EP3216127A2

EP3216127A2 - Method and device for detecting steering wheel contact

Info

Publication number: EP3216127A2
Application number: EP15823496.3A
Authority: EP
Inventors: Hans-Jürgen BOSSLER; Martin Kreuzer; Guido Hirzmann; Alexander Lammers; Helga Heist
Original assignee: TRW Automotive Safety Systems GmbH
Current assignee: ZF Automotive Safety Systems Germany GmbH
Priority date: 2014-11-07
Filing date: 2015-11-09
Publication date: 2017-09-13
Also published as: US20170334477A1; DE102014016422A1; WO2016071002A2; CN107107939A; US10501107B2; WO2016071002A3

Abstract

A device for detecting steering wheel contact comprises at least a first electrode (12) which is provided in a steering wheel (10) and which forms, together with a human body acting as a second electrode and a dielectric situated therebetween, at least one sensor capacitor (26). The device also comprises an evaluation circuit (24) having a reference capacitor (30) of known capacitance which can be connected parallel to the sensor capacitor (26), a direct current voltage source (34) which can be connected to the reference capacitor (30), and a measuring device for measuring the voltage at the reference capacitor (30). A method for detecting steering wheel contact using such a device comprises the following successive steps: charging the reference capacitor (30) by applying a known reference voltage, or charging the reference capacitor (30) and subsequently measuring a first voltage at the reference capacitor (30); connecting, in parallel, the sensor capacitor (26) to the reference capacitor (30) so that a portion of the charge of the reference capacitor (30) is transmitted to the sensor capacitor (26); measuring a second voltage at the reference capacitor (30); and determining the capacitance of the sensor capacitor (26) from the known capacitance of the reference capacitor (30), the reference voltage or the first voltage and the second voltage.

Description

Device and method for detecting a steering wheel touch

The invention relates to a device for detecting a steering wheel contact. The invention further relates to a method for detecting a steering wheel touch by means of such a device.

Apart from optical detection systems, there are basically two ways to detect a steering wheel touch sensor: capacitive and resistive. In resistive systems, an electrical signal is generated by the pressure of the hand on the steering wheel. Influences due to differently tight or less tight leather sheaths and thus different output forces acting on the sensor make a clear detection difficult.

In capacitive systems, trouble-free detection is currently not possible or only with great technical effort. Problems such as insufficient grounding, temperature influences and detection of the initial state when starting the vehicle are to be solved. In particular, the sensors are very prone to drift. In known "open" systems in which the human body forms a capacitor electrode with a variable distance to an electrode fixed in the steering wheel and their evaluation is based on a resonant circuit, the frequency shift (detuning of the resonant circuit) is measured, but often already becomes an approximation evaluated as a touch to the steering wheel, so that a reliable detection of a real touch is not guaranteed.

From US 8 836 350 B2 a method for capacitive touch detection is known, which was developed for the query of touch-sensitive keyboards. In the context of touch detection, the voltage is monitored on an internal capacitor of an integrated circuit, which can be coupled to an external capacitor.

CONFIRMATION COPY DE 10 2012 024 903 A1 shows a sheet with a carrier, at least two electrical functional elements and at least one seam, in which the strand-shaped functional elements are integrated. Such a sheet is intended to be used in particular in electrical heaters. The object of the invention is to enable a reliable and robust detection of a steering wheel touch.

This object is achieved by a device having the features of claim 1 and by a method having the features of claim 20. Advantageous and expedient embodiments of the device according to the invention and of the method according to the invention are specified in the associated subclaims.

The device according to the invention for detecting a steering wheel contact comprises at least one first electrode arranged in a steering wheel, which forms at least one sensor capacitor together with a human body functioning as a second electrode and an intervening dielectric. The device according to the invention further comprises an evaluation circuit with a reference capacitor of known capacitance, which is switchable parallel to the sensor capacitor, a DC voltage source which can be connected to the reference capacitor, and a measuring device for measuring the voltage at the reference capacitor.

Compared to a "closed" system, in which two opposing electrodes are installed in the steering wheel, which must be moved towards each other to generate an output signal by pressure and therefore require a highly elastic dielectric, the device according to the invention sees an "open" system before, in which the human body is used as a second capacitor electrode. By approaching and finally touching the steering wheel, the capacitance of the sensor capacitor thus formed changes significantly. Thus, can be dispensed with a second capacitor electrode in the steering wheel, and the required structural changes in the steering wheel are limited. Thanks to the inclusion of the reference capacitor, which can be switched in parallel with the sensor capacitor, the structure of the evaluation device according to the invention makes possible a reliable and robust detection of a steering wheel contact even under varying conditions, in particular in the With regard to temperature (drift), humidity, etc. The typical problems in the monitoring of resonant circuits do not occur in the device according to the invention, since only working with DC voltage, which can be tapped advantageous from the electrical system, and no frequencies must be measured. The device according to the invention is overall very cost-effective and easily scalable. The results of the evaluation can be transferred to various vehicle assistance systems in order to influence their control accordingly.

The compact design of the evaluation circuit contributes to the fact that it can at least partially be arranged on a chip. In particular, the arrangement of the reference capacitor on a chip is advantageous because the chip provides protection against external influences.

The at least one first electrode of the sensor capacitor is formed in the preferred embodiments of the invention by an arranged in a steering wheel rim of the steering wheel electrical conductor.

The electrical conductor from which the first electrode is formed, may comprise one or more wires, wherein it is z. B. may be commercially available enameled wires. In principle, wires can be easily bent and placed in a form suitable for an electrode. As an electrical conductor but also one or more conductive strips or bundles of several strands can be used.

According to a particular embodiment of the invention, the electrical conductor to an electrically conductive paint or an electrically conductive paste. The application of such a varnish or paste to a support allows a free shaping of the first electrode. Another advantage is that the paint or the paste before the foaming on the blank, which later forms the foam around the steering wheel skeleton, can be applied or introduced into the tool. This may have procedural advantages over a subsequent introduction of the first electrode. Another way to provide the electrical conductor for the first electrode is to use a (prefabricated) metal plate or foil. According to the preferred embodiments of the invention, the electrical conductor is disposed within a foam surrounding a steering wheel skeleton. There, the electrical conductor is largely protected against mechanical stress and other disturbing influences. In addition, the outer, facing away from the steering wheel skeleton part of the foam surrounds a part of the dielectric of the sensor capacitor, which is advantageous in view of the insulating properties of the foam.

In principle, however, other embodiments are possible in which the electrical conductor is applied to a foam surrounding a steering wheel skeleton. In this case, the electrical conductor can be applied either on the inner side facing the steering wheel skeleton or on the outside facing away from the steering wheel skeleton.

For example, the electrical conductor immediately below an outer shell of the steering wheel rim, z. As a leather or synthetic leather sheathing, be arranged. The term sheathing should also decorative attachments z. B. plastic, wood material, textile, etc. include.

Starting from an elongate base material (wires, bands, etc.), the electrical conductor can run essentially along the toroidal circumferential direction of the steering wheel rim or essentially along the poloidal circumferential direction of the steering wheel rim. "Substantially" is intended here to mean that curvatures are permitted or even desired as long as the electrical conductor as a whole extends in the particular direction mentioned.

According to one embodiment of the invention, the electrical conductor is part of a metallic fabric or a metallic knitwear. Such fabrics or knits are more flexible in comparison to metal plates and better conform to their bearing surface.

In a further embodiment of the first electrode, the electrical conductor is applied to a carrier, in particular on a textile, a mat or a film. The electrical conductor, which is preferably formed in the latter case by thread-like wires, can be attached as in embroidering by means of pulling or sewing on one or both sides of the carrier. Such electrically conductive fabrics have been proven, inter alia, in electric steering wheel heating, but can also be used as a capacitor electrode within the scope of the invention.

The electrical conductor, from which the first electrode of the sensor capacitor is formed, does not necessarily consist of a metal or a metal alloy. The electrical conductor can also be formed from a semiconductor material.

In order to minimize electrical interference and thus the susceptibility of the device according to the invention to error or to improve the quality of the results, shielding is desirable. A passive shield may be provided by a metallic skeleton of the steering wheel or an electrical conductor disposed between the steering wheel skeleton and the first electrode, which is at a constant potential or grounded.

An active shielding can be achieved in that the evaluation circuit is set up such that a metallic skeleton of the steering wheel or an electrical conductor arranged between the steering wheel skeleton and the first electrode is charged to a predetermined potential when the sensor capacitor and the reference capacitor are connected in parallel.

For a more sophisticated, differentiated evaluation, which goes beyond the mere detection of a steering wheel touch, the invention provides a plurality of distributed over the steering wheel rim arranged first electrodes, which are galvanically separated from each other. In this way, the number of evaluable sensor capacitors is increased accordingly. Depending on the number and arrangement of the first electrodes, conclusions can be given about the specific location of the steering wheel contact. It is also possible to detect whether one or two hands are resting on the steering wheel, and also movement patterns can be detected.

The invention also provides a method for detecting a steering wheel touch by means of the device according to the invention as defined above. The procedure comprises the following, successive steps: - Charging the reference capacitor by applying a known reference voltage, or charging the reference capacitor and then measuring a first voltage on the reference capacitor;

- Connecting the sensor capacitor in parallel to the reference capacitor, so that a portion of the charge of the reference capacitor is transferred to the sensor capacitor;

- measuring a second voltage on the reference capacitor; and

Determining the capacitance of the sensor capacitor from the known capacitance of the reference capacitor, the reference voltage or first voltage and the second voltage.

As already mentioned, the skillful inclusion of the reference capacitor with known constant capacitance ensures that even small changes in the capacitance of the sensor capacitor can be reliably detected. Before charging the reference capacitor and the charge transfer from the reference capacitor to the sensor capacitor at least the sensor capacitor is completely discharged. Of course, during the charge transfer, the reference capacitor is disconnected from the DC source.

In particular, in order to detect changes in the capacitance of the sensor capacitor, which suggest movements of the driver, in particular of his hand or finger, the above-mentioned method steps are to be repeated continuously.

For a high resolution of the detection, the time interval between the repetitions of the method steps should be as small as possible, preferably in the millisecond range.

To suppress interference and improve the quality of the method according to the invention can be charged when paralleling the sensor capacitor and the reference capacitor serving as (active) shielding electrical conductor in the steering wheel, such as the metallic steering wheel skeleton, to a predetermined, suitable for interference suppression potential. As part of the evaluation can be used for a reproducible detection of a steering wheel touch on at least one threshold for the capacitance of the sensor capacitor, which is preferably based on empirically determined empirical values. The evaluation circuit can easily be extended to monitor several sensor capacitors, more precisely several first electrodes. This allows a more sophisticated, differentiated evaluation, which goes beyond the mere detection of a steering wheel touch. Depending on the number and arrangement of the first electrodes can provide information about the specific location of the steering wheel touch. It is also possible to detect whether one or two hands are resting on the steering wheel, and also movement patterns can be detected.

Further features and advantages of the invention will become apparent from the following description and from the accompanying drawings, to which reference is made. In the drawings show:

- Figure 1 is a vehicle steering wheel with schematically illustrated, distributed electrodes arranged a device according to the invention for detecting a steering wheel contact;

- Figure 2 is a schematic section through the rim of the steering wheel of Figure 1 with an enlarged detail; and

- Figure 3 is a schematic diagram of an evaluation circuit of the device according to the invention.

For the capacitive detection of a touch of a steering wheel 10 in a motor vehicle, as shown by way of example in FIG. 1, at least one sensor capacitor is required, which is designed to be "open" in the device for detecting a steering wheel contact described below.

The first electrode 12 of the sensor capacitor is an electrically conductive surface which is arranged immovably in the steering wheel rim 14 and is not visible from the outside. (To illustrate, in the left half of FIG. 1, several points of the steering wheel rim 14 are schematically marked, at which first electrodes 12 can be located according to an exemplary arrangement Various possibilities of the design and arrangement of the first electrode 12 will be described in more detail later.

A second electrode represents the human body of the driver. Accordingly, the distance between the first electrode 12 and the second electrode is variable. The distance between the two electrodes is minimal when the driver, in particular his fingers or hand, touches the steering wheel 10 immediately above the first electrode 12.

The dielectric of the sensor capacitor is formed by the layers of the steering wheel rim 14, including the casing 20, radially outward of the first electrode 12, and optionally by the air between the casing 20 and the driver's hand.

Figure 2 shows a section through the steering wheel rim 14 including an enlarged detail. A metallic steering wheel skeleton 16 is surrounded by a plastic foam 18, z. As polyurethane (PUR) surrounded. The foam 18 is in turn covered by a sheath 20 made of leather, synthetic leather, wood, fabric, paint, plastic, rubber or other electrically non-conductive material. Of course, the steering wheel rim 14 itself may have other components, but here are of minor importance.

As already mentioned, at least one first electrode 12 is located in the rim 14 of the steering wheel. As can be seen in the detailed enlargement of FIG. 2, this can be formed by a plurality of interconnected wires 22 which are arranged inside the foam casing 18. Instead of a plurality of wires 22, only a single, possibly multi-twisted wire 22 may be provided. For the sake of simplicity, the following will always assume a plurality.

In the embodiment of Figure 2, the wires 22 extend longitudinally to the toroidal circumferential direction T of the steering wheel rim 14. However, the wires 22 can equally well run transversely thereto and wind completely or at least partially in the poloidal circumferential direction P about the steering wheel skeleton 16. In both cases, only the basic direction of the wires 22 is indicated, ie, a wave or meandering course should be included in each case. It is important in any case that the wires 22 are arranged overall so that they form a total of more or less an electrically conductive surface that can act as a capacitor electrode.

It is desirable that the wires 22 do not rest on the inside and / or outside of the skirt 18, where they may slip or be sensed by the driver, but are disposed within the foam 18. In principle, the wires 22 can be introduced into the foam casing 18 either before or after foaming. In the latter case, the foam 18 must be cut later. In order for the wires 22 to be held securely and reliably in the foam 18, a special method of insertion can be used which allows for a fast and uniform laying of the wires 22. In this case, the wires 22 are introduced into the cut at the same time as cutting the foam casing 18 at the same time, so that no additional working step is required in order, for example, to disassemble the cut edges for inserting the wires 22 again.

The wires 22 may be coated with an insulating lacquer layer (eg, enameled copper wire) and / or may be part of a metallic fabric or a metallic knit fabric. The term "woven and knitted fabric" is intended to generally mean here any more or less two-dimensional structure of wires or the like, in particular single-layered or multi-layered woven fabrics as well as knitted fabrics and knitted fabrics.

Furthermore, the wires 22 may also be applied to a carrier. In particular, as the first electrode 12, a sheet is suitable, as used in an electric steering wheel heating. Such a sheet has a carrier, which may preferably be flat and at least partially a textile, a mat or a film. The wires 22 may be fastened like threads by pulling or sewing on one or both sides of the support. The course and area occupied by the wires 22 on the carrier should be chosen so that they can function as a capacitor electrode. Instead of wires 22, other electrical conductors, such as electrically conductive bands, bundles of multiple strands or the like, may be used to form the first electrode 12.

In another variant, the first electrode 12 is formed from an electrically conductive lacquer or an electrically conductive paste. The paint or paste can basically on the steering wheel skeleton 16 facing inside or on the outside of the foam 18, z. B. directly under the Ummante- ment 20, be applied. In the latter case, the paint or paste can be applied directly to the blank before it is foamed. Another possible embodiment of the first electrode 12 is a metal plate or foil which is inserted between the foam casing 18 and the casing 20.

Instead of a metal or an alloy, a semiconductor material can also be used for the electrical conductor. If a spatial resolution is desired in the detection of steering wheel touch, according to the schematic representation of the left half of Figure 1, a plurality of distributed in the toroidal circumferential direction T of the steering wheel rim arranged first electrodes 12 may be provided, which are monitored simultaneously. In addition, the first electrodes 12 may also be distributed (still) in the circumferential poloidal direction P of the steering wheel rim 14 and / or arranged only at locations of the steering wheel rim 14 selected according to specific criteria.

Hereinafter, the detection of a steering wheel contact by the driver using the arranged in the steering wheel rim 14 first electrode 12 will be described. The detection is based on the capacitance change of the sensor capacitor, which occurs when the driver acting as the second electrode, in particular his hand or finger, approaches the first electrode 12.

Figure 3 shows the basic structure of an evaluation circuit 24, which is used in the device described here. The sensor capacitor is here generally symbolized by 26. Its second electrode variable in the distance to the first electrode 12, which is formed by the driver's human body, is designated here by 36. By means of a first switch 28, the sensor capacitor 26 can be connected in parallel with at least one reference capacitor 30. The reference capacitor 30 has a known capacitance C _Re f and can be arranged together with other components of the evaluation circuit 24 on a chip (IC), for example as part of an analog-to-digital converter. The capacitance C _{Ref of} the reference capacitor 30 is chosen such that it lies approximately in the region of the capacitance C _Se n of the sensor capacitor 26 when a steering wheel contact takes place.

A DC voltage source 34 can be connected to the reference capacitor 30 via a second switch 32. By means of a measuring device, in each case the voltage at the reference capacitor 30 can be measured.

The following explanations clarify the basic measuring principle. By closing the second switch 32, a known reference voltage U _{Ref is applied} to the reference capacitor 30. According to the equation

Alternatively, the reference _capacitor 30 can be charged and, after charging, a first voltage on the reference _capacitor 30 can be measured, which for the sake of simplicity is also referred to as U _Ref1 .

By opening the second switch 32 and closing the first switch 28, the reference capacitor 30 is disconnected from the DC voltage source 34 and connected in parallel with the sensor capacitor 26, which was previously completely discharged. In this switching state, a charge compensation takes place, in which a part of the charge Q of the reference capacitor 30 is transferred to the parallel-connected sensor capacitor 26. For this charge transfer about 20 milliseconds are estimated.

The total charge Q remains the same in this process, but is now divided between the two capacitors 26, 30 (Q = Q _Ref + Qsen) - In contrast, the now measurable at the reference capacitor 30 second voltage U _Re f2 is less than the (first) reference voltage U _Ref1 , since the total capacity of the parallel th capacitors 26, 30 (C _Re f + C _Sen ) is greater than the capacitance C _{Ref of} the reference capacitor 30 alone:

From the above relationships, the actual capacitance C _Se n of the sensor capacitor 26 can now be easily calculated:

CRef * Ussefl ⁼ (C _Re f + Csen) ^* U _Re f2

Csen ⁼ CRef * (U _ef i - U _Re f ₂ ) / U _Re f2

With a known capacitance C _{Ref of} the reference capacitor 30, only the DC voltages U _Re fi nd U _Re f2 have to be measured in each case in order to be able to determine the actual capacitance C _{Sen of} the sensor capacitor 26.

The processes described above are at short intervals, z. B. two milliseconds, repeated continuously. The continuous determination of the capacitance C _{Sen of} the sensor capacitor 26 makes it possible to detect changes in this capacitance. On the basis of carefully defined threshold values which can be based on empirically determined empirical values Exceeding these thresholds close to a touch or a release of the steering wheel 10.

A more complex evaluation makes it possible to detect an approach to the steering wheel 10 or a distance from the steering wheel 10. Also, a distinction between a touch / approach with the hand and a touch / approach with a finger can be made.

If a plurality of galvanically separated first electrodes 12 are mounted on the steering wheel 10, the evaluation of all sensor capacitors 26 thus formed can provide information about the location of the steering wheel 10 at which a contact takes place and whether one or two hands rest on the steering wheel 10. In addition, gestures can also be recognized, such as a strum of a hand on the steering wheel rim 14th

The results of the evaluation can be fed to various vehicle assistance systems as input parameters. To suppress interference, the metallic steering wheel skeleton 16 or another electrical conductor disposed between the steering wheel skeleton 16 and the first electrode 12 can be used as a shield. Passive shielding is achieved by placing this electrical conductor at a constant potential or ground. As a result, in particular the charging and discharging operations on the capacitors 26, 30 and thus the evaluation of the measured voltages are more robust against disturbances. Additionally or alternatively, an active shield may be provided, in which this electrical conductor in the above-described parallel switching sensor capacitor 26 and reference capacitor 30 is charged to a suitable for noise suppression potential.

The first electrode 12 of the sensor capacitor 26 may simultaneously be a heating element of a steering wheel heater, or it may be connected to such or similar device. Additionally or alternatively, the at least one first electrode 12 may also be arranged on the airbag module of the steering wheel 10, in particular in or under its upper cover.

LIST OF REFERENCE NUMBERS

10 steering wheel

12 first electrode

14 steering wheel rim

16 steering wheel skeleton

18 foam around

20 sheathing

22 wires

24 evaluation circuit

26 sensor capacitor

28 first switch

30 reference capacitor

32 second switch

34 DC voltage source

36 second electrode

T toroidal circumferential direction

P poloidal circumferential direction

Claims

claims

Device for detecting a steering wheel touch, with

at least one first electrode (12) arranged in a steering wheel (10) and forming at least one sensor capacitor (26) together with a human body functioning as a second electrode and an intervening dielectric, and

an evaluation circuit (24) with

a reference capacitor (30) of known capacitance, which is switchable parallel to the sensor capacitor (26),

a DC voltage source (34) which is connectable to the reference capacitor (30), and

a measuring device for measuring the voltage at the reference capacitor (30).

Apparatus according to claim 1, characterized in that the evaluation circuit is at least partially disposed on a chip.

Device according to claim 1 or 2, characterized in that the first electrode (12) is formed by an electrical conductor arranged in a steering wheel rim (14) of the steering wheel (10).

Apparatus according to claim 3, characterized in that the electrical conductor comprises one or more wires (22).

Apparatus according to claim 3 or 4, characterized in that the electrical conductor comprises one or more conductive strips or bundles of a plurality of strands.

Device according to one of claims 3 to 5, characterized in that the electrical conductor comprises an electrically conductive paint or an electrically conductive paste.

Device according to one of claims 3 to 6, characterized in that the electrical conductor comprises a metal plate or foil.

8. Device according to one of claims 3 to 7, characterized in that the electrical conductor is arranged within a foam casing (18) surrounding a steering wheel skeleton (16).

9. Device according to one of claims 3 to 7, characterized in that the electrical conductor is applied to a foam casing (18) surrounding a steering wheel skeleton (16).

10. Device according to one of claims 3 to 7, characterized in that the electrical conductor is arranged directly below an outer Ummante- ment (20) of the steering wheel rim (20). 11. Device according to one of claims 3 to 10, characterized in that the electrical conductor extends substantially along the toroidal circumferential direction of the steering wheel rim (14).

12. Device according to one of claims 3 to 10, characterized in that the electrical conductor extends substantially along the poloidalen circumferential direction of the steering wheel rim (14).

13. Device according to one of claims 3 to 12, characterized in that the electrical conductor is part of a metallic fabric or a metallic knitwear.

14. Device according to one of claims 3 to 13, characterized in that the electrical conductor is applied to a support, in particular on a textile, a mat or a film.

15. The apparatus according to claim 14, characterized in that the electrical conductor is attached by means of pulling or sewing on one or both sides of the carrier.

16. Device according to one of claims 3 to 14, characterized in that the electrical conductor is formed of a semiconductor material.

17. Device according to one of the preceding claims, characterized in that a metallic skeleton (16) of the steering wheel (10) or between the steering wheel skeleton (16) and the first electrode (12) arranged electrical conductor is at a constant potential or ground. 18. Device according to one of the preceding claims, characterized in that the evaluation circuit is arranged so that a metallic skeleton (16) of the steering wheel (10) or between the steering wheel skeleton (16) and the first electrode (12) arranged electrical conductor at a parallel connection of the sensor capacitor (26) and the Referenzkondensa- sector (30) is charged to a predetermined potential.

19. Device according to one of the preceding claims, characterized in that a plurality of distributed over the steering wheel rim (14) arranged first electrodes (12) are provided.

20. A method for detecting a steering wheel touch by means of a device according to one of the preceding claims, comprising the following, successive steps:

Charging the reference capacitor (30) by applying a known reference voltage, or charging the reference capacitor (30) and then measuring a first voltage at the reference capacitor (30);

Connecting in parallel the sensor capacitor (26) to the reference capacitor (30) so that a portion of the charge of the reference capacitor (30) is transferred to the sensor capacitor (26);

Measuring a second voltage on the reference capacitor (30); and determining the capacitance of the sensor capacitor (26) from the known capacitance of the reference capacitor (30), the reference voltage or the first voltage and the second voltage. 1 . A method according to claim 20, characterized in that the method steps are repeated continuously.

22. The method according to claim 21, characterized in that the time interval between the repetitions of the method steps is in the millisecond range.

23. The method according to any one of claims 20 to 22, characterized in that during parallel connection of the sensor capacitor (26) and the reference capacitor (30) serving as a shielding electrical conductor is charged in the steering wheel to a predetermined potential.

24. The method according to any one of claims 20 to 23, characterized in that for detecting a steering wheel touch on at least one threshold value for the capacitance of the sensor capacitor (26) is used, which is preferably based on empirically determined empirical values.

25. The method according to any one of claims 20 to 24, characterized in that using the evaluation circuit a plurality of sensor capacitors (26) are monitored.