, so that the second plate 7b extends substantially parallel to the support 3 and to the first plate 7a.

[0081] The spacer columns 15 mechanically connect the third plate 7c to the first plate 7a. The spacer columns 15 extend in continuity with the first support columns 11, on the opposite side of the first plate 7a, and through the passage openings 9 of the second plate 7b. The spacer columns 15 define third central conduits 16 for engaging the fixing members.

[0082] The spacer columns 15 have identical lengths L3. The length L3 of the spacer columns 15 is determined from the length Li of the first support columns 11 and the length L2 of the second support columns 13 so that a distance Ei separating the first plate 7a from the second plate 7b is equal to a distance E2 separating the second plate 7b from the third plate 7c.

[0083] More particularly, the sum of the distance Ei separating the first plate 7a from the second plate 7b, of the distance E2 separating the second plate 7b from the third plate 7c and of a thickness of the second plate 7b is equal to the length L3 of the spacing columns 15.

[0084] The thickness of the second plate 7b being much less than the length L3 of the spacer columns 15, the variations in the thickness of the second plate 7b due to thermal expansion are negligible compared to those of the length L3.

[0085] The spacer columns 15 are made of a second metallic material having a second coefficient of thermal expansion o2. The second metallic material may be the same as the first metallic material or different.

[0086] The first support columns 11 are made of a third metallic material having a third coefficient of thermal expansion o3. The second support columns 13 are made of a fourth metallic material, having a fourth coefficient of thermal expansion eu.

[0087] The third metallic material may be the same as the second metallic material and / or the first metallic material, or different.

[0088] In addition, the fourth metallic material may be identical to the third metallic material. In particular, when the number of plates 7 of the capacitive sensor 1 is odd, as in the case shown in FIGS. 1 to 3, the second metallic material, the third metallic material and / or the fourth metallic material may be different and have different thermal expansion coefficients.

[0089] In particular, when the number of plates 7 is even, it is necessary for the third material of the first support columns 11 and the fourth metallic material of the second support columns 13 to be identical to the second material forming the spacer columns 15. Such a choice is in particular necessary in order to maintain variations in the distance Ei separating the first plate 7a and the second plate 7b and the variations in the distances separating the other pairs of facing plates.

[0090] Such conditions make it possible to predict in a simple and reliable manner the variations of the capacitance of the capacitive sensor 1 with temperature. The insulating fixing elements 17 are provided to mechanically fix the first support columns 11 and the second support columns 13 to the support 3 while electrically insulating them from the support 3.

[0091] The insulating fixing elements 17 are, for example, of the type known as a barrel-socket. The insulating fixing elements 17 have, for example, an elongated tubular shape defining a central passage 18 for engaging the fixing member, completed with a transverse washer 19 intended to be interposed between the first support column 11 and the support 3 and / or between the second support column 13 and the support 3.

[0092] The fixing elements 17 may be made from a hard and rigid polymer material, having a low coefficient of thermal expansion over the temperature range considered. Thus, the thermal expansions of the insulating fixing elements 17 following temperature variations are negligible compared to those of the support 3, the plates 7, the first support columns 11, the second support columns 13 and the spacer columns 15.

[0093] During temperature variations, in the range considered, the length Li of the first support columns 11, the length L2 of the second support columns 13 and the length of the spacer columns 15 vary linearly with the corresponding second thermal expansion coefficient a2, and the third thermal expansion coefficient o3.

[0094] Similarly, the surfaces of the plates 7 facing each other vary, to the first order, as twice the first coefficient of thermal expansion ai.

[0095] For a two-plate plane capacitor, the capacitance is calculated by the formula where C is the capacity,

[0096] £ is the dielectric permittivity of the medium separating the plates,

[0097] S is the surface area of ​​the plates facing each other, and

[0098] E is the gap between the plates.

[0099] For a capacitor with three or more plates, the total capacitance is the sum of the capacitances of the pairs of plates 7 opposite each other, calculated with the previous formula.

[0100] Thus, the coefficient of linear variation of the capacitance of the capacitive sensor 1 with the temperature, as a consequence of the thermal expansions of the plates 7 and the columns, is equal to where AC is the change in capacitance between the initial temperature and a measured temperature, Co is the capacitance at initial temperature,

[0101] AT is the temperature variation between the initial temperature and the measured temperature, ai is the first coefficient of thermal expansion of the first metallic material, and o2 is the second coefficient of thermal expansion of the second metallic material.

[0102] The value of the coefficient of linear variation of the capacitance makes it possible to predict drifts in the measurement of the dielectric permittivity £ of the fuel based on knowledge of the thermal expansion coefficients of the first metallic material and the second metallic material.

[0103] Advantageously, the choice of the first metallic material and the second metallic material, in order to have the second coefficient of thermal expansion o2 equal to twice the first coefficient of thermal expansion ai, makes it possible to have a substantially zero temperature drift of the capacitance of the capacitive sensor.

[0104] According to an exemplary embodiment of the capacitive sensor 1 according to the first embodiment of the invention, the first metallic material constituting the plates 7 and the support 3 is aluminum, with a first coefficient of thermal expansion ai ai = 23.9 ppm / °C (parts per million per degree Celsius) = 23.9.10 -6 °C-1 .

[0105] More particularly, in the exemplary embodiment presented, the capacitive sensor 1 comprises three plates 7.

[0106] The first support columns 11, the second support columns 13 and the spacer columns 15 may be made of stainless steel, in particular of the type designated by the acronym Inox 304.

[0107] The second metallic material and the third metallic material can then be identical and have coefficients of thermal expansion, respectively the second coefficient of thermal expansion o2 and the third coefficient of thermal expansion a3, equal: o2 = o3 = 17.3 ppm / °C (parts per million per degree Celsius) = 17.3.10 -6 °C -1 .

[0108] The polymer material of the insulating fixing elements 17 may be polyetheretherketone, also designated by the acronym “PEEK” for “polyetheretherketone” in English, in particular comprising 30% by mass of glass fibers.

[0109] The coefficient of linear variation of the capacitance of the capacitive sensor 1 was measured over the temperature range considered and three values ​​of +27 ppm / °C, +29 ppm / °C and +30 ppm / °C were obtained. Such values ​​show a good agreement with the theoretical value calculated according to the previous formula,

[0110] Figure 4 is a schematic representation of the capacitive sensor 1 according to a second embodiment of the invention.

[0111] The capacitive sensor 1 according to the second embodiment comprises five plates 7. In particular, a first plate 7a, a third plate 7c and a fifth plate 7e form a first set of plates identical to each other and electrically connected. Furthermore, a second plate 7b and a fourth plate 7d form a second set of plates identical to each other and electrically connected.

[0112] The third plate 7c, the fourth plate 7d and the fifth plate 7e constitute additional plates with respect to the first plate 7a and the second plate 7b of the plurality of plates 7.

[0113] The assembly of the support 3, the first plate 7a, the second plate 7b and the third plate 7c by means of the first support columns 11, the second support columns 13, the spacer columns 15 and the insulating fixing elements 17 is identical to that of the first embodiment described previously.

[0114] The fourth plate 7d is fixed and electrically connected to the second plate 7b by a set of spacer columns 15 passing through the passage openings 9 of the third plate 7c.

[0115] The fifth plate 7e is fixed and electrically connected to the third plate 7c by a set of spacer columns 15 passing through the passage openings 9 of the fourth plate 7d.

[0116] The five plates 7 of the capacitive sensor 1 according to the second embodiment form between them four air gaps having respective distances

[0117] Ei, separating the first plate 7a from the second plate 7b,

[0118] E2, separating the second plate 7b from the third plate 7c,

[0119] E3, separating the third plate 7c from the fourth plate 7d, and

[0120] E4 separating the fourth plate 7d from the fifth plate 7e, equal, receiving the fuel contained in the tank.

[0121] Such an arrangement makes it possible, for surfaces of the metal plates 7 and a length L3 of the spacer columns 15 that are identical, to have a total capacitance twice that of the capacitive sensor 1 according to the first embodiment, thus improving the accuracy of the fuel dielectric permittivity measurements. According to another exemplary embodiment of the capacitive sensor 1 according to the second embodiment, the first metallic material constituting the metal plates 7 and the support 3 is titanium, with a first coefficient of thermal expansion eu ai = 8.64 ppm / °C (parts per million per degree Celsius) = 8.64.10 -6 °C -1 The coefficient of linear variation of the capacitance of the capacitive sensor 1 was measured over the temperature range considered. Several series of measurements give an average value of the coefficient of linear variation of the capacitance which is substantially zero.

[0122] Such values ​​show a good agreement with the theoretical value calculated according to the previous formula, of 0.02 ppm / °C

Claims

CLAIMS 1. Sensor (1), in particular a capacitive sensor (1), capable of enabling a measurement of a dielectric permittivity (s) of a medium, in particular a fuel contained in an aircraft tank, comprising: - a support (3), - a plurality of insulating fixing elements (17) fixed to the support (3), - a plurality of plates (7) comprising at least a first plate (7a) and a second plate (7b) extending substantially parallel to the support (3) and aligned along a stacking axis (X) perpendicular to the support (3), - at least one first support column (11) having a first length (Li) and connecting the first plate (7a) to one of the insulating fixing elements (17), - at least one second support column (13) having a second length (L2) greater than the first length (Li) and connecting the second plate (7b) to one of the insulating fixing elements (17), - at least one additional plate (7c, 7d, 7e), and - at least one spacer column (15) having a third length (L3) and connecting the additional plate (7c, 7d, 7e) to a plate of the plurality of plates (7) which is the second closest neighbor of the additional plate (7c, 7d, 7e) in the direction of the support (3) along the stacking axis (X), characterized in that the support (3) and the plates (7) are made of the same first metallic material, and in that the spacer column (15) is made of a second metallic material.

2. Capacitive sensor (1) according to the preceding claim, in which a second coefficient of thermal expansion (a2) of the second metallic material is substantially equal to twice a first coefficient of thermal expansion (eu) of the first metallic material.

3. Sensor (1) according to any one of the preceding claims, wherein the insulating fixing elements (17) are made of a polymer material, in particular polyetheretherketone, in particular comprising glass fibers, specifically at least 25% glass fibers.

4. Sensor (1) according to any one of the preceding claims, wherein the first support column (11) is made of a third metallic material and / or the second support column (13) is made of a fourth metallic material.

5. Sensor (1) according to the preceding claim, wherein the plurality of plates (7) comprises an even number of plates (7).

6. Sensor (1) according to the preceding claim, in which the second metallic material and the third metallic material, and in particular the fourth metallic material, are identical.

7. Sensor (1) according to any one of claims 1 to 4, wherein the plurality of plates (7) comprises an odd number of plates (7), in particular three or five plates (7).

8. Sensor (1) according to any one of the preceding claims, in which it comprises the same number of first support columns (11), second support columns (13) and spacer columns (15).

9. Aircraft fuel tank comprising a sensor (1) according to one of the preceding claims.

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