Vibronic measuring device

Abstract

The invention relates to testing of the functionality of vibronic measuring devices (1). Here, there is a change to the phase shift (φ) of the excitation signals (AS1, AS2) which are used to excite a mechanically oscillating unit (111). The test is carried out using the received signals (RS1, RS2) received by the mechanically oscillating unit (111).

Description

[0001] Vibronic measuring device

[0002] The invention relates to the functionality testing of vibronic measuring devices.

[0003] In automation technology, field devices are often used to record and / or influence process variables. Sensors are used to record process variables, for example, to measure fill level, limit level, flow, pressure, temperature, pH value, conductivity, or dielectric value. When measuring a medium in a container, the term "container" in the context of the invention also includes open containers, such as pools, lakes, or flowing waters.

[0004] For point level measurement, for example, measuring devices based on the vibronic measuring principle are used. One such measuring device type is shown in the publication DE 10 2012 101 667 A1. Such measuring devices basically use vibronic sensors with a mechanically vibrating element, such as a tuning fork, and a feedback amplifier unit (see, for example, DE 100 50 299 A1). To determine the viscosity of a medium, for example, the frequency-phase curve is used in WO 02 / 31471 A2. It is also known to measure process variables based on their decay behavior (see DE 102 03 461 A1).

[0005] To convert mechanical vibrations into electrical signals, a piezo element or a similar component is mechanically coupled to the tuning fork, allowing it to be mechanically vibrated by an electrical excitation signal. The mechanical vibrations are used to generate an electrical reception signal whose frequency corresponds to that of the vibrations.

[0006] The received signal is either tapped from the piezo element that also excites the mechanical vibration. In this case, the received signal must be separated from the excitation signal according to the state of the art. Alternatively, a separate, second piezo element, which is also mechanically coupled to the tuning fork or the mechanically oscillating element, can serve as the output for the received signal.

[0007] The signal output of the mechanically oscillating unit for the received signal is routed to the input of the amplifier unit, while the output of the amplifier unit is connected to the input for the electrical excitation signal on the tuning fork. If the amplifier unit ensures a constant phase shift between the incoming received signal and the outgoing excitation signal, and if sufficient signal amplification is provided, an oscillating circuit is formed in which the frequency of the excitation signal matches that of the received signal. This frequency depends on the one hand on the phase shift specified by the amplifier unit (see, for example, WO 2017 / 215875 A1). On the other hand, the frequency is influenced by the resonance of the tuning fork and thus by any coverage of the tuning fork with the medium whose density or limit level is to be determined.Accordingly, by measuring the current frequency of the excitation signal or the received signal, a possibly reached limit level and / or the density of the medium can be determined.

[0008] The vibronic measuring principle functions flawlessly as long as the oscillating circuit is not overlaid by external interference, such as vibrations or electromagnetic interference fields. Vibrations can be caused, for example, by the operation of the process plant, such as the operation of conveyor belts and pumps. In this case, there is a risk that the corresponding interference signals will overlay the excitation or reception signal, which can distort the point level or density measurement. This is particularly problematic if the process plant in which the measuring device is used does not recognize that the measuring device is no longer functioning reliably. Depending on the country and process plant, specific specifications or approval levels for field devices apply in this regard. These define the verifiability of the correct function of field devices, for example the "Safety Integrity Level (SIL)" defined within the IEC 61508 / IEC 61511 series of standards.

[0009] Different testing options for measuring instruments are already known in the state of the art.

[0010] DE 20 2005 008 627 U1 describes a measuring device with an oscillating unit and a functional test unit. The functional test unit interacts with the signal received from the oscillating unit and generates a test signal, which is processed by a downstream evaluation unit. The test signal corresponds to a defined measured value. Thus, the electronics downstream of the oscillating unit are tested. To also test the electronics, EP 1 091 199 B1 stipulates that a signal generator generates a test signal.

[0011] DE 10 2019 131 485 A1 describes the monitoring of a coil as part of the sensor. The coil is subjected to an excitation signal, and the received signal is evaluated. Thus, a component as part of the converter unit is tested between electrical signals and mechanical vibrations. According to WO 2017 / 097528 A1, the mechanically oscillating unit is excited to oscillate at different frequencies in a test mode in order to infer fault conditions from the received signals. According to DE 10 2021 129 416 A1, the spectra resulting from a frequency sweep can be processed using a neural network.

[0012] In WO 2016 / 128217 A1, the mechanically oscillating unit is excited by a signal consisting of a carrier signal with a carrier frequency and a modulation signal with a modulation frequency. The received signal consists of a carrier signal and a modulation signal, each of which allows the determination of a process variable. A phase shift between the modulation signals is used to determine attenuation, which can indicate whether, for example, deposits, corrosion, or aging effects have occurred.

[0013] In DE 10 2016 120 326 A1, the state of the resonant circuit is determined from the temporal development of the quality of the resonant circuit, which comprises the oscillating unit and a piezo element.

[0014] In WO 95 / 20749 A1, the oscillating circuit consisting of an amplifier circuit and the converter unit, which excites the oscillating unit to oscillate and receives its oscillations and converts them into electrical signals, is opened during a test phase. Errors are detected from the decay behavior and, in particular, from the phase difference between the output signal of the amplifier circuit and the signal tapped at the converter unit.

[0015] WO 2020 / 165560 A1 stipulates that, in a test mode, the mechanically oscillating unit is excited to oscillate using an excitation signal whose frequency was measured during the previous measurement. This occurs with the oscillating circuit open. The temporal development of the amplitudes of the received signals is used to determine the functional status of the measuring device by comparing them with the corresponding threshold values.

[0016] DE 10 2004 050 494 A1 stipulates that an oscillating unit is excited to two different vibration modes. One of the two modes is not influenced by the medium and therefore allows monitoring of the sensor itself.

[0017] To detect the presence of faults, according to DE 10 2005 009 580 B4, the amplitudes and frequencies of the vibrations are evaluated and the resulting results are compared. For testing an oscillating unit, DE 103 18 445 A1 stipulates that a component of the oscillating unit can be manually damped. This deliberately creates a faulty mechanical state.

[0018] In DE 10 2012 102 589 A1, by passing through a frequency range which is higher than the frequency range occurring during normal operation, it is determined whether the absence of vibrations is the result of a covering or corrosion of the vibrating unit.

[0019] In order to monitor the polarization of a piezoelectric element used to drive the oscillating unit, its capacitance is determined and evaluated in DE 10 2017 130 530 A1.

[0020] If material adheres to the oscillating unit, this can also lead to a fault condition, since free oscillations are not possible even in the uncovered state.

[0021] To detect the aforementioned buildup, DE 103 28 296 A1 defines a limit for the oscillation frequency, below which an alarm is triggered. DE 10 2009 045 204 A1 stipulates that the presence of buildup should be detected based on the amplitude curve of the received oscillations during a frequency scan.

[0022] DE 10 2012 101 667 A1 deals with external vibrations. These are detected by using the mechanically vibrating unit without excitation to detect vibrations. This idea is also pursued by DE 10 2016 124 740 A1.

[0023] The invention is based on the object of providing a measuring device which can be checked for its correct function, in particular with regard to possible malfunctions.

[0024] The object is achieved by a measuring device for determining and / or monitoring a measured variable of a medium, with a vibronic sensor and a control unit, wherein the vibronic sensor has a mechanically oscillating unit and an amplifier unit, wherein the mechanically oscillating unit can be excited by an electrical excitation signal with an excitation oscillation frequency and generates a received signal with an oscillation frequency, wherein the amplifier unit is designed to generate the electrical excitation signal with a defined phase shift in relation to the received signal and thus excite the mechanically oscillating unit, wherein the control unit is designed to determine the oscillation frequency and / or an amplitude of the received signal and to determine the measured variable of the medium based on the determined oscillation frequency and / or the determined amplitude of the received signal, wherein the control unit is designedto control the amplifier unit for testing the measuring device in such a way that the amplifier unit generates at least two excitation signals with the same signal strength and different predeterminable phase shifts, wherein the control unit is designed to assess the sensor with regard to its functionality depending on how the amplitudes and / or oscillation frequencies of the received signals behave as a result of the different phase shifts, and wherein the amplifier unit and the mechanically oscillatable unit form a closed oscillating circuit.

[0025] According to the invention, the control unit is characterized by the fact that it can control the amplifier unit for testing the measuring device in such a way that the phase shift of the excitation signal is modulated or changed. If the received signal then changes as expected based on the targeted change in the phase shift, this means that the sensor is functioning reliably. If the received signal does not change as expected, this means that signals are being received from the mechanically oscillating element that are not attributable to the actual oscillations. For example, extraneous oscillations may be present.However, it is also possible that there is a malfunction in the oscillating circuit formed by the mechanically oscillating unit and the amplifier unit, for example, a cable break, depolarization of a piezoelectric element as part of the transducer unit, detachment of such a piezoelectric element, or other damage to the measuring device. In this case, the vibronic sensor itself cannot oscillate.

[0026] For the test, the mechanically oscillating unit is subjected to at least two excitation signals, particularly at different times and preferably alternatingly, which differ from each other in terms of their phase shifts. Accordingly, in the fault-free case, it is expected that the received signals received upon application of the associated excitation signals differ from each other in terms of their amplitudes and / or frequencies. The two excitation signals each have the same signal strength.

[0027] In one embodiment, the two excitation signals have the same excitation oscillation frequency fA.

[0028] This type of testing primarily allows the determination and / or monitoring of the medium's measured variable to continue without interruption. If there is no interference, the measured variable can be derived and / or monitored from the received signals. The testing of the measuring device can thus be performed without any loss of measuring capability and can thus be described as a type of live test performed parallel to the measurement. When the mechanically oscillating unit is excited with the excitation signals, the oscillating circuit consisting of the mechanically oscillating unit and the amplifier unit is closed to create a feedback loop.

[0029] To test the sensor, it is possible to evaluate the respective received signals relative to each other. The received signals can be evaluated individually in relation to the measured variable. The received signals thus fulfill a dual function.

[0030] In one embodiment, the alternation between a first excitation signal with a first phase shift and a second excitation signal with a second phase shift is performed several times. Each alternation allows the functionality of the measuring device to be checked. Preferably, the alternating alternation between the two excitation signals with different phase shifts occurs continuously during use of the measuring device. In one embodiment, it is provided that, for example, under special application conditions, the alternation of the signal strengths can be deactivated and therefore does not occur continuously.

[0031] In one embodiment, the control unit is designed to assess functionality by evaluating an envelope of the received signals. In this embodiment, preferably, several oscillations of the oscillating unit are received as a received signal when an excitation signal is present and evaluated using an envelope.

[0032] One embodiment includes the control unit being designed to classify the sensor as non-functional if the amplitudes and / or oscillation frequencies of the received signals do not change due to the different excitation signals. In this embodiment, it is checked and, if necessary, signaled whether the received signal does not change in accordance with the change in the excitation signal.

[0033] In a supplementary or alternative embodiment, it is provided that the control unit is designed to classify the sensor as functional if the amplitudes and / or oscillation frequencies of the received signals change sufficiently or significantly due to the different excitation signals. If the necessary changes occur in the received signal, the sensor is classified as functional. In particular, no extraneous vibrations are received that are dominant over the actual oscillation of the mechanically oscillating unit in the oscillating circuit. It can also be assumed that there is no interruption, in particular no cable break or similar, in the oscillating circuit of the amplifier unit and the mechanically oscillating unit. In general, the following core of the invention is implemented: The phase shifts of the excitation signals are modulated.If a corresponding modulation is also evident in the received signals, the measuring device and, above all, the sensor consisting of the mechanically oscillating unit and the amplifier unit are functioning properly. Therefore, the measured variable can also be reliably determined or monitored from the received signals. If the phase shifts of the excitation signals are modulated, the corresponding modulation would be the change in the amplitudes and / or oscillation frequencies of the received signals. However, if no modulation or the corresponding modulation is not evident in the received signals, then an error exists that prevents the mechanically oscillating unit from oscillating at the resonant frequency, which should actually be established due to the oscillating circuit and the medium. This is signaled accordingly.

[0034] One embodiment involves the control unit being designed to use a relative change in amplitude and / or frequency of the received signals to evaluate the sensor. Thus, absolute measurements are not required. A relative change in amplitude is, for example, the difference between the amplitudes of the two received signals relative to one of the two amplitudes.

[0035] One embodiment consists in specifying the phase shifts in such a way that the magnitudes of the differences between the phase shifts and 90° are different. This embodiment takes into account that the course of the amplitudes of the received signals, depending on the phase shift between the excitation signal and the received signal, behaves symmetrically to the phase shift of 90°. At a phase shift of 90°, the amplitude is maximum. Changing the phase shift to, for example, 80° (= 90° - 10°) therefore results in the same reduction in amplitude relative to the amplitude at 90° as setting the phase shift to 100° (= 90° + 10°). Therefore, this embodiment provides for the magnitudes of the differences between the phase shifts and 90° to be unequal, for example 80° and 95°.

[0036] According to one embodiment, the mechanically oscillating unit is a tuning fork, a single rod or a membrane.

[0037] According to the state of the art, if appropriately designed, the control unit can determine the density of the medium and / or an upper or lower limit level, i.e. generally the fill level of the medium in a container, based on the frequency of the received signal or the excitation signal as a measured variable. Frequency evaluation preferably takes place when liquids are the medium. In one embodiment, the control unit is designed as an integral component of the sensor. Alternatively, a process control center or a decentralized server functions as the control unit. In general, the term "unit" within the scope of the invention refers to any electronic circuit, device, or component that is suitably designed for its intended use. Depending on the requirements, it can therefore be an analog circuit for generating or processing corresponding analog signals. However, it can also be a (semiconductor-based) digital circuit such as an FPGA.

[0038] The invention is explained in more detail with reference to the following figures. Shown are:

[0039] Fig. 1 : a schematic arrangement of a measuring device according to the invention on a container,

[0040] Fig. 2: the oscillating circuit of the measuring device in the representation as an electrical circuit and

[0041] Fig. 3: a graph showing the relationship between phase shift and amplitude as a function of frequency.

[0042] For a general understanding of the measuring device 1 according to the invention, Fig. 1 shows a schematic arrangement of the measuring device 1 on a container 2. A medium 3 is located in the container 2, for example, its density is to be determined or a change in fill level relative to a limit level is to be detected. The medium 3 can be a liquid or bulk material.

[0043] For example, measuring device 1 is connected here to a remote control unit 12, for example, a process control system. This can be used to transmit the respective measured value or indicate a change. However, other information about the general operating status of measuring device 1 can also be communicated. Alternatively, control unit 12 and sensor 11 are integral components of measuring device 1.

[0044] For measurement, the measuring device 1 is arranged laterally at a connection of the container 2, such as a flange connection. If a density is to be determined, the measuring device 1 must be arranged in relation to the container height in such a way that, at least at the time of measurement, the covering of the measuring device 1 or its tuning fork 111 with medium 3 is ensured. In the case of a point level measurement, the measuring device 1 must be arranged horizontally at the height of the container 2 that corresponds to the limit level to be detected. The functioning of the measuring device 1 is explained using the schematic circuit of an exemplary embodiment in Fig. 2.

[0045] Shown is a vibronic sensor 11 which, as a mechanically oscillatable element, comprises the oscillating fork 111 and an amplifier unit 112 coupled thereto. The amplifier unit 112 can, for example, be based on at least one operational amplifier. For mechanical excitation, at least one first piezo element 1111 is coupled to the oscillating fork 111. As a result, when the first piezo element 1111 is subjected to an electrical excitation signal AS1, AS2, the oscillating fork 111 is mechanically set into oscillation at the corresponding frequency fR. The oscillating fork 111 also has a second piezo element 1112. As a result, the mechanical oscillation of the oscillating fork 111 generates an electrical received signal RS1, RS2, the frequency fR of which in turn corresponds to that of the mechanical oscillation. The two piezo elements 1111, 1112 thus form a transducer unit.

[0046] As an alternative to the embodiment shown in Fig. 2, it is also possible to tap the received signal RS1, RS2 at the first piezo element 1111, so that no second piezo element 1112 is required. This requires that the signals AS1, RS1 and AS2, RS2 are separated from each other, for example, by considering one signal as a current signal and the other as a voltage signal.

[0047] The received signal RS1, RS2 originating from the tuning fork 111 is routed to an input 1121 of the amplifier unit 112, while the output 1122 of the amplifier unit 112 is connected to the first piezo element 1111 or the input for the electrical excitation signal AS1, AS2 on the tuning fork 111. This results in a closed feedback loop.

[0048] The amplifier unit 112 establishes a constant phase shift cp between the incoming received signal RS1, RS2 and the outgoing excitation signal AS1, AS2 and has a constant or controlled amplification factor. This means that the amplitude AR of the received signal RS1, RS2 is amplified and output at the output 1122 of the amplifier unit 112. The amplification and the phase shift cp, which is controlled to a constant value or specified by appropriate filters, form the resonant circuit 11, so that the frequency fR of the excitation signal AS1, AS2 matches that of the received signal RS1, RS2.

[0049] The frequency fR depends, on the one hand, on the phase shift cp, which is specified by the amplifier unit 112. On the other hand, the frequency fR is also determined by the resonant oscillation of the tuning fork 111, which depends on any coverage of the tuning fork 111 with medium 2. Accordingly, by measuring the current frequency fR of the excitation signal AS1, AS2 or the received signal RS1, RS2, a possibly reached limit level and / or the density of the medium 2 can be determined or monitored.

[0050] The graphs of Fig. 3 show the dependencies between phase shift cp, amplitude AR of a received signal RS1, RS2 and frequency fR of the respective excitation signal AS1, AS2.

[0051] The amplitude AR (solid line) reaches its maximum at the resonance frequency fR,0, which results from the sensor's electromechanical oscillating circuit, which consists of the mechanically oscillating unit and the amplifier unit. The resonance frequency fR,0 results when the phase shift specified by the amplifier unit is 90°. With increasing or decreasing frequencies, the amplitude AR decreases symmetrically to the resonance frequency fR,0.

[0052] The phase shift cp has an inverted S-shaped curve depending on the frequency fR, with the inflection point of the curve at 90°. The phase shift cp also affects the frequency fR. Therefore, in a functioning sensor, a change in the phase should lead to a change in the oscillation frequency.

[0053] At the same time, the phase shift cp affects the amplitude AR through the relationship between the phase shift cp and the amplitude AR of the received signal. Three different phase shifts cp are entered for this purpose: 120°, 90°, and 60°. At 90°, the amplitude AR is maximum. At 120° and 60°, the amplitude is reduced by the same amount, since the magnitudes of the differences between the phase shifts cp and 90° are equal. Therefore, it is recommended to select the phase shifts cp so that the magnitudes are different in order to implement condition monitoring.

Claims

Patent claims 1. A measuring device (1) for determining and / or monitoring a measured variable of a medium (2), comprising a vibronic sensor (11) and a control unit (12), wherein the vibronic sensor (11) comprises a mechanically oscillating unit (111) and an amplifier unit (112), wherein the mechanically oscillating unit (111) is excitable by an electrical excitation signal (AS1, AS2) with an excitation oscillation frequency (fA) and generates a received signal (RS) with an oscillation frequency (fR), wherein the amplifier unit (112) is designed to generate the electrical excitation signal (AS1, AS2) with a defined phase shift (cp1, cp2) in relation to the received signal (RS) and thus excite the mechanically oscillating unit (111), wherein the control unit (12) is designed,to determine the oscillation frequency (fR) and / or an amplitude (AR) of the received signal (RS) and to determine the measured variable of the medium (2) based on the determined oscillation frequency (fR) and / or the determined amplitude (AR) of the received signal (RS), wherein the control unit (12) is designed to control the amplifier unit (112) for testing the measuring device (1) in such a way that the amplifier unit (112) generates at least two excitation signals (AS1, AS2) with the same signal strength and different predeterminable phase shifts (cp1, cp2), wherein the control unit (12) is designed to assess the sensor (11) with regard to its functionality depending on how the amplitudes (AR) and / or oscillation frequencies (fR) of the received signals (RS1, RS2) behave as a result of the different phase shifts (cp1, cp2), and wherein the amplifier unit (112) and the mechanically oscillatable unit (111) form a closed oscillating circuit., 2. Measuring device according to claim 1, wherein the control unit (12) is designed to control the amplifier unit (112) in such a way that the amplifier unit (112) essentially constantly and alternately generates two excitation signals (AS1, AS2) with the same signal strength and different predeterminable phase shifts (cp1, cp2).

3. Measuring device according to claim 1 or 2, wherein the control unit (12) is designed to assess the functionality by evaluating an envelope of the received signals (RS1, RS2).

4. Measuring device according to one of claims 1 to 3, wherein the control unit (12) is designed the sensor (1) is to be classified as non-functional if the amplitudes (AR) and / or oscillation frequencies (fR) of the received signals (RS1, RS2) do not change due to the different excitation signals (AS1, AS2).

5. Measuring device according to one of claims 1 to 4, wherein the control unit (12) is designed to classify the sensor (1) as functional if the amplitudes (AR) and / or oscillation frequencies (fR) of the received signals (RS1, RS2) change due to the different excitation signals (AS1, AS2).

6. Measuring device according to one of claims 1 to 5, wherein the control unit (12) is designed to use a relative amplitude change and / or frequency change of the received signals (RS1 , RS1 ) for the assessment of the sensor (11).

7. Measuring device according to one of claims 1 to 6, wherein the phase shifts (cp1, cp2) are predetermined such that the amounts of the differences between the phase shifts (cp1, cp2) and 90° are different.

8. Measuring device according to one of claims 1 to 8, wherein the mechanically oscillatable unit (111) is a tuning fork, a single rod or a membrane.

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