Vibration sensor

Abstract

The invention discloses a vibration sensor, wherein a transducer device (2) excites a mechanically oscillating unit (1) to mechanical vibrations and / or receives mechanical vibrations from the mechanically oscillating unit (1). An oscillating element (10) of the mechanically oscillating unit (1) has different dimensions in two orthogonal directions, with a wide side (11) and a narrow side (12). The transducer device (2) has a bimorph drive (20) and four electrodes (5, 6). The four electrodes (5, 6) are separated from each other by two insulating gaps (7) that intersect at a right angle. The four electrodes (5, 6) form two electrode pairs, the electrodes (5, 6) of which are diametrically opposed to each other. One electrode (5, 6) of each electrode pair is located adjacent to the two electrodes (6, 5) of another electrode pair.The wide side (11) of the oscillating element (10) is oriented at a 45° angle to the two insulating gaps (7).

Description

[0001] The invention relates to a vibration sensor. The vibration sensor is used, for example, to determine and / or monitor a process variable of a medium. The process variable is, for example, the fill level or density.

[0002] In the prior art, tuning forks are known as an example of a vibration sensor that oscillates with different frequencies and qualities depending on medium properties, such as density or viscoelasticity. The two oscillating elements, also known as fork tines, are located on a membrane and can contact the medium. They form a mechanical resonator or closed oscillating circuit, whereby the oscillation frequency and oscillation amplitude depend only on the medium properties. The energy generated by resonant oscillations in such a vibration sensor is retained in the sensor regardless of the type and design of the process connection. Energy losses are possible due to damping in the resonator and due to dissipation in the medium. For example, in viscous fluids, energy is lost due to molecular friction.

[0003] The low internal energy loss in the resonator of the vibration sensor is an important prerequisite for the reliable detection of gases or liquids. Opposing reaction forces of the vibrating elements, i.e., the prongs, should be balanced in the membrane.

[0004] With regard to the application of the sensors, there are differences in the evaluation of the vibration signals: The frequency of the mechanical vibrations is a measurement parameter for applications with liquids. This is due to the significantly higher densities than with gases and the fact that low-viscosity liquids dampen the vibrations little. Conversely, the amplitude of the vibrations is a preferred measurement parameter for bulk solids, as such media always exhibit high damping regardless of the bulk density. Therefore, the quality factor or vibration amplitude is a measure for bulk solid detection.

[0005] A uniform deposit of sediment on both tines of the tuning fork does not impair sensor performance within a certain range. However, if there is a risk of solid structures such as chips or lumps becoming trapped between the tines, or of crystallization processes or the hardening of material (e.g., bitumen) on the fork tines, so-called single-rod units can be used as mechanically oscillating units. This means that only one oscillating element in the form of a rod is present.

[0006] In such single-rod sensors, mechanical compensation is achieved through additional vibration elements to prevent vibration energy from being lost through the process connection. Such compensation elements do not come into contact with the medium. The required force compensation can generally only be achieved in the uncovered or immersed state. If the medium builds up on the single-rod, or if the single-rod sensor, which is compensated for vibration in air, is immersed in liquid, the reaction forces are no longer compensated or are no longer sufficiently compensated. As a result, the vibrations can collapse, or the vibration frequency can be unexpectedly high or low.

[0007] Single rods are mostly used for bulk solids applications and are mechanically compensated for air, i.e., when uncovered. When immersed in the bulk solid, compensation is no longer important, as the bulk solids dampen the vibrations so strongly that the sensor does not vibrate at all when covered. This is in contrast to a liquid as the medium. The single rod continues to vibrate when covered, and the medium imparts an additional moment of inertia to the tines, depending on the density of the medium. This causes a change in frequency and also means that the sensor is no longer compensated. Without compensation, vibration energy is lost. In conjunction with the influence of components coupled to the sensor, such as the container wall or fittings, the vibrations can break down or the vibrating unit can oscillate at an incorrect frequency.

[0008] Another issue arises from the temperature dependence of the stiffness and damping of the material of the oscillating unit. For example, the stiffness of the material of a tuning fork decreases with increasing temperature, leading to a reduction in the oscillation frequency. The mechanical quality factor of the resonator is also temperature-dependent and can vary by up to a factor of 10 depending on the application conditions. These temperature changes can, for example, lead to switching thresholds being exceeded incorrectly. Therefore, to expand the application range with regard to the process temperature, temperature sensors are used, for example, to account for temperature drift in the switching threshold. However, this makes switching reliability dependent on these temperature sensors.

[0009] Overall, there is a need to develop vibration sensors that function reliably under as many application conditions as possible and preferably independent of temperature and approach.

[0010] For example, it is known to excite the mechanically oscillating unit to oscillate in different directions in order to obtain different information about a medium to be measured or about the mechanically oscillating unit itself (see, for example, DE 100 14 724 A1).

[0011] If a flat prong vibrates parallel to the prong surface, a covering liquid will only slightly reduce the oscillation frequency, and the frequencies in air and the medium will be approximately the same. However, if such a prong vibrates perpendicular to the prong surface, the covering with a liquid creates a large frequency difference between the oscillations and the oscillations in air. This is due to the co-moving mass of medium covering the prong, which depends on the density and effective area of ​​the oscillating element, as well as the viscoelasticity of the medium. The same applies to the oscillation amplitude, which, for resonators with forced oscillations, depends on the mechanical quality of the vibration sensor.

[0012] Thus, a measurement can be performed by evaluating, for example, the frequency or amplitude ratio between two orthogonal oscillations, especially between parallel and perpendicular oscillations. The change in frequency and amplitude depends on the type of medium. For example, parallel and perpendicular oscillations do not differ in frequency and amplitude in gaseous media. In liquids, the frequency of parallel oscillations is higher than that of perpendicular oscillations. The same applies to the amplitude. In highly viscous liquids or bulk materials, no oscillation may be possible at all.

[0013] As already mentioned above, the oscillating unit is no longer optimally compensated due to the medium being covered, and energy is transferred to the process connection. The extent to which the amplitude or frequency of the oscillations changes depends on the coupling of the oscillator to the process connection. However, since a relative measurement of the two oscillation directions is performed and the energy losses are the same for both directions, the energy loss due to the covering plays only a minor role.

[0014] To excite or receive vibrations, a transducer device is usually present that converts between electrical signals and mechanical vibrations. Piezoelectric elements are often used for this purpose. These can be arranged in a stack, for example (see, for example, WO 01 / 66269 A1). Alternatively, they are so-called bimorph drives (see, for example, DE10 2004 010 992 B3 or DE 100 14 724 A1). These are usually discs polarized in opposite directions.

[0015] However, the prior art does not contain any concrete description of how a vibration sensor is to be designed with respect to the transducer device in such a way that the mechanically oscillatable unit can execute oscillations in different directions.

[0016] The invention is based on the object of proposing a vibration sensor whose mechanically oscillating unit can oscillate in different directions.

[0017] The object is achieved by a vibration sensor with at least one mechanically oscillatable unit and a transducer device, wherein the transducer device excites the mechanically oscillatable unit to mechanical vibrations and / or receives mechanical vibrations from the mechanically oscillatable unit, wherein the mechanically oscillatable unit has an oscillating element, wherein the oscillating element is designed such that the oscillating element has different dimensions in two orthogonal directions, in that the oscillating element has a wide side and a narrow side, wherein the transducer device has at least one bimorph drive and four electrodes on one side of the bimorph drive, wherein the four electrodes are separated from one another by two insulating gaps, wherein the two insulating gaps intersect at a substantially right angle, wherein the four electrodes are arranged in two electrode pairs such thatthat the electrodes of each electrode pair are diametrically opposed and one electrode of each electrode pair is adjacent between the two electrodes of another electrode pair, and wherein the vibrating element is arranged relative to the four electrodes such that the wide side of the vibrating element is oriented substantially at a 45° angle to the two insulating gaps.

[0018] The vibration sensor comprises a mechanically oscillating unit and a transducer device that excites the mechanically oscillating unit to mechanical vibrations and / or receives mechanical vibrations from the mechanically oscillating unit. The mechanically oscillating unit comprises an oscillating element that has different dimensions in two orthogonal directions (e.g., length and width). It can therefore also be referred to as a flat paddle. This results in a wide side – in the paddle plane – and a narrow side (perpendicular to the paddle plane).

[0019] The converter device has a bimorph drive and four electrodes on one side of the bimorph drive. The electrodes are separated from each other by two insulating gaps that intersect at a right angle. The four electrodes form two electrode pairs. The two electrodes of each pair are diametrically opposed. Furthermore, one electrode of each pair is located between the two electrodes of the other pair. This means that the electrodes of the two pairs alternate around a central region: an electrode of one - the first - pair is followed by an electrode of the other - the second - pair, followed by the other electrode of the first and then the other electrode of the second pair.

[0020] The oscillating element, which is to be excited to oscillate or whose oscillations are to be detected, is arranged relative to the four electrodes such that the wide side of the oscillating element is oriented essentially at a 45° angle to the two insulating gaps. Due to this orientation, the oscillating element is located primarily above the two electrodes of a first electrode pair and is perpendicular to the orientation of the two electrodes of the other, i.e., second, electrode pair.

[0021] If the electrodes of the first pair of electrodes, above which the oscillating element is primarily located, are subjected to an excitation signal, the oscillating element oscillates parallel to its wide side and thus in the paddle plane. If the electrodes of the second pair of electrodes, which are located parallel to the narrow side, are subjected to an excitation signal, the oscillating element performs vertical oscillations. A bimorph drive is used, which is therefore partially polarized in two different directions.

[0022] One embodiment consists in the mechanically oscillatable unit having exactly one oscillating element. Thus, in this embodiment, only a single oscillating element is present.

[0023] One embodiment provides for the oscillating element to be mounted above a diaphragm and the transducer device to be mounted below it, and for the side of the bimorph drive on which the four electrodes are arranged to face away from the diaphragm. In this embodiment, the transducer device and the oscillating element are indirectly coupled to one another via a diaphragm. The side of the bimorph drive on which the four electrodes are fixed faces away from the diaphragm. This arrangement particularly simplifies the contacting of the electrodes.

[0024] One embodiment consists in the four electrodes being configured essentially as equally sized circular sectors. In this embodiment, the four electrodes together form a circle. They are each the same size. Therefore, they also excite equally sized sections of the bimorph drive to oscillate. In this embodiment, each pair of electrodes covers essentially half of the side of the bimorph drive covered by the electrodes. This embodiment is preferably accompanied by the bimorph drive being configured essentially circular.

[0025] One embodiment provides for the oscillating element to be essentially designed as a paddle. Preferably, it is a flat paddle. The paddle is arranged, in particular, vertically on a membrane, to which it is coupled or connected for the transmission of forces and torques.

[0026] One embodiment involves the vibration sensor having a control device, and the control device applying excitation signals to the four electrodes in such a way that those areas of the bimorph drive that are in contact with the electrodes of an electrode pair execute opposing mechanical movements. For example, to generate vibrations, a region of the bimorph drive covered by one electrode contracts, while the diametrically opposite region covered by the other electrode expands. This results in a movement of the vibrating element along a connecting axis between the electrodes of the respective electrode pair.

[0027] One embodiment provides that the control device applies rectangular signals as excitation signals to the four electrodes.

[0028] One embodiment consists in that the vibrating element is connected to a housing, that the housing is connected to a connection unit, and that the connection unit is a pipe extension, a flange or a cable. The housing is preferably connected to the diaphragm and preferably also houses connection components for the transducer device. Adjacent to this housing is a connection unit, which is e.g. a pipe extension, a solid flange or simply a cable. Since the two vibration types are evaluated relative to one another, the further process connection, i.e. the type of fixing of the vibration sensor at the measuring location, can be selected as desired.

[0029] An alternative embodiment consists in the transducer device having at least four separate piezo elements, and in the four electrodes being arranged on the end faces of the piezo elements. Due to the aforementioned grouping or arrangement of the electrodes, a connecting axis exists between the electrodes of each electrode pair. Therefore, in this embodiment, the oscillating element is arranged such that the wide side is arranged along the connecting axis of an electrode pair. (The connecting axis in the case of the previously discussed bimorph drive is therefore the angle bisector between the insulating gaps.) The four individual piezo elements are, in particular, polarized in only one direction and each have an electrode on one end face. Preferably, the affected end faces all face towards or away from the mechanically oscillatable unit.

[0030] One embodiment consists in the fact that the sums of the surface areas of the electrodes assigned to the same electrode pair are substantially equal. In this embodiment, the sum of the surface areas of the electrodes of one electrode pair is substantially equal to the sum of the surface areas of the other electrode pair. In a further embodiment, the surface areas of the four electrodes are substantially identical.

[0031] The invention is explained in more detail with reference to the following figures. Fig. 1 shows a spatial representation of a vibration sensor, Fig. 2 shows a longitudinal section through the vibration sensor of the Fig. 1, Fig. 3 shows a view inside the housing of the vibration sensor of the Fig. 1 on the side of the bimorph drive, which is provided with the four electrodes and the indicated position of the oscillating element, Fig. 4 shows an example of the electrical wiring of the bimorph drive of the vibration sensor of the Fig. 1, Fig. 5 shows the expansion behavior of the bimorph drive (a)) and its effect on the oscillating element (b)), Fig. 6 shows a first embodiment of a process connection of the vibration sensor of the Fig. 1, Fig. 7 shows a second embodiment of a process connection of the vibration sensor of the Fig. 1 and Fig. Figure 8 shows a view of an alternative design of the transducer device with four individual piezo elements.

[0032] The vibration sensor of the Fig. 1 has a single oscillating element 10 for the mechanically oscillating unit 1. The oscillating element 10 is designed in the shape of a flat paddle. It is important that the oscillating element 10 has a significantly greater extension in one direction—in the paddle plane—than in a direction perpendicular to it. The length and width therefore differ significantly from each other. Here, the oscillating element 10 is flat. Alternatively, it has a structure. The oscillating element 10 therefore has a wide side 11 and a narrow side 12.

[0033] The vibrating element 10 is mounted on a membrane 3, the inside of which is located in a housing 4 that is connected to the membrane 3.

[0034] Furthermore, a control device 8 is provided, which, together with the converter device 2 discussed below, excites the oscillating element 10 to oscillate. These oscillations are, in particular, in two orthogonal directions, as indicated here by the arrows. Thus, the oscillating element 10 moves in one excitation mode along its long side 11 (these are the so-called parallel oscillations) and in another excitation mode perpendicular to the long side 11 and thus along its narrow side 12 (this is referred to here as perpendicular oscillation).

[0035] If the frequency and / or amplitude values ​​of the two (parallel and perpendicular) oscillations are related to each other, a process variable of interest in a medium, e.g., fill level or density, can be determined very reliably.

[0036] In the Fig. Figure 2 shows that the transducer device 2 is located below the membrane 3 and thus below the vibrating element 10. The transducer device 2 converts between electrical signals and mechanical vibrations. Thus, when the transducer device 2 is subjected to an excitation signal, it generates the mechanical vibrations of the vibrating element 10. Conversely, the transducer device 2 converts mechanical vibrations of the vibrating element 10 into electrical signals, which are evaluated with respect to the measured variable of interest.

[0037] The transducer device 2 is protected within the housing 4 and is designed as a bimorph drive 20. It is thus a piezoelectric element with two regions of different polarization. One polarization is toward the membrane 3 and the other polarized away from the membrane 3.

[0038] The bimorph drive 20 is designed as a flat disc and has two sides 200, 201. One side 201 is located on the inside of the membrane 3 and the other side 200 faces away from the membrane 3 and carries - as explained below - the electrode structure with the four electrodes 5, 6.

[0039] The Fig. 3 shows the four electrodes 5, 6 on the side 200 of the bimorph drive 2 which, when installed, faces away from the membrane 3. The membrane 3, which is circular here, surrounds the bimorph drive 2, which is also circular in shape. On side 200 there are four electrodes 5, 6 which are implemented as equally sized circular segments and are divided into two electrode pairs. The two electrodes 5, 6 of each pair are located diametrically opposite one another. Adjacent between the two electrodes of one pair 5, 6 there is an electrode of the other pair 6, 5. The electrodes of the two pairs thus alternate with one another around the circumference. The circular segments of the two pairs each have an axis of symmetry. Between the electrodes 5, 6 there are two insulating gaps 7 which are perpendicular to one another at a center point of the arrangement of the electrodes 5, 6.

[0040] The position of the oscillating element 10 relative to the four electrodes 5, 6 is shown. It can be seen that the paddle shape of the oscillating element 10 results in a rectangle when projected onto the side of the bimorph drive 2.

[0041] The oscillating element 10 is arranged so that the long side 11 (cf. Fig. 1 and Fig. 2) forms a 45° angle with the insulating gaps 7. Therefore, the long side 11 is also arranged along the axis of symmetry of a pair of electrodes 5.

[0042] If the electrodes of the electrode pair 5, above which the long side 11 extends, are subjected to an electrical excitation signal, the oscillating element 10 oscillates parallel to its long side 11 - and thus also within the paddle plane - at the frequency Fp. If the electrodes 6 of the pair to which the long side 11 is perpendicular are excited, the oscillating element 10 performs the vertical oscillations at the frequency Fs.

[0043] A single piezoelectric device is used to excite the two oscillations. One possible electronic circuit is shown in the Fig. 4. Shown are the two electrodes of a pair of electrodes.

[0044] On the side 201 facing the membrane 3 in the assembled state, there is a common electrode 202 that is not electronically connected. Of the two electrodes 5, one electrode is connected to the ground of the electronic circuit via a resistor. The other electrode is connected to the control device 8, from which it receives a rectangular signal.

[0045] The arrangement can be understood in such a way that the common electrode 202 forms a virtual mass between two series-connected piezo elements, which are located between the control device 8 and - via the resistor - the mass of the circuit.

[0046] Since both segments of the bimorph drive 20 have the same polarization direction under the opposing electrodes 5 and the three electrodes 5, 202 form a voltage divider, the excitation signal causes one part of the piezoelectric bimorph drive 20 to expand and another to contract. This shows the Fig. 5 a).

[0047] The two segments of the bimorph drive below electrodes 5 have the same polarization direction, as indicated by the arrows pointing upwards on the sides. For example, if the left electrode 5 is applied with a positive voltage +E, it expands in the polarization direction and shrinks in the perpendicular direction (indicated here by the two opposing arrows within the piezo element). Since the two electrodes 5 are connected as a voltage divider (see Fig. 4), the right electrode 5 and thus the segment below it receives a negative voltage -E with respect to the common electrode 202. Therefore, this segment behaves inversely to the left one: It contracts in the direction of polarization and expands in the plane of the piezo element (indicated by the arrows facing away from each other).

[0048] In the Fig. 5 b) shows how the bimorph drive affects the oscillating element 10 through the force-locking connection with the membrane.

[0049] The condition of the Fig. 5 a) with the contraction and expansion of the two segments of the piezo element. The deformation of the membrane causes a tilting oscillation, and in particular, the oscillating element 10 oscillates to the left.

[0050] In the Fig. Figure 6 shows a variant in which the vibrating element 10 is located on a diaphragm 3. The diaphragm 3 is followed by a housing 3, which merges into a pipe extension as a connection unit 9. Another connection unit 9 is designed as a solid flange.

[0051] The Fig. 7 shows an alternative embodiment in which a cable is arranged as a connection unit 9 behind the housing 4.

[0052] The Fig. Figure 8 shows an embodiment in which the transducer device 2 is formed by four separate piezo elements 30, which are arranged at the same distance from a common center point of the circular membrane 3. The piezo elements 30 are each polarized in the same direction, here as an example out of the plane of the drawing. The electrodes 5, 6 are shown, which are located on the end faces 31 of the circular cylindrical piezo elements 30. The electrodes 5, 6 of the electrode pairs are each connected to one another by a connecting axis 50, which in the embodiment shown passes through the centers of the circular base surfaces of the electrodes 5 and 6, respectively. For the sake of clarity, only the connecting axis 50 of the electrodes 5 of an electrode pair is shown here. It can be seen that - as in the embodiment of the Fig.3 - the oscillating element 10 is arranged so that the wide side is oriented along the connecting axis 50.

[0053] In general, the vibration sensor offers the following advantages: • The sensor is particularly suitable for digital excitation of the mechanically oscillating unit and for the evaluation of the oscillations or the signals generated from the oscillations. • A uniform switching logic can be used for different media such as liquids and bulk materials. • There is an alternative to temperature-compensated vibration sensors that uses temperature sensors. This is based on the fact that temperature affects both frequencies Fp and Fs equally, and that the temperature dependency is automatically taken into account using a ratio of the frequencies (e.g., Fp / Fs or Fs / Fp). • Sensors with two orthogonal vibration modes can operate with significantly more approach than other sensors because the approach affects the frequencies of the two vibrations essentially the same. • Sensor calibration is not required. • The shape of the oscillating unit, in particular that of the oscillating element, allows it to be used with media that tend to crystallize (ice formation, melting, etc.) or that contain components that can jam (e.g. fruit pulp in yogurt, fibers). List of reference symbols 1 mechanically oscillating unit 2 converter device 3 Membran 4 housings 5 electrode of an electrode pair 6 Electrode of another electrode pair 7 Insulation gap 8 Control device 9 Connection unit 10 Oscillating element 11 wide side of the oscillating element 12 narrow side of the oscillating element 20 bimorph drive 30 piezo element 31 front side 50 Connecting axis of an electrode pair 200 one side of the bimorph drive 201 opposite side of the bimorph drive 202 common electrode QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 100 14 724 A1 [0010, 0014] WO 01 / 66269 A1

[0014] DE 10 2004 010 992 B3

[0014]

Claims

[1] Vibration sensor, with at least one mechanically oscillatable unit (1) and a transducer device (2), wherein the transducer device (2) excites the mechanically oscillatable unit (1) to mechanical oscillations and / or receives mechanical oscillations from the mechanically oscillatable unit (1), wherein the mechanically oscillatable unit (1) comprises an oscillating element (10), wherein the oscillating element (10) is designed such that the oscillating element (10) has different dimensions in two orthogonal directions, in that the oscillating element (10) has a wide side (11) and a narrow side (12), wherein the converter device (2) has at least one bimorph drive (20) and four electrodes (5, 6) on one side (200) of the bimorph drive (20), wherein the four electrodes (5, 6) are separated from each other by two insulating gaps (7), the two insulating gaps (7) intersecting at a substantially right angle, wherein the four electrodes (5, 6) are arranged in two electrode pairs such that the electrodes (5, 6) of each electrode pair are diametrically opposed and one electrode (5, 6) of one electrode pair is located adjacent between the two electrodes (6, 5) of another electrode pair, and wherein the oscillating element (10) is arranged relative to the four electrodes (5, 6) such that the wide side (11) of the oscillating element (10) is oriented substantially at a 45° angle to the two insulating gaps (7). [2] Vibration sensor according to claim 1, wherein the mechanically oscillatable unit (1) has exactly one oscillating element (10). [3] Vibration sensor according to claim 1 or 2, wherein the oscillating element (10) is mounted above and the transducer device (2) is mounted below a membrane (3), and wherein the side (200) of the bimorph drive (20) on which the four electrodes (5, 6) are arranged faces away from the membrane (3). [4] Vibration sensor according to one of claims 1 to 3, wherein the four electrodes (5, 6) are designed substantially as circular sectors of equal size. [5] Vibration sensor according to one of claims 1 to 4, wherein the oscillating element (10) is designed essentially as a paddle. [6] Vibration sensor according to one of claims 1 to 5, wherein the vibration sensor has a control device (8), and wherein the control device (8) applies excitation signals to the four electrodes (5, 6) in such a way that those regions of the bimorph drive (20) which are in contact with the electrodes (5, 6) of an electrode pair execute opposite mechanical movements. [7] Vibration sensor according to claim 6, wherein the control device (8) applies rectangular signals as excitation signals to the four electrodes (5, 6). [8] Vibration sensor according to one of claims 1 to 7, wherein the oscillating element (10) is connected to a housing (4), wherein the housing (4) is connected to a connection unit (9), and wherein the connection unit (9) is a pipe extension, a flange or a cable.

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