Capacitive strain gage and method

• US 6,575,041 B2
• Filed: 02/05/1999
• Issued: 06/10/2003
• Est. Priority Date: 02/05/1999
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Northrop Grumman Corporation
• Inventors: Papazian, John M., Kesselman, Martin, Schwarz, Robert C.
• Primary Examiner(s): NOORI, MAX H
• Application Number:
  US09/246,666
• Publication Number:
  US 20020043112A1
• Time in Patent Office: 1,586 Days
• Field of Search: 737/80, 737/75, 738/626.4, 737/76, 737/18, 738/785, 731/41, 737/24, 737/06
• US Class Current: 73/780
• CPC Class Codes: G01L 1/142 using capacitors

Abstract

A capacitive strain gage assembly (10) for measuring strain in a component (40) includes a housing (12) attachable to the component (40) at a first location (42). The strain gage assembly (10) includes a first target sensor (24,26) and second target sensor (28,30) disposed in the housing (12). The strain gage assembly (10) also includes a sensor member (14) attachable to the component (40) at a second location (44) in movable relation to the housing (12). The sensor member (14) includes a sensor element (32) operable to transmit capacitive signals to the first and second target sensors (24,26,28,30). The sensor element (32) is disposed between the first and second target sensors (24,26,28,30). The strain gage assembly (10) may also be constructed using generally buckle resistant flexible materials for nonplanar strain measurement applications.

Claims

1. A capacitive strain gage assembly, comprising:

a housing attachable to a component at a first location;

a first capacitive target sensor disposed within the housing;

a second capacitive target sensor disposed within the housing in a generally parallel relationship with the first capacitive target sensor;

a third capacitive target sensor disposed within the housing in a generally parallel relationship with the second capacitive target sensor;

a fourth capacitive target sensor disposed within the housing in a generally parallel relationship with the third capacitive target sensor;

a sensor member positioned in a generally parallel relationship with the first, second, third, and fourth capacitive target sensors and disposed between the second and third capacitive target sensors, the sensor member attachable to the component at a second location and movable relative to the housing and the first, second, third, and fourth capacitive target sensors, the sensor member having a capacitive sensor element movable with the sensor member in a direction generally parallel with the first, second, third, and fourth capacitive target sensors to vary the capacitance between the sensor member and the first capacitive target sensor, the second capacitive target sensor, the third capacitive target sensor, and the fourth capacitive target sensor; and

a pressure element disposed within the housing and positioned to maintain the sensor member in contact with the second and third capacitive target sensors.

2. The assembly of claim 1, wherein:

the first and second capacitive target sensors comprise a first target member; and

the third and fourth capacitive target sensors comprise a second target member.

3. The assembly of claim 2, wherein the sensor member and the first and second target members comprise a generally buckle resistant flexible material.

4. The assembly of claim 1, further comprising a pressure element disposed within the housing and positioned to maintain the sensor member in contact with the second and third capacitive target sensors.

5. The assembly of claim 4, wherein the pressure element comprises a foam material.

6. The assembly of claim 1, wherein the sensor member includes a first surface and a second surface, and the sensor element is symmetrically disposed between the first and second surfaces.

7. The assembly of claim 1, further comprising an alignment element disposed within the housing and operable to maintain generally unidirectional movement of the sensor member relative to the first, second, third, and fourth capacitive target sensors.

8. The assembly of claim 1, wherein:

the first capacitive target sensor comprises a first cross sectional area in a plane generally parallel with the first capacitive target sensor and the second capacitive target sensor comprises a second cross sectional area in a plane generally parallel with the second capacitive target sensor, the first cross sectional area larger than the second cross sectional area; and

the third capacitive target sensor comprises a third cross sectional area in a plane generally parallel with the third capacitive target sensor and the fourth capacitive target sensor comprises a fourth cross sectional area in a plane generally parallel with the fourth capacitive target sensor, the fourth cross sectional area larger than the third cross sectional area.

9. The assembly of claim 1, wherein the first capacitive target sensor, the second capacitive target sensor, the third capacitive target sensor, and the fourth capacitive target sensor compensate for non-planar displacements of the sensor member.

10. A capacitive strain gage assembly, comprising:

a housing attachable to a first location of a component;

a first capacitive target sensor disposed within the housing and having a first cross sectional area in a plane generally parallel with the first capacitive target sensor;

a second capacitive target sensor disposed within the housing in a generally parallel relationship with the first capacitive target sensor and having a second cross sectional area in a plane generally parallel with the second capacitive target sensor, the second cross sectional area smaller than the first cross sectional area;

a sensor member positioned in a generally parallel relationship with the first and second capacitive target sensors, the second capacitive target sensor disposed between the first capacitive target sensor and the sensor member, the sensor member attachable to the component at a second location and movable relative to the housing and the first and second capacitive target sensors, the sensor member having a capacitive sensor element movable with the sensor member in a direction generally parallel with the first and second capacitive target sensors to vary the capacitance between the sensor member and the first capacitive target sensor and the second capacitive target sensor; and

a pressure element disposed within the housing and positioned to maintain the sensor member in contact with the second and third capacitive target sensors.

11. The assembly of claim 10, wherein the sensor member includes a first surface and a second surface, the sensor element symmetrically disposed between the first and second surfaces.

12. The assembly of claim 10, further comprising a pressure element disposed within the housing and positioned to maintain the sensor member in contact with the second capacitive target sensor.

13. The assembly of claim 10, wherein the first and second capacitive target sensors comprise a target member, and the target member and the sensor member comprise a generally buckle resistant flexible material.

14. The assembly of claim 10, further comprising an alignment element disposed within the housing and operable to maintain generally unidirectional movement of the sensor member relative to the first and second capacitive target sensors.

15. A capacitive strain gage assembly, comprising:

a housing attachable to a component at a first location;

a first capacitive target sensor disposed within the housing;

a second capacitive target sensor disposed within the housing in a generally parallel relationship with the first capacitive target sensor;

a sensor member positioned in a generally parallel relationship with the first and second capacitive target sensors and disposed between and in contact with the first and second capacitive target sensors, the sensor member attachable to the component at a second location and movable relative to the housing and the first and second capacitive target sensors, the sensor member having a capacitive sensor element movable with the sensor member in a direction generally parallel with the first and second capacitive target sensors to vary the capacitance between the sensor member and the first capacitive target sensor and the second capacitive target sensor; and

a pressure element disposed within the housing and positioned to maintain the sensor member in contact with the first and second capacitive target sensors.

16. The assembly of claim 15, wherein:

the first capacitive target sensor comprises a first target member; and

the second capacitive target sensor comprises a second target member.

17. The assembly of claim 16, further comprising a pressure element disposed within the housing and positioned to maintain the sensor member in contact with the first and second target members.

18. The assembly of claim 16, wherein the sensor member and the first and second target members comprise a generally buckle resistant flexible material.

19. The assembly of claim 16, wherein the sensor member comprises:

a first surface disposed adjacent the first target member; and

a second surface disposed adjacent the second target member;

wherein the sensor element is symmetrically disposed between the first and second surfaces.

20. The assembly of claim 15, further comprising:

a third capacitive target sensor, the first capacitive target sensor disposed between the third capacitive target sensor and the sensor member; and

a fourth capacitive target sensor, the second capacitive target sensor disposed between the fourth capacitive target sensor and the sensor member.

21. The assembly of claim 20, wherein the third capacitive target sensor comprises a first cross sectional area in a plane generally parallel with the third capacitive target sensor and the first capacitive target sensor comprises a second cross sectional area in a plane generally parallel with the first capacitive target sensor, the first cross sectional area larger than the second cross sectional area.

22. The assembly of claim 15, wherein the first capacitive target sensor and the second capacitive target sensor compensate for non-planar displacements of the sensor member.

23. A method for measuring strain in a component, comprising:

attaching a housing comprising a generally parallel first capacitive target sensor and second capacitive target sensor to a first location of a component, a cross sectional area of the first capacitive target sensor in a plane generally parallel with the first capacitive target sensor larger than a cross sectional area of the second capacitive target sensor in a plane generally parallel with the second capacitive target sensor;

attaching a sensor member to a second location of the component, the sensor member positioned generally parallel to the first capacitive target sensor and the second capacitive target sensor, the second capacitive target sensor disposed between the first capacitive target sensor and the sensor member, the sensor member movable relative to the housing and the first and second capacitive target sensors in a direction generally parallel with the first and second capacitive target sensors, the sensor member comprising a capacitive sensor element movable with the sensor member;

attaching a pressure element disposed within the housing and positioned to maintain the sensor member in contact with the first and second capacitive target sensors;

detecting the change in capacitance between the sensor element, the first capacitive target sensor, and the second capacitive target sensor as the sensor member moves relative to the first and second capacitive target sensors; and

determining the strain occurring in the component based on the change in capacitance.

24. A method for measuring strain in a component, comprising:

attaching a housing comprising a generally parallel first capacitive target sensor and second capacitive target sensor to a first location of a component;

attaching a sensor member to a second location of the component, the sensor member positioned generally parallel to the first capacitive target sensor and the second capacitive target sensor and disposed between and in contact with the first capacitive target sensor and the second capacitive target sensor, the sensor member movable relative to the housing and the first and second capacitive target sensors in a direction generally parallel with the first and second capacitive target sensors, the sensor member comprising a capacitive sensor element movable with the sensor member;

attaching a pressure element disposed within the housing and positioned to maintain the sensor member in contact with the first and second capacitive target sensors;

detecting the change in capacitance between the sensor element, the first capacitive target sensor, and the second capacitive target sensor as the sensor member moves relative to the first and second capacitive target sensors; and

determining the strain occurring in the component based on the change in capacitance.

25. A capacitive strain gage assembly, comprising:

a support attachable to a component at a first location;

a first capacitive target sensor coupled to the support;

a second capacitive target sensor coupled to the support in a generally parallel relationship with the first capacitive target sensor;

a third capacitive target sensor coupled to the support in a generally parallel relationship with the second capacitive target sensor;

a fourth capacitive target sensor coupled to the support in a generally parallel relationship with the third capacitive target sensor;

a sensor member positioned in a generally parallel relationship with the first, second, third, and fourth capacitive target sensors and disposed between the second and third capacitive target sensors, the sensor member attachable to the component at a second location and movable relative to the support and the first, second, third, and fourth capacitive target sensors, the sensor member having a capacitive sensor element movable with the sensor member in a direction generally parallel with the first, second, third, and fourth capacitive target sensors to vary the capacitance between the sensor member and the first capacitive target sensor, the second capacitive target sensor, the third capacitive target sensor, and the fourth capacitive target sensor; and

a pressure element coupled to the support and positioned to maintain the sensor member in contact with the second and third capacitive target sensors.

26. The assembly of claim 25, further comprising a pressure element disposed within the housing and positioned to maintain the sensor member in contact with the second and third capacitive target sensors.

27. The assembly of claim 25, further comprising an alignment element coupled to the support and operable to maintain generally unidirectional movement of the sensor member relative to the first, second, third, and fourth capacitive target sensors.

28. The assembly of claim 25, wherein:

the first capacitive target sensor comprises a first cross sectional area in a plane generally parallel with the first capacitive target sensor and the second capacitive target sensor comprises a second cross sectional area in a plane generally parallel with the second capacitive target sensor, the first cross sectional area larger than the second cross sectional area; and

the third capacitive target sensor comprises a third cross sectional area in a plane generally parallel with the third capacitive target sensor and the fourth capacitive target sensor comprises a fourth cross sectional area in a plane generally parallel with the fourth capacitive target sensor, the fourth cross sectional area larger than the third cross sectional area.

29. The assembly of claim 25, wherein the first capacitive target sensor, the second capacitive target sensor, the third capacitive target sensor, and the fourth capacitive target sensor compensate for non-planar displacements of the sensor member.

30. A capacitive strain gage assembly, comprising:

a support attachable to a first location of a component;

a first capacitive target sensor coupled to the support and having a first cross sectional area in a plane generally parallel with the first capacitive target sensor;

a second capacitive target sensor coupled to the support in a generally parallel relationship with the first capacitive target sensor and having a second cross sectional area in a plane generally parallel with the second capacitive target sensor, the second cross sectional area smaller than the first cross sectional area;

a sensor member positioned in a generally parallel relationship with the first and second capacitive target sensors, the second capacitive target sensor disposed between the first capacitive target sensor and the sensor member, the sensor member attachable to the component at a second location and movable relative to the support and the first and second capacitive target sensors, the sensor member having a capacitive sensor element movable with the sensor member in a direction generally parallel with the first and second capacitive target sensors to vary the capacitance between the sensor member and the first capacitive target sensor and second capacitive target sensor; and

a pressure element coupled to the support and positioned to maintain contact between the sensor member and the second capacitive target sensor.

31. The assembly of claim 30, further comprising an alignment element coupled to the support and operable to maintain generally unidirectional movement of the sensor member relative to the first and second capacitive target sensors.

32. The assembly of claim 30, further comprising a pressure element disposed within the housing and positioned to maintain the sensor member in contact with the second capacitive target sensor.

33. A capacitive strain gage assembly, comprising:

a support attachable to a component at a first location;

a first capacitive target sensor coupled to the support;

a second capacitive target sensor coupled to the support in a generally parallel relationship with the first capacitive target sensor;

a sensor member positioned in a generally parallel relationship with the first and second capacitive target sensors and disposed between and in contact with the first and second capacitive target sensors, the sensor member attachable to the component at a second location and movable relative to the support and the first and second capacitive target sensors, the sensor member having a capacitive sensor element movable with the sensor member in a direction generally parallel with the first and second capacitive target sensors to vary the capacitance between the sensor member and the first capacitive target sensor and second capacitive target sensor; and

a pressure element coupled to the support to maintain the sensor member in contact with the first and second capacitive target sensors.

34. The assembly of claim 33, further comprising a pressure element disposed within the housing and positioned to maintain the sensor member in contact with the first and second capacitive target sensors.

35. The assembly of claim 33, further comprising:

a third capacitive target sensor, the first capacitive target sensor disposed between the third capacitive target sensor and the sensor member; and

a fourth capacitive target sensor, the second capacitive target sensor disposed between the fourth capacitive target sensor and the sensor member.

36. The assembly of claim 35, wherein the third capacitive target sensor comprises a first cross sectional area in a plane generally parallel with the third capacitive target sensor and the first capacitive target sensor comprises a second cross sectional area in a plane generally parallel with the first capacitive target sensor, the first cross sectional area larger than the second cross sectional area.

37. The assembly of claim 33, wherein the first capacitive target sensor and the second capacitive target sensor compensate for non-planar displacements of the sensor member.

38. A method for measuring strain in a component, comprising:

attaching a support comprising a generally parallel first capacitive target sensor and second capacitive target sensor to a first location of a component, a cross sectional area of the first capacitive target sensor in a plane generally parallel with the first capacitive target sensor greater than a cross sectional area of the second capacitive target sensor in a plane generally parallel with the second capacitive target sensor;

attaching a sensor member to a second location of the component, the sensor member positioned generally parallel to the first capacitive target sensor and the second capacitive target sensor, the second capacitive target sensor disposed between the first capacitive target sensor and the sensor member, the sensor member movable relative to the support and the first and second capacitive target sensors in a direction generally parallel with the first and second capacitive target sensors, the sensor member comprising a capacitive sensor element movable with the sensor member;

attaching a pressure element coupled to the support to maintain the sensor member in contact with the first and second capacitive target sensors;

detecting the change in capacitance between the sensor element, the first capacitive target sensor, and the second capacitive target sensor as the sensor member moves relative to the first and second capacitive target sensors; and

determining the strain occurring in the component based on the change in capacitance.

39. A method for measuring strain in a component, comprising:

attaching a support comprising a generally parallel first capacitive target sensor and second capacitive target sensor to a first location of a component;

attaching a sensor member to a second location of the component, the sensor member positioned generally parallel to the first capacitive target sensor and the second capacitive target sensor and disposed between and in contact with the first capacitive target sensor and the second capacitive target sensor, the sensor member movable relative to the support and the first and second capacitive target sensors in a direction generally parallel with the first and second capacitive target sensors, the sensor member comprising a capacitive sensor element movable with the sensor member;

attaching a pressure element coupled to the support to maintain the sensor member in contact with the first and second capacitive target sensors;

detecting the change in capacitance between the sensor element, the first capacitive target sensor, and the second capacitive target sensor as the sensor member moves relative to the first and second capacitive target sensors; and

determining the strain occurring in the component based on the change in capacitance.