System to detect poor process ground connections

• US 7,921,734 B2
• Filed: 05/12/2009
• Issued: 04/12/2011
• Est. Priority Date: 05/12/2009
• Status: Active Grant

• Current Assignee: Micro Motion Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Foss, Scot R., Totenhagen, William G.
• Primary Examiner(s): PATEL, HARSHAD R
• Application Number:
  US12/464,409
• Publication Number:
  US 20100288054A1
• Time in Patent Office: 700 Days
• Field of Search: None
• US Class Current: 73/861.12
• CPC Class Codes: G01F 1/60 Circuits therefor
• CPC Class Codes: G01F 25/10 of flowmeters

Abstract

A magnetic flowmeter is disclosed that includes a pair of electrodes coupled to a process fluid and a diagnostic circuit coupled to the pair of electrodes to monitor a common mode signal. The diagnostic circuit is adapted to detect a poor ground connection associated with at least one electrode of the pair of electrodes based on line noise associated with the common mode signal.

Claims

1. A field device comprising:

a pair of electrodes coupled to a process fluid;

a diagnostic circuit coupled to the pair of electrodes to monitor a common mode signal and adapted to detect a poor ground connection associated with at least one electrode of the pair of electrodes based on line noise associated with the common mode signal; and

wherein the diagnostic circuit is adapted to detect the poor ground connection when the line noise exceeds a noise threshold.

2. The field device of claim 1, further comprising:

coils to apply a magnetic field to the process fluid to induce an electromagnetic force (EMF) in the process fluid; and

measurement circuitry coupled to the pair of electrodes and adapted to determine a fluid flow rate based on the EMF.

3. The field device of claim 2, wherein the measurement circuitry comprises at least one operational amplifier coupled to the pair of electrodes.

4. The field device of claim 3, wherein the at least one operational amplifier is saturated when the line noise associated with the common mode signal exceeds a noise threshold, and wherein the at least one operational amplifier distorts a common mode signal when saturated.

5. The field device of claim 1, wherein the diagnostic circuit is adapted to measure a noise amplitude associated with the line noise in the common mode signal, and wherein the noise threshold represents an amplitude sufficient to saturate at least one amplifier coupled to at least one electrode of the pair of electrodes.

6. The field device of claim 1, wherein the diagnostic circuit is further adapted to measure a differential mode signal to detect leakage of process fluid.

7. A magnetic flow meter for measuring a flow rate of a fluid in a pipe, the flow meter comprising:

a flow tube adapted to couple to a pipe to carry a fluid;

a magnetic coil disposed adjacent to the flow tube to induce an electromagnetic force (EMF) within the fluid;

at least two electrodes disposed within the pipe, the at least two electrodes being electrically isolated from the flowtube and one another;

measurement circuitry coupled to the at least two electrodes and adapted to measure flow rate based on the induced EMF; and

diagnostic circuitry coupled to the at least two electrodes to monitor a common mode signal and adapted to detect a poor electrical ground connection when line noise associated with the common mode signal exceeds a noise threshold.

8. The magnetic flow meter of claim 7, wherein the measurement circuitry comprises an operational amplifier coupled to at least one of the at least two electrodes.

9. The magnetic flow meter of claim 8, wherein the noise threshold comprises a power level sufficient to saturate the operational amplifier.

10. The magnetic flow meter of claim 7, wherein the diagnostic circuitry is further adapted to monitor a differential mode signal to determine a differential mode line noise, wherein the diagnostic circuitry includes logic to use the common mode line noise and the differential mode line noise to identify a particular electrode of the at least two electrodes having the poor electrical ground connection.

11. The magnetic flow meter of claim 10, wherein the diagnostic circuitry is adapted to monitor the common mode line noise and within a selected frequency range.

12. The magnetic flow meter of claim 11, wherein the selected frequency range comprises a frequency range from approximately 50 Hz to 60 Hz.

13. The magnetic flow meter of claim 11, wherein the selected frequency range is adjustable to detect line noise at particular frequencies.

14. The magnetic flow meter of claim 7, further comprising logic adapted to generate an alert in response to detecting the poor electrical ground connection.

15. A magnetic flow meter comprising:

a flow tube coupled to a pipe and adapted to carry a process fluid;

first and second electrodes extending into the flow tube and coupled to the process fluid; and

a diagnostic circuit coupled to the first and second electrodes to monitor a common mode signal between the first and second electrodes, the diagnostic circuit adapted to detect a poor electrical ground connection associated with at least one of the first and second electrodes when common mode line noise associated with the common mode signal exceeds a noise threshold.

16. The magnetic flow meter of claim 15, further comprising coils adapted to apply a magnetic field to the process fluid to induce an electromagnetic force (EMF) within the process fluid.

17. The magnetic flow meter of claim 16, further comprising measurement circuitry coupled to the first and second electrodes and adapted to measure the EMF to determine a flow rate associated with the process fluid.

18. The magnetic flow meter of claim 15, wherein the diagnostic circuit is further adapted to monitor differential mode line noise associated with the first and second electrodes.

19. The magnetic flow meter of claim 18, further comprising logic adapted to determine which electrode of the first and second electrodes has the poor electrical ground connection based on the common mode line noise and the differential mode line noise.