CONDITION MONITORING OF A VIBRONIC SENSOR

Description

[0001] The invention relates to a method for monitoring the condition of a coil, which coil is part of a device for determining at least one process variable of a medium in a container, and to a device designed to carry out a method according to the invention. The device is a vibronic sensor. The device comprises a mechanically oscillatable unit, a drive / receiver unit, and an electronics unit. The process variable is given, for example, by the fill level, in particular a limit level, the flow rate of the medium or also by its density or viscosity. The medium is located, for example, in a container, e.g. a vessel, a tank, or also in a pipeline. The medium itself is, for example, given by a liquid, a gas, or a bulk material.

[0002] Vibronic sensors are widely used in process and / or automation technology. In the case of level measuring devices, they comprise at least one mechanically oscillating unit, such as a tuning fork, a single rod, or a diaphragm. During operation, this unit is excited to mechanical vibrations by a drive / receiver unit, often in the form of an electromechanical transducer unit, which in turn can be, for example, a piezoelectric drive or an electromagnetic drive. The applicant manufactures a wide variety of corresponding field devices and markets them, for example, under the names LIQUIPHANT or SOLIPHANT. The underlying measurement principles are known from numerous publications. The drive / receiver unit excites the mechanically oscillating unit to mechanical vibrations using an electrical excitation signal.Conversely, the drive / receiver unit can receive the mechanical vibrations of the mechanically oscillating unit and convert them into an electrical received signal. Accordingly, the drive / receiver unit is either a separate drive unit and a separate receiving unit, or a combined drive / receiver unit. In many cases, the drive / receiver unit is part of a feedback electrical oscillating circuit, which is used to excite the mechanically oscillating unit to mechanical vibrations. For example, for a resonant vibration, the oscillating circuit condition must be met, according to which the gain factor is ≥1 and all phases occurring in the oscillating circuit are a multiple of 360°. To excite and fulfill the oscillating circuit condition, a certain phase shift between the excitation signal and the received signal must be ensured.Therefore, a predeterminable value for the phase shift, i.e., a target value for the phase shift between the excitation signal and the received signal, is often set. A wide variety of solutions, both analog and digital methods, have become known from the prior art, as described, for example, in documents DE102006034105A1, DE102007013557A1, DE102005015547A1, DE102009026685A1, DE102009028022A1, DE102010030982A1, or DE 102017115147 A1. DE 102017115147 A1 discloses a method for monitoring the condition of a coil with at least two connecting wires, which coil is part of a device for determining at least one process variable of a medium in a container. The method of this document includes determining a target value of a total ohmic resistance for the coil and the connecting wires and determining a status indicator of the coil.

[0003] Both the excitation signal and the received signal are characterized by their frequency ω, amplitude A and / or phase Φ. Accordingly, changes in these variables are usually used to determine the respective process variable. The process variable can be, for example, a fill level, a predetermined fill level, or the density or viscosity of the medium, as well as the flow rate. In a vibronic point level switch for liquids, for example, a distinction is made between whether the oscillating unit is covered by the liquid or is oscillating freely. These two states, the free state and the covered state, are differentiated based on different resonance frequencies, i.e., based on a frequency shift.

[0004] Density and / or viscosity, in turn, can only be determined with such a measuring device if the oscillating unit is covered by the medium. Various methods for determining density and / or viscosity have also become known from the prior art, such as those disclosed in documents DE10050299A1, DE102007043811A1, DE10057974A1, DE102006033819A1, DE102015102834A1, or DE102016112743A1.

[0005] Piezoelectric and / or electromagnetic drive / receiver units are particularly used for the drive / receiver unit itself. Drive / receiver units based on the piezoelectric effect can generally achieve a comparatively high level of efficiency. However, they are only partially suitable for use at high temperatures, particularly at temperatures above 300°C. An alternative in this context is electromagnetic drive / receiver units, as described, for example, in the publications WO2007 / 113011, WO2007 / 114950 A1, DE102015104533A1, and DE102016112308A1. The conversion of electrical energy into mechanical energy occurs via a magnetic field. The described converter units therefore typically comprise at least one coil and one permanent magnet.The coil generates an alternating magnetic field that permeates the magnet, and a periodic force is transferred to the oscillating unit via the magnet. Depending on the materials used, such a drive / receiver unit can be used in a temperature range between -200°C and 500°C, for example.

[0006] To ensure the reliable operation of a vibronic sensor, various methods for diagnosing potential malfunctions have been developed in the state of the art. The ability to monitor condition or diagnose potential malfunctions is particularly important in safety-critical applications, such as the use of a point level switch in a container filled with a flammable liquid. Depending on the specific application, the respective measuring device must meet different safety requirements. In this regard, for example, the so-called SIL standard (SIL stands for Safety Integrity Level) according to the IEC 61508 standard has become known. This standard distinguishes between four different levels for specifying the safety integrity requirements of safety functions.The safety integrity level essentially represents a measure of the reliability of the respective system or measuring device in relation to a potential hazard. Typically provided safety functions to ensure a certain level of safety integrity include, for example, emergency shutdowns or the shutdown of overheated devices.

[0007] To determine a safety integrity level, for example, the failure behavior of individual components is examined. Furthermore, the presence of redundant structures is checked, as is the ratio between safe and unsafe, or dangerous, errors. From such considerations, an overall failure rate can then be determined, which is used to assign the respective safety integrity level to the respective system or measuring device. DE102004027397A1 describes a method for detecting a cable break in a vibronic sensor. Documents DE10014724A1 and DE102009045204A1 disclose diagnostic options for detecting buildup in the area of ​​the vibrating unit. However, diagnostics in the area of ​​the drive / receiver unit of a vibronic sensor have also been proposed in a wide variety of designs, such as the diagnosis of a piezoelectric drive / receiver unit described in DE102008032887A1.With regard to an electromagnetic drive / receiver unit, it has again become known from DE102017115147A1 to assess the condition of the drive / receiver unit based on the total ohmic resistance.

[0008] Based on the prior art, the present invention is based on the object of providing a measuring device with which high safety requirements can be met in a particularly simple manner.

[0009] This object is achieved by the method according to claim 1 and by the device according to claim 11.

[0010] With regard to the method, the object is achieved by a method for monitoring the condition of a coil, which coil is part of a device for determining at least one process variable of a medium in a container. The method comprises the following steps: Applying an electrical excitation signal to the coil and receiving an electrical reception signal from the coil, determining a first value for the reception signal at a first predeterminable measurement time, comparing the first value for the reception signal at the first measurement time with a reference value, and determining a status indicator for the coil based on the comparison, the condition indicator is a statement about a defective electrical contact or a cable break in the area of ​​the coil or at least two connecting wires, wherein the presence of a defective electrical contact or a cable break in the area of ​​the coil is detected by means of a deviation of the step response of the received signal in reaction to the sudden change in the excitation signal from a reference step response.

[0011] In particular, the step response of the received signal exhibits an exponential curve in the case of an intact coil. In this case, a deviation from such a curve indicates the presence of a defective electrical connection.

[0012] Diagnosing the drive / receiver unit by comparing a measured value at a predefined initial measurement time with a reference value is particularly easy to implement. In particular, the design effort is very minimal – implementing condition monitoring requires only a few additional components for the respective sensor.

[0013] It is advantageous if the first predefined measurement time is selected so that it lies within a predefined first time interval in which the received signal reacts to a sudden change in the excitation signal in the form of a step response. A step response is particularly easy to evaluate. It is accompanied by a characteristic curve of the received signal as a function of time, which can be easily detected.

[0014] It is also advantageous if the excitation signal is a square-wave signal. A square-wave signal is characterized by a periodic alternation between two constant values, which, plotted as a function of time, exhibits a rectangular waveform. It is thus characterized by periodic rising and falling edges. In this context, it is also advantageous if the sudden change in the excitation signal is a rising or falling edge of the excitation signal.

[0015] Preferably, a statement regarding the extent of damage to the coil is determined. This allows not only a qualitative but also a quantitative statement regarding the coil's condition. Depending on the diagnosis of the coil's condition made using the condition indicator, it is also conceivable to generate messages, for example, regarding maintenance or the presence of a defect.

[0016] It is also preferred to determine the status indicator based on the deviation of the measured value from the reference value. In particular, it is possible to check whether the deviation exceeds a predefined limit.

[0017] According to one embodiment of the method, the status indicator is a statement about the presence of at least one turn short circuit in the area of ​​the coil.

[0018] In this respect, it is advantageous if the presence of a turn-to-turn short circuit in the coil area is detected by the reference value being undershot at the first preset measuring time.

[0019] In one embodiment, the method includes determining a second value for the received signal at a second predeterminable measurement time, and determining a temperature of the medium based on the second value for the received signal. In addition to condition monitoring, the method according to the invention allows temperature determination without the implementation of an additional temperature-sensitive sensor element.

[0020] The temperature of the medium in the area of ​​a vibronic sensor has a significant influence on the determination of the respective process variable, in particular on the received signal. To compensate for this influence, it has become known from DE102006007199A1 or DE102009029490A1 to equip a vibronic measuring device with an additional temperature sensor. However, the integration of an additional sensor into a measuring device generally involves a certain amount of design effort. With regard to an electromagnetic drive / receiver unit, it has become known from DE102017114315A1 to determine the temperature of the medium based on an electrical parameter, for example, the ohmic resistance of the coil. However, even such a measure is associated with comparatively high effort, since means for determining the respective parameter must be provided.The inventive temperature determination based on a second value for the received signal at a second predeterminable measurement time, however, can be implemented in a particularly simple manner. This second value for the received signal or the second predeterminable measurement time is selected such that the second measured value is directly dependent on the temperature of the medium and can be used directly to determine the temperature. Thus, no additional sensor elements or complex design measures for temperature determination are necessary.

[0021] With regard to the temperature of the medium, it is also conceivable that the influence of temperature on the received signal is compensated during continuous measurement of the device. In this way, the measurement accuracy of the respective sensor can be increased when determining the respective process variable.

[0022] In this context, it is advantageous if the temperature of the medium is determined by comparing the second value for the received signal with at least one characteristic curve of the received signal as a function of temperature.

[0023] It is also advantageous if the second preset measurement time is chosen so that it lies outside the first time interval.

[0024] Preferably, the second predeterminable measuring time is selected such that it lies within a second time interval in which the received signal is substantially constant.

[0025] The object underlying the invention is further achieved by a device for determining and / or monitoring at least one process variable of a medium in a container, comprising at least one coil, which device is designed to carry out a method according to at least one of the described embodiments. The method according to the invention can be carried out simultaneously with continuous measurement operation or alternately with continuous operation during a diagnostic operating mode of the device. A manual, punctual triggering of the method, for example, by a switching operation or the like, is also conceivable.

[0026] With regard to the device, it is advantageous if the device is a vibronic sensor comprising a mechanically oscillatable unit, a drive / receiver unit with at least one coil, which drive / receiver unit is designed to excite the mechanically oscillatable unit to mechanical oscillations by means of an electrical excitation signal and to receive the mechanical oscillations from the oscillatable unit and to convert them into an electrical received signal, and an electronics unit which is designed to generate the excitation signal based on the received signal and to determine the at least one process variable from the received signal.

[0027] In one embodiment of the device, the oscillating unit is a membrane, a single rod or a tuning fork.

[0028] A further embodiment of the device includes that the process variable is a, in particular predeterminable, fill level, the density or the viscosity of the medium.

[0029] Within the scope of the present invention, a wide variety of configurations of a vibronic sensor, in particular different drive / receiver units, can be used. Two particularly preferred configurations for a drive / receiver unit are described below, without limiting the scope.

[0030] In one embodiment, the drive / receiving unit comprises at least a membrane that can be set into mechanical vibrations, two rods fastened to the membrane perpendicular to a base surface of the membrane, a housing, wherein the membrane forms at least a partial region of a wall of the housing, and wherein the two rods extend into the interior of the housing, two magnets, wherein a magnet is fastened in each case in the end region of one of the two rods facing away from the membrane, and a coil with a core, which is fastened inside the housing above the magnets and which coil can be supplied with an electrical alternating current signal, wherein the coil is designed to generate a magnetic field, which magnetic field sets the two rods into mechanical vibrations by means of the two magnets, and wherein the two rods are fastened to the membrane in such a way that the vibrations of the two rods result in vibrations of the membrane.

[0031] In a further embodiment, the drive / receiving unit comprises at least a membrane capable of mechanical vibration, at least three rods attached to the membrane perpendicular to a base surface of the membrane, a housing, wherein the membrane forms at least a partial region of a wall of the housing, and wherein the rods extend into the interior of the housing, at least three magnets, wherein a magnet is attached to each of the at least three rods in the end region facing away from the membrane, and a coil with a core, which is attached within the housing above the magnets, and which coil can be supplied with an electrical alternating current signal, wherein the coil is designed to generate a magnetic field, which magnetic field causes the rods to vibrate mechanically by means of the magnets, wherein the rods are attached to the membrane in such a way that vibrations of the rods result in vibrations of the membrane, wherein at least one of the rods is attached to the membrane substantially at a location along the base surface of the membrane, at which location the second derivative of the deflection of the membrane from a rest position as a function of the location on the base surface is substantially zero.

[0032] Furthermore, a drive / receiving unit according to one of the two embodiments described by way of example with four rods is particularly preferred.

[0033] Furthermore, it should be noted that the embodiments described in connection with the method according to the invention are also applicable mutatis mutandis to the device according to the invention and vice versa.

[0034] The invention and its advantageous embodiments are described below with reference to the figures Fig. 1 - Fig. 4 described in more detail. It shows: Fig. 1 : a representation of a vibronic sensor according to the state of the art, Fig. 2 : two possible designs of a vibronic sensor with preferred electromagnetic drive / receiver units, Fig. 3 a diagram of the coil's reception signal to illustrate the condition monitoring according to the invention; and Fig. 4 a diagram of the coil's received signal to illustrate the temperature determination according to the invention.

[0035] In Fig. 1aA vibronic level gauge 1 is shown. A sensor unit 2 with a mechanically oscillating unit 3 in the form of a tuning fork is partially immersed in a medium 4 located in a container 5. The oscillating unit 3 is excited to mechanical vibrations by the drive / receiver unit 6, usually an electromechanical transducer unit, and can be, for example, a piezoelectric stack or bimorph drive, but also an electromagnetic or magnetostrictive drive / receiver unit. However, it goes without saying that other designs of a vibronic level gauge are also possible. Furthermore, an electronics unit 7 is shown, by means of which the signal is acquired, evaluated, and / or fed.

[0036] Fig. 1bshows a more detailed view of an oscillating unit 3 in the form of a tuning fork, such as is used, for example, for the vibronic sensor marketed by the applicant under the name LIQUIPHANT. A membrane 8 and an oscillating element 9 connected to it are shown. The oscillating element 9 has two oscillating rods 10a, 10b, each of which has a paddle 11a, 11b molded onto its end. During operation, the oscillating fork 3 performs oscillating movements according to the oscillation mode with which it is excited. Each of the two oscillating rods 10a, 10b essentially behaves like a so-called flexural oscillator. In the fundamental oscillation mode, the two oscillating rods 10a, 10b oscillate, for example, in antiphase with each other.

[0037] Although numerous different designs for the drive / receiver unit 6 can be used within the scope of the present invention, the following description refers, without restriction of generality, to electromagnetic drive / receiver units 6 with at least one coil, as described in the documents DE102015104533A1 or DE102016112308A1.

[0038] In Fig. 2aa schematic view of such a drive / receiver unit 6 is shown. A housing 12 terminates with the lower wall with a membrane 8, which is to be assigned to the oscillatable unit 3. For the embodiment shown here, the housing 12 is cylindrical and the disc-shaped membrane 8 has a circular cross-sectional area A. However, it goes without saying that other geometries are also conceivable and fall within the scope of the present invention. Two rods 15a, 15b are attached to the membrane 8 perpendicular to the base area A of the membrane 8 and extending into the interior of the housing 12. This is in particular a force-fitting connection. The base area A of the membrane 8 then lies in a plane perpendicular to the longitudinal direction of the rods 15a, 15b.

[0039] In the end region of the rods 15a, 15b facing away from the membrane 8, a magnet 16a, 16b, in particular an SmCo or Alnico magnet, is fastened in each case. The magnets 16a, 16b are preferably all aligned or oriented in the same way. A coil 17 is arranged above the magnets 16a, 16b, which comprises a wire wound around the core 18. The core 18 of the coil 17 is part of a pot-shaped armature unit 19 with a base 20 and a peripheral wall 21. For example, the base 20, like the base area A of the membrane 8, can have a circular cross-sectional area. From the base 20 of the pot-shaped armature unit 19, the core 18 of the coil 17 extends centrally in the form of a nozzle into the interior of the armature unit 19. In this case, the peripheral wall 21 then functions as a magnetic field return. The rods 15a, 15b with the magnets 16a and 16b do not touch the coil 17 and the core 18.During continuous operation, coil 17 is subjected to an alternating current signal to generate an alternating magnetic field. For this purpose, the coil has two . Fig. 2a connecting wires not shown.

[0040] Due to the alternating field, the rods 15a and 15b are deflected horizontally, i.e., perpendicularly or transversely to their longitudinal axes, via the magnets 16a and 16b, causing them to vibrate. On the one hand, the rods 15a and 15b then exert a lever effect, through which the bending of the rods 15a and 15b caused by the horizontal deflection is transferred to the diaphragm 8, causing the diaphragm 8 to vibrate. On the other hand, the combination of the two rods 15a and 15b and the diaphragm 8 constitutes a separate resonator.

[0041] Fig. 2b shows an electromechanical converter unit 6 similar to that in Fig. 2a with the difference that according to Fig. 2bthree rods 15a-15c and three magnets 16a-16c are present, as disclosed in DE102016112308A1. However, the electromechanical converter unit can also have four or more rods.

[0042] In the case of the arrangements from Fig. 2 The two vibrating rods 10a, 10b of the mechanically vibratable unit 3 and the rods 15a-15c of the transducer unit 6 each form a mechanical resonator with the diaphragm 8. The diaphragm 8 is preferably, but not necessarily, designed as a single piece. In particular, it can be assigned to both the oscillatory unit 3 and the transducer unit 6.

[0043] According to the invention, a condition monitoring of a vibronic sensor is carried out, in which a comparison of a first value of the received signal at a predeterminable first measuring time is carried out with a reference value.

[0044] An embodiment of the inventive condition monitoring of the coil 17 is shown in Fig. 3 illustrated. The received signal U of the coil 17 is shown as a function of the sampling point AP, in principle therefore as a function of time, for different temperatures T1, T2 and T3 and for a fully functional coil (1) and a partially short-circuited coil (2). The predeterminable first measurement time t1 for the embodiment shown is AP=500 and is selected such that it lies in a predeterminable first time interval Δt1, in which a step response S of the received signal U can be detected as a result of a sudden change in the excitation signal A [not shown here]. 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 or falling edge of the excitation signal.

[0045] The first measurement time t1 is chosen so that it occurs after the jump S of the received signal U. For the configuration shown here, the first measurement time t1 lies in a range in which the received signal U decays exponentially after the step response S. The curve shown corresponds to a rising edge of the excitation signal A.

[0046] The diagram shows that at the first measurement time t1, the values ​​for the received signal U for all selected temperatures T1, T2, T3 for the functional coils 17 (1) are each greater than for the partially short-circuited coils (2). In this range, i.e., in the range of the first measurement interval Δt1, a reference value U ref can be defined, which is such that all values ​​U>U ref for the received signal U correspond to functional coils 17 and all values ​​U

[0047] ​However, the method according to the invention can also generate other statements about the state of the coil 17. For example, by comparing the step response S with a reference step response S ref , the presence of a defective electrical contact or a cable break in the area of ​​the coil can be detected.

[0048] Furthermore, the method according to the invention allows, in a further embodiment, to make a statement 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 predeterminable measuring time t2, as in Fig. 4 As shown in Fig. 3 are in Fig. 4Curves for the received signal U are shown for three different temperatures T1, T2, T3, each for a functional coil 17 (1) and a partially short-circuited coil 17 (2). In a second predeterminable time interval Δt2, which lies outside the first time interval Δt1, the received signal U has a substantially constant value. The values ​​for the received signals U differ for each of the selected temperatures T1, T2, T3. At the same temperature T, however, the values ​​for the received signals U for functional coils 17 (1) and those that are at least partially short-circuited (2) hardly differ.Accordingly, a statement about the temperature T of the medium 4 can be derived from a second value U2 for the received signal U at the second predeterminable measuring time t2, for example by comparing the second value U2 for the received signal U with a characteristic curve for the received signal U as a function of the temperature T, which can be stored, for example, in the electronics unit 7 of the device 1.

[0049] In summary, the present invention enables condition monitoring for a vibronic sensor 1 and, if appropriate, additional determination of the temperature T of the medium 4 in a particularly simple manner and in particular without integrating further components into the vibronic sensor 1. The condition monitoring can be carried out in parallel or alternately with the normal measuring operation of the sensor 1. List of reference symbols

[0050] 1Vibronic sensor 2Sensor unit 3Oscillating unit 4Medium 5Container 6Drive / receiving unit 7Electronic unit 8Membrane of the oscillating unit 9Oscillating element 10a, 10bOscillating rods 11a, 11bPaddles 12Housing of the electromechanical transducer unit 15a-15cRods 16a-16cMagnets 17Coil 18Coil core 19Pot-shaped armature unit 20Base 21Circumferential wall T,T1-T3Temperature of the medium AExcitation signal UEceived signal U1, U2Values ​​for the received signal .....U ref Reference value for the received signal Δt1 , Δt2Specifiable time interval t1, t2Measurement time APSample point f, f 1 -f 3 Frequency of the excitation signal SStep response

Claims

1. Method for monitoring the state of a coil (17), which coil (17) is part of a device (1) for determining at least one process variable of a medium (4) in a container (5), comprising the following method steps: - applying an electrical excitation signal (A) to the coil (17) and receiving an electrical received signal (U) from the coil (17), determining a first value (U1) for the received signal (U) at a first predeterminable measurement time (t1), comparing the first value (U1) for the received signal (U) at the first measurement time (t1) with a reference value (Uref), and determining a status indicator for the coil (17) on the basis of the comparison, the state indicator being a statement about a defective electrical contact or a cable break in the area of the coil (17) or of at least two connecting wires, wherein the presence of a defective electrical contact or a cable break in the area of the coil (17) is detected on the basis of a deviation of the step response (S) of the received signal (U) in response to the step change of the excitation signal (A) from a reference step response (Sref).

2. Method according to claim 1, wherein the first predeterminable measurement time (t1) is selected such that it lies in a predeterminable first time interval (Δt1) in which the received signal (U) reacts in the form of a step response (S) to a step change in the excitation signal (A).

3. Method according to claim 1 or 2, wherein the excitation signal (A) is a square-wave signal.

4. Method according to claim 2 and 3, wherein the abrupt change in the excitation signal (A) is an ascending or descending edge of the excitation signal (A).

5. Method according to at least one of the previous claims, wherein the state indicator is a statement about the presence of at least one interturn fault in the area of the coil (17).

6. Method according to claim 5, wherein the presence of a turn-to-turn fault in the region of the coil (17) is detected on the basis of the reference value (Uref) being undershot at the first predeterminable measurement time (t1).

7. Method according to at least one of the previous claims, wherein a second value (U2) for the received signal (U) is determined at a second predeterminable measurement time (t2), and wherein a statement about a temperature (T) of the medium (4) is determined on the basis of the second value (U2) for the received signal (U).

8. Method according to claim 7, wherein the temperature (T) of the medium (4) is determined by means of a comparison of the second value (U2) for the received signal (U) with at least one characteristic curve of the received signal (U) as a function of the temperature (T).

9. Method according to one of the previous claims, wherein the second predeterminable measurement point in time (t2) is selected such that it lies outside the first time interval (Δt1).

10. Method according to one of the previous claims, wherein the second predeterminable measurement time (t2) is selected such that it lies within a second time interval (Δt2) in which the received signal (U) is essentially constant.

11. 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), which device (1) is configured to carry out a method according to at least one of the previous claims.

12. Device according to claim 11, wherein the device (1) is a vibronic sensor comprising - a mechanically vibratable unit (4), - a drive / receive unit (6) having at least one coil (17), which drive / receive unit (6) is designed to excite the mechanically vibratable unit (4) to mechanical vibrations by means of an electrical excitation signal (A) and to receive the mechanical vibrations from the vibratable unit (4) and to convert them into an electrical receive signal (E), and - an electronic unit (7) which is designed to generate the excitation signal (A) on the basis of the received signal (E) and to determine the at least one process variable from the received signal (E), acts.