Method for operating a Coriolis mass flowmeter and Coriolis mass flowmeter

Abstract

A method for operating a Coriolis mass flowmeter having at least one measuring tube, at least one oscillation generator, at least two oscillation sensors, and at least one control and evaluation unit, the oscillation generator and the oscillation sensors being arranged on the measuring tube, wherein the measuring tube has a medium flowing through it, wherein the oscillation generator puts the measuring tube into a harmonic oscillation with the excitation frequency f.sub.0 and the excitation amplitude A.sub.0, wherein the first and the second oscillation sensors detect the oscillation of the measuring tube and wherein the first oscillation sensor forwards the oscillation to the control and evaluation unit as first measuring signal and wherein the second oscillation sensor forwards the oscillation to the control and evaluation unit as second measuring signal, and wherein at least one comparison measurement signal is determined from the first measuring signal and/or the second measuring signal.

Claims

1. A method for determining occurrence of a condition affecting accuracy of measurements produced during operation of a Coriolis mass flowmeter that comprises at least one measuring tube, at least one oscillation generator, at least two oscillation sensors, and at least one control and evaluation unit, the oscillation generator and the oscillation sensors being arranged on the measuring tube, the method comprising: directing a flow of medium through the measuring tube, causing the oscillation generator to put the measuring tube into a harmonic oscillation with an excitation frequency f.sub.0 and an excitation amplitude A.sub.0, using the first and the second oscillation sensors to detect the oscillation of the measuring tube, forwarding oscillation detected by the first oscillation sensor to the control and evaluation unit as a first measuring signal, forwarding oscillation detected by the second oscillation sensor to the control and evaluation unit as a second measuring signal, determining at least one comparison measurement signal from at least one of the first measuring signal and the second measuring signal, determining mass flow of the medium flowing through the measuring tube from the amplitude and phase of the oscillation detected by the first and the second oscillation sensors, using the excitation frequency f.sub.0 and the excitation amplitude A.sub.0 to calculate an expected value of at least one of the amplitude, phase and a variable derived therefrom at at least one evaluation frequency, wherein the evaluation frequency corresponds to the excitation frequency f.sub.0 and/or a harmonic of the excitation frequency f.sub.0, determining at least one of the amplitude, the phase and the variable derived therefrom of the comparison measurement signal exclusively at the evaluation frequency, comparing the expected value of the at least one of the amplitude, the phase and the variable derived therefrom to a corresponding value of the comparison measurement signal and determining a measure for a deviation of the comparison measurement signal from the expected value, issuing an alert when the deviation of the expected value exceeds an upper limit value or falls below a lower limit value, and determining the existence of a mechanical problem with the flowmeter when an alert is issued.

2. The method according to claim 1, wherein the mechanical problem is one of mechanical disengagement of the oscillation sensors, misalignment of the oscillation generator and misalignment of at least one of the oscillation sensors.

3. The method according to claim 1, wherein the at least one variable derived from the amplitude is at least one of power, a value for harmonic distortion, a value for the harmonic distortion including noise, a distortion factor and a signal-to-noise ratio.

4. The method according to claim 1, wherein the comparison measurement signal is one of the first measuring signal, the second measuring signal, a sum signal of the first measuring signal and the second measuring signal, a difference signal of the first and second measuring signals, and a functional relationship between the first and the second measuring signals.

5. The method according to claim 1, wherein more than one said expected value of at least one of the amplitude, the phase and the variable derived therefrom is determined, and wherein the expected values are compared to the corresponding values of the comparison measurement signal.

6. The method according to claim 5, wherein the at least one additional expected value is determined at an evaluation frequency which lies in at least one of a frequency interval around 20 to 50% of the excitation frequency around the excitation frequency f.sub.0, and a frequency interval around the harmonic of the excitation frequency f.sub.0, of 20 to 50% of the frequency of the harmonic around the frequency of the harmonic; and wherein the at least one of the amplitude, the phase and a variable derived therefrom of the comparison measurement signal is detected at the evaluation frequency and compared to the corresponding expected value.

7. The method according to claim 1, wherein the determination of at least one of the expected value of the amplitude, the phase and the variable derived therefrom, and the comparison to the corresponding values of the comparison measurement signal are performed periodically.

8. A Coriolis mass flowmeter comprising: at least one measuring tube, at least one oscillation generator on the at least one measuring tube for producing a harmonic oscillation of the at least one measuring tube with an excitation frequency f.sub.0 and an excitation amplitude A.sub.0, at least two oscillation sensors on the at least one measuring tube for detecting oscillation of the measuring tube, and at least one control and evaluation unit, wherein the first and the second oscillation sensors are connected to the at least one control and evaluation unit so as to forward the oscillation detected to the control and evaluation unit as first and second measuring signals, respectively, and wherein the at least one control and evaluation unit comprises: means for producing at least one comparison measurement signal using at least one of the first measuring signal and the second measuring signal, means for determining mass flow of a medium flowing through the at least one measuring tube from the amplitude and phase of the oscillation detected by the first and second oscillation sensors, means for calculating an expected value of at least one of amplitude, phase and at least one variable derived therefrom using the excitation frequency f.sub.0 and the excitation amplitude A.sub.0 at at least one evaluation frequency, means for determining the at least one of the amplitude and the phase and the variable derived therefrom of the comparison measurement signal exclusively at the evaluation frequency, means for comparing the expected value to a corresponding value of the comparison measurement signal and for determining a measure of deviation of the comparison measurement signal from the expected value, and means for issuing an alert when the deviation determined exceeds an upper limit value or falls below a lower limit value.

9. Coriolis mass flowmeter according to claim 8, wherein the alert represents the existence of a mechanical problem with the flowmeter.

10. Coriolis mass flowmeter according to claim 9, wherein mechanical problem is one of mechanical disengagement of the at least one of the oscillation sensors, misalignment of the oscillation generator and misalignment of at least one of the oscillation sensors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a flow chart for a first embodiment of a method according to the invention,

(2) FIG. 2 is a flow chart for a second embodiment of a method according to the invention and

(3) FIG. 3 is a schematic representation of a first embodiment of a Coriolis mass flowmeter according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) FIG. 1 shows a first embodiment of a method 1 for operating a Coriolis mass flowmeter 2, which is shown, for example, in FIG. 3. The Coriolis mass flowmeter 2 has at least one measuring tube 3, at least one oscillation generator 4, at least two oscillation sensors 5 and at least one control and evaluation unit 6, wherein the oscillation generator 4 and the oscillation sensors 5 are arranged on the measuring tube 3, wherein the measuring tube 3 has medium flowing through it.

(5) In a first step 7 of the illustrated method 1, the oscillation generator 4 puts the measuring tube 3 into a harmonic oscillation with the excitation frequency f.sub.0 and the excitation amplitude A.sub.0. It should be noted that the measuring tube 3 oscillates during the entire method 1, and, in this respect, the excitation step 7 continues during the method 1.

(6) In a next step 8, the first and second oscillation sensors 5 detect the oscillation of the measuring tube 3, and the first oscillation sensor 5 forwards the oscillation to the control and evaluation unit 6 as a first measuring signal, and the second oscillation sensor 5 forwards the oscillation to the control and evaluation unit 6 as a second measuring signal.

(7) Comparison measurement signals 10 are then determined in a further step 9 from the first measuring signal and the second measuring signal. In the illustrated embodiment, the comparison measurement signals 10 correspond to the first measuring signal.

(8) In a next step 11, the amplitude and the phase of the comparison measurement signal 10 are determined at the excitation frequency f.sub.0. At the same time, the total power P.sub.ges of the comparison measurement signal 10 is determined 12.

(9) The mass flow of the medium flowing through the measuring tube 3 is determined 13 from the measured amplitude and the phase as well as from comparison with the second measuring signal. In addition, the power P.sub.0 of the comparison measurement signal 10 is determined from the amplitude at the excitation frequency f.sub.0. The THD+N value of the comparison measurement signal 10 is determined in a next step 15 from the total power P.sub.ges and the power P.sub.0 of the comparison measurement signal 10 at the excitation frequency f.sub.0. The THD+N value is compared to a stored expected value, which has been determined earlier, in a next step 16, and the deviation between the THD+N value and the expected value is determined in step 16 by subtraction. If the deviation exceeds an upper limit value, an alert is issued in a next step 17.

(10) FIG. 2 shows a second embodiment of a method 1 for operating a Coriolis mass flowmeter 2, as described, for example, above and illustrated in FIG. 3. First, the measuring tube 3 is excited in a first step 7 to harmonic oscillation with the excitation frequency f.sub.0 and the amplitude A.sub.0. The oscillation sensors 5 detect the oscillation and forward it in step 8 to the control and evaluation unit 6 as a first and a second measuring signal. Next, a comparison measurement signal 10 is determined 9, which corresponds to the first measuring signal in the illustrated embodiment.

(11) Based on the excitation frequency f.sub.0 and the amplitude A.sub.0, an expected value for the THD value, i.e., the ratio of the powers P.sub.h to P.sub.0, is determined 18.

(12) In a subsequent step 19, the THD value of the comparison measurement signal 10 is determined. For this, the amplitude of the comparison measurement signal is determined at the excitation frequency f.sub.0 and at the second and third harmonics, and the THD value is calculated therefrom.

(13) In a next step 20, the THD value of the comparison measurement signal 10 is compared to the expected value of the THD value and a measure for the deviation is determined by subtraction.

(14) In a next step 17, an alert is issued if the deviation exceeds an upper limit value.

(15) As already described, FIG. 3 shows an embodiment of a Coriolis mass flowmeter 2. In addition to the components already described, the Coriolis mass flowmeter 2 has a display unit 21, via which the alert is issued in the event of an upper limit value being exceeded or a lower limit value being exceeded.