Vibration-type sensor
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2024-03-26
- Publication Date
- 2026-03-18
AI Technical Summary
Vibration sensors face mechanical stress and potential damage due to thermal expansion coefficient differences between metal and ceramic materials at high temperatures, leading to mechanical failures and depolarization of piezoelectric elements.
A vibration sensor design featuring a star-shaped contacting electrode with alternating club-like elevations and trough-like depressions reduces radial stresses and distributes forces evenly, using materials with low thermal expansion coefficients and avoiding solid connections to prevent mechanical stress and damage at high temperatures.
The sensor can operate effectively at temperatures greater than 200°C by minimizing mechanical stress and preventing damage from thermal expansion, ensuring reliable performance in high-temperature applications.
Smart Images
Figure EP2024058111_14112024_PF_FP_ABST
Abstract
Description
[0001] VIBRATION TYPE SENSOR
[0002] 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.
[0003] Vibration sensors are known in the art, for example, in the form of tuning forks or single-rod sensors. Such sensors have a mechanically oscillating unit that is excited to mechanical vibrations by a drive / receiver unit. The vibrations resulting from the interaction with a medium to be measured or monitored are received by the drive / receiver unit and sent for evaluation. For example, a measurement exploits the fact that the vibration frequency or amplitude changes when the mechanically oscillating unit transitions from an uncovered state to one covered by the medium. This allows, for example, the monitoring of the fill level of the medium in a container.
[0004] The drive / receiver unit typically contains a converter device that converts between electrical signals and mechanical vibrations. Piezoelectric elements forming a stack are often used for this purpose. Contacting electrodes, also known as solder tabs, are used to make contact. WO 01 / 66269 A1 discloses two examples of the shape of contacting electrodes with molded tabs. In one variant, the contacting electrode is star-shaped, with the spaces between the fingers intended to accommodate adhesive used during manufacturing. In another embodiment, the contacting electrode is circular, which has the advantage of a larger and more uniform contact surface.
[0005] The construction of the transducer device requires that different materials—metal for the contact electrode and ceramic for the piezoelectric element—are connected to each other in a force-locking manner. Particularly in high-temperature applications, the different thermal expansion coefficients can lead to mechanical stresses that may even exceed the fracture stress of the ceramic. In extreme cases, such stresses can also cause depolarization of the piezoelectric elements. However, the more common case is the breakage of the ceramic elements. An electrode, often at least partially made of silver, is usually located on the piezoelectric element. Thus, the construction is: ceramic of the piezo element, electrode of the piezo element, and on top of it the contact electrode.
[0006] Designs of contacting electrodes can be taken from the state of the art, such as EP 1 134 038 A1 or DE 102019 132 021 A1 , which each show solder lugs with rectangularly opening tips.
[0007] DE 10 2004 007 767 A1 shows an electrode that extends between several piezo elements.
[0008] The invention is based on the object of proposing a vibration sensor in which mechanical stresses in the transducer device are reduced.
[0009] The object is achieved by a vibration sensor with a mechanically oscillatable unit and with 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 transducer device has at least one piezo element and at least one contacting electrode, wherein the contacting electrode has an outer border, wherein the outer border is designed essentially star-shaped with alternating elevations and depressions, wherein the depressions are designed essentially trough-like and / or the elevations are designed essentially club-like and wherein the elevations have a rounded shape.
[0010] The contacting electrode (an alternative name is soldering lug) of the vibration sensor according to the invention is star-shaped, with elevations (which can also be referred to as rays, peaks or points of the star) and depressions (which can also be referred to as valleys) alternating around it. The depressions have a trough shape, so that they have a rather flat or largely uniform course in a central section. The elevations have a club shape, so that they have a rather broad and preferably rounded course in the radially outer region. The elevations therefore do not end in a point. The largely uniform course of the depressions and the rather broad elevations reduce possible radial stresses that can occur particularly in the area of the outer edge.The rounded shape of the elevations prevents mechanical stress peaks that can occur when the contacting electrode is connected to the electrode of a piezo element.
[0011] Extensive studies have shown that the outer edge area of the contacting electrode, in particular, is subjected to high stress during temperature increases. Therefore, the outer edge of the invention is designed such that the recesses draw material from the contacting electrode from the outer area into a radially deeper area, while the elevations leave less material in the radially outer area. The elevations are widened, particularly by their cone shape, so that forces occurring there are distributed as evenly as possible over a larger area. Therefore, pointed ends are avoided.
[0012] Due to the alternation of elevations and depressions of the contacting electrode, in an exemplary embodiment, only approximately 22% of a circular ring around a center point of the contacting electrode is covered by material.
[0013] The special design of the converter device, which can alternatively also be referred to as a drive, offers the advantage that the vibration sensor can also be used at high temperatures, in particular greater than 200 °C.
[0014] In the current state of the art, the contact electrodes are usually made of copper alloys (bronze, brass) with a gold, silver, or tin coating. If temperatures exceed 150 °C for extended periods, such contact electrodes are welded to the silver electrodes of the piezo elements. If temperatures exceed 200 °C during application, this can lead to a loss of contact pressure due to the different thermal expansion coefficients of the piezoceramic (approx. 6*1 O'). 6 1 / K) and the force-welded contact electrode (approx. 19*10 -6 1 / K) can cause damage to the vibration sensors. This is prevented by the invention.
[0015] The contacting electrode is preferably designed as a flat disc.
[0016] In one embodiment, the ends of the elevations form an imaginary circle around a center point of the contacting electrode. In this embodiment, the geometry of the contacting electrode is determined from a circle in which the depressions are present as a reduction of the material.
[0017] In one embodiment, the transducer device has a plurality of piezo elements, preferably arranged in a stack. Preferably, each of the piezo elements is assigned at least one contacting electrode. Alternatively or additionally, the transducer device has a plurality of contacting electrodes, which may be identical or different.
[0018] One embodiment of the vibration sensor provides for at least one contact tab to be formed on the contacting electrode. Such a contact tab is described, for example, in WO 01 / 66269 A1. Both the contacting electrode and the contact tab are electrically conductive.
[0019] In one embodiment, the contact flag is designed as a strip.
[0020] One embodiment includes the contact tab being arranged such that the contact tab merges into a raised portion of the outer edge of the contacting electrode. In this embodiment, the geometries of the raised portion and the contact tab partially merge, forming a single unit in a transition region. One embodiment provides for the outer edge to have four raised portions. In this embodiment, the contacting electrode is a four-pointed star; in another embodiment, one point can be extended by the contact tab.
[0021] One embodiment of the vibration sensor includes the contacting electrode being at least partially configured as a mesh. In this embodiment, the contacting electrode does not have a completely closed surface, but is formed from overlapping and interwoven threads—particularly made of metal. This allows for a certain degree of compensation for thermal expansion effects within the contacting electrode itself.
[0022] One embodiment provides that the contacting electrode has at least one recess within its outer border - preferably in the form of a through hole or gap. One embodiment of the vibration sensor includes multiple recesses within the outer border. In this embodiment, the contacting electrode therefore has multiple recesses, preferably designed as holes. One embodiment of the vibration sensor provides that at least a majority of the recesses are arranged point-symmetrically to a center point of the contacting electrode. This symmetry is intended to ensure a uniform distribution of the forces that can result from thermal expansion. One embodiment of the vibration sensor includes at least some of the recesses being arranged at a substantially equal radial distance from the center point of the contacting electrode.In this embodiment, the recesses are partially arranged in a circle around the center of the contacting electrode. One embodiment of the vibration sensor provides that the at least one recess, or at least some of the recesses, are circular or star-shaped. One embodiment provides that a center point of the contacting electrode is located within the at least one recess. In this embodiment, there is thus a recess in the center of the contacting electrode. The remaining material of the contacting electrode is preferably arranged substantially symmetrically around this recess.
[0023] A supplementary or alternative embodiment provides that the contact lug has at least one recess.
[0024] Preferably, the contacting electrode is characterized by a structure that is as symmetrical as possible in terms of its geometry.
[0025] One embodiment is characterized in that the contacting electrode and an electrode applied to the piezoelectric element are made of materials that essentially do not form solid bonds at high temperatures or whose alloying elements do not dissolve into each other. A suitable combination of materials is intended to prevent solid bonds between the contacting electrode and an electrode applied directly to the piezoelectric element, which could lead to problems in the application due to different thermal expansion coefficients.
[0026] One embodiment consists in the contacting electrode containing a chromium component. This embodiment is particularly advantageous when the electrode located on the piezoelectric element is at least partially made of silver. The aim is to prevent the silver of the silver electrode from diffusing into the metal structure of the contacting electrode and from forming a solid bond or solid solution with each other. Preferably, an oxide layer is additionally created on the metal of the electrode of the piezoelectric element and / or the contacting electrode at high temperatures (450°C to 550°C), which represents an additional barrier to diffusion.
[0027] According to one embodiment, the contacting electrode consists at least partially of a stainless alloy with a thermal expansion coefficient of less than 15 * 10' 61 / K. In one embodiment, the material of the contacting electrode is a Fe-Cr alloy 1.4016 (or AISI 430) with a low thermal expansion coefficient of 10.5* 10' 6 1 / K, with a relatively high chromium content and a ferrite structure. Cr-Fe alloys are characterized by their very low thermal expansion coefficients within the stainless steel material group. Alternatively, nickel-based alloys such as Hastelloy 2.4819 (or C-276) with a thermal expansion coefficient of 12.5 x 10' 6 1 / K or INCONEL alloy 783 with an expansion coefficient of approx. 10.5* 10' 6 1 / K can be used.
[0028] A further embodiment provides that the converter device has at least two contacting electrodes, and that the at least two contacting electrodes are connected to one another via a contact tab. The contacting electrodes are preferably of identical design. The contact tab connects the contacting electrodes to one another.
[0029] A supplementary embodiment includes the converter device having at least two piezo elements arranged in a stack, and each of the two contacting electrodes being connected, i.e., contacted, to another of the two piezo elements. If multiple piezo elements are arranged in a stack, this embodiment allows the piezo elements to each be contacted with a contacting electrode and thus interconnected, since the individual contacting electrodes are in turn connected to one another via the contact tab. In addition to electrical contact, the contact tab also ensures that the height difference between the layers on which the contacting electrodes are inserted is overcome.
[0030] The invention is explained in more detail with reference to the following figures.
[0031] Fig. 1 shows schematically the structure of a vibration sensor,
[0032] Fig. 2 shows a first embodiment of the contacting electrode,
[0033] Fig. 3 shows a second embodiment,
[0034] Fig. 4 shows a third embodiment and
[0035] Fig. 5 shows a fourth embodiment. Fig. 1 shows a so-called tuning fork as an example of a design of the vibration sensor.
[0036] The mechanically oscillating unit 1 has two so-called fork tines connected to a diaphragm. On the opposite side of the diaphragm, in a housing indicated here, there is a transducer device 2, which in the example shown has two disc-like piezo elements 3. For electrical contacting of the two ceramic piezo elements 3—and the electrodes 3.1 applied thereto, preferably made of silver—a contacting electrode 4 is provided in the example shown. This contacting electrode 4 is made at least partially of a metal and is connected to a wire 6 as an exemplary embodiment of an electrical conductor.
[0037] Fig. 2 shows a first example of the contacting electrode 4. The outer border 40 is designed as a four-pointed star with four elevations 41 and four depressions 42 arranged between them. The depressions 42 are trough-shaped. Thus, there are two gently sloping flanks at the sides with a slight gradient, between which there is a rather flat intermediate section with only a slightly changing radius. This can also be described as the depressions 42 being significantly wider than they are deep. The elevations 41 are shaped like a club towards the radially outward direction. Therefore, the elevations 41 do not end in points, but rather have rounded and wider end regions.
[0038] A protrusion 41 merges into a contact lug 5, which is strip-shaped here.
[0039] Fig. 2 shows a closed surface of the contacting electrode 4 within its outer border 40. In the embodiment of Fig. 3, it has two recesses 43 within the outer border 40 or in the transition area between the contacting electrode 4 and the contact lug 5.
[0040] Fig. 4 shows an extension of the embodiment of Fig. 2.
[0041] There are three contact electrodes 4, each connected to each other by a contact tab 5. This design is used in a transducer device in which several piezo elements are stacked one above the other. Each contact electrode 4 is connected to a different piezo element, and the contact tabs 5 between the contact electrodes 4 are located on the side of the stack of piezo elements and serve to overcome the height difference.
[0042] In the variant of Fig. 5, two contacting electrodes 4 are connected to each other by a contact lug 5. Another contact lug 5 serves to contact this series circuit from the outside, e.g., by a wire 6 as indicated in Fig. 1.
[0043] Reference symbols
[0044] 1 mechanically oscillating unit
[0045] 2 Transducer device 3 Piezo element
[0046] 3.1 Electrode
[0047] 4 Contacting electrode
[0048] 5 Contact flag
[0049] 6 wire 40 outer border
[0050] 41 Survey
[0051] 42 Deepening
[0052] 43 recess
Claims
Patent claims 1 . Vibration sensor, with a mechanically oscillatable unit (1 ) and with a transducer device (2), wherein the transducer device (2) excites the mechanically oscillatable unit (1 ) to mechanical vibrations and / or receives mechanical vibrations from the mechanically oscillatable unit (1 ), wherein the transducer device (2) has at least one piezo element (3) and at least one contacting electrode (4), wherein the contacting electrode (4) has an outer border (40), wherein the outer border (40) is designed essentially star-shaped with alternating elevations (41 ) and depressions (42), wherein the depressions (42) are designed essentially trough-like and / or the elevations (41 ) are designed essentially club-like, and wherein the elevations (41) have a rounded shape.
2. Vibration sensor according to claim 1, wherein at least one contact lug (5) is formed on the contacting electrode (4), and wherein the contact lug (5) is arranged such that the contact lug (5) merges into a raised portion (41) of the outer perimeter (40) of the contacting electrode (4).
3. Vibration sensor according to claim 1 or 2, wherein the outer border (40) has four elevations (41).
4. Vibration sensor according to one of claims 1 to 3, wherein the contacting electrode (4) is at least partially designed as a mesh.
5. Vibration sensor according to one of claims 1 to 4, wherein the contacting electrode (4) has at least one recess (43) within the outer border (40).
6. Vibration sensor according to one of claims 1 to 5, if dependent on claim 2, wherein the contact tab (5) has at least one recess (43).
7. Vibration sensor according to one of claims 1 to 6, wherein the contacting electrode (4) and an electrode (3.1) applied to the piezo element (3) consist of materials which essentially do not form solid bonds at high temperatures or whose alloying elements do not dissolve into one another.
8. Vibration sensor according to one of claims 1 to 7, wherein the contacting electrode (4) has a chromium content.
9. Vibration sensor according to one of claims 1 to 8, wherein the contacting electrode (4) is at least partially made of a stainless alloy with a thermal expansion coefficient of less than 15 * 10' 6 1 / K exists.
10. Vibration sensor according to one of claims 1 to 9, wherein the transducer device (2) has at least two contacting electrodes (4), and wherein the at least two contacting electrodes (4) are connected to one another via a contact tab (5).