Method for operating a measuring device having at least one oscillator and measuring device for carrying out said method

Abstract

The invention discloses a method for operating a measuring device, the measuring device having a measuring sensor, the measuring sensor having an oscillator, the oscillator having at least one vibrating measuring tube for guiding a medium, the measuring sensor being in particular a measuring sensor for determining the mass flow, density and / or viscosity of the medium; the method comprising the following steps: determining a current value of a resonant frequency of a vibration mode of the oscillator (110); exciting a non-resonant vibration (120) at an excitation frequency different from the current value of the resonant frequency; and determining an amplitude (130) of a sensor signal representing the non-resonant vibration; characterized in that, in order to determine the amplitude of the sensor signal of the non-resonant vibration, the sensor signal of the vibration sensor of the oscillator is determined by means of a low-pass filter having a time constant of not less than 1000 period lengths of the non-resonant vibration, for example not less than 10000 period lengths of the non-resonant vibration, more specifically not less than 100000 period lengths of the non-resonant vibration. The invention also relates to a measuring device for carrying out the method.

Description

Technical Field

[0001] The invention relates to a method for operating a measuring device, which has a measuring sensor with an oscillator, wherein the oscillator has at least one vibrating measuring tube for conducting a medium, wherein the measuring device is used in particular to determine the mass flow, density and / or viscosity of the medium. The invention also relates to a measuring device for implementing the method. The oscillator of a universal measuring sensor is usually excited to a vibration with a resonant frequency of the mode of use. The vibration amplitude at resonance, which is related to the excitation power, can indicate the mass of the oscillator and thus the viscosity of the medium. The resonant frequency of the bending vibration mode enables the density to be determined, and the amplitude of the superimposed vibration in the Coriolis mode enables the mass flow to be determined, which can be detected in the form of a time offset between the vibration signal of the vibration sensor on the inlet side and the vibration signal of the vibration sensor on the outlet side. The determined value of the measured variable described is valid as long as the assumptions about the mechanical properties of the oscillator reflected in the coefficients of the calculation model of the measured variable apply. This is guaranteed when the measuring sensor is put into operation, but the mechanical properties can change for various reasons, depending on the conditions of use. Therefore, it is meaningful to monitor the mechanical properties of the oscillator. In this regard, for example, International Publication WO 2012 062551 A1 teaches non-resonant excitation. Background Art

[0002] With a typical value of at least some 100 for the oscillator mass, in the case of an excitation at s times the resonance frequency, for s>1.05 or s<0.95, the amplitude x is given by x=F / (D*(s 2 -1)), where F is the modal excitation force and D is the modal stiffness. Converted into measurable observables Us, Ie and f, for example, the following equation is obtained, which defines a suitable monitoring variable H, for example:

[0003] H:=Us / (Ie*f 2 )=K / (D*(s 2 -1)),

[0004] Here K is a constant which is in particular a factor which binds the transfer function between the excitation current Ie and the excitation force and the transfer function between the speed of the oscillator and the sensor voltage Us of the electromagnetic sensor.

[0005] Apart from temperature dependencies and / or pressure dependencies which may have to be modeled, the modal stiffness D does not change in the case of a complete measuring sensor. Therefore, for a given value s, the ratio of the observables given on the left side of the equation should not change.

[0006] If, on the other hand, a change is found in the ratio of the observables, this means that the underlying variable of the modal stiffness D or the factor s or the constant K has changed. For example, wear or corrosion can cause a change in the modal stiffness; a deviation in the factor s means an error in the frequency control; a change in the factor K, for example, would indicate a hardware fault in the sensor electronics. Whatever the cause. In any case, such a deviation should be reliably identified and signaled.

[0007] With the available excitation power, it is impractical to measure the current value of the monitored variable H and simultaneously determine the mass flow measurement value with sufficient accuracy. Therefore, it is customary to determine the current value of the monitored variable only at occasional checks, when the measuring operation may be interrupted for a short time. The object of the present invention is to find a remedy in this respect. Summary of the invention

[0008] This object is achieved according to the invention by a method for operating a measuring sensor according to independent claim 1 and by a measuring sensor according to independent claim 8 .

[0009] The method according to the invention is used to operate a measuring device having a measuring sensor with an oscillator, wherein the oscillator has at least one vibrating measuring tube for guiding a medium, wherein the measuring device is used in particular to determine at least one of the measured variables of mass flow, density and viscosity of the medium; wherein the method comprises the following steps:

[0010] determining a current value of a resonant frequency of a vibration mode of the oscillator;

[0011] exciting non-resonant vibration at an excitation frequency different from a current value of the resonant frequency; and

[0012] determining an amplitude of a sensor signal of the non-resonant vibration;

[0013] wherein the amplitude of the excitation current signal that excites the non-resonant vibration is not greater than the amplitude of the excitation current signal that simultaneously excites the resonant vibration;

[0014] Therein, in order to determine the amplitude of the sensor signal of the non-resonant vibration, the sensor signal of the vibration sensor of the oscillator is detected via a low-pass filter as follows: the time constant of the low-pass filter is not less than 1000 cycle lengths of the non-resonant vibration, for example, not less than 10,000 cycle lengths of the non-resonant vibration, in particular not less than 100,000 cycle lengths of the non-resonant vibration.

[0015] In a development of the invention, a non-resonant vibration is superimposed on the vibration at the resonant frequency.

[0016] In a development of the invention, the sensor signal of the vibration sensor is damped at the resonant frequency of the oscillator in order to determine the amplitude of the sensor signal of the non-resonant vibration by means of a filter, in particular a notch filter.

[0017] In a further development of the present invention, the amplitude of the excitation current signal by which the non-resonant oscillation is excited does not exceed 4 mA, for example, does not exceed 1.5 mA.

[0018] In a further development of the present invention, the amplitude of the excitation current signal by which the non-resonant oscillation is excited is not less than 0.25 mA, in particular not less than 0.5 mA, preferably not less than 0.75 mA.

[0019] In a development of the invention, the amplitude of the excitation current signal by which the non-resonant vibrations are excited is no more than half, in particular no more than a quarter, in particular no more than an eighth of the amplitude of the excitation current signal by which the resonant vibrations are simultaneously excited. These ratios of the excitation current signal amplitudes are particularly suitable for measuring sensors that have a high power requirement for exciting the resonant frequency, so that, given a predetermined available electrical power, a greater current share for exciting the non-resonant vibrations leads to an impairment of the measuring performance of the measured variables of mass flow and / or density. This is particularly the case in particular in the case of measuring sensors with a rigid measuring tube, i.e., in general, for measuring sensors with a larger nominal width (e.g., DN50 and above).

[0020] In a further development of the invention, the sampling frequency at which the sensor signal is sampled is not less than four times, for example not less than eight times, in particular not less than sixteen times, the resonant frequency of the oscillator.

[0021] In a development of the invention, a value of a transfer function is formed which relates the amplitude of the sensor signal of the non-resonant oscillation to the amplitude of the excitation signal of the non-resonant oscillation. In a currently preferred development of the invention, the excitation frequency of the non-resonant oscillation differs from the resonant frequency by a constant factor s.

[0022] In an alternative embodiment, the excitation frequency of the non-resonant vibration may differ from the resonant frequency by a particularly constant offset. In this case, in order to achieve a correct interpretation of the relationship between the amplitude of the sensor signal of the non-resonant vibration and the amplitude of the excitation signal of the non-resonant vibration, it is still necessary to determine which factor s corresponds to the ratio of the excitation frequency to the resonant frequency.

[0023] In a development of the invention, the excitation frequency of the non-resonant oscillation differs from the current resonant frequency by at least 5% of the current resonant frequency.

[0024] In a development of the invention, the value of the transfer function represents the modal stiffness or deflection of the oscillator.

[0025] In a development of the invention, cross-sensitivities of the oscillator with respect to temperature and pressure of the medium conducted in the measuring tube of the oscillator as well as cross-sensitivities of the vibration sensor and the actuator with respect to temperature are compensated during the determination of the transfer function.

[0026] The measuring device according to the invention is used to determine the mass flow, density and / or viscosity of a medium, and has a measuring sensor and a measuring and operating circuit, wherein the measuring sensor comprises: an oscillator having at least one vibrating measuring tube for conducting the medium; at least one exciter, the at least one exciter being used to excite the measuring tube to vibrate; at least one sensor, the at least one sensor being used to detect the vibration of the measuring tube; wherein the measuring and operating circuit is configured to drive the exciter, to detect a signal of the at least one sensor, and to perform the method according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will now be explained based on the exemplary embodiments shown in the accompanying drawings. It is shown as follows:

[0028] Figure 1a : Resonance curves of oscillators of various qualities Q;

[0029] Figure 1b : Figure 1a The difference between the resonance curves of

[0030] Figure 1c : The difference between the resonance curves of oscillators of the same mass but with altered stiffness;

[0031] Figure 2a : Flowchart of an exemplary embodiment of the present invention;

[0032] Figure 2b : Used to determine Figure 2a A detailed flow chart of the amplitude of the non-resonant vibration in an exemplary embodiment of the present invention;

[0033] Figure 3 : A schematic diagram of a filter stage used in the above exemplary embodiment; and

[0034] Figure 4 : Schematic diagram of an exemplary embodiment of a measuring device according to the invention. DETAILED DESCRIPTION

[0035] Reference Figure 1a to Figure 1c , explaining the basic process of monitoring the state of an oscillator.

[0036] Figure 1aThe resonance curves shown in derive from oscillators that differ only in mass, wherein values ​​of Q=200, Q=1,000 or Q=5,000 are assumed. These are typical values ​​that may occur during operation of a Coriolis mass flow meter. In the case of a static deflection, the amplitude A(X) is shown here with the same force relative to the amplitude A(0). The normalized excitation frequency X=f / f0 is plotted on the abscissa, where f0 is the natural frequency of the vibration mode of the considered oscillator.

[0037] Figure 1b Show Figure 1a , where in each case the curves for Q=200 and Q=1,000 are subtracted from the curve for Q=5,000. In the region marked by the shaded boxes above and below the resonance peak at X=1, changes in mass have virtually no effect any more; even a mass change from Q=5,000 to Q=200 results in an amplitude change of less than 0.1%.

[0038] Figure 1c The difference in the resonance curves for different measuring tube stiffnesses (99.5% or 99.0% of the reference stiffness) for a mass of Q=1,000 is shown. The difference curves are normalized to the static deflection of the oscillator with 100% initial stiffness (X=0). Figure 1b and Figure 1c A comparison of the difference curves shows that, at a sufficient distance from the resonance peak, the effect of the described stiffness change is several orders of magnitude stronger than the effect of the mass change. Thus, by observing the vibration amplitude, for example in the marked frequency range, the mechanical integrity of the oscillator can be monitored. However, since the absolute value of the amplitude in the mentioned frequency range is very small, e.g. Figure 1a As shown, the amplitude measurement according to the prior art is therefore only performed for specific diagnostic processes, in which the measuring operation can at best be limited to a limited extent. In the diagnostic process, a large excitation power is provided for the vibration in the frequency range of X>1.1 or X<0.9 in order to achieve an amplitude sufficient for measurement during the diagnostic process. This is disadvantageous because the fault can only be identified later, i.e., only when the diagnosis is performed after the fault has occurred. In addition, the data basis for the statistical analysis is very small.

[0039] In contrast, the method according to the invention is based on exciting non-resonant vibrations with very little power, so that the measuring operation at the resonant frequency is firstly not disturbed in practice and secondly the oscillator can be monitored continuously. As long as the signal component of the non-resonant vibration in the sensor signal is approximately 80 dB below the signal component of the resonant vibration, special measures are taken to determine the required information, namely the amplitude of the non-resonant vibration.

[0040] exist Figure 2aAn exemplary embodiment of a method 100 according to the invention is shown in . The current resonant frequency of an oscillator is continuously determined 110 by adjusting the excitation of the bending vibration use mode according to a resonance criterion. This can be, for example, an amplitude maximization or a 90° phase angle between the excitation signal and the oscillator deflection or a 0° phase angle between the excitation signal and the speed signal of the oscillator. The excitation at resonance is continuously superimposed 120 with a non-resonant excitation, in particular with a constant amplitude, wherein the non-resonant excitation occurs at an excitation frequency that differs from the resonant frequency by a constant factor s, wherein, for example, s≤0.9 or s≥1.1. The excitation force of the non-resonant excitation is, for example, not greater than one tenth of the excitation force of the excitation at resonance.

[0041] Subsequently, an amplitude signal of the non-resonant vibration of the oscillator is determined 130 .

[0042] The ratio of the determined amplitude signal to the excitation signal is formed and, if necessary, a correction for cross-sensitivities such as temperature, pressure and / or aging of the sensor or actuator is performed at 140. Suitable correction functions are disclosed, for example, in the not yet published German patent application with the document number DE 10 2019 122 094.7. The data prepared in this way are recorded and can be used for further evaluations such as trend analysis and threshold value comparison 150.

[0043] Now refer to Figure 2b The determination of the amplitude signal 130 is explained in more detail.

[0044] The analog sensor signal S1(t) is sampled at a sampling frequency of, for example, 32 kHz, the amplitude of which is proportional to the vibration amplitude of the measuring tube. After decimation by a factor of 2, two digital value sequences (X, Y) with a repetition frequency of 16 kHz are provided by means of an orthogonal mixer, which are phase-shifted by 90° relative to each other with respect to the frequency of the non-resonant excitation. The two sequences are combined by means of a signal of the type T(Z, w) = ((1-w) / (1-wZ -1 )) filter sequence is used, where w = a, b, c, d are different weighting coefficients.

[0045] In the first filter stage (T(Z,a)) 2 Between the second filter stage T(Z,b), further decimation occurs at a frequency of 2 KHz or 1 KHz.

[0046] After the third filter stage T(Z,c), a notch filter is used to suppress the signal in the case of vibrations at a resonant frequency which is some order of magnitude stronger than the signal of the non-resonant vibration of interest, for example about 80 dB.

[0047] Subsequently, a further time-average formation was carried out at a frequency of 54 Hz.

[0048] The average values ​​X, Y are detected within one second and their standard deviation is determined. If the standard deviation is less than a threshold, the average value averaged within one second is taken and, optionally after further compensation for cross-sensitivities, is fed for further long-term analysis; otherwise, the average value is discarded. The long-term analysis includes a filter stage T(Z, d) and takes into account values ​​over a period of time of the order of 1,000 seconds. Based on the values ​​thus prepared for X and Y, finally according to A=(X 2 +Y 2 ) 1 / 2 Determine the amplitude A of the non-resonant vibration signal. This amplitude should be constant relative to the excitation signal of the non-resonant vibration.

[0049] Cross-sensitivities to be taken into account are, for example, related to the temperature of the actuator or sensor and / or the measuring tube and the medium pressure present in the measuring tube. Finally, if the vibration sensor is an electromagnetic sensor, the density or the resonance frequency can also have an influence on the sensor signal. In this case, the velocity represented by the vibration signal is proportional firstly to the expected amplitude of the deflection and secondly to the density-dependent vibration frequency.

[0050] Filter stages upstream of the notch filter can be implemented as fixed point filters, especially 32-bit or 64-bit filters, while floating point filter stages are currently preferred for the last filter stage.

[0051] exist Figure 3 The implementation of the filter stage T(Z,w) is shown in FIG. This arrangement is derived from the previous output value out of the filter stage. n-1 and the nth input value in n The nth output value out of the filter stage is obtained from n :

[0052] out n =(1-w)x(in n -out n-1 )+out n-1

[0053] The factor 1-w can be expressed in fixed-point calculation as

[0054] 1–w=(2 L –2 N ) / 2 L ,

[0055] For example, where L=32 and N=18...24.

[0056] Using the method described, the ratio between the amplitude of the non-resonant vibration and the associated excitation current at a minimum value of, for example, only 1 mA can be determined accurately enough to be able to identify changes in the measuring tube at an early stage. Due to the low power consumption of this monitoring function, sufficient power is still available to excite the measuring tube vibration at resonance and thus to fulfill the actual measuring task of the measuring sensor.

[0057] Figure 4 The exemplary embodiment of the measuring device 1 according to the invention shown comprises an oscillator 10, which comprises a pair of vibrating measuring tubes 14 guided in parallel, which extend between an inlet-side flange 11 and an outlet-side flange 12, wherein each of the flanges comprises a flow divider or collector into which the measuring tubes 14 lead. The flow dividers are connected to one another via a rigid housing 15, so that the vibrations of the flow dividers accommodating the measuring tubes are effectively damped in the vibration frequency range of the useful bending vibration modes of the oscillator. The measuring tubes 10 are rigidly connected to an inlet-side node plate 20 and an outlet-side node plate 21, wherein the node plates define the vibration nodes of the oscillator 10 formed by the two measuring tubes 14, thereby largely establishing the frequencies of the useful bending vibration modes. The oscillator 10 is excited to vibrate by an electrodynamic exciter 17 acting between the two measuring tubes 14, wherein the vibrations are detected by means of two vibration sensors 18, 19, which detect the relative movement of the measuring tubes 14 relative to one another. The exciter 17 is operated by an operating and evaluation circuit 30, wherein the operating and evaluation circuit 30 also detects and evaluates the signals of the vibration sensor in order to determine a density measurement value and optionally a mass flow measurement value. According to the invention, the operating and evaluation circuit 30 is also configured to carry out the method according to the invention, i.e. in particular to determine the amplitude of the non-resonant vibration and to correlate this amplitude with an excitation signal representing the excitation force. Figure 4 Unlike the one shown in , the operating and evaluation circuit can also include several spatially separated modules. Thus, the amplitude of the non-resonant vibration can also be calculated in a remote computing module, to which the required raw data are transmitted wirelessly, for example.

Claims

1. A method for operating a measuring device, the measuring device having a measuring sensor, the measuring sensor having at least one oscillator, wherein the oscillator has at least one vibrating measuring tube for conducting a medium; in, The method comprises the following steps: determining a current value of a resonant frequency of a vibration mode of the oscillator (110); exciting non-resonant vibration (120) at an excitation frequency different from the current value of the resonant frequency, wherein the non-resonant vibration is superimposed on the vibration at the resonant frequency, wherein the amplitude of the excitation current signal through which the non-resonant vibration is excited does not exceed half the amplitude of the excitation current signal through which the resonant vibration is simultaneously excited; and determining an amplitude (130) of a sensor signal representative of the non-resonant vibration, wherein, in order to determine the amplitude of the sensor signal of the non-resonant vibration, the sensor signal of the vibration sensor of the oscillator is detected via a low-pass filter having a time constant of not less than 1,000 periods of the non-resonant vibration, Therein, the sensor signal of the vibration sensor is damped at the resonant frequency of the oscillator in order to determine the amplitude of the sensor signal of the non-resonant vibration by means of a filter.

2. The method according to claim 1, wherein: The time constant of the low pass filter is not less than 10,000 cycles of the non-resonant vibration.

3. The method according to claim 1, wherein: The time constant of the low pass filter is not less than 100,000 cycles of the non-resonant vibration.

4. The method according to claim 3, wherein: The amplitude of the excitation current signal by which the non-resonant oscillation is excited does not exceed 4 mA.

5. The method according to claim 3, wherein: The amplitude of the excitation current signal by which the non-resonant oscillation is excited does not exceed 1.5 mA.

6. The method according to any one of claims 1 to 5, wherein: The sensor signal of the vibration sensor is suppressed at the resonant frequency of the oscillator in order to determine the amplitude of the sensor signal of the non-resonant vibration by means of a notch filter.

7. The method according to any one of claims 1 to 5, wherein: The amplitude of the excitation current signal by which the non-resonant vibration is excited is not less than 0.25 mA.

8. The method according to any one of claims 1 to 5, wherein: The amplitude of the excitation current signal by which the non-resonant vibration is excited is not less than 0.5 mA.

9. The method according to any one of claims 1 to 5, wherein: The amplitude of the excitation current signal by which the non-resonant vibration is excited is not less than 0.75 mA.

10. The method according to any one of claims 1 to 5, wherein: The amplitude of the excitation current signal by which the non-resonant oscillation is excited does not exceed one quarter of the amplitude of the excitation current signal by which the resonant oscillation is simultaneously excited.

11. The method according to any one of claims 1 to 5, wherein: The amplitude of the excitation current signal by which the non-resonant oscillation is excited does not exceed one eighth of the amplitude of the excitation current signal by which the resonant oscillation is simultaneously excited.

12. The method according to any one of claims 1 to 5, wherein: The sampling frequency at which the sensor signal is sampled is not less than four times the resonant frequency of the oscillator.

13. The method according to any one of claims 1 to 5, wherein: The sampling frequency at which the sensor signal is sampled is not less than eight times the resonant frequency of the oscillator.

14. The method according to any one of claims 1 to 5, wherein: The sampling frequency at which the sensor signal is sampled is not less than sixteen times the resonant frequency of the oscillator.

15. The method according to any one of claims 1 to 5, wherein: A value of a transfer function is formed which relates the amplitude of the sensor signal of the non-resonant oscillation to the amplitude of the excitation signal of the non-resonant oscillation.

16. The method according to claim 15, wherein: The value of the transfer function represents the modal stiffness or deflection of the oscillator.

17. The method according to claim 15, wherein: When determining the transfer function, cross-sensitivities of the oscillator with respect to temperature and of the pressure of the medium conducted in the measuring tube of the oscillator as well as cross-sensitivities of the vibration sensor and of the exciter for exciting the vibration of the measuring tube with respect to temperature are compensated.

18. The method according to claim 16, wherein: When determining the transfer function, cross-sensitivities of the oscillator with respect to temperature and of the pressure of the medium conducted in the measuring tube of the oscillator as well as cross-sensitivities of the vibration sensor and of the exciter for exciting the vibration of the measuring tube with respect to temperature are compensated.

19. The method according to any one of claims 1 to 5, wherein: The excitation frequency of the non-resonant vibration differs from the resonant frequency by at least 5% of the resonant frequency.

20. The method according to any one of claims 1 to 5, wherein: The excitation frequency of the non-resonant vibration differs from the current value of the resonant frequency by a constant factor s.

21. A measuring device (1) for determining the mass flow, density and / or viscosity of a medium, the measuring device (1) comprising: Measuring sensors; as well as a measuring and operating circuit (30), The measuring sensor comprises: an oscillator having at least one vibrating measuring tube (10) for conducting the medium; at least one exciter (17), the at least one exciter (17) being used to excite the measuring tube to vibrate; at least one sensor (18, 19) for detecting vibrations of the measuring tube; The measuring and operating circuit (30) is configured to drive the actuator (17), to detect the signal of the at least one sensor (18, 19), and to perform the method according to any one of claims 1 to 20.

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