Method and apparatus for self-calibration of capacitive sensors

• US 6,545,495 B2
• Filed: 04/17/2001
• Issued: 04/08/2003
• Est. Priority Date: 04/17/2001
• Status: Expired due to Fees

• Current Assignee: UT-Battelle LLC
• Original Assignee: UT-Battelle LLC
• Inventors: Warmack, Robert J., Britton, Charles L. Jr.
• Primary Examiner(s): Oda, Christine
• Assistant Examiner(s): KERVEROS, JAMES C
• Application Number:
  US09/837,142
• Publication Number:
  US 20020158637A1
• Time in Patent Office: 721 Days
• Field of Search: 324/762, 324/761, 324/765, 324/752, 324/658, 324/690, 324/750, 324/244, 250/306, 250/216, 250/324, 731/05, 73/240.6
• US Class Current: 324/601
• CPC Class Codes: G01R 35/005 Calibrating; Standards or reference devices, e.g. voltage or resistance standards, "golden" references G01R33/0035, G01R35/002 take precedence

Abstract

A method for determining operational characteristics of capacitive sensors adapted for self-calibration includes the steps of providing at least one capacitive sensor having a suspended element and a pick-up plate. An AC input signal is applied to one of the plates, while force is provided between the plates, the force capable of variation. At least one operational characteristic of capacitive sensors can be determined, such as the suspended element spring constant and the gap distance between the plates. A capacitive sensor system adapted for self-calibration includes at least one capacitive sensor having a suspended element and a pick-up plate, a self-testing and calibration network, and a structure for generating force between the plates, the force capable of variation.

Claims

1. A method for determining operational characteristics of capacitive sensors, comprising the steps of:

providing at least one capacitive sensor, said capacitive sensor having plates, said plates including a suspended element and a pick-up plate, said suspended element capable of displacement relative to said pick-up plate by application of a force, said displacement and said force related by a spring constant;

applying an AC input signal to one of said plates,

providing force between said plates, said force capable of variation,

measuring a displacement current signal or a signal derived from said displacement current which is induced by application of said AC input signal to one of said plates, and

determining at least one operational characteristic of said capacitive sensor from parameters including said measured displacement current signal or a signal derived from said displacement current signal.

2. The method of claim 1, wherein said AC input signal is a voltage signal.

3. The method of claim 1, wherein said AC input signal is a current signal.

4. The method of claim 1, wherein a distance (gap) between said plates is determined in said determining step.

5. The method of claim 4, wherein said force includes an electrostatic force provided by applying a DC bias between said plates.

6. The method of claim 5, further comprising the steps of varying an amplitude of said DC bias and measuring at least one resulting output signal, wherein a spring constant for said suspended element is determined during said determining step.

7. The method of claim 5, wherein said force includes a magnetic force between said plates.

8. The method of claim 7, wherein said magnetic force is a repulsive force.

9. The method of claim 7, wherein said magnetic force is substantially equal in magnitude and opposite in direction to said electrostatic force.

10. The method of claim 7, further comprising the step of varying an amplitude of said DC bias and measuring at least one resulting output signal, wherein a spring constant for said suspended element is determined during said determining step.

11. The method of claim 10, further comprising the step of applying at least one of said spring constant and said gap to field measurements made by said capacitive sensor.

12. The method of claim 11, wherein at least one of said spring constant and said gap is automatically updated during said field use.

13. The method of claim 12, wherein at least one of said spring constant and said gap is continuously updated during said field use.

14. The method of claim 1, wherein said force includes a magnetic force.

15. The method of claim 14, further comprising the step of providing magnetic ones of said suspended element and said pick-up plate.

16. The method of claim 15, wherein said providing magnetic ones includes providing said plates with at least one source of magnetism selected from the group of magnetically polarizable material and electromagnets.

17. The method of claim 1, wherein a spring constant and a gap is determined in said determining step, further comprising the step of storing said spring constant and said gap.

18. A method for operating capacitive sensors, comprising the steps of:

providing at least one capacitive sensor and at least one self-testing and calibration network, said capacitive sensor having plates, said plates including a suspended element and a pick-up plate, said suspended element capable of displacement relative to said pick-up plate by application of a force, said displacement and said force related by a spring constant;

applying an AC input signal to one of said plates,

providing force between said plates, said force capable of variation,

measuring a displacement current signal or a signal derived from said displacement current which is induced by application of said AC input to one of said plates, and

determining at least one operational characteristic of said capacitive sensor from parameters including said measured displacement current signal or a signal derived from said displacement current signal.

19. The method of claim 18, wherein at least one of a spring constant for said suspended element and a gap between said plates is determined during said determining step.

20. The method of claim 18, further comprising the step of storing at least one of said spring constant and said gap.

21. The method of claim 19, wherein at least one of said spring constant and said gap are updated automatically during field use of said capacitive sensor.

22. The method of claim 19, further comprising the steps generating a measurement signal responsive to the detection of the presence of at least one material, and applying at least one of said spring constant and said gap to said measurement signal.

23. The method of claim 21, wherein both said spring constant and said gap are applied to said measurement signal.

24. A method for determining gap distances between capacitive sensor plates, comprising the steps of:

providing at least one capacitive sensor, said capacitive sensor having plates, said plates including a suspended element and a pick-up plate, said suspended element capable of displacement relative to said pick-up plate by application of a force, said displacement and said force related by a spring constant;

applying an AC input signal to one of said plates for generating a displacement current signal across said capacitive sensor;

measuring said displacement current signal or a signal derived from said displacement current, and

determining a gap distance between said plates from said resulting output signal from parameters including said measured displacement current signal or a signal derived from said displacement current signal.

25. The method for claim 24, further comprising the step of storing said gap.

26. The method for claim 24, further comprising the step of automatically updating said gap during field use of said capacitive sensor.

27. The method for claim 26, wherein said gap is continuously updated during said field use.

28. The method for claim 24, further comprising the steps of generating a measurement signal during field use responsive to the detection of the presence of at least one material, and applying said gap to said measurement signal.

29. The method of claim 28, wherein said gap is automatically updated during said field use.

30. The method of claim 29, wherein said gap is continuously updated during said field use.

31. A method for determining at least one operational charactertistic of capacitive sensors, comprising the steps of:

providing at least one capacitive sensor, said capacitive sensor having plates, said plates including a suspended element and a pick-up plate, said suspended element capable of displacement relative to said pick-up plate by application of a force, said displacement and said force related by a spring constant;

providing force between said plates, said force capable of variation;

applying an AC input signal to one of said plates to produce a displacement current through said capacitive sensor;

directing an optical beam onto said suspended element to generate a reflected beam;

detecting said reflective optical beam, and

determining at least one operational characteristic of said capacitive sensor from parameters including said measured displacement current signal or a signal derived from said displacement current signal.

32. The method of claim 31, wherein said operational characteristic include a gap between said plates.

33. The method of claim 32, further comprising the step of optically determining said gap using said detected optical beam, wherein a measured electrical gap can be compared to said optically determined gap.

34. A capacitive sensor system, comprising:

at least one capacitive sensor, said capacitive sensor having plates, said plates including a suspended element and a pick-up plate, said suspended element capable of displacement relative to said pick-up plate by application of a force, said displacement and said force related by a spring constant;

at least one self-testing and calibration network switchably connected to said capacitive sensor, said self-testing and calibration network including a processor, structure for generation of force between said plates, said force capable of variation, structure for applying an AC input signal to one of said plates, and structure for measuring a displacement current signal or a signal derived from said displacement current which is induced by application of said AC input signal across said capacitive sensor.

35. The system of claim 34, wherein said processor is adapted for determining parameters including at least one of a gap distance between said plates and a spring constant of said suspended element.

36. The system of claim 34, further comprising a memory.

37. The system of claim 34, wherein said processor is adapted to automatically apply at least one of said spring constant and said gap to measurement signals resulting from the detection of the presence of at least one material obtained during field use of said capacitive sensor.

38. The system of claim 37, wherein at least one of said spring constant and said gap is periodically updated during said field use.

39. The system of claim 38, wherein at least one of said spring constant and said gap is continuously updated during said field use.

40. The system of claim 34, wherein said structure for generation of force includes a DC bias applied between said plates, said DC bias creating an electrostatic force between said plates.

41. The system claim 34, wherein structure for generation of force includes magnetic ones of said suspended element and said pick-up plate, said magnet ones creating a magnetic force between said plates.

42. The system of claim 41, wherein said magnetic ones includes at least one source of magnetism selected from the group of magnetically polarizable material and electromagnets.

43. The system of claim 41, wherein said magnetic force is substantially equal in magnitude and opposite in direction to an electrostatic force between said plates.

44. The system of claim 34, further comprising a bulk substrate material having a plurality of die, at least one of said capacitive sensors formed on at least one of said plurality of die.

45. The system of claim 44, further comprising a package for bonding die to said package, wherein said self-testing and calibration network is contained in said package with at least one of said capacitive sensors.

46. The system of claim 45, wherein said self-testing and calibration network and at least one of said capacitive sensors are formed on a single one of said plurality of die.

47. The system of claim 45, further comprising at least one oscillator circuit for generating an AC signal from a DC input.

48. The system of claim 47, wherein said self-testing and calibration network, said oscillator circuit and at least one of said capacitive sensors formed on at least one of said plurality of die.