Method and apparatus for measuring pressure in a coriolis mass flowmeter

• US 5,734,112 A
• Filed: 08/14/1996
• Issued: 03/31/1998
• Est. Priority Date: 08/14/1996
• Status: Expired due to Term

• Current Assignee: Micro Motion Incorporated (Emerson Electric Company)
• Original Assignee: Micro Motion Incorporated (Emerson Electric Company)
• Inventors: Pankratz, Anthony William, Derby, Howard Vincent, Bose, Tamal, Levien, Andrew Keith
• Primary Examiner(s): Dombroske, George M.
• Assistant Examiner(s): AMROZOWICZ, PAUL DOUGLAS
• Application Number:
  US08/689,839
• Time in Patent Office: 594 Days
• Field of Search: 73/861.355, 73/861.356, 73/861.357, 73/861.354
• US Class Current: 73/861.56
• CPC Class Codes: G01F 1/8413 means for influencing the flowmeter's motional or vibrational behaviour, e.g., conduit support or fixing means, or conduit attachments
• CPC Class Codes: G01F 1/8431 electronic circuits
• CPC Class Codes: G01F 1/8436 signal processing
• CPC Class Codes: G01F 1/8477 with multiple measuring conduits
• CPC Class Codes: G01F 15/024 involving digital counting

Abstract

A method for determining pressure in an operating Coriolis effect mass flowmeter. The Coriolis flowmeter flow tubes are vibrated in both a bending mode (as is normal for measuring mass flow rate) and in a twisting mode. The ratio of the fundamental frequencies at which the flow tubes vibrate in each of the two vibration modes is proportional to the pressure within the flow tubes. In the preferred embodiment, a sum/difference method initially isolates the superposed sinusoids representing the fundamental frequencies of the two vibrational modes. Fast conjugate gradient (FCG) digital filters are then used to rapidly estimate the fundamental frequencies in each of the two vibration modes. The estimated frequencies are then used by filter chains including digital notch and band pass filters as well as recursive maximum likelihood (RML) digital filter techniques to enhance the bending mode and twisting mode fundamental frequency estimates. The enhanced bending mode and twisting mode frequency estimates are used to determine the pressure within the flow tubes as a function of the ratio of the two frequencies as well as to center the notch and band pass filter chains used to enhance the bending mode frequency of the two vibration sensor channels for mass flow rate computations. The pressure so determined may then be used to correct mass flow rate computations or for other pressure measurement purposes per se.

Claims

1. A method for determining the pressure within a flowmeter having vibrating flow tube means comprising the steps of:

vibrating said flow tube means of said flowmeter in a first vibration mode;

determining a first resonant frequency of said flow tube means in response to said first vibration mode of said flow tube;

vibrating said flow tube means of said flowmeter in a second vibration mode;

determining a second resonant frequency of said flow tube means in response to said second vibration mode; and

determining said pressure within said flowmeter by calculating the ratio between said first resonant frequency and said second resonant frequency.

2. The method of claim 1 wherein said pressure is determined independently of said density of said material.

3. The method of claim 1 wherein said pressure is determined at a zero mass flow rate of said material.

4. The method of claim 1 wherein said pressure is determined at mass flow rates of said material greater than zero.

5. The method of claim 1 wherein said pressure is determined independently of said density of said material.

6. The method of claim 1 wherein said pressure is determined at a zero mass flow rate of said material.

7. The method of claim 1 wherein said pressure is determined at mass flow rates of said material greater than zero.

8. The method of claim 1 wherein said flowmeter is a Coriolis mass flowmeter.

9. The method of claim 1 wherein said flowmeter is a vibrating tube densitometer.

10. The method of claim 1 wherein said first vibration mode vibrates said flow tube means in a bending mode and wherein said second vibration mode vibrate said flow tube means in a twisting mode.

11. The method of claim 1 further comprising the steps of:

determining an uncorrected mass flow rate corresponding to material flowing through said flowmeter; and

deriving a corrected mass flow rate for material flowing through said flowmeter in response to the determination of said uncorrected mass flow rate and said pressure determination.

12. The method of claim 1 wherein the step of determining said first resonant frequency includes the steps of:

sensing a first signal generated by motion of a first sensor associated with the vibrating flow tube means of said flowmeter;

sensing a second signal generated by motion of a second sensor associated with the vibrating flow tube means of said flowmeter; and

filtering said first signal and said second signal to extract a signal component corresponding to said first resonant frequency.

13. The method of claim 12 wherein said first vibration mode vibrates said flow tubes in a bending mode.

14. The method of claim 13 wherein the step of filtering includes the steps of:

adding said first signal to said second signal to produce an isolated signal having a strong frequency component in the bending mode frequency; and

enhancing said isolated signal to eliminate undesired components in said isolated signal to produce an enhanced signal in the bending mode frequency.

15. The method of claim 13 further comprising the steps of:

determining an uncorrected mass flow rate corresponding to material flowing through said flowmeter in response to said vibrations of said flow tube means in said bending mode; and

deriving a corrected mass flow rate for material flowing through said flowmeter in response to the determination of said uncorrected mass flow rate and said pressure determination.

16. The method of claim 1 wherein the step of determining said second resonant frequency includes the steps of:

sensing a first signal generated by motion of a first sensor attached to the vibrating flow tube means of said flowmeter;

sensing a second signal generated by motion of a second sensor attached to the vibrating flow tube means of said flowmeter; and

filtering said first signal and said second signal to extract a signal component corresponding to said second resonant frequency.

17. The method of claim 16 wherein said second vibration mode vibrates said flow tube means in a twisting mode.

18. The method of claim 17 wherein the step of filtering said first signal and said second signal includes the steps of:

subtracting said second signal from said first signal to produce an isolated signal having a strong frequency component in the twisting mode frequency; and

enhancing said isolated signal to eliminate undesired components in said isolated signal to produce an enhanced twisting mode frequency signal.

19. The method of claim 1 wherein said flow tube means is vibrated concurrently in both said first vibration mode and said second vibration mode.

20. The method of claim 1 wherein said flow tube means is vibrated sequentially, one mode at a time in said first vibration mode and said second vibration mode.

21. The method of claim 1 wherein said step of determining said pressure includes:

determining a ratio between said first resonant frequency and said second resonant frequency;

compensating said ratio for changes in said ratio caused by changes in a parameter of said flowmeter; and

wherein said pressure is determined in response to said compensated ratio.

22. The method of claim 21 wherein said parameter is the mounting condition of said flowmeter and wherein said compensating step comprises:

determining a first calibration ratio when said flowmeter is calibrated at a first mounting condition;

determining a second calibration ratio when said flowmeter is installed in its intended application at a second mounting condition;

generating a ratio compensation value in response to said determination of said first calibration ratio and said second calibration ratio;

compensating said ratio with said ratio compensation value; and

wherein said pressure is determined in response to said compensated ratio.

23. The method of claim 21 wherein said parameter is the temperature of said vibrating flow tube means and wherein said compensating step comprises:

determining a first calibration ratio when said flowmeter is calibrated at a first temperature;

determining a second calibration ratio when said flowmeter is calibrated at a second temperature;

generating a ratio compensation value in response to said determination of said first calibration ratio and said second calibration ratio;

measuring the temperature of said vibrating flow tube means;

compensating said ratio with said ratio compensation value in response to said measured temperature; and

wherein said pressure is determined in response to said compensated ratio.

24. The method of claim 21 wherein said parameter is the density of said material in said vibrating flow tube means and wherein said compensating step comprises:

determining a first calibration ratio when said flowmeter is calibrated with said material having a first density;

determining a second calibration ratio when said flowmeter is calibrated with said material having a second density;

generating a ratio compensation value in response to said determination of said first calibration ratio and said second calibration ratio;

measuring the density of said material in said vibrating flow tube means;

compensating said ratio with said ratio compensation value in response to said measured density; and

wherein said pressure is determined in response to said compensated ratio.

25. The method of claim 11 wherein said step of determining said ratio includes:

determining a first calibration ratio when said flowmeter is calibrated with said material having a first mass flow rate;

determining a second calibration ratio when said flowmeter is calibrated with said material having a second mass flow rate;

generating a ratio compensation value in response to said determination of said first calibration ratio and said second calibration ratio;

compensating said ratio with said ratio compensation value in response to said corrected mass flow rate; and

wherein said pressure is determined in response to said compensated ratio.

26. An apparatus for determining pressure within a flowmeter having vibrating tube means comprising:

means for vibrating said flow tube means in a first vibration mode;

means for vibrating said flow tube means in a second vibration mode;

sensor means attached to said vibrating flow tube means operable to generate signals responsive to the motion of said flow tube means;

means responsive to said sensor means for determining a first frequency of said first vibration mode;

means responsive to said sensor means for determining a second frequency of said second vibration mode;

determining a ratio between said first frequency and said second frequency;

measuring the density of said material in said vibrating flow tube means;

compensating said ratio in response to said measured density; and

determining said pressure in response to said compensated ratio.

27. A method for determining the pressure within a flowmeter having vibrating flow tube means comprising the steps of:

vibrating said flow tube means of said flowmeter in multiple vibration modes, a first vibration mode being sensitive to changes in said pressure within said flowmeter and a second vibration mode being substantially insensitive to changes in said pressure within said flowmeter;

determining a first resonant frequency of said flow tube means in response to said first vibration mode of said flow tube; and

determining said pressure within said flowmeter in response to the determination of said first resonant frequency.