Vibration sensor
The vibration sensor employs piezoelectric elements with opposite polarizations and orthogonal orientation to enhance measurement accuracy by independently exciting and detecting two modes, addressing energy loss and sensitivity issues in viscous liquids.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- ENDRESS & HAUSER GMBH & CO KG
- Filing Date
- 2025-11-25
- Publication Date
- 2026-06-25
AI Technical Summary
Existing vibration sensors face challenges in accurately measuring process parameters such as fill level, density, and viscosity due to high energy loss and low sensitivity in different vibration modes, particularly in viscous liquids, as the oscillation frequency and amplitude are influenced differently by fluid properties.
A vibration sensor design utilizing two piezoelectric elements with opposite polarizations and a 90° rotation, stacked to generate orthogonal vibrations, allowing independent excitation and detection of two modes (SW1 and SW2) to enhance measurement accuracy and sensitivity across varying viscosities.
The design enables precise determination of viscosity and density by selectively using modes SW1 and SW2, improving measurement accuracy and efficiency in liquids with different viscosities, especially in low- and high-viscosity media.
Smart Images
Figure EP2025084074_25062026_PF_FP_ABST
Abstract
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 of a medium. The process parameter is, for example, fill level, density, or viscosity, wherein the medium is, for example, a liquid, a gas, or a bulk material.
[0003] The tuning fork is an example of a classic vibration sensor that oscillates at different frequencies and / or Q factors depending on the medium's properties, such as density or viscoelasticity. The two tines, which typically have a paddle with a wide and a narrow side, are located on a membrane and, in application, contact the medium. The paddles are aligned parallel to each other. The tines form a mechanical resonator or closed resonating circuit, where the oscillation frequency and amplitude depend solely on the medium's properties, including, for example, the fill level, i.e., the degree to which the tines are covered by the medium. The energy stored in the oscillating unit through resonant vibrations is partially lost through damping in the resonator and primarily through dissipation in the medium.For example, in highly viscous liquids, the vibrational energy is dissipated by molecular friction. Minimal energy loss within the tuning fork resonator itself is crucial for optimal measurement, especially when using liquids as the medium. The opposing reaction forces of the vibrating tines are intended to cancel each other out within the membrane.
[0004] It has been shown that only two modes of the tuning fork are characterized by enabling a closed resonant circuit with minimal energy losses through the process connection and high mechanical quality. The first is a basic mode, or mode SW1, in which the two fork tines oscillate perpendicular to their paddle surfaces. They thus move periodically towards and away from each other. The second is mode SW2, in which the fork tines move in a plane parallel to their paddle surfaces, and thus in the direction of their greatest extent. They therefore tilt alternately laterally away from each other and towards each other. These modes are described, for example, in connection with build-up detection in DE 100 14 724 A1 or for liquid detection in level switches in DE 10 2016 118 445 A1.
[0005] A fluid has a different effect on harmonic oscillations in the vibration modes SW1 and SW2. In mode SW1, the frequency and amplitude of the oscillations are strongly influenced by the fluid's density and viscosity, as the effective surface area A for the interaction between the paddle and the medium is large. The effective area A of the paddle and its moment of inertia can be described by a sensitivity coefficient Sswi.
[0006] The oscillation frequency F is therefore given as a function of the liquid density p at a constant temperature:
[0007] F is involved o the oscillation frequency in a vacuum.
[0008] The more viscous the liquid medium, the more the mechanical quality of the oscillating unit is reduced.
[0009] For mode SW2, the moment of inertia of the tines changes only negligibly compared to mode SW1. Since the effective paddle area (essentially just the edge of the paddle) is significantly smaller, the sensitivity coefficient is also significantly smaller, such that: Ssw2 « Sswi. Therefore, the oscillation frequency in mode SW2 decreases much less with a change in density than in mode SW1, resulting in low sensitivity to density.
[0010] Therefore, the two modes allow different process variables to be measured with a single vibration sensor. To enable this, both modes must be excited and the respective vibrations detected.
[0011] The object underlying the invention is therefore to propose a vibration sensor in which vibrations of the two aforementioned modes are possible.
[0012] The invention solves the problem by means of a vibration sensor, comprising a mechanically oscillating unit and a transducer device, wherein the transducer device excites the mechanically oscillating unit to mechanical vibrations and / or receives mechanical vibrations from the mechanically oscillating unit, wherein the transducer device comprises at least a first piezoelectric element and a second piezoelectric element, wherein the first piezoelectric element and the second piezoelectric element each have a first segment with a first polarization and a second segment with a second polarization, wherein the first polarization and the second polarization are each oppositely oriented to each other, wherein the first piezoelectric element and the second piezoelectric element are arranged in a stack with a longitudinal axis, and wherein the first piezoelectric element and the second piezoelectric element are arranged relative to each other in such a way thatthat the first piezoelectric element and the second piezoelectric element are rotated at an angle of 90° to each other around the longitudinal axis.
[0013] The sensor according to the invention uses at least two piezoelectric elements, such as those described in WO 2004 / 057283 A1. Each piezoelectric element has two segments or portions that are polarized in opposite directions. Thus, the piezoelectric elements are partially polarized from bottom to top and partially from top to bottom. The end faces of each piezoelectric element are coated with a metallic electrode for electrical contact. In one embodiment, the metallic electrode completely covers the respective end face and thus both segments. In an alternative embodiment, the electrode is divided and covers each segment separately. The advantage of the differently polarized segments is as follows: When electrical signals are applied, one segment contracts while the other expands. This allows, for example,A paddle located above the piezoelectric element tilts.
[0014] The two piezoelectric elements are stacked on top of each other. This allows the stack to be easily mounted, for example, below a paddle of the vibration sensor. The unique feature is that the two piezoelectric elements are rotated 90° relative to each other around the stack's longitudinal axis. If the piezoelectric elements were not rotated relative to each other, the segments with the same polarization would be aligned. With the 90° rotation, the segments are partially overlapping.
[0015] To understand the orientation of the piezoelectric elements, consider the configuration where the first and second segments are essentially the same size. The segments are therefore each half-disc. For further explanation, consider the configuration where the two piezoelectric elements have essentially the same base area. In this case, the piezoelectric elements each have the same size and shape. Finally, for the sake of clarity, consider the case where the second piezoelectric element is located above the first. This results in the following: Due to the 90° rotation, half of the two segments of the second piezoelectric element are located above the first segment of the first piezoelectric element. The other half of the two segments of the first piezoelectric element are located above the second segment of the first piezoelectric element.Since the piezoelectric elements have the same base area, the halves of the segments of the second piezoelectric element completely cover the segments of the first piezoelectric element. As described in the aforementioned publication WO 2004 / 057283 A1, appropriate application of force to the piezoelectric elements used here generates a tilting motion. By rotating the piezoelectric elements 90° relative to each other, two tilting motions offset by an angle of 90° can be generated. This is utilized according to the invention to cause the mechanically oscillating unit to oscillate in two orthogonal directions. Since a stacked drive is used overall, higher mechanical forces can be generated by adding further piezoelectric elements, which increases the accuracy of the measurements.
[0016] One embodiment consists in the mechanically oscillating unit having at least one oscillating element, and the oscillating element being designed such that it has different dimensions in two orthogonal directions, namely a narrow side and a wide side. In this embodiment, the mechanically oscillating unit has at least one oscillating element, which can also be described, for example, as a paddle or flat prong. The converter device is preferably oriented relative to the at least one oscillating element such that the two tilting movements coincide with the two sides of the oscillating element. Thus, one tilting movement occurs in the direction of the narrow side and the other in the direction of the wide side (or, alternatively, perpendicular to the direction of the narrow side).During vibration parallel to the tine surface (i.e., in the direction of the greater extent through the wide side), the vibration frequency and the mechanical quality factor are reduced less by the medium than during vibration perpendicular to the paddle surface (i.e., in the direction of the smaller extent through the narrow side, which can also be referred to as the edge of the paddle). The viscosity of the medium determines Lehr's damping coefficient, so that for low-viscosity liquids, the viscosity can be determined from the vibration amplitude or a decay function using mode SW1. For high-viscosity liquids, mode SW2 is preferably used to determine the viscosity, as this mode is associated with less damping by the medium.
[0017] One embodiment includes the vibration sensor further comprising an electronic unit, and the electronic unit controlling the transducer device in such a way that the vibrating element performs mechanical vibrations in at least one of the two orthogonal directions.
[0018] One design involves the piezoelectric elements being disc-shaped.
[0019] One embodiment involves each piezoelectric element having a circular base. Another embodiment involves each piezoelectric element having a square base. The square structure of the piezoelectric elements simplifies their relative orientation during manufacturing.
[0020] One design involves the piezoelectric elements each having a ring shape.
[0021] One embodiment of the transducer device comprises additional piezoelectric elements, arranged in the stack with the first and second piezoelectric elements. These additional piezoelectric elements are mechanically coupled to either the first or the second piezoelectric element and are oriented in the same way. This embodiment increases the drive efficiency by force-fit connection of additional or supplementary piezoelectric elements to the first or the second piezoelectric element. The arrangement is mirror-symmetrical between each additional piezoelectric element and the first or second piezoelectric element, respectively. The additional piezoelectric elements have, in particular, the same basic structure as the first and second piezoelectric elements, i.e., they also have two differently oriented polarizations. Furthermore, the additional piezoelectric elements are preferably provided with an electrode on both end faces.An electrical parallel circuit is preferably created overall.
[0022] One embodiment includes the transducer device having contact electrodes that are arranged in the stack between the piezoelectric elements and are electrically contacted with them.
[0023] One embodiment consists in the mechanically oscillating unit having two oscillating elements, and in each oscillating element being assigned a stack with at least one first piezoelectric element and one second piezoelectric element.
[0024] The invention is explained in more detail with reference to the following figures. They show:
[0025] Fig. 1: a spatial representation of a vibration sensor design,
[0026] Fig. 2: a side view of a stack of piezoelectric elements of a transducer device,
[0027] Fig. 3: an exploded view of part of a first embodiment of a stack of piezoelectric elements,
[0028] Fig. 4: an exploded view of a second embodiment of a stack of piezoelectric elements, Fig. 5: a section through part of a vibration sensor according to a first embodiment,
[0029] Fig. 6: a schematic representation of the circuitry of the converter device of the sensor of Fig. 5,
[0030] Fig. 7: a section through part of a vibration sensor according to a second embodiment and
[0031] Fig. 8: a section through a part of a vibration sensor according to a third embodiment.
[0032] Fig. 1 shows a so-called vibrating fork as an example of a vibration sensor design. The mechanically vibrating unit 1 has two so-called fork tines or paddles as vibrating elements 10, which are connected to a diaphragm 5. On the inner side of the diaphragm 6 (not shown here), a transducer device 2 is located in a housing 6 (indicated here). This transducer device is supplied with electrical excitation signals by the electronic unit 3 and transmits electrical reception signals to the electronic unit 3.
[0033] The oscillating elements 10 each have a wide side 12 and a narrow side 11. Both oscillating elements 10 are intended to be capable of performing mechanical oscillations in the two directions R1 and R2 (indicated by the arrows). Direction R1 is the direction in which the oscillating elements 10 have their greater extent through the wide side 12. The narrow side 11, which can also be described as the edge of the paddle, is located in the perpendicular direction R2.
[0034] Figure 2 shows a stack of four piezoelectric elements 21, 22, 24, each shaped as a disk belonging to the transducer device 2, arranged and connected to form a stack 23. Each piezoelectric element 21, 22, 24 consists of two segments S1, S2 with polarization in opposite directions. This is indicated by the arrows.
[0035] The piezoelectric elements 21, 22, and 24 each form groups such that the first piezoelectric element 21 and another piezoelectric element 24 belong together, and the second piezoelectric element 22 and another piezoelectric element 24 belong together. Within these groups, segments S1 and S2 are arranged one above the other. The two groups, or rather the first piezoelectric element 21 and the second piezoelectric element 22, are rotated 90° relative to each other about the longitudinal axis L of the stack 23. Thus, only one segment—here, for example, S1—of the second piezoelectric element 22 is located above the two segments S1 and S2 of the first piezoelectric element 21.
[0036] Since one advantage of these special piezo elements 21 , 22, 24 is that they can generate a tilting movement in one direction, the stack 23 shown can accordingly generate tilting movements in two directions that are perpendicular to each other.
[0037] Figure 3 shows how the first piezoelectric element 21 and another piezoelectric element 24 are arranged relative to each other as part of a stack 23. Their orientation is mirror-symmetrical, so that the polarities of the touching segments are opposite each other or point away from each other. This is indicated here by the positive and negative poles (see also the arrows in Figure 2). This amplifies the drive force and creates an electrical parallel circuit.
[0038] Between the two piezoelectric elements 21, 24 – or more precisely: between the electrodes located on the end faces of the piezoelectric elements 21, 24 – is a contact electrode 4, which can also be referred to as a solder lug. The contact electrode 4 serves to apply and receive electrical signals for determining and / or monitoring process parameters, such as viscosity.
[0039] It can be seen that the piezoelectric elements 21 and 24 have a square base and a through-hole. They are therefore, overall, angular, disc-shaped rings. The contact electrode 4 also has a through-hole.
[0040] Figure 4 shows the components of an alternative embodiment of the stack of a transducer device. The piezoelectric elements 21, 22, 24 are each round, disc-shaped rings. The contact electrodes 4 located between them also have corresponding recesses in the center and have the same outer diameter as the piezoelectric elements 21, 22, 24. The 90° rotation of the lower piezoelectric elements 21, 24 and the upper piezoelectric elements 22, 24 relative to each other is clearly visible. By applying electrical signals to the contact electrodes 4, the desired two tilting movements can therefore be generated.
[0041] The ring-shaped piezo elements 21 , 22, 24 and the ring-shaped contact electrodes 4 simplify the attachment of the drives in a tuning fork.
[0042] Figure 5 shows a vibration sensor setup with a tuning fork as the mechanically oscillating unit 1. Each of the two oscillating elements 10 is associated with a stack of a first piezoelectric element 21 and a second piezoelectric element 22. The housing 6 has a diaphragm 5 with two prongs 10. Four piezoelectric elements 21 and 22 are arranged on the inside of the diaphragm 5, secured by two screws 7. A beam serves as a connecting plate 8 between the mounting elements 7.
[0043] The two lower piezoelectric elements 22, located directly on the diaphragm 5, are oriented such that applying, for example, a harmonic electrical voltage at the resonant frequency excites only mode SW2, but not mode SW1. The piezoelectric elements 21 above them are oriented such that applying the harmonic electrical voltage at the resonant frequency excites only mode SW1. Thus, the special design of the piezoelectric elements 21 and 22 allows each of them to excite only one mode separately.
[0044] The contact electrodes 4 and the signal lines connected to them are connected to an electronic unit 3, as shown schematically in Fig. 6.
[0045] The piezoelectric elements 21 and 22 are operated in pairs to determine and evaluate the oscillation frequency and mechanical quality factor. The density and Newtonian viscosity of a liquid can be determined from the oscillation frequency and quality factor. For low-viscosity liquids, such as aqueous solutions, mode SW1 is used. For high-viscosity liquids, the viscosity can be determined using mode SW2. In one embodiment, the viscosity is determined using both modes to generate an average result or to perform a self-test. In another variant, mode SW2 is used additionally to determine the density in media with a high density greater than 1,500 kg / m³. 3 to determine.
[0046] Fig. 6 shows two stacks 23 for two oscillating elements 10, each consisting of a first piezoelectric element 21 and a further piezoelectric element 24, and of a second piezoelectric element 22 and a further piezoelectric element 24. All outer electrodes of the piezoelectric elements 21, 22, 24 are short-circuited and form a ground rail (upper connecting lead). The term "outer electrodes" here also includes those electrodes belonging to the first piezoelectric element 21 and the second piezoelectric element 22 that are opposite each other in the respective stack 23.
[0047] The signal electrodes of the first two piezoelectric elements 21 – located in the lower row – which excite mode S2, are also electrically connected to each other and are connected to a signal line. The signal electrodes of the second piezoelectric elements 22, which excite mode S1, are also short-circuited and are connected to another signal line. Figure 7 shows an alternative embodiment of the vibration sensor.
[0048] The transducer device 2 consists – as shown in the embodiment of Fig. 5 – of two stacks of piezoelectric elements, each of which is assigned to one of the two fork prongs of the mechanically oscillating unit 1. The elements are fastened by a screw and a stud bolt 7. The screw is tightened onto the stud bolt, thereby pressing the connecting plate 8, located above the two piezoelectric stacks, against the diaphragm 5. For this purpose, the connecting plate 8 has a continuous recess through which the stud bolt passes. In the embodiment shown, the piezoelectric elements are square and designed as continuous discs, i.e., without an inner recess.
[0049] In the embodiment shown in Fig. 8, as in the variant shown in Fig. 7, two stacks of piezoelectric elements are located between a connecting plate 8 and the diaphragm 5. Here, the pressure is built up by a circular cylindrical pressure screw as a fastening element 7, which is screwed into the internal thread of the housing 6.
[0050] Exemplary dimensions are: The thickness of the membrane 5 is between 0.5 mm and 3 mm, preferably between 1.5 mm and 2.5 mm. The diameter of the membrane 5 is between 20 mm and 50 mm, preferably between 25 mm and 45 mm. The connecting plate 8, which can also be referred to as a beam, is made, for example, of stainless steel, e.g., steel 1.4435, 1.4404, or 1.4462. An optimal thickness of the plate 8 is between 9 mm and 12 mm. The described piezoelectric disks of the transducer device 2 preferably have outer dimensions (in the case of circular disks, the diameter) between 7 mm and 14 mm, with a thickness between 1 mm and 2 mm. If the disks are annular, the inner diameter is preferably between 3 mm and 5 mm.
[0051] Reference symbol
[0052] 1 mechanically oscillating unit
[0053] Converter device
[0054] Electronic unit
[0055] Contact electrode
[0056] 5 Membran
[0057] Housing
[0058] 7 Fastening element
[0059] 8 Connecting plate
[0060] 10 oscillating element
[0061] 11 narrow side of the oscillating element
[0062] 12 wide side of the oscillating element
[0063] 21 first piezoelectric element
[0064] 22 second piezoelectric element
[0065] 23 stacks
[0066] 24 additional piezoelectric elements
[0067] L Longitudinal axis
[0068] R1 direction
[0069] R2 direction
[0070] S1 first segment
[0071] S2 second segment
Claims
Patent claims 1. Vibration sensor, comprising a mechanically oscillating unit (1) and a transducer device (2), wherein the transducer device (2) excites the mechanically oscillating unit (1) to mechanical vibrations and / or receives mechanical vibrations from the mechanically oscillating unit (1), wherein the transducer device (2) comprises at least one first piezoelectric element (21) and one second piezoelectric element (22), wherein the first piezoelectric element (21) and the second piezoelectric element (22) each have a first segment (S1) with a first polarization and a second segment (S2) with a second polarization, wherein the first polarization and the second polarization are each oppositely oriented, wherein the first piezoelectric element (21) and the second piezoelectric element (22) are arranged in a stack (23) are arranged with a longitudinal axis (L), and wherein the first piezo element (21) and the second piezo element (22) are arranged relative to each other such that the first piezo element (21) and the second piezo element (22) are rotated relative to each other by an angle of 90° about the longitudinal axis (L).
2. Vibration sensor according to claim 1, wherein the mechanically oscillating unit (1) has at least one oscillating element (10), wherein the oscillating element (10) is designed such that the oscillating element (10) has different dimensions in two orthogonal directions (R1, R2) by having a narrow side (11) and a wide side (12), and wherein the vibration sensor further comprises an electronic unit (3), and wherein the electronic unit (3) controls the transducer device (2) such that the oscillating element (10) performs mechanical oscillations in at least one of the two orthogonal directions (R1, R2).
3. Vibration sensor according to claim 1 or 2, wherein the piezo elements (21 , 22) are disc-shaped.
4. Vibration sensor according to one of claims 1 to 3, wherein the piezo elements (21 , 22) each have a square base area.
5. Vibration sensor according to one of claims 1 to 4, wherein the piezo elements (21 , 22) each have a ring shape.
6. Vibration sensor according to one of claims 1 to 5, wherein the transducer device (2) has further piezoelectric elements (24), wherein the further piezoelectric elements (24) are arranged in the stack (23) with the first piezoelectric element (21) and the second piezo element (22), wherein the further piezo elements (24) are mechanically coupled to the first piezo element (21) or to the second piezo element (22) and are oriented in the same way.
7. Vibration sensor according to one of claims 1 to 5, wherein the mechanically oscillating unit (1) has two oscillating elements (10), and wherein each oscillating element (10) is associated with a stack (23) with at least one first piezoelectric element (21) and a second piezoelectric element (22).