Vibronic sensor

Abstract

The present invention relates to a vibronic sensor (1) having a membrane (5) that is able to be set into vibration, a piezoelectric drive (3) for setting the membrane (5) into vibration and for sensing vibrations of the membrane (5), a mechanical vibration unit (11), arranged on the membrane (5), for transmitting vibrations of the membrane (5) to a medium surrounding the mechanical vibration unit (11), wherein a resonant frequency of the mechanical vibration unit (11) is dependent on the medium surrounding it, wherein the piezoelectric drive (3) has at least one piezo element (4) and electrodes (7) for making electrical contact with the at least one piezo element (4), wherein the drive (3) is embodied in an encapsulated manner with a casing (2) that surrounds the drive (3) at a distance therefrom and is filled with a liquid filling (9), wherein the filling (9) has an electrical disruptive strength of more than 3 kV / mm, wherein the casing (2) is designed to be flexible at least in some portions.

Description

[0001] Vibronic sensor

[0002] The present invention relates to a vibronic sensor according to the preamble of claim 1.

[0003] In process and factory automation, electronic sensors are used to monitor and control processes. Such sensors can be used, for example, to detect fill levels, limit levels, or monitor process parameters.

[0004] A widely used type of such sensor is vibronic sensors. Vibronic sensors for detecting fill or limit levels operate by exciting a mechanical vibration unit at its resonant frequency. Covering the vibrating element with a liquid or bulk material changes its resonant frequency. Exceeding a predetermined value of this change generates a switching command.

[0005] Vibronic sensors for determining and / or monitoring the fill level of a medium in a container or for determining the density of a medium in the container comprise a housing, a membrane, a mechanical vibration unit, a drive / receiver unit (hereinafter also referred to as the drive), and a control / evaluation unit (hereinafter also referred to as the sensor electronics). The membrane closes one end of the housing, and the mechanical vibration unit is arranged on the membrane. The drive is arranged inside the housing in such a way that it causes the membrane and the mechanical vibration unit to vibrate and detects their vibrations. The sensor electronics uses the detected vibrations to determine whether the specified fill level has been reached or the density of the medium.

[0006] Such vibration detectors are available in a wide variety of designs. The mechanical vibration unit consists of at least one vibrating rod, which—as already mentioned—is attached directly to a membrane. The membrane is set into vibration by an electromechanical transducer, e.g., a piezoelectric element. Due to the vibrations of the membrane, the mechanical vibration unit attached to the membrane also performs vibrations.

[0007] Vibration detectors designed as level gauges take advantage of the fact that the vibration frequency and amplitude depend on the respective coverage of the oscillating unit: While the mechanical vibration unit can oscillate freely and undamped in air, it experiences a change in frequency and amplitude as soon as it is partially or completely immersed in the product. A predetermined frequency change (typically, the frequency is measured, not the amplitude), thus clearly indicates whether a predetermined fill level of a product in the container has been reached. Level gauges of this type are also used primarily as a means of protecting against overfilling or preventing a pump from running idle.

[0008] Furthermore, the damping of the vibration of the oscillating element is also influenced by the density of the respective product. With a constant coverage of at least one oscillating element, there is a correlation with the density of the product, making vibration detectors ideal for both detecting a given limit state and detecting the density.

[0009] In practice, to monitor and detect the fill level or density of the product in the container, the vibrations of the membrane are recorded and converted into electrical signals. Typically, at least one piezoelectric element is used for this purpose. The electrical signals are then evaluated by an evaluation electronics unit. During level determination, the evaluation electronics monitors the vibration frequency and / or the vibration amplitude of the vibrating unit and signals the "sensor covered" or "sensor uncovered" status as soon as the measured values ​​fall below or exceed a specified reference value. This can be reported to the operating personnel visually and / or acoustically. Alternatively or additionally, a switching process is triggered, for example, opening or closing an inlet or outlet valve on the container.Piezoceramic elements are predominantly used as electromechanical transducers in the drive unit of such a sensor. Various concepts are known for this. Screwed stack drives, in which the piezo actuators are secured by a screw fastening, and bonded drives, in which a piezo element is bonded to the membrane, are widely used.

[0010] To use such piezoceramic elements as electromechanical transducers in a drive, they must first be polarized. Polarizing such piezoceramics requires high electric fields in the range of 3000 V / mm and above.

[0011] In ceramic piezoelectric elements, the internal dipoles are still disordered after the sintering process, which is why no piezoelectric properties are exhibited. The Weiss domains, or domains, have a random spatial orientation and cancel each other out. By polarizing the piezoelectric element using an external DC electric field, the individual dipole moments are aligned. The material can be heated to just below the Curie temperature and then cooled again, which further enhances the aforementioned effect. The imprinted orientation is partially retained after polarization (remanent polarization) and is referred to as the polarization direction.

[0012] Such drives can generally be used at process temperatures exceeding 300°C. At these high process temperatures, depolarization phenomena in the piezoceramics used already occur, reducing the sensor's drive performance. Likewise, any overshoots in the process temperature can cause further depolarization. Such depolarization phenomena can also occur due to aging of sensors.

[0013] Due to depolarization of the drive's piezo elements, the drive loses power over time, meaning that for the same amount of electrical energy applied, less mechanical energy is generated, and vice versa. This means that the mechanical oscillation unit is excited to oscillate with less mechanical energy and consequently oscillates at a lower amplitude. Furthermore, this also means that a lower amplitude is available for oscillation detection, resulting in a smaller electrical signal for oscillation detection.

[0014] The object of the invention is to propose a concept for the construction of a vibronic level sensor which enables continuous operation of the sensor with constant signal quality.

[0015] This object is achieved by a sensor having the features of patent claim 1. Advantageous embodiments and variants of the invention emerge from the subclaims and the following description. The features listed individually in the subclaims can be combined with each other in any technically reasonable manner, as well as with the features explained in more detail in the following description, and represent other advantageous embodiments of the invention.

[0016] A vibronic sensor according to the invention comprising a membrane capable of vibrating, a piezoelectric drive for causing the membrane to vibrate and for detecting vibrations of the membrane, and a mechanical vibration unit arranged on the membrane for transmitting vibrations of the membrane to a medium surrounding the mechanical vibration unit. A resonance frequency of the mechanical vibration unit depends on the surrounding medium. The piezoelectric drive has at least one piezoelement and electrodes for electrically contacting the at least one piezoelement. According to the invention, the vibronic sensor is characterized in that the drive is encapsulated with a shell surrounding the drive at a distance and an insulating gaseous or liquid filling, wherein the filling has an electrical breakdown field strength of more than 3 kV / mm.The casing for encapsulating the drive is designed to be flexible at least in sections and is preferably designed such that a thermal expansion of the filling is absorbed by a corresponding expansion of the casing.

[0017] Essential to the present invention is that the drive of the vibronic sensor is encapsulated and filled with an insulating medium having a breakdown field strength of at least 3 kV / mm. If the field strength applied to a piezo element is 3 kV / mm, the piezo element can be repolarized. Aging and degradation effects caused by excessively high temperatures, aging, or excessive mechanical stress can be remedied by repolarizing the piezo element, thus increasing the sensor's service life and—if depolarization is consciously accepted—also achieving a wider operating window. This means that the sensor can be used at higher temperatures, for example, since any resulting depolarization of the piezo material and the associated decline in drive performance can be remedied by timely repolarization.

[0018] In the context of this application, "encapsulated" means that the drive is housed in a closed enclosure filled with an insulator. The enclosure encloses the drive circumferentially, seals it off at the rear, and is also sealed off at the front, preferably by the membrane.

[0019] Insulation with a breakdown field strength of more than 3 kV / mm ensures that when an electric field sufficient for repolarization is applied, no arcing occurs, which would lead to damage to the piezo material or the electrical contacts, e.g. by burning off the electrodes.

[0020] In this context, repolarization means that the affected piezo element is subjected to a field strength sufficiently high to achieve saturation polarization of the piezoelectric material. If this saturation field strength is maintained for a certain polarization time, a remanent polarization remains after the electric field is switched off, allowing the piezo element to be remanently polarized.

[0021] A liquid or gas as an insulator in the casing has the advantage over a potting or a solid as an insulator in that gases and liquids only have a reduced influence on the dynamic properties of the sensor. Potting compounds dampen the movement of the drive and the diaphragm to an excessive extent, making them unsuitable for practical use. Using insulating liquids simplifies sealing of the casing and also prevents diffusion. In arrangements to compensate for thermally induced effects, compensation to the atmosphere is standard. The pressure difference is compensated to the atmosphere by a corresponding, permeable element. In this case, a pressure difference occurring due to thermal expansion is mitigated by the deformation of a defined, impermeable element.

[0022] In a further development, the vibronic sensor is characterized in that the insulator is a liquid and in particular a mineral oil, in particular transformer oil, in particular saturated pentaerythritol tetrafatty acid esters, and preferably consists thereof.

[0023] Enriching the insulating fluid with the aforementioned substances increases the dielectric strength. Transformer oils are well-known and well-tested from their use in liquid-insulated transformers. Transformer oils are therefore also commercially available and have been well-researched with regard to their chemical, electrical, and thermal properties.

[0024] Preferably, the liquid volume within the enclosure is as small as possible while still meeting the necessary insulation requirements. This means that the distances between the enclosure and the electrodes are selected such that, when the enclosure is filled with the insulator, penetration to the enclosure and membrane is also prevented. The liquid preferably has a volume of less than 3 ml, more preferably less than 2 ml, and even more preferably less than 1 ml.

[0025] High voltages in the range of 3000 V / mm are required to polarize piezo elements. To prevent voltage arcing between the anode and cathode, the piezo elements are surrounded by non-conductive oil during polarization. Since vibronic sensors, such as those subject to the present application, must be usable in all installation positions, the insulating filling must be held in position by a suitable casing. When the device heats up, the filling expands with a thermal expansion coefficient that differs from that of the casing. This creates an overpressure or underpressure that, without compensation, would prestress the diaphragm. This would lead to a change in stiffness and, consequently, to a shift in the natural frequency of the vibronic sensor. The change in internal pressure also causes a load that stresses the diaphragm in addition to the process pressure.Without appropriate compensation, this load would have to be counteracted by a thicker fork membrane, which would dampen the vibration amplitude.

[0026] The present vibronic sensor thus enables liquid-insulated repolarization of the drive in the installed state.

[0027] In order to generate a sufficiently high field strength for repolarization of the piezo, the sensor has high-voltage electronics to provide a polarization voltage.

[0028] In a preferred embodiment, the casing has a sleeve-shaped section and a contact feedthrough. The sleeve-shaped section surrounds the drive in the radial direction, while the contact feedthrough closes off the casing at the rear. The contact feedthrough can be plate-shaped, for example. In one embodiment, the contact feedthrough can have glazed-in contact pins.

[0029] In one embodiment of the vibronic sensor, the sleeve-shaped section can be designed to be deformable in the radial direction. This deformability in the radial direction minimizes movement in the axial direction. This has the advantage that, for example, the contact feedthrough can be designed so that it is not deformable.

[0030] In one embodiment, the sleeve-shaped section can be preformed concavely in the radial direction. In this case, a concavely preformed sleeve-shaped section means a pre-deformation of the sleeve-shaped section directed inward, i.e., toward the drive. This allows the sleeve-shaped section to be attached to an edge of the membrane and, thanks to the inward-facing pre-deformation, still has a compensation space into which the sleeve-shaped section can deform upon thermal expansion of the filling, thus increasing the volume of the casing. The concave shape also prevents the "cracking frog" effect. The sleeve never exceeds a completely cylindrical shape, but always has a curvature inward. This prevents the bending direction from suddenly changing. If the sleeve-shaped section were strictly cylindrical, there would be no possibility of radial expansion if it were attached to the edge of the membrane.

[0031] In a preferred embodiment, the sleeve-shaped section is welded to the membrane at the front. Welding the sleeve-shaped section to the membrane has the advantage of creating a secure and permanently sealed connection between the sleeve-shaped section and the membrane, which has a high temperature resistance.

[0032] Alternatively, gluing or soldering is also possible.

[0033] If the sleeve-shaped section has a smaller diameter in a rear area where it is connected to the contact feedthrough than in a front area where it is connected to the membrane, it is also particularly easy to attach the sleeve-shaped section, possibly with the contact feedthrough already attached, to the edge of the membrane. Attachment can be achieved by welding, in particular by laser welding.

[0034] As already described, the sleeve-shaped section can be sealingly connected to the contact feedthrough on the back, in particular also welded.

[0035] In this way, the part of the shell consisting of the contact feedthrough and the sleeve-shaped section can be prefabricated as an assembly, which can then be connected to the assembly consisting of the membrane and drive.

[0036] In a preferred embodiment, the sleeve is designed, at least in sections, with a wall thickness that is 1 / 3 and 1 / 4 or less of the thickness of the membrane. This design ensures that the sleeve deforms in a predetermined region upon thermally induced expansion of the filling. Preferably, the sleeve is designed in the region of the sleeve-shaped portion such that it deforms in this region. In one embodiment, the piezo elements have an electrical contact with variable length in the axial direction of the sleeve. Variable-length electrical contacts can compensate for a displacement of the contact leadthrough in the axial direction upon deformation of the sleeve-shaped portion.

[0037] For example, the variable-length electrical contact can be realized by designing the electrical contact as at least one contact pin with at least one correspondingly designed sliding sleeve. For example, a socket in the form of a sliding sleeve can be arranged on the drive, while a pin is arranged on the contact feedthrough, which engages into the socket. Depending on the thermal expansion and corresponding deformation of the sleeve-like section, there can be a varying degree of overlap between the socket and the pin. Alternatively, the pin can be arranged on the piezo element and the socket on the contact feedthrough.

[0038] The present invention will be explained in detail below using exemplary embodiments with reference to the accompanying figures. They show:

[0039] Figure 1 shows an embodiment of a vibronic sensor 1 according to the present application.

[0040] In the figures, unless otherwise stated, the same reference symbols designate the same or corresponding components with the same function.

[0041] Figure 1 shows an embodiment of a vibronic sensor 1 according to the present application.

[0042] The vibronic sensor 1 comprises a membrane 5 capable of mechanical vibrations in an axial direction A, as well as an electromechanical transducer, referred to as the drive 3, for generating and detecting these mechanical vibrations. In the present embodiment, a mechanical vibration unit 11 in the form of two paddle-shaped oscillators is arranged on the membrane 5, by means of which vibrations of the membrane can be transmitted to a medium surrounding the mechanical vibration unit 11. In the present embodiment, the drive 3 comprises a piezo element 4, which is bonded to the membrane 5 with the interposition of a so-called compensating ceramic 6 to compensate for different thermal expansion coefficients of the membrane 5 and the piezo element 4.The piezo element 4 is electrically contacted via electrodes 7 so that an electrical signal can be applied to excite the piezo element 4 and an electrical voltage resulting from a mechanical excitation of the piezo element 4 can be tapped.

[0043] The electrodes 7 are designed here as contact pins glued to the piezo element, which extend in the axial direction A at the rear of the piezo element 4.

[0044] In the present embodiment, the drive 3 is surrounded by a casing 2 filled with a filling 9, in this case a transformer oil. The casing 2 is formed at the front by the diaphragm 5, in the circumferential direction by a sleeve-shaped section 21, and at the rear by a contact feedthrough 23. The sleeve-shaped section 21 is welded by means of a first weld seam 25 at a transition from the diaphragm 5 and an edge 15 extending from the rear of the diaphragm 5 in the axial direction A, and is thus sealingly connected to the diaphragm 5. At the rear, the sleeve-shaped section 21 is fastened to the contact feedthrough 23 by a second weld seam 27. In this way, the drive 3 is sealingly enclosed and thus encapsulated from the rest of the housing of the vibronic sensor 1.

[0045] The transformer oil has a dielectric strength of 5-30 kV / mm and is therefore optimally suited for insulating the drive from the shell 2 and the housing.

[0046] Two electrically conductive contacts arranged corresponding to the electrodes 7 and designed as sleeves for receiving the electrodes 7, which are designed as contact pins, are arranged through the contact feedthrough 23. The contacts, together with the electrodes 7, form a type of sliding sleeve, thus forming an electrical contact with a variable length in the axial direction A.

[0047] This length variability is necessary in the present exemplary embodiment because the sleeve-shaped section 21 of the casing 2 is pre-formed concavely. In the present exemplary embodiment, this means that the sleeve-shaped section 21 is pre-formed inwards, i.e. in the direction of the drive 3. This pre-forming creates a compensation space 24 in the radial direction R, into which the casing 2 can expand. Due to temperature-induced expansion of the filling, the filling expands within the casing 2 and presses the sleeve-shaped section 21 outwards in the radial direction. Due to this deformation, the extension of the sleeve-shaped section 21 increases in the axial direction A, whereby a distance between the piezo element 4 and the contact feedthrough 23 increases. As a result of the increase in the distance, the electrodes 7 slide further out of the contacts of the contact feedthrough 23, while at the same time electrical contact is maintained.

[0048] An increased volume of the filling 9 and the resulting increased internal pressure inside the casing 2 are compensated for by a deformation of the casing wall of the sleeve-shaped section 21 and a concomitant increase in the enclosed volume. The contact feedthrough 23 and also the membrane 5 are significantly stiffer than the casing wall, which is why the internal pressure acts on this wall first.

[0049] If the casing 2 were completely rigid, the frequency would shift significantly in response to a pressure difference under the influence of temperature, as the stiffness of the system would increase. Membrane 5 and the cable gland would deform under the influence of the internal pressure.

[0050] The sleeve-shaped section 21 of the casing 2 is located inside the vibronic sensor 1, which is why it is not subject to process pressure. Thus, the sleeve-shaped section 21 does not need to be designed for the process pressure, but only needs to be able to withstand the temperature-induced pressure difference. By appropriately adapting the design of the casing 2, in particular the sleeve-shaped section 21, the volume of the filling 9 can be minimized, thus also minimizing the thermal expansion of the filling 9. Reference numeral

[0051] Vibronic sensor

[0052] Covering

[0053] drive

[0054] Piezo element

[0055] membrane

[0056] Leveling ceramics

[0057] Electrodes

[0058] filling

[0059] Mechanical oscillating unit

[0060] Contact implementation

[0061] edge

[0062] Sleeve-shaped section

[0063] Contact implementation

[0064] Compensation space

[0065] First weld

[0066] Second weld

Claims

Patent claims 1. A vibronic sensor (1) comprising a membrane (5) capable of vibrating, a piezoelectric drive (3) for vibrating the membrane (5) and for detecting vibrations of the membrane (5), a mechanical vibration unit (11) arranged on the membrane (5) for transmitting vibrations of the membrane (5) to a medium surrounding the mechanical vibration unit (11), wherein a resonance frequency of the mechanical vibration unit (11) is dependent on the medium surrounding it, wherein the piezoelectric drive (3) comprises at least one piezo element (4) and electrodes (7) for electrically contacting the at least one piezo element (4), characterized in that the drive (3) is encapsulated with a casing (2) surrounding the drive (3) at a distance and filled with a liquid filling (9), wherein the filling (9) has an electrical breakdown field strength of more than 3 kV / mm,wherein the sheath (2) is at least partially flexible., 2. Vibronic sensor (1) according to claim 1, characterized in that Sheath (2) has a sleeve-shaped section (21) and a contact feedthrough (23).

3. Vibronic sensor (1) according to one of the preceding claims, characterized in that the sleeve-shaped section (21) is designed to be deformable in the radial direction.

4. Vibronic sensor (1) according to one of the preceding claims, characterized in that the sleeve-shaped section (21) is pre-formed concavely in the radial direction (R).

5. Vibronic sensor (1) according to one of the preceding claims, characterized in that the sleeve-shaped section (21) is welded to the membrane (5) on the front side.

6. Vibronic sensor (1) according to one of the preceding claims, characterized in that the sleeve-shaped section (21) is sealingly connected, in particular welded, to the rear of the contact leadthrough (23).

7. Vibronic sensor (1) according to one of the preceding claims, characterized in that the casing (2) is designed at least in sections with a wall thickness which is between 1 / 3 and 1 / 4 or less of a thickness of the membrane (5).

8. Vibronic sensor (1) according to one of the preceding claims, characterized in that the piezo elements (4) have an electrical contact which is variable in length in the axial direction (A).

9. Vibronic sensor (1) according to claim 8, characterized in that the electrical contact is designed as at least one contact pin with at least one correspondingly designed sliding sleeve.

10. Vibronic sensor (1) according to one of the preceding claims, characterized in that the electrical contact is prestressed in the axial direction (A).

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