DE102023112237A1 - vibronic sensor - Google Patents

Abstract

The present invention relates to a vibronic sensor (1) with a membrane (5) that can be set into vibration, a piezoelectric drive (3) for setting the membrane (5) into vibration 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) has at least one piezo element (4) and electrodes (7) for electrically contacting the at least one piezo element (4), wherein the drive (3) is encapsulated with a casing (2) surrounding the drive (3) at a distance from it 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 casing (2) is designed to be flexible at least in sections.

Description

The present invention relates to a vibronic sensor according to the preamble of patent claim 1.

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 to monitor process parameters.

A widely used type of such sensor is vibronic sensors. Vibronic sensors for detecting fill levels or limit levels work by exciting a mechanical vibration unit with its resonance frequency. Covering the vibration element with a liquid or bulk material changes its resonance frequency. If this change exceeds a predetermined value, a switching command is generated.

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 have 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, with the drive inside the housing being arranged in such a way that it causes the membrane and the mechanical vibration unit to vibrate and detects their vibrations, and the sensor electronics use the vibrations detected to determine whether the specified fill level has been reached or the density of the medium.

Such vibration detectors are known in a wide variety of designs. The mechanical vibration unit is at least one vibrating rod which - as already mentioned - is attached directly to a membrane. The membrane is set into vibration via an electromechanical transducer, e.g. a piezoelectric element. Due to the vibrations of the membrane, the mechanical vibration unit attached to the membrane also carries out vibrations.

Vibration detectors designed as level measuring devices make use of the fact that the vibration frequency and the vibration amplitude depend on the respective degree of coverage of the oscillating unit: While the mechanical vibration unit can perform its vibrations freely and undamped in air, it experiences a change in frequency and amplitude as soon as it is partially or completely immersed in the filling material. From a predetermined frequency change (typically the frequency is measured rather than the amplitude), it can be clearly deduced whether a predetermined fill level of a product in the container has been reached. Level measuring devices of this type are also used primarily as a means of protecting against overfilling or to prevent a pump from running idle.

In addition, the damping of the vibration of the vibrating element is also influenced by the density of the respective product. With a constant degree of coverage of at least one vibrating element, there is a connection with the density of the product, so that vibration detectors are ideally suited both for detecting a given limit state and for detecting the density.

In practice, the vibrations of the membrane are recorded and converted into electrical reception signals to monitor and detect the fill level or density of the product in the container. Typically, at least one piezoelectric element is used for this purpose. The electrical reception signals are then evaluated by an evaluation electronics. When determining the fill level, the evaluation electronics monitor the vibration frequency and/or the vibration amplitude of the oscillating unit and signal the status "sensor covered" or "sensor uncovered" 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 and, for example, an inlet or outlet valve on the container is opened or closed.

Piezoceramic elements are predominantly used as electromechanical transducers in the drive unit of such a sensor. Different concepts are known for this. Screwed stack drives, in which the piezo actuators are fixed by a screw fastening, and glued drives, in which a piezo element is glued to the membrane, are widespread.

In order to use such piezoceramic elements as electromechanical transducers in the drive, they must first be polarized. To polarize such piezoceramics, high electric fields in the range of 3000 V/mm and more are necessary.

In ceramic piezo elements, the internal dipoles are still disordered after the sintering process, which is why no piezoelectric properties are apparent. The Weiss domains have an arbitrary spatial orientation and balance each other out. The individual dipole moments are aligned by polarizing the piezo element using an external DC electric field. 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.

Such drives can generally be used at process temperatures of up to 300°C. At these high process temperatures, depolarization phenomena in the piezoceramics used already occur, which reduces the drive power of the sensor. Any overshoots in the process temperature can also cause further depolarization. Such depolarization phenomena can also occur due to aging of sensors.

Due to depolarization of the piezo elements of the drive, the drive loses power over time, i.e. less mechanical energy is generated for the same amount of electrical energy used and vice versa. This means that the mechanical vibration unit is excited to vibrate with less mechanical energy and consequently vibrates with a lower amplitude. This also means that a lower amplitude is available for detecting the vibration and therefore only a smaller electrical signal is available for vibration detection.

It is the object of the invention to propose a concept for a construction of a vibronic level sensor, which enables continuous operation of the sensor with constant signal quality.

This object is achieved by a sensor with 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 and with the features explained in more detail in the following description and represent other advantageous embodiments of the invention.

A vibronic sensor according to the invention with a membrane that can be set into vibration, a piezoelectric drive for setting the membrane into vibration and for detecting vibrations of the membrane, 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 medium surrounding it. The piezoelectric drive has at least one piezo element and electrodes for electrically contacting the at least one piezo element. According to the invention, the vibronic sensor is characterized in that the drive is encapsulated with a casing surrounding the drive at a distance and an insulating gaseous or liquid filling, the filling having an electrical breakdown field strength of more than 3 kV/mm. The casing for encapsulating the drive is 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.

It is essential for the present invention that the drive of the vibronic sensor is encapsulated and filled with an insulating medium that has 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. Ageing and degradation effects that occur due to excessive temperatures, ageing or excessive mechanical stress can be remedied by repolarizing the piezo, so that an increased service life of the sensor and - if depolarization is consciously accepted - a larger operating window can be achieved. This means that the sensor can be used at higher temperatures, for example, because any depolarization of the piezo material that occurs as a result and any associated drop in performance of the drive can be remedied by timely repolarization.

In the context of the present application, encapsulated means that the drive is arranged in a closed casing filled with an insulator. The casing encloses the drive in the circumferential direction, closes it off at the back and is also closed off at the front, preferably by the membrane.

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 that could damage the Piezomate. materials or the electrical contacts, e.g. due to burn-off of the electrodes.

In this context, repolarization means that the piezo element in question is subjected to a sufficiently high field strength to achieve saturation polarization of the piezoelectric material. If this saturation field strength is present for a polarization time, a remanent polarization remains after the electric field is switched off, with which the piezo element can be remanently polarized.

A liquid or gas as an insulator in the casing has the advantage over a potting compound or a solid as an insulator that gases and liquids only have a reduced effect on the dynamic properties of the sensor. Potting compounds dampen the movement of the drive and the membrane to such an extent that they cannot be used in practice. Using insulating liquids makes it easier to seal the casing and also prevents diffusion.

In arrangements for compensating thermally induced effects, compensation to the atmosphere is carried out as standard. The pressure difference is compensated to the atmosphere by a corresponding, permeable element. In this case, a pressure difference caused by thermal expansion is mitigated by the deformation of a defined, impermeable element.

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.

Enriching the insulation liquid 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 available on the market and have been well researched in terms of their chemical, electrical and thermal properties.

Preferably, the volume of liquid inside the casing is as small as possible, but still meets the necessary insulation requirements. This means that the distances between the casing and the electrodes are chosen so that when the casing is filled with the insulator, breakdowns to the casing and membrane are also prevented. The liquid preferably has a volume of less than 3 ml, more preferably less than 2 ml, more preferably less than 1 ml.

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

The present vibronic sensor thus enables a liquid-isolated repolarization of the drive in the installed state.

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.

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 in the rear direction. The contact feedthrough can be designed in the form of a plate, for example. In one embodiment, the contact feedthrough can have glazed contact pins.

In one embodiment of the vibronic sensor, the sleeve-shaped section can be designed to be deformable in the radial direction. Movement in the axial direction is minimized by deformability in the radial direction. This has the advantage that, for example, the contact feedthrough can be designed in such a way that it is not deformable.

In one embodiment, the sleeve-shaped section can be preformed concavely in the radial direction. A concavely preformed sleeve-shaped section means in the present case an inwardly, i.e. pre-deformation of the sleeve-shaped section in the direction of the drive. This means that the sleeve-shaped section can be attached to one 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 when the filling expands thermally, thus increasing the volume of the casing. The concave shape also prevents the "cracking frog" effect. The sleeve never exceeds the completely cylindrical shape, but always has a curvature inwards. This means that the bending direction cannot suddenly change.

If the sleeve-shaped section were strictly cylindrical, there would be no possibility of expansion in the radial direction when attached to the edge of the membrane.

In a preferred embodiment, the sleeve-shaped section is welded to the membrane on the front side. Welding the sleeve-shaped section to the membrane has the advantage that welding creates a secure and permanently tight connection between the sleeve-shaped section and the membrane, which has a high temperature resistance.

Alternatively, gluing or soldering is also possible.

If the sleeve-shaped section has a smaller diameter in a rear area, in which it is connected to the contact feedthrough, than in a front area, in which 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 it, to the edge of the membrane. The attachment can be carried out by welding, in particular by laser welding.

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

In this way, the part of the casing 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 the drive.

In a preferred embodiment, the casing 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. A corresponding design ensures that the casing deforms in a predetermined area when a thermally induced expansion of the filling occurs. The casing is preferably designed in the area of the sleeve-shaped section so that it deforms in this area.

In one embodiment, the piezo elements have an electrical contact with variable length in the axial direction of the sleeve. By means of electrical contacts with variable length, a displacement of the contact leadthrough in the axial direction can be compensated for when the sleeve-like section is deformed.

For example, the length-variable electrical contact can be implemented 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 that engages in the socket is arranged on the contact leadthrough. Depending on the thermal expansion and corresponding deformation of the sleeve-like section, there can be a varying amount of overlap between the socket and the pin. Alternatively, the pin can be arranged on the piezo element and the socket on the contact leadthrough.

• 1 ein Ausführungsbeispiel eines vibronischen Sensors 1 gemäß der vorliegenden Anmeldung.

The present invention is explained in detail below using exemplary embodiments with reference to the attached figures. They show:

• 1 an embodiment of a vibronic sensor 1 according to the present application.

In the figures, unless otherwise stated, identical reference symbols designate identical or corresponding components with identical functions.

1 shows an embodiment of a vibronic sensor 1 according to the present application.

The vibronic sensor 1 has a membrane 5 that can be set into mechanical vibrations in an axial direction A and an electromechanical transducer, referred to as 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 vibrators 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 has a piezo element 4, which is glued 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.

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

In the present embodiment, the drive 3 is surrounded by a casing 2 which is filled with a filling 9, in this case a transformer oil. The casing 2 is formed at the front by the membrane 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 to a transition from the membrane 5 and an edge 15 extending from the membrane 5 at the rear in the axial direction A and is thus connected to the membrane 5 in a sealing manner. At the rear, the sleeve-shaped section 21 is attached to the contact feedthrough 23 with a second weld seam 27. In this way, the drive 3 is sealed and thus encapsulated from the rest of the housing of the vibronic sensor 1.

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

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

This length variability is necessary in the present embodiment because the sleeve-shaped section 21 of the casing 2 is pre-formed concavely. In the present 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-related expansion of the filling, it expands within the casing 2 and pushes the sleeve-shaped section 21 outwards in the radial direction. Due to this deformation, the extension of the sleeve-shaped section 21 in the axial direction A increases, which increases the distance between the piezo element 4 and the contact feedthrough 23. As the distance increases, the electrodes 7 slide further out of the contacts of the contact feedthrough 23, but at the same time an electrical contact is maintained.

An increased volume of the filling 9 and a resulting increased internal pressure inside the casing 2 is compensated by a deformation of the casing wall of the casing-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.

If the casing 2 were completely rigid, the frequency would shift significantly in the event of a pressure difference under the influence of temperature, as the stiffness of the system would increase. Membrane 5 and cable duct would deform under the influence of the internal pressure.

The sleeve-shaped section 21 of the casing 2 is located in the interior of the vibronic sensor 1, which is why a process pressure does not act on it. The sleeve-shaped section 21 therefore does not have to be designed for the process pressure, but only has to be able to withstand the temperature-indicated 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, so that the thermal expansion of the filling 9 is also minimized.

reference sign

1

vibronic sensor

2

Covering

3

drive

4

piezo element

5

membrane

6

leveling ceramics

7

electrodes

9

filling

11

Mechanical oscillating unit

13

contact implementation

15

edge

21

sleeve-shaped section

23

contact implementation

24

compensation space

25

First weld

27

Second weld

Claims (10)

Vibronic sensor (1) with a membrane (5) that can be set into vibration, a piezoelectric drive (3) for setting the membrane (5) into vibration 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) has 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 therefrom, which is filled with a liquid filling (9), wherein the filling (9) has an electrical breakdown field strength of more than 3 kV/mm, wherein the casing (2) is designed to be flexible at least in sections. Vibronic sensor (1) according to patent claim 1 , characterized in that the sheath (2) has a sleeve-shaped portion (21) and a contact feedthrough (23). Vibronic sensor (1) according to one of the preceding claims, characterized in that the sleeve-shaped portion (21) is designed to be deformable in the radial direction. Vibronic sensor (1) according to one of the preceding claims, characterized in that the sleeve-shaped portion (21) is preformed concavely in the radial direction (R). 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. 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 side of the contact leadthrough (23). 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). Vibronic sensor (1) according to one of the preceding claims, characterized in that the piezo elements (4) have an electrical contact that is variable in length in the axial direction (A). Vibronic sensor (1) according to patent claim 8 , characterized in that the electrical contact is designed as at least one contact pin with at least one correspondingly designed sliding sleeve. Vibronic sensor (1) according to one of the preceding claims, characterized in that the electrical contact is prestressed in the axial direction (A).

Images