Monitoring the condition of vibration sensors

Abstract

The invention relates to a method for monitoring the condition of a vibration sensor (1) in order to determine and / or monitor at least one process variable of a medium (2VC) in a container (2a), the sensor having at least one sensor unit (3) having a mechanically oscillatable unit (4), the method comprising the following method steps: A ) excites the mechanically oscillatable unit (4) to perform mechanical oscillations, and receives a signal (U E ) in the form of receiving mechanical vibrations; ‑ determining the received signal (U E ) of the measured value of amplitude (A) and the measured value of frequency (f); compare the measured values ​​of amplitude (A) and frequency (f) with the reference value of amplitude (A ref ) and the reference value of frequency (f ref ) for comparison; and ‑ determining a status indicator based on the comparison.

Description

Technical Field

[0001] The invention relates to a method for monitoring the condition of a vibration sensor for determining and / or monitoring at least one process variable, in particular a physical or chemical process variable, of a medium in a container. The vibration sensor comprises a sensor unit having a mechanically oscillatable unit. For example, the process variable to be monitored can be the fill level or flow rate of the medium, but also the density or viscosity of the medium. For example, the container is a vessel or a pipeline. Background Art

[0002] Vibration sensors are widely used in process and / or automation technology. In the case of a fill level measuring device, they have at least one mechanically oscillatable unit, for example an oscillating fork, a single tine or a membrane. This is excited during operation by means of a drive / receiving unit, usually in the form of an electromechanical transducer unit, so that a mechanical oscillation is performed. The electromechanical transducer unit can in turn be a piezoelectric drive or an electromagnetic drive, for example.

[0003] Corresponding field devices are manufactured by the applicant in a wide variety and are marketed, for example, under the LIQUIPHANT and SOLIPHANT trademarks in the case of fill level measuring devices. The basic measuring principle is in principle known from a large number of published documents. The drive / receiver unit excites the mechanically oscillatable unit by means of an electrical excitation signal so that a mechanical oscillation is performed. Conversely, the drive / receiver unit can receive the mechanical oscillations of the mechanically oscillatable unit and convert them into electrical reception signals. The drive / receiver unit can be a separate drive unit and a separate reception unit or a combined drive / receiver unit.

[0004] In this case, the drive / receiver unit is in many cases part of a feedback electrical oscillating circuit, by means of which the excitation of the mechanically oscillatable unit occurs so that a mechanical oscillation is performed. For example, for a resonant oscillation, the following oscillating circuit conditions must be met: the amplification factor ≥ 1, and the sum of all phases occurring in the oscillating circuit must be a multiple of 360°.

[0005] In order to excite and meet the oscillation circuit conditions, a certain phase shift between the excitation signal and the received signal must be ensured. Therefore, a predeterminable value for the phase shift, that is, an expected value for the phase shift between the excitation signal and the received signal, is frequently set. The most diverse solutions known in the prior art for this purpose include both analog methods and digital methods. In principle, the setting of the phase shift can be performed, for example, by using a suitable filter, or controlled to a predeterminable phase shift, an expected value, by means of a control loop. For example, it is known from DE102006034105A1 to use an adjustable phase shifter. In contrast, DE102007013557A1 describes the additional integration of an amplifier with an adjustable gain factor for additional control of the oscillation amplitude. DE102005015547A1 proposes the use of an all-pass filter. Furthermore, as disclosed in DE 10 2009 026 685 A1, DE 10 2009 028 022 A1 and DE 10 2010 030 982 A1, the phase shift can be established by means of a so-called frequency sweep. However, the phase shift can also be controlled to a predeterminable value by means of a phase control loop (phase-locked loop, PLL). Such an excitation method is the subject of DE 10 2010 030 982 A1.

[0006] Both the excitation signal and the received signal are characterized by a frequency ω, an amplitude A and / or a phase Φ. Accordingly, changes in these variables are usually taken into account to determine specific process variables, such as a predetermined level of a medium in a container or a density and / or viscosity of a medium or a flow rate of a medium through a pipe or pipeline. For example, in the case of a vibrating limit switch for a liquid, it can be distinguished whether the oscillating unit is covered by a liquid or is oscillating freely. These two states (free state and covered state) are distinguished in this case, for example, based on different resonant frequencies (i.e., frequency shifts). For example, as described in DE10050299A1, DE102007043811A1, DE10057974A1, DE102006033819A1 and DE102015102834A1, the density and / or viscosity can be ascertained with such a measuring device only when the oscillating unit is covered by a medium.

[0007] In order to ensure the reliable operation of the vibration sensor, the prior art provides different methods, with the help of which information about the condition of the vibration sensor can be obtained. For example, a method for monitoring the quality of a vibration sensor is known from DE102005036409A1. A measuring device includes at least one power measuring unit, which monitors the energy requirement of the exciter / receiving unit at least for the situation of the resonant oscillation. In this way, information about the quality of the vibration sensor can be obtained. The higher the quality, the less energy is required to excite the resonant oscillation. Therefore, if the energy required to excite the resonant oscillation rises within a predeterminable time period, or exceeds a predeterminable limit value of the quality ascertained during the production of the sensor, it can be inferred that there are defects, accretion, etc. in the area of ​​the oscillatable unit.

[0008] Furthermore, from DE 10 2007 008 669 A1 a vibration sensor is known which has an electronics unit comprising a phase measurement unit, an adjustable phase shifter and a phase adaptation unit which controls the setting of the phase shift between the excitation signal and the received signal. Control parameters can be updated and stored at predeterminable time intervals during operation of the sensor. Furthermore, monitoring of the situation can be performed based on a comparison between the stored control parameters and the current control data.

[0009] DE 10 2017 111 392 A1 describes a method for monitoring the condition of a vibration sensor, in which case information about the condition of the sensor is ascertained as a function of the excitation frequency based on the frequency spectrum of the received signal or a variable derived therefrom.

[0010] A method for monitoring the condition of a vibration sensor based on a comparison of a measured value of a physical and / or chemical variable of a sensor characteristic of the sensor (e.g., frequency) with a reference value is known from DE 10 2017 102 550 A1. In this way, for example, information about corrosion, wear or accretion in the region of the oscillatable unit can be obtained.

[0011] However, with the known methods it is usually only possible to state in principle that corrosion, wear or accretion is present. In contrast, the precise location and exact determination of the cause of the altered oscillation behavior of a specific sensor is usually not ascertainable from the measurement signal alone. Summary of the invention

[0012] It is therefore an object of the present invention to provide a method for monitoring the condition of a vibration sensor which is able to monitor the condition as accurately as possible.

[0013] According to the invention, this object is achieved by a method for monitoring the condition of a vibration sensor for determining and / or monitoring at least one process variable of a medium in a container and having at least one sensor unit having a mechanically oscillatable unit, the method comprising the following method steps:

[0014] - exciting the mechanically oscillatable unit by means of an excitation signal so that it performs a mechanical oscillation, and receiving the mechanical oscillation in the form of a received signal,

[0015] - determining a measured value of the amplitude and a measured value of the frequency of the received signal,

[0016] - compare the measured values ​​of amplitude and frequency with reference values ​​of amplitude and frequency, and

[0017] - Determining a status indicator based on the comparison.

[0018] Since both frequency and amplitude are considered simultaneously, a particularly precise monitoring of the condition can be achieved. Advantageously, when monitoring the condition, the process in which the sensor is located does not need to be interrupted. By recording the measured values ​​of frequency and amplitude, changes in the sensor over time can be observed.

[0019] Furthermore, the method of the invention advantageously enables the performance of predictive maintenance. Based on the determined measured values ​​of amplitude and frequency, it is possible, for example, to estimate when maintenance of the sensor will be required.

[0020] In an embodiment of the method, a deviation between a measured value of frequency and / or amplitude and a reference value is determined, and a condition indicator is ascertained based on the deviation. For this purpose, it is advantageous to ascertain whether the deviation between the measured value of frequency and / or amplitude and the reference value exceeds a predeterminable limit value.

[0021] In a further embodiment of the method, the reference values ​​for amplitude and frequency are in each case values ​​(in particular measured values) of amplitude and frequency which correspond to the resonant oscillation of the oscillatable unit in the fundamental oscillation mode and in air. For example, the reference values ​​can be ascertained during the production of the sensor and stored, for example, in a storage unit, a database or a data table. The reference values ​​in turn correspond to the delivery condition of the sensor. However, the reference values ​​can also be ascertained at the customer's plant and after installation in the receptacle. Since the reference values ​​are determined individually for each sensor, common variations in the values ​​caused by production tolerances can be directly processed.

[0022] An embodiment of the method comprises that the mechanically oscillatable unit is excited in air to mechanical resonant oscillations in a fundamental oscillation mode, and wherein the received signal represents the resonant oscillations of the oscillatable unit in the fundamental oscillation mode. Thus, in this case, the monitoring condition is performed when the oscillatable unit is not covered by the medium.

[0023] In a preferred embodiment of the method, the condition indicator is: information about accretion, corrosion or wear in the area of ​​the oscillatable unit; information about defects in the area of ​​a drive / receiving unit, with the aid of which excitation of the oscillatable unit occurs so that mechanical oscillations are performed; or information about defects in the electronics of a sensor comprising the oscillatable unit.

[0024] Advantageously, the oscillatable unit is an oscillatory fork comprising a membrane and two oscillating tines fixed to the membrane.

[0025] The condition monitoring according to the invention provides a number of different information about the oscillatable unit, some particularly preferred condition monitoring variants of which are as follows:

[0026] An embodiment of the method provides that, in the absence of an ascertainable measured value for the amplitude and a measured value for the frequency being less than a reference value, this indicates the presence of erosion and / or wear on the oscillation tines and / or the presence of hard accretion in the membrane region.

[0027] An embodiment of the method provides that, if the deviation between the measured value of the amplitude and a reference value is less than a predeterminable reference value and the measured value of the frequency is less than a limit value, this indicates the presence of hard accretion in the region of the oscillation tines and / or the presence of corrosion and / or wear in the region of the membrane.

[0028] A further embodiment of the method comprises that the presence of soft accretion in the region of the oscillation tines and / or the presence of medium residues in the region of the oscillatable unit is indicated when the measured value of the amplitude and the measured value of the frequency are less than their reference values.

[0029] A further embodiment comprises, when the measured value of the amplitude is smaller than a reference value and the deviation between the measured value of the frequency and the reference value is smaller than a predeterminable limit value, this may indicate a defect in the area of ​​the drive / receiving unit (in particular in the area of ​​at least one piezoelectric element of the drive / receiving unit) and / or a defect in the area of ​​the electronic unit.

[0030] In case the oscillatable unit performs oscillations with a predeterminable shedding frequency, this can indicate, for example, the presence of a defect in the drive / receiving unit, a bad contact, a cable break, a blockage of the oscillation tines or a defect in the electronic unit.

[0031] In the case that no measured value of the ascertainable frequency is available, a defect of the electronic unit may be indicated.

[0032] Another preferred embodiment includes implementing the method according to the IO-Link standard. The IO-Link standard is defined in the IEC61131-9 standard and is a technology widely distributed in automation technology for connecting intelligent actuators and sensors. IO-Link relates to a serial point-to-point connection, via which data is transmitted between a so-called IO-Link master and one or more connected IO-Link devices as slaves. For example, the master is a fieldbus module or a PLC interface module, which has one or more ports for connecting IO-Link devices. The IO-Link master represents the connection between the IO-Link device and the automation system, and communicates, for example, via a fieldbus. In contrast, an IO-Link device is a sensor, an actuator, an interactive element, or a display unit. In this case, the IO-Link device is described via a so-called IODD description file (IO-Link device description), which contains, for example, device-specific configuration parameters of the IO-Link device.

[0033] In summary, the invention enables accurate monitoring of the condition of a vibration sensor. The monitoring is very easy to implement and can be performed while the process is continuing. Since both frequency and amplitude are taken into account, an accurate monitoring of the condition is possible, based on which not only the presence of a defect can be indicated, but also the location of the defect within the sensor can be determined and the cause of a specific change in behavior can be determined. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention and its advantages will now be described more precisely based on the accompanying drawings, which show the following:

[0035] Figure 1 A vibration sensor according to the prior art is shown, and

[0036] Figure 2 An oscillatable unit of a vibration sensor in the form of an oscillatory fork is shown. DETAILED DESCRIPTION

[0037] Figure 1 A vibration sensor 1 is shown, which comprises a sensor unit 3 having an oscillatory unit 4 in the form of an oscillatory fork, which is partially immersed in a medium 2 located in a container 2a. The oscillatory unit 4 is excited by means of an exciter / receiving unit 5 so that a mechanical oscillation is performed. For example, the exciter / receiving unit 5 can be a piezoelectric stack or a bimorph driver. However, it should be understood that other embodiments of the vibration sensor also fall within the scope of the present invention. An electronic unit 6 is also included, by means of which signal recording, evaluation and / or feeding occurs.

[0038] Figure 2An oscillatable unit 4 in the form of an oscillating fork is shown in a side view, for example as used in a vibration sensor 1 sold by the applicant under the trademark LIQUIPHANT. The oscillating fork 4 comprises two oscillating tines 8a, 8b formed on a membrane 7, at the ends of which are formed two paddles 9a, 9b. The oscillating tines 8a, 8b together with the paddles 9a, 9b are also generally referred to as fork tines. In order for the mechanically oscillatable unit 4 to perform mechanical oscillations, a force is applied to the membrane 7 by means of a drive / receiving unit 5, which is fixed to the side of the membrane 7 opposite to the side carrying the oscillating tines 8a, 8b by material bonding. The drive / receiving unit 5 is an electromechanical transducer unit and comprises, for example, a piezoelectric element or an electromagnetic drive (not shown). The drive / receiving unit 5 consists of separate drive and receiving units, or is a combined drive / receiving unit. In the case where the drive / receiving unit 5 comprises a piezoelectric element 9, the force applied to the membrane 7 is exerted by applying, for example, an excitation signal U in the form of an alternating voltage. A The change of the applied voltage affects the change of the geometric shape of the driving / receiving unit 5, that is, the contraction or expansion inside the piezoelectric element, so that the applied AC voltage is used as the excitation signal U A The membrane 7 connected to the drive / receiver unit 5 by material bonding causes an oscillation. Conversely, the mechanical oscillation of the oscillatable unit is transmitted to the drive / receiver unit 5 via the membrane and converted into an electrical reception signal U E Then, based on the received signal U E To determine certain process variables, for example a predeterminable fill level of the medium 2 in the container 2 a , or the density or viscosity of the medium 2 .

[0039] An option for monitoring the condition of a vibration sensor based on comparing the measured frequency f and the measured amplitude A of the oscillatable unit 4 will now be explained for an excitation in the fundamental oscillation mode of the oscillatable unit 4. In a first step, reference values ​​f of amplitude and frequency are determined. ref , A ref , wherein the oscillatable unit 4 is excited to perform a resonant oscillation in the air.

[0040] In order to determine information about the condition of the sensor 1 during ongoing operation, the oscillatable unit is activated by means of an excitation signal U A is re-excited so that mechanical oscillation is performed in the fundamental oscillation mode, and the received signal U representing the oscillation E are received and evaluated for frequency f and amplitude A. At this time, the oscillatable unit 4 is not covered by the medium. These values ​​f, A are then compared, for example, with their reference values ​​f ref , A ref Compare and determine the measured value f, A and the reference value f ref , A ref deviation.

[0041] For example, a predeterminable limit value may be defined. If the deviation exceeds the limit value, then in a given case there is a problem or the sensor 1 requires repair. The method of the invention thus offers the advantage of predictive maintenance.

[0042] Reference numerals

[0043] 1 Vibration sensor

[0044] 2 Medium

[0045] 2a Container

[0046] 3 Sensor unit

[0047] 4 Oscillating units

[0048] 5. Drive / receive unit

[0049] 6 Electronic unit

[0050] 7 Membrane

[0051] 8a, 8b Oscillating tines

[0052] 9a, 9b Paddle

[0053] U A stimulus signal

[0054] U E Receiving Signals

[0055] f Frequency

[0056] f ref Reference value of frequency

[0057] A amplitude

[0058] A ref Reference value of amplitude

Claims

1. A method for monitoring the condition of a vibration sensor (1) for determining and / or monitoring at least one process variable of a medium (2) in a container (2a) and having at least one sensor unit (3) with a mechanically oscillatable unit (4), the method comprising the following method steps: - With the help of the excitation signal (U A ) excites the mechanically oscillatable unit (4) so ​​that it performs mechanical oscillations, and receives a signal (U E ) in the form of receiving the mechanical oscillation, - Determine the received signal (U E )’s amplitude (A) and frequency (f), - compare the measured value of the amplitude (A) and the measured value of the frequency (f) with the reference value of the amplitude (A ref ) and the reference value of frequency (f ref ) for comparison, and - ascertaining the status indicator based on the comparison, in, When there is no detectable amplitude (A) measurement value and the frequency (f) measurement value is less than the reference value (f ref ), then it indicates the presence of corrosion and / or wear on the oscillation tines (8a, 8b) of the mechanically oscillatable unit (4) and / or the presence of hard accretion in the region of the membrane (7) of the mechanically oscillatable unit (4), Among them, the measured value of the amplitude (A) and the reference value (A ref ) is less than a predeterminable limit value and the measured value of the frequency (f) is less than a reference value (f ref ), then it indicates the presence of hard accretion in the region of the oscillation tines (8a, 8b) of the mechanically oscillatable unit (4) and / or the presence of corrosion and / or wear in the region of the membrane (7) of the mechanically oscillatable unit (4), Among them, the measured value of amplitude (A) and the measured value of frequency (f) are less than their reference value (f ref , A ref ), it indicates the presence of soft accretion in the region of the oscillation tines (8a, 8b) of the mechanically oscillatable unit (4) and / or the presence of medium residues in the region of the mechanically oscillatable unit (4), and Among them, when the measured value of the amplitude (A) is less than the reference value (A ref ) and the measured value of the frequency (f) is different from the reference value (f ref ) is less than a predeterminable limit value, this indicates that there is a defect in the area of ​​the drive / receiving unit (5) and / or a defect in the area of ​​the electronic component (6).

2. The method according to claim 1, in, Reference value of amplitude (A ref ) and the reference value of frequency (f ref ) are the values ​​of amplitude and frequency corresponding to the fundamental oscillation mode and the resonant oscillation of the mechanically oscillatable unit (4) in air.

3. The method according to claim 2, in, The mechanically oscillatable unit (4) is excited in air into mechanically resonant oscillations in the fundamental oscillation mode, and wherein the received signal represents the resonant oscillations of the mechanically oscillatable unit (4) in the fundamental oscillation mode.

4. The method according to any one of claims 1 to 3, in, The mechanically oscillatable unit (4) is an oscillatory fork comprising the membrane (7) and two oscillating tines (8a, 8b) fixed to the membrane (7).

5. The method according to claim 1, in, When the measured value of the amplitude (A) is less than the reference value (A ref ) and the measured value of the frequency (f) is different from the reference value (f ref ) is less than a predeterminable limit value, this indicates that a defect is present in the region of at least one piezoelectric element of the drive / receiving unit (5).

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