Vibronic sensor and method for operating such a sensor

Abstract

The present invention relates to a vibronic sensor (1) having a membrane (5) which can be made to vibrate, a piezoelectric drive (3) for making the membrane (5) vibrate and for detecting vibrations of the membrane (5), a mechanical vibration unit (11), which is arranged on the membrane (5), for transmitting vibrations of the membrane (5) to a medium surrounding a 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 has at least one piezo element (37) and electrodes for electrically contacting the at least one piezo element (37), wherein the drive (3) is encapsulated with a casing (9), which surrounds the drive (3) at a distance, and an insulating gaseous or liquid filling (7), wherein the filling (7) has an electrical breakdown field strength of greater than 3 kV / mm.

Description

[0001] Vibronic sensor and method for operating such a sensor

[0002] The present invention relates to a vibronic sensor according to the preamble of claim 1 and a method for operating such a sensor.

[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 can be a membrane, a vibrating rod attached to the membrane, or a vibrating fork attached to the 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 vibrates.

[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 specified limit level 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 a design 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 and an insulating gaseous or liquid filling, wherein the filling has an electrical breakdown field strength of more than 3 kV / mm.

[0017] A key feature of 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 extending 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 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] Using a liquid or gas as an insulator in the enclosure has the advantage over a potting compound or a solid insulator in that gases and liquids only have a reduced impact on the dynamic properties of the sensor. Potting compounds dampen the movement of the actuator and diaphragm to an excessive extent, making them unsuitable for practical use. Using insulating liquids simplifies the sealing of the enclosure and also prevents diffusion.

[0022] In a further development, the vibronic sensor is characterized by the fact that the insulator is a liquid, specifically a mineral oil, especially transformer oil, especially saturated pentaerythritol tetrafatty acid esters, and preferably consists of it. Enriching the insulating 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 with regard to their chemical, electrical, and thermal properties.

[0023] 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, breakdowns to the enclosure and membrane are also prevented. The liquid preferably has a maximum volume of 2 ml. The volume is preferably 0.3–1.4 ml, depending on the further technical design. The volume is particularly preferably 1 ml.

[0024] 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.

[0025] The high-voltage electronics can be designed to be switchable on and off, so that the sensor can continue to be operated in an energy-saving manner and the energy required for the high-voltage electronics is only consumed when polarization of the piezo material is necessary.

[0026] In particular, the high-voltage electronics can include a voltage booster circuit. The voltage booster circuit serves to generate the high voltage required for repolarization of the piezo material from the vibronic sensor's supply voltage, thus generating the saturation field strength.

[0027] The voltage booster circuit can be designed, in particular, as a semiconductor circuit, preferably as a voltage multiplier, boost converter, or charge pump, or as a combination of these circuits. Since vibronic sensors are often designed as two-wire sensors powered according to the 4-20 mA standard, the available supply power is limited. The supply voltage for such two-wire sensors is typically in the range of 12-48 V and must then be boosted using a suitable circuit arrangement.

[0028] A two-wire sensor or two-wire field device according to the present invention is understood to be a field device that is connected to a higher-level unit via two lines, wherein both a power supply and a measured value transmission take place via these two lines.

[0029] The power and / or signal transmission between the two-wire field device and the higher-level unit follows the well-known 4 mA to 20 mA standard, which uses a 4 mA to 20 mA current loop, i.e., a two-wire line, between the field device and the higher-level unit. In addition to the analog signal transmission, the measuring devices can transmit or receive additional information to or from the higher-level unit using various other protocols, particularly digital protocols. Examples include the HART protocol and the Profibus PA protocol.

[0030] These field devices are also powered via the 4 mA to 20 mA current signal, eliminating the need for an additional power supply cable besides the two-wire cable. To minimize wiring and installation effort, as well as safety measures, for example, when used in explosion-proof areas, it is also not desirable to provide additional power supply cables.

[0031] With two-wire field devices, the available input power is significantly limited. The electronics in the field device must be designed to operate reliably even at a minimum signal current of 4 mA. This also presents a major challenge when repolarizing piezo elements.

[0032] To enable the gradual storage of available energy, it is advisable for the high-voltage electronics to incorporate an energy buffer, particularly a capacitor for temporarily storing the polarization voltage. Such a capacitor can buffer energy for polarization during phases in which the sensor exhibits reduced energy consumption and subsequently be used for polarization.

[0033] In a preferred embodiment, the high-voltage electronics are designed such that a polarization voltage for a field strength of at least 3 kV / mm is available for a polarization duration, in particular for at least 30 s, in particular 30-60 s. A duration of at least 30 s is generally sufficient to achieve alignment of the depolarized regions.

[0034] A method according to the invention for operating a vibronic sensor, comprising 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, wherein a resonance frequency of the mechanical vibration unit is dependent on the medium surrounding it, wherein the piezoelectric drive has at least one piezo element and electrodes for electrically contacting the at least one piezo element, and the drive is encapsulated with a shell surrounding the drive and an insulating gaseous or liquid filling, wherein the filling has an electrical breakdown field strength of more than 3 kV / mm, is characterized in thatthat a repolarization of the at least one piezo element is carried out during operation of the vibronic sensor.,

[0035] The vibronic sensor can be designed in particular according to the above description.

[0036] The core of the present method for operating a vibronic sensor is that a repolarization of at least one piezo element is carried out during operation of the sensor.

[0037] It should be emphasized at this point that repolarization, within the meaning of the present application, specifically means the application of an electric field such that an already existing polarization of the piezo element, i.e., an already existing polarization direction, is enhanced. This does not mean, in particular, that repolarization, as understood here, requires the existing polarization to first be canceled by applying an electric field before applying an electric field for repolarization.

[0038] In order to operate the present vibronic sensor in the most energy-saving and efficient way possible, it is advisable not to carry out the proposed repolarization according to a mere time schedule, e.g. every x operating hours, but additionally or alternatively to monitor a characteristic of the sensor, whereby the repolarization is triggered when a threshold value of the monitored characteristic is undershot or exceeded.

[0039] In this way, one variant ensures that repolarization is performed even when, for example, the specified time schedule does not yet allow for repolarization. This increases the availability and efficiency of the sensor. The other variant also ensures that the sensor operates as energy-efficiently as possible by only performing polarization when necessary due to the monitored parameter.

[0040] Suitable parameters can be, for example, an amplitude of the generated oscillations, a frequency or the result of impedance spectroscopy or combinations thereof.

[0041] If the oscillation amplitude decreases under known conditions, for example, when the sensor is oscillating freely in air at a known temperature, this may be due to depolarization of the drive's piezo elements and can be remedied by repolarization. If the oscillation amplitude is still reduced after repolarization, this may be due to other problems, such as buildup and the resulting increase in the mass of the mechanical oscillation unit.

[0042] Impedance spectroscopy, i.e., determining the alternating current resistance of the piezoelectric elements as a function of the frequency of the applied alternating voltage, can also be used to determine the polarization of the piezoelectric elements. Repolarization of the piezoelectric elements can, in particular, involve the following steps:

[0043] - Activating high-voltage electronics,

[0044] - storing energy in an energy buffer,

[0045] - Increasing an input voltage in the high-voltage electronics up to the polarization voltage,

[0046] - Applying the polarization voltage to the at least one piezo element for a polarization duration.

[0047] When repolarization is required, the high-voltage electronics are activated, and energy is then collected in the energy buffer. If a sufficient amount of energy is available for repolarization, it is converted into the polarization voltage required for repolarization in the high-voltage range, and the polarization voltage is applied to at least one piezo element for a predetermined polarization duration. The required polarization voltage depends on the piezo material used and the thickness of the piezo elements. If both are known, the field strength required for repolarization can be determined, which then allows the required polarization voltage to be calculated.

[0048] The present invention will be explained in detail below using exemplary embodiments with reference to the accompanying figures. They show:

[0049] Figure 1 shows an embodiment of a vibronic sensor,

[0050] Figure 2 shows the polarization of a piezo material depending on an applied electric field,

[0051] Figure 3 shows the mechanical behavior of a piezo element depending on an applied electric field and

[0052] Figure 4 shows a highly simplified representation of the polarization behavior of a piezoelectric material as a function of temperature. Unless otherwise indicated, identical reference numerals in the figures denote identical or corresponding components with the same function.

[0053] Figure 1 shows an embodiment of a vibronic sensor according to the present application.

[0054] The vibronic sensor 1 has a housing 2, in which a sensor electronics unit 30, shown only schematically here, is arranged in a rear section. The sensor electronics unit 30 is designed as a control and evaluation unit, which, during normal operation of the vibronic sensor 1, controls a drive 3 arranged in a front section of the housing 2 and evaluates the sensor signals. The drive 3 is designed as a piezo stack drive with two piezo elements 37 as electromechanical transducers for generating a movement based on a voltage applied to the drive 31.

[0055] The drive of such vibronic sensors 1 is typically based on piezoceramics used as electromechanical transducers. Lead zirconate titanate (PZT) is the most common material class. To use such ceramics as piezo elements 37 in the drive, they must first be polarized. High electric fields in the range of 3000 V / mm and more are required to polarize such piezoceramics.

[0056] The vibronic sensor 1 is designed in such a way that it is capable of repolarizing the piezo elements 37 of the drive 31 after depolarization phenomena. For this purpose, the vibronic sensor 1 comprises various components, which are explained in more detail below.

[0057] Firstly, the drive 3 is encapsulated. For this purpose, the drive is arranged in a casing 9 that is spaced apart from the drive 3 and designed to accommodate an insulating filling 7, in this case a liquid insulator in the form of an insulating oil. In addition, a front section of the housing 2 is separated from the rest of the housing by a partition 35. In this way, for example, further explosion protection requirements can be met. The drive 3 is connected by cable 33 to high-voltage electronics 34, which can provide the high voltage in the range of 3000 V on the output side necessary for repolarization of the piezo elements 37. Via the high-voltage electronics 34, the drive 31 is also connected to the sensor electronics 30, which performs the normal sensor function.In addition to the normal sensor function, i.e. in particular the limit level monitoring, the sensor electronics 30 is designed to carry out an impedance spectroscopy of the piezo elements 37 of the drive 3.

[0058] The high-voltage electronics are designed in such a way that they are only active when a repolarization of the piezo elements of the drive 3 is to take place, ie the high-voltage electronics are inactive during normal measuring operation of the sensor 1.

[0059] For repolarization, the measuring operation of sensor 1 can be briefly interrupted. The high-voltage generation and the application of the high voltage to the drive can thus be switched on and off. The high voltage generation can be carried out, for example, by a boost converter in the high-voltage electronics 34, but is not limited to this method.

[0060] In the present embodiment, the high-voltage electronics 34 comprises an energy storage device in the form of a capacitor that is charged with the high voltage. When a sufficient charge level is reached, the capacitor is connected to the drive 3, and the piezo elements 37 are thereby subjected to high voltage at the polarization voltage level. The required components must have the highest possible dielectric strength. Furthermore, the design of the structure must also maintain the required distances between the components and the conductor tracks to prevent arcing.

[0061] Figure 2 shows the classic polarization curve of a piezo element 37, as it can be used in the drive 3 according to Figure 1.

[0062] Shown is the polarization P of the piezo element 37 as a function of the field strength E of an applied electric field. By applying an electric field of field strength E, the polarization P of the piezo element 37 increases up to a saturation polarization Ps. If the field strength is reduced again, a so-called remanent polarization PR remains even without an applied electric field, by means of which the piezo element 37 can then be operated as an electromechanical actuator. An electric field with the opposite sign can cancel the polarization P again starting at a so-called coercive field strength -Ec. If the field strength E of the electric field is further increased, a saturation polarization -Ps in the opposite direction is reached, whereby a negative remanent polarization -P is also maintained here.

[0063] Figure 3 shows the mechanical strain S of a piezoelectric element as a function of the electric field strength E. The mechanical strain also exhibits saturation in this curve and is limited by the dielectric strength of the piezoelectric element. This is based on the existing polarization P, with repolarization of the piezoelectric material beginning at the coercive field strength Ec.

[0064] During measurement operation of a vibronic sensor 1, as is the subject of the present application, the piezo elements are typically operated with unipolar electric fields and are not driven into the range of mechanical strain saturation. Therefore, during normal operation, no change in polarization P is generated.

[0065] Figure 4 shows in a highly simplified manner the behavior of the polarization P of a piezo element as a function of the temperature T.

[0066] From a limit temperature T G, which is about 50% of the Curie temperature T c of the piezo material used, the polarization P decreases until it is completely lost at the Curie temperature Tc. In addition to the temperature T, the polarization P can also decrease due to excessively high electric fields (see Fig. 2) and excessive mechanical loads. If the polarization P of the piezo element decreases, it expands less, which leads to a reduced drive power of the drive 3 of the vibronic sensor 1.

[0067] By targeted repolarization by applying an electric field with a field strength E above the coercive field strength Ec, the polarization of the piezo element 37 can be restored, so that the drive power is restored.

[0068] Reference symbol

[0069] 1 vibronic sensor

[0070] 2 housings

[0071] 3 Drive

[0072] 5 Membran

[0073] 7 Filling

[0074] 9 Cover

[0075] 11 mechanical vibration unit

[0076] 30 Sensor electronics

[0077] 33 connection cables

[0078] 34 High-voltage electronics

[0079] 37 Piezo element

[0080] P Polarization

[0081] T Temperature

[0082] E field strength

[0083] S mechanical elongation

[0084] SR remanent strain

[0085] Ec coercive field strength

[0086] Ps saturation polarization

[0087] P remanent polarization

[0088] Tc Curie temperature

Claims

Patent claims 1. 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 piezoelement (37) and electrodes for electrically contacting the at least one piezoelement (37), characterized in that the drive (3) is encapsulated with a casing (9) surrounding the drive (3) in a spaced-apart manner and an insulating gaseous or liquid filling (7), wherein the filling (7) has an electrical breakdown field strength of more than 3 kV / mm.

2. Vibronic sensor (1) according to claim 1, characterized in that the liquid comprises, preferably is, a mineral oil, in particular transformer oil, in particular saturated pentaerythritol tetrafatty acid esters.

3. Vibronic sensor (1) according to one of the preceding claims, characterized in that the liquid has a maximum volume of 2 ml, preferably 0.3-1.4 ml, particularly preferably 1 ml.

4. Vibronic sensor (1) according to one of the preceding claims, characterized in that the sensor has high-voltage electronics (34) for providing a polarization voltage.

5. Vibronic sensor (1) according to claim 4, characterized in that the high-voltage electronics (34) has a voltage booster circuit.

6. Vibronic sensor (1) according to claim 5, characterized in that the voltage booster circuit is designed as a semiconductor circuit, in particular a voltage multiplier, boost converter or charge pump.

7. Vibronic sensor (1) according to one of claims 4 to 6, characterized in that the high-voltage electronics (34) has an energy buffer, in particular a capacitor for temporarily storing the polarization voltage.

8. Vibronic sensor (1) according to one of the preceding claims, characterized in that the high-voltage electronics (34) are designed such that a polarization voltage for a field strength of at least 3 kV / mm is available for a polarization duration, in particular for at least 30 s, in particular 30-60 s.

9. Method for operating a vibronic sensor (1), 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 comprises at least one piezo element (37) and Electrodes for electrically contacting the at least one piezo element (37), and the drive (3) is encapsulated with a casing (9) surrounding the drive (3) and an insulating gaseous or liquid filling (7), wherein the filling (7) has an electrical breakdown field strength of more than 3 kV / mm, in particular according to one of the preceding claims, characterized in that a repolarization of the at least one piezo element (37) is carried out during operation of the vibronic sensor (1).

10. Experience according to claim 9, characterized in that a characteristic of the sensor (1) is monitored and if a threshold value of a monitored characteristic is undershot or exceeded, the repolarization is triggered.

11. Experience according to claim 10, characterized in that the characteristic comprises an amplitude, a frequency or an impedance spectroscopy or combinations thereof.

12. Method according to one of claims 9 or 10, characterized in that the repolarization comprises the following steps: - Activating high-voltage electronics, - storing energy in an energy buffer, - Increasing an input voltage in the high-voltage electronics up to the polarization voltage, - Applying the polarization voltage to the at least one piezo element for a polarization duration.

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