CORIOLIS MASS FLOWMETER AND METHOD FOR MONITORING A CORIOLIS MASS FLOWMETER

Abstract

Monitoring a mass flowmeter includes ascertaining a resonant frequency of a bending oscillation, wanted mode, and a density measured value of a medium as a function of the frequency. A bending oscillation is excited outside of resonance with an excitation signal having an amplitude and a frequency (? times the resonant frequency of the bending oscillation, wanted mode). An amplitude of a sensor signal of the bending oscillation outside of resonance is ascertained. A value of an integrity function of the measuring tube depending on a ratio of the sensor signal amplitude to the excitation signal amplitude of the bending oscillation is ascertained. The integrity function depends further on a density term of a transfer function that models contributions of a plurality of oscillation modes to the sensor signal. This function is reduced to reference conditions, and/or transformed to an integrity value, which has no cross sensitivities for media density.

Claims

1-11. (canceled)

12. A method for monitoring a Coriolis mass flowmeter having at least one oscillatable measuring tube for conveying a medium, comprising: ascertaining a resonant frequency of a bending oscillation, wanted mode of the Coriolis mass flowmeter; ascertaining a density measured value of a medium conveyed in the measuring tube as a function of the resonant frequency; exciting a bending oscillation outside of resonance with an excitation signal, which has an excitation signal amplitude and an excitation frequency, which amounts to ? times the resonant frequency of the bending oscillation, wanted mode; registering a sensor signal and ascertaining a sensor signal amplitude of the bending oscillation outside of resonance; ascertaining a value of an integrity function of the measuring tube depending on a ratio of the sensor signal amplitude of the bending oscillation to the excitation signal amplitude of the bending oscillation, wherein the integrity function depends further on a density dependent term of a transfer function, wherein the density dependent term of the transfer function models contributions of a plurality of oscillation modes to the sensor signal, wherein the integrity function by means of the density dependent term of the transfer function is reduced to reference conditions, and/or transformed to an integrity value, which has no cross sensitivities to media density.

13. The method as claimed in claim 12, wherein the transfer function models a ratio of sensor signal amplitude and excitation signal amplitude as a function of the excitation frequency and the media density.

14. The method as claimed in claim 12, wherein the density dependent term of the transfer function comprises summands, which are proportional to the contribution of individual oscillation modes to the sensor signal, wherein the contribution of an oscillation mode depends on the ratio of the excitation frequency to the eigenfrequency of the oscillation mode.

15. The method as claimed in claim 14, wherein the density dependent term of the transfer function models the eigenfrequencies of the oscillation modes as proportional to one another.

16. The method as claimed in claim 15, wherein the density dependent term of the transfer function models a mode specific proportionality factor between the eigenfrequencies of two oscillation modes as a function of density measured value.

17. The method as claimed in claim 16, wherein the function is a linear function of density measured value.

18. The method as claimed in claim 12, wherein the density dependent term of the transfer function models contributions of individual oscillation modes to the sensor signal, in each case, free of damping.

19. The method as claimed in claim 14, wherein the density dependent term of the transfer function models contributions of individual oscillation modes to the sensor signal as proportional to an effective modal excitability.

20. The method as claimed in claim 19, wherein the density dependent term of the transfer function relates the modal excitabilities of two oscillation modes by a proportionality factor.

21. The method as claimed in claim 12, wherein the density dependent term of the transfer function models besides the contribution of the bending oscillation, wanted mode the contribution to the sensor signal of only one additional oscillation mode.

22. A Coriolis mass flowmeter, comprising: at least one oscillatable measuring tube for conveying a medium; at least one exciter for exciting bending oscillations of the measuring tube as a function of an excitation signal; at least one oscillation sensor for registering the bending oscillations of the at least one measuring tube and for outputting an oscillation dependent sensor signal; and at least one measuring- and operating circuit for supplying the exciter with an excitation signal and for registering the sensor signal, wherein the measuring- and operating circuit is adapted to perform the following method: ascertaining a resonant frequency of a bending oscillation, wanted mode of the Coriolis mass flowmeter; ascertaining a density measured value of a medium conveyed in the measuring tube as a function of the resonant frequency; exciting a bending oscillation outside of resonance with an excitation signal, which has an excitation signal amplitude and an excitation frequency, which amounts to ? times the resonant frequency of the bending oscillation, wanted mode; registering a sensor signal and ascertaining a sensor signal amplitude of the bending oscillation outside of resonance; ascertaining a value of an integrity function of the measuring tube depending on a ratio of the sensor signal amplitude of the bending oscillation to the excitation signal amplitude of the bending oscillation, wherein the integrity function depends further on a density dependent term of a transfer function, wherein the density dependent term of the transfer function models contributions of a plurality of oscillation modes to the sensor signal, wherein the integrity function by means of the density dependent term of the transfer function is reduced to reference conditions, and/or transformed to an integrity value, which has no cross sensitivities to media density.

Description

[0037] The invention will now be explained in greater detail based on the examples of embodiments in the drawing, the figures of which show as follows:

[0038] FIG. 1a a side view of an example of an embodiment of a Coriolis mass flowmeter of the invention;

[0039] FIG. 1b a perspective view of the example of an embodiment of the Coriolis mass flowmeter of the invention shown in FIG. 1a;

[0040] FIG. 2 a flowchart of an example of an embodiment of a method of the invention;

[0041] FIG. 3a a graph with values of the density dependent simplified integrity function HBSI.sub.raw(?);

[0042] FIG. 3b a graph of density dependent correction functions f.sub.dens(?), whose coefficients are determined based on the values of the density dependent simplified integrity function HBSI.sub.raw(?) in FIG. 3a; and

[0043] FIG. 3c a graph with density dependent functions, whose coefficients are determined based on the values of the density dependent simplified integrity function HBS.sub.raw(?) in FIG. 3a, and which serve to reduce values of the density dependent simplified integrity function HBSI.sub.raw(?) to values of the density independent integrity function HBSI.

[0044] Shown in FIGS. 1a and 1b is an example of an embodiment of a Coriolis mass flowmeter 2 of the invention, which is adapted for performing the method of the invention. The Coriolis mass flowmeter 2 includes two oscillatably held measuring tubes A and B, each of which is bent and extends parallel to the other. Coriolis mass flowmeter 2 is insertable in such a manner into a pipeline that a fluid flowing in the pipeline flows through the two measuring tubes A, B. Forming the interfaces, respectively, at the inlet side, and at the outlet side, between the measuring tubes A, B and the pipeline are manifolds 4, 6, which are connected rigidly together by a support tube ST. In this way, the inlet side and outlet side end sections of the measuring tubes are also coupled with the support tube ST, whereby relative movements between the inlet side and outlet side end sections of the measuring tubes are effectively suppressed. Arranged between the two measuring tubes A, B is an electrodynamic exciter 8, as a result of which the two measuring tubes A, B are excitable to execute bending oscillations relative to one another, wherein a free oscillatory length of the measuring tubes A, B is established by coupling elements 10, 11, with which the measuring tubes are mechanically coupled together at the inlet side and at the outlet side. Arranged between the two measuring tubes A, B at, respectively, inlet side and outlet side sections are electrodynamic oscillation sensors 14, 16. While FIGS. 1a and 1b show an example of an embodiment of a Coriolis mass flowmeter having a pair of bent measuring tubes in the rest position, the invention relates equally to Coriolis mass flowmeters having a single measuring tube or those with a plurality of pairs of measuring tubes. Equally, instead of the illustrated, bent measuring tubes having a mirror symmetry relative to a measuring tube transverse plane, also S shaped measuring tubes or straight measuring tubes can be used for implementing the invention.

[0045] Coriolis mass flowmeter 2 includes, additionally, an operating- and evaluation circuit 18 for supplying the exciter 8 with an exciter current and for registering and evaluating measurement signals of the electrodynamic oscillation sensors 14, 16. The measuring- and operating circuit is especially adapted to perform the method of the invention for monitoring the Coriolis mass flowmeter. This includes the determining of values of an integrity function, in order to be able early to detect changes in the Coriolis mass flowmeter.

[0046] An example of an embodiment of the method of the invention will now be explained based on FIG. 2.

[0047] The method 100 begins with the ascertaining 110 of a resonant frequency f.sub.1 of a bending oscillation, wanted mode of the Coriolis mass flowmeter. Such can in the case of the device shown in FIGS. 1a and 1b especially be the so-called f.sub.1 mode, in which the two measuring tubes oscillate with opposite phase relative to one another without oscillation nodes between the coupling elements. Based on such resonant frequency f.sub.1, there occurs in a second step 120 the determining of density measured value for the medium conveyed in the measuring tubes.

[0048] For ascertaining the integrity function, there occurs the exciting 130 of the bending oscillation, wanted mode outside of resonance at ? times the resonant frequency with an exciter current, which has an exciter electrical current amplitude I, wherein ? assumes especially the value 1.2.

[0049] There follows the registering 140 of a sensor signal and the ascertaining 145 of a sensor signal amplitude U of the bending oscillation outside of resonance.

[0050] Based on the sensor signal amplitude U and the exciter electrical current amplitude I, there follows then the ascertaining 150 of a value of an integrity function. Such can be, for example, a value of an integrity function HBSI or a value of a simplified integrity function HBSI.sub.raw(?.sub.ref) reduced to a reference density, wherein the reference density ?.sub.ref is especially the density of water.

[0051] As explained above, there follows for the integrity function HBSI based on Equations (XV) and (XVI):

[00013]$\mathrm{HBSI}=\frac{U}{I}\frac{1-{?}^2}{?f_1}\frac{1}{1+?C\left(?\right)}$$\mathrm{where}:$$C\left(?\right)=\frac{1-{?}^2}{1-{\left(\frac{?}{?+?\left(?-{?}_{\mathrm{ref}}\right)}\right)}^2}$

[0052] Correspondingly, there follows for the simplified integrity function HBSI.sub.raw(?.sub.ref) based on Equations (VIII), (XI) and XVII):

[00014]${\mathrm{HBSI}}_{\mathrm{raw}}\left({?}_{\mathrm{ref}}\right)=\frac{U}{I}\frac{1-{?}^2}{?f_1}{f}_{\mathrm{dens}}\left(?\right)$$\mathrm{where}:$$f_{\mathrm{dens}}\left(?\right)=\frac{1+?C\left({?}_{\mathrm{ref}}\right)}{1+?C\left(?\right)}$

[0053] The implementing of the method of the invention for a given Coriolis mass flowmeter requires only the determining of three parameters, namely the coefficients ?, ? of the linear function of the density dependence and the excitability coefficient ?. In such case, it has been found that the coefficients ?, ? of the linear function of the density dependence are essentially constant for a device type, and only the excitability coefficient ? exhibits individual variations. Thus, it is, in most cases, sufficient to determine the coefficients of the linear function of the density dependence once for a device type and only check the excitability coefficient example specifically. The procedure for this, will now be explained based on FIGS. 3a to 3c.

[0054] For characterizing especially a factory new Coriolis mass flowmeter, its measuring tubes are supplied with media over a large density range and caused to oscillate at ? times the resonant frequency of a bending oscillation, wanted mode, especially the f.sub.1mode, in order to ascertain the particular transfer function U/I and therewith the values of the density dependent simplified integrity function HBSI.sub.raw(?).

[00015]${\mathrm{HBSI}}_{\mathrm{raw}}\left(?\right)=\frac{U}{I}\frac{1-{?}^2}{?f_1}$

[0055] Suitable media for this are water as medium with a reference density p.sub.ref of, for instance, 1000 kg/m.sup.3, and air with a density ? of, for instance, 1.2 kg/m.sup.3, while higher densities up to 3100 kg/m.sup.3 can be achieved with solutions of sodium polytungstate.

[0056] FIG. 3a shows observed values of the density dependent simplified integrity function HBSI.sub.raw(?). The illustrated data points in such case come from different examples of the same measuring device type, wherein equal symbols designate data from a specific example.

[0057] FIG. 3b shows example specific correction functions f.sub.dens(?), with which values of HBSI.sub.raw(?) are reduced to the value of HBSI.sub.raw(?.sub.ref), wherein f.sub.dens(?.sub.ref) according to definition assumes the value 1 and holds in general.

[0058] Using

[00016]$f_{\mathrm{dens}}\left(?\right)=\frac{1+?C\left({?}_{\mathrm{ref}}\right)}{1+?C\left(?\right)}$$\mathrm{and}$$C\left(?\right)=\frac{1-{?}^2}{1-{\left(\frac{?}{?+?\left(?-{?}_{\mathrm{ref}}\right)}\right)}^2}$

the unknown, example specific coefficients ?, ? and ? are determined based on the data in FIG. 3a, wherein it has been found that the coefficients ?, ? for examples of a device type can be assumed to be constant. Solely the coefficient ? exhibits individual variations, such that it is ascertained example specifically.

[0059] Thus, with earlier ascertained coefficients, the example specific functions 1/(1+?*C(?)) shown in FIG. 3c can be ascertained, with which a measured value of the density dependent simplified integrity function HBSI.sub.raw(?) can be reduced to a value of the density independent integrity function HBSI according to

[00017]$\mathrm{HBSI}={\mathrm{HBSI}}_{\mathrm{raw}}\left(?\right)\frac{1}{1+?C\left(?\right)}$