WO2018059754A1

WO2018059754A1 - Vibration sensor and method for operating a vibration sensor

Info

Publication number: WO2018059754A1
Application number: PCT/EP2017/066226
Authority: WO
Inventors: Levin Dieterle
Original assignee: Vega Grieshaber Kg
Priority date: 2016-09-29
Filing date: 2017-06-29
Publication date: 2018-04-05
Also published as: DE102016118445A1

Abstract

The invention relates to a vibration sensor (1) comprising a membrane (5) that can be stimulated so as to oscillate by means of a piezoelectric drive (3), a mechanical oscillator (10) which is arranged on the membrane and has at least two eigenmodes (100, 110, 120, 130), and an electronic system for controlling the drive (3) and for evaluating oscillations detected by the drive (3), wherein the drive (3) comprises at least one piezoelectric element (7), the piezoelectric element (7) having at least four separately controllable sectors (71, 72, 73, 74).

Description

Vibration sensor and method for operating a vibration

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

From the prior-art vibration sensors exhibit a piezoelectric actuator to oscillate stimulable membrane having a membrane arranged on the mechanical ^¬ African transducer and an electronic unit for controlling the drive and for the evaluation of the detected vibrations.

These are often used as vibratory

Level switch with a vibratable Memb ^¬ ran and a drive for moving the membrane in oscillation and / or for picking up a vibration of the membrane and ei ^¬ nem arranged on the membrane mechanical oscillator, often being used as a drive komkom ^¬ piezoelectric elements. Such vibration level switches are used insbeson ^¬ particular for the detection of levels or limit levels in Behäl ^¬ tern for flowable and fluidizable media, especially for liquids or solids. The vibrato ^¬ ons level switch are sorted by level in the container with the medium in contact with or not, so that a Schwingfre ^acid sequence of the membrane or of which is arranged on the membrane mecha ^¬ African transducer is influenced by the contact with the medium.

The known from the state of vibration sensors are often operated with a glued drive, wherein in this on ^¬ triebsart a disc-shaped design piezo element under Interposition of a compensation element for adjusting the thermal expansion coefficients of membrane and piezoelectric element is glued to the membrane.

To enable the diaphragm to vibrate and detect vibrations of the membranes and to be able to be converted into a measurement signal, the piezoelectric element know at least two electrical PLEASE CONTACT ^¬ requirements. 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 applied flat Metallisierun ^¬ gen be used for contacting of the piezo element.

In the case of the piezoelectric drives known from the prior art, the underside of the piezo element, ie a surface of the piezo element facing the diaphragm, generally contacts the entire surface while the top of the piezo element, ie the surface of the piezo element facing away from the diaphragm, either with is one or provided with several electrical contacts ^¬ rule. For example, the upper surface of the piezoelectric element may be contacted four times segmented and it can be in each case two diagonally opposite contacted at ^¬ parent segments together electrically. 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 Amplitu ^¬ de the membrane via the other two contacts. If the surface is not contacted segmen ^¬ advantage, as excitation and detection can be performed only se ^¬ quentiell.

In the piezo elements typically used, a voltage is generated by the piezoelectric element, which is proportional to is a curvature of the piezoelectric element and can be tapped on the Kontaktie ^¬ ments. Conversely, an applied voltage is converted into a proportional curvature of the piezo element and thus of the membrane.

As mechanical oscillators are often tuning fork similar from ^¬ formed arrangements are used, in which two symmetrically arranged on the Memb ^¬ ran arranged and parallel to each other ^¬ tete paddles for vibration transmission. These paddles usually have a rod-shaped coupling to the membrane and, like a paddle widened flat free end.

The known from the prior art vibration sensors measure only one when immersing a mechanical

Oscillator occurring in the medium to be detected Fre ^¬ quenzverschiebung relative to the frequency of a Leerresonanz and derive from this frequency shift from a switching command. By the above described excitation of two diagonally opposite ^¬ opposing segments, the so-called Clap-mode is excited, in which moving the paddles of the mechanical oscillator along a surface normal of the paddle towards and away from each other.

Is at the used in the prior art drives entwe ^¬ the coupled resonant frequency of the Clap-mode at the full-surface contact with the piezoelectric element and the mechanical ^¬ rule oscillator starts due to the injected frequency in the corresponding eigenmode to swing or it will at segmented contacted piezo drives coupled to two segments, the natural frequency of the Clap mode and performed on the two remaining segments, a detection of the forming vibration frequency. For measurements in highly viscous media it occurs frequently on to Be ^¬ cover the mechanical oscillator and subsequent deduction of the medium to buildup or between the paddles of the mechanical oscillator, which also lead to a Frequenzver ^¬ shift, which as a covering of the mechanical

Schwingers can be interpreted. This situation leads to the withdrawal of the medium to a significantly delayed change ^¬ tion of the switching state of the known from the prior art ^¬ th sensors. Sometimes it can also misdetections occur. This is perceived in the prior art as a significant after ^¬ part.

It is the object of the present invention to provide a vibrato ^¬ onssensor available, the onsverhalten a better Detekti- and has fewer incorrect detections. 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 dependent Patentansprü ^¬ chen.

An inventive vibration sensor with a stimulable via a piezoelectric drive Memb ^¬ ran with a arranged on the membrane mechanical Schwin ^¬ ger, which has at least two eigenmodes, and a Elekt ^¬ ronik for controlling the drive and for the evaluation of The vibrations detected by means of the drive are characterized in that the drive comprises at least one piezoelectric element, the piezoelectric element having at least four sectors which 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 Oscillator opposite and orthogonal to their respective surfaces ^¬ normal swing, can be excited.

Thus the present invention is based on the knowledge Grun de ^¬ that has a coverage for the detection of the mechanical oscillator with a medium, the Clap mode 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-cross mode and causes an equally large rela ^¬ tive frequency shift in both modes compared to the respective idle resonance, so the presence of an adhesion can be detected.

In contrast to the prior art, the vorlie ^¬ constricting invention will be driven all the four sectors of the piezoelectric element is equal ^¬ early and specifically in accordance with, so that by excitation of a ge ^¬ aimed deformation of the diaphragm sub targeted excitation different eigenmodes of the mechanical oscillator can be done.

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 occupy substantially a quarter of the base area and korres ^¬ pondierend each other, in particular congruent, are arranged.

According to the present application, a substantially circular base area also includes ground surfaces deviating ^therefrom , which, for example, have an elliptical shape

and / or have in their shape a base surface adapted to a deformation of the membrane that forms during oscillation of the mechanical oscillator. Likewise, can be the one adapted to be forming at a vibration of the mechanical oscillator in the deformation of the diaphragm Wesentli ^¬ chen a quarter of the base engaging electrodes. By a corresponding adaptation, for example an increased electrical amplitude can be obtained, so that a further improved ver ^¬ detection is made possible.

A corresponding arrangement of electrodes and sectors means, in particular, that each one sector of the piezoelement is assigned an electrode and electrically contacts this sector. A simple electrical control of the piezoelectric element with a simple design can be achieved if the piezoelectric element has a full-surface back ^¬ side electrode on a second side. Characterized in that the piezoelectric element on the first side, in particular the upper side, is selectively kontak ^¬ advantage and on the second side, in particular the lower ^¬ side has a full-surface back electrode, ei ^¬ ne simple coupling of the ground potential to the back and a simple coupling a voltage excitation or tapping of voltages for detection on the top he ^¬ follow.

It is advantageous to have when the piezoelectric element, the electrodes and the electronics configured and matched to each other that when a first drive mode two mutually Benach ^¬ disclosed sectors of the piezoelectric element and in a second Ansteu ^¬ ermodus two mutually diagonally opposite sectors of the piezo element in such a way are excitable that the Piezoe ^¬ element deformed rectified in these sectors. The membrane whereby the Clap-transverse mode can be excited, in a directly or indirectly bonded with the membrane Piezoe ^¬ lement by these two drive modes either substantially curved in the same direction, whereby the Clap mode can be excited, or oppositely curved. The oppositely curved membrane resembles a hyperbolic paraboloid or egg ^¬ ner saddle surface. The sector boundary lines of the piezo form here in each case the nodal lines of the mechanical vibration of, said rod-shaped connection of the paddle of the mechanical ^¬ shear vibrator is ideally placed directly off against this nodal line.

When the piezo element at least two portions different ^¬ Licher polarization direction can including The indicated above deformations are even easier coupled into the membrane and the ground potential of the back electrode is due to the necessary for a rectified deformation countervailing starting voltage in each case constant between the on ^¬ regespannungen. 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 invention, a method for operating a vibration sensor, in which a arranged on a diaphragm mechanical oscillator by means of a drive to a

Oscillation is excited and detects a vibration of the mechanical oscillator for detecting a coverage state of the mechanical ^¬ rule vibrator, wherein the method is characterized in that for detecting the state of coverage of the mechanical oscillator and buildup on the mechanical ^¬ African vibrator at least two different eigenmodes of the mechanical oscillator are alternately excited and a respective resulting vibration 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. Alternately, however, this means in particular that in one measurement cycle in each case probably the first and the second excitation mode is excited.

Advantageously, the two different Eigenmo ^¬ a different vibration direction, which are preferably substantially orthogonal to each other. 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 is intended as a surface normal of nor ^¬ paint vector on the largest of the surfaces of the mechanical

Vibrator or the respective oscillating elements understood ^¬ who.

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

For detecting a coverage of the mechanical oscillator with a medium or adhesion of a frequency and / or amplitude and / or attenuation and / or Pha ^¬ senbeziehung of the mechanical oscillator are preferably analyzed after excitation. As used under different ^¬ union eigenmodes of the mechanical oscillator according to the present invention, all ^¬ sec tors of the piezo element for selective excitation of, it is not possible simultaneously to excite a To perform detection. Therefore, a sequential ^proceeding 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 oscillation takes place ,

In order to realize the greatest possible vibration amplitude, it is appropriate to adjust the frequency to excite the respective eigenmode to the last detected in this eigenmode Fri ^acid sequence or to track them. In this way it is ensured that the mechanical oscillator always Be ^¬ rich its respective actual resonance frequency, whether, is excited with or without attachment covered or uncovered.

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 beneficial for 50 ms the

Oscillation is detected.

At a resonant frequency of the mechanical oscillator of about 1 kHz, this corresponds to an excitation of approximately 20 oscillations ^¬ supply cycles of the mechanical oscillator, wherein the Mög ^¬ friendliness is, the duration of the excitation at each of not ^¬ manoeuvrable time for 20 cycles of oscillation, wherein Adjust the last detected resonant frequency.

Furthermore, it is also conceivable that the number of vibration ^¬ cycles adapted to the attenuation occurring through the medium is to get in a certain range, a largely constant amplitude in the case of reception.

Furthermore, it has proven to be advantageous for the evaluation of the decay behavior to evaluate a sum signal generated from the voltages applied to the individual piezoelectric sectors. This prevents that a superimposition of the two vibration modes in 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. In the drawings: Figure 1 is a mechanical oscillation unit of a vibration ^¬ sensor according to the present application.

Fig. 2 four in the mechanical vibration unit

according to FIG. 1, mainly self-developed modes,

Fig. 3 shows a piezoelectric element with differently polarized

Sectors and the polarity of the applied electrical see voltage for generating the eigenmodes 100 and

130 according to Figure 2 and

4 shows the maximum oscillation amplitudes occurring upon excitation of the piezoelectric element 201 according to FIG. 3 in accordance with the control specified in FIGS. 211 and 221.

1 shows a perspective view of a me chanical ^¬ oscillation unit of a vibration sensor according to the present application. The mechanical vibration unit is basically capable of oscillation from egg ^¬ ner in a circumferential edge 30 is spun ^¬ th membrane 5, wherein on the diaphragm 5 a mechanical ^¬ shear oscillator 10 is disposed. The mechanical oscillator 10 is formed in the present embodiment as a tuning fork with two vibration elements, the two

Vibration elements are each formed as attached via a coupling 102 on the diaphragm 5 paddle 101. The pad ^¬ del 101 are aligned substantially parallel to each other and symmetrically to a plane of symmetry S via the couplings 102 on the membrane 5 attached.

FIG. 1 also shows the surface ^normal n of one of the pads 101. Advantageously, the paddles are planarized to maximize the difference in interaction with the media / adhesion between the two stimulable modes. But since the paddles may have more complex geometries Oberflächengeo ^¬ is considered a surface normal n of norma ^¬ len vector to the gebil- by the largest surface extension finished level.

FIG. 2 shows a top view of the membrane 5 of the mechanical vibration unit from FIG. 1, wherein the vibration elements of the mechanical oscillator 10 are shown schematically in this plan view. In particular, the paddles 101 and their coupling 102 can be seen, wherein the couplings 102 are arranged symmetrically to the plane of symmetry S and the paddles 101 are aligned parallel to each other. In the four sub-figures from Figure 2, which are denoted by 100, 110, 120 and 130 which occur primarily in a mechanical vibrating assembly according to Figure 1 first four eigenmodes of the mechanical oscillator 10 are schematic ^¬ table located. With 100 the so-called Clap mode or gossip mode is marked in ^¬ characterized wherein both paddle 101 against the same and in the opposite ^¬ clock along the surface normal of the paddle 101 n oscillation gene. This symmetry plane S forms a Spie ^¬ gelebene for this mode.

110 is the so-called Sway-mode characterized in the two paddles 101 rectified, d. H. oscillate in the same direction in the direction of the surface normal n of the paddle 101.

120 the so-called Sway-transverse-mode is characterized, in which the paddle 101 in the common mode perpendicular to the surfaces ^¬ normal n of the paddles and thus parallel to the symmetry plane S, that is, they oscillate in the plane defined by the paddle FLAE ^¬ surface.

Finally, at 130, the clap-cross mode is indicated, in which both paddles 101 are equal to each other, i. H. in the push-pull perpendicular to the surface normal n of the paddle 101 and thus swing parallel to the plane of symmetry S.

With the drawn arrows each movement Rich ^¬ processing of the paddle is ben 101 angege- 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 coverage of the mechanical vibrator 10 with medium or for adhesion detection, the clap or clap-transverse mode are particularly suitable. The use of clap Mode for detecting a coverage with medium corresponds to the prior art. If the mechanical oscillator 10 immerses in a medium to be detected, the resonance frequency and amplitude of the Clap mode change as a function of 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. Swinging the mechanical oscillator 10 but is free and has a strong adhesion of medium affects the inertial mass of the adhesion to both modes alike and the relative frequency shift ^¬ environment due to adhesion is similar 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 drive 3 suitable for this purpose can be accomplished, for example, by a sectored polarized piezoelectric element 7 with 90 ° electrodes 71, 71, 73, 74. Three possible polarizations of the piezoelectric element 7 are shown in the sections 201, 202 and 203 in FIG. 3.

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

This variant is shown in section 201.

In a second embodiment, which is shown in section 202, the piezoelectric element 7 is divided into four segments, these being circulating alternately polarized in each case. The segments are each 90 ° large with this ^¬ correspond to the electrodes 71, 72, 73, 74 is provided. Furthermore, a uniform polarization of the piezoelectric element 7 would also be possible, wherein a division into sectors is effected by four 90 ° electrodes.

In Figure 3 ® symbolizes a polarization in the plane ^¬ into and O out of the plane of the drawing.

The counteracted respectively to the shown piezoelectric elements 7 is polarized piezo elements describe this Piezoele ^¬ elements 7 of the same type. At the same deformation of the entgegenge ^¬ is polarized piezoelectric element 7 in comparison to the Darge ^¬ presented polarizations to reach, must only the Po ^¬ larity of the applied AC voltage to be rotated. For the same applied alternating voltage, the same deformation ei ^¬ ne half period later assumed (180 ° out of phase).

211, 212 and 213 shows the polarity of the AC voltage to the Clap mode ^{¬ on} and 221, 222 and 223, the polarity of the AC voltage for exciting the clap-transverse mode.

Particularly advantageous is the variant marked 201, since for the two modes to be excited (Clap and Clap- quer) two adjacent sectors, ie one half of the piezoelectric element 7 with a first alternating voltage in Figure 3 with "+" and the other half with a phase shifted by 180 ° alternating voltage, in Figure 3 - labeled in ^¬ records must be excited This excitation is shown in Figure 3 symbolically in sections 211 and 221 ".".

Characterized in that both voltages are phase shifted by 180 °, the ground potential is always exactly between the two clamping ^¬ voltages. Thus, the ground electrode on the rear side of the piezoelectric element does not have to be contacted separately and can capacitive capacitive to the membrane 5 to which the piezoelectric element 7 is adhered, are coupled.

The actuation of the piezoelectric element 7 is advantageously carried out by a sinusoidal voltage on the Resonanzfre ^¬ frequency, since the frequency spectrum has only one frequency and ^¬ not, for example, in a square-wave voltage has a plurality of harmonics, which in addition to the ^{¬ to} moving Mode can stimulate other vibration modes. To ensure optimum drive, the relative

Orientation of the adhered to the membrane piezoelectric element 7 relative to the tuning fork be such that the connection of the paddle 102 to the diaphragm 5 is exactly on one of Sektorli ^¬ nien of the piezoelectric element 7.

For the evaluation of the decay behavior, it has been found to be particularly advantageous, the voltages which arise at the piezoelectric element 7 by the mechanical deflection of the Memb ^¬ ran 5 by the decay of the mechanical oscillator 10, 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 the polarization according to 201, Vll is the voltage applied to the electrode 11, V12 to the electrode 12, V13 to the

Electrode 13 and corresponding to V14 abuts the electrode 14 during the swinging out of the piezoelectric element 7. 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 + for For V11) or V12 + V11- (V13 + V14) ^¬ values. The signs in the summation arise by the polarization or bending of the individual Sekto ^¬ ren 71,72,73,74.

Fig. 4 shows the logarithmic applied vibration is amplitude (simulation) as a function of the excitation frequency at the tip of a paddle 101 along the surface of Norma ^¬ len n of the paddle 101 (x-direction) and perpendicular thereto (y direction) when excited with a sectored piezoelectric element 7 according to the embodiment shown in 201. The amplitude in the x-direction with polarity of the starting voltage according to 211 is shown at 310. 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 y-direction 312 with polarity of the alternating voltage according to 221 or the amplitude in x-direction 313 with polarity of AC voltage according to 211. This shows that it is possible to stimulate the resonance of the clap and clap-cross mode largely independently of each other, so that the movement in the direction perpendicular to the an ^¬ excited mode is negligible. Furthermore, the two Sway modes are only very slightly excited by this drive, so that they have no influence.

LIST OF REFERENCE NUMBERS

I vibration sensor 3 drive

5 membrane

7 piezo element

10 mechanical oscillator

II first electrode

12 second electrode

13 third electrode

14 fourth electrode

15 rear side electrode

30 edge

71 first sector

72 second sector

73 third sector

74 fourth sector

101 paddles

102 coupling

100 clap mode

110 Sway Fashion

120 Sway-cross-fashion

130 Clap-Quer-Mode frequency

Ampl itude

Attenuation n Surface normal S Symmetry axis

Claims

Vibration sensor (1) having a piezoelectric ^¬ a rule drive (3) excitable to a vibrating diaphragm

(5) with a diaphragm arranged on the mechanical oscillator (10) with at least two eigenmodes (100, 110, 120, 130) and electronics for controlling the on ^¬ drive (3) and for evaluation by means of the drive

(3) detected vibrations,

d a d u r c h e c e n c i n e s that

the drive (3) comprises at least one piezoelectric element (7), wherein the piezoelectric element (7) has at least four separated at ^¬ controllable sectors (71, 72, 73, 74).

A vibration sensor (1) according to claim 1,

d a d u r c h e c e n c i n e s that

the four sectors on a first side of the piezoelectric element are contacted by four substantially equal-sized electrodes (11, 12, 13, 14).

Vibration sensor (1) according to one of the preceding claims ^¬

d a d u r c h e c e n c i n e s that

the piezoelectric element (7) is disc-shaped with circular in Western base and the Sekto ^¬ ren (71, 72, 73, 74) and the electrodes (11, 12, 13, 14) each substantially assume a quarter of the base area a ^¬ and are arranged corresponding to each other.

Vibration sensor (1) according to one of the preceding claims ^¬,

d a d u r c h e c e n c i n e s that

the piezoelectric element (7) on a second side has a complete having flat rear side electrode.

Vibration sensor (1) according to one of the preceding claims ^¬,

d a d u r c h e c e n c i n e s that

the piezoelectric element (7), the electrodes (11, 12, 13, 14) and designed the electronics so and are coordinated so that each diagonal in a first driving mode in each case two Einan ^¬ the adjacent sectors of the piezoelectric element and in a second control mode, two against ^¬ overlying sectors of the piezoelectric element are excitable such that the piezoelectric element (7) deformed rectified in these sectors.

Vibration sensor (1) according to one of the preceding claims ^¬,

d a d u r c h e c e n c i n e s that

has the piezoelectric element (7) at least two portions under ^¬ schiedlicher polarization.

A method of operating a vibration sensor (1) in which is arranged an at a membrane (5) mechanical oscillator (10) is excited to oscillations ^¬ supply by means of one drive (3) and a vibration of the mechanical oscillator (10) for detecting a cover state the mechanical oscillator (10) is detected,

d a d u r c h e c e n c i n e s that

for detecting the state of coverage of the mechanical oscillator (10) and buildup on the mechanical oscillator (10) at least two different Eigenmo ^¬ (100, 110, 120, 130) of the mechanical oscillator (10) are excited alternately and each resultieren- de vibration is detected. Method according to claim 7,

d a d u r c h e c e n c i n e s that

the eigenmodes (100, 110, 120, 130) have a different one

Have direction of vibration.

Method according to claim 8,

d a d u r c h e c e n c i n e s 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,

d a d u r c h e c e n c i n e s that

a first eigenmode (100) for detecting a blanket along a surface normal (s) of the mechanical

Oscillator oscillates and a second eigenmode (130) for detecting an adhesion perpendicular to the surface normal ^¬ len (n) oscillates.

Method according to claim 10,

d a d u r c h e c e n c i n e s that

to excite the first eigenmode (100) for each two nebenei ^¬ Nander opposite sectors of the piezo element (7) are set into a rectified deformation and at ^¬ excitation of the second eigenmode (130) for each two diagonally ge ^¬ genüberliegende sectors of the piezo element (7) to a rectified deformation are excited.

Method according to one of claims 7 to 11,

d a d u r c h e c e n c i n e s that

for detecting a frequency (f) and / or amplitude (A) and / or damping of the mechanical oscillator (10) according to the suggestion is analyzed. Method according to claim 12,

d a d u r c h e c e n c i n e s that

the frequency (f) for excitation of the respective eigenmode (100, 110, 120, 130) is adapted to the frequency last detected at this eigenmode (100, 110, 120, 130).

Method according to one of claims 7 to 13,

d a d u r c h e c e n c i n e s that

each one eigenmode (100, 110, 120, 130) between 20 ms and 80 ms, in particular between 40 ms and 60 ms, wei ^¬ ter in particular for 50 ms, excited and then between 20 ms and 80 ms, especially between 40 ms and 60 ms, in particular for 50 ms is detected.

Method according to one of claims 7 to 14,

d a d u r c h e c e n c i n e s that

is adapted to the number of the excitation cycles, corresponding to occur ^¬ the attenuation by the medium to be detected, to ensure an approximately constant reception amplitude in a pre-enclosed measuring area.

Method according to one of the preceding claims 7 to 15,

d a d u r c h e c e n c i n e s that

an evaluation by the addition of electrodes (11, 12, 13, 14) takes place in each case perpendicular to the direction of oscillation of the excited fashion contiguous applied clamping voltages ^¬ (Vll, V12, V13, V14), and the difference of the sum ^¬ men.

17. The method according to claim 16,

d a d u r c h e c e n c i n e s that

the voltages (Vll, V12, V13, V14) of the electrodes (11, 12, 13, 14) received according to their polarization positive or negative in the summation, the Spannun ^¬ gene (Vll, V12, V13, V14) at a polarization positive in the drawing plane and negative in the case of polarization from the drawing plane.

18. The method according to claim 16 or 17,

d a d u r c h e c e n c i n e s that

If the difference such that a cross-talk ^¬ chen detected by the first mode voltage sum to a detected from the second mode voltage sum vermie ^¬ is.