DE102008064019A1 - Capacitive filling level sensor for detecting filling level of e.g. petrol, in container in motor vehicle, has evaluation circuit determining filling level of fluid, and inductor-capacitor element formed from capacitor and inductance - Google Patents

Abstract

The sensor has electrodes lying opposite to each other and extending over filling height of a container. Capacitance of a measuring capacitor is changed depending on a filling level of a fluid in the container. An evaluation circuit (26) determines the filling level of the fluid in the container from the capacitance of the measuring capacitor. The evaluation circuit has an inductor-capacitor element formed from the measuring capacitor and an inductance. The electrodes are electrically insulated from each other. The evaluation circuit is arranged on hybrid ceramic.

Description

The The invention relates to a filling level sensor for detecting a level of a liquid in a container, in particular a capacitive level sensor according to the Preamble of claim 1.

Capacitive level sensors are known in various embodiments. It is in this context by way of example to the publications DE 29 41 652 C2 . DE 42 04 212 A1 . DE 197 54 093 C2 . EP 0 152 644 A2 . EP 1 528 375 A1 . US 5,802,728 and WO 99/10714 A1 directed. All these level sensors have in common that they have at least two extending over the Füllhöhenbereich the liquid container electrodes forming a measuring capacitor, and that an evaluation circuit determines the level of the liquid in the container in dependence on the capacitance of this measuring capacitor. It is exploited that the level-dependent wetting of the electrodes with the liquid, which has a certain dielectric constant, changes the capacitance of the measuring capacitor.

Out some of the above pamphlets are as well known, at least one of the two opposite each other to divide lying electrodes and so two measuring capacitors form. The division is preferably carried out such that the width of the sub-electrodes over the Füllhöhenbereich changed the liquid in the container. By a suitable combination of the capacity values the two measuring capacitors thus formed can be the Liquid level independent determine their dielectric constant.

in the Area of automotive technology can such level sensors For example, be used in fuel tanks. Problematic There are possible corrosive to the in the fuel Immersive electrodes may arise over time. The contact resistance caused by such corrosion can negatively affect the determination of the level of the fuel by means of a capacitive level sensor impact.

task It is therefore an improved level sensor of the present invention to create the type described above, in a simple manner maintains its functionality in the long term.

These Task is by a level sensor for detecting a level of a liquid in a container solved with the features of claim 1. advantageous Embodiments and developments of the invention are in the dependent Claims specified.

Of the Level sensor for detecting a level contains a liquid in a container a first electrode and at least one second electrode extending lying opposite each other over a Füllhöhenbereich extend the container, wherein the capacity one formed from the opposite electrodes Measuring capacitor depending on the level the liquid in the container changes, as well an evaluation circuit consisting of the capacitance of the measuring capacitor the level of the liquid in the container certainly. The level sensor of the invention is characterized in that the evaluation circuit is one of the measuring capacitor and an inductance formed LC element.

Of the Inventive level sensor has an evaluation circuit, one from the measuring capacitor and a Having inductance formed LC element. Such LC element in the evaluation circuit is compared to those in the Evaluation circuits of conventional capacitive level sensors used RC members much less sensitive to ohmic resistances. Contact resistance, which, over time, attach to the electrodes in the respective fluid can thus have in determining the level none or at most with this capacitive level sensor low impact. The level sensor retains thereby its functionality in the long term, without that Electrodes consuming and expensive compared to them preserved or opposite contacting liquid this would have to be isolated.

Of the Level sensor of the invention is therefore particularly in Motor vehicle area can be used advantageously. For example can reduce the levels of fuels (Gasoline, diesel, etc.), lubricants (lubricating oil, etc.) and auxiliaries (coolants, reducing agents, etc.) in the respective containers reliable and easier Be determined manner.

In addition, of course, the level sensor according to the invention also offers the advantages of conventional capacitive level sensors, such as the absence of moving components, the usability for different liquids, the usability for heterogeneous liquids, integration into complex container shapes, level measurement in different container locations, continuous filling level measurement and the like.

In A preferred embodiment of the invention can be two second electrodes may be provided, each over extend the filling height region of the container and electrically isolated from each other to face with the one lying first electrode to form two measuring capacitors, wherein the width of at least one of the two second electrodes over changed the filling height range of the container. The evaluation circuit can then have two LC elements, each from one of the two measuring capacitors and an inductance are formed.

By Use of two such measuring capacitors leaves the level of the liquid in the container regardless of some physical quantities (in particular the dielectric constant of the liquid, temperature, etc.) by passing over the two measuring capacitors certain capacity values in a suitable way be linked.

• a) Ermitteln des Füllstandes der Flüssigkeit im Behälter aus einer Verknüpfung der Kapazitäten der beiden Messkondensatoren als einen ersten Messwert;
• b) Bestimmen einer Dielektrizitätszahl der Flüssigkeit im Behälter aus einer Messfrequenz der Auswerteschaltung und dem als ersten Messwert ermittelten Füllstand; und
• c) Ermitteln des Füllstandes der Flüssigkeit im Behälter aus der Dielektrizitätszahl der Flüssigkeit und der Kapazität des Messkondensators bei zusammengeschalteten zweiten Elektroden als einen zweiten Messwert und Vergleichen dieses zweiten Messwerts mit dem ersten Messwert. Vorzugsweise kann die die Auswerteschaltung weiter so ausgestaltet sein, dass sie den folgenden Verfahrensschritt durchführen kann:
• d) Korrigieren der in Schritt b) bestimmten Dielektrizitätszahl der Flüssigkeit im Behälter auf Basis eines oder mehrerer Vergleiche gemäß Schritt c).

In the case of such a fill level sensor, the two second electrodes can preferably also be electrically interconnectable, and the evaluation circuit can be configured such that it can carry out the following method steps:

• a) determining the level of the liquid in the container from a combination of the capacitances of the two measuring capacitors as a first measured value;
• b) determining a dielectric constant of the liquid in the container from a measuring frequency of the evaluation circuit and the level determined as the first measured value; and
• c) determining the fill level of the liquid in the container from the dielectric constant of the liquid and the capacitance of the measuring capacitor with interconnected second electrodes as a second measured value and comparing this second measured value with the first measured value. Preferably, the evaluation circuit can be further configured so that it can perform the following method step:
• d) correcting the determined in step b) the dielectric constant of the liquid in the container based on one or more comparisons according to step c).

In In another embodiment of the invention, a measurement frequency the evaluation circuit to the dielectric constant of the liquid be adjusted in the container.

In Another embodiment of the invention, the evaluation circuit essentially between the first and second electrodes of the measuring capacitor or the measuring capacitors in the end region of the electrodes at the top Be arranged end of the Füllhöhenbereichs. Preferably the evaluation circuit can in this case on a hybrid ceramic be arranged and encapsulated in a metal housing.

at This embodiment is generally only small Capacitance changes of the measuring capacitor or the measuring capacitors by the arrangement of the evaluation circuit not directly on the electrodes of the level sensor falsified by the otherwise required, longer connecting lines. In addition, offers in this embodiment the possibility of without an additional temperature sensor also the temperature of the liquid in the tank to measure with.

Above as well as other features, advantages and applications will become apparent from the following description of a preferred embodiment of the invention in conjunction with the accompanying drawings better understandable. Show:

1 a schematic representation of a liquid container with a level sensor of the invention;

2 a schematic side sectional view of a level sensor according to a preferred embodiment of the invention;

3 a schematic plan view of the level sensor of 2 without counterelectrode; and

4 a capacity level diagram for explaining the operation of the level sensor of 2 and 3 ,

1 schematically shows a capacitive level sensor 10 in a container 12 for receiving a liquid 14 such as is arranged in a fuel tank of a motor vehicle. The filling level sensor 10 extends substantially over the entire Füllhöhenbereich of the container 12 to the level 16 the liquid 14 in the container 12 capture. He is also connected via a connection cable 18 connected to other devices, such as controls, displays and the like, to this the determined level of the liquid 14 for further processing, display or the like.

As in 2 and 3 illustrated includes the capacitive level sensor 10 a sheath 24 , which also serves as the first electrode or counter electrode. For this purpose, the sheath is made, for example, of metal or of a metal vapor-deposited plastic.

Inside this sheath 24 is a carrier 20 , for example, made of an epoxy resin, arranged on the two substantially triangular plates 22a . 22b are applied, which serve as second electrodes. These plates 22a . 22b are made of aluminum for this purpose, for example.

Shape and size of vehicle 20 , first electrode 24 and second electrodes 22a . 22b are not on the embodiment of 2 and 3 limited. For the operation of the level sensor 10 but it is advantageous if the counter electrode 24 and the first electrodes 22a . 22b are adapted to each other in size and shape so that they are substantially over the entire Füllhöhenbereich of the container 12 lie opposite each other in a substantially constant distance and form in this way a measuring capacitor or two measuring capacitors. Typical basic forms in this context are, for example, plate and cylinder capacitors, without the present invention being intended to be limited thereto.

The evaluation circuit 26 is preferably in an end region (left in 2 and 3 ) of the level sensor 10 , which is in the area of the upper end of the level 16 the liquid 14 lies directly on the second electrodes 22a . 22b and thus between the electrodes 22a . 22b and 24 mounted and over pins 30 with the second electrodes 22a . 22b connected. The jacket 24 is typically connected to ground. The evaluation circuit 26 is for example arranged on a hybrid ceramic and in a metallic housing 28 (eg circulating soldered sheet metal lid) encapsulated. The hybrid ceramic changes the capacitance value of the measuring capacitor only very slightly.

In this arrangement, the evaluation circuit 26 are no connecting lines between it and the one from the electrodes 22a . 22b . 24 formed measuring capacitor necessary so that even small changes in capacitance of this measuring capacitor are not distorted. In addition, this arrangement allows, without the use of an additional temperature sensor, the temperature of the liquid 14 in the container 12 to measure with.

The evaluation circuit 26 has two LC-links, consisting of the one measuring capacitor, formed from an aluminum plate 22a and the sheath 24 , and an inductance or from the other measuring capacitor, formed from the other aluminum plate 22b and the sheath 24 , and an inductance are formed. The LC elements can be constructed either as a parallel circuit or as a series circuit of the respective measuring capacitor and the respective inductance. In addition, the two LC elements may optionally include two separate inductors, one common inductor, or two inductors coupled together.

The two aluminum plates 22a . 22b on the carrier 20 are electrically isolated from each other, so that together with the sheath opposite this 24 two separate measuring capacitors can be formed. The evaluation circuit 26 but is preferably designed so that these two second electrodes 22a . 22b can be interconnected and created in this way a single measuring capacitor.

As in 3 shown, the width of the two second electrodes changes in each case 22a . 22b over the filling height area of the container 12 , The left end corresponds to the electrodes 22a . 22b in 3 the upper end portion of the Füllhöhenbereichs and the right end of the electrodes 22a . 22b in 3 the lower end of the Füllhöhenbereich. Preferably, the two second electrodes 22a . 22b formed as two triangles, which match each other in opposite directions to each other on the support 20 are arranged as in 3 illustrated.

Be the electrodes 22a . 22b and 24 according to the level 16 the liquid 14 in the container 12 with the liquid 14 wetted, the capacitances C ₁ and C _{2 of} the two measuring capacitors change due to the liquid 14 (Dielectric) between the first and second electrodes 22a . 22b . 24 , there C ₁ denotes the capacity of the one second electrode 22a and the sheath 24 formed measuring capacitor and C _2, the capacity of the other second electrode 22b and the sheath 24 formed other measuring capacitor.

Is the level sensor 10 not with liquid 14 wetted, the two capacitances C ₁ and C _{2 are the} same size. Will now be the level sensor 10 ie, its electrodes become 22a . 22b and 24 from below (right in 2 and 3 ) upwards (left in 2 and 3 ) wets, so the capacitance C _{2 of} the measuring capacitor increases with the second electrode wider in the lower region 22b due to the relative permittivity of the liquid 14 , which is generally significantly greater than 1, faster than the capacitance C _{1 of} the other measuring capacitor, as in the diagram of 4 exemplified. When the maximum level 16 is reached, ie when the electrodes of the level sensor 10 completely with the liquid 14 are wetted, the two capacitances C ₁ and C _{2 are the} same size again.

Since the LC elements used instead of an RC element of the evaluation circuit 26 Insensitive to ohmic resistances, these capacitance values C ₁ and C _{2 are} replaced by contact resistances, which may be due to corrosion of the electrodes 22a . 22b . 24 in the liquid 14 arise, not influenced. Because of this, the electrodes must be 22a . 22b . 24 at the level sensor 10 the invention is not against the liquid 14 be isolated or preserved in a special way.

oder aus der Formel (2) zur Berechnung der Kapazität C eines Zylinderkondensators mit dem Radius R₁ der inneren Elektrode, dem Radius R₂ der äußeren Elektrode und der Zylinderlänge l:

jeweils mit ε₀ als der Dielektrizitätskonstanten des Vakuums. Bei beiden Formeln (1) und (2) würden sich die Dielektrizitätszahlen ε der Flüssigkeit bei der Berechnung des Verhältnisses von C₁/C₂ herauskürzen. Mit anderen Worten wirken sich Änderungen der Dielektrizitätszahlen der Flüssigkeit 14 auf beide Flächen 22a und 22b in gleicher Weise aus. Diese Überlegung gilt in analoger Weise auch für andere Kondensatorformen. Neben der Bildung des Verhältnisses C₁/C₂ sind auch andere Verknüpfungen der beiden Kapazitätswerte denkbar, bei denen die Dielektrizitätszahl ε der Flüssigkeit 14 im Behälter 12 herausfällt.The evaluation circuit 26 can now from the ratio of the two determined capacitances C ₁ and C ₂ calculate the level of the liquid as a first measured value h ₁ . This first measured value h ₁ is independent of the permettivity or dielectric constant ε of the liquid 14 in the container as it does not change the ratio of the two capacitance values. This can be seen, for example, from the formula (1) for calculating the capacitance C of a plate capacitor having a plate area A and a plate spacing d:

or from the formula (2) for calculating the capacitance C of a cylindrical capacitor having the radius R _{1 of} the inner electrode, the radius R _{2 of} the outer electrode and the cylinder length l:

each with ε ₀ as the dielectric constant of the vacuum. In both formulas (1) and (2), the dielectric numbers ε of the liquid would be abbreviated when calculating the ratio of C ₁ / C ₂ . In other words, changes in the dielectric constants of the liquid affect 14 on both surfaces 22a and 22b in the same way. This consideration applies analogously to other capacitor forms. In addition to the formation of the ratio C ₁ / C ₂ , other combinations of the two capacitance values are conceivable in which the dielectric constant ε of the liquid 14 in the container 12 fall out.

A change in the dielectric and thus its relative permittivity ε, for example due to a changed mixing ratio of the fuel used (ethanol, methanol, benzene, methyl ester, gasoline, diesel fuel, etc.) influences the determination of the filling level 16 with the capacitive level sensor 10 the invention thus not.

Has the evaluation circuit 26 determines the ratio C ₁ / C ₂ and from this the level 16 the liquid 14 As calculated as the first measured value h ₁ , it can now use the measuring frequency f to determine the dielectric constant ε of the liquid 14 determine. From the Thomson equation of vibration ( 3 ) for electrical resonant circuits, it can be seen that this is basically possible for an LC element even with more complex circuit structures:

Is in this way the dielectric constant ε of the liquid 14 in the container 12 known, the evaluation circuit switches 26 the two second electrodes 22a and 22b together so that these along with the sheath 24 form a common measuring capacitor. This reference measurement provides with the help of, for example, formula (1) or (2) a capacitance value C, from which a second measured value h ₂ for the level 16 the liquid 14 in the container 12 can derive.

The evaluation circuit 26 can now compare the two measured values h ₁ and h ₂ with each other and thus the level determined by it 16 check. In this case, the evaluation circuit 26 be adjusted so that on the basis of the corrections of he mediated dielectric constant ε of the liquid 14 the same measured values h ₁ and h ₂ for the filling level 16 with separate and interconnected second electrodes 22a . 22b result. In particular, the measuring frequency f of the evaluation circuit 26 based on the wetting area of the electrodes 22a . 22b adapted to the determined dielectric.

Even if the liquid 14 in the container 12 is inhomogeneous (eg, water in the lower bottom area of a fuel tank), this distorts the above-described determination of the level 16 hardly, since always the entire measuring range is used as reference range.

The capacitive level sensor 10 The invention can be used in a particularly advantageous manner in the field of automotive engineering, since the liquids used 14 (Fuel, lubricants, aids) typically cause corrosion of the coming into contact with them electrodes and thus the emergence of appropriate contact resistance.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• DE 2941652 C2 [0002]
• - DE 4204212 A1 [0002]
• DE 19754093 C2 [0002]
• EP 0152644 A2 [0002]
• EP 1528375 A1 [0002]
• US 5802728 [0002]
• WO 99/10714 A1 [0002]

Claims (7)

Level sensor ( 10 ) for detecting a fill level ( 16 ) of a liquid ( 14 ) in a container ( 12 ), with a first electrode ( 24 ) and at least one second electrode ( 22a . 22b ) lying opposite one another over a filling height region of the container ( 12 ), wherein the capacitance of one of the opposing electrodes ( 22a . 22b ; 24 ) measured capacitor depending on the level ( 16 ) of the liquid ( 14 ) in the container ( 12 ) changes; and an evaluation circuit ( 26 ), from the capacity of the measuring capacitor the level ( 16 ) of the liquid ( 14 ) in the container ( 12 ), characterized in that the evaluation circuit ( 26 ) comprises an LC element formed from the measuring capacitor and an inductance. Level sensor according to claim 1, characterized in that two second electrodes ( 22a . 22b ) are provided, each extending over the Füllhöhenbereich of the container ( 12 ) and are electrically isolated from each other in order to connect with the one opposite first electrode ( 24 ) to form two measuring capacitors, wherein the width of at least one of the two second electrodes ( 22a . 22b ) over the filling height area of the container ( 12 ) changed; and the evaluation circuit ( 26 ) has two LC-members, each formed from one of the two measuring capacitors and an inductor. Level sensor according to claim 2, characterized in that the two second electrodes ( 22a . 22b ) are electrically interconnectable; and the evaluation circuit ( 26 ), a) the level ( 16 ) of the liquid ( 14 ) in the container ( 12 ) from a combination of the capacitances (C ₁ , C ₂ ) of the two measuring capacitors as a first measured value (h ₁ ), b) from a measuring frequency (f) and the level determined as the first measured value (h ₁ ) ( 16 ) a dielectric constant (ε) of the liquid ( 14 ) in the container ( 12 ) and c) the level ( 16 ) of the liquid ( 14 ) in the container ( 12 ) from the relative permittivity (ε) of the liquid ( 14 ) and the capacitance (C) of the measuring capacitor with interconnected second electrodes ( 22a . 22b ) as a second measured value (h ₂ ) and to compare with the first measured value (h ₁ ). Level sensor according to claim 3, characterized in that the evaluation circuit ( 26 ), d) the dielectric constant (ε) of the liquid determined in step b) ( 14 ) in the container ( 12 ) based on one or more comparisons according to step c). Level sensor according to one of the preceding claims, characterized in that a measuring frequency (f) of the evaluation circuit ( 26 ) to the relative permittivity (ε) of the liquid ( 14 ) in the container ( 12 ) is adjusted. Level sensor according to one of the preceding claims, characterized in that the evaluation circuit ( 26 ) substantially between the first and second electrodes ( 22a . 22b ; 24 ) of the measuring capacitor or of the measuring capacitors in the end region of the electrodes is arranged at the upper end of the filling height region. Level sensor according to claim 6, characterized in that the evaluation circuit ( 26 ) arranged on a hybrid ceramic and in a metal housing ( 28 ) is encapsulated.