Device for determining and / or monitoring at least one process variable of medium

Abstract

The invention relates to a device (1) for determining and / or monitoring at least one process variable of a medium (2), comprising an electronics unit (3) and a sensor unit (4) having a mechanically oscillatory unit (5) and at least a first tubular piezoelectric element (6), the device (1) being embodied in such a way that the mechanically oscillatory unit (5) is excited by means of a first excitation signal and the mechanically oscillatory unit (5) is excited by means of a second excitation signal. The electronic unit (3) is adapted to determine at least one process variable on the basis of the received signal, and wherein the mechanical oscillation is carried out in a first oscillation mode and the mechanical oscillation of the mechanically oscillatable unit (5) in the first oscillation mode is received and converted into an electrical received signal, and wherein the electronic unit (3) is adapted to determine at least one process variable on the basis of the received signal. The mechanically oscillatable unit (5) comprises, in the end region (7), at least a first cylindrical bore (8) into which the at least a first piezoelectric element (6) is at least partially inserted.

Description

Technical Field

[0001] The present invention relates to a device for determining and / or monitoring at least one process variable of a medium. Background Art

[0002] Electromechanical vibration sensors are widely used in process and / or automation technology. For level measurement devices, such devices have at least one mechanically oscillatable unit, such as, for example, an oscillating fork, a single tooth or a diaphragm. This unit is excited during operation by an excitation / reception unit, usually in the form of an electromechanical transducer unit, so as to generate mechanical oscillations. The excitation / reception unit can in turn be, for example, a piezoelectric actuator or an electromagnetic actuator. The applicant manufactures a variety of corresponding field devices and sells them under brands such as LIQUIPHANT or SOLIPHANT. Its basic measurement principle has been disclosed in principle in a large number of documents. The excitation / reception unit excites the mechanically oscillatable unit by an electrical excitation signal so as to perform mechanical oscillations. Conversely, the excitation / reception unit can receive the mechanical oscillations of the mechanically oscillatable unit and convert them into an electrical reception signal. The excitation / reception unit can be a separate drive unit and a separate reception unit, or a combined excitation / reception unit.

[0003] In such a case, the excitation / reception unit is in many cases part of a feedback electrical oscillation circuit, through which the mechanically oscillatable unit is excited so as to perform mechanical oscillations. For example, the conditions for resonance of the oscillation circuit must be met, that is, the amplification factor ≥1, and all phases occurring in the oscillation circuit are multiples of 360°. In order to excite and meet the oscillation circuit conditions, it is necessary to ensure a certain phase shift between the excitation signal and the reception signal. Therefore, it is usually necessary to set a predeterminable phase shift value, that is, the expected value of the phase shift between the excitation signal and the reception signal. For this purpose, the prior art provides a variety of solutions, including analog methods and digital methods, such as, for example, the methods described in DE102006034105A1, DE102007013557A1, DE102005015547A1, DE102009026685A1, DE102009028022A1, DE102010030982A1 or DE00102010030982A1.

[0004] Both the excitation signal and the reception signal are characterized by a frequency ω, an amplitude A and / or a phase Φ. Correspondingly, changes in these variables are usually considered to determine a specific process variable. The process variable can be, for example, a level, a predetermined level, or the density or viscosity of a medium, as well as a flow rate. For example, for an electromechanical vibration limit switch for liquids, it is possible to distinguish whether the oscillatable unit is covered by a liquid or oscillates freely. In this case, these two states (free state and covered state) are distinguished, for example, based on different resonance frequencies (i.e., based on a frequency shift).

[0005] The excitation / reception unit is generally embodied as a piezoelectric element in the form of a rectangular disk and is mounted in the bag-shaped hollow space of the oscillatable unit. Then, the hollow space is filled with a potting compound such that the excitation / reception unit is connected by force interlocking, for example, friction-fixed to the wall of the hollow space. The potting compound is preferably selected such that very good force coupling is achieved between the excitation / reception unit and the oscillatable unit through the potting compound. For example, such an excitation / reception unit is described in DE102012100728A1. In order to produce the hollow space, the area of the oscillatable unit provided for the hollow space is generally etched by means of electrical discharge machining. However, such a material removal method is complex and costly. Summary of the Invention

[0006] Accordingly, it is an object of the present invention to provide a device in which the excitation / reception unit can be inserted into the oscillatable unit in a simple manner.

[0007] According to the present invention, the object is achieved by a device for determining and / or monitoring at least one process variable of a medium, the device comprising an electronic unit and a sensor unit having a mechanically oscillatable unit and at least a first tubular piezoelectric element,

[0008] wherein the device is embodied to excite the mechanically oscillatable unit by a first excitation signal such that a mechanical oscillation is executed in a first oscillation mode, and to receive the mechanical oscillation of the mechanically oscillatable unit in the first oscillation mode and convert it into an electrical reception signal,

[0009] wherein the electronic unit is adapted to ascertain at least one process variable based on the reception signal, and

[0010] wherein the mechanically oscillatable unit includes at least a first cylindrical hole in an end region, and at least the first piezoelectric element is at least partially inserted into the at least first cylindrical hole.

[0011] In the device according to the invention, at least the first piezoelectric element is inserted into the first cylindrical hole. Compared with the bag-shaped hollow space known from the prior art for receiving piezoelectric elements, the production of such a hole is much easier. Instead of using a more complex corrosive production method, simpler and more economical machining methods, such as drilling, can be used.

[0012] Furthermore, by using at least a first tubular piezoelectric element, the thermomechanical stress in the region of the at least first tubular piezoelectric element can be reduced. Generally, the mechanical oscillating unit, the at least first piezoelectric element, and (where applicable) the coefficient of thermal expansion of the potting compound that connects the at least first piezoelectric element to the at least first cylindrical hole are different from each other, such that under the influence of temperature stress, all of these occur in the region of the at least first piezoelectric element. Due to the tubular embodiment of the at least first piezoelectric element, it expands particularly in the direction of its longitudinal axis. Conventional disk-shaped or sheet-shaped piezoelectric elements generate strain both radially and axially, and thus generate greater thermomechanical stress compared to tubular piezoelectric elements. The at least first piezoelectric element is embodied in the form of a tube or a tube section.

[0013] The mechanical oscillating unit is, for example, a diaphragm, a single tooth, an arrangement of at least two oscillating elements, or an oscillating fork. The at least first piezoelectric element can be used on the one hand as an excitation / reception unit for generating the mechanical oscillation of the mechanical oscillating unit generated by means of an excitation signal. In turn, when the oscillating unit is covered by a medium, the mechanical oscillation is affected by the properties of the medium, so that information about at least one process variable can be generated based on the received signal representing the oscillation of the oscillating unit.

[0014] In one embodiment, the sensor unit includes at least a second tubular piezoelectric element,

[0015] wherein the mechanical oscillating unit is an oscillating fork having a first oscillating element and a second oscillating element,

[0016] wherein the at least first piezoelectric element is at least partially inserted into a first cylindrical hole of the first oscillating element, and the at least second piezoelectric element is at least partially inserted into a second cylindrical hole of the second oscillating element.

[0017] In a further embodiment, the device includes a temperature sensor. Temperature affects the mechanical properties of the sensor unit as well as various different process variables of the medium. By additionally determining and / or monitoring the temperature, the measurement accuracy of the device can be significantly improved and the functions of the device can be extended. In particular, the temperature sensor is a thermocouple or a resistive element.

[0018] In a further development, the temperature sensor is located in the at least first cylindrical hole. By positioning the temperature sensor together with the at least first piezoelectric element in the at least first cylindrical hole, space can be saved and the process temperature can be optimally evaluated.

[0019] In a further embodiment, the temperature sensor is arranged in a hollow space surrounded by the at least first piezoelectric element. Thus, the temperature sensor is located in the tube of the at least first piezoelectric element, that is, inside the tube, and is thus arranged in a particularly space-saving manner.

[0020] Alternative embodiments provide that the temperature sensor is arranged in the end region of at least the first piezoelectric element. Preferably, the temperature sensor is arranged in the end region of at least the first piezoelectric element remote from the mechanically oscillatable unit.

[0021] In a further development, at least the first piezoelectric element is electrically connected to the end region of the mechanically oscillatable unit by means of a conductive adhesive or a solder joint.

[0022] In a further embodiment, the outer surface of at least the first piezoelectric element is embodied as a first conductive electrode at least in a first section, and the inner surface of at least the first piezoelectric element is embodied as a second conductive electrode at least in a second section. Thus, the first conductive electrode section and the second conductive electrode section are partially or entirely formed by the outer surface and the inner surface.

[0023] In a further development, at least the first piezoelectric element includes a circumferential contact (i.e., it extends around the piezoelectric element), which is embodied such that the first conductive electrode can be electrically contacted through a third section of the inner surface, or such that the second conductive electrode can be electrically contacted through a fourth section of the outer surface. The circumferential contact ensures that both the first conductive electrode and the second conductive electrode can be contacted on the inner surface or the outer surface. If the first conductive electrode is formed by the outer surface at least in the first section, the first section can extend in the direction of the inner surface, thereby forming a third section in the region of the inner surface. Alternatively, when the second conductive electrode is formed by at least the second section of the inner surface, the second section can extend in the direction of the outer surface, thereby forming a fourth section of the outer surface.

[0024] In another embodiment, at least the first piezoelectric element is electrically connected to the electrical connection element through the outer surface and / or the inner surface. In particular, the electrical connection element is a cable or a circuit board. In a given case, the temperature sensor can also be arranged on the circuit board.

[0025] Preferably, the mechanically oscillatable unit is connected to the housing through its end region.

[0026] Advantageously, at least the first piezoelectric element at least partially contacts the wall of at least the first cylindrical hole. Description of the Drawings

[0027] The present invention will now be explained in more detail on the basis of the drawings, in which Figures 1 to 7 The following is shown:

[0028] Figure 1 : Schematic view of the device of the present invention mounted on a container.

[0029] Figure 2 : Schematic view of an embodiment of a first form of the sensor unit.

[0030] Figure 3 : Schematic diagram of an embodiment of the second form of the sensor unit.

[0031] Figure 4a : Cross-sectional view of an embodiment of the third form of the sensor unit.

[0032] Figure 4b : Schematic diagram of an embodiment of the third form of the sensor unit.

[0033] Figure 5 : Detailed view of at least the first piezoelectric element.

[0034] Figure 6 : Embodiment of the first form of the surrounding contact.

[0035] Figure 7 : Embodiment of the second form of the surrounding contact. Detailed Description

[0036] Figure 1 Exemplarily shown is a device 1 of the present invention, which has an electronic unit 3 and a sensor unit 4. The device 1 is mounted on a container 30, such as a pipe or a tank for example. A medium 2 is contained in the container 30. The sensor unit 4 includes a mechanically oscillatable unit 5 and at least a first piezoelectric element 6 (here hidden by the housing 28 of the device 1). The device 1, in particular the electronic unit 3, is embodied to excite the mechanically oscillatable unit 5 by a first excitation signal such that a mechanical oscillation is executed in a first oscillation mode, and to receive the mechanical oscillation of the mechanically oscillatable unit 5 in the first oscillation mode and convert it into an electrical received signal. In addition, the electronic unit 3 is additionally embodied to ascertain at least one process variable of the medium 2 based on the received signal. Process variables of the medium 2 are, for example, level, density or viscosity.

[0037] Figure 2 An embodiment of the first form of the sensor unit is shown. The mechanically oscillatable unit 5 includes at least a first cylindrical hole 8 in an end region 7, and at least the first piezoelectric element 6 is at least partially inserted into the hole. For example, at least the first piezoelectric element 6 is electrically connected to the end region 7 of the mechanically oscillatable unit 5 by a conductive adhesive or a solder joint. Generally, the piezoelectric element has, for example, a coefficient of thermal expansion. The mechanically oscillatable unit can be made of, for example, stainless steel having a coefficient of thermal expansion. Conventional adhesives used as potting compounds generally have a coefficient of thermal expansion. Due to the different coefficients of thermal expansion, thermo-mechanical stresses are generated. These stresses of at least the first piezoelectric element 6 in the tubular embodiment are reduced compared to at least the first piezoelectric element in a disc-shaped or sheet-shaped embodiment, because only large regions exist axially and these regions do not expand radially.

[0038] The mechanically oscillatable unit 5 is optionally connected to the housing 28 in its end region 7. The device 1 may optionally include a temperature sensor 14. The temperature sensor 14 may be mounted in at least a first cylindrical bore 8 and, for example, arranged in the end region 16 of at least a first piezoelectric element 6, such as Figure 2 as shown. Preferably, the temperature sensor 14 is arranged in the end region 16 of at least a first piezoelectric element 6 remote from the mechanically oscillatable unit 5, such as Figure 3 as shown.

[0039] Figure 3 A second example of an embodiment of the sensor unit 4 is shown. In this example, the sensor unit 4 includes an oscillating fork 10 having a first oscillating element 11 and a second oscillating element 12. The first oscillating element 11 includes a first cylindrical bore 8 into which at least a first piezoelectric element 6 is inserted. The second oscillating element 12 includes a second cylindrical bore 13 into which at least a second piezoelectric element 9 is inserted. At least a first piezoelectric element 6 and at least a second piezoelectric element 9 are preferably arranged such that they each at least partially contact the walls 29 of their cylindrical bores 8, 13. Figure 3 A temperature sensor 14 is shown that is provided only in the first oscillating element 11. However, there may also be a second redundant temperature sensor, especially in the second cylindrical bore 13.

[0040] Alternatively, the temperature sensor 14 can even be arranged in the hollow space 15 surrounded by at least a first piezoelectric element 6, such as Figures 4a to 4b as shown. For this alternative, the dimensions of the temperature sensor 14 need to be designed to be smaller than the inner radius of at least a first piezoelectric element 6 in at least two dimensions.

[0041] Figure 5 A detailed view of an example of at least a first piezoelectric element 6 is shown. At least a first piezoelectric element 6 includes an outer surface 17 and an inner surface 20. At least one first section 18 of the outer surface 17 is embodied as a first conductive electrode 19. At least one second section 21 of the inner surface 20 is embodied as a second conductive electrode 22. In the selected example, the inner surface 20 is entirely embodied as a second conductive electrode 22, and the outer surface 17 is entirely realized as a first conductive electrode 19.

[0042] To enable simple electrical contact, at least a first piezoelectric element 6 may have a surrounding contact 23, as Figure 6 and Figure 7 shown. For example, the surrounding contact is embodied such that the second conductive electrode 22 can be electrically contacted through a fourth section 25 of the outer surface 17 (compare Figure 6 ). Electrical contact of the first conductive electrode 19 and the second conductive electrode 20 can be made from the outer surface 17 through an electrical connection element 26 (not shown).

[0043] Alternatively, the circumferential contact 23 may be embodied such that the first conductive electrode 19 can be in electrical contact through a third section 24 of the inner surface 20, as Figure 7 shown. In such a case, the first conductive electrode 19 and the second conductive electrode 20 can be electrically connected in a simple manner by an electrical connection element 26 (such as a cable or a circuit board). In the case of using a circuit board as the electrical connection element 26, the temperature sensor 14 can also be arranged on the circuit board.

[0044] List of reference numerals

[0045] 1 Device

[0046] 2 Medium

[0047] 3 Electronic unit

[0048] 4 Sensor unit

[0049] 5 Mechanically oscillatable unit

[0050] 6 First piezoelectric element

[0051] 7 End region of the mechanically oscillatable unit

[0052] 8 First cylindrical hole

[0053] 9 Second piezoelectric element

[0054] 10 Oscillating fork

[0055] 11 First oscillating element

[0056] 12 Second oscillating element

[0057] 13 Second cylindrical hole

[0058] 14 Temperature sensor

[0059] 15 Hollow space

[0060] 16 End region of the first piezoelectric element

[0061] 17 Outer surface of the first piezoelectric element

[0062] 18 First section

[0063] 19 First conductive electrode

[0064] 20 Inner surface of the first piezoelectric element

[0065] 21 Second section

[0066] 22 Second conductive electrode

[0067] 23 Surrounding contact

[0068] 24 Third section

[0069] 25 Fourth section

[0070] 26 Electrical connection component

[0071] 28 Housing

[0072] 29 Wall of the first cylindrical hole

[0073] 30 Container

Claims

1. An apparatus (1) for determining and / or monitoring at least one process variable of a medium (2), comprising an electronic unit (3) and a sensor unit (4), the sensor unit having a mechanically oscillatable unit (5) and at least a first tubular piezoelectric element (6), Among them, The apparatus (1) is embodied to excite the mechanically oscillatable unit (5) by a first excitation signal such that a mechanical oscillation is performed in a first oscillation mode, and to receive the mechanical oscillation of the mechanically oscillatable unit (5) in the first oscillation mode and convert it into an electrical received signal, wherein the electronic unit (3) is adapted to ascertain the at least one process variable based on the received signal, and wherein the mechanically oscillatable unit (5) includes at least a first cylindrical hole (8) in an end region (7), and at least the first piezoelectric element (6) is at least partially inserted into the at least first cylindrical hole.

2. The apparatus (1) according to claim 1, Among them, the sensor unit (4) includes at least a second tubular piezoelectric element (9), wherein the mechanically oscillatable unit (5) is a tuning fork (10) having a first oscillating element (11) and a second oscillating element (12), wherein at least the first piezoelectric element (6) is at least partially inserted into a first cylindrical hole (8) of the first oscillating element (11), and at least the second piezoelectric element (9) is at least partially inserted into a second cylindrical hole (13) of the second oscillating element (12).

3. The apparatus (1) according to one of claims 1 to 2, Among them, the apparatus (1) further includes a temperature sensor (14).

4. The apparatus (1) according to claim 3, Among them, the temperature sensor (14) is inserted into the at least first cylindrical hole (8).

5. The apparatus (1) according to one of claims 3 to 4, Among them, the temperature sensor (14) is arranged in a hollow space (15) surrounded by the at least first piezoelectric element (6).

6. The apparatus (1) according to one of claims 3 to 4, Among them, the temperature sensor (14) is arranged in an end region (16) of the at least first piezoelectric element (6).

7. The apparatus (1) according to one of claims 3 to 6, Among them, the temperature sensor (14) is a thermocouple or a resistive element.

8. The apparatus (1) according to one of claims 1 to 7, Among them, the at least first piezoelectric element (6) is electrically connected to the end region (7) of the mechanically oscillatable unit (5) by a conductive adhesive or a solder joint.

9. The apparatus (1) according to one of claims 1 to 8, Among them, an outer surface (17) of the at least first piezoelectric element (6) is embodied as a first conductive electrode (19) at least in a first section (18), and an inner surface (20) of the at least first piezoelectric element (6) is embodied as a second conductive electrode (22) at least in a second section (21).

10. The apparatus (1) according to claim 9, Among them, The at least first piezoelectric element (6) includes a surrounding contact member (23) embodied such that the first conductive electrode (19) can be electrically contacted through a third section (24) of the inner surface (20), or such that the second conductive electrode (22) can be electrically contacted through a fourth section (25) of the outer surface (17).

11. The device (1) according to one of claims 9 to 10, Among them, The at least first piezoelectric element (6) is electrically connected to an electrical connection element (26) through the outer surface (17) and / or the inner surface (20).

12. The device (1) according to claim 11, Among them, The electrical connection element (26) is a cable or a circuit board.

13. The device (1) according to one of claims 1 to 12, Among them, The mechanically oscillatable unit (5) is connected to the housing (28) through the end region (7).

14. The device (1) according to one of claims 1 to 13, Among them, The at least first piezoelectric element (6) at least partially contacts the wall (29) of the first cylindrical hole (8).

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