WO2024132411A1

WO2024132411A1 - Apparatus and method for capacitive fill-level measurement

Info

Publication number: WO2024132411A1
Application number: PCT/EP2023/083408
Authority: WO
Inventors: Stefan Klehr
Original assignee: Siemens Aktiengesellschaft
Priority date: 2022-12-22
Filing date: 2023-11-28
Publication date: 2024-06-27

Abstract

The invention relates to an apparatus (3) for capacitive fill-level measurement on an outer wall of a fluid container (2) filled with at least one first fluid (14), comprising: - at least a first capacitor (7a) and a second capacitor (7b), which are arranged parallel to one another along a longitudinal direction (L) of the capacitors (7a, 7b) and which can be mounted on the outer wall of the fluid container (2), - a sensing device (13), which is designed to apply a voltage to the capacitors (7a, 7b), with temporal staggering, and to sense an electric charge of each of the capacitors (7a, 7b) in order to metrologically determine a capacitance of each of the capacitors (7a, 7b), wherein the sensing device (13) is also designed to compare the metrologically sensed capacitances of the capacitors (7a, 7b) with one another and to determine a fill level (F) of the fluid container (2) on the basis of this comparison, taking into account a positioning specification with respect to the fluid container.

Description

Device and method for capacitive level measurement

The invention relates to a device for capacitive level measurement on an outer wall of a fluid container filled with at least one first fluid. The invention also relates to a system with a device and a fluid container. The invention also relates to a method for capacitive measurement of a level of a fluid container filled with at least one first fluid.

In the chemical industry, glass or plastic containers are often used for storage or for chemical reactions. To determine the fill level in the containers, a simple number scale printed on the container wall for manual reading is often sufficient (see measuring cup). Measuring equipment for determining the fill level is often not used for various reasons, such as costs or no contact with the media.

Furthermore, there are often different liquids in one container, so that a boundary layer forms (e.g. water and oil). In order to distinguish or separately measure the fill levels of both fluids, simple, non-media-contacting instrumentation is not possible.

Many containers are not stationary, so a hard-wired measuring device for detecting the fill level is not desirable. A battery-supported and wireless solution would be advantageous.

WO 2023/036423 Al discloses a capacitive reading device for a pointer instrument.

The invention is based on the object of providing a device and an associated method for the capacitive measurement of a fill level, which can be done with little effort and minimally invasively.

This object is achieved by a device for capacitive level measurement on an outer wall of a fluid container filled with at least a first fluid according to claim 1. In addition, the object is achieved by a system according to claim 8. In addition, the object is achieved by a method for capacitive measurement of a level of a fluid container filled with at least a first fluid according to claim 9. Advantageous further developments emerge from the dependent claims.

A device according to the invention for capacitive level measurement on an outer wall of a fluid container filled with at least a first fluid comprises the following steps:

- at least one first capacitor and one second capacitor, which are arranged parallel to each other along a longitudinal direction of the capacitors and which can be attached to the outer wall of the fluid container,

- a detection device which is designed to apply an electrical voltage to the capacitors, preferably offset in time, and to detect a respective electrical charge of the capacitors in order to determine a respective capacitance of the capacitors by measurement, wherein the detection device is further designed to compare the measured capacitances of the capacitors with one another and to determine a fill level of the fluid container on the basis of this comparison, taking into account a positioning indication with regard to the fluid container.

The device according to the invention can be easily attached to the outer wall of a fluid container. The capacitors are arranged parallel to one another along a longitudinal direction. In other words, the capacitors are arranged parallel and offset from one another. The detection device of the device according to the invention is designed to apply an electrical voltage, preferably staggered in time, to the capacitors. Depending on the permittivity of a fluid in the fluid container, a certain electrical charge is established in the respective capacitor. This charge is measured by the detection device. From the measured charge and the applied voltage, the detection device determines a capacitance of the respective capacitor. The electrical voltage can be applied to several or all capacitors at the same time. However, they can also advantageously be subjected to the electrical voltage staggered in time or one after the other in order to prevent or at least reduce mutual parasitic influences on the capacitances determined later.

The detection device is also designed to compare the determined capacitances of the two capacitors. If the detection device determines, for example, that both capacitors have a capacitance expected for the medium air, it can be assumed that the fill level in the fluid container is very low and does not reach the level of the first capacitor. If, on the other hand, the capacitance of both capacitors is significantly higher, it can be assumed that the fill level in the fluid container is high and exceeds both capacitors. If the capacitances of the two capacitors differ from one another within the scope of the measurement accuracy, it can be assumed that the fill level in the fluid container reaches the level of the first capacitor.

In order for the device according to the invention to be able to indicate an absolute fill level, the detection device must be given a positioning specification. This specifies the height of the fluid container at which the device is to be installed. It thus provides a so-called offset, which in the simplest case is zero. The detection device knows the distance between the capacitors. Together with the positioning information, it can use this to calculate exactly what level the fluid container has.

The detection device can be located in close proximity to the capacitors and in particular can be arranged within a common housing. However, at least the part of the detection device that compares the capacitances and determines the fill level can also be designed remotely from the capacitors, for example in a cloud-based computer environment.

The device according to the invention has the advantages that no moving mechanical parts are required for measuring the fill level. In addition, only readily available standard components can be used. In addition, the device does not intervene in the fluid container, which would be particularly problematic with chemically reactive fluids. The measurement of the fill level is also not limited to a single fluid in the fluid container. Rather, with N capacitors, up to N different fluids can be detected.

It is obvious that the invention is not limited to just two capacitors. Rather, the device can have a plurality of three or more capacitors. The following applies: the higher the number of capacitors, the higher the spatial resolution. With a large number of capacitors, the fill level can therefore be specified more precisely.

Preferably, the device has a shield for electromagnetic shielding of the capacitors, in particular against each other. The device according to the invention is designed to detect changes in capacitance. detect the electromagnetic fields that arise due to different permittivities of different fluids within the fluid container. For this purpose, it is advisable if the capacitors are shielded against undesirable electromagnetic influences by means of suitable shielding. In particular, the capacitors can be electromagnetically shielded from one another in order to prevent or at least minimize falsification of the measurement results.

The detection device may comprise a capacitance-digital converter, which preferably has a measurement resolution of at least 24 bits.

In addition, the detection device can comprise a multiplexer which has at least two voltage outputs, each of which is conductively connected to a capacitor (more precisely: the voltage outputs are each connected to an electrode of a capacitor). In the case of three or more capacitors, the multiplexer can have correspondingly more voltage outputs.

Preferably, the electrical potential of a voltage output of the multiplexer to which no electrical voltage is applied corresponds to the electrical potential of the shield. As a result, the capacitor or capacitors that are not currently supplied with voltage act as additional shielding for the voltage-supplied capacitor that is currently being used to determine the capacitance.

A printed circuit board (PCB) can serve as a carrier element for the electrodes/capacitors. Alternatively, the carrier element can be a printed, flexible film such as a polyimide or a PU film. In a preferred development of the invention, the device comprises at least a first, a second and a third capacitor, which are arranged parallel to one another along the longitudinal direction of the capacitors. The second capacitor is arranged longitudinally between the first and the third capacitor. One voltage output of the multiplexer is conductively connected to the first and the third capacitor, while another voltage output of the multiplexer is electrically connected to the second capacitor. The underlying idea here is that the voltage outputs of a multiplexer can be connected to several capacitors. This makes it possible to achieve a higher resolution for the fill level without needing a multiplexer with so many voltage outputs. More detailed information on this can be found in the description of the exemplary embodiments.

The previously explained object is also achieved by a system which comprises a fluid container filled with one or more fluids and a device which is designed as previously explained. The device is attached to the outer wall of the fluid container, with the longitudinal direction of the capacitors of the device being aligned in the direction of the fluid container fill level to be measured. In other words, the device is aligned vertically on the fluid container and is attached there using suitable means (gluing, clamping, etc.).

The problem is also solved by a detection system which comprises a plurality of devices as previously explained, which are connected to one or more higher-level evaluation units, wherein the devices are preferably arranged in a pipeline. The outer wall of the fluid container is preferably made of plastic or glass. The previously explained object is also achieved by a method for capacitively measuring a fill level of a fluid container filled with at least a first fluid, which method comprises the following method steps: a) attaching a device to an outer wall of the fluid container, the device having at least a first capacitor and a second capacitor, which are arranged parallel to one another along a longitudinal direction of the capacitors, and a detection device, b) applying an electrical voltage, preferably offset in time, to the capacitors by the detection unit, c) detecting a respective electrical charge of the capacitors resulting from the application of the electrical voltage by the detection unit, d) metrologically determining the respective capacitance of the capacitors on the basis of the applied electrical voltage and the detected electrical charge by the detection unit, e) comparing the metrologically detected capacitances of the capacitors in order to determine the fill level of the fluid container therefrom.

The detection device can comprise a capacitance-digital converter, which preferably has a measurement resolution of at least 24 bits, and which detects the respective electrical charge of the capacitors resulting from the application of the electrical voltage and determines the respective capacitance of the capacitors by measurement.

The detection device can also comprise a multiplexer which has at least two voltage outputs, each of which is conductively connected to a capacitor, and by means of which the respective electrical voltage is applied to the capacitors.

Preferably, the device comprises at least a first, a second and a third capacitor arranged along the Longitudinal direction of the capacitors are arranged parallel to one another, wherein the second capacitor is arranged longitudinally between the first and the third capacitor, and wherein a first voltage output of the multiplexer is conductively connected to the first and the third capacitor, and wherein a second voltage output of the multiplexer is electrically conductively connected to the second capacitor, wherein an electrical voltage is applied to the first and the third capacitor by means of the first voltage output of the multiplexer, and wherein an electrical voltage is applied to the second capacitor by means of the second voltage output of the multiplexer.

The above-described properties, features and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more readily understood in connection with the following description of the embodiments, which is explained in more detail in connection with the figures. They show:

FIG 1 is a diagram of a system comprising a device and a fluid container;

FIG 2 shows a schematic diagram for measuring a capacitance by means of a device according to the invention;

FIG 3 is a circuit diagram of part of a device according to the invention;

FIG 4 is a perspective view of an embodiment of a device according to the invention;

FIG 5 is a side view of a device according to the invention;

FIG 6 is a perspective view of an embodiment of a device according to the invention; FIG 7 shows an aspect of a method according to the invention;

FIG 8 is a circuit diagram of part of another device according to the invention;

FIG 9 shows an aspect of a device according to the invention;

FIG 10 is a side view of a device according to the invention; and

FIG 11 is a diagram of a measurement carried out by means of a device according to the invention.

FIG 1 shows the side view of a system 1 according to the invention. The system 1 comprises a fluid container 2, a device 3 and a detection device 13 (see FIG 3). The fluid container 2 is made of glass in the present case and can be filled with a fluid such as water or oil. A fill level F of the fluid container is measured in FIG 1 from the lower edge (bottom) of the fluid container 2 in a vertical direction upwards.

Only a part of the device 3 is shown in FIG. 1. This part comprises four supply electrodes 4a, 4b, 4c, 4d, a sensor electrode 5 and an electromagnetic shield 6 arranged to the right of the sensor electrode 5 in FIG. 1 as well as a further shield 8 arranged to the left of the supply electrodes 4a, 4b, 4c, 4d. The shields 6, 8 can be made of aluminum, for example, and can have the same electrical potential.

The feed electrodes 4a, 4b, 4c, 4d together with the sensor electrode 5 form four capacitors 7a, 7b, 7c, 7d, which are indicated in FIG 1 with dashed borders. The electrodes 4a, 4b, 4c, 4d, 5 are arranged on a longitudinal axis L, for example on a flexible, printed film or a printed circuit board, which are not shown in FIG 1 for reasons of clarity (cf. FIG 9).

The part of the device 3 shown in FIG. 1 is attached to an outer wall of the fluid container 2 (i.e. outside the fluid container 2) by means of suitable means (adhesive, clamping means, etc.). The part of the device 3 is arranged such that the longitudinal axis L of the feed electrodes 4a, 4b, 4c, 4d or the sensor electrode 5 coincides with a direction of the fill level F.

The feed electrodes 4a, 4b, 4c, 4d are subjected to an electrical voltage at different times, for which the detection device 13 of the device 3 shown in FIG. 3 is responsible. FIG. 2 shows in a sectional view, as an example for a capacitor 7a of the four capacitors 7a, 7b, 7c, 7d from FIG. 1, which is formed from a feed electrode 4a and the sensor electrode 5, the electrical field E resulting from the application of the electrical voltage. A part of this electrical field E runs through the fluid container wall 2 (sectional view) and is influenced by a permittivity of the fluid in the fluid container 2.

The electrical voltage is provided by the detection device 13 of the device 3 shown in FIG. 3. FIG. 3 shows an electrical wiring diagram of the detection device 13 of the device 3. Three of the capacitors 7a, 7b, 7c are shown on the left-hand side. The feed electrodes 4a, 4b, 4c of the capacitors 7a, 7b, 7c are each connected to a voltage output 9a, 9b, 9c of a multiplexer 10. The multiplexer 10 is designed to apply an electrical voltage to the voltage outputs 9a, 9b, 9c at different times. It receives this electrical voltage from a voltage output 11 ("EXC") of a capacitance-digital converter 12 of the detection device 13. The capacitance-digital converter 12 can be, for example, an AD7745 from Analog Devices. This can detect a capacitance range from 0 to 8 pF with a resolution of 24 bits. The capacitance is determined via an input 14 ("CIN( + )"), which is connected to the sensor electrode 5. The internal structure of the capacitance-digital converter 12 is not the subject of this invention and is therefore not explained in more detail.

The 3-8-line decoder 74xx238 from Texas Instruments can be used as the multiplexer 10, for example. The multiplexer 10 sets the voltage outputs 9a, 9b, 9c, which are not supplied with a voltage, to the same electrical potential as the shield 6. As a result, all feed electrodes 4a, 4b, 4c, 4d, which are not supplied with a voltage, serve as a shield for the feed electrode 4a, 4b, 4c, 4d that is supplied with a voltage. Reference is also made to FIG. 4, which has eight feed electrodes 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h, a sensor electrode 5 and a shield 6, and is attached to the fluid container 2. In FIG. 4, only one of the feed electrodes 4c is supplied with a voltage. Only the capacitor 7c formed by this supply electrode 4c and the sensor electrode 5 is used for the capacitance measurement at the time shown in FIG. 4.

FIG 5 illustrates how the capacitance values determined by the detection device 13 can be assigned to the fill level F in the fluid container 2. The detection device 13 applies an electrical voltage to the first three capacitors 7a, 7b, 7c (from bottom to top) one after the other and determines the capacitance of the respective capacitor 7a, 7b, 7c. This is largely determined by a permittivity of a in the fluid container 2. With the transition to the fourth capacitor 7d, there is no longer any fluid 14 in the fluid container 2, which influences the electric field generated by the capacitor 7d and ultimately the determined capacitance. By comparing the capacitance measured on the fourth capacitor 7d with the capacitance measured on the third capacitor 7c, the fill level F in the fluid container 2 can be determined. The distance between the capacitors 7a, 7b, 7c, 7d determines the resolution with which the fill level can be determined.

Each feed electrode 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h, 4i, 4j must be connected to a voltage output 9a, 9b, 9c of the multiplexer 10. The number of electrode numbers can therefore be the same as the number of electrodes that can be switched for measurement. With an 8-fold multiplexer 10, one of the eight feed electrodes 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h can be activated for measurement. If these eight electrodes are simply placed parallel to one another, the fill level can only be determined within the measuring section in which the eight electrodes are lined up.

If one now wants to record the fill level of the entire container with a resolution of, for example, 1% of the measuring range, one would have to arrange 100 feed electrodes linearly according to the above scheme. This would require a 100-fold multiplexer 10, which would be complex in terms of circuitry. A solution for this is provided by the embodiment shown in FIG. 6. Each voltage output 9a, 9b, 9c of the multiplexer 10 is connected to two or more feed electrodes 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h, 4i, 4j, 4k, 41. In the case of multiple use, the capacitance measuring range of the detection unit 13 is dimensioned such that the signal control is only reached to 100% when the fluid has reached all supply electrodes 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h, 4i, 4j, 4k, 41 belonging to a respective voltage output 9a, 9b, 9c, and to 50% when it has reached the Half of the corresponding feed electrodes 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h, 4i, 4j, 4k, 41.

In this way, with relatively few voltage outputs of the multiplexer 10, a large number of feed electrodes 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h, 4i, 4j, 4k, 41 can be placed and thus a high spatial resolution can be achieved.

A further explanatory example is shown in FIG 7. The device 3 here has 16 supply electrodes 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h, 4i, 4j, 4k, 41, 4m, 4n, 4o, 4p and a sensor electrode 5. The multiplexer 10 here has four voltage outputs 9.1, 9.2, 9.3, 9.4 (analogous to the voltage outputs 9a, 9b, 9c from the previous figures), which are each connected to four of the supply electrodes 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h, 4i, 4j, 4k, 41, 4m, 4n, 4o, 4p. For example, the first voltage output 9.1 is connected to the first feed electrode 4a, the fifth feed electrode 4e, the ninth feed electrode 4i and the thirteenth feed electrode 4m. The distance between the feed electrodes 4a, 4e, 4i and 4m connected to the voltage output is therefore four feed electrodes each. The same applies to the other voltage outputs 9.2, 9.3 and 9.4.

On the right-hand side, different filling levels 14 of fluids (water and oil) are shown. The table below shows the level of control of the capacity determined in relation to a voltage output 9.1, 9.2, 9.3, 9.4. For the first column, the level of the water reaches up to the second feed electrode 4b or up to the second capacitor. The first feed electrode 4a is connected to the first voltage output 9.1, the second feed electrode 4b to the second voltage output 9.2. The level of control of the capacity determined in relation to the voltage output 9.1, 9.2 is 25% in each case, since the level does not reach up to the other the two voltage outputs 9.1, 9.2 connected feed electrodes. Analogous levels of control are shown for the second, third and fourth columns. For the fifth column, the water reaches up to the ninth feed electrode 4i. This requires a control of 50% for the capacitances for the second, third and fourth voltage outputs 9.2, 9.3, 9.4 and 75% for the capacitance for the first voltage output 9.1.

For the capacities of the second, third and fourth voltage outputs 9.2, 9.3, 9.4, an additional 3% control is added, since the oil only brings with it a smaller increase in capacity than the water. This allows the detection unit 13 to easily determine how high the water level and how high the oil level are - and only requires one multiplexer 10 with four voltage outputs.

FIG 8 shows a further embodiment of the detection device 13 of the apparatus 3.

FIG. 9 shows further embodiments of the feed electrodes 4a, 4b, 4c with the sensor electrode 5. The circuit can be manufactured with a large length and then cut to the length required for the respective fluid container (for example by cutting). On the right-hand side of FIG. 9 it is shown that the feed electrodes 4a, 4b, 4c do not have to be arranged directly parallel to one another, but can also be arranged alternately around the sensor electrode 5. The only important thing is that their axes K are arranged essentially parallel to one another.

FIG 10 shows a sectional view of the first part of a device 3 according to the invention. The feed electrodes 4a, 4b, 4c, 4d visible in FIG 10 (and the sensor element not visible in FIG 10 hidden by the feed electrodes 4a, 4b, 4c, 4d) Electrodes 4a, 4b, 4c, 4d and shields 6, 8 are attached to a flexible film 16 and are made of copper or a conductive ink. Electrodes 4a, 4b, 4c, 4d, 5 have a thickness of 25 micrometers. On top of this (to the right in FIG. 10) there is another shielding layer 17. This can, for example, have a thickness of 35 micrometers.

FIG 11 shows the results of a measurement carried out with 24 feed electrodes (3x8 with an 8-way multiplexer). The electrodes are 20x10 mm in size and are spaced 1 mm apart (in the direction of the longitudinal axis L). The measuring range is 270 mm. The fill level in a fluid container was increased in 5 mm steps after each measurement.

The capacitance changes gradually in 33% control steps (due to 3 electrodes connected together, i.e. 3 capacitors connected in parallel). The X-axis shows the fill level in the fluid container and the Y-axis the determined capacitance for the eight feed electrodes (or the capacitor formed by them). From a fill level of 100mm, the channel assigned to the first feed electrode (designated K in FIG 11) measures approx. 1.7pF (66% capacitance control). From 190mm, the channel is fully controlled with 2.6pF (100%).

Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited to the disclosed example and other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims

Patent claims

1. Device (3) for capacitive level measurement on an outer wall of a fluid container (2) filled with at least a first fluid (14), comprising:

- at least one first capacitor (7a) and one second capacitor (7b) which are arranged parallel to one another along a longitudinal direction (L) of the capacitors (7a, 7b) and can be attached to the outer wall of the fluid container (2),

- a detection device (13) which is designed to apply an electrical voltage to the capacitors (7a, 7b), preferably at different times, and to detect a respective electrical charge of the capacitors (7a, 7b) in order to determine a respective capacitance of the capacitors (7a, 7b) by measurement, wherein the detection device (13) is further designed to compare the measured capacitances of the capacitors (7a, 7b) with one another and to determine a fill level (F) of the fluid container (2) on the basis of this comparison, taking into account a positioning indication with regard to the fluid container.

2. Device (3) according to claim 1, which has a shield (6, 8, 17) for electromagnetic shielding of the capacitors (7a, 7b), in particular against each other.

3. Device (3) according to one of the preceding claims, wherein the detection device (13) comprises a capacitance-digital converter (12) which preferably has a measurement resolution of at least 24 bits.

4. Device (3) according to one of the preceding claims, wherein the detection device (13) comprises a multiplexer (10) which has at least two voltage outputs (9a, 9b), each of which is conductively connected to a capacitor (7a, 7b).

5. Device (3) according to claim 2 and 4 or according to claim 2, 3 and 4, wherein the electrical potential of a voltage output (9a, 9b) of the multiplexer (10), to which no electrical voltage is applied, corresponds to the electrical potential of the shield (6, 8, 17).

6. Device (3) according to one of the preceding claims, in which the capacitors (7a, 7b, 7c) are arranged on a circuit board or a flexible film.

7. Device (3) according to at least claim 4, which comprises at least a first (7a), a second (7b) and a third capacitor (7c) which are arranged parallel to one another along the longitudinal direction (L) of the capacitors (7a, 7b, 7c), wherein the second capacitor (7b) is arranged in the longitudinal direction (L) between the first capacitor (7a) and the third capacitor (7c), and wherein a voltage output (9a) of the multiplexer (10) is conductively connected to the first capacitor (7a) and the third capacitor (7c), and wherein another voltage output (9b) of the multiplexer (10) is electrically conductively connected to the second capacitor (7b).

8. System (1) comprising a fluid container (2) filled with one or more fluids and a device (3) according to one of the preceding claims, which is attached to the outer wall of the fluid container (2), wherein the longitudinal direction

(L) of the capacitors (7a, 7b, 7c) of the device (1) is aligned in the direction of the filling level (F) of the fluid container (2) to be measured.

9. System (1) according to claim 8, wherein the outer wall of the fluid container (2) is made of plastic or glass.

10. Method for capacitive measurement of a fill level (F) of a fluid container (2) filled with at least a first fluid, comprising: a) attaching a device (3) to an outer wall of the fluid container (2), the device (3) having at least a first capacitor (7a) and a second capacitor (7b), which are arranged parallel to one another along a longitudinal direction (L) of the capacitors (7a, 7b), and a detection device (13), b) applying an electrical voltage, preferably offset in time, to the capacitors (7a, 7b) by the detection device (13), c) detecting a respective electrical charge of the capacitors (7a, 7b) resulting from the application of the electrical voltage by the detection device (13), d) metrologically determining the respective capacitance of the capacitors (7a, 7b) on the basis of the applied electrical voltage and the detected electrical charge by the detection device (13), e) comparing the metrologically detected capacitances of the capacitors (7a, 7b) in order to determine the fill level of the fluid container (2) therefrom.

11. Method according to claim 10, wherein the detection device (13) comprises a capacitance-digital converter, which preferably has a measurement resolution of at least 24 bits, and which detects the respective electrical charge of the capacitors (7a, 7b) resulting from the application of the electrical voltage and determines the respective capacitance of the capacitors (7a, 7b) by measurement.

12. Method according to claim 10 or 11, wherein the detection device (13) comprises a multiplexer (10) which has at least two voltage outputs (9a, 9b), which are each conductively connected to a capacitor (7a, 7b), and by means of which the respective electrical voltage is applied to the capacitors (7a, 7b).

13. Method according to claim 12, wherein the device (3) comprises at least a first, a second and a third capacitor (7a, 7b, 7c) arranged along the longitudinal direction (L) of the capacitors (7a, 7b, 7c) are arranged parallel to one another, wherein the second capacitor (7b) is arranged in the longitudinal direction (L) between the first capacitor (7a) and the third capacitor (7c), and wherein a first voltage output (9a) of the multiplexer (10) is conductively connected to the first capacitor (7a) and the third capacitor (7c), and wherein a second voltage output (9b) of the multiplexer (10) is electrically conductively connected to the second capacitor (7b), wherein an electrical voltage is applied to the first capacitor (7a) and the third capacitor (7c) by means of the first voltage output (9a) of the multiplexer (10), and wherein an electrical voltage is applied to the second capacitor (7b) by means of the second voltage output (9b) of the multiplexer (10).