METHOD FOR OPERATING A VIBRONIC SENSOR

Description

[0001] The invention relates to a method for determining and / or monitoring a predeterminable fill level of a medium in a container using a vibronic sensor with the features of the preamble of the first claim. In addition to a, particularly predeterminable, fill level of a medium, vibronic sensors can also be used to determine the flow rate, density, or viscosity of the medium. The container is, for example, a tank or a pipeline. A generic method is disclosed in DE 10 2005 009 580 A1.

[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 a piezoelectric drive or an electromagnetic drive, for example.

[0003] The applicant manufactures a wide variety of corresponding field devices and, in the case of level measuring devices, markets them, for example, under the names LIQUIPHANT or SOLIPHANT. The underlying measuring 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 reception signal. The drive / receiver unit is accordingly either a separate drive unit and a separate receiver unit or a combined drive / receiver unit.

[0004] In many cases, the drive / receiver unit is part of a feedback electrical oscillating circuit, which excites the mechanically oscillating unit to mechanical oscillations. For example, for a resonant oscillation, the oscillating circuit condition must be met, according to which the gain factor is ≥1 and all phases occurring in the oscillating circuit are multiples of 360°.

[0005] To excite and fulfill the resonant circuit condition, a certain phase shift between the excitation signal and the received signal must be ensured. For this reason, 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 state of the art for this purpose. In principle, the phase shift can be adjusted, for example, by using a suitable filter, or it can be controlled to a predeterminable phase shift, the target value, using a control loop. DE102006034105A1, for example, discloses the use of an adjustable phase shifter.The additional integration of an amplifier with an adjustable gain factor for additional control of the oscillation amplitude was described in DE102007013557A1. DE102005015547A1 proposes the use of an all-pass filter. The phase shift can also be adjusted using a so-called frequency sweep, as disclosed, for example, in DE102009026685A1, DE102009028022A1, and DE102010030982A1. However, the phase shift can also be controlled to a preset value using a phase-locked loop (PLL). An excitation method based on this method is the subject of DE102010030982A1.

[0006] 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, such as a specified fill level of a medium in a container, or the density and / or viscosity of a medium, or the flow of a medium through a pipe. In the case of a vibronic point level switch for liquids, for example, a distinction is made as to whether the oscillating unit is covered by the liquid or is freely oscillating. These two states, the free state and the covered state, are differentiated, for example, based on different resonance frequencies, i.e., a frequency shift (see, for example, DE 33 48 119 C2).

[0007] The density and / or viscosity, in turn, can only be determined with such a measuring device if the oscillating unit is covered by the medium, as for example in the documents DE10050299A1, DE102007043811A1, DE10057974A1, DE102006033819A1, or DE102015102834A1.

[0008] To ensure the reliable operation of a vibronic sensor, numerous methods have become known for monitoring the sensor's condition, as described, for example, in documents DE102005036409A1, DE102007008669A1, DE102017111392A1, or DE102017102550A1. See also US 2005 / 0210954 A1.

[0009] In DE 103 28 296 A1, build-up on a tuning fork is detected by evaluating the frequency of the vibrations.

[0010] A discussion of the evaluation of amplitude and frequency can be found in DE 10 2005 009 580 A1, DE 102 42 970 A1 or DE 197 20 519 A1.

[0011] The present invention is based on the object of expanding the application range of vibronic sensors.

[0012] This object is achieved according to the invention by a method having the features of the first claim.

[0013] According to the invention, a reference value is defined for the amplitude and for the frequency, by means of which the amplitude and the frequency of the received signal are compared.

[0014] By considering the two variables amplitude and frequency, more information regarding the process variable can be obtained. For example, a change in frequency or amplitude can be traced back to different causes. By additionally considering the other characteristic variable, it is possible to distinguish between the various possible causes of the change.

[0015] In one embodiment, it is checked whether the frequency exceeds or falls below a predefined frequency limit. In another embodiment, it is also checked whether the amplitude exceeds or falls below a predefined amplitude limit.

[0016] One embodiment includes recording the frequency and / or amplitude of the received signal as a function of time. In this way, temporal developments of the sensor can also be observed. The method according to the invention thus also advantageously enables the implementation of maintenance, in particular predictive maintenance.

[0017] One embodiment of the method according to the invention includes the conclusion that, in the event of a change in frequency, particularly if the predeterminable frequency limit is exceeded or undershot, and the amplitude remains essentially constant, the oscillating unit is covered by a fluid. If only the frequency changes while the amplitude remains essentially constant, a conclusion can be drawn about the predeterminable fill level being reached.

[0018] According to the invention, in the event of a change in amplitude, particularly if the predeterminable amplitude limit is exceeded or undershot, a conclusion is drawn as to whether the oscillating unit is covered by foam or as to the presence of sediment in the medium. Advantageously, the present invention allows the detection of foams and sediments that are usually not detectable or only detectable with considerable effort using conventional evaluation methods.

[0019] In this context, the invention concludes that sediment has deposited in the area of ​​the vibrating unit if the frequency remains essentially constant. This is particularly advantageous if the vibronic sensor is used to determine a minimum limit level in a container. In this case, for example, sediment can be detected in the bottom area of ​​the container, which can distort the statement about the limit level. With precise knowledge of the presence of sediment in the bottom area, a significantly more accurate determination of the limit level is possible.

[0020] Alternatively, the invention provides that in the event of a change in frequency, where the predeterminable frequency limit is not exceeded or undershot, the presence of sediment in the medium or coverage by foam is inferred. A slight change in frequency, however, where the frequency limit is not yet exceeded or undershot, is, according to the invention, an indicator of foam or sediment dissolved in the medium.

[0021] In one embodiment of the method, the amplitude reference value and / or the frequency reference value are each a value for the amplitude and / or the frequency, which value corresponds to a resonant oscillation of the oscillating unit in the fundamental vibration mode and in air. For example, these reference values ​​can be determined during production of the respective sensor and stored, for example, in a memory unit, in a database, or on a data sheet. The reference values ​​then correspond to the delivery state of the sensor. However, they can also be determined by the customer and after installation in the respective container. By determining the reference values ​​individually for each sensor, the usual variances in these values ​​resulting from manufacturing tolerances can be directly counteracted.

[0022] It is also advantageous within the scope of the method according to the invention if the mechanically oscillatable unit is excited to mechanical resonance oscillations in the fundamental oscillation mode, wherein the received signal represents the resonance oscillations of the oscillatable unit in the fundamental oscillation mode.

[0023] Finally, a further embodiment includes the oscillating unit being a tuning fork with a membrane and two vibrating rods attached to the membrane.

[0024] In summary, the present invention allows for precise determination and / or monitoring of the specified fill level using a vibronic sensor, which is very easy to implement. By simultaneously considering frequency and amplitude, a more accurate determination of the process variable is possible, and various negative factors influencing the process variable determination, such as the presence of foam or sediment, can be detected, thus eliminating their influence on the resulting conclusion.

[0025] The invention and its advantages are illustrated by the following figures Fig. 1 and Fig. 3 described in more detail. It shows: Fig. 1 a state-of-the-art vibronic sensor, Fig.2 an oscillating unit of a vibronic sensor in the form of a tuning fork, and Fig. 3 Frequency and amplitude diagrams for different media to illustrate the inventive procedure.

[0026] In Fig. 1 1 shows a vibronic sensor 1. It depicts a sensor unit 3 with an oscillating unit 4 in the form of a tuning fork, which is partially immersed in a medium 2 located in a container 2a. The oscillating unit 4 is excited to mechanical vibrations by means of the excitation / receiving unit 5 and can, for example, be a piezoelectric stack or bimorph drive. However, it goes without saying that other embodiments of a vibronic sensor also fall within the scope of the invention. Furthermore, an electronics unit 6 is depicted, by means of which the signal is detected, evaluated, and / or fed.

[0027] In Fig. 2 A side view of an oscillating unit 4 in the form of a oscillating fork, such as that integrated in the vibronic sensor 1 marketed by the applicant under the name LIQUIPHANT, is shown. The oscillating fork 4 comprises two oscillating rods 8a, 8b formed on a membrane 7, to which two paddles 9a, 9b are formed at the ends. The oscillating rods 8a, 8b together with the paddles 9a, 9b are often also referred to as fork tines. In order to set the mechanically oscillating unit 4 into mechanical oscillations, a force is applied to the membrane 8 by means of a drive / receiver unit 5 integrally attached to the side of the membrane 8 facing away from the oscillating rods 7a, 7b. The drive / receiver unit 5 is an electromechanical transducer unit and comprises, for example, a piezoelectric element or an electromagnetic drive (not shown).The drive unit 5 and the receiving unit are either constructed as two separate units or as a combined drive / receiving unit. If the drive / receiving unit 5 comprises a piezoelectric element 9, the force exerted on the membrane 7 is generated by applying an excitation signal UA, for example in the form of an alternating electrical voltage. A change in the applied electrical voltage causes a change in the geometric shape of the drive / receiving unit 5, i.e. a contraction or relaxation within the piezoelectric element such that the application of an alternating electrical voltage as excitation signal UA causes an oscillation of the membrane 7, which is integrally connected to the drive / receiving unit 5. Conversely, the mechanical oscillations of the oscillatable unit are transmitted via the membrane to the drive / receiving unit 5 and converted into an electrical reception signal UE.The predeterminable fill level of the medium 2 in the container 2a can then be determined based on the received signal UE , for example based on an amplitude A, frequency f, or phase of the received signal UE .

[0028] The method according to the invention now allows for significantly higher accuracy in determining the preset fill level in an expanded range of applications. Various preferred embodiments are described in this context by way of example in Fig. 3 shown.

[0029] In a first step, reference values ​​f ref , A ref for the amplitude and the frequency are determined, whereby the oscillating unit 4 is excited to resonant oscillations in air. In order to determine a statement about the predeterminable fill level during continuous operation, the oscillating unit 4 is excited to mechanical oscillations in the fundamental oscillation mode by means of an excitation signal UA and the received signal UE representing the oscillations is received and evaluated with regard to the frequency f and amplitude A. The values ​​f, A are compared with the respective reference values ​​f ref , A ref and, for example, a deviation of the measured values ​​f, A from the reference values ​​f ref , A ref is determined, or it is checked whether the frequency f and / or amplitude A exceed or fall below the respective predeterminable limit value f ref or A ref.

[0030] In Fig. 3 Exemplary diagrams of the frequency f and amplitude A are shown, in which the frequency f and amplitude A are shown for different situations as a function of the immersion depth t of the oscillating unit 4 into the medium 2. In the case of covering the oscillating unit 4 with a liquid medium 2, there is a change in the frequency f with an essentially constant amplitude A or only a slight change in the amplitude A, as in Fig. 3a illustrated.

[0031] If, however, a change in the amplitude A can be detected, in particular beyond a predeterminable amplitude limit value A ref , it can be concluded that the oscillating unit 4 is covered by a foam or that a sediment is present in the medium, as in Fig. 3b illustrated. Without this additional observation, it would be impossible to distinguish between a freely vibrating, oscillating unit 4 and a covering of foam or sediment. This can lead to significant problems, as reaching the specified limit may not be indicated correctly.

[0032] In order to be able to distinguish between a sediment and a foam, for example, in the presence of a change in the amplitude A, as in the case of the Fig. 3b , an additional consideration of the frequency f may be helpful. If the frequency f remains essentially constant, then, for example, a deposit of sediment may be present in the area of ​​the oscillating unit 4. If, on the other hand, a change in the frequency f is also detected, but which, for example, does not exceed a predeterminable limit value for the frequency, then the presence of sediment in the medium 2 or the covering of the oscillating unit 4 by a foam can be concluded.

[0033] In addition to the possibilities mentioned, numerous further embodiments of the method according to the invention are conceivable, which allow further conclusions to be drawn from the consideration of the frequency and amplitude with regard to the determination of a statement about the limit level, which also fall within the scope of the present invention. Bezugszeichen

[0034] 1Vibronic sensor 2Medium 2aContainer 3Sensor unit 4Oscillating unit 5Drive / receiving unit 6Electronics unit 7Membrane 8a,8bOscillating rods 9a,9bPaddles UA Excitation signal UE Received signal fFrequency f ref Reference value for the frequency AAmplitude A ref Reference value for the amplitude afreely oscillating oscillatory unit bmedium-covered oscillatory unit tImmersion depth of the oscillatory unit

Claims

1. Method for determining and / or monitoring a predeterminable filling level of a medium (2) in a container (2a) by means of a vibronic sensor (1), with at least one sensor unit (3) with a mechanically vibrating unit (4) comprising the following method steps: - Excitation of the mechanically vibrating unit (4) to mechanical vibrations by means of an excitation signal (UA) and reception of the mechanical vibrations in the form of a reception signal (UE), - Determine an amplitude (A) and a frequency (f) of the received signal (UE), - comparing the frequency (f) and the amplitude (A) of the received signal (UE) with a predeterminable frequency limit value (fref) and a predeterminable amplitude limit value (Aref), and - Determining whether the predeterminable fill level has been reached by comparing the frequency (f) and the amplitude (A) of the received signal (UE) with the predeterminable frequency limit value (fref) and the predeterminable amplitude limit value (Aref), characterized in that that in the event of a change in amplitude (A), it is concluded that the vibrating unit (4) is covered by a foam or that a sediment is present in the medium (2) in the area of the vibrating unit (4), that in the event of a change in amplitude (A) and in the event of a constant frequency (f), the presence of the sediment in the medium (2) in the region of the oscillating unit (4) and that in the event of a change in the amplitude (A) and in the event of a change in the frequency (f), whereby the predeterminable frequency limit value (fref) is not exceeded or undershot, the presence of the sediment in the medium (2) in the region of the oscillating unit (4) or the coverage by the foam is concluded.

2. Method according to claim 1, wherein the amplitude reference value (Aref) and / or the frequency reference value (fref) is in each case a value for the amplitude (A) and / or the frequency (f), which value corresponds to a resonant oscillation of the oscillating unit (4) in the fundamental oscillation mode and in air.

3. Method according to claim 1 or 2, wherein the vibrating unit (4) is a tuning fork with a diaphragm and two vibrating rods attached to the diaphragm.