DE102016118445A1 - Vibration sensor and method for operating a vibration sensor - Google Patents

Abstract

Vibration sensor (1) having a diaphragm (5) which can be excited to vibrate by means of a piezoelectric drive (3) with a mechanical oscillator (10) arranged on the diaphragm with at least two eigenmodes (100, 110, 120, 130) and electronics for controlling the drive (3) and for evaluating vibrations detected by means of the drive (3), wherein the drive (3) comprises at least one piezoelectric element (7), wherein the piezoelectric element (7) has at least four separately controllable sectors (71, 72, 73, 74).

Description

The present invention relates to a vibration sensor according to the preamble of patent claim 1 and to a method for operating a vibration sensor according to the preamble of patent claim 7.

Vibration sensors known from the prior art have a membrane which can be excited to vibrate by means of a piezoelectric drive, with a mechanical oscillator | DL1 arranged on the diaphragm, as well as electronics for controlling the drive and for evaluating detected vibrations.

These are often used as a vibration level switch with a vibratable membrane and a drive for moving the membrane in vibration and / or tapping a vibration of the membrane and a membrane arranged on the mechanical vibrator, often being used as a drive piezoelectric elements , Such vibration level limit switches are used in particular for the detection of levels or limit levels in containers for flowable and fluidizable media, in particular for liquids or bulk solids. Depending on the level in the container, the vibration level switches are in contact or not with the medium, so that an oscillation frequency of the membrane or of the mechanical oscillator arranged on the membrane is influenced by the contact with the medium.

The vibration sensors known from the prior art are often operated with a glued drive, wherein in this drive a disc-shaped piezoelectric element is glued with the interposition of a compensating element for adjusting the thermal expansion coefficient of the diaphragm and piezoelectric element with the membrane.

In order to set the membrane in vibration and to detect vibrations of the membranes and to be able to convert them into a measuring signal, the piezoelectric element has at least two electrical contacts. At least one first electrical contacting is applied to an upper side of the piezoelectric element and at least one second electrical contacting is applied to a lower side of the piezoelectric element. Typically, surface-applied metallizations are used for contacting the piezoelectric element.

In the piezoelectric actuators known from the prior art, the underside of the piezoelectric element, i. a membrane-facing surface of the piezoelectric element, usually contacted over the entire surface while the top of the piezoelectric element, d. H. the membrane of the pioneering surface of the piezoelectric element is provided either with one or more electrical contacts. For example, the upper side of the piezoelectric element can be contacted in a four-part segmented manner and two diagonally opposite segments can be electrically contacted together in each case. In this way it is possible via two of the contacts to couple a movement in the membrane and simultaneously detect a frequency and / or amplitude of the membrane via the other two contacts. If the surface is not contacted in a segmented manner, excitation and detection can only be carried out sequentially.

In the piezo elements typically used, a voltage is generated by the piezoelectric element which is proportional to a curvature of the piezoelectric element and can be tapped off via the contacts. Conversely, an applied voltage is converted into a proportional curvature of the piezo element and thus of the membrane.

Tuning fork-like arrangements are frequently used as mechanical oscillators, in which two paddles arranged symmetrically on the membrane and oriented parallel to one another are used for vibration transmission. These paddles usually have a rod-shaped coupling to the membrane and, like a paddle widened flat free end.

The vibration sensors known from the prior art merely measure a frequency shift occurring when a mechanical oscillator is immersed in the medium to be detected relative to the frequency of a zero resonance and derive a switching command from this frequency shift. The above-described excitation of two diagonally opposite segments stimulates the so-called clap mode, in which the paddles of the mechanical oscillator move toward and away from each other along a surface normal of the paddle.

In the drives used in the prior art, the resonant frequency of the Clap mode is coupled either in the full-surface contact of the piezoelectric element and the mechanical oscillator begins to oscillate due to the coupled frequency in the corresponding eigenmode or it is in segmented contacted piezoelectric actuators on two segments Natural frequency of the Clap mode coupled and performed on the two remaining segments, a detection of the forming vibration frequency.

When measurements are taken in highly viscous media, after covering the mechanical oscillator and subsequent removal of the medium, adhesions often occur or between the paddles of the mechanical oscillator, which likewise lead to a frequency shift, which can be interpreted as covering the mechanical oscillator. This situation leads after the withdrawal of the medium to a significantly delayed change in the switching state of the known from the prior art sensors. Sometimes it can also misdetections occur. This is perceived as a significant disadvantage in the prior art.

It is the object of the present invention to provide a vibration sensor which has a better detection behavior and fewer misdetections. Further, it is an object of the present invention to provide a method of operating such a vibration sensor by which misdetections are avoided.

These objects are achieved by a vibration sensor having the features of patent claim 1 and by a method for operating a vibration sensor having the features of patent claim 7.

Advantageous developments are specified in the dependent claims.

A vibration sensor according to the invention having a diaphragm which can be excited to vibrate by means of a piezoelectric drive and has a mechanical oscillator arranged on the diaphragm, which has at least two eigenmodes, and electronics for controlling the drive and for evaluating vibrations detected by means of the drive is characterized in that the drive comprises at least one piezoelectric element, wherein the piezoelectric element has at least four sectors that can be controlled separately from one another.

By a corresponding configuration of the piezoelectric element with at least four separately controllable sectors is achieved that in addition to the Clap mode used in the prior art exclusively for level detection also another eigenmode, in particular the clap-transverse mode in which the vibration elements of the mechanical Vibrate opposite and orthogonal to their respective surface normal, can be excited.

The present invention is based on the finding that for the detection of a covering of the mechanical oscillator with a medium, the Clap mode has a particularly high frequency shift relative to the empty resonance. If the clap-transverse mode is examined with the same coverage of the mechanical oscillator, this shows that this has a significantly lower frequency shift. In contrast, an adhesion to the mechanical oscillator acts as a sluggish mass on both the Clap mode and the Clap-Quer mode and causes an equally large relative frequency shift for both modes in comparison to the respective empty resonance, so that the presence an adhesion can be detected.

In contrast to the prior art, according to the present invention, all four sectors of the piezoelectric element are controlled simultaneously and specifically, so that a targeted excitation of different eigenmodes of the mechanical oscillator can take place by exciting a targeted deformation of the membrane.

For a targeted excitation of the different eigenmodes of the mechanical oscillator by coupling a targeted deformation in the membrane, it is advantageous if the four sectors of the piezoelectric element are contacted at least on one side of the piezoelectric element by four, substantially equal electrodes. Advantageously, the piezoelectric element is disc-shaped with a substantially circular base surface, wherein the sectors and the electrodes in each case occupy substantially a quarter of the base surface and corresponding to each other, in particular congruent, are arranged.

According to the present application, a substantially circular base area also includes deviating base surfaces which, for example, have an elliptical shape and / or have in their shape a base surface which is adapted to a vibration of the mechanical oscillator forming the membrane. Likewise, the electrodes, which occupy substantially one quarter of the base area, can be adapted to a deformation of the membrane which forms during a vibration of the mechanical oscillator. By an appropriate adaptation, for example, an increased electrical amplitude can be achieved, so that a further improved detection is made possible.

A corresponding arrangement of electrodes and sectors means in particular that each one sector of the piezoelectric element is associated with an electrode and electrically contacted this sector.

A simple electrical control of the piezoelectric element with a simple structure can be achieved if the piezoelectric element has a full-surface backside electrode on a second side. Due to the fact that the piezo element on the first side, in particular the upper side, is selectively contacted and on the second side, in particular the underside has a full-surface back electrode, a simple coupling of the ground potential to the back and a simple coupling a voltage excitation or tapping voltages for detection on the top can be done ,

It is advantageous if the piezoelectric element, the electrodes and the electronics are designed and matched to one another such that two adjacent sectors of the piezoelectric element and in a second control mode each two diagonally opposite sectors of the piezoelectric element can be excited in such a way that the piezoelectric element deforms rectified in these sectors. In a piezoelectric element glued directly or indirectly to the membrane, the membrane is either essentially curved in the same direction as a result of these two activation modes, as a result of which the clap mode can be excited or curved in opposite directions, whereby the clap-transverse mode can be excited. The oppositely curved membrane resembles a hyperbolic paraboloid or a saddle surface. The sector boundary lines of the piezo in each case form the node lines of the mechanical oscillation, wherein the rod-shaped connection of the paddle of the mechanical oscillator is ideally mounted directly on this node line.

If the piezoelectric element has at least two sections of different polarization direction, the above-mentioned deformations can be more easily coupled into the membrane and the ground potential of the backside electrode is in each case constant between the starting voltages due to the equivalent starting voltage necessary for a rectified deformation. By a bipolar excitation voltage, the ground potential can thus be coupled capacitively and a direct electrical contacting of the return electrode is not necessary.

According to the present invention, there is further provided a method of operating a vibration sensor, wherein a mechanical vibrator disposed on a diaphragm is excited to vibrate by means of a drive, and vibration of the mechanical vibrator is detected to detect a coverage state of the mechanical vibrator, the method thereby being characterized in that at least two different eigenmodes of the mechanical oscillator are alternately excited to detect the coverage state of the mechanical oscillator and of adhesions to the mechanical oscillator, and a respectively resulting oscillation is detected.

By alternately exciting different eigenmodes of the mechanical oscillator, on the one hand a covering of the mechanical oscillator with medium can be detected and on the other hand an adhesion to the mechanical oscillator can be detected.

Alternate within the meaning of the present application does not mean that the second excitation mode always necessarily follows the first excitation mode and vice versa, but may also mean in the sense of the present application that one of the excitation modes occurs several times. In this case, however, alternating means in particular that both the first and the second excitation mode are excited in one measurement cycle in each case.

Advantageously, the two different eigenmodes have a different direction of oscillation, these preferably being substantially orthogonal to one another. A particularly good detection can be achieved if a first eigenmode for detecting a covering oscillates along a surface normal of the mechanical oscillator and a second eigenmode for detecting an adhesion oscillates perpendicular to this surface normal and perpendicular to a longitudinal axis of the mechanical oscillator.

In the present application should be understood as surface normal of the normal vector to the largest of the surfaces of the mechanical vibrator or the respective vibrating elements.

In one embodiment of the present method, all sectors of the piezoelectric element to excite the first eigenmode are excited to a rectified deformation, wherein for the excitation of the second eigenmode each two diagonally opposite sectors of the piezoelectric element are excited to a rectified deformation.

For detecting a covering of the mechanical oscillator with medium or an adhesion, a frequency and / or an amplitude and / or an attenuation and / or phase relationship of the mechanical oscillator after the excitation are preferably analyzed. Since, according to the present invention, all sectors of the piezoelectric element are used for the targeted excitation of the different eigenmodes of the mechanical oscillator, it is not possible at the same time to perform a detection for excitation. Therefore, a sequential procedure is carried out in which the mechanical oscillator is first excited to oscillate in one of the eigenmodes and after the excitation a detection of frequency and / or Phase and / or amplitude and / or attenuation of the then decaying vibration takes place.

In order to realize the largest possible oscillation amplitude, it makes sense to adapt the frequency for excitation of the respective eigenmode to the frequency last detected at this eigenmode or to track this frequency. In this way, it is ensured that the mechanical oscillator is always excited in the range of its respective current resonant frequency, whether covered or uncovered, with or without adhesion.

It has proved to be advantageous if in each case an eigenmode between 20 ms and 80 ms, in particular between 40 ms and 60 ms and particularly advantageously for 50 ms excited and then between 20 ms and 80 ms, especially between 40 ms and 60 ms and especially advantageous for 50 ms the vibration is detected.

At a resonant frequency of the mechanical oscillator of about 1 kHz, this corresponds to an excitation of about 20 oscillation cycles of the mechanical oscillator, whereby it is also possible to adjust the duration of the excitation to the respectively necessary time for 20 oscillation cycles at the last detected resonant frequency.

Furthermore, it is also conceivable that the number of oscillation cycles is adapted to the attenuation occurring through the medium in order to obtain a largely constant amplitude in the case of reception in a certain measuring range.

Furthermore, it has proven to be advantageous for the evaluation of the decay behavior to evaluate a generated from the voltage applied to the individual piezoelectric sockets sum signal. This prevents that a superimposition of the two modes of vibration with a short time sequence of the two excitation modes changed the measurement result unfavorable.

The present invention will be explained below in detail with reference to the accompanying figures. Show it:

1 a mechanical vibration unit of a vibration sensor according to the present application,

2 four in accordance with the mechanical vibration unit 1 mainly self-developed modes,

3 a piezoelectric element with differently polarized sectors and the polarity of the applied electrical voltage for generating the eigenmodes 100 and 130 according to 2 and

4 the maximum occurring vibration amplitudes upon excitation of the piezoelectric element 201 according to 3 according to the in 211 and 221 specified control.

1 shows in a perspective view a mechanical vibration unit of a vibration sensor according to the present application.

The mechanical vibration unit is essentially made of a vibrating in a peripheral edge 30 clamped membrane 5 built up, taking on the membrane 5 a mechanical oscillator 10 is arranged. The mechanical oscillator 10 is formed in the present embodiment as a tuning fork with two vibration elements, wherein the two vibration elements each as via a coupling 102 on the membrane 5 attached paddles 101 are formed. The paddles 101 are aligned substantially parallel to each other and symmetrical to a plane of symmetry S via the couplings 102 on the membrane 5 attached.

In 1 Further, the surface normal is one of the paddles 101 located. Advantageously, the paddles are planarized to maximize the difference in interaction with the media / adhesion between the two stimulable modes. However, since the paddles can also have more complex surface geometries, the surface vector n of the normal vector is understood to be the plane formed by the largest surface extent.

In 2 is a top view of the membrane 5 of the mechanical vibration unit 1 shown, wherein the vibration elements of the mechanical oscillator 10 are shown schematically in this plan view. In particular, there are the paddles 101 as well as their coupling 102 to recognize, with the couplings 102 arranged symmetrically to the plane of symmetry S and the paddles 101 are aligned parallel to each other. In the four subfigures of 2 , with 100 . 110 . 120 and 130 are designated, which are in a mechanical vibration arrangement according to 1 mainly occurring first four eigenmodes of the mechanical vibrator 10 schematically drawn.

With 100 is the so-called clap-mode or gossip mode in which both paddles 101 opposite and in push-pull along the surface normals n of the paddles 101 swing. This plane of symmetry S forms a mirror plane for this mode.

With 110 is the so-called Sway-mode, in which both paddles 101 rectified, ie in common mode in the direction of the surface normal n of the paddles 101 swing.

With 120 is the so-called Sway-Quer-Mode, in which the paddles 101 in normal mode perpendicular to the surface normal n of the paddle and thus parallel to the plane of symmetry S, ie they swing in the plane defined by the paddle surface.

With 130 Finally, the clap-cross fashion is featured in both paddles 101 opposite, ie in push-pull perpendicular to the surface normal n of the paddle 101 and thus swing parallel to the plane of symmetry S.

The direction of movement of the paddles is indicated by the arrows 101 indicated at the beginning of a period of oscillation.

Depending on the design of the paddle 101 , Membrane 5 and connection 102 to the membrane 5 The order of the modes may differ in their frequency.

For the detection of the covering of the mechanical oscillator 10 with medium or for adhesion detection, the clap or clap-cross mode are particularly suitable. The use of the Clap mode for detecting a coverage with medium corresponds to the state of the art. Dive the mechanical oscillator 10 In a medium to be detected, the resonance frequency and amplitude of the Clap mode change depending on the density and viscosity of the medium. With complete coverage with medium, the clap-cross mode has a significantly lower frequency shift compared to the clap mode. The mechanical oscillator vibrates 10 however, free and with a strong adhesion of medium, the sluggish mass of adhesion acts on both modes equally and the relative frequency shift due to the adhesion is comparable in both modes. Thus, by using and comparing the frequencies of clap and clap-cross modes, adhesion can be actively detected and sensor mismatch due to adhesions and the resulting frequency shift avoided.

A suitable drive for this purpose 3 can be, for example, a sectored polarized piezoelectric element 7 with 90 ° electrodes 71 . 71 . 73 . 74 accomplish. Three possible polarizations of the piezoelectric element 7 are in 3 in the sections 201 . 202 and 203 shown.

On the one hand opposite halves of the piezoelectric element can be polarized equal, each half each two 90 ° electrodes 71 . 74 ; 72 . 73 having.

This variant is in section 201 shown.

In a second embodiment, in section 202 is shown, is the piezoelectric element 7 divided into four segments, these are each polarized alternately. The segments are each 90 ° in size and with corresponding electrodes 71 . 72 . 73 . 74 Mistake.

Furthermore, it would also be a uniform polarization of the piezoelectric element 7 possible, with a division into sectors by four 90 ° electrodes.

In 3 symbolizes ⊗ a polarization into the drawing plane and ⊙ out of the drawing plane.

Each of the illustrated piezoelectric elements 7 opposite polarized piezo elements describe piezo elements 7 the same kind. To the same deformation of the oppositely polarized piezo element 7 In comparison with the illustrated polarizations, only the polarity of the applied AC voltage has to be rotated. At the same applied AC voltage, the same deformation is assumed half a period later (180 ° out of phase).

211 . 212 and 213 shows the polarity of the AC voltage to excite the Clap mode and 221 . 222 and 223 the polarity of the AC voltage to excite the clap-cross mode.

Particularly advantageous proves to be 201 characterized variant, since for the two modes to be excited (Clap and Clapquer) each two adjacent sectors, ie one half of the piezoelectric element 7 with a first AC voltage in 3 marked with "+" and the other half with a 180 ° out of phase AC voltage, in 3 must be stimulated with "-" marked. This suggestion is in 3 symbolic in the sections 211 and 221 shown. Due to the fact that both voltages are phase-shifted by 180 °, the ground potential is always exactly between the two voltages. Thus, the ground electrode on the back of the piezoelectric element does not have to be contacted separately and can simply capacitive to the membrane 5 on the the piezo element 7 is glued, coupled.

The activation of the piezo element 7 takes place in an advantageous manner by a sinusoidal voltage on the resonant frequency, since the frequency spectrum has only one frequency and not, for example. In a square-wave voltage has a plurality of harmonics, which can stimulate other vibrational modes in addition to the mode to be excited. In order to ensure optimum drive, the relative orientation of the piezo element glued to the membrane should be 7 be related to the tuning fork such that the connection of the paddles 102 to the membrane 5 exactly on one of the sector lines of the piezoelement 7 lies.

For the evaluation of the decay behavior, it has been found to be particularly advantageous, the voltages which at the piezoelectric element 7 due to the mechanical deflection of the membrane 5 by the decay of the mechanical oscillator 10 arise, according to the polarization and deflection of the individual sectors 71 . 72 . 73 . 74 to add. As a result, a crosstalk of the decay phases is prevented in the corresponding subsequent decay phase of the other mode, since corresponding voltage components cancel each other out. In the case of polarization according to 201 V11 is the voltage applied to the electrode 11 , V12 the electrode 12 , V13 to the electrode 13 and corresponding to V14, the electrode 14 during swinging out of the piezo element 7 is applied. In the case of the clap mode, it is thus advantageous to have the voltage V12 + V13 - (V11 + V14) or V11 + V14 - (V12 + V13) and for the clap (transverse) mode the voltage V13 + V14 - (V12 + V11) or V12 + V11 - (V13 + V14). The signs in the summation result from the polarization or deflection of the individual sectors 71 . 72 . 73 . 74 ,

4 shows the logarithmically plotted oscillation amplitude (simulation) as a function of the excitation frequency at the tip of a paddle 101 along the surface normals n of the paddle 101 (x-direction), and perpendicular thereto (y-direction) when excited with a sectored piezoelectric element 7 according to the in 201 illustrated embodiment. With 310 is the amplitude in the x-direction with polarity of the starting voltage according to 211 shown. 311 shows the amplitude in y-direction with polarity of the starting voltage according to 221 , These two amplitudes are many times greater than the amplitude in the y direction 312 with polarity of the AC voltage according to 221 or the amplitude in the x direction 313 with polarity of the AC voltage according to 211 , This shows that it is possible to excite the resonance of the clap and clap-cross modes largely independently of each other, so that the movement in the direction perpendicular to the excited mode is negligible. Furthermore, the two Sway modes are excited only very slightly by this drive, so that they remain without influence.

LIST OF REFERENCE NUMBERS

1

vibration sensor

3

drive

5

membrane

7

piezo element

10

mechanical oscillator

11

first electrode

12

second electrode

13

third electrode

14

fourth electrode

15

Back electrode

30

edge

71

first sector

72

second sector

73

third sector

74

fourth sector

101

paddle

102

coupling

100

Clap mode

110

Sway mode

120

Sway-bar mode

130

Clap-transverse mode

f

frequency

A

amplitude

D

damping

n

surface normal

S

axis of symmetry

Claims (18)

Vibration sensor ( 1 ) with a piezoelectric drive ( 3 ) to a vibration excitable membrane ( 5 ) with a arranged on the membrane mechanical oscillator ( 10 ) with at least two eigenmodes ( 100 . 110 . 120 . 130 ) and an electronics for controlling the drive ( 3 ) and for evaluation by means of the drive ( 3 ) detected vibrations, characterized in that the drive ( 3 ) at least one piezoelectric element ( 7 ), wherein the piezoelectric element ( 7 ) at least four separately controllable sectors ( 71 . 72 . 73 . 74 ) having. Vibration sensor ( 1 ) according to claim 1, characterized in that the four sectors on a first side of the piezoelectric element by four substantially equal-sized electrodes ( 11 . 12 . 13 . 14 ) are contacted. Vibration sensor ( 1 ) according to one of the preceding claims, characterized that the piezo element ( 7 ) is disc-shaped with a western circular base and the sectors ( 71 . 72 . 73 . 74 ) and the electrodes ( 11 . 12 . 13 . 14 ) occupy each substantially a quarter of the base area and are arranged corresponding to each other. Vibration sensor ( 1 ) according to one of the preceding claims, characterized in that the piezo element ( 7 ) has on a second side a full-surface back electrode. Vibration sensor ( 1 ) according to one of the preceding claims, characterized in that the piezo element ( 7 ), the electrodes ( 11 . 12 . 13 . 14 ) and the electronics are designed and matched to each other such that in a first drive mode two adjacent sectors of the piezoelectric element and in a second drive mode, two diagonally opposite sectors of the piezoelectric element can be excited such that the piezoelectric element ( 7 ) deformed rectified in these sectors. Vibration sensor ( 1 ) according to one of the preceding claims, characterized in that the piezo element ( 7 ) has at least two sections of different polarization. Method for operating a vibration sensor ( 1 ), in which a on a membrane ( 5 ) arranged mechanical oscillator ( 10 ) by means of one drive ( 3 ) is excited to a vibration and a vibration of the mechanical vibrator ( 10 ) for detecting a covering state of the mechanical vibrator ( 10 ) is detected, characterized in that for detecting the covering state of the mechanical oscillator ( 10 ) and adhesions to the mechanical oscillator ( 10 ) at least two different eigenmodes ( 100 . 110 . 120 . 130 ) of the mechanical oscillator ( 10 ) are alternately excited and a respective resulting vibration is detected. Method according to claim 7, characterized in that the eigenmodes ( 100 . 110 . 120 . 130 ) have a different direction of vibration. Method according to claim 8, characterized in that the oscillation directions of the eigenmodes ( 100 . 110 . 120 . 130 ) are substantially orthogonal to each other. Method according to one of claims 7 to 9, characterized in that a first eigenmode ( 100 ) for detecting a covering along a surface normal (s) of the mechanical oscillator oscillates and a second eigenmode ( 130 ) for detecting adhesion perpendicular to the surface normal (s). Method according to claim 10, characterized in that for excitation of the first eigenmode ( 100 ) two adjacent sectors of the piezoelectric element ( 7 ) are excited to a rectified deformation and to excite the second eigenmode ( 130 ) two diagonally opposite sectors of the piezoelectric element ( 7 ) are excited to a rectified deformation. Method according to one of claims 7 to 11, characterized in that for detection a frequency (f) and / or amplitude (A) and / or damping of the mechanical oscillator ( 10 ) is analyzed after the suggestion. Method according to claim 12, characterized in that the frequency (f) for excitation of the respective eigenmode ( 100 . 110 . 120 . 130 ) to the last in this eigenmode ( 100 . 110 . 120 . 130 ) detected frequency is adjusted. Method according to one of Claims 7 to 13, characterized in that a respective eigenmode ( 100 . 110 . 120 . 130 ) between 20 ms and 80 ms, in particular between 40 ms and 60 ms, in particular for 50 ms, excited and then between 20 ms and 80 ms, in particular between 40 ms and 60 ms, in particular for 50 ms is detected. Method according to one of claims 7 to 14, characterized in that the number of pickup cycles, in accordance with the attenuation occurring by the medium to be detected, is adjusted in order to ensure an approximately constant receiving amplitude in a predetermined measuring range. Method according to one of the preceding claims 7 to 15, characterized in that an evaluation by addition of electrodes ( 11 . 12 . 13 . 14 ) at respectively perpendicular to the oscillation direction of the excited mode adjacent adjacent voltages (V11, V12, V13, V14) and subtraction of the sums takes place. Method according to claim 16, characterized in that the voltages (V11, V12, V13, V14) of the electrodes ( 11 . 12 . 13 . 14 ) according to their polarization positively or negatively enter into the summation, wherein the voltages (V11, V12, V13, V14) are considered positive in a polarization in the drawing plane and negative in a polarization from the plane of the drawing. A method according to claim 16 or 17, characterized in that the difference formation takes place in such a way that crosstalk of the voltage sum detected by the first mode to a voltage sum detected by the second mode is avoided.

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