Drive circuit modal filter for a vibrating tube flowmeter

• US 6,199,022 B1
• Filed: 07/11/1997
• Issued: 03/06/2001
• Est. Priority Date: 07/11/1997
• Status: Expired due to Term

• Current Assignee: Micro Motion Incorporated (Emerson Electric Company)
• Original Assignee: Micro Motion Incorporated (Emerson Electric Company)
• Inventors: Cunningham, Timothy J.
• Primary Examiner(s): WACHSMAN, HAL D
• Application Number:
  US08/890,785
• Time in Patent Office: 1,334 Days
• Field of Search: 702/33, 702/36, 702/39, 702/41, 702/45, 702/48, 702/50, 702/54, 702/56, 702/77, 702/100, 702/103, 702/106, 702/113, 702/114, 702/124, 702/126, 702/142, 702/150, 702/189, 702/190, 702/191, 702/194, 702/196, 702/197, 702/198, 702/199, 702/FOR.123, 702/FOR.126, 702/FOR.128, 702/FOR.124, 702/FOR.172, 702/FOR.150, 702/FOR.164, 702/FOR.166, 702/FOR.170, 700/281, 700/282, 700/280, 700/275, 700/279, 738/613-4-8, 73/32.A, 73/11.6, 73/13.5, 73/18.2, 735/78, 735/70, 735/79, 736/68, 736/62--65, 731/523.2, 738/610.2, 738/610.3, 738/611.8, 738/611.9, 738/612.3, 738/612.4
• US Class Current: 702/54
• 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: G01N 2009/006 vibrating tube, tuning fork

Abstract

A drive system for a vibrating tube-based measurement instrument employing a spatial filter to produce a drive signal having modal content only at a desired vibration mode. Multiple feedback sensors located at different locations along a vibrating tube produce multiple feedback sensors. Each feedback signal has applied to it a weighting or gain factor. All of the weighted feedback signals are then summed to produce a drive signal, or a signal proportional to a drive signal, having improved modal content as compared to any of the feedback signals by themselves. The weighting factors are selected by any of several means. One method is to build the eigenvector matrix for the vibrating flow tube by extracting the eigenvectors from a finite element model of the vibrating structure. The inverse or psuedo-inverse of the eigenvector matrix is calculated to obtain the modal filter vector. The appropriate set of weighting coefficients are selected from the modal filter vector.

Claims

1. A drive system for vibrating a fluid container, comprising:

drive means positioned adjacent said fluid container and responsive to a drive signal for vibrating said fluid container;

first sensor means attached to a first location on said fluid container for producing a first motion signal indicative of the movement of said fluid container at said first location;

second sensor means attached to a second location on said fluid container for producing a second motion signal indicative of the movement of said fluid container at said second location;

said first and second motion signals representing a plurality of vibration modes of said fluid container; and

spatial filter means for receiving said first and second motion signals and generating said drive signal, said drive signal representing less vibration modes than said plurality of vibration modes.

2. The drive system of claim 1 wherein said fluid container is a flow tube through which fluid flows.

3. The drive system of claim 2 wherein said flow tube is part of a Coriolis mass flowmeter.

4. The drive system of claim 2 wherein said flow tube is part of a vibrating tube densitometer.

5. The drive system of claim 2 wherein said drive means is attached to said flow tube.

6. The drive system of claim 2 wherein said spatial filter means includes:

first weighting means for applying a first weighting factor to said first motion signal to develop a first weighted signal;

second weighting means for applying a second weighting factor to said second motion signal to develop a second weighted signal; and

summing means for combining said first weighted signal and said second weighted signal to produce said drive signal.

7. The drive system of claim 6 wherein said summing means includes:

summing means for combining said first weighted signal and said second weighted signal to produce a modally filtered signal, which is a signal filtered of some of said plurality of vibration modes; and

amplification means for amplifying said modally filtered signal to produce said drive signal.

8. The drive system of claim 7 further comprising:

a third sensor means attached to a third location on said flow tube for producing a third motion signal indicative of the movement of said flow tube at said third location.

9. The drive system of claim 8 wherein said third location is near a position at which said drive means interacts with said flow tube.

10. The drive system of claim 8 further comprising:

third weighting means for applying a third weighting factor to said third motion signal to develop a third weighted signal; and

summing means for combining said first, second and third weighted signals to produce said drive signal.

11. The drive system of claim 6 wherein said first and second weighting means are analog amplifiers.

12. The drive system of claim 6 wherein said spatial filter means further includes:

an analog to digital converter for converting said first and second motion signals to digital signals; and

said first and second weighting means being digital amplifiers.

13. The drive system of claim 1 wherein said first and second sensor means are velocity sensors.

14. The drive system of claim 1 wherein said first and second sensor means are position sensors.

15. The drive system of claim 1 wherein said first and second sensor means are acceleration sensors.

16. The drive system of claim 1 wherein said first and second sensor means are strain gauges.

17. The drive system of claim 1 further including:

amplitude control means, responsive to said drive signal and a reference voltage, for maintaining a maximum vibration amplitude of said fluid container at a substantially constant level.

18. A method for vibrating a fluid container, comprising the steps of:

receiving a first motion signal indicative of the movement of said fluid container at a first location on said fluid container, said first motion signal representing a plurality of vibration modes of said fluid container;

receiving a second motion signal indicative of the movement of said fluid container at a second location on said fluid container, said second motion signal representing a plurality of vibration modes of said fluid container;

spatially filtering said first and second motion signals to generate a drive signal, said drive signal having less vibration modes than said plurality of vibration modes; and

applying said drive signal to a driver operative to cause said fluid container to vibrate in response to said drive signal.

19. The method of claim 18 wherein said applying step includes:

applying said drive signal to a driver operative to cause said fluid container to vibrate in response to said drive signal where said fluid container is a flow tube of a vibrating tube flowmeter.

20. The method of claim 18 wherein said spatially filtering step includes:

applying a first weighting factor to said first motion signal to develop a first weighted signal;

applying a second weighting factor to said second motion signal to develop a second weighted signal; and

summing said first weighted signal and said second weighted signal to produce said drive signal.

21. The method of claim 20 wherein said summing step includes:

summing said first weighted signal and said second weighted signal to produce a modally filtered signal; and

amplifying said modally filtered signal to produce said drive signal.

22. The method of claim 21 further comprising:

receiving a third motion signal indicative of the movement of said fluid container at a third location on said fluid container, said third motion signal having modal content at a plurality of vibration modes.

23. The method of claim 22 further comprising:

applying a third weighting factor to said third motion signal to develop a third weighted signal; and

summing said first, second and third weighted signals to produce said drive signal.

24. The method of claim 18 further comprising:

controlling a maximum vibration amplitude of said fluid container, responsive to said drive signal and a reference voltage, for maintaining said maximum vibration amplitude of said fluid container at a substantially constant level.

25. A method for generating modal filter weighting factors for a modal filter in a drive system for a measurement instrument employing a vibrating tube, comprising the steps of:

building an eigenvector matrix for the motion of said vibrating tube at N locations on said vibrating tube wherein N is an integer number greater than or equal to one;

solving for the inverse of the psuedo-inverse of the eigenvector matrix to obtain a modal filter vector for said vibrating tube, said modal filter vector containing N sets of coefficients wherein each one of said N sets of coefficients relates to one of a plurality of vibration modes present on said vibrating tube; and

selecting one of said N sets of coefficients as said modal filter weighting factors to be applied to feedback signals from feedback sensors located at said N locations on said vibrating tube.

26. The method of claim 25 wherein said building step includes:

performing an experimental modal analysis on said vibrating tube to generate eigenvectors for said eigenvector matrix.

27. The method of claim 25 wherein said building step includes;

developing a finite element model of said vibrating tube; and

extracting eigenvectors from said finite element model for said eigenvector matrix.

28. The method of claim 25 wherein said solving step includes:

solving the equation x=φ

η

for η

where;

x is a vector of physical response coordinates

φ

is said eigenvector matrix, and

η

is said modal filter vector containing said N sets of coefficients.

29. The method of claim 28 wherein said selecting step includes:

determining which of said plurality of vibration modes present on said vibrating tube is to be extracted as a drive signal for causing said vibrating tube to vibrate; and

selecting, responsive to said determining step, a desired set of coefficients from said N sets of coefficients as said modal filter weighting factors.

30. A method for characterizing a modal filter in a drive system for a vibrating tube flowmeter, comprising the steps of:

choosing a temporary first weighting factor and a temporary second weighting factor;

receiving a first feedback signal from a first feedback sensor attached at a first location on a flow tube of said vibrating tube flowmeter,

receiving a second feedback signal from a second feedback sensor attached at a second location on said flow tube of said vibrating tube flowmeter;

applying said temporary first weighting factor to said first feedback signal to produce a first weighted signal;

applying said temporary second weighting factor to said second feedback signal to produce a second weighted signal;

summing said first and second weighted signals to produce a drive signal;

determining whether said drive signal has modal contents of desired modes as compared to said first and second feedback signals; and

selecting said temporary first weighting factor as an operational first weighting factor and said temporary second weighting factor as an operational second weighting factor in response to determining that said drive signal has said modal content of said desired modes.