Vibration sensor with reduced vulnerability to bubble formation

Abstract

The present invention relates to an apparatus (1) for determining and / or monitoring at least one process variable (P) of a medium (M), comprising a mechanically vibrating unit (4), a driver / receiver unit (5), and an electronic unit (6), wherein the driver / receiver unit (5) is designed to excite the mechanically vibrating unit (4) by means of an electrical excitation signal to generate mechanical vibration, and to receive the mechanical vibrations of the mechanically vibrating unit (4) and convert them into electrical received signals, the electronic unit (6) is designed to generate an excitation signal based on the received signal, set the frequency of the excitation signal such that there is a predetermined phase shift between the excitation signal and the received signal, and to determine at least one process variable (P) based on the received signal, the mechanically vibrating unit (4) comprising a diaphragm (7), and the process-facing surface (0) of the diaphragm (7) is curved.

Description

Technical Field

[0001] This invention relates to an apparatus for determining and / or monitoring at least one process variable of a medium, comprising a unit capable of mechanical vibration, a driver / receiver unit, and an electronic unit. The medium is thus located in a container, for example, in a storage tank or in a pipeline. The process variable is, in particular, a predetermined fill level, flow rate, density, or viscosity of the medium. Background Technology

[0002] Vibration sensors are commonly used in process and / or automation engineering. In the case of leveling measuring devices, they have at least one unit capable of mechanical vibration, such as, for example, a vibrating fork, a single rod, or a diaphragm. In operation, it is excited to generate mechanical vibration by means of an actuator / receiver unit, which is typically in the form of an electromechanical transducer unit, which can then be, for example, a piezoelectric actuator or an electromagnetic actuator. The applicant has manufactured a variety of corresponding field devices, and these corresponding field devices are distributed, for example, under the names LIQUIPHANT or SOLIPHANT. The basic measurement principle is known in principle from numerous publications. The actuator / receiver unit excites the mechanically vibrating unit to generate mechanical vibration by means of an electrical excitation signal. Conversely, the actuator / receiver unit is capable of receiving the mechanical vibrations of the mechanically vibrating unit and converting them into electrical received signals. Therefore, the actuator / receiver unit can be either a separate actuator unit and a separate receiver unit, or a combined actuator / receiver unit.

[0003] In many cases, the driver / receiver unit is thus part of an electrical resonant feedback circuit, by means of which the unit capable of mechanical vibration is excited to generate mechanical vibration. For example, for resonant vibration, the resonant circuit condition must be met, according to which the amplification factor ≥ 1 and all phases appearing in the resonant circuit result in multiples of 360°. In order to excite and satisfy the resonant circuit condition, a defined phase shift must be ensured between the excitation signal and the received signal. Therefore, a predefined value for the phase shift is often set, thus setting a set value for the phase shift between the excitation signal and the received signal. For this purpose, various solutions are known from the prior art, including analog and digital methods, such as those described, for example, in documents DE102006034105A1, DE102007013557A1, DE102005015547A1, DE102009026685A1, DE102009028022A1, DE102010030982A1, or DE00102010030982A1.

[0004] Both the excitation and received signals are characterized by their frequency ω, amplitude A, and / or phase Φ. Therefore, changes in these variables are often used to determine the corresponding process variables. For example, process variables can be the fill level, a specified fill level, or the density or viscosity of the medium and its flow rate. For instance, given a vibrating level switch for a liquid, it's important to distinguish whether the vibrating unit is covered by the liquid or vibrating freely. These two conditions, free condition and covered condition, are differentiated, for example, based on different resonant frequencies, i.e., based on frequency shifts.

[0005] If the vibrating element is completely covered by the medium, then density and / or viscosity can only be determined using such a measuring device. Different possibilities for determining density and / or viscosity are also known from the prior art, such as those disclosed in documents DE10050299A1, DE102007043811A1, DE10057974A1, DE102006033819A1, DE102015102834A1, or DE102016112743A1.

[0006] One problem with sensors based on mechanical vibration is related to the presence of air bubbles in different media. Air bubbles have a significant impact on the viscoelasticity of liquids. Therefore, undesirable changes in the vibration frequency of the vibrating element, independent of the corresponding process variable being considered, can occur along with falsified measurements of the corresponding process variable.

[0007] Various factors, such as stirring or pumping operations during the process, degassing of dissolved air due to pressure reduction in the medium, or even changes in the medium temperature, can lead to the formation of bubbles in a liquid medium. Bubbles are particularly common in fresh or aqueous solutions. These separate bubbles thus function, distributed both within the medium and, consequently, on the surface of the corresponding sensor unit of the sensor, which includes a vibrating element.

[0008] For example, a method for determining the physical parameters of an aerated liquid using a Coriolis flowmeter is known from DE102015122661A1. A vibrating element is excited in two different vibration modes to produce mechanical vibrations that depend to varying degrees on the presence of air bubbles within the medium. The effect of the air bubbles on the measurement can be determined and corrected based on the ratio of values ​​calculated for density and / or mass flow rate under the two vibration modes.

[0009] However, it is desirable to avoid the adhesion of air bubbles and the resulting measurement inaccuracies from the outset. Therefore, starting with the cited prior art, this invention is based on the aim of improving the measurement accuracy of vibration sensors. Summary of the Invention

[0010] This objective is achieved by a device for determining and / or monitoring at least one process variable of a medium, the device comprising a mechanically vibrating unit according to the invention, an actuator / receiver unit, and an electronic unit. The actuator / receiver unit is designed to excite the mechanically vibrating unit to generate mechanical vibrations by means of an electrical excitation signal, and to receive the mechanical vibrations of the mechanically vibrating unit and convert them into an electrical received signal. The electronic unit is then designed to generate an excitation signal based on the received signal, setting the frequency of the excitation signal such that a predetermined phase shift exists between the excitation signal and the received signal, and to use the received signal to determine at least one process variable. According to the invention, the mechanically vibrating unit includes a diaphragm, wherein the process-facing surface of the diaphragm is curved.

[0011] The vibrating unit is part of the sensor unit of the device. For example, this is a diaphragm vibrator, a single rod, or a vibrating fork, and a given vibrating unit includes a diaphragm and / or one or more vibrating elements integrally formed on the diaphragm. The actuator / receiver unit can include at least one piezoelectric element, but it can also be an electromagnetic actuator / receiver unit. On one hand, it functions as an actuator / receiver unit to generate mechanical vibrations of the mechanically vibrating unit by means of an electrical excitation signal, which can be, for example, a sine wave or a rectangular wave signal. However, it is also used to receive mechanical vibrations and convert them into an electrically received signal. When the vibrating unit is covered by a medium, the mechanical vibrations of the mechanically vibrating unit are affected by the properties of the medium, making it possible to generate conclusions about at least one process variable using the received signal representing the vibrations of the mechanically vibrating unit. The mechanically vibrating unit is preferably excited at least temporarily to produce resonant vibrations.

[0012] The curved design of the diaphragm significantly reduces bubble deposition and / or adhesion in the diaphragm region. In contrast, on a flat surface, bubble deposition preferably occurs in the diaphragm region, where, due to Archimedes' principle, the bubble remains and cannot escape. Conversely, on a curved surface, the bubble moves outward from the center region of the surface due to buoyancy and is able to escape from there.

[0013] Many variations are conceivable for the specific design of the curved diaphragm, all of which fall under the scope of this invention. Therefore, the exact shape and properties of the diaphragm can be appropriately selected depending on the application, such as the sensor's mounting location or the medium. Some preferred embodiments of the diaphragm according to the invention are illustrated below by way of example.

[0014] In one embodiment of the invention, the surface of the diaphragm is therefore at least partially convex. This is particularly advantageous if the device is attached to the container via the upper or side wall of the container.

[0015] In an alternative embodiment of the invention, the surface of the diaphragm is at least partially concave. Such an embodiment is therefore advantageous given the integration of the device from the lower wall of the container.

[0016] In another embodiment, the surface is designed as a conical, truncated conical, hemispherical, or spherical shape.

[0017] Another embodiment includes a circular surface, particularly in the region at the center point of the surface.

[0018] An alternative embodiment includes a surface with a tip, particularly in the region at the center of the surface. A surface with a tip is particularly capable of accelerating the separation process of bubbles located on the diaphragm surface.

[0019] In another embodiment, the surface is designed to be symmetrical with respect to its center point. Symmetrical embodiments are advantageous relative to the vibrational characteristics of the vibrating elements.

[0020] It is advantageous if at least one vibrating element is integrally formed on the diaphragm. It is equally advantageous if at least two vibrating elements are integrally formed on the diaphragm. Therefore, a single rod or vibrating fork is preferred. However, more than two vibrating elements can also be integrally formed on the diaphragm.

[0021] In this regard, it is advantageous if the transition region between the diaphragm and the vibrating element has a correspondingly predetermined transition radius. In this way, the adhesion of air bubbles in the transition region can be significantly reduced or avoided.

[0022] Another embodiment includes a plane parallel to the longitudinal axis passing through the diaphragm with an angle less than or equal to 45° between it and the tangent to the diaphragm surface. Such embodiments are particularly reasonable if the device is an electronic vibration multisensor, as described, for example, in a previously unpublished international patent application with document number PCT / EP2019 / 064724 or a previously unpublished German patent application with reference number 102018127526.9. These applications will be referenced in their entirety below. In another embodiment, the device includes at least one first piezoelectric element serving as a driver / receiver unit, wherein the device is designed to excite a mechanically vibrating unit to generate mechanical vibration by means of an excitation signal, receive the mechanical vibrations of the vibrating unit and convert them into a first received signal, transmit a transmission signal, and receive a second received signal, wherein the electronic device is designed to use the first and / or second received signals to determine at least one process variable of the medium.

[0023] The transmitted signal is preferably an ultrasonic signal, especially a pulsed ultrasonic signal, particularly at least one ultrasonic pulse. Therefore, as a second measurement method used, an ultrasonic-based measurement is performed within the scope of the invention. The corresponding transmitted signal at least partially passes through the medium and is affected by the medium in terms of its characteristics. Therefore, conclusions regarding different media can also be drawn using the correspondingly received second received signal.

[0024] It is advantageous if the device further includes at least one second piezoelectric element, wherein the first and second piezoelectric elements are designed to excite mechanically vibrating units to generate mechanical vibration by means of an excitation signal, and to receive the mechanical vibrations of the vibrating units and convert them into a first received signal, wherein the first piezoelectric element is designed to transmit a transmission signal, and wherein the second piezoelectric element is designed to receive the transmitted signal in the form of a second received signal.

[0025] Therefore, these embodiments relate to a multi-sensor in which two different measurement principles are combined.

[0026] It is also advantageous if the mechanically vibrating unit is a vibrating fork having first and second vibrating elements, wherein the first piezoelectric element is at least partially arranged in the first vibrating element, and the second piezoelectric element is at least partially arranged in the second vibrating element. Corresponding embodiments of the sensor unit have been described, for example, in documents DE102012100728A1 and DE102017130527A1. These two applications are also mentioned in their entirety within the scope of this invention. The possible embodiments of the sensor unit described in these two documents are exemplary possible structural embodiments of the sensor unit. For example, it is not absolutely necessary to exclusively arrange the piezoelectric elements in the region of the vibrating element. Rather, the individual piezoelectric elements used may also be arranged in the region of the diaphragm, or in another vibrating element that is not used for vibration excitation but is also applied to the diaphragm.

[0027] Finally, it is advantageous if the extent of the diaphragm parallel to the longitudinal axis of the vibrating element is less than 20% of the length of the vibrating rod parallel to the longitudinal axis. Attached Figure Description

[0028] The invention will be explained in more detail with reference to the following figures. These are shown below:

[0029] Figure 1 This is a schematic diagram of a vibration sensor based on existing technology.

[0030] Figure 2 It is a vibrating unit in the form of a vibrating fork, having (a) a planar surface and (b) a curved diaphragm surface, and

[0031] Figure 3 These are various exemplary embodiments for use with a bent diaphragm. Detailed Implementation

[0032] In the accompanying drawings, the same elements are given the same reference numerals.

[0033] Figure 1 A vibration sensor 1 with sensor unit 2 is shown. The sensor has a mechanically vibrating unit 4 in the form of a vibrating fork, part of which is immersed in a medium M located in a storage 3. The vibrating unit 4 is excited by a driver / receiver unit 5 to generate mechanical vibration, and can be, for example, a piezoelectric stacked driver or a dual-crystal driver. Other vibration sensors, for example, have an electromagnetic driver / receiver unit 5. A single driver / receiver unit 5 may be used for both exciting and detecting the mechanical vibrations. However, it is also conceivable that one driver unit and one receiver unit are implemented separately. Figure 1 The document also describes an electronic unit 6, by means of which signal acquisition, evaluation and / or feeding are performed.

[0034] Figure 2 The image depicts a corresponding vibrating unit 4 in the form of a vibrating fork. Unit 4 has a diaphragm 7 on which two vibrating elements 8a and 8b are integrally formed in the form of a medium M when introduced into the container 3. Figure 2 In step a, the surface O of the diaphragm 7 is designed to be planar. As a result, bubbles contained in the medium M can accumulate on the surface O of the diaphragm 7 and be trapped there according to Archimedes' principle.

[0035] In such Figure 2 In the variant of the invention depicted in b, the surface O of the diaphragm 7 is curved in contrast, allowing bubbles to pass from the center of surface O to its edge and escape from there. Therefore, the curved design significantly reduces the sensitivity of the vibration sensor relative to bubble adhesion and the associated adverse measurement effects.

[0036] For the diaphragm according to the invention, many embodiments are conceivable, some of which are... Figure 3 The examples are depicted in the form of cross-sectional views. For instance, surface O can be designed as convex (e.g., Figure 3 (as shown in a) or concave (as shown in a) Figure 3 (as shown in b), depending particularly on the mounting location of the vibration sensor 1. In both cases, surface O is circular. Alternatively, surface O can also have a hemispherical or spherical cross-section shape. Furthermore, as... Figure 3 As depicted in c, it is also conceivable that it has a tip or, for example, has such a tip. Figure 3The surface O is the shape of the truncated cone shown in d. Besides the embodiment shown here, many other possibilities are conceivable, all of which fall within the scope of this invention. It should also be noted that surface O does not necessarily need to be designed to be symmetrical, as... Figure 3 The same applies to the variants depicted. Instead, surface O can be selected specific to the application.

[0037] List of reference numerals

[0038] 1. Vibration sensor

[0039] 2 Sensor Unit

[0040] 3. Storage

[0041] 4. Vibrating units

[0042] 5 Driver / Receiver Units

[0043] 6 electronic units

[0044] 7. Diaphragm

[0045] 8a First Vibration Element

[0046] 8b Second Vibration Element

[0047] 9. Bubbles

[0048] M medium

[0049] P process variable

Claims

1. An apparatus for determining and / or monitoring at least one process variable (P) of a medium (M), comprising a unit (4) capable of mechanical vibration, at least one first piezoelectric element serving as a driver / receiver unit (5), and an electronic unit (6). in, The driver / receiver unit (5) is designed to excite the mechanically vibrating unit (4) to generate mechanical vibration by means of an electrical excitation signal, and to receive the mechanical vibration of the mechanically vibrating unit (4) and convert them into a first electrical receiving signal. The electronic unit (6) is designed to generate the electrical excitation signal based on the first electrical received signal, and the frequency of the electrical excitation signal is set such that a predetermined phase shift exists between the electrical excitation signal and the first electrical received signal. The device further includes at least one second piezoelectric element. The first and second piezoelectric elements are designed to excite the mechanically vibrating unit (4) to generate mechanical vibration by means of the electrical excitation signal, and to receive the mechanical vibration of the mechanically vibrating unit (4) and convert it into the first electrical receiving signal. The first piezoelectric element is designed to transmit a signal, which is an ultrasonic signal. The second piezoelectric element is designed to receive the transmitted signal in the form of a second received signal. The electronic unit (6) is designed to determine at least one process variable (P) of the medium (M) using the first electrical receiving signal and / or the second receiving signal. The mechanically vibrating unit (4) is a vibrating fork having a first vibrating element (8a) and a second vibrating element (8b). The first piezoelectric element is at least partially arranged in the first vibrating element (8a), and the second piezoelectric element is at least partially arranged in the second vibrating element (8b). The mechanically vibrating unit (4) includes a diaphragm (7). The process-facing surface (O) of the diaphragm (7) is curved, and The surface (O) of the diaphragm (7) is at least partially convex.

2. The device according to claim 1, in, The surface (O) is conical, truncated conical, hemispherical, or has a spherical truncated shape.

3. The device according to claim 1, in, The surface (O) is circular in the region at the center point of the surface (O).

4. The device according to claim 1, in, The surface (O) has a tip in the region of the center point of the surface (O).

5. The device according to claim 1, in, The surface (O) is designed to be symmetrical with respect to the center point of the surface (O).

6. The device according to claim 1, in, The angle between a plane parallel to the longitudinal axis passing through the diaphragm (7) and the tangent to the surface (O) of the diaphragm (7) is less than or equal to 45°.

7. The device according to claim 1, in, The extent of the diaphragm (7) parallel to the longitudinal axis of the first vibrating element (8a) and the second vibrating element (8b) is less than 20% of the length of the vibrating rod parallel to the longitudinal axis.

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