Pressure sensor fault detection

• US 20070169557A1
• Filed: 01/26/2006
• Published: 07/26/2007
• Est. Priority Date: 01/26/2006
• Status: Active Grant

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Willcox, Charles, Klosinski, Andrew, Harasyn, Donald
• Granted Patent: US 7,308,830 B2
• Time in Patent Office: Days
• US Class Current: 73/718
• CPC Class Codes: G01L 27/007 Malfunction diagnosis, i.e. diagnosing a sensor defect
• CPC Class Codes: G01L 9/12 by making use of variations in capacitance , i.e. electric circuits therefor

Abstract

A diagnostic system for a pressure sensor having a cavity configured to receive on applied pressure is provided. The cavity has a first and a second wall. A deflectable diaphragm is positioned in the cavity and configured to form a first and a second capacitance with the first wall and a third and a fourth capacitance with the second wall which change in response to the applied pressure. The capacitances form a first transfer function and a second transfer function. Changes in the first transfer function relative to the second transfer function are detected to provide a diagnostic output.

Claims

1. A diagnostic system for a pressure sensor, comprising:

a cavity configured to receive an applied pressure, the cavity having a first and a second wall;

a deflectable diaphragm in the cavity configured to form a first and a second capacitance with the first wall and a third and a fourth capacitance with the second wall which change in response to the applied pressure, the first and third capacitances forming a first transfer function and the second and the fourth capacitances forming a second transfer function; and

circuitry configured to provide a diagnostic output as a function of the first and second transfer functions.

2. The apparatus of claim 1 wherein the circuitry is configured to provide the diagnostic over a two-wire process control loop.

3. The apparatus of claim 1 wherein the circuitry calculates a present value of H based upon the first and second transfer functions.

4. The apparatus of claim 3 wherein the circuitry is further configured to compare the calculated present value of H with a nominal value H₀.

5. The apparatus of claim 1 wherein:

the first transfer function is in the form of;

TF_M=α

_MDP+b_M

and the second transfer function is in the form of;

TF_R=α

_RDP+b_R

where α

_R and α

_M are gage factors, DP is an applied differential pressure and b_M and b_R are y-intercepts and the subscript M identifies the first and third capacitances and the subscript R identifies the second and fourth capacitances.

6. The apparatus of claim 5 wherein the circuitry is further configured to calculate a present value of H as:

$H\equiv <br><br>\frac{{\mathrm{TF}}_R}{{\mathrm{\alpha <br><br>}}_R}-\frac{{\mathrm{TF}}_M}{{\mathrm{\alpha <br><br>}}_M}.$

7. The apparatus of claim 1 wherein the diagnostic output is indicative of condition of an electrode.

8. The apparatus of claim 1 wherein the diagnostic output is indicative of the group of diagnostic outputs consisting of center diaphragm leaks, a failing pressure sensor, electrode loss, contamination, loss of an electrical connection and damage to wire leads.

9. The apparatus of claim 1 wherein a fill fluid on one side of the deflectable diaphragm has a dielectric constant which differs from a dielectric constant of a fill fluid on the other side of a deflectable diaphragm.

10. A method of diagnosing operation of a pressure sensor, comprising:

measuring first, second, third, and fourth capacitances of the pressure sensor;

calculating a present value of H based upon the measured first, second, third and fourth capacitances; and

diagnosing operation of the pressure sensor based upon changes in the calculated value.

11. The method of claim 10 wherein the diagnostic output is indicative of a leak in the center diaphragm.

12. The method of claim 10 including providing an output based upon changes in the calculated value.

13. The method of claim 10 wherein the present value of H is based upon a first and a second transfer function.

14. The method of claim 13 including comparing the present value of H with a nominal value.

15. The method of claim 13 wherein:

the first transfer function is in the form of;

$\frac{M_1-M_2}{M_1+M_2}={\mathrm{TF}}_M={\mathrm{\alpha <br><br>}}_M<br><br>\mathrm{DP}+b_M;<br><br>$

the second transfer function is in the form of;

$\frac{R_1-R_2}{R_1+R_2}={\mathrm{TF}}_R={\mathrm{\alpha <br><br>}}_R<br><br>\mathrm{DP}+b_{R;<br><br>}$

where M₁ and M₂ refer to capacitances between main electrodes and a center diaphragm. And R₁ and R₂ refer to capacitances between the ring electrodes and the center diaphragm. α

_M and α

_R are gage factors, DP is an applied differential pressure and b_M and b_R are y-intercepts and the subscript M identifies the first and third capacitances and the subscript R identifies the second and fourth capacitances.

16. The method of claim 13 wherein including calculating the present value as:

$H\equiv <br><br>\frac{{\mathrm{TF}}_R}{{\mathrm{\alpha <br><br>}}_R}-\frac{{\mathrm{TF}}_M}{{\mathrm{\alpha <br><br>}}_M}.$

17. The method of claim 10 wherein changes in the present value are indicative of a condition of an electrode.

18. The method of claim 10 including implementing the steps of the method in a microcomputer in a process transmitter.

19. The method of claim 10 wherein a fill fluid on one side of a deflectable diaphragm has a dielectric constant which differs from a dielectric constant of a fill fluid on the other side of a deflectable diaphragm.

20. The method of claim 19 wherein diagnosing operation comprises detecting leaks in the deflectable diaphragm.