Coriolis mass flow rate meter having a substantially increased noise immunity

• US 4,934,196 A
• Filed: 06/02/1989
• Issued: 06/19/1990
• Est. Priority Date: 06/02/1989
• Status: Expired due to Term

• Current Assignee: Micro Motion Incorporated (Emerson Electric Company)
• Original Assignee: Micro Motion Incorporated (Emerson Electric Company)
• Inventors: Romano, Paul
• Primary Examiner(s): Goldstein, Herbert
• Application Number:
  US07/361,000
• Time in Patent Office: 382 Days
• Field of Search: 73/861.37, 73/861.38, 73/510
• US Class Current: 73/861.356
• 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

Apparatus and accompanying methods for use therein for a Coriolis mass flow rate meter which is substantially immune to noise, and more particularly, to such a meter that is substantially unaffected by noise that occurs at substantially any frequency different from a fundamental frequency at which the flow tube(s) in the meter vibrate. Specifically, the meter relies on measuring mass flow rate by determining the phase difference that occurs between real and imaginary components of the discrete fourier transform (DFT) of both the left and right velocity sensor waveforms evaluated at the fundamental frequency at which the flow tubes vibrate. The fundamental frequency is located, during an initialization operation, by providing a power spectrum, determined through use of the DFT, of one of the sensor signals and then selecting that frequency at which the magnitude of the power spectrum reaches a maximum value. In addition, the frequency at which both velocity sensor signals is sampled is readjusted in response to any change in the phase of one of the velocity sensor signals, as transformed using the DFT, in order to assure that the sampling frequency always remains substantially equal to a pre-defined integer multiple of the fundamental frequency. Furthermore, the meter, through use of the numerical value of any such phase change, also provides a density indication which is also substantially immune to such noise.

Claims

1. Apparatus for measuring mass flow rate of a fluid using a Coriolis meter having a flow conduit with first and second sensors associated therewith for providing first and second sensor signals indicative of movement of the flow tube, said apparatus comprising:

means for driving said flow conduit in a sinusoidal vibratory pattern and at a resonant frequency thereof while said fluid flows therethrough;

means for sampling said first and second sensor signals at a sampling frequency to yield first and second sampled sensor signals;

means, responsive to one of the sampled sensor signals, for producing a power spectrum for a pre-selected sequence of frequencies contained within said one sampled sensor signal and for selecting a particular one of said frequencies in said sequence at which said power spectrum reaches a maximum value as being a fundamental frequency at which said conduit is resonantly vibrating;

means, responsive to said selecting means and connected to said sampling means, for setting the sampling frequency equal to a pre-defined multiple of said one frequency;

means, responsive to said first and second sampled sensor signals, for transforming said first and second sensor signals using a pre-defined transformation from a time domain to a frequency domain so as to yield corresponding first and second frequency values, wherein said first and second frequency values are produced by separately evaluating said transformation for said first and second sensor sampled signals at the fundamental frequency;

means responsive to said first and second frequency values for determining a phase difference occurring therebetween; and

means, responsive to said phase difference, for providing a value of the mass flow rate of said fluid.

2. The apparatus in claim 1 wherein the transformation is a Fourier, Discrete Fourier, Fast Fourier, Hanning or Hilbert transform.

3. The apparatus in claim 2 wherein said producing and selecting means comprises:

means, responsive to said one sampled sensor signal, for transforming said one sampled sensor signal using the pre-defined transformation into a corresponding frequency value;

first means for determining a magnitude of the corresponding frequency value produced by the transformation at each successive frequency in a series of frequencies occurring at a first resolution within a pre-defined range and situated within said sequence;

means, responsive to said first determining means, for locating a first frequency within said series and occurring at said first resolution at which the magnitude of said corresponding frequency value reaches a maximum value;

second means for determining the magnitude of the corresponding frequency value for each successive frequency in a second series of frequencies occurring at a second resolution and situated within a band of frequencies centered at said first located frequency and extending to corresponding limit frequencies situated on respective sides thereof as defined by said first resolution, wherein said second resolution is higher than said first resolution; and

means, responsive to said second determining means, for locating a second frequency occurring at said second resolution at which the magnitude of said corresponding frequency value reaches a maximum magnitude and selecting said second located frequency as said fundamental frequency.

4. The apparatus in claim 3 wherein said first magnitude determining means comprises:

means for successively varying a frequency index throughout the pre-defined range by an incremental value specified by said first resolution; and

means for determining the magnitude of the corresponding frequency value at each successive value of the frequency index occurring at said first resolution within said pre-defined range while maintaining the sampling frequency at a constant value.

5. The apparatus in claim 4 wherein said first and second frequency values have first and second real and imaginary components, respectively, and wherein said phase difference determining means comprises means, responsive to said first and second real and imaginary components, for producing said phase difference, and wherein said mass flow rate providing means comprises:

means for providing a time interval value which defines a time interval that occurs between a time when the velocity of a pre-defined point on said flow conduit reaches a pre-defined reference value and a time when the velocity of a second pre-defined point situated on said flow conduit and oppositely located from said first point reaches a corresponding pre-defined reference value and wherein said time difference is proportional to the value of the mass flow rate of the fluid; and

means for generating said mass flow rate value in response to said time interval value.

6. The apparatus in claim 5 wherein said sampling means comprises means for sampling the first and second sensor signals on an interleaved basis at a sampling frequency of "128" times the fundamental frequency so as to produce 32 different discrete frequency components of each of said first and second sampled sensor signals.

7. The apparatus in claim 3 wherein said second magnitude determining means comprises:

means for successively varying the sampling frequency by an incremental value specified by said second resolution from the first one of the limit frequencies in said second series to the second one of the limit frequencies in said second series; and

means for determining the magnitude of the corresponding frequency value at each successive frequency in said second series.

8. The apparatus in claim 7 wherein said first and second frequency values have first and second real and imaginary components, respectively, and wherein said phase difference determining means comprises means, responsive to said first and second real and imaginary components, for producing said phase difference, and wherein said mass flow rate providing means comprises:

means for providing a time interval value which defines a time interval that occurs between a time when the velocity of a pre-defined point on said flow conduit reaches a pre-defined reference value and a time when the velocity of a second pre-defined point situated on said flow conduit and oppositely located from said first point reaches a corresponding pre-defined reference value and wherein said time difference is proportional to the value of the mass flow rate of the fluid; and

means for generating said mass flow rate value in response to said time interval value.

9. The apparatus in claim 8 wherein said sampling means comprises means for sampling the first and second sensor signals on an interleaved basis at a sampling frequency of "128" times the fundamental frequency so as to produce 32 different discrete frequency components of each of said first and second sampled sensor signals.

10. The apparatus in claim 2 further comprising:

means, responsive to either said first or second frequency value, for ascertaining a phase value of the corresponding first or second frequency value; and

means, responsive to the ascertained phase value, for varying the sampling frequency in order to compensate for a change in the fundamental frequency caused by a substantially simultaneously occurring variation in density of the fluid flowing through the meter, whereby the sampling frequency substantially remains at the pre-defined multiple of the fundamental frequency.

11. The apparatus in claim 10 wherein said sampling frequency varying means further comprises:

means for multiplying the determined phase value of the first or second frequency value by a pre-determined constant to yield a scaled phase value; and

means for varying the sampling frequency in response to said scaled phase value.

12. Apparatus for a Coriolis mass flow rate meter comprising:

a Coriolis metering assembly for measuring the mass flow rate of a fluid flowing therethrough, said assembly comprising;

first and second flow tubes;

inlet and outlet manifolds for respectively conducting said fluid into and out of both of said flow tubes, wherein said inlet manifold divides the fluid between both of said tubes and said outlet manifold combines the fluid exiting from both of said tubes;

first and second mounting blocks, which receive corresponding ends of both of said flow tubes and are fixedly secured to said inlet and outlet manifolds, for respectively dividing the fluid emanating from said inlet manifold and routing said divided fluid into both of said flow tubes or combining the fluid emanating from both of said flow tubes and routing the combined fluid into said outlet manifold,

means, responsive to a drive signal, for vibrating both of said flow tubes in an opposing sinusoidal pattern and at a resonant frequency thereof while said fluid flows therethrough; and

first and second sensors for sensing movement of both of said flow tubes caused by opposing Coriolis forces induced by passage of the fluid through said flow tubes and for producing first and second sensor signals responsive to sensed movement; and

circuit means, responsive to said sensor signals, for providing a measured value of the mass flow rate of the fluid that is passing through said metering assembly, said circuit means comprising;

means, responsive to at least one of said sensor signals, for producing said drive signal;

means for sampling said first and second sensor signals at a sampling frequency to yield first and second sampled sensor signals;

means, responsive to one of the sampled sensor signals, for producing a power spectrum for a pre-selected sequence of frequencies contained within said one sampled sensor signal and for selecting a particular one of said frequencies in said sequence at which said power spectrum reaches a maximum value as being a fundamental frequency at which said conduit is resonantly vibrating;

means, responsive to said selecting means and connected to said sampling means, for setting the sampling frequency equal to a pre-defined multiple of said one frequency;

means, responsive to said first and second sampled sensor signals, for transforming said first and second sensor signals using a pre-defined transformation from a time domain to a frequency domain so as to yield corresponding first and second frequency values, wherein said first and second frequency values are produced by separately evaluating said transformation for said first and second sensor sampled signals at the fundamental frequency;

means responsive to said first and second frequency values for determining a phase difference occurring therebetween; and

means, responsive to said phase difference, for providing a value of the mass flow rate of said fluid.

13. The apparatus in claim 12 wherein the transformation is a Fourier, Discrete Fourier, Fast Fourier, Hanning or Hilbert transform.

14. The apparatus in claim 13 wherein said producing and selecting means comprises:

means, responsive to said one sampled sensor signal, for transforming said one sampled sensor signal using the pre-defined transformation into a corresponding frequency value;

first means for determining a magnitude of the corresponding frequency value produced by the transformation at each successive frequency in a series of frequencies occurring at a first resolution within a pre-defined range and situated within said sequence;

means, responsive to said first determining means, for locating a first frequency within said series and occurring at said first resolution at which the magnitude of said corresponding frequency value reaches a maximum value;

second means for determining the magnitude of the corresponding frequency value for each successive frequency in a second series of frequencies occurring at a second resolution and situated within a band of frequencies centered at said first located frequency and extending to corresponding limit frequencies situated on respective sides thereof as defined by said first resolution, wherein said second resolution is higher than said first resolution; and

means, responsive to said second determining means, for locating a second frequency occurring at said second resolution at which the magnitude of said corresponding frequency value reaches a maximum magnitude and selecting said second located frequency as said fundamental frequency.

15. The apparatus in claim 14 wherein said first magnitude determining means comprises:

means for successively varying a frequency index throughout the pre-defined range by an incremental value specified by said first resolution; and

means for determining the magnitude of the corresponding frequency value at each successive value of the frequency index occurring at said first resolution within said pre-defined range while maintaining the sampling frequency at a constant value.

16. The apparatus in claim 14 wherein said second magnitude determining means comprises:

means for successively varying the sampling frequency by an incremental value specified by said second resolution from the first one of the limit frequencies in said second series to the second one of the limit frequencies in said second series; and

means for determining the magnitude of the corresponding frequency value at each successive frequency in said second series.

17. The apparatus in claim 13 further comprising:

means, responsive to either said first or second frequency value, for ascertaining a phase value of the corresponding first or second frequency value; and

means, responsive to the ascertained phase value, for varying the sampling frequency to compensate for a change in the fundamental frequency caused by a substantially simultaneously occurring variation in density of the fluid flowing through the meter, whereby the sampling frequency substantially remains at the pre-defined multiple of the fundamental frequency.

18. The apparatus in claim 17 wherein said sampling frequency varying means further comprises:

means for multiplying the determined phase value of the first or second frequency value by a pre-determined constant to yield a scaled phase value; and

means for varying the sampling frequency in response to said scaled phase value.

19. A method for measuring mass flow rate of a fluid using a Coriolis meter having a flow conduit with first and second sensors associated therewith for providing first and second sensor signals indicative of movement of the flow tube, said apparatus comprising the steps of:

driving said flow conduit in a sinusoidal vibratory pattern and at a resonant frequency thereof while said fluid flows therethrough;

sampling said first and second sensor signals at a sampling frequency to yield first and second sampled sensor signals;

producing, in response to one of the sampled sensor signals, a power spectrum for a pre-selected sequence of frequencies contained within said one sampled sensor signal and selecting a particular one of said frequencies in said sequence at which said power spectrum reaches a maximum value as being a fundamental frequency at which said conduit is resonantly vibrating;

setting the sampling frequency equal to a pre-defined multiple of said one frequency;

transforming said first and second sensor signals using a pre-defined transformation from a time domain to a frequency domain so as to yield corresponding first and second frequency values, wherein said first and second frequency values are produced by separately evaluating said transformation for said first and second sensor sampled signals at the fundamental frequency;

determining, in response to said first and second frequency values, a phase difference occurring therebetween; and

providing, in response to said phase difference, a value of the mass flow rate of said fluid.

20. The method in claim 19 wherein the transformation is a Fourier, Discrete Fourier, Fast Fourier, Hanning or Hilbert transform.

21. The method in claim 20 wherein said producing and selecting step comprises the steps of:

transforming said one sampled sensor signal using the pre-defined transformation into a corresponding frequency value;

first determining a magnitude of the corresponding frequency value produced by the transformation at each successive frequency in a series of frequencies occurring at a first resolution within a pre-defined range and situated within said sequence;

locating, in response to said first determining step, a first frequency within said series and occurring at said first resolution at which the magnitude of said corresponding frequency value reaches a maximum value;

second determining the magnitude of the corresponding frequency value for each successive frequency in a second series of frequencies occurring at a second resolution and situated within a band of frequencies centered at said first located frequency and extending to corresponding limit frequencies situated on respective sides thereof as defined by said first resolution, wherein said second resolution is higher than said first resolution; and

locating, in response to said second determining step, a second frequency occurring at said second resolution at which the magnitude of said corresponding frequency value reaches a maximum magnitude and selecting said second located frequency as said fundamental frequency.

22. The method in claim 21 wherein said first magnitude determining step comprises the steps of:

successively varying a frequency index throughout the pre-defined range by an incremental value specified by said first resolution; and

determining the magnitude of the corresponding frequency value at each successive value of the frequency index occurring at said first resolution within said pre-defined range while maintaining the sampling frequency at a constant value.

23. The method in claim 22 wherein said first and second magnitude determining steps each further comprise the step of determining the sum of the square of the values of the real and imaginary components of the corresponding frequency value produced by the transformation.

24. The method in claim 23 wherein said first and second frequency values have first and second real and imaginary components, respectively, and wherein said phase difference determining step comprises the step of producing, in response to said first and second real and imaginary components, said phase difference, and wherein said mass flow rate providing step comprises the steps of:

providing a time interval value which defines a time interval that occurs between a time when the velocity of a pre-defined point on said flow conduit reaches a pre-defined reference value and a time when the velocity of a second pre-defined point situated on said flow conduit and oppositely located from said first point reaches a corresponding pre-defined reference value and wherein said time difference is proportional to the value of the mass flow rate of the fluid; and

generating said mass flow rate value in response to said time interval value.

25. The method in claim 24 wherein the pre-defined multiple is a pre-defined integer multiple.

26. The method in claim 25 wherein said sampling step comprises the step of sampling the first and second sensor signals on an interleaved basis at a sampling frequency of "128" times the fundamental frequency so as to produce 32 different discrete frequency components of each of said first and second sampled sensor signals.

27. The method in claim 21 wherein said second magnitude determining step comprises the steps of:

successively varying the sampling frequency by an incremental value specified by said second resolution from the first one of the limit frequencies in said second series to the second one of the limit frequencies in said second series; and

determining the magnitude of the corresponding frequency value at each successive frequency in said second series.

28. The method in claim 27 wherein said first and second magnitude determining steps each further comprise the step of determining the sum of the square of the values of the real and imaginary components of the corresponding frequency value produced by the transformation.

29. The method in claim 28 wherein said first and second frequency values have first and second real and imaginary components, respectively, and wherein said phase difference determining step comprises the step of producing, in response to said first and second real and imaginary components, said phase difference, and wherein said mass flow rate providing step comprises the steps of:

providing a time interval value which defines a time interval that occurs between a time when the velocity of a pre-defined point on said flow conduit reaches a pre-defined reference value and a time when the velocity of a second pre-defined point situated on said flow conduit and oppositely located from said first point reaches a corresponding pre-defined reference value and wherein said time difference is proportional to the value of the mass flow rate of the fluid; and

generating said mass flow rate value in response to said time interval value.

30. The method in claim 29 wherein the pre-defined multiple is a pre-defined integer multiple.

31. The method in claim 30 wherein said sampling step further comprises the step of sampling the first and second sensor signals on an interleaved basis at a sampling frequency of "128" times the fundamental frequency so as to produce 32 different discrete frequency components of each of said first and second sampled sensor signals.

32. The method in claim 20 further comprising the steps of:

ascertaining, in response to either said first or second frequency value, a phase value of the corresponding first or second frequency value; and

varying the sampling frequency, in response to the ascertained phase value, in order to compensate for a change in the fundamental frequency caused by a substantially simultaneously occurring variation in density of the fluid flowing through the meter, whereby the sampling frequency substantially remains at the pre-defined multiple of the fundamental frequency.

33. The method in claim 32 wherein said sampling frequency varying step further comprises the steps of:

multiplying the determined phase value of the first or second frequency value by a pre-determined constant to yield a scaled phase value; and

varying the sampling frequency in response to said scaled phase value.

34. The method in claim 33 wherein the pre-defined multiple is a pre-defined integer multiple.

35. A method for use in a mass flow rate metering system for determining mass flow rate of a fluid passing therethrough, wherein said system utilize a Coriolis meter having a flow conduit with first and second sensors associated therewith for providing first and second sensor signals indicative of movement of said flow conduit induced by Coriolis forces caused by movement of the fluid through the conduit, said method comprising the steps of:

driving the flow conduit in a sinusoidal vibratory pattern and at a resonant frequency thereof while said fluid flows therethrough;

sampling said first and second sensor signals at a sampling frequency to yield first and second sampled sensor signals;

during initialization;

setting the sampling frequency to an initial pre-defined value;

producing, in response to one of the sensor signals sampled at said initial value, a power spectrum for a pre-selected sequence of frequencies contained within said one sampled sensor signal and selecting a particular one of said frequencies in said sequence at which said power spectrum reaches a maximum value as being a fundamental frequency at which said conduit is resonantly vibrating; and

setting the sampling frequency to a second sampling frequency value, equal to a pre-defined integer multiple of said one frequency, for initial use during mass flow rate measurement;

during mass flow rate measurement;

transforming said first and second sensor signals using a pre-defined transformation from a time domain to a frequency domain so as to yield corresponding first and second frequency values, wherein said first and second frequency values are produced by separately evaluating said transformation at the fundamental frequency for said first and second sampled sensor signals;

determining in response to said first and second frequency values a phase difference occurring therebetween;

providing a value of the mass flow rate of the fluid in response to said phase difference;

ascertaining, in response to either said first or second frequency value, a phase value of the corresponding first or second frequency value; and

varying the sampling frequency, in response to the ascertained phase value, in order to compensate for a change in the fundamental frequency caused by a substantially simultaneously occurring variation in density of the fluid flowing through the meter, and

iteratively repeating the sensor signals transforming, phase difference determining, mass flow rate value providing, phase value ascertaining and sampling frequency varying steps so as to provide a succession of mass flow rate measurement values, whereby the sampling frequency that is used to produce substantially all of these mass flow rate measurement values substantially remains at the pre-defined integer multiple of the fundamental frequency.

36. The method in claim 35 wherein the transformation is a Fourier, Discrete Fourier, Fast Fourier, Hanning or Hilbert transform.

37. The method in claim 36 wherein said producing and selecting step comprises the steps of:

transforming said one sampled sensor signal using the pre-defined transformation into a corresponding frequency value;

first determining a magnitude of the corresponding frequency value produced by the transformation at each successive frequency in a series of frequencies occurring at a first resolution within a pre-defined range and situated within said sequence;

locating, in response to said first determining step, a first frequency within said series and occurring at said first resolution at which the magnitude of said corresponding frequency value reaches a maximum value;

second determining the magnitude of the corresponding frequency value for each successive frequency in a second series of frequencies occurring at a second resolution and situated within a band of frequencies centered at said first located frequency and extending to corresponding limit frequencies situated on respective sides thereof as defined by said first resolution, wherein said second resolution is higher than said first resolution; and

locating, in response to said second determining step, a second frequency occurring at said second resolution at which the magnitude of said corresponding frequency value reaches a maximum magnitude and selecting said second located frequency as said fundamental frequency.

38. The method in claim 37 wherein said first magnitude determining step comprises the steps of:

successively varying a frequency index throughout the pre-defined range by an incremental value specified by said first resolution; and

determining the magnitude of the corresponding frequency value at each successive value of the frequency index occurring at said first resolution within said pre-defined range while maintaining the sampling frequency at a constant value.

39. The method in claim 37 wherein said second magnitude determining step comprises the steps of:

successively varying the sampling frequency by an incremental value specified by said second resolution from the first one of the limit frequencies in said second series to the second one of the limit frequencies in said second series; and

determining the magnitude of the corresponding frequency value at each successive frequency in said second series.