METHOD FOR OPERATING A MAGNETO-INDUCTIVE FLOWMETER, AND MAGNETO-INDUCTIVE FLOWMETER

Abstract

The present disclosure relates to a method for operating a magneto-inductive flowmeter, wherein the magneto-inductive flowmeter has a measuring tube for conducting a flowable medium, at least two measurement electrodes for detecting a flow speed-based measurement voltage induced in the medium, and a magnetic field-generating device for generating a magnetic field which passes through the measuring tube, said magnetic field generating device having a coil. The method has the steps of applying a control voltage to the coil in order to generate a change in the coil current over time, and determining the change in the coil current over time in a change-over region, wherein the coil current is changed in the change-over region until a target coil current target is reached.

Claims

1-12. (canceled)

13. A method for operating a magneto-inductive flowmeter, wherein the magneto-inductive flowmeter has a measuring tube for conducting a flowable medium, at least two measurement electrodes for detecting a flow-velocity-dependent measurement 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; comprising the method steps: applying a control voltage to the coil in order to generate a change in the coil current over time; and determining the change in the coil current over time in a change-over region, wherein, in the change-over region, a change in the coil current is present until a target coil current I_target is reached.

14. The method according to claim 13, wherein the change-over region comprises a γ time range (γ), wherein the change in the coil current over time in the γ time range (γ) is determined, wherein the γ time range (γ) is immediately followed by a δ time range (δ), in which the change in the coil current has an opposite sign to the change in the coil current in the γ time range (γ).

15. The method according to claim 14, wherein eddy currents occurring in the γ time range (γ) are constant over time.

16. The method according to claim 14, wherein the γ time range (γ) has a γ time range start (T_(γ start)), wherein, in the γ time range start (T_(γ start)), a value of the coil current is at least 75% of a value of the target coil current I_target.

17. The method according to claim 13, wherein the change in the coil current over time in an ε time range (ε) is determined, wherein the coil current in the ε time range (ε) changes flow direction once.

18. The method according to claim 17, wherein the ε time range (ε) has an ε time range start (T_(ε start)), wherein a value of the coil current at the ε time range start (T_(ε start)) is less than 15% of the value of the target coil current I_target.

19. The method according to claim 17, wherein the ε time range (ε) has an ε time range end (T_(ε end)), wherein a value of the coil current at the ε time range end (T_(ε end)) is less than 15% of the value of the target coil current I_target.

20. The method according to claim 13, comprising the method steps: measuring a coil voltage applied to the coil; and determining a self-inductance of the coil by means of the change in the coil current over time and the coil voltage.

21. The method according to claim 13, comprising the method step: outputting the change in the coil current over time or a quantity dependent on the change in the coil current.

22. The method according to claim 13, comprising the method steps: determining a correction term assigned to a determined deviation of the change in the coil current over time from a reference value, and calculating a corrected measurement voltage value U_corr taking into account the correction term and a measured measurement voltage value U_meas and/or a corrected quantity dependent on the measured measurement voltage value.

23. The method according to claim 13, wherein the reference value is determined by means of a mathematical model, calibration method and/or simulation program, wherein the deviation of the change in the coil current over time from the reference value is in each case assigned a correction term.

24. A magneto-inductive flowmeter, wherein: the flowmeter has an operating, measurement and/or evaluation circuit, which is configured to carry out the following method steps: applying a control voltage to a coil in order to generate a change in a coil current over time; and determining the change in the coil current over time in a change-over region, wherein, in the change-over region, a change in the coil current is present until a target coil current I_target is reached.

Description

[0072] The invention is explained in greater detail with reference to the following figures. The following are shown:

[0073] FIG. 1: a magneto-inductive flowmeter; and

[0074] FIG. 2: a diagram showing the variation over time of the coil current and of the coil voltage.

[0075] The structure and measuring principle of the magnetic-induction 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 or a plastic tube. A magnetic-field generating device 4 is mounted in such a way that the magnetic field lines are oriented perpendicularly 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 measuring tube 2, which is tapped by two measurement electrodes 3 mounted opposite each other on the inner wall of the measuring tube 2. In general, two measurement electrodes 3 are used, which measurement 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. In order to prevent the measurement voltage applied to the first and second measurement electrodes 3 from being dissipated via the tube, the inner wall is lined with an insulating material or a plastic liner.

[0076] 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 a control voltage with a characteristic curve to the coil arrangement, with which the coil current is regulated. A characteristic curve of the control voltage can be seen in FIG. 2.

[0077] The polarity reversal of the voltage source 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 measurement 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 magneto-inductive flowmeter 1 the measurement 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 measure the coil voltage actually applied to the coils and the coil current or the change in the coil current over time. A display unit (not shown) outputs the determined change in the coil current over time or a quantity dependent on the change in the coil current. Alternatively, a message or a warning message can be output if these deviate from the stored reference value. The reference value is determined by means of a mathematical model, calibration method and/or simulation program. However, this is not sufficient in particular in applications in the drinking water sector. For this reason, the measurement and/or evaluation circuit 7 is configured to correct the determined deviation by means of a stored correction factor.

[0078] FIG. 2 shows a diagram that depicts the variation over time of the coil current B and of the coil voltage A.

[0079] The depicted curve of the coil voltage A is characterized by two constant voltage values, which are in each case applied to the coils for a certain period of time, wherein the two voltage values and the respectively assigned time periods differ. However, operating methods for magneto-inductive flowmeters are also known, in which only a constant voltage value with alternating polarity or a voltage curve is applied in which the voltage value is adjusted with respect to time. However, the advantageous time ranges in which the influences on the self-inductance of the coil are minimal also apply to other operating methods. The illustrated curve A shows a time interval that is repeated over time and the sign of which alternates. In the prior art, voltage curves are known, which, between the time periods in which a voltage is applied to the coils, have a rest phase in which no voltage is applied; rather, only the decay behavior of the voltage is determined.

[0080] The depicted curve of the coil current B has two regions. In a first region, the coil current changes due to a change in the applied coil voltage. This region is also referred to as the change-over region. In the second region, coil current and coil voltage are essentially constant. For this reason, before the measurement voltage is tapped at the measurement electrodes and the flow measurement value is determined, there is therefore usually a wait until the coil current has settled and is constant or has reached the target value. The second region is also referred to as the measuring range. In addition, the coil current curve has three further characteristic periods (ε, δ and γ).

[0081] The ε time range covers the period in which the coil current changes flow direction. This is also referred to as the zero crossing. Precisely at the zero crossing, no coil current flows through the coil and the influence of the electrical resistance of the coil on the self-inductance of the coil is negligibly small. This is particularly advantageous, since influences due to the temperature-dependent conductivity of the coil winding material are thereby eliminated. The ε time range has an ε time range start (T.sub.ε start) and an ε time range end (T.sub.ε end). The ε time range begins when the value of the coil measurement current is less than 15%, in particular less than 10% and preferably less than 5% of the value of the target coil current I.sub.target. The ε time range ends when the value of the coil measurement current is greater than 15%, in particular greater than 10% and preferably greater than 5% of the value of the target coil current I.sub.target, wherein I.sub.target, according to the development depicted, is the peak coil current.

[0082] The γ and δ time ranges lie immediately one after the other and essentially cover the time period in which the coil current overshoots or settles. The δ time range comprises the time period in which the sign of the change in the coil current is opposite to the sign in they time range. In addition, the change in coil current decreases in the δ time range until the coil current has reached the target value and the second region begins. The γ time range comprises the time period immediately before the δ time range, wherein however a coil current flows throughout the entire time range. That is to say, the zero crossing of the coil current does not lie within the γ time range.

[0083] The coil current values of the γ time range are preferably characterized in that they are greater than the target value of the coil current in the second region or in the measuring range. Furthermore, the γ time range is selected such that the coil current values lie on a straight line. It has been found that the eddy currents assume a stable state in the γ time range and do not change or change only slightly. Their influence on the self-inductance of the coil is thus also minimal. As a result, even the smallest deviations of self-inductance from the reference value that arise as a result of external influences can already be detected and compensated. The γ time range start (T.sub.γ start) begins when the value of the coil measurement current is at least 75%, in particular at least 85% and preferably 95% of a value of the target coil current I.sub.target, wherein, according to the depicted development, the target coil current I.sub.target corresponds to the peak coil current. The γ time range end (T.sub.γ end) is defined by the start of the δ time range, in particular by the time at which the coil current changes the flow direction. The measured values for determining self-induction preferably derive exclusively from the γ time range.

LIST OF REFERENCE SIGNS

[0084] 1 Magneto-inductive flowmeter

[0085] 2 Measuring tube

[0086] 3 Measurement electrode

[0087] 4 Magnetic-field generating device

[0088] 5 Coil

[0089] 6 Operating circuit

[0090] 7 Measurement and/or evaluation circuit

[0091] ε ε time range

[0092] γ γ time range

[0093] δ δ time range

[0094] I.sub.target Target coil current

[0095] T.sub.γ start γ time range start

[0096] T.sub.γ end γ time range end

[0097] T.sub.ε start ε time range start

[0098] T.sub.ε end ε time range end