Vibration sensor

Abstract

The invention relates to a vibration sensor. A transducer device (2) excites a mechanically vibrating unit (1) so as to mechanically vibrate and / or receives mechanical vibrations from the mechanically vibrating unit (1). The transducer device (2) has a piezo element (3). A vibrating element (5) of the mechanically vibrating unit (1) is secured in a contact region (6) of a support unit (4). The piezo element (3) is arranged in a depression (7) of the support unit (4) outside of the contact region (6).

Description

[0001] Vibration sensor

[0002] The invention relates to a vibration sensor. The vibration sensor is preferably used to determine and / or monitor a process variable of a medium. The process variable is, for example, the fill level or density, wherein the medium is, for example, a liquid, a gas, or a bulk solid.

[0003] In the prior art, it is known to use vibration sensors for liquids or bulk solids to detect whether the media has reached or fallen below a limit level. Such sensors also allow, for example, the measurement of the density of the medium. The measuring principle is based on the fact that a mechanically oscillating unit is excited to mechanical vibrations by a transducer device, and that received vibrations are influenced by the interaction of the oscillating unit with the medium. The generated vibrations usually belong to the fundamental mode. The mechanically oscillating unit usually has one (in which case the sensor is a so-called single-rod) or two vibrating elements (this is a so-called tuning fork). The transducer device often has piezo elements, which serve to convert electrical signals into mechanical vibrations.Exemplary embodiments of vibration sensors can be found in DE 103 08 087 A1, EP 0 950 879 A1, WO 2011 / 039 266 A1, CN 2 02 255 551 U or CN 2 04 788 576 U.

[0004] For example, WO 01 / 66269 A1 describes a vibration sensor in which a stack of piezo elements is clamped force-fit between a mounting aid and a diaphragm. The transducer device consists of metallic and ceramic components that have different thermal expansion coefficients. Therefore, different preloads can arise at different temperatures during the process. This can even lead to the transducer device losing contact with the diaphragm or to a significant reduction in the mechanical preload in the piezo stack, resulting in the vibration sensor failing. Aging effects, such as a drift in the oscillation frequency, often occur. Therefore, regular calibrations may be necessary, especially in applications involving density evaluation.The membrane also requires that applications be limited to a maximum pressure, otherwise plastic deformation could occur.

[0005] From DE 10 2012 100 728 A1, for example, it is known to insert disc-shaped piezo elements directly into the root area of ​​vibrating elements. This eliminates the issue of necessary preload. The advantage is that the vibrating elements are directly excited to vibrate. Since no thin membrane is required, but rather a possibly thick carrier unit to which the vibrating elements are attached can be used, vibration sensor designs for high process pressures are also possible. However, manufacturing is complex, as the piezo elements must be inserted into the narrow pockets of the carrier unit with high precision. Vibration sensors that have a tube extension with a plastic coating, such as PFA or ECTFE, are particularly problematic. These coatings are usually produced at temperatures above the Curie temperature of the piezo elements.Therefore, the piezo elements can only be installed later. However, if the housing is long and the distance to the pockets is greater, installation becomes very laborious.

[0006] The invention is based on the object of proposing a vibration sensor which has the widest possible range of applications and the highest possible resistance to aging and is characterized by the simplest possible production.

[0007] The invention solves the problem by means of a vibration sensor having at least one mechanically oscillatable unit and a transducer device, wherein the transducer device excites the mechanically oscillatable unit to mechanical oscillations and / or receives mechanical oscillations from the mechanically oscillatable unit, wherein the transducer device has at least one piezo element, wherein the mechanically oscillatable unit has at least one carrier unit and an oscillating element, wherein the oscillating element is fastened to the carrier unit in a contact region, wherein the carrier unit has a recess, wherein the recess lies outside the contact region of the oscillating element, and wherein the piezo element is arranged in the recess.

[0008] In the invention, a piezo element is inserted into a recess in a support unit. The support unit has sufficient strength or thickness for this purpose. The recess is located primarily outside a contact area of ​​the vibrating element, at or in which the vibrating element is connected to the support unit. This is particularly associated with the fact that the recess is not located in the vibrating element itself. In other words: the recess and therefore the piezo element are arranged on the support unit laterally offset from the vibrating element.

[0009] In the vibration sensor, in particular the recess and the contact area are arranged relative to one another in such a way that the carrier unit transmits mechanical vibrations from the piezo element to the oscillating element and mechanical vibrations from the oscillating element to the piezo element. The mechanical vibrations are therefore transmitted from the piezo element to the oscillating element and vice versa via the carrier unit. The carrier unit therefore assumes an intermediary function. During operation, potential energy is stored in the carrier unit. In one variant, this occurs through significant deformations in the carrier unit and the resulting moderate mechanical stress. In an alternative variant, there is slight deformation of the carrier unit but high mechanical stress. Which variant of energy storage is used also depends on the design of the carrier unit, i.e. whether it is stiffer, for example, due to a greater thickness.If the carrier unit is, for example, a thick plate, high mechanical stress is generated with very little deformation of the plate.

[0010] The vibration sensor according to the invention simplifies production and the sensor properties show higher reproducibility.

[0011] In one design, the support unit has a thickness between 2 mm and 12 mm, or alternatively between 5 mm and 10 mm. This allows the vibration sensor, e.g., made of stainless steel (1.4435, 1.4404, 1.4462, etc.) or Hastelloy (C22, C276, etc.), to be used in pressure ranges up to 500 bar.

[0012] In one embodiment, the piezo element consists of a ceramic, e.g. based on lead titanate zirconate (PZT) or based on a lead-free ceramic, such as potassium sodium niobate (KNN), bismuth sodium titanate (BNT) or barium titanate (BT).

[0013] In one embodiment, the recess and the piezo element are designed and coordinated with each other, and the piezo element is arranged in the recess in such a way that the piezo element protrudes from the recess. This projection of the piezo element beyond the recess simplifies electrical contact with the piezo element. If the piezo element has a height between 5 mm and 13 mm in one embodiment, the piezo element protrudes beyond the recess by, for example, 1 mm to 3 mm.

[0014] In one embodiment, the piezo element is glued into the recess. This simplifies manufacturing.

[0015] In one embodiment, exactly one recess for the transducer device is provided in the carrier unit. In one embodiment, exactly one recess is located in a central region between the contact areas of the preferably two oscillating elements.

[0016] One embodiment consists in the recess having a circular-cylindrical inner shape and / or the piezo element having a circular-cylindrical outer shape. In one embodiment, the recess is a circular bore, i.e., preferably a circular blind hole. In an alternative or supplementary embodiment, the piezo element is a circular cylinder. In one embodiment, exactly one circular-cylindrical piezo element is present.

[0017] In one embodiment, the circular-cylindrical piezo element is inserted into the recess such that the cylinder's longitudinal axis is aligned along the recess's longitudinal axis or parallel to it. In one embodiment, the two longitudinal axes coincide.

[0018] In an additional embodiment, the recess is also circular-cylindrical, so that the piezo element and the recess are arranged coaxially to each other.

[0019] One embodiment provides that the mechanically oscillatable unit has two oscillating elements, that the two oscillating elements are each attached to the support unit in a contact area, and that the recess is located outside the contact areas of the two oscillating elements. In this embodiment, the vibration sensor is designed as a tuning fork. The recess for the piezo element is offset from both contact areas. Thus, the piezo element is not located above one of the two oscillating elements, which can also be referred to as prongs.

[0020] In one embodiment, the recess on the carrier unit is located centrally between the two contact areas and thus has essentially the same distance to the two oscillating elements.

[0021] One embodiment consists in the piezo element having a continuous recess, an outer side, and an inner side, and at least one electrode on each of the outer and inner sides. In this embodiment, the piezo element is hollow. At least one electrode (e.g., a metallic coating) is located on each of the inner and outer sides for electrical contact.

[0022] In one embodiment, the wall thickness of the piezo element is between 0.5 mm and 2 mm. Advantageously, the wall thickness for sensors with a diameter of 1 to 0.5 inches (thus suitable for typical industrial process connections) is between 0.75 mm and 1.25 mm. The outer tube diameter preferably depends on the temperature range in which the vibration sensor is to be used, as well as on the type of steel from which the mechanically vibrating unit is made. For austenitic stainless steels such as 1.4404 or 1.4435, Hastelloy 2.4819, or duplex steel 1.4462, and a temperature range of -50 °C to 160 °C, the outer diameter of the piezo element is less than or equal to 8 mm.

[0023] In one embodiment, the piezoelectric element, which has electrodes on an outer and inner side, is radially polarized. In this embodiment, the two electrodes, which are mounted on the inner and outer surfaces of the hollow piezoelectric element, preferably each have a different sign (plus or minus).

[0024] One embodiment provides for at least one electrode to be arranged on the outside and one on the inside. In this embodiment, contacting occurs when an electrode is located inside and outside. This has the advantage that the contacting of both electrodes can be made via one side—either inside or outside. This also simplifies manufacturing.

[0025] In one embodiment, at least two electrodes are applied to the inside or outside. In another embodiment, a single electrode (on the outside or inside) is located opposite the two electrodes.

[0026] One embodiment consists of two electrodes arranged on the inside and one full-surface electrode on the outside. The two electrodes on the inside of the hollow piezo element are electrically separated from each other. In one embodiment, the two internal electrodes have essentially the same surface area.

[0027] In an extended embodiment, the piezo element is radially polarized such that regions of the piezo element between the two inner electrodes and the outer electrode are polarized in different directions. The piezo element is thus radially polarized overall, but not in a uniform direction: If the polarization between an inner electrode and the outer electrode runs from the inside out, the region between the other inner electrode and the outer electrode is polarized inversely, from the outside in.

[0028] In a related design, only the two inner electrodes are connected to an electronic unit. In a complementary design, the outer electrode is connected to ground. Whether or not ground is connected depends on the design of the electronics that applies an excitation signal to the piezo element and / or processes the received signal generated by the piezo element. An elegant solution is to leave the outer electrode unconnected.

[0029] According to one embodiment, the piezoelectric element has a circular-cylindrical outer shape and two end faces, with at least one electrode arranged on each end face. The piezoelectric element is preferably polarized along its longitudinal axis. In one embodiment, the piezoelectric element has a continuous recess, i.e., it is hollow-cylindrical with a circular base.

[0030] One embodiment consists in that at least one sensor is arranged in the recess, and that the sensor is designed to measure a temperature and / or humidity and / or hydrogen content. In this embodiment, the hollow shape of the piezo element is used to bring a sensor close to the carrier unit or close to the process. The sensor is, for example, a temperature sensor, a humidity sensor, or a hydrogen sensor. The sensor can also be in the form of a chip. The sensor can therefore help monitor the process in which the vibration sensor is used. This makes it possible, for example, to compensate for the effects of temperature on the oscillation frequency. Alternatively or additionally, the sensor helps to monitor the vibration sensor. For example, increased humidity can serve as a warning signal that the vibration sensor is leaking.

[0031] One embodiment provides that the vibration sensor has a plug for electrical contact, and that the transducer device is directly connected to a plug. In this embodiment, the transducer device is not contacted via electrical conductors, e.g., stranded wires, but rather a plug. The plug has, for example, a circuit board and / or a microcontroller. Preferably, the sensor according to one of the aforementioned embodiments is also connected to the plug. The microcontroller is preferably designed to process the signals from the sensor and / or the transducer device.

[0032] One embodiment consists in the vibration sensor further comprising a housing, the support unit enclosing the housing on one side, and the support unit being rigidly coupled to the housing. In this embodiment, the transducer device is surrounded by a housing. The support unit and the housing form an overall unit. The support unit and the housing—e.g., adjoining it laterally—are rigidly coupled to one another. For example, in one embodiment, the support unit merges integrally into the housing.

[0033] An alternative embodiment provides that the vibration sensor further comprises a housing, that the carrier unit closes off the housing on one side, and that the carrier unit is connected to the housing via a decoupling component. In this embodiment, for example, the carrier unit is also integrally connected to the housing, with a reduction in the wall thickness around the carrier unit serving to decouple the mechanical connection between the carrier unit and the housing.

[0034] In one embodiment, the carrier unit is circular and the housing is circular-cylindrical.

[0035] In one embodiment, the vibration sensor further comprises a mounting aid, wherein the mounting aid serves to insert and / or center the piezo element in the housing. In this embodiment, the piezo element is inserted into the housing and thus also into the recess via a mounting aid and / or is centered there. In one embodiment, the mounting aid has an outer diameter such that it is guided along the inner wall of the housing during insertion. The piezo element is attached to the mounting aid at such a location that it reaches the recess of the carrier unit when the mounting aid is inserted into the housing.

[0036] The invention is explained in more detail with reference to the following figures.

[0037] Fig. 1 shows a section through a first embodiment of a schematically illustrated vibration sensor,

[0038] Fig. 2 shows a spatial representation of a first embodiment of a cylindrical piezo element,

[0039] Fig. 3 shows a section through the piezo element of Fig. 2,

[0040] Fig. 4 shows a section through a second embodiment of a piezo element,

[0041] Fig. 5 shows a section through a second embodiment of a schematically illustrated vibration sensor,

[0042] Fig. 6 shows a spatial representation of a third embodiment of a cylindrical piezo element,

[0043] Fig. 7 shows a section through the piezo element of Fig. 6,

[0044] Fig. 8 shows a section through a third embodiment of a schematically illustrated vibration sensor and

[0045] Fig. 9 shows a section through a fourth embodiment of a cylindrical piezo element.

[0046] Fig. 1 shows the basic structure of a vibration sensor.

[0047] The housing 9, which is circularly cylindrical here, ends at the front with a very thick support unit 4. On the support unit 4 there are two oscillating elements 5, which are designed as prongs of the mechanically oscillating unit 1. The areas of the

[0048] The support unit 4, above the oscillating elements 5, contains the contact areas 6 for the oscillating elements 5. In the support unit 4, centrally between the contact areas 6, there is a circular-cylindrical bore in the form of a recess 7. A piezo element 3 of the transducer device 2 is arranged in the recess 7. Preferably, this is only a single piezo element 3. The piezo element 3 is designed as a hollow circular cylinder that is radially polarized.

[0049] Electrodes (see, for example, Fig. 3) are located on the inside and outside of the piezo element 3. These electrodes are connected to an electronic unit 12 via electrical lines 13, e.g., in the form of stranded wires. When an alternating voltage is applied to the electrodes, and thus to the sides of the piezo element 3, the piezo element expands or shrinks harmoniously in diameter. These movements lead to lateral forces that are transferred to the carrier unit 4 and a slight deformation of the carrier unit 4. The carrier unit 4 transfers these forces to the oscillating elements 5. The oscillations generated by the oscillating elements 5 are preferably at the fundamental resonant frequency.

[0050] Conversely, the mechanical vibrations of the vibrating elements 5 are transmitted via the carrier unit 4 to the piezo element s and thus converted into electrical signals. These signals can then be evaluated by the electronics unit 12 with regard to the process variable of interest—e.g., fill level, density, or viscosity.

[0051] It can be seen that the piezo element 3 protrudes beyond the recess 7. This simplifies contact with the lines 13. The piezo element 3 and the recess 7 are arranged coaxially with each other. In the illustrated embodiment, this means, in particular, that the longitudinal axes of the circular-cylindrical recess 7 and the circular-cylindrical piezo element 3 coincide.

[0052] In the illustrated embodiment, the central recess 30 of the piezo element 3 is used to guide a sensor 8 and bring it closer to the process. Sensor 8 is a temperature sensor, whose measured values ​​allow the electronics unit 12 to compensate for a temperature-dependent drift of the resonant frequency of the mechanically oscillating unit 1. Alternatively, it can be a humidity sensor or a hydrogen sensor.

[0053] Fig. 2 shows a hollow cylindrical piezo element 3 with a circular base. The piezo element 3 is radially polarized, so that the electrodes on the inner side 32 and the outer side 31 each have a different polarity. The end face 33 is relevant for the design of Fig. 4, in which an electrode is guided from the inside to the outside over the end face 33. Fig. 3 shows a section through the piezo element 3 of Fig. 2.

[0054] The continuous recess 30, the electrode E1 on the outside 31 and the electrode E2 on the inside 32 can be seen. If an alternating voltage is applied to the electrodes E1, E2, the piezo element 3 alternately expands and contracts.

[0055] In the embodiment shown in Fig. 4, a contact reconnection has been implemented. This involves extending the electrode E1 on the outer side 31 over the end face 33 to the inner side 32. This makes it possible to contact both sides (inner side 32 and outer side 31) of the piezo element 3 from one side (here: inner side 32). This simplifies manufacturing.

[0056] The design of Fig. 5 is similar to the variant of Fig. 1 , so only the differences are discussed.

[0057] The support unit 4 and the housing 9 are decoupled from each other by an annular recess around the support unit 4. The recess is therefore an exemplary embodiment of a decoupling component 10. Therefore, in particular, essentially no vibrations are transmitted between the support unit 4 and the housing 9. The support unit 4 is therefore essentially coupled only to the two oscillating elements 5.

[0058] Furthermore, a connector 11 is provided here, which is directly connected to the piezo element 3. In one embodiment, the connector 11 has an evaluation unit that processes the electrical signals of the piezo element 3. For example, as in the embodiment of Fig. 1, a sensor 8 can also be provided, the measurement signals of which are used to evaluate the signals of the piezo element 3.

[0059] Figures 6 and 7 show a design of the piezo element, which is designed as a one-piece hollow cylinder with a circular base.

[0060] On the inner side 32, there are two separate electrodes E2, and on the outer side, there is a full-surface electrode E1. The two inner electrodes E2 are polarized inversely to the outer electrode E1.

[0061] The piezoelectric element is radially polarized, resulting in two oppositely polarized parts or regions: one polarization runs from inside to outside and one polarization runs from outside to inside. In each case, a polarization region is located between an inner electrode E2 and the outer electrode E1. Consider the section in Fig. 7. The region of the piezoelectric element between the inner electrode E2 (located on the left here) and the outer electrode E1 is radially polarized from inside to outside. Conversely, the region of the piezoelectric element between the right inner electrode E2 and the outer electrode E1 is polarized from outside to inside.

[0062] Only the two inner electrodes E2 are connected to the electronics unit. The outer electrode E1 is only a virtual ground that is not further connected.

[0063] One advantage is that the active electrodes E2 are electrically insulated from the metallic housing (see Fig. 1) by the piezoelectric material. Furthermore, the electrodes E2 only need to be electrically contacted on one side of the piezoelectric element, and in particular, only on the inside.

[0064] Fig. 8 shows an embodiment in which the piezo element 3 is arranged between a mounting aid 14 and the carrier unit 4.

[0065] For assembly, the piezo element s is glued to the mounting aid 14, resulting in a one-piece assembly component. The mounting aid 14—here, for example, a plate-like one—is wide enough to be guided along the inner wall of the housing 9 during assembly. By correctly positioning the piezo element 3 on the mounting aid 14, the piezo element 3 then enters the recess 7. This support during assembly is particularly helpful when dealing with a sensor with a so-called tube extension, i.e., a very long housing 9.

[0066] Fig. 9 shows a piezoelectric element with a circular-cylindrical configuration and a continuous recess 30. An electrode E1, E2 is located on each of the two end faces 33. The piezoelectric element 3 itself is polarized in the longitudinal direction, i.e., from end face to end face 33. In one embodiment, additional electrodes may also be present.

[0067] List of reference symbols

[0068] 1 mechanically oscillating unit

[0069] converter device

[0070] Piezo element

[0071] carrier unit

[0072] 5 Oscillating element

[0073] Contact area

[0074] 7 Deepening

[0075] 8 Sensor

[0076] 9 housings

[0077] 10 Decoupling component

[0078] 11 plugs

[0079] 12 Electronic unit

[0080] 13 Management

[0081] 14 Assembly aid

[0082] 30 continuous recess

[0083] 31 Outside of the piezo element

[0084] 32 Inside of the piezo element

[0085] 33 Front side of the piezo element

[0086] E1, E2 electrode

Claims

Patent claims 1 . Vibration sensor, with at least one mechanically oscillating 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 transducer device (2) has at least one piezo element (3), wherein the mechanically oscillatable unit (1) has at least one carrier unit (4) and one oscillating element (5), wherein the oscillating element (5) is fastened to the carrier unit (4) in a contact region (6), wherein the carrier unit (4) has a recess (7), wherein the recess (7) lies outside the contact region (6) of the oscillating element (5), and wherein the piezo element (3) is arranged in the recess (7) - and preferably glued in the recess (7).

2. Vibration sensor according to claim 1, wherein the recess (7) and the piezo element (3) are designed and matched to one another and the piezo element (3) is arranged in the recess (7) in such a way that the piezo element (3) protrudes from the recess (7).

3. Vibration sensor according to claim 1 or 2, wherein the recess (7) has a circular cylindrical inner shape and / or the piezo element (3) has a circular cylindrical outer shape.

4. Vibration sensor according to one of claims 1 to 3, wherein the mechanically oscillatable unit (1) has two oscillating elements (5), wherein the two oscillating elements (5) are each fastened to the carrier unit (4) in a contact region (6), and wherein the recess (7) lies outside the contact regions (6) of the two oscillating elements (5).

5. Vibration sensor according to one of claims 1 to 4, wherein the piezo element (3) has a continuous recess (30), an outer side (31) and an inner side (32), and wherein at least one electrode (E1, E2) is present on the outer side (31) and on the inner side (32).

6. Vibration sensor according to claim 5, wherein at least one electrode (E1, E2) is arranged on the outer side (31) and on the inner side (32).

7. Vibration sensor according to claim 5 or 6, wherein two electrodes (E2) are arranged on the inner side (32) and one full-surface electrode (E1) on the outer side (31), and wherein the piezo element (3) is radially polarized such that regions of the piezo element (3) between the two electrodes (E2) on the inner side (32) and the electrode (E1) on the outer side (31) are polarized in different directions.

8. Vibration sensor according to one of claims 1 to 7, wherein the piezo element (3) has a circular cylindrical outer shape and two end faces (33), and wherein at least one electrode (E1, E2) is arranged on each of the end faces (33).

9. Vibration sensor according to one of claims 5 to 8, wherein at least one sensor (8) is arranged in the recess (30), and wherein the sensor (8) is designed to measure a temperature and / or a humidity and / or a hydrogen content.

10. Vibration sensor according to one of claims 1 to 9, wherein the vibration sensor further comprises a housing (9), wherein the carrier unit (4) closes off the housing (9) on one side, and wherein the carrier unit (4) is rigidly coupled to the housing (9). 1 1. Vibration sensor according to one of claims 1 to 9, wherein the vibration sensor further comprises a housing (9), wherein the carrier unit (4) closes off the housing (9) on one side, and wherein the carrier unit (4) is connected to the housing (9) via a decoupling component (10).

12. Vibration sensor according to claim 10 or 11, wherein the vibration sensor further comprises a mounting aid (14), and wherein the mounting aid (14) for inserting and / or centering the piezo element (3) into the housing (9).

13. Vibration sensor according to one of claims 1 to 12, wherein the vibration sensor has a plug (11) for electrical contact, and wherein the transducer device (2) is directly connected to a plug (11).

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