DE102007001175A1 - Method and device for level measurement - Google Patents

Abstract

The present invention relates to a method and a device for measuring the level of aggressive media in a tank, in particular a urea solution for exhaust aftertreatment in an internal combustion engine. In order to provide an inexpensive and reliable method for measuring the level of aggressive media and a corresponding device, it is proposed that the device (1) comprises a device for capacitive level measurement, for measuring a liquid level (h) by a wall (6) of a respective Tanks (2) formed through and connected to an evaluation circuit (12).

Description

The The present invention relates to a method and an apparatus for level measurement aggressive media in a tank, especially a urea solution for Exhaust gas aftertreatment in an internal combustion engine.

Modern internal combustion engines in motor vehicles must increasingly meet stricter emission standards. A particularly efficient method of exhaust gas purification for diesel vehicles is exhaust gas aftertreatment by means of selective catalytic reduction, abbreviated to SCR. In this process, the harmful NO _x components of the exhaust gas in an SCR catalyst are almost completely converted into atmospheric nitrogen N ₂ and water vapor H ₂ O by chemical reduction. As reducing agent, a urea or urea solution is injected into the hot combustion exhaust gas upstream of the SCR catalytic converter. Due to the heat in the exhaust system, urea or CH ₄ N ₂ O decomposes and releases ammonia or NH ₃ . Then a reaction takes place according to the following equation: NO _x (eg, NO NO ₂ , N ₂ O, etc.) + NH ₃ => N ₂ + H ₂ O.

Without More fuel consumption is thus the reduction of pollutant emissions ammonia required especially in diesel-powered commercial vehicles provided.

Under the term "aggressive Media "are in the Within the scope of the present invention, fluid substances having corrosive properties Understood. A urea solution is therefore considered only as an example of such a fluid, stored in a container and dosed a running process in a certain way supplied becomes. These are z. B. a 32.5% aqueous Urea solution, uniformly designated by the industry as Adblue, whose. composition regulated in DIN 70070. Another example of one such adjuvant is known under the name deoxium become mixture of urea and formic acid.

Out The prior art is for measuring the liquid level in tanks various measuring methods known. For liquids that do not freeze, become common Float used, which are attached to a lever or cable and whose position is read electromechanically. To do this Grinding potentiometer or contact switch used. One at the Lever mounted magnet allows the use of reed contact switches, Tongue switches, such. Eg at the Siemens VDO MAAPS fuel gauge, as well as HALL or MR / GMR magnetic sensors.

Farther is a mounting of two or more metallic contact plates at a certain height known in the tank. Is the level an electrically conductive liquid above the contact plates, forms the liquid to be sensed an electrically conductive connection that is measured. These contacts are used to detect a minimum level, for each level is its own Provide contact.

If but a partially filled with a urea solution tank in a motor vehicle freezes and partially thaws again, forms on a float system or other internals a lump of ice, the float or other Built-in z. B. breaks at the drive-induced vibration or damage. by virtue of the volume increase during freezing also occur on very stable, floatless installations always damage on when these cavities have, since filled cavities freeze up.

It Object of the present invention, a cheap and reliable working Method for level measurement aggressive media and to create a corresponding device.

These The object is solved by the features of the independent claims. advantageous Further developments are the subject of the respective subclaims.

For stationary systems and ships have been systems for exhaust aftertreatment for a long time In the meantime trucks are increasingly being equipped and retrofitted. there becomes a urea solution as a liquid carried in a separate tank and by metering the exhaust gas of the respective internal combustion engine fed. For correct operation of the SCR dosing system is an accurate level indicator of the storage tank desirable. starting point The present invention recognizes that the urea solutions used are very aggressive but also i.d.R. already freeze at about -10 ° C. Without special Heating devices can Vehicles, however, cool in parts below -40 ° C. But Even in these temperature ranges have internal combustion engines or motor vehicles in compliance with particular emission standards functional stay and be ready.

According to the invention, a method for measuring the level of aggressive media in a tank is characterized in that a fill level or level measurement is performed capacitively, through which a respective liquid level is measured through a tank wall. Accordingly, a device according to the invention has a capacitive level measurement, which is used to measure a liquid level through a tank wall of a respective tank Tanks is formed through. It is therefore dispensed with separate internals in a tank. Preferably, an inventive measuring method with respect to the aggressive fluid is even designed as non-contact measurement of a current fluid level. For this purpose, the invention utilizes the property of aggressive media that they are well electrically conductive.

In an embodiment The invention are on or in the tank wall in a mounting position two or more conductive Strips mounted in a vertical direction, with electronics for measuring the capacity are connected between the strips. If the liquid level is low, so is also the capacity the entire arrangement low. At high liquid levels form the Strip two capacitors in series. In this embodiment the tank wall forms the dielectric, the strips in each case the one Electrode that forms urea fluid the other electrode.

In another embodiment According to the invention, the strips form one electrode of two parallel switched capacitor assemblies, of which in an installed position lower capacitor as a dielectric which has two tank walls, the other capacitor, however, by the absence of the well-conducting Urea-liquid next to the two tank walls also has air as a dielectric. A different high level exercises with it a relevant one Influence on the capacity the overall arrangement.

In an alternative embodiment The invention is the urea liquid by a mounted in the tank electrically conductive passage z. B. with the vehicle ground or a point in a measurement circuit connected. In this case receives a capacitor, wherein the tank wall is the dielectric and the electrodes through metal strips outside the tank and the electrical conductive liquid be formed in the tank. The fluid level defined by the conductivity the fluid opposite a tank wall an electrode surface variable size, through the one capacity the arrangement is determined.

advantageously, incorrect measurements are made by inclination in a training Capacitive measurement by means of mounted on the tank opposite and Strip avoided. By connecting the strip electrically a kind of averaging of the levels is effected.

For the measurement very small capacity differences Various technical solutions are known. For this purpose, there are proven circuits, eg. B. as a vibrator in humidity sensors or on a microcontroller basis, such as. Eg Siemens VDO door handle sensor. The market also offers special ASICS for capacity measurement differences in the lower pF range, such as the ICs QT117 or QT300 of the company Quantum, can be found on the Internet www.qprox.com.

According to the invention is a non-contact measurement realized whose device requires no mechanical parts, which Such a device is in principle subject to no wear. It Furthermore, there are no tank internals, so no risk of breakage thawing after icing. As in the proposed Measuring principle no cavities are provided in Ureatank, there no danger of their destruction by freezing the tank contents. A measurement with two opposite Strip guarantees a precise level measurement even in the case of a crooked tank.

Further Features and advantages of the invention will be described below of exemplary embodiments with reference to the figures of the drawing. In the Drawing show in schematized form:

1 a three-dimensional view of a urea tank with two conductive strips for capacitive level measurement through a tank wall therethrough;

2a and 2 B :
Cuts in the levels AA, BB of 1 ;

3 a section through a urea tank with an electrically conductive strip for capacitive level measurement through a tank wall, wherein a counter electrode is formed by the urea liquid itself by being contacted in an electrically conductive manner;

4 a modification of the embodiment according to 1 in which two conductive strip for capacitive level measurement are mounted through a tank wall on opposite sides of the tank in order to achieve as accurate as possible even with an inclined tank and

5 a modification of the embodiment according to 4 in which an outer electrode is divided into horizontally to a free fluid surface extending parallel stripes.

About the various embodiments and figures will hereafter be given the same reference numerals and names for same function or assemblies used.

The picture of 1 shows a device 1 for capacitive level measurement in one three-dimensional view. At a urea tank 2 are two conductive stripes 3 . 4 in an outdoor area 5 on a tank wall 6 arranged. With this an electric field penetrates 7 the tank wall 6 and also a tank interior 8th at least partially. So is through the tank wall 6 Through a level h measured by a capacitance change, as a urea solution 9 forms a counter electrode with a variable depending on their level h surface. Depending on the level h, a more or less large area of the tank wall 6 via a relative permittivity ε _rel > 1 of its material in the role as the dielectric a contribution to a current capacity of the measuring arrangement.

To this embodiment shows 2a as a section in the plane AA of 1 the concrete structure in the area of the two conductive strips 3 . 4 hinting at the course of some field lines 10 of the electric field 7 , Accordingly, the conductive strips 3 . 4 outside on the tank wall 6 applied and with a protective cover 11 Mistake. The field lines 10 run between the conductive strips 3 . 4 depending on a current level h partially by air, see 2a , but sometimes also by the urea solution 9 , please refer 2 B with the representation of a section in the plane BB of 1 , Thus, the capacitance of the overall arrangement results from a parallel connection of two capacitors in a multi-conductor system. The capacity of the overall arrangement is achieved by connecting an evaluation circuit or logic 12 via lines 13 to the conductive strips 3 . 4 determined and forwarded accordingly or displayed as a warning.

As in 1 indicated, but can also be an electrical contact 14 be provided by the urea solution 9 itself to an electrically connected electrode in a measurement order for determining capacity. The contact 14 takes place according to this embodiment 1 in the range of only indicated metering pump. The measurement is then carried out with two interconnected electrode strips 3 . 4 and the urea liquid 9 as a counter electrode, but no further conductive contacting of the liquid is necessary. This principle is in 3 reproduced in a sectional view. Here, the current level h is the size of the quasi-electrode 9 opposite the conductive strips 3 . 4 constant height H as the counter electrode, wherein the tank wall 6 with ε _rel <1 forms the dielectric.

The picture of 4 now shows a modification of the first, too 1 described embodiment, in the outside of the urea tank 2 or in the tank wall 6 electrically conductive strips are mounted as an electrode for capacitive measurement of the level. To 4 are advantageously again both electrodes 3 . 4 outside the tank 2 or the interior 8th arranged, but now opposite each other. This has a tilting of the tank 2 no great influence on a measurement more. In the present case, a parallel connection of two capacitors is evaluated. The capacitors have a quasi-layered dielectric, as with a capacitor between the tank walls 6 with ε _ges = ε _o · ε _rel and air with ε _ges = ε _o · 1, but in the second capacitor arrangement with electrically conductive urea solution only with the tank walls 6 , So only with ε _ges = ε _o · ε _{rel is} arranged.

Of course, this measuring arrangement can also be used to measure with two opposing strips 3 . 4 perform, with the strips 3 . 4 are connected or their signal in the evaluation circuit 12 be added. Again, accurate level measurement or level measurement is possible even when the tank is tilted 2 allows. Thus, without any internals in the tank interior 8th Fill levels h are determined exactly.

5 shows a modification of the embodiment according to 4 in which an outer electrode 3 in stripes 3a . 3b , ... 3i is divided. These stripes 3a . 3b , ... 3i run parallel to a free fluid surface. Further, the stripes 3a . 3b , ... 3i electrically connected, resulting in mutually parallel partial capacitors. In order for partial height electrodes have been mounted at different heights, which now a level h can be detected in discrete stages.

1

contraption

2

Urea tank

3

conductive strip 3a . 3b , ... 3i partial strip

4

conductive strip

5

outer space

6

tank wall

7

electrical field

8th

Tank interior

9

Urea solution

10

cover the conductive one strip

11

protective cover

12

evaluation

13

cables

H

Level / level

H

Height of a conductive strip 3 . 4

ε _rel

relative permittivity

ε _o

physical constant

Claims (9)

Device for measuring the level of aggressive media in a tank, in particular a urea solution for exhaust aftertreatment in an internal combustion engine, thereby characterized shows that it has a device for capacitive level measurement, which is used to measure a liquid level (h) through a wall ( 6 ) of a respective tank ( 2 ) formed through and with an evaluation circuit ( 12 ) connected is. Device according to claim 1, characterized in that on or in the tank wall ( 6 ) in an installed position two or more conductive strips ( 3 . 4 ) are mounted in the vertical direction, with an electronics ( 12 ) for measuring the capacitance between the strips ( 3 . 4 ) are connected. Device according to one of the preceding claims, characterized in that the urea liquid ( 9 ) by one in the tank ( 2 ), in particular in the region of a pump, with the vehicle mass or a point in a measuring circuit is connected, wherein the tank wall ( 6 ) represents the dielectric and the electrodes by metal strips ( 3 . 4 ) outside the tank ( 2 ) on the one hand and the liquid ( 9 ) on the other hand. Device according to one of the preceding claims, characterized in that the electrodes or electrically conductive strips ( 3 . 4 ) on the tank ( 2 ) are mounted opposite each other. Device according to one of the preceding claims, characterized in that an electrode ( 3 ) in the form of strips ( 3a . 3b , ... 3i ), which run parallel to a free fluid surface, so that on a wall ( 6 ) of the tank ( 2 ) result in electrically parallel to each other partial capacitors. Method for measuring the level of aggressive media in a tank, in particular a urea solution for exhaust aftertreatment in an internal combustion engine, characterized in that a level or level measurement is performed without contact by a respective liquid level (h) by a tank wall ( 6 ) is measured indirectly in the form of an evaluated capacitance change. Method according to the preceding claim, characterized in that a capacitance change is evaluated on the basis of a level-related change in the relative dielectric constant (ε _rel ). Method according to one of the two preceding claims, characterized in that a capacitance change is evaluated on the basis of a level-related change in the size of a capacitor plate by at least one electrode strip ( 3 . 4 ) on or in the tank wall ( 6 ) as an electrode and the urea liquid ( 9 ) itself can be used as a counter electrode for determining a capacitance. Method according to one of the three preceding claims, characterized in that on a wall ( 6 ) of the tank ( 2 ) electrically parallel to each other partial capacitors in the form of parallel to egg ner free fluid surface extending strips ( 3a . 3b , ... 3i ) can be used to detect a level (h) in discrete steps.