METHOD FOR OPERATING A CORIOLIS MASS FLOWMETER

Abstract

A method for operating a Coriolis mass flowmeter that has at least one measuring tube with medium flowing through it involves exciting the measuring tube excited to oscillation, detecting the oscillations of the measuring tube and determining the density of the medium. Detection of the state and a change in the state of a Coriolis mass flowmeter is achieved by determining a calibration temperature and a calibration density sensitivity of the Coriolis mass flowmeter using the detected oscillations, at a temperature differing from the calibration temperature, and a density sensitivity of the flowmeter determined using the detected oscillations. A measurement rate of change of the density sensitivity is determined and a forecast rate of change of the density sensitivity is calculated using a forecast algorithm, and at a given deviation of the measurement rate of change from the forecast rate of change r.sub.p, a deviation signal is generated.

Claims

1. A method for operating a Coriolis mass flowmeter, wherein the Coriolis mass flowmeter has at least one measuring tube with medium flowing through it, and an electronic control and evaluation unit for controlling the measuring tube with a control signal and for setting excitation frequencies, comprising: exciting the at least one measuring tube to oscillation in at least one frequency and/or in at least one eigenform, using oscillation sensors for detecting oscillations of the measuring tube, and using the electronic control and evaluation unit for determining a density d of the medium by evaluating the detected oscillations, wherein: at a calibration temperature T.sub.k, a calibration density sensitivity E.sub.d,k(T.sub.k) of the Coriolis mass flowmeter is determined by the electronic control and evaluation unit using oscillations of the measuring tube detected by the oscillation sensors, at a temperature T that differs from the calibration temperature T.sub.k and which is independent of the detected oscillations of the measuring tube, a density sensitivity E.sub.d(T) of the Coriolis mass flowmeter is determined by the electronic control and evaluation unit using the oscillations of the measuring tube detected by the oscillation sensors, a measurement rate of change r.sub.m of density sensitivity E.sub.d is determined by the electronic control and evaluation unit using the calibration density sensitivity E.sub.d,k(T.sub.k) determined using the detected oscillations and the density sensitivity E.sub.d(T) at the temperature T, a forecast rate of change r.sub.p of the density sensitivity is calculated by the control and evaluation unit using a forecast algorithm which is dependent on the temperature T, and at a given deviation of the measurement rate of change r.sub.m from the forecast rate of change r.sub.p, a deviation signal is generated by the electronic control and evaluation unit which indicates that a change in a dynamic system behavior of the Coriolis mass flowmeter has occurred.

2. A method according to claim 1, wherein the change in the dynamic system behavior indicates a structural change of the measuring tube due to at least one of wear of the measuring tube or deposition of material in the measuring tube.

3. A method according to claim 1, wherein the measurement rate of change r.sub.m is determined by formation of a quotient from the density sensitivity E.sub.d(T) determined at the temperature T and from the density sensitivity E.sub.d,k(T.sub.k) determined at the calibration temperature T.sub.k using the electronic control and evaluation unit.

4. A method according to claim 1, wherein the density sensitivity E.sub.d is determined by means of a mathematical model G.sub.1(s) of the Coriolis mass flowmeter of at least second order in that the measuring tube is excited to oscillation in a first eigenform at an eigenfrequency F.sub.01 of the first eigenform and at two additional frequencies f.sub.ZA and f.sub.ZB.

5. A method according to claim 1, wherein the forecast algorithm for the forecast rate of change r.sub.p is a polynomial in the temperature difference of the temperature T differing from the calibration temperature T.sub.k and the calibration temperature T.sub.k.

6. A method according to claim 5, wherein the factor r.sub.p0 of a linear member of the temperature difference (T−T.sub.k) is determined by determining at least two density sensitivities E.sub.d at least two different temperatures, of which one temperature is the calibration temperature T.sub.k.

7. A method according to claim 1, wherein the method is carried out in a test mode, in which the measuring tube is excited to oscillation with an amplitude that is less than in the measurement operating mode.

8. A method according to claim 1, wherein evaluation of the oscillation of the measuring tube in a second eigenform takes place simultaneously with determination of the mass flow through the measuring tube.

9. A method according to claim 1, wherein at least one of the method steps of determining the calibration density sensitivity E.sub.d,k(T.sub.k) (100), determining the density sensitivity E.sub.d(T) at a temperature T differing from the calibration temperature T.sub.k, determining the measurement rate of change r.sub.m of the density sensitivity E.sub.d, determining the forecast rate of change r.sub.p of the density sensitivity using a forecast algorithm, and generating a deviation signal is carried out in a test device attached to the Coriolis mass flowmeter.

10. A method according to claim 1, wherein determining the calibration density sensitivity E.sub.d,k(T.sub.k) and determining the forecast algorithm for the forecast rate of change r.sub.p are carried out during factory calibration of the Coriolis mass flowmeter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 schematically shows a Coriolis mass flowmeter, with which the claimed method is carried out,

[0029] FIG. 2 is a flow chart of the claimed method for operation a Coriolis mass flowmeter, and

[0030] FIG. 3 is a flow chart, enhanced compared to the representation in FIG. 2, for illustrating the claimed method.

DETAILED DESCRIPTION OF THE INVENTION

[0031] In FIGS. 1 to 3, a method for operating a Coriolis mass flowmeter 2 is basically shown, wherein FIG. 1 essentially addresses the structural design of the Coriolis mass flowmeter 2. The Coriolis mass flowmeter 2 has a measuring tube 3 with a medium that is not shown in detail flowing through it.

[0032] The measuring tube 3 is excited to oscillations in at least one frequency and/or in at least one eigenform. An electromagnetic actuator 4 arranged centrally in respect to the longitudinal extension of the measuring tube 3 is used for this, with which a force can be exerted centrally on the measuring tube 3, with which the measuring tube 3 is excited into a first oscillation mode. When medium is flowing through the measuring tube 3, an oscillation of the measuring tube 3 in the second oscillation mode is automatically engaged due to the operative Coriolis forces, this is sometimes also called the Coriolis mode.

[0033] The resulting oscillations of the measuring tube 3 can be detected by means of the oscillation sensors 5 arranged to the left and right of the actuator 4. Different measuring variables can be determined by evaluating the measuring tube oscillations. The primary measuring variable, i.e., the mass flow, can be determined by evaluating the oscillations in the second eigenform of the measuring tube 3. The density of the medium can be determined by evaluating the measuring tube oscillations in the first eigenform, as is generally known.

[0034] In addition to the mechanical construction, the Coriolis mass flowmeter 2 also has an electronic control and evaluation unit 6, which is usually integrated in a standard housing of the Coriolis mass flowmeter 2, which is not shown here. The control and evaluation unit 6 is used, on the one hand, for properly controlling the measuring tube 3 of the Coriolis mass flowmeter 2 with a control signal, for updating the excitation frequency into the resonance frequency of the measuring tube 3, for setting a frequency intentionally deviating from the resonance frequency, for evaluating the sensor signal of the oscillation sensors 5 as well as for carrying out different methods for operating the Coriolis mass flowmeter such as determining primary measuring variables, determining secondary measuring variables and observing the Coriolis mass flowmeter 2. To this end, the control and evaluation unit 6 is provided with an electronic computing unit 7 and, in the present case, with a display unit 8. Data can be exchanged between the Coriolis mass flowmeter 2 and an external device 10 via an interface 9. The external device 10 can be a control center, an operating device or also a test device.

[0035] The claimed method for operating the Coriolis mass flowmeter 2 is shown in FIGS. 2 and 3. The idea of the method consists of determining rates of change r of the density sensitivity E.sub.d for testing the Coriolis mass flowmeter 2 and a deviation from the rate of change r being an indication of a change of the dynamic behavior of the Coriolis mass flowmeter 2.

[0036] To this end, a rate of change r of the density sensitivity E.sub.d, namely the measurement rate of change r.sub.m is determined in dependence on the detected oscillations of the measuring tube 3, so that changed system dynamics also affect the determination of the measurement rate of change r.sub.m.

[0037] Another rate of change of the density sensitivity, namely the forecast rate of change r.sub.p, is also determined, wherein the forecast algorithm for calculating this forecast rate of change r.sub.p is not dependent on the detected oscillations of the measuring tube 3 and thus not on changed system dynamics of the Coriolis mass flowmeter 2. In this manner, conclusions can be made about the state or, respectively about a changed state of the Coriolis mass flowmeter 2 based on the determination of the rate of the change of the sensitivity of density measurement.

[0038] The measuring tube 3 is shown initially schematically and representatively for the entire Coriolis mass flowmeter 2 in FIGS. 2 and 3. Oscillations are detected from the measuring tube 3 that have one or several frequencies Furthermore, the measuring tube has a temperature T.

[0039] In method step 100, the calibration density sensitivity E.sub.d,k of the Coriolis mass flowmeter 2 is determined at a calibration temperature T.sub.k using the detected oscillations of the measuring tube 3. This is indicated in FIGS. 2 and 3 by the detected measuring tube oscillation with the frequency f.sub.k and the temperature T.sub.k.

[0040] In the method step 110, the density sensitivity E.sub.d(T) of the Coriolis mass flowmeter 2 is determined at a temperature T differing from the calibration temperature T.sub.k using the detected oscillations of the measuring tube 3. Since the determination of the density sensitivity E.sub.d(T) takes place using the detected oscillations of the measuring tube 3, the density sensitivity E.sub.d(T) is dependent on the dynamic behavior of the Coriolis mass flowmeter 2 or, respectively, the measuring tube 3.

[0041] In method step 120, using the previously-obtained data, a measurement rate of change r.sub.m of the density sensitivity E.sub.d is determined from the calibration density sensitivity E.sub.d,k(T.sub.k) and the density sensitivity E.sub.d(T). In the present case, the quotient of these two density sensitivities is formed.

[0042] In the subsequentially shown method step 130, a forecast rate of change r.sub.p of the density sensitivity E.sub.d is calculated with a forecast algorithm, which, namely, is dependent on the temperature T differing from the calibration temperature T.sub.k, however is not dependent on the detected oscillations of the measuring tube 3, which is indicated in that only the temperature T affects the determination of the forecast rate of change r.sub.p.

[0043] Finally, in the method step 140, a deviation of the measurement rate of change r.sub.m from the forecast rate of change r.sub.p is determined and, in the case of a deviation or the exceedance of a certain deviation threshold, a deviation signal is generated.

[0044] In FIGS. 2 and 3, a sequence of all method steps 100, 110, 120, 130, 140 is shown. This does not necessarily have to be so. In fact, the forecast rate of change r.sub.p can be calculated independently from the previously shown method steps, i.e., for example, simultaneously with these method steps. It is only required for the last method step of determining a deviation of the measurement rate of change r.sub.m from the forecast rate of change r.sub.p that these rates of change are known in full.

[0045] It is shown in FIG. 3 that the determination of the calibration density sensitivity E.sub.d,k(T.sub.k) in method step 100 and the determination of the forecast algorithm for determining the forecast rate of change r.sub.p of the density sensitivity E.sub.d in preliminarily take place in a method step 150, preferably during calibration of the Coriolis mass flowmeter 2, for example during initial factory calibration. The corresponding data are then preferably stored in the Coriolis mass flowmeter, since they characterize the individual Coriolis mass flowmeter.