Method for monitoring the state of a coil and device for determining and / or monitoring a process variable

Abstract

The invention relates to a method for monitoring the state of a coil (17) which is part of a device (1) for determining at least one process variable of a medium (4) in a container (5), the method comprising the following method steps: applying an electrical excitation signal (A) to the coil (17) and receiving an electrical reception signal (U) from the coil (17), determining a first value (U1) of the reception signal (U) at a first predefinable measuring time (t1), comparing the first value (U1) of the reception signal (U) at the first measuring time (t1) to a reference value (U1), ref ) and determining a status indicator for the coil (17) based on the comparison. The invention also relates to a device (1) designed to carry out the method according to the invention.

Description

Technical Field

[0001] The invention relates to a method for monitoring the state of a coil, which is part of a device for determining at least one process variable of a medium in a container, and to a device designed to implement the method according to the invention. The device is a vibration sensor. The device comprises a unit with mechanical vibration capability, a drive / receiving unit and an electronic unit. The process variable is given, for example, by the filling level, in particular the limit level, the volume flow of the medium, or also by its density or viscosity. For example, the medium is located in a receptacle, for example a container, a tank or a pipeline. The medium itself is given, for example, by a liquid, a gas or a bulk material. Background Art

[0002] Vibration sensors are often used in process and / or automation engineering. In the case of filling level measuring devices, they have at least one unit with mechanical vibration capability, such as a swing fork, a single rod or a membrane. During operation, it is excited into mechanical vibration by a drive / receiver unit, which is usually in the form of an electromechanical transducer unit, which in turn can be, for example, a piezoelectric drive or an electromagnetic drive. The applicant has produced a variety of corresponding field devices and distributed them, for example, under the name LIQUIPHANT or SOLIPHANT. In principle, the basic measuring principle is known from many publications. The drive / receiver unit excites the unit with mechanical vibration capability into mechanical vibration by an electrical excitation signal. On the contrary, the drive / receiver unit can receive the mechanical vibration of the unit with mechanical vibration capability and convert it into an electrical receiving signal. Accordingly, the drive / receiver unit is either a separate drive unit and a separate receiving unit, or a combined drive / receiver unit.

[0003] In many cases, the drive / receive unit is thus part of a feedback electrical resonant circuit, by means of which the unit capable of mechanical vibration is excited into mechanical vibration. For example, for resonant vibration, the resonant circuit condition according to which the amplification factor is ≥ 1 and all phase results occurring in the resonant circuit are multiples of 360° must be met. In order to excite and meet the resonant circuit condition, a defined phase shift between the excitation signal and the received signal must be ensured. Therefore, a predeterminable value of the phase shift, i.e. a target value of the phase shift between the excitation signal and the received signal, is frequently set. For this purpose, various solutions, including analog and digital methods, are known from the prior art as described, for example, in DE 10 2006 034 105 A1, DE 10 2007 013 557 A1, DE 10 2005 015 547 A1, DE 10 2009 026 685 A1, DE 10 2009 028 022 A1, DE 10 2010 030 982 A1 or DE 00 10 2010 030 982 A1.

[0004] Both the excitation signal and the received signal are characterized by their frequency ω, amplitude A and / or phase Φ. Therefore, the change of these variables is usually used to determine the corresponding process variable. For example, the process variable can be the filling level, the specified filling level or the density or viscosity of the medium, and the flow rate. For example, in the case of a vibrating level switch for liquids, it is distinguished whether the unit with vibration capability is covered by liquid or vibrates freely. For example, the two states, free state and covered state, are distinguished based on different resonance frequencies, i.e. based on frequency shift.

[0005] If the unit with vibration capability is covered by the medium, the density and / or viscosity can only be determined with such a measuring device. Regarding the determination of density and / or viscosity, different possibilities are also known from the prior art, such as those disclosed in documents DE10050299A1, DE102007043811A1, DE10057974A1, DE102006033819A1, DE102015102834A1 or DE102016112743A1.

[0006] Piezoelectric and / or electromagnetic drive / receiver units are particularly used for the drive / receiver unit itself. Given a drive / receiver unit based on the piezoelectric effect, relatively high efficiency can usually be achieved. However, they are only suitable for use at high temperatures, particularly at temperatures above 300°C, under certain conditions. In this context, an alternative is provided by an electromagnetic drive / receiver unit, such as described in publications WO2007 / 113011, WO2007 / 114950A1, DE102015104533A1, DE102016112308A1. Here, the conversion of electrical energy to mechanical energy is carried out via a magnetic field, respectively. Therefore, the transducer unit described generally includes at least one coil and a permanent magnet. An alternating magnetic field passing through the magnet is generated by the coil, and a periodic force is transmitted to the vibrating unit via the magnet. Such a drive / receiver unit can be used, for example, in a temperature range between -200°C and 500°C, depending on the material used.

[0007] In order to ensure the reliable operation of the vibration sensor, a variety of methods for diagnosing possible faults of the vibration sensor are known from the prior art. The possibility of condition monitoring or diagnosis is very important, in particular in the case of safety-critical applications, such as the use of limit level switches in containers with flammable liquids. Depending on the specific application, the corresponding measuring device must meet different safety requirements. In this regard, for example, the so-called SIL standard (SIL stands for safety integrity level) is widely known from the IEC61508 standard specification, which distinguishes four different levels to specify the safety integrity requirements of the safety function. In principle, the safety requirement level is a measure of the reliability of the corresponding system or measuring device depending on the potential danger. For example, a safety function that usually provides a defined level for ensuring safety integrity is provided by emergency shutdown or shutting down overheating equipment.

[0008] For example, the failure behavior of individual components is investigated to establish the safety integrity level. In addition, the presence of redundant structures and the relationship between safe and unsafe or dangerous errors are checked. Based on such considerations, it is then possible to determine the total failure rate, which is used to associate the corresponding safety integrity level with the corresponding system or measuring device.

[0009] DE102004027397A1 describes a possibility for detecting a cable break in a vibration sensor. From documents DE10014724A1 and DE102009045204A1, diagnostic options for detecting coatings in the region of a unit with vibration energy are known. However, in various embodiments, diagnostics in the region of a drive / receiver unit of a vibration sensor are also proposed, such as the diagnosis of a piezoelectric drive / receiver unit described in DE102008032887A1. With regard to electromagnetic drive / receiver units, it is again known from DE102017115147A1 to evaluate the state of the drive / receiver unit based on the total ohmic resistance. Summary of the invention

[0010] Starting from this prior art, the object of the invention is to provide a measuring device with which high safety requirements can be met in a particularly simple manner.

[0011] This object is achieved by a method and an apparatus.

[0012] With regard to the method, the object is achieved by a method for monitoring the state of a coil, which is part of a device for determining at least one process variable of a medium in a container. The method comprises the following method steps:

[0013] - applying an electrical excitation signal to the coil and receiving an electrical reception signal from the coil,

[0014] - determining a first value of the received signal at a first predefinable measuring time,

[0015] - comparing a first value of the received signal at a first measurement time with a reference value, and

[0016] - Determining a state indicator of the coil based on the comparison.

[0017] Diagnosing the driver / receiving unit by comparing the measured value at a predefinable first measuring time with a reference value can be realized in a particularly simple manner. In particular, the design outlay is very low—implementation of the condition monitoring requires only few additional components for the corresponding sensors.

[0018] It is advantageous if the first predefinable measuring time is selected such that it lies within a first predefinable time interval in which the received signal reacts to the sudden change in the excitation signal in the form of a step response. The step response is advantageously particularly easy to evaluate. It is accompanied by a characteristic curve diagram of the received signal as a function of time, which characteristic curve diagram can be detected in a simple manner.

[0019] It is also advantageous if the excitation signal is a square wave signal. A square wave signal is characterized by a periodic change between two constant values, which has a rectangular curve graph plotted as a function of time. It is therefore characterized by periodic rising and falling edges. In this context, it is advantageous if the abrupt change of the excitation signal is a rising or falling edge of the excitation signal.

[0020] Conclusions about the degree of damage to the coil are preferably determined. Thus, both qualitative and quantitative conclusions about the state of the coil are possible. Based on the coil state diagnosis given on the basis of the state indicator, it is also conceivable to generate messages, for example, about maintenance to be performed or about the presence of defects.

[0021] It is also preferred that the status indicator is determined as a function of the deviation of the measured value from the reference value. In particular, it can be checked whether the deviation exceeds a predefinable limit value.

[0022] According to one embodiment of the method, the status indicator is a conclusion regarding the presence of at least one winding short circuit in the region of the coil.

[0023] In this respect, it is advantageous if the presence of a winding short circuit in a region of the coil is detected on the basis of falling below a reference value at a first predefinable measuring time.

[0024] According to a further embodiment of the method, the status indicator is a conclusion regarding a defective electrical contact or a cable break in the region of the coil or at least two connecting lines.

[0025] In this regard, it is also advantageous if the deviation of the step response of the received signal in response to a sudden change in the excitation signal from a reference jump response is used to detect the presence of a defective electrical contact or a cable break in the region of the coil. In the case of an intact coil, the step response of the received signal has, in particular, an exponential curve. In this case, deviations from such a curve indicate the presence of a defective electrical contact.

[0026] In one embodiment, the method comprises determining a second value of the received signal at a second predefinable measuring time, and wherein a conclusion about the temperature of the medium is determined based on the second value of the received signal. In addition to state monitoring, the method according to the invention allows the temperature to be determined without implementing an additional temperature-sensitive sensor element.

[0027] The temperature in the region of the vibration sensor of the corresponding medium has a great influence on the determination of the corresponding process variable, in particular, has a great influence on the corresponding received signal. In order to compensate for this influence, it is known from DE 102006007199A1 or DE102005029490A1 that a vibration measuring device with an additional temperature sensor is provided. However, integrating another sensor into the measuring device is basically associated with a certain design cost. From DE102017114315A1, it is further known that, with respect to the electromagnetic drive / receiving unit, the temperature of the medium is determined based on an electrical parameter, for example, based on the ohmic resistance of the coil. However, this measure is also associated with a relatively high cost, because a device for determining the corresponding parameter is provided. In contrast, the temperature determination according to the present invention can be realized in a particularly simple manner based on the second value of the received signal at the second predefinable measuring time. The second value of the received signal or the second predefinable measuring time is selected so that the second measured value directly depends on the temperature of the medium and can be directly used to determine the temperature. Therefore, the temperature determination does not require other sensor elements or complex design measures.

[0028] With regard to the temperature of the medium, it is also conceivable that the influence of the temperature on the received signal is compensated in the continuous measuring operation of the device. In this way, the measuring accuracy of the corresponding sensor can be increased when determining the corresponding process variable.

[0029] In this context, it is advantageous if a comparison of the second value of the received signal with at least one characteristic curve of the received signal as a function of temperature is used to determine the temperature of the medium.

[0030] It is likewise advantageous if the second predefinable measuring time is selected such that it lies outside the first time interval.

[0031] Preferably, the second predefinable measuring time is selected such that it lies within a second time interval in which the received signal is substantially constant.

[0032] The object of the invention is also achieved by a device for determining and / or monitoring at least one process variable of a medium in a container, the device comprising at least one coil, the device being designed to implement a method according to at least one of the described embodiments. The method according to the invention can thus be implemented simultaneously for continuous measuring operation or alternately with continuous operation during a diagnostic operating mode of the device. It is also conceivable to trigger the method manually, selectively, for example via a switch operation or the like.

[0033] With regard to the device, it is advantageous if the device comprises a vibration sensor, which comprises a unit with mechanical vibration capability, a drive / receiver unit having at least one coil, the drive / receiver unit being designed to excite the unit with mechanical vibration capability by means of an electrical excitation signal to generate mechanical vibrations, and to receive mechanical oscillations from the unit with vibration capability and convert them into an electrical reception signal; and an electronic unit being designed to generate an excitation signal starting from the reception signal and to determine at least one process variable from the reception signal.

[0034] In one embodiment of the device, the vibratable unit is a membrane, a monorod or a vibrating fork.

[0035] A further embodiment of the device comprises that the process variable is, in particular, a predefinable filling level, a density or a viscosity of the medium.

[0036] Within the scope of the invention, various embodiments of the vibration sensor, in particular different drive / receive units, can be used. In the following, without limiting the generality, two particularly preferred embodiments for the drive / receive unit are specified.

[0037] In one embodiment, the driving / receiving unit comprises at least

[0038] - a membrane capable of being set into mechanical vibration,

[0039] - two rods fastened to the membrane perpendicularly to its base surface,

[0040] - a housing, wherein the membrane forms at least a partial area of ​​a wall of the housing, and wherein the two rods extend into the interior of the housing,

[0041] - two magnets, wherein one magnet is fastened in each case in the end region of one of the two rods facing away from the membrane, and

[0042] - a coil having a core mounted in a housing above the magnet and which can be excited with an alternating current signal,

[0043] Therein, the coil is designed to generate a magnetic field which, via two magnets, sets the two rods into mechanical vibration, and

[0044] Therein, two rods are fastened to the membrane in such a way that the vibrations of the membrane are caused by the vibrations of the two rods.

[0045] In a further embodiment, the driving / receiving unit comprises at least

[0046] - a membrane capable of being set into mechanical vibration,

[0047] - at least three rods fastened to the membrane perpendicularly to its base surface,

[0048] - a housing, wherein the membrane forms at least a partial area of ​​a wall of the housing, and wherein the rod extends into the interior of the housing,

[0049] at least three magnets, wherein one magnet is fastened to each of the at least three rods in the end region facing away from the membrane, and

[0050] - a coil having a core mounted in a housing above the magnet and which can be excited with an alternating current signal,

[0051] The coil is designed to generate a magnetic field which, through the magnet, causes the rod to vibrate.

[0052] wherein the rod is fastened to the membrane in such a way that the vibration of the membrane is caused by the vibration of the rod,

[0053] wherein at least one rod is attached to the diaphragm substantially at a position along the base surface of the membrane,

[0054] At this position, the second derivative of the deflection of the diaphragm from the rest position as a function of the position on the base surface is substantially zero.

[0055] Furthermore, a drive / receiving unit according to one of the two embodiments described by way of example with four rods is particularly preferred. With regard to the two preferred embodiments described for the drive / receiving unit, reference is made to the entire contents of documents DE 10 2015 104 533 A1 and DE 10 2016 112 308 A1.

[0056] It should also be noted that embodiments described in connection with the method according to the invention can also be applied mutatis mutandis to the device according to the invention and vice versa. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Reference below Figure 1-Figure 4 The present invention and its advantageous embodiments are described in more detail as follows:

[0058] Figure 1 : Representation of a vibration sensor according to the prior art,

[0059] Figure 2: Two possible embodiments of a vibration sensor with a preferably electromagnetic drive / receiver unit,

[0060] Figure 3 : A diagram for illustrating a received signal of a coil according to state monitoring of the present invention; and

[0061] Figure 4 : Diagram of the received signal of the coil for illustrating the temperature determination according to the invention. DETAILED DESCRIPTION

[0062] Figure 1 1 shows a vibrating fill level measuring device 1. A sensor unit 2 with a mechanically vibrable unit 3 is depicted in the form of a vibrating fork which is partially immersed in a medium 4, which is located in a container 5. The unit 3 with the ability to vibrate is excited to mechanical vibrations by a drive / receiver unit 6, which is usually an electromechanical transducer unit and can be, for example, a piezoelectric stack or a bimorph driver, but can also be an electromagnetic or magnetostrictive drive / receiver unit. However, it is naturally clear that other embodiments of the vibration sensor are also possible. In addition, an electronic unit 7 is depicted, by which signal detection, evaluation and / or supply are performed.

[0063] Figure 1 b shows a more detailed view of a unit 3 with vibration capability in the form of a vibrating fork, for example for a vibration sensor sold by the applicant under the name LIQUIPHANT. A membrane 8 and a vibrating element 9 connected thereto can be seen. The vibrating element 9 has two vibrating rods 10a, 10b, at the end of each of which a paddle 11a, 11b is integrally formed. In operation, the vibrating fork 3 performs a vibrating movement according to the vibration mode in which it is excited. Each of the two vibrating rods 10a, 10b behaves substantially like a so-called bending vibrator. In the fundamental mode, for example, the two vibrating rods 10a, 10b oscillate in antiphase with respect to one another.

[0064] Although many different embodiments for the drive / receive unit 6 can be used within the scope of the invention, the following description relates without general limitation to an electromagnetic drive / receive unit 6 with at least one coil, as they are described in documents DE 10 2015 104 533 A1 or DE 10 2016 112 308 A1. In the context of the present application, reference is made to the entire contents of these two patent applications.

[0065] Figure 2A shows a schematic diagram of such a drive / receiving unit 6. The housing 12 ends at a lower wall with a membrane 8, which is associated with the vibrating unit 3. For the embodiment shown here, the housing 12 is cylindrical and the disc-shaped membrane 8 has a circular cross section A. However, it goes without saying that other geometric shapes are also conceivable and fall within the present invention. Two rods 15a, 15b are fastened to the membrane 8 perpendicular to the bottom surface A of the membrane 8 and extend to the interior of the housing 12. This is in particular a non-positive connection. The bottom surface A of the membrane 8 is then located in a plane perpendicular to the longitudinal direction of the rods 15a, 15b.

[0066] Magnets 16a, 16b, in particular samarium cobalt or alnico magnets, are fastened in the end region of the rods 15a, 15b facing away from the membrane 8, respectively. The magnets 16a, 16b are preferably all oriented identically. A coil 17 comprising a wire wound around a core 18 is arranged above the magnets 16a, 16b. The core 18 of the coil 17 is part of a cup-shaped anchoring unit 19 having a base 20 and a peripheral wall 21. For example, the base 20, like the base surface A of the membrane 8, can have a circular cross-sectional area. From the base 20 of the cup-shaped anchoring unit 19, the core 18 of the coil 17 extends centrally into the interior of the anchoring unit 19 in the form of a connecting piece. In this case, the peripheral wall 21 then has the function of internal magnetic field guidance. The rods 15a, 15b with the magnets 16a and 16b do not contact the coil 17 and the core 18. In continuous operation, the coil 17 is subjected to an alternating signal to generate an alternating magnetic field. For this purpose, the coil has two connecting wires, Figure 2 Not shown in a.

[0067] Due to the alternating field, the rods 15a and 15b are deflected horizontally via the magnets 16a and 16b, i.e. perpendicularly or transversely to their longitudinal axis, so that they are set in vibration. On the one hand, the rods 15a and 15b thus have a lever effect, by means of which the bending of the rods 15a and 15b generated by the horizontal deflection is transmitted to the membrane 8, in this way setting the membrane 8 in vibration. On the other hand, however, the combination of the two rods 15a and 15b and the membrane 8 is a separate resonator.

[0068] Figure 2 b shows a similar Figure 2 The electromechanical transducer unit 6 of a, except that Figure 2 b there are three rods 15a - 15c and three magnets 16a - 16c as disclosed in DE 10 2016 112 308 A1. However, the electromechanical transducer unit can also have four or more rods.

[0069] exist Figure 2In the case of the embodiment of the invention, the two swinging rods 10a, 10b form the unit 3 with mechanical vibration capability, and the rods 15a-15c of the transducer unit 6 respectively form a mechanical resonator with the membrane 8. The membrane 8 is preferably, but not necessarily, embodied in one piece. In particular, it can be assigned to both the vibrating unit 3 and the transducer unit 6.

[0070] According to the invention, the state monitoring of the vibration sensor is performed based on a comparison of a first value of the received signal at a predefinable first measuring time, which is compared with a reference value.

[0071] exist Figure 3 An embodiment of the state monitoring of the coil 17 according to the invention is shown in FIG. The received signal U of the coil 17 as a function of the sampling point AP, i.e. in principle as a function of time, is depicted for different temperatures T1, T2 and T3 and for a fully functional coil (1) and a partially short-circuited coil (2), respectively. For the illustrated embodiment, the predefinable first measuring time t1 is at AP=500 and is selected such that it is within a predefinable first time interval Δt1, wherein a step response S of the received signal U is detectable as a result of a sudden change in the excitation signal A. The excitation signal A is, for example, a square wave signal, and the step response S of the received signal U occurs in response to a rising edge or a falling edge of the excitation signal.

[0072] The first measuring time t1 is selected such that it is located temporally after the jump S of the received signal U. For the exemplary embodiment shown here, the first measuring time t1 lies in the range in which the received signal U decays exponentially after the step response S. The graph shown thus corresponds to a rising edge of the excitation signal A.

[0073] As can be seen from the figure, at the first measuring time t1, the values ​​of the received signals U of all selected temperatures T1, T2, T3 of the functional coil 17 (1) are respectively greater than those of the partially short-circuited coil (2). Within this range, i.e. within the range of the first measuring interval Δt1, the reference value U can be established. ref , providing this reference value so that all values ​​of the received signal U>U ref corresponds to the functional coil 17 and receives all values ​​U of the signal U ref Corresponds to a partially short-circuited coil. Therefore, if the first value of the received signal U1 at the predefinable first measuring time t1 is below the reference value U ref , it can be concluded from the undershoot that there is a partial short circuit in the area of ​​coil 17.

[0074] However, other conclusions about the state of the coil 17 can also be generated by the method according to the invention. For example, by using the step response S and the reference jump response S ref ​By comparison, defective electrical contacts or cable breaks in the coil region can be detected.

[0075] Furthermore, in another embodiment, the method according to the invention can draw conclusions about the temperature T of the medium 4 in the container 5. For this purpose, a second value U2 of the received signal U is determined at a second predefinable measuring time t2, such as Figure 4 As shown. Figure 3 Same as in Figure 4 , a graph of the received signal U for three different temperatures T1, T2, T3 and respectively for the functional coil 17 (1) and the partially short-circuited coil 17 (2) is shown. In a second predefinable time interval Δt2 outside the first time interval Δt1, the received signal U has a substantially constant value. The value of the received signal U is thus different for each of the selected temperatures T1, T2, T3. In contrast, at the same temperature T, the value of the received signal U for the functional coil 17 (1) and the at least partially short-circuited coil 17 (2) hardly differs. Based on the second value U2 of the received signal U at the second predefinable measuring time T2, conclusions can be drawn accordingly about the temperature T of the medium 4, for example by comparing the second value U2 of the received signal U with a characteristic curve of the received signal U as a function of the temperature T, which characteristic curve of the received signal U as a function of the temperature T can be stored, for example, in the electronic unit 7 of the device 1.

[0076] In summary, the invention enables condition monitoring of vibration sensor 1 and optionally additional determination of temperature T of medium 4 in a particularly simple manner and in particular without integrating further components into vibration sensor 1. Condition monitoring can be performed in parallel or alternately with normal measuring operation of sensor 1.

[0077] Reference Symbols List

[0078] 1 Vibration sensor

[0079] 2 Sensor unit

[0080] 3 units with vibration capability

[0081] 4 Medium

[0082] 5. Container

[0083] 6. Drive / receive unit

[0084] 7 Electronic unit

[0085] 8. The membrane of the unit with the ability to vibrate

[0086] 9 Vibration element

[0087] 10a, 10b Vibrating rod

[0088] 11a, 11b paddle

[0089] 12 Housing of electromechanical transducer unit

[0090] 15a, 15c rod

[0091] 16a, 16c magnet

[0092] 17 Coil

[0093] 18 Coil core

[0094] 19 Cup-shaped anchor unit

[0095] 20 base

[0096] 21 Wall

[0097] T, T1-T3 medium temperature

[0098] A Excitation signal

[0099] U Receive signal

[0100] U1, U2 receive signal value

[0101] U ref Reference signal for received signal

[0102] Δt1, Δt2 are predefined time intervals

[0103] t1,t2 measurement time

[0104] AP sampling point

[0105] f,f1-f3 frequency of the excitation signal

[0106] S step response.

Claims

1. A method for monitoring the state of a coil (17) which is part of a device (1) for determining at least one process variable of a medium (4) in a container (5), The method comprises the following method steps - applying an electrical excitation signal (A) to the coil (17) and receiving an electrical reception signal (U) from the coil (17), - determining a first value (U1) of said electrical reception signal (U) at a first predefinable measuring time (t1), - comparing said first value (U1) of said electrical reception signal (U) at said first predefinable measurement time (t1) with a reference value (U ref ) for comparison, and - determining a status indicator of said coil (17) based on said comparison, in, The status indicator is a conclusion about a defective electrical contact or a cable break in the region of the coil (17) or at least two connecting lines, wherein the first predefinable measuring time (t1) is selected such that the first predefinable measuring time (t1) is within a predefinable first time interval (Δt1), wherein the electrical reception signal (U) reacts to a sudden change in the electrical excitation signal (A) in the form of a step response (S), Wherein, based on the step response (S) of the electrical receiving signal (U) in response to the sudden change in the electrical excitation signal (A) and the reference jump response (S ref ) to detect the presence of a defective electrical contact or a cable break in the region of the coil (17).

2. The method according to claim 1, in, The electrical excitation signal (A) is a square wave signal.

3. The method according to claim 1, in, The sudden change of the electrical excitation signal (A) is a rising edge or a falling edge of the electrical excitation signal (A).

4. The method according to claim 1, in, A second value (U2) of the electrical reception signal (U) is determined at a second predefinable measuring time (t2), and a conclusion about the temperature (T) of the medium (4) is determined based on the second value (U2) of the electrical reception signal (U).

5. The method according to claim 4, in, The temperature (T) of the medium (4) is determined based on a comparison of the second value (U2) of the electrical reception signal (U) with at least one characteristic curve of the electrical reception signal (U) as a function of the temperature (T).

6. The method according to claim 4, in, The second predefinable measuring time (t2) is selected such that the second predefinable measuring time is outside the first time interval (Δt1).

7. The method according to one of claims 4 to 6, in, The second predefinable measuring time (t2) is selected such that the second predefinable measuring time is within a second time interval (Δt2) in which the electrical receive signal (U) is substantially constant.

8. A device (1) for determining and / or monitoring at least one process variable of a medium (4) in a container (5), comprising at least one coil (17), the device (1) being designed to implement the method according to one of claims 1 to 7.

9. The device according to claim 8, in, The device (1) is a vibration sensor include - a unit (3) capable of mechanical vibration, - a driving / receiving unit (6) having at least one coil (17), the driving / receiving unit (6) being designed to excite the unit (3) capable of mechanical vibration to mechanical vibration by means of an electrical excitation signal (A), and to receive the mechanical vibration from the unit (3) capable of mechanical vibration and convert the mechanical vibration into an electrical receiving signal (U), and - an electronic unit (7) designed to generate the electrical excitation signal (A) starting from the electrical reception signal (U) and to determine the at least one process variable as a function of the electrical reception signal (U).

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