Vibration sensor

Abstract

The invention relates to a vibration sensor. A control device (3) applies excitation signals to a transducer device (2), whereby the transducer device (2) excites mechanical vibrations in a unit (1) that can vibrate mechanically. The transducer device (2) has a piezoelectric hollow cylinder (20) with an inner surface (21), an outer surface (22), and five electrodes (23, 24). Four electrodes (23) have the same surface area and are provided on the inner surface (21) or the outer surface (22), and the fifth electrode (24) is provided on the outer surface (22) or the inner surface (21).

Description

[0001] Vibration sensor

[0002] The invention relates to a vibration sensor. The vibration sensor serves, for example, to determine and / or monitor a process parameter such as the fill level, viscosity, or density of a medium. The medium is, for example, a liquid.

[0003] In the field of process automation, it is common practice to monitor or determine process variables using vibration sensors. These sensors incorporate a mechanically vibrating unit, such as a tuning fork, a single rod, or a diaphragm. The amplitude and / or frequency of the mechanical vibrations is influenced by the interaction with the surrounding medium, allowing conclusions to be drawn about the process variables based on the aforementioned characteristic vibration parameters.

[0004] Various transmitter / receiver units (alternatively called transducer devices) are known for exciting the oscillating unit and converting the mechanical vibrations into electrical signals available for further processing and evaluation. Components made of piezoelectric ceramics are frequently used. When such ceramics are subjected to electrical signals, they deform and can therefore set the oscillating unit into vibration via a mechanical coupling. Conversely, if the oscillating unit exerts a mechanical load on the ceramic, an electrical signal is generated. The transducer device thus converts between electrical signals and mechanical vibrations, and vice versa.

[0005] Therefore, to ensure the longest possible and consistently reliable use of vibration sensors, attention must also be paid to the design of the transducer device or any piezoelectric components used.

[0006] Therefore, the objective underlying the invention is to propose a vibration sensor that is characterized by durability and high reliability.

[0007] The object is achieved according to the invention by a vibration sensor comprising at least one mechanically oscillating unit, a transducer device, and a control device, wherein the control device supplies the transducer device with excitation signals, wherein the transducer device excites the mechanically oscillating unit to mechanical vibrations based on the excitation signals, wherein the transducer device comprises at least one piezoelectric hollow cylinder – preferably with a substantially circular base – wherein the hollow cylinder has an inner surface, an outer surface, and at least five electrodes for supplying the hollow cylinder with the excitation signals, wherein four of the at least five electrodes have substantially the same area and are arranged on the inner surface or the outer surface of the hollow cylinder, and wherein the fifth of the five electrodes is located on the outer surface or the outer surface.is located on the inner surface.

[0008] The transducer device of the vibration sensor according to the invention has at least one piezoelectric component designed as a hollow cylinder. This includes an inner surface and an outer surface. In one embodiment, the hollow cylinder has a circular base, with the wall thickness preferably being uniform throughout. In another embodiment, the hollow cylinder is radially polarized.

[0009] Electrodes are applied to the inner and outer surfaces of the hollow cylinder to apply electrical signals and preferably also to tap off these signals. These electrodes are made of materials such as silver, gold, platinum, palladium, copper, nickel, or a suitable alloy and are applied directly to the piezoceramic. For further signal transmission, the electrodes are connected to electrical conductors or, for example, solder lugs or weld lugs.

[0010] A total of five electrodes are mounted on the hollow cylinder. Four of the electrodes have the same surface area. They are therefore all the same size and are located either on the inner surface or on the outer surface. The remaining fifth electrode is located on the other surface of the hollow cylinder: on the outer surface if the four electrodes are on the inner surface, and on the inner surface if the four electrodes are on the outer surface.

[0011] When the four electrodes are subjected to different electrical potentials, the hollow cylinder partially contracts or expands in the case of radial polarization. This change in thickness is accompanied by a change in height. However, if the height changes, an inclined surface results at the end faces of the hollow cylinder, which then leads to a tilting of a component coupled to the hollow cylinder (i.e., in particular, the oscillating unit).

[0012] The robust design as a hollow cylinder results in a longer service life and greater intrinsic protection against temperature shocks and mechanical stresses, such as those encountered in industrial environments. The cylindrical shape is particularly associated with increased pressure and deformation resistance. Depending on the design, the fifth electrode can also be electrically charged, serve as a common ground, or remain uncontacted. In one configuration, the fifth electrode can be left uncontacted.

[0013] The excitation signal is preferably an alternating voltage signal.

[0014] The mechanically oscillating unit is preferably excited to oscillation in the fundamental mode.

[0015] One embodiment provides that the four electrodes are essentially designed as strips extending along a longitudinal axis of the hollow cylinder. In this embodiment, the four electrodes have an elongated shape, so that the electrical signals can preferably act along the entire height of the hollow cylinder.

[0016] One embodiment includes a recess, extending essentially along the longitudinal axis, between each pair of the four electrodes to electrically isolate them from one another. In this embodiment, the electrical isolation is achieved by a recess or gap between the electrodes. This can be created, for example, during the application of the electrode material. Alternatively, the affected area of ​​the hollow cylinder is fully metallized, and the electrodes are subsequently coated with a protective lacquer, with the recesses created by etching.

[0017] One embodiment provides that the surface area of ​​the fifth electrode is such that it is at least partially opposite each of the four other electrodes. According to the basic design, the fifth electrode lies on one face of the hollow cylinder opposite the four electrodes on the other face. This embodiment ensures that there is also a corresponding overlap with respect to the projections of the electrodes onto the opposite face. The dimensions also depend on whether the fifth electrode is located on the inner or outer surface.

[0018] One embodiment involves the fifth electrode essentially covering the entire outer or inner surface. In this embodiment, the fifth electrode completely covers one of the two surfaces of the hollow cylinder.

[0019] One embodiment provides that the hollow cylinder is polarized in the radial direction.

[0020] One embodiment involves the control device supplying the transducer device with excitation signals depending on which process parameter of a medium is to be determined and / or monitored by the vibration sensor. In this embodiment, the excitation signal applied to the transducer device is changed according to the respective process parameter and preferably also according to the respective medium. For example, a different excitation signal is used for determining or monitoring viscosity than for determining density as the process parameter. The four electrodes, each with its own distinct control capability, thus provide a wide range for adaptation and optimization to the process parameter or the medium.

[0021] In one embodiment, for example, a temperature sensor is provided to determine and / or monitor the viscosity as a process parameter of the medium, in order to detect or monitor the temperature of the medium.

[0022] In one embodiment, vibrations in the fundamental mode of the mechanically vibrating unit are performed with low-viscosity media, using the largest possible effective area between the vibrating unit and the medium. Above a certain predetermined viscosity, the system switches to vibrations with the smallest possible effective area, thus minimizing damping by the medium.

[0023] One embodiment provides that the mechanically oscillating unit has at least one oscillating element, and that the oscillating element is designed to have different dimensions in two orthogonal directions. The oscillating element has, for example, a paddle shape, which can also be achieved by fixing the paddle to a rod or tube. In this embodiment, the oscillating element is characterized by having at least two sides of different sizes.

[0024] One embodiment involves the control device supplying the transducer device with excitation signals in such a way that, if the process variable is the viscosity of a medium, the oscillating element vibrates in the direction of its smaller extent. This embodiment generates vibrations that, due in part to the shape of the oscillating element, exhibit minimal interaction with the medium. This prevents, for example, the medium from having an excessively damping effect on the vibrations. If the oscillating element vibrates in the direction of its smaller extent, it also vibrates perpendicular to the direction of its larger extent.

[0025] One embodiment provides that, in cases where the process variable is the density or fill level of a medium, the control device supplies the transducer device with the excitation signals in such a way that the oscillating element vibrates in the direction in which it has a larger extent. In this embodiment, the oscillating element moves in the direction of the larger extent and thus perpendicular to the direction of the smaller extent. Therefore, less medium is moved by the oscillating element, so the medium has a less damping effect on the vibrations. This is relevant, for example, for measurements in highly viscous media.

[0026] One embodiment includes a mechanically oscillating unit comprising at least two oscillating elements – preferably attached to a support element designed as a membrane or solid component – ​​and the two oscillating elements being designed such that they each have different extensions in two orthogonal directions. In this embodiment, the mechanically oscillating unit is a vibrating fork, which, for example, has two paddle-shaped vibrating elements – specifically, elements that terminate in a paddle. In this embodiment, the vibrating fork can therefore also be referred to as fork tines.

[0027] One embodiment provides that the vibration sensor has two piezoelectric hollow cylinders, and that each oscillating element is assigned one hollow cylinder. In this embodiment, the transducer device has two hollow cylinders, one of which excites a oscillating element to vibration or receives its vibrations.

[0028] One embodiment involves the control device applying the excitation signals to the converter device in such a way that the vibration elements oscillate in opposite directions. In this embodiment, the two vibration elements oscillate out of phase. This ensures that as little force and torque as possible from the vibrations are transmitted to the support element to which the two vibration elements are attached.

[0029] One embodiment provides that the converter device receives mechanical vibrations from the mechanically vibrating unit.

[0030] One embodiment involves connecting the control device and the four electrodes in such a way, and / or applying the excitation signals to the four electrodes, that two of the four electrodes are always paired at the same electrical potential. In this embodiment, the four electrodes are grouped into two pairs with respect to the excitation and are each applied to the same electrical potential, so that the sections of the hollow cylinder located under the electrodes expand or contract in the same direction.

[0031] The invention is explained in more detail with reference to the following figures. Figure 1 shows a schematic representation of one embodiment of the vibration sensor.

[0032] Fig. 2: a spatial representation of a first embodiment of the hollow cylinder,

[0033] Fig. 3: a top view of the hollow cylinder of Fig. 2,

[0034] Fig. 4: a top view of a second embodiment of the hollow cylinder,

[0035] Fig. 5: Top view of Fig. 3 with illustration of the polarization,

[0036] Fig. 6: a section through the hollow cylinder of Fig. 2,

[0037] Fig. 7: a top view of the vibration sensor of Fig. 1 ,

[0038] Fig. 8: a section through a variant of the vibration sensor,

[0039] Fig. 9: a section through a second variant of the vibration sensor,

[0040] Fig. 10: a section through a third variant of the vibration sensor and

[0041] Fig. 11 : two different geometries of the vibration elements.

[0042] Figure 1 schematically shows a design of a vibration sensor.

[0043] In the illustrated example, the mechanically oscillating unit 1 is a so-called tuning fork with two oscillating elements 10, each paddle-shaped. Due to the paddle shape, the oscillating elements 10 have significantly different extents in two directions: there is the large actual paddle surface and, perpendicular to it, the narrow edge. This also results in a significantly different interaction area with the surrounding medium (not shown here).

[0044] If the oscillating elements 10 oscillate in a direction perpendicular to their paddle surfaces, a large amount of medium is moved. If the oscillation occurs perpendicular to this direction, the medium is moved only from the paddle edge. The directions of oscillation are indicated by the two pairs of arrows above the oscillating elements 10.

[0045] The mechanically vibrating unit 1 is excited to oscillate by the transducer device 2, which is housed in the casing 12. The transducer device 2 also serves to convert the mechanical vibrations into electrical signals. The transducer device 2 is supplied with excitation signals by the control device 3. This allows the type of vibration to be set. This relates in particular to the direction of the vibrations, which is selected depending, among other things, on which process parameter is to be measured or monitored and what type of medium is involved. For example, when measuring viscosity and / or density, vibrations with the largest possible interaction area are used. However, if, for example, a highly viscous medium is involved and the fill level, density, or viscosity is to be determined, vibrations along the axis with the smaller area of ​​the paddles are used.

[0046] The vibration elements 10 are located here on a membrane as an example of a carrier element 11. Each vibration element 10 is to be located above a hollow cylinder 20 of the converter device 2 (see the following explanations).

[0047] The spatial representation of Fig. 2 and the top view of Fig. 3, which are both described together here, show a hollow cylinder 20 made of a piezoelectric material, which serves as part of the control device 3.

[0048] The hollow cylinder 20 is circularly cylindrical and homogeneously radially polarized. On the outer surface

[0049] Four electrodes 23, made of silver, for example, are located at position 22 and serve to apply the electrical signals. Based on the electrical excitation signals, the respective areas below the electrodes 23 contract or expand.

[0050] The four electrodes 23 each have the same area and are essentially strips along the longitudinal axis 25 of the hollow cylinder 20. For the electrical insulation of the electrodes

[0051] There are recesses 26 or gaps between the electrodes 23.

[0052] On the inner surface 21 there is a single electrode 24, which can also be called the fifth electrode, and covers the entire inner surface 21 of the hollow cylinder 20.

[0053] When electrodes 23, 24 are subjected to the excitation signals, the height of the cylinder changes due to the change in its thickness. This, in turn, causes a vibrating element 10, located above the hollow cylinder 20, to tilt.

[0054] Consider the example in Fig. 3: If the two right electrodes 23 are subjected to a positive potential, while the two left electrodes 23 are subjected to a negative potential, the oscillating element 10 (see Fig. 1) tilts in the direction of the paddle surface, i.e., with its maximum extent. However, if the two upper electrodes 23 are subjected to the negative potential and the two lower electrodes 23 to the positive potential, the movement occurs along the axis with the narrow edge.

[0055] Figure 4 shows a top view of an alternative embodiment of the hollow cylinder 20, in which the four equally sized electrodes 23 are located on the inner surface 21 and the fifth electrode 24 is located on the outer surface 22. This is achieved, for example, by completely metallizing the outer surface 22. This variant has the advantage that the necessary electrical conductors and contacts are located inside the hollow cylinder 20 and are thus protected by it.

[0056] The top view of Fig. 5 and the centrally cut, spatially represented hollow cylinder 20 of Fig. 6 further illustrate the relationship between the polarization and the application of the electrical excitation signal to the electrodes 23, 24.

[0057] The polarization runs - as indicated by the arrows - in a radial direction and, in the example shown, particularly from the inside out.

[0058] The two electrodes 23 in Fig. 6 are subjected to different voltage potentials. This causes one quadrant of the hollow cylinder – the left one here – to expand, while the other – the right one – contracts (each indicated by the vertically oriented arrows). This causes a rocker element 10, located above the hollow cylinder 20, to tilt to the right.

[0059] The interaction between hollow cylinder 20 and oscillating element 10 and the relationship with their relative arrangement to each other is illustrated by Fig. 7.

[0060] Fig. 7 shows a top view of a support element 11 on which two paddle-shaped oscillating elements 10 are located. The oscillating elements 10 are arranged such that their broad paddle sides are oriented parallel to each other. The upper arrows indicate the oscillations perpendicular to the direction along which the oscillating elements 10 have their smallest extent. Perpendicular to this, the oscillations occur with the narrow side of the paddles 10.

[0061] Below the vibration elements 10 – and therefore below the support element 11 – are the circularly arranged electrodes 23. The recesses 26 between these define the axes of the vibration elements 10 and the vibrations.

[0062] Figures 8 to 10 show three different versions of

[0063] Vibration sensors, each with two oscillating elements 10, each of which is associated with a hollow cylinder 20 of the transducer device 2. Depending on the design, the hollow cylinders 20 can be clamped, glued, or fixed by a potting compound.

[0064] In the variant shown in Fig. 8, each vibrating element 10 has a wider shoulder in which a hollow cylinder 20 is located. The shoulders transition into a shaft 110 in the reinforced support element 11, which thus also serves to mechanically decouple the vibrating elements 10 from the housing 12.

[0065] In the variant of Fig. 8, if the converter device 2 is located in the mechanically oscillating unit 1, then in the variant of Fig. 9 the hollow cylinders 20 are recessed and are located adjacent to the support element 11 in the housing 12.

[0066] The hollow cylinders 20 are each arranged in a pedestal 111 (alternatively called a tube), which extend from the support element 11 into the interior of the housing 12. The wall thicknesses of the pedestals 111 are essentially the same as the wall thickness of the support element 11. Preferably, the same material or alloy is used.

[0067] In the variant shown in Fig. 10, recesses or bores are located in the very solid and thick support element 11 above each of the two vibration elements 10. A hollow cylinder 20 is located in each of these recesses. In this example, the hollow cylinders 20 are thus arranged in alignment behind the vibration elements 10, so that the longitudinal axes of the hollow cylinders 20 and the vibration elements 10 coincide. The variant shown in Fig. 10 is particularly suitable for applications involving high process pressures.

[0068] Fig. 11 shows two embodiments: a) and b) of the paddle shape of the oscillating elements 10, i.e. in particular the fork tines of the variant of Fig. 1 .

[0069] If the vibrations occur horizontally, there is a large effective area. With vibrations along the vertical axis, only the edges displace the medium.

[0070] The width of the oscillating element 10 perpendicular to the respective direction of oscillation has a twofold effect: Firstly, it affects the measurement sensitivity with respect to density. Secondly, it affects the damping coefficient of the oscillations, which is primarily dependent on the viscosity of the medium being measured or monitored. The damping coefficient can be used, for example, to determine the Newtonian viscosity.

[0071] For low-viscosity liquids, both forms shown in Fig. 11 a) and b) exhibit the same measurement quality, as they both have the same width a. However, when measurements are taken in high-viscosity liquids, the vibrations preferably occur along the long axis, i.e., along the vertical. In this case, the variant shown in Fig. 11 b) is more advantageous than the variant shown in Fig. 11 a), since the width b2 is smaller than the width b1.

[0072] For liquids with medium viscosity, the variant shown in Fig. 11 a) is suitable due to its larger width b1, as it allows for a more precise determination of the viscosity. The geometry of the vibration elements 10 must be selected, in particular, depending on the density and viscosity of the medium.

[0073] List of reference symbols for mechanically oscillating units

[0074] Converter device

[0075] Control device

[0076] Vibration element

[0077] Support element

[0078] Housing

[0079] Hollow cylinder

[0080] Inner surface

[0081] Outdoor area

[0082] electrode

[0083] electrode

[0084] Longitudinal axis

[0085] recess

[0086] Shaft of the support element

[0087] Podium

Claims

Patent claims 1. Vibration sensor, comprising at least one mechanically oscillating unit (1), a transducer device (2), and a control device (3), wherein the control device (3) applies excitation signals to the transducer device (2), wherein the transducer device (2) excites the mechanically oscillating unit (1) to mechanical vibrations based on the excitation signals, wherein the transducer device (2) comprises at least one piezoelectric hollow cylinder (20) – preferably with a substantially circular base –, wherein the hollow cylinder (20) has an inner surface (21), an outer surface (22), and at least five electrodes (23, 24) for applying the excitation signals to the hollow cylinder (20), wherein four electrodes (23) of the at least five electrodes (23, 24) have substantially the same area and are arranged on the inner surface (21) or the outer surface (22) of the hollow cylinder (20).and wherein the fifth electrode (24) of the five electrodes (23, 24) is arranged on the outer surface (22) or the inner surface (21).

2. Vibration sensor according to claim 1, wherein the four electrodes (23) are essentially designed as strips extending along a longitudinal axis (25) of the hollow cylinder (20).

3. Vibration sensor according to claim 2, wherein a recess (26) extending substantially along the longitudinal axis (25) is located between each pair of the four electrodes (23) for electrical separation of the electrodes (23) from each other.

4. Vibration sensor according to one of claims 1 to 3, wherein the area of ​​the fifth electrode (24) is such that the fifth electrode (24) is at least partially opposite each of the four electrodes (23).

5. Vibration sensor according to one of claims 1 to 4, wherein the control device (3) supplies the converter device (2) with the excitation signals depending on which process parameter of a medium is to be determined and / or monitored by the vibration sensor.

6. Vibration sensor according to one of claims 1 to 5, wherein the mechanically oscillating unit (1) has at least one oscillating element (10), and wherein the oscillating element (10) is designed such that it has different dimensions in two orthogonal directions.

7. Vibration sensor according to claim 5 and claim 6, wherein, in the case that the process variable is a viscosity of a medium, the control device (3) supplies the transducer device (2) with the excitation signals in such a way that the vibrating element (10) performs vibrations in the direction in which the vibrating element (10) has a smaller extent.

8. Vibration sensor according to claim 5 and claim 6 or according to claim 7, wherein, in the case that the process variable is a density or a fill level of a medium, the control device (3) supplies the transducer device (2) with the excitation signals in such a way that the vibrating element (10) performs vibrations in the direction in which the vibrating element (10) has a greater extent.

9. Vibration sensor according to one of claims 1 to 8, wherein the mechanically oscillating unit (1) has at least two oscillating elements (10) – preferably attached to a support element (11) designed as a membrane or solid component – ​​and wherein the two oscillating elements (10) are designed such that they each have different dimensions in two orthogonal directions.

10. Vibration sensor according to claim 9, wherein the vibration sensor has two piezoelectric hollow cylinders (20), and wherein each vibrating element (10) is associated with a hollow cylinder (20).

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