Process pressure measurement devices with improved error compensation

• US 6,295,875 B1
• Filed: 05/14/1999
• Issued: 10/02/2001
• Est. Priority Date: 05/14/1999
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Broden, David A., Frick, Roger L., Rud, Stanley E. Jr.
• Primary Examiner(s): OEN, WILLIAM L
• Application Number:
  US09/312,411
• Time in Patent Office: 872 Days
• Field of Search: 73/718, 73/724, 361/283.1, 361/283.3, 361/283.4
• US Class Current: 73/718
• CPC Class Codes: G01L 13/025 using diaphragms
• CPC Class Codes: G01L 9/0072 using variations in capacitance
• CPC Class Codes: G01L 9/0075 using a ceramic diaphragm, e.g. alumina, fused quartz, glass
• CPC Class Codes: G01L 9/125 with temperature compensating means non electric temperature compensating means G01L19/04

Abstract

A device and method are able to compensate for diaphragm deformation (offset) and varying dielectric constants present in a process field environment. The device can be implemented as a pressure sensor or a transmitter having a pressure sensor. The pressure sensor, filled with a dielectric fill-fluid, includes at least three capacitor plates, disposed about a diaphragm. At least two capacitor plates are placed on one side of a conductive diaphragm, and are arranged in a particular manner with respect to the diaphragm. At least one capacitor plate is placed on the other side of the diaphragm. The method compensates for both diaphragm offset and variances in the dielectric constant of the fill-fluid. An error compensated measurement of differential pressure is a function of the amount of diaphragm deflection detected at the edge region subtracted from the amount of diaphragm deflection.

Claims

1. A pressure sensor, the pressure sensor comprising:

a cell body having a first interior wall and a second interior wall generally opposite the first interior wall, the first and second interior walls defining an interior cavity;

a deflectable diaphragm having a conductive portion, the diaphragm coupled to the cell body between the first and second interior walls, the diaphragm separating the interior cavity into a first cavity and a second cavity,

wherein the first cavity is defined by the first interior wall and the diaphragm, and the second cavity is generally opposite the first cavity and defined by the second interior wall and the diaphragm,

wherein the first and second cavities each contain a dielectric fill-fluid, the fill fluids each adapted to receive a pressure and exert a corresponding force on the diaphragm, and the diaphragm is deflectable in response to differences in the pressures received by the fill-fluids in the first and second cavities, and

wherein the first interior wall includes an edge region and a center region, the edge region surrounding the center region;

a first electrode coupled to the first interior wall generally within the center region and within the first cavity, the first electrode generally opposite the conductive portion of the diaphragm and capacitively coupled to the conductive portion to form a first variable capacitor;

a second electrode coupled to the first interior wall generally within the edge region, the second electrode generally opposite the conductive portion of the diaphragm and capacitively coupled to the conductive portion to form a second variable capacitor; and

a third electrode coupled to the second interior wall, the third electrode capacitively coupled to the conductive portion of the diaphragm to form a third variable capacitor.

2. The pressure sensor of claim 1 wherein the cell body is formed of a first cell half having the first interior wall and a second cell half having the second interior wall, and wherein the diaphragm is stretched between the two cell halves.

3. The pressure sensor of claim 2 wherein the diaphragm is in contact with the cell body and adjacent to the first and second interior walls.

4. The pressure sensor of claim 2 wherein a portion of the first and second interior walls are formed of an insulator material.

5. The pressure sensor of claim 4 wherein the electrodes are sputtered on the insulator material of the first and second interior walls.

6. The pressure sensor of claim 1 wherein the deflectable diaphragm is a thin metal membrane, and the conductive portion includes the entire diaphragm.

7. The pressure sensor of claim 1 wherein the dielectric fill-fluid is a silicone oil.

8. A pressure sensor, the pressure sensor comprising:

a cell body having a concave first interior wall and a concave second interior wall generally opposite the first interior wall, wherein the first and second interior walls each include an edge region and a center region, the edge region surrounding the center region on each interior wall, the first and second interior walls defining an interior cavity;

a conductive and deflectable diaphragm, the diaphragm coupled to the cell body between the first and second interior walls, the diaphragm separating the interior cavity into a first cavity and a second cavity,

wherein the first cavity is defined by the first interior wall and the diaphragm, and the second cavity is generally opposite from the first cavity and defined by the second interior wall and the diaphragm, and

wherein the first and second cavities each include a dielectric fill-fluid adapted to receive a pressure and exert a force on the diaphragm, and the diaphragm is deflectable in response to differences in the pressures received by the fill-fluids in the first and second cavities;

a first conductive plate attached to the first interior wall generally within the center region, the first conductive plate capacitively coupled to the conductive diaphragm to form a first variable capacitor;

a second conductive plate attached to the first interior wall generally within the edge region, the second conductive plate spaced-apart from the first conductive plate and capacitively coupled to the conductive diaphragm to form a second variable capacitor;

a third conductive plate attached to the second interior wall generally within the center region and opposite the first conductive plate, the third conductive plate capacitively coupled to the conductive diaphragm to form a third variable capacitor; and

a fourth conductive plate attached to the second interior wall generally within the edge region, the fourth conductive plate spaced-apart from the third conductive plate, the fourth conductive plate capacitively coupled to the conductive diaphragm to form a fourth variable capacitor.

9. The pressure sensor of claim 8 wherein the concave first and second interior walls are generally spherical.

10. The pressure sensor of claim 9 wherein the concave interior walls provide an over-pressure stop for the diaphragm deflected under the differences in pressure.

11. The pressure sensor of claim 8 wherein an axis extending perpendicularly from the center of the diaphragm extends through the center regions of the first and second interior walls.

12. The pressure sensor of claim 11 wherein a first distance along the first interior wall from the first electrode to the axis is shorter than a second distance along the interior wall from the second electrode to the axis.

13. The pressure sensor of claim 11 wherein the first electrode is substantially circular and has a center corresponding with the axis on the first interior wall, and the second electrode is at least a partial ring disposed about the first electrode.

14. The pressure sensor of claim 13 wherein the second electrode is a complete ring.

15. The pressure sensor of claim 11 wherein the third electrode is substantially circular and has a center corresponding with the axis on the second interior wall, and the fourth electrode is at least a partial ring disposed about the third electrode.

16. A pressure sensor, the pressure sensor comprising:

a cell body having generally opposite concave first and second interior walls at least partially defining an interior cavity;

a deflectable diaphragm having a conductive portion, the diaphragm attached to the cell body to form a first cavity defined by the first interior wall and the diaphragm, and forming a second cavity defined by the second interior wall and the diaphragm,

wherein the first and second cavities are each adapted to receive a variable pressure that exerts a force on the diaphragm, and the diaphragm is deflectable in response to differences in the pressures received in the first and second cavities;

a first electrode coupled to the first interior wall and capacitively coupled to the conductive portion of the diaphragm to form a first variable capacitor;

a second electrode coupled to the first interior wall and spaced-apart from the first electrode, the second electrode capacitively coupled to the diaphragm to form a second variable capacitor;

a third electrode coupled to the second interior wall and capacitively coupled to the conductive portion of the diaphragm to form a third variable capacitor.

17. A process pressure transmitter, comprising:

a transmitter electronics disposed within a housing, the transmitter electronics including a communications circuit coupled to a processing system;

an analog to digital converter disposed within the housing, the analog to digital converter electrically coupled to the transmitter electronics;

a pressure sensor comprising;

a cell body having a first interior wall and a second interior wall generally opposite the first interior wall, the first and second interior walls defining an interior cavity;

a deflectable diaphragm having a conductive portion, the diaphragm coupled to the cell body between the first and second interior walls, the diaphragm separating the interior cavity into a first cavity and a second cavity,

wherein the first cavity is defined by the first interior wall and the diaphragm, and the second cavity is generally opposite the first cavity and defined by the second interior wall and the diaphragm,

wherein the first and second cavities each contain a dielectric fill-fluid, each of the fill fluids adapted to receive a pressure and exert a corresponding force on the diaphragm, and the diaphragm is deflectable in response to differences in the pressures received by the fill-fluids in the first and second cavities, and

wherein the first interior wall includes an edge region and a center region, the edge region surrounding the center region;

a first electrode coupled to the first interior wall generally within the center region, the first electrode capacitively coupled to the conductive portion to form a first variable capacitor;

a first lead wire electrically connected to the first electrode and extending from the cell body;

a second electrode coupled to the first interior wall generally within the edge region, the second electrode capacitively coupled to the conductive portion to form a second variable capacitor;

a second lead wire electrically connected to the second electrode and extending from the cell body;

a third electrode coupled to the second interior wall generally within the center region of the second interior wall, the third electrode capacitively coupled to the conductive portion of the diaphragm to form a third capacitor;

a third lead wire electrically coupled to the third electrode and extending from the cell body;

a fourth electrode coupled to the second interior wall generally within the edge region of the second interior wall, the fourth electrode electrically coupled to the conductive portion to form a fourth capacitor; and

a fourth lead wire electrically coupled to the fourth electrode and extending from the cell body;

wherein the four lead wires are electrically coupled to the analog to digital converter.

18. The pressure transmitter of claim 17 wherein the first and fourth electrodes are electrically coupled together at a first node outside of the cell body, and wherein the second and third electrodes are electrically coupled together at a second node outside of the cell body.

19. The pressure sensor of claim 18 wherein the first and second nodes are electrically coupled to the analog to digital converter.

20. The pressure sensor of claim 18 wherein the first and second nodes are located within the analog to digital converter.

21. The pressure sensor of claim 18 wherein a fifth capacitor is electrically coupled between the fourth electrode and the first node, and a sixth capacitor is electrically coupled between the second electrode and the second node.

22. The pressure transmitter of claim 17 wherein the second electrode is electrically coupled to a first inverting charge amplifier outside of the cell body, the first inverting charge amplifier electrically coupled to the first electrode at a third node, and the fourth electrode is electrically coupled to a second inverting charge amplifier outside of the cell body, the second inverting charge amplifier coupled to the third electrode at a fourth node.

23. The pressure transmitter of claim 22 wherein the third and fourth nodes are electrically coupled to the analog to digital converter.

24. The pressure transmitter of claim 22 wherein the inverting charge amplifiers are located within the analog to digital converter.

25. A method of measuring differential pressure, comprising:

applying a first process pressure to a first amount of fill-fluid on a first side of a deflectable diaphragm;

applying a second process pressure to a second amount of fill-fluid on a second side of the deflectable diaphragm;

measuring an amount of deflection of a center region of the diaphragm from a selected position;

measuring an amount of deflection of an edge region of the diaphragm from the selected position; and

subtracting the amount of deflection at the edge region from the amount of deflection at the center region.

26. The method of claim 25 wherein the selected position is a diaphragm-at-rest plane.

27. The method of claim 25 and further including the step of generating an output substantially related to the amount of deflection at the edge region subtracted from the amount of deflection at the center region.

28. The method of claim 25 wherein the diaphragm is opposite a cavity wall having a wall edge region opposite the edge region of the diaphragm and a wall center region opposite the center region of the diaphragm, the method further including:

dividing the amount of deflection at the edge region subtracted from the amount of deflection at the center region by a value equal to a distance of the wall edge region to the selected position added to a distance of the wall center region to the selected position.

29. The method of claim 28 wherein the distance of the edge region of the cavity wall to the selected position is not equal to the distance of the center region of the cavity wall to the selected position.

30. The method of claim 25 wherein the diaphragm is opposite a cavity wall having a wall center region opposite the center region of the diaphragm, the method further including:

dividing the amount of deflection at the edge region subtracted from the amount of deflection at the center region by a value equal to a distance of the wall center region to the selected position.

31. A method of error compensation in measuring differential pressure with a sensor having a conductive and deflectable diaphragm, first and second electrodes on one side of the diaphragm, and third and fourth electrodes on another side of the diaphragm, wherein the four electrodes are each separately coupled to the diaphragm to form four capacitors, the method comprising:

adding a first capacitance signal related to an electrical signal at the first electrode to a fourth capacitance signal related to an electrical signal at the fourth electrode to obtain a first sum;

adding a third capacitance signal related to an electrical signal at the third electrode to a second capacitance signal related to an electrical signal at the second electrode to obtain a second sum;

adding the first sum to the second sum to obtain a denominator;

subtracting the second sum from the first sum to obtain a numerator; and

dividing the numerator by the denominator.

32. The method of claim 31 wherein,

the first capacitance signal includes a first main capacitance signal and a first stray capacitance signal;

the second capacitance signal includes a second main capacitance signal and a second stray capacitance signal;

the third capacitance signal includes a third main capacitance signal and a third stray capacitance signal; and

the fourth capacitance signal includes a fourth main capacitance signal and a fourth stray capacitance signal.

33. The method of claim 32 wherein the first, second, third and fourth electrical signals at the electrodes are multiplied by a gain.

34. The method of claim 33 wherein the first, second, third and fourth stray capacitance signals cancel each other in an output.

35. The method of claim 32 wherein the step of adding the first capacitance signal to the fourth capacitance signal includes subtracting a first linearization capacitance signal related to an electrical signal at a first linearization capacitor from the first or fourth capacitance signals to obtain the first sum;

and

wherein the step of adding the third capacitance signal to the second capacitance signal includes subtracting a second linearization capacitance signal related to an electrical signal at a second linearization capacitor from the second or third capacitance signals to obtain the second sum.

36. The method of claim 35 wherein the second capacitance signal is a second effective capacitance signal and includes a second adjustment capacitance signal related to an electrical signal at a second adjustment capacitor multiplied by the sum of the second main capacitance signal and second stray capacitance signal and divided by the sum of the second adjustment capacitance signal, second main capacitance signal and second stray capacitance signal;

and the fourth capacitance signal is a fourth effective capacitance signal and includes a first adjustment capacitance signal related to an electrical signal at a first adjustment capacitor multiplied by the sum of the fourth main capacitance signal and fourth stray capacitance signal and divided by the sum of the first adjustment capacitance signal, fourth main capacitance signal and fourth stray capacitance signal.

37. The method of claim 31 wherein at least some of the electrical signals at the first, second, third, and fourth electrodes are amplified to provide signals representative of the respective first, second, third, or fourth capacitances signal.

38. The method of claim 37 wherein the electrical signals at the second and fourth electrodes are amplified by a gain of less than unity.

39. A method of error compensation in measuring process pressure with a process measurement transmitter having a sensor generating a plurality of electrical signals representative of a first, second, third and fourth capacitance signals, the method comprising:

subtracting the fourth capacitance signal from the second capacitance signal to obtain a first difference;

subtracting the third capacitance signal from the first capacitance signal to obtain a second difference;

subtracting the first difference from the second difference to obtain a numerator; and

dividing the numerator by a denominator, wherein the denominator includes a first sum of the second and fourth capacitance signals subtracted from a second sum of the first and third capacitance signals.

40. The method of claim 39 wherein the first sum is added to the second sum to obtain a third sum, and the denominator includes a fourth sum of linearization capacitances subtracted from the third sum.

41. The method of claim 39 wherein the four capacitance signals each include a main signal and a stray capacitance signal.

42. A method of error compensation in measuring differential pressure with a sensor having a conductive and deflectable diaphragm, first and second electrodes on one side of the diaphragm, and a third electrode on another side of the diaphragm, wherein the three electrodes are each separately coupled to the diaphragm to form three capacitors, the method comprising:

multiplying a first capacitance signal related to a first electrical signal at the first electrode by a third capacitance signal related to a third electrical signal at the third electrode and by a first constant to obtain a first value;

dividing the first value by a second capacitance signal related to a second electrical signal at the second electrode to obtain a second value;

subtracting the second value from the third capacitance signal to obtain a fourth value;

subtracting the fourth value from the first capacitance signal to obtain a numerator; and

dividing the numerator by the sum of the first capacitance signal, third capacitance signal and a second constant.