METHOD OF OPERATING A MAGNETICALLY-INDUCTIVE FLOWMETER

Abstract

A method for operating a magnetically-inductive flowmeter, wherein the magnetically-inductive flowmeter includes: a measuring tube for guiding a flowable medium; at least two measuring electrodes for detecting a flow velocity-dependent measuring voltage induced in the medium; and a magnetic field-generating device for generating a magnetic field passing through the measuring tube, wherein the magnetic field-generating device includes a coil system with at least one coil, includes determining a deviation σ of a reactance of the coil system or of a variable dependent upon the reactance of the coil system from a desired value. A magnetically inductive flowmeter is also disclosed.

Claims

1-11. (canceled)

12. A method for operating a magnetically inductive flowmeter, comprising: providing the magnetically inductive flowmeter, including: a measuring tube for guiding a flowable medium; at least two measuring electrodes for detecting a flow velocity-dependent measuring voltage induced in the medium; and a magnetic field-generating device for generating a magnetic field passing through the measuring tube, wherein the magnetic field-generating device has a coil system with at least one coil; and determining a deviation σ of a reactance of the coil system or of a variable dependent upon the reactance of the coil system from a desired value.

13. The method according to claim 12, further comprising: determining a corrected flow measurement value Q.sub.V using a polynomial function, wherein the polynomial function is a linear function on the basis of the deviation σ and the currently-detected measuring voltage or a variable dependent upon the detected measuring voltage, wherein the polynomial function optionally has a correction factor k.

14. The method according to claim 12, further comprising: providing an excitation signal at the coil system, wherein the excitation signal includes a pulse sequence at one frequency, at least two pulse sequences each at at least one frequency, and/or at least one sinusoidal signal.

15. The method according to claim 14, further comprising: determining a measurement signal at the coil system, wherein the variable dependent upon the reactance of the coil system is determined at least for a monitoring frequency f.sub.Ü via a transform, including an integral transform and/or a Fourier analysis of a temporal section of the excitation signal and of the measurement signal or of a temporal section of a variable dependent upon the excitation signal and/or the measurement signal.

16. The method according to claim 14, wherein a change in the reactance or the variable dependent upon the reactance of the coil system is determined for a monitoring frequency f.sub.Ü.

17. The method according to claim 16, wherein the following applies for the monitoring frequency f.sub.Ü:
0.1 Hz≤f.sub.Ü≤10 kHz.

18. The method according to claim 12, wherein the desired value of the reactance or of the variable dependent upon the reactance of the coil system describes the reactance or the variable dependent upon the reactance of the coil system in the adjusted state.

19. The method according to claim 12, wherein the variable dependent upon the reactance of the coil system is the apparent resistance of the coil system.

20. The method according to claim 14, wherein the excitation signal corresponds to a coil exciter signal, wherein the coil exciter signal has at least one measurement phase in which a coil current is constant and in which a measurement of the induced measuring voltage takes place, and wherein the coil exciter signal has a transient phase between two successive measurement phases in which transient phase a coil current and/or a coil current direction in the coil system changes.

21. The method according to claim 14, wherein the excitation signal corresponds to a coil exciter signal and an additionally impressed diagnostic signal, wherein the coil exciter signal has at least one measurement phase in which a coil current is constant and in which a measurement of the induced measuring voltage takes place, wherein the coil exciter signal and the diagnostic signal each include a pulse sequence at one frequency, at least two pulse sequences each at at least one frequency, and/or at least one sinusoidal signal, and wherein the at least one frequency of the diagnostic signal differs from the at least one frequency of the diagnostic signal, and/or an amplitude of the diagnostic signal differs from an amplitude of the coil exciter signal.

22. A magnetically inductive flowmeter, comprising: a measuring tube for guiding a flowable medium; at least two measuring electrodes for detecting a flow velocity-dependent measuring voltage induced in the medium; and a magnetic field-generating device for generating a magnetic field passing through the measuring tube, wherein the magnetic field-generating device includes a coil system with at least one coil; and an operating, measurement, and/or evaluation circuit configured to determine a deviation σ of a reactance of the coil system or of a variable dependent upon the reactance of the coil system from a desired value.

Description

[0057] The invention is explained in greater detail with reference to the following figures. The following are shown:

[0058] FIG. 1: a perspectival view of a magnetically-inductive flowmeter according to the invention;

[0059] FIG. 2: an embodiment of an excitation signal B and of a measurement signal A in the time domain and in the associated frequency domain;

[0060] FIG. 3: two further embodiments of the excitation signal B and measurement signal A in the time domain.

[0061] The structure and measuring principle of the magnetically-inductive flowmeter 1 is known in principle (see FIG. 1). A medium having an electrical conductivity is conducted through a measuring tube 2. The measuring tube 2 usually comprises a metallic tube with an electrically-insulating liner or a plastic or ceramic tube. A magnetic-field generating device 4 is mounted such that the magnetic field lines are oriented to be substantially perpendicular to a longitudinal direction defined by the measuring tube axis. A saddle coil or a pole shoe with a mounted coil 5 is preferably suitable as the magnetic-field-generating device 4. When the magnetic field is applied, a potential distribution is produced in the flowing medium in the measuring tube 2, which potential distribution is tapped by two measuring electrodes 3 mounted opposite each other on the inner wall of the measuring tube 2. In general, two measuring electrodes 3 are used, which measuring electrodes are arranged diametrically and form an electrode axis that runs perpendicular to an axis of symmetry of the magnetic field lines and of the longitudinal axis of the measuring tube 2. On the basis of the measured measurement voltage and taking into account the magnetic flux density, the flow rate of the medium can be determined and, taking into account the cross-sectional area of the tube, the volumetric flow rate can be determined. If the density of the medium is known, it will be possible to determine the mass flow rate.

[0062] The magnetic field built up by means of the coil and pole-shoe arrangement is generated by a clocked direct current of alternating flow direction. An operating circuit 6 is connected to the two coils 5 and is configured to apply an excitation voltage with a characteristic curve to the coil system, with which the coil current or the coil voltage is regulated.

[0063] Advantageous embodiments of the characteristic curve of the excitation signals B are shown in FIGS. 1 and 2. The polarity reversal of the coil voltage ensures a stable zero point and renders measurement insensitive to influences from multi-phase substances, inhomogeneities in the liquid, or low conductivity. A measurement and/or evaluation circuit 7 reads the voltage applied to the measuring electrodes 3 and outputs the flow rate and/or the calculated volume flow rate and/or the mass flow rate of the medium. In the cross-section, shown in FIG. 1, of a magnetically-inductive flowmeter 1, the measuring electrodes 3 are in direct contact with the medium. However, coupling can also take place capacitively. According to the invention, the measurement and/or evaluation circuit 7 is additionally configured to determine a measurement signal A at the coil system. The measurement signal A comprises the coil voltage actually present at the coil system and/or the coil current through the coil system.

[0064] According to the invention, the measurement and/or evaluation circuit is further configured to transform the excitation signal B and the measurement signal A or a variable dependent upon the excitation signal B and measurement signal A into a frequency spectrum, and, therefrom, to determine a deviation σ of the reactance from a desired value, and to correct the determined flow measurement value as a function of the determined deviation σ.

[0065] A display unit (not shown) outputs the determined deviation σ or a variable dependent upon the determined deviation σ. Alternatively, a message or a warning can be output if these deviate from the stored setpoint value or setpoint interval. The setpoint value is determined by means of a mathematical model, calibration method, and/or simulation program. However, this is not sufficient, particularly in applications in the drinking water sector. Therefore, the measurement and/or evaluation circuit 7 is configured to correct the measured measuring voltage or a flow measurement variable dependent upon the measuring voltage by the determined deviation σ. The deviation σ is not necessarily determined over the entire frequency spectrum or for all individual frequencies, but for a selected monitoring frequency f.sub.Ü.

[0066] FIG. 2 shows an embodiment of an excitation signal B and a measurement signal A in the time domain, and the resulting frequency spectra E and F in the frequency domain. According to the embodiment, the excitation signal B comprises a coil voltage, and the measurement signal A comprises a coil current. The coil voltage comprises two clocked pulses with different pulse amplitudes and pulse widths. Such an excitation signal B corresponds to a typical coil exciter signal D.

[0067] After the transform of a temporal section of the measurement signal and the excitation signal, a frequency spectrum with discrete frequencies is obtained in each case. In the case of deviations from a desired value, influences by external magnetic fields can then be deduced from the frequency-dependent reactance. The measurement and/or evaluation unit is configured to monitor the change in the reactance for a set monitoring frequency f.sub.Ü. According to the embodiment shown, the monitoring frequency is approximately 100 Hz.

[0068] FIG. 3 shows two embodiments of the excitation signal B and of the measurement signal A. In both embodiments, the excitation signal B comprises a coil voltage, and the measurement signal A comprises a coil current. Both embodiments differ from the embodiment of FIG. 1 in that, in addition to the coil exciter signal D, a diagnostic signal C is applied to the coil system. The two embodiments shown differ in how the diagnostic signal C is related to the coil exciter signal D.

[0069] The first of the two embodiments shows a characteristic excitation signal B in which the diagnostic signal C is applied, in addition to the coil exciter signal D. The excitation signal B is a superposition of the coil exciter signal D and of the diagnostic signal C. That is, the coil exciter signal D and the diagnostic signal C overlap. The measurement signal A depends upon the excitation signal B and therefore has a reaction of the coil system to the diagnostic signal C. The diagnostic signal C must be temporally offset with the coil exciter signal D such that the diagnostic signal C does not extend into the measurement phase. The reaction of the measurement signal A to the excitation signal B is sensitive to external magnetic fields. Therefore, the frequency and/or the amplitude of the diagnostic signal C is defined independently of the coil exciter signal D such that external influences can be resolved with the measurement and/or evaluation circuit.

[0070] The second of the two embodiments also shows a characteristic excitation signal B in which the diagnostic signal C is applied in addition to the coil exciter signal D. However, the coil exciter signal D is interrupted for a time period in which the diagnostic signal C is applied. The diagnostic signal C and the coil exciter signal D thus alternate.

LIST OF REFERENCE SIGNS

[0071] 1 Magnetically-inductive flowmeter [0072] 2 Measuring tube [0073] 3 Measuring electrode [0074] 4 Magnetic-field-generating device [0075] 5 Coil [0076] 6 Operating circuit [0077] 7 Measurement and/or evaluation circuit [0078] A Measurement signal [0079] B Excitation signal [0080] C Diagnostic signal [0081] D Coil exciter signal [0082] E Frequency spectrum of measurement signal [0083] F Frequency spectrum of excitation signal [0084] f.sub.Ü Monitoring frequency