High sensitivity pressure sensor with long term stability

• US 6,813,954 B2
• Filed: 05/23/2002
• Issued: 11/09/2004
• Est. Priority Date: 05/25/2001
• Status: Expired due to Fees

• Current Assignee: Panametrics, Inc. (General Electric Company)
• Original Assignee: Panametrics, Inc. (General Electric Company)
• Inventors: Gokhfeld, Yuzef
• Primary Examiner(s): Lefkowitz, Edward
• Assistant Examiner(s): ALLEN, ANDRE J
• Application Number:
  US10/153,948
• Publication Number:
  US 20020174706A1
• Time in Patent Office: 901 Days
• Field of Search: 737/18, 737/24, 737/16, 737/36, 361/283
• US Class Current: 73/718
• CPC Class Codes: G01L 13/025 using diaphragms
• CPC Class Codes: G01L 19/02 Arrangements for preventing, or for compensating for, effects of inclination or acceleration of the measuring device; Zero-setting means for aneroid barometers G01L7/14
• CPC Class Codes: G01L 9/0072 using variations in capacitance
• CPC Class Codes: G01L 9/12 by making use of variations in capacitance , i.e. electric circuits therefor
• CPC Class Codes: G01L 9/125 with temperature compensating means non electric temperature compensating means G01L19/04

Abstract

A high sensitivity pressure sensor with long term stability including a housing with first and second chambers, a membrane separating the first and second chambers, a first electrode located in the first chamber and spaced from one side of the membrane forming a first capacitor therewith, and a second electrode located in the second chamber and spaced from an opposite side of the membrane forming a second capacitor therewith. A measuring circuit is connected across the first and second capacitors for measuring membrane displacement by detecting differences in capacitance between the first and second capacitors and a compensation circuit is configured to apply an electric field to the membrane as a compensating force by reducing the voltage difference between the first electrode and the membrane and simultaneously increasing the voltage difference between the second electrode and the membrane or vice versa to provide long term stability. The invention also features a unique compensation circuit for a pressure sensor and a method of making and operating a pressure sensor.

Claims

1. A high sensitivity pressure sensor with long term stability comprising:

a housing including first and second chambers;

a membrane separating the first and second chambers;

a first electrode located in the first chamber and spaced from one side of the membrane forming a first capacitor therewith;

a second electrode located in the second chamber and spaced from an opposite side of the membrane forming a second capacitor therewith;

a measuring circuit connected across the first and second capacitors for measuring membrane displacement by detecting differences in capacitance between the first and second capacitors; and

a compensation circuit configured to apply an electric field to the membrane as a compensating force and to reduce the voltage difference between the first electrode and the membrane and to increase the voltage difference between the second electrode and the membrane to provide long term stability.

2. The pressure sensor of claim 1 in which the membrane is made of metal.

3. The pressure sensor of claim 2 in which the metal membrane is not under significant tension in the absence of a differential pressure between the two chambers.

4. The pressure sensor of claim 3 in which the tension on the membrane is less then 0.36 N/m.

5. The pressure sensor of claim 3 in which the membrane displacement versus pressure due to the bending force of the membrane is Y_bend, the membrane displacement versus pressure due to tension is Y_tens, and Y_bend<

<

Y_tens.

6. The pressure sensor of claim 1 in which the housing is made of a material having a coefficient of thermal expansion the same as or substantially the same as the coefficient of thermal expansion of the material of the membrane.

7. The pressure sensor of claim 6 in which the housing is made of titanium and the membrane is made of titanium.

8. The pressure sensor of claim 1 in which the housing includes first and second base plates each with an inner chamber and the membrane is disposed between the two base plates separating the inner chambers thereof.

9. The pressure sensor of claim 8 in which the two base plates are secured together under compression and then released to slightly tension the membrane to prevent negative tension thereof.

10. The pressure sensor of claim 8 in which the first electrode is attached to a first holder affixed to the first base plate over its inner chamber and the second electrode is attached to a second holder affixed to the second base plate over its inner chamber.

11. The pressure sensor of claim 10 further including an insulator between the first holder and the first base plate and an insulator between the second holder and the second base plate.

12. The pressure sensor of claim 10 further including a first seal about the first electrode sealing it with respect to the first base plate inner chamber and a second seal about the second electrode sealing it with respect to the second base plate inner chamber.

13. The pressure sensor of claim 8 in which the first base plate includes a conduit in communication with the inner chamber thereof and the second base plate includes a conduit in communication with the inner chamber thereof.

14. The pressure sensor of claim 1 in which the membrane has a thickness of between 5-15 micrometers.

15. The pressure sensor of claim 1 in which the volume of the first chamber is substantially the same as or the same as the volume of the second chamber.

16. The pressure sensor of claim 1 in which the spacing between the first electrode and the membrane is the same as or substantially the same as the spacing between the second electrode and the membrane.

17. The pressure sensor of claim 16 in which the said spacing is between 10-30 micrometers.

18. The pressure sensor of claim 1 in which the first electrode has a substantially planar surface facing the membrane and the second electrode has a substantially planar surface facing the membrane.

19. The pressure sensor of claim 1 in which the compensation circuit includes a voltage divider connected to the first and second electrodes, and an amplifier connected to a middle point of the voltage divider for monitoring the electrical potential of the middle point.

20. The pressure sensor of claim 1 in which the measuring circuit includes a bridge circuit connected to the first and second electrodes and having a feedback connected to the compensation circuit.

21. The pressure sensor of claim 20 in which the bridge circuit includes a high frequency voltage source.

22. The pressure sensor of claim 21 in which the high frequency voltage source is the secondary of a transformer, the primary of which is connected to a high frequency voltage generator.

23. The pressure sensor of claim 19 in which the compensation circuit further includes a lock-in amplifier connected between the amplifier and the measuring circuit.

24. The pressure sensor of claim 23 in which the lock-in amplifier includes a phase shifter connected to a high frequency voltage source and a phase detector connected to the amplifier.

25. The pressure sensor of claim 24 further including a low pass filter disposed between the output of the phase detector and the amplifier.

26. A high sensitivity pressure sensor with long term stability comprising:

a housing including first and second chambers;

a metal membrane separating the first and second chambers, the metal membrane not under significant tension in the absence of a differential pressure between the two chambers;

a first electrode located in the first chamber and spaced from one side of the metal membrane forming a first capacitor,

a second electrode located in the second chamber and spaced from an opposite side of the metal membrane forming a second capacitor;

a measuring circuit for measuring membrane displacement by detecting differences in capacitance between the first and second capacitors; and

a compensation circuit including a voltage divider connected to the first and second electrodes and having a middle point which when unbalanced applies a greater electric field to one side of the membrane than the other side of the membrane.

27. A high sensitivity pressure sensor with long term stability comprising:

a housing including first and second chambers;

a metal membrane separating the first and second chambers;

a first electrode located in the first chamber and spaced from one side of the metal membrane forming a first capacitor;

a second electrode located in the second chamber and spaced from an opposite side of the metal membrane forming a second capacitor,

the housing made of a material having a coefficient of thermal expansion the same as or substantially the same as the coefficient of thermal expansion of the material of the membrane;

a measuring circuit connected across the first and second capacitors for measuring membrane displacement by detecting differences in capacitance between the first and second capacitors; and

a compensation circuit configured to apply an electric field to the membrane as a compensating force and to reduce the voltage difference between the first electrode and the membrane and to increase the voltage difference between the second electrode and the membrane.

28. A high sensitivity pressure sensor with long term stability comprising:

a housing including first and second chambers;

a metal membrane separating the first and second chambers;

a first electrode located in the first chamber and spaced from one side of the metal membrane forming a first capacitor, and

a second electrode located in the second chamber and spaced from an opposite side of the metal membrane forming a second capacitor,

the metal membrane not under significant tension in the absence of a differential pressure between the two chambers.

29. The pressure sensor of claim 28 in which the tension on the membrane is less then 0.36 N/m.

30. The pressure sensor of claim 28 in which membrane displacement versus pressure due to the bending force of the membrane is Y_bend, membrane displacement versus pressure due to tension is Y_tens, and Y_bend<

<

Y_tens.

31. The pressure sensor of claim 28 in which the housing is made of a material having a coefficient of thermal expansion the same as or substantially the same as the coefficient of thermal expansion of the material of the membrane.

32. The pressure sensor of claim 31 in which the housing is made of titanium and the membrane is made of titanium.

33. The pressure sensor of claim 28 in which the housing includes first and second base plates each with an inner chamber, the membrane disposed between the two base plates separating the inner chambers thereof.

34. The pressure of sensor of claim 33 in which the two base plates are secured together under compression and then released to slightly tension the membrane to prevent negative tension thereof.

35. The pressure sensor of claim 33 in which the first electrode is attached to a first holder affixed to the first base plate over its inner chamber and the second electrode is attached to a second holder affixed to the second base plate over its inner chamber.

36. The pressure sensor of claim 35 further including an insulator between the first holder and the first base plate and an insulator between the second holder and the second base plate.

37. The pressure sensor of claim 35 further including a first seal about the first electrode sealing it with respect to the first base plate inner chamber and a second seal about the second electrode sealing it with respect to the second base plate inner chamber.

38. The pressure sensor of claim 33 in which the first base plate includes a conduit in communication with the inner chamber thereof and the second base plate includes a conduit in communication with the inner chamber thereof.

39. The pressure sensor of claim 28 in which the membrane has a thickness of between 5-15 micrometers.

40. The pressure sensor of claim 28 in which the volume of the first chamber is substantially the same as or the same as the volume of the second chamber.

41. The pressure sensor of claim 28 in which the spacing between the first electrode and the membrane is the same as or substantially the same as the spacing between the second electrode and the membrane.

42. The pressure sensor of claim 41 in which the said spacing is between 10-30 micrometers.

43. The pressure sensor of claim 28 in which the first electrode has a substantially planar surface facing the membrane and the second electrode has a substantially planar surface facing the membrane.

44. The pressure sensor of claim 28 further including a controller comprising:

a measuring circuit connected across the first and second capacitors for detecting differences in capacitance between the first and second capacitors; and

a compensation circuit.

45. A high sensitivity pressure sensor with long term stability comprising:

a housing including first and second chambers;

a metal membrane separating the first and second chambers;

a first electrode located in the first chamber and spaced from one side of the metal membrane forming a first capacitor;

a second electrode located in the second chamber and spaced from an opposite side of the metal membrane forming a second capacitor,

the housing made of a material having a coefficient of thermal expansion the same as or substantially the same as the coefficient of thermal expansion of the material of the membrane; and

a compensation circuit configured to apply an electric field to the metal membrane as a compensating force and to reduce the voltage difference between the first electrode and the metal membrane and to increase the voltage difference between the second electrode and the metal membrane to provide long term stability.

46. A control circuit for a differential pressure sensor including first and second capacitors and a membrane common to both capacitors, the control circuit comprising:

a measuring circuit connected across the first and second capacitors for measuring membrane displacement by detecting differences in capacitance between the first and second capacitors; and

a compensation circuit configured to initially apply an equal voltage and electric field to both sides of the membrane, and apply a compensating force by reducing the voltage difference between the first electrode and the membrane simultaneously increasing the voltage difference between the second electrode and the membrane or vice versa to provide long term stability.

47. A method of making and using a high sensitivity pressure sensor with long term stability, the method comprising:

forming a housing to include first and second chambers;

disposing a membrane to separate the first and second chambers;

placing a first electrode in the first chamber spaced from one side of the membrane forming a first capacitor therewith;

placing a second electrode in the second chamber spaced from an opposite side of the membrane forming a second capacitor therewith;

connecting a measuring circuit across the first and second capacitors for measuring membrane displacement by detecting differences in capacitance between the first and second capacitors; and

initially applying an equal strength electric field to each side of the membrane, and applying a compensating force by reducing the voltage difference between the first electrode and the membrane and simultaneously increasing the voltage difference between the second electrode and the membrane or vice versa to provide long term stability.

48. The method of claim 47 in which the membrane is made of metal.

49. The method of claim 48 in which the metal membrane is not under significant tension in the absence of a differential pressure between the two chambers.

50. The method of claim 49 in which the tension on the membrane is less then 0.36 N/m.

51. The method of claim 49 in which the membrane displacement versus pressure due to the bending force of the membrane is Y_bend, the membrane displacement versus pressure due to tension is Y_tens, and Y_bend<

<

Y_tens.

52. The method of claim 47 in which the housing is made of a material having a coefficient of thermal expansion the same as or substantially the same as the coefficient of thermal expansion of the material of the membrane.

53. The method of claim 52 in which the housing is made of titanium and the membrane is made of titanium.

54. The method of claim 47 in which the housing includes first and second base plates each with an inner chamber and the membrane is disposed between the two base plates separating the inner chambers thereof.

55. The method of claim 54 in which the two base plates are secured together under compression and then released to slightly tension the membrane to prevent negative tension thereof.

56. The method of claim 55 in which the first electrode is attached to a first holder affixed to the first base plate over its inner chamber and the second electrode is attached to a second holder affixed to the second base plate over its inner chamber.

57. The method of claim 56 further including disposing an insulator between the first holder and the first base plate and disposing an insulator between the second holder and the second base plate.

58. The method of claim 56 further including disposing a first seal about the first electrode sealing it with respect to the first base plate inner chamber and disposing a second seal about the second electrode sealing it with respect to the second base plate inner chamber.

59. The method of claim 55 in which the first base plate includes a conduit in communication with the inner chamber thereof and the second base plate includes a conduit in communication with the inner chamber thereof.

60. The method of claim 47 in which the membrane has a thickness of between 5-15 micrometers.

61. The method of claim 47 in which the volume of the first chamber is substantially the same as or the same as the volume of the second chamber.

62. The method of claim 47 in which the spacing between the first electrode and the membrane is the same as or substantially the same as the spacing between the second electrode and the membrane.

63. The method of claim 62 in which the spacing is between 10-30 micrometers.

64. The method of claim 47 in which the first electrode has a substantially planar surface facing the membrane and the second electrode has a substantially planar surface facing the membrane.

65. The method of claim 47 in which the compensation circuit includes a voltage divider connected to the first and second electrodes, and an amplifier connected to a middle point of the voltage divider for monitoring the electrical potential of the middle point.

66. The method of claim 47 in which a bridge circuit is connected to the first and second electrodes and having a feedback connected to the compensation circuit.

67. The method of claim 66 in which the bridge circuit includes a high frequency voltage source.

68. The method of claim 62 in which the high frequency voltage source is the secondary of a transformer, the primary of which is connected to a high frequency voltage generator.

69. The method of claim 65 in which the compensation circuit further includes a lock-in amplifier connected between the amplifier and the measuring circuit.

70. The method of claim 69 in which the lock-in amplifier includes a phase shifter connected to a high frequency voltage source and a phase detector connected to the amplifier.

71. The method of claim 70 further including a low pass filter disposed between the output of the phase detector and the amplifier.

72. A method of operating a high sensitivity pressure sensor to have long term stability wherein the pressure sensor includes a housing with first and second chambers, a metal membrane separating the first and second chambers, a first electrode located in the first chamber and spaced from one side of the membrane forming a first capacitor, a second electrode located in the second chamber and spaced from an opposite side of the metal membrane forming a second capacitor, the method comprising:

measuring membrane displacement by detecting differences in capacitance between the first and second capacitors; and

initially applying an electric field of equal magnitude to each side of the membrane, and applying a compensating force by reducing the voltage difference between the first electrode and the membrane and simultaneously increasing the voltage difference between the second electrode and the membrane or vice versa.

73. A high sensitivity pressure sensor with long term stability comprising:

a housing including first and second chambers;

a metal membrane separating the first and second chambers;

a first electrode located in the first chamber and spaced from one side of the metal membrane forming a first capacitor;

a second electrode located in the second chamber and spaced from an opposite side of the metal membrane forming a second capacitor,

the metal membrane not under significant tension in the absence of a differential pressure between the two chambers; and

a compensation circuit configured to apply an electric field to the metal membrane as a compensating force and to reduce the voltage difference between the first electrode and the metal membrane and to increase the voltage difference between the second electrode and the metal membrane to provide long term stability.

74. The pressure sensor of claim 73 in which the tension on the membrane is less then 0.36 N/m.

75. The pressure sensor of claim 73 in which membrane displacement versus pressure due to the bending force of the membrane is Y_bend, membrane displacement versus pressure due to tension is Y_tens, and Y_bend<

<

Y_tens.

76. The pressure sensor of claim 73 in which the housing is made of a material having a coefficient of thermal expansion the same as or substantially the same as the coefficient of thermal expansion of the material of the membrane.

77. The pressure sensor of claim 76 in which the housing is made of titanium and the membrane is made of titanium.

78. The pressure sensor of claim 73 in which the housing includes first and second base plates each with an inner chamber, the membrane disposed between the two base plates separating the inner chambers thereof.

79. The pressure of sensor of claim 78 in which the two base plates are secured together under compression and then released to slightly tension the membrane to prevent negative tension thereof.

80. The pressure sensor of claim 78 in which the first electrode is attached to a first holder affixed to the first base plate over its inner chamber and the second electrode is attached to a second holder affixed to the second base plate over its inner chamber.

81. The pressure sensor of claim 80 further including an insulator between the first holder and the first base plate and an insulator between the second holder and the second base plate.

82. The pressure sensor of claim 80 further including a first seal about the first electrode sealing it with respect to the first base plate inner chamber and a second seal about the second electrode sealing it with respect to the second base plate inner chamber.

83. The pressure sensor of claim 78 in which the first base plate includes a conduit in communication with the inner chamber thereof and the second base plate includes a conduit in communication with the inner chamber thereof.

84. The pressure sensor of claim 73 in which the membrane has a thickness of between 5-15 micrometers.

85. The pressure sensor of claim 73 in which the volume of the first chamber is substantially the same as or the same as the volume of the second chamber.

86. The pressure sensor of claim 73 in which the spacing between the first electrode and the membrane is the same as or substantially the same as the spacing between the second electrode and the membrane.

87. The pressure sensor of claim 86 in which the said spacing is between 10-30 micrometers.

88. The pressure sensor of claim 73 in which the first electrode has a substantially planar surface facing the membrane and the second electrode has a substantially planar surface facing the membrane.

89. The pressure sensor of claim 73 further including a controller comprising:

a measuring circuit connected across the first and second capacitors for detecting differences in capacitance between the first and second capacitors.

90. A high sensitivity pressure sensor with long term stability comprising:

a housing including first and second chambers;

a metal membrane separating the first and second chambers;

a first electrode located in the first chamber and spaced from one side of the metal membrane forming a first capacitor,

a second electrode located in the second chamber and spaced from an opposite side of the metal membrane forming a second capacitor,

the metal membrane not under significant tension in the absence of a differential pressure between the two chambers; and

a compensation circuit including a voltage divider connected to the first and second electrodes and having a middle point which when unbalanced applies a greater electric field to one side of the metal membrane than the other side of the metal membrane.

91. A high sensitivity pressure sensor with long term stability comprising:

a housing including first and second chambers;

a metal membrane separating the first and second chambers;

a first electrode located in the first chamber and spaced from one side of the metal membrane forming a first capacitor;

a second electrode located in the second chamber and spaced from an opposite side of the metal membrane forming a second capacitor,

the housing made of a material having a coefficient of thermal expansion the same as or substantially the same as the coefficient of thermal expansion of the material of the membrane; and

a compensation circuit including a voltage divider connected to the first and second electrodes and having a middle point which when unbalanced applies a greater electric field to one side of the metal membrane than the other side of the metal membrane.