Method for determining and / or monitoring a process variable and vibration sensor

Abstract

The invention relates to a method for determining and / or monitoring at least one process variable of a medium. A mechanically vibratable unit (1), which is immersed in the medium or is covered by the medium, is excited to produce mechanical vibrations. Mechanical vibrations dependent on an interaction with the medium are received and evaluated. A gradient measure for a gradient of a signal curve is determined from the received mechanical vibrations, from which an amplitude measure is determined. The invention further relates to a vibration sensor.

Description

[0001] Method for determining and / or monitoring a process variable and vibration sensor

[0002] The invention relates to a method for determining and / or monitoring at least one process parameter of a medium. The invention further relates to a vibration sensor. The vibration sensor preferably serves to determine and / or monitor a process parameter of a medium. The process parameter is, for example, fill level, viscosity, or density, wherein the medium is, for example, a liquid, a gas, or a bulk material.

[0003] It is known in the art to use vibration sensors to detect whether liquids or bulk solids have reached or fallen below a limit level. Such sensors also allow, for example, the measurement of the medium's density or viscosity. The measuring principle is based on the fact that a mechanically vibrating unit is excited to mechanical vibrations by a transducer device, and that the vibrations received by the transducer device are influenced by the interaction of the vibrating unit with the medium. The generated vibrations generally belong to the fundamental mode. The mechanically vibrating unit has, for example, one vibrating element (in which case the sensor is a so-called single-rod sensor) or two vibrating elements (this is a so-called two-rod sensor).

[0004] (Tingling fork). The transducer device often incorporates piezoelectric elements that serve to convert between electrical signals and mechanical vibrations.

[0005] DE 102016 112743 A1 states that the density or viscosity of the medium can be determined from the amplitude of the received vibrations. For this purpose, the damping – preferably the so-called Lehr damping – is determined from the amplitude maxima of the received signals.

[0006] Difficulties arise in determining amplitude values ​​when the received signals are highly noisy or when they are clipped, for example, by an unfavorably set gain of the electronics. The invention is based on the objective of improving the monitoring and determination of process variables using the vibronic measurement principle.

[0007] The invention solves the problem by a method for determining and / or monitoring at least one process parameter of a medium, wherein the method comprises at least the following steps: wherein a mechanically oscillating unit, at least partially immersed in or at least partially covered by the medium, is excited to mechanical vibrations, wherein mechanical vibrations dependent on an interaction with the medium are received and evaluated with respect to the process parameter, and wherein the evaluation comprises at least the following steps: that at least one slope measure for a slope of a signal waveform in the received mechanical vibrations is determined, that at least one amplitude measure for the signal waveform is determined as a function of the slope measure, and that the process parameter is determined and / or monitored as a function of the amplitude measure.According to the invention, the slope of the signal curve of the received mechanical vibrations, which depend on the process variable, is determined in order to determine the amplitude of the signal curve.

[0008] In the method according to the invention, the signals—preferably electrical—resulting from the interaction of the mechanically vibrating unit with the medium are evaluated in such a way that a slope measure for the gradient of a signal waveform is first obtained. This slope measure, which can also be referred to as gradient, then allows the calculation of at least one amplitude measure, i.e., an amplitude value. This is preferably the maximum amplitude. By determining the slope, the necessary data for determining the process variable can therefore be obtained even if the electrical signals resulting from the received vibrations are clipped—"dipped." Finally, a statement about, for example, the density or viscosity of the medium can be derived from the amplitude measure.

[0009] One embodiment of the method provides that the slope is determined at a zero point of the signal waveform. Another embodiment provides that a measure of attenuation – preferably Lehr attenuation – is determined from the amplitude. The relationship between attenuation and amplitude has already been sufficiently described in DE 10 2016 112743 A1.

[0010] To determine the damping, in one embodiment the mechanically oscillating unit is excited to mechanical vibrations – preferably by sinusoidal excitation signals. After excitation ceases, the free vibrations and those damped by the medium are received and converted into electrical signals. The maximum amplitudes are then determined from these electrical signals according to the invention. In one variant, the damping D is calculated from the maximum amplitudes x0 and x1 of two consecutive vibrations, e.g., using the following method.

[0011] Correlation identified:

[0012]

[0013] The following relationship exists between the period T of the oscillations, or at least one oscillation, and the angular frequency coO: ln(x0 / x1) = CÜO * D ​​* T. The oscillation frequency wD due to damping is given by wD = wO * 1 - D 2 .

[0014] One embodiment of the method includes a further step in the evaluation process: determining a measure of the vibration frequency from the received mechanical vibrations. In this embodiment, the frequency at which the mechanically vibrating unit oscillates in the medium is also determined.

[0015] One embodiment of the method involves exciting the mechanical vibrations with a sinusoidal signal. The received vibrations therefore also have a sinusoidal signal waveform.

[0016] One embodiment of the method involves determining or monitoring the density or viscosity of the medium as a process variable. Furthermore, the invention solves the problem by means of a vibration sensor for determining and / or monitoring at least one process variable, comprising at least one mechanically vibrating unit, a transducer device, and an electronic device, wherein the transducer device excites the mechanically vibrating unit to mechanical vibrations and receives mechanical vibrations from the mechanically vibrating unit, and wherein the electronic device processes the received mechanical vibrations and / or signals dependent thereon and evaluates them with regard to at least one process variable.and wherein the electronic device performs at least the following steps during evaluation: that the electronic device determines at least one slope measure for a slope of a signal waveform in the received mechanical vibrations, that the electronic device determines at least one amplitude measure for the signal waveform as a function of the slope measure, and that the electronic device determines and / or monitors the process variable as a function of the amplitude measure.

[0017] The electronic device of the vibration sensor is designed according to one embodiment such that it executes the method according to one of the preceding or following embodiments. Therefore, the explanations for the method also apply appropriately to the vibration sensor, so a repetition is omitted here.

[0018] The invention is explained in more detail with reference to the following figures.

[0019] Fig. 1 schematically shows a vibration sensor and

[0020] Fig. 2 shows the schematic path of a received signal and the graphical representation of the processing.

[0021] Fig. 1 shows the basic structure of a vibration sensor.

[0022] The mechanically oscillating unit 1 is designed here as a tuning fork, the fork tines of which are located on a membrane. On the inside of the membrane is a transducer device 2, which consists of piezoelectric elements. Starting from sinusoidal electrical excitation signals, the transducer device 2 excites the mechanically oscillating unit 1 to mechanical vibrations. If the fork tines are immersed in a medium (not shown here), the medium influences the mechanical vibrations, which manifests itself in the mechanical vibrations received by the transducer device 1. To evaluate the received vibrations, the transducer device 2 generates electrical signals, which are processed by the electronic device 3.

[0023] Fig. 2 schematically shows the signal waveform of an electrical received signal (solid line), the peaks of which have been clipped by overdriving an amplifier component.

[0024] To determine the damping of the vibrations by the medium, information about the amplitude of the received signal is required. The signal is clearly unsuitable for this purpose if only a maximum amplitude is being sought.

[0025] The signal amplitude (on the y-axis, denoted as f(t)) is plotted against time t on the x-axis. It can be seen that the signal waveform is a sine curve, since the oscillations are also generated with sine signals.

[0026] To still obtain the maximum amplitude, the slope of the signal waveform at a zero crossing (let's call this time to) is determined as a measure of the slope. Based on this slope, the maximum amplitude A is calculated as an amplitude measure relative to the oscillation frequency w determined from the signal. The following relationship applies:

[0027]

[0028] The dashed-dotted line represents the derivative of the signal waveform (f'(t)) and therefore has a cosine shape. Thus, the electronic device 3 determines the slope of the signal waveform at a zero crossing of the signal in order to calculate the maximum amplitude A. The attenuation is then determined using this amplitude. Reference symbol list

[0029] mechanically oscillating unit converter device

[0030] electronic device

Claims

Patent claims 1. Method for determining and / or monitoring at least one process parameter of a medium, the procedure includes at least the following steps: wherein a mechanically oscillating unit (1) that is at least partially immersed in or at least partially covered by the medium is excited to mechanical oscillations, wherein mechanical vibrations dependent on an interaction with the medium are received and evaluated in relation to the process parameter, and the evaluation includes at least the following steps: that at least one slope measure is determined for the slope of a signal waveform in the received mechanical vibrations, that, depending on the slope measure, at least one amplitude measure for the signal waveform is determined, and that the process size is determined and / or monitored depending on the amplitude measure.

2. Method according to claim 1, where the slope measurement is determined at a zero point of the signal waveform.

3. Method according to claim 1 or 2, wherein a measure of damping - preferably of Lehr damping - is determined from the amplitude measure.

4. Method according to any one of claims 1 to 3, the evaluation further includes the following step, that a measure of the vibration frequency is determined from the received mechanical vibrations.

5. Method according to one of claims 1 to 4, where the mechanical vibrations are excited by a sine wave signal.

6. Method according to any one of claims 1 to 5, where the density or viscosity of the medium is determined or monitored as a process parameter.

7. Vibration sensor for determining and / or monitoring at least one process parameter, with at least one mechanically oscillating unit (1), with a converter device (2) and with an electronic device (3), wherein the converter device (2) excites the mechanically oscillating unit (1) to mechanical vibrations and receives mechanical vibrations from the mechanically oscillating unit (1), wherein the electronic device (3) processes the received mechanical vibrations and / or signals dependent thereon and evaluates them with regard to at least one process variable, and wherein the electronic device (3) performs at least the following steps during the evaluation: that the electronic device (3) determines at least one slope measure for a slope of a signal waveform in the received mechanical vibrations, that the electronic device (3) determines at least one amplitude measure for the signal waveform as a function of the slope measure, and that the electronic device (3) determines and / or monitors the process variable as a function of the amplitude measure.

8. Vibration sensor according to claim 7, wherein the electronic device (3) performs the method according to any one of claims 2 to 6.

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