Capacitive position sensor and sensing methodology

• US 20050030049A1
• Filed: 07/01/2004
• Published: 02/10/2005
• Est. Priority Date: 07/01/2003
• Status: Active Grant

• Current Assignee: Tiax, LLC
• Original Assignee: Tiax, LLC
• Inventors: Chertok, Allan
• Granted Patent: US 7,141,988 B2
• Time in Patent Office: Days
• US Class Current: 324/661
• CPC Class Codes: G01D 5/2412 by varying overlap

Abstract

A simple and robust variable coaxial capacitive sensor and detection method for monitoring the position of a rapidly reciprocating member such as a piston or displacer in a free piston Stirling engine. The coaxial capacitor of the present invention in a preferred embodiment thereof is configured to modulate capacitive electrode area rather than inter-electrode spacing and as a result a highly linear transfer function can be achieved. Also disclosed are detection methods which derive and process signals in connection with applications which have small sensor capacitance variations while suppressing stray capacitance error.

Claims

1. A sensor for determining the position of a reciprocating member, said sensor comprising:

a fixed electrode support structure comprising a fixed electrode and a guard electrode, said fixed electrode being electrically isolated from said guard electrode; and

a movable electrode, said movable electrode positioned to move in direct proportion to the movement of said reciprocating member;

wherein said fixed electrode and said movable electrode form a capacitive structure, said capacitive structure having a variable capacitance which is directly proportional to the instantaneous position of said reciprocating member.

2. The sensor of claim 1 further comprising a voltage source and wherein said guard electrode is driven at a potential which is the same as the potential at said fixed electrode.

3. The sensor of claim 1 wherein said movable electrode is structurally and electrically in contact with a piston of an engine.

4. The sensor of claim 3 wherein said engine is a free piston Stirling engine.

5. The sensor of claim 4 wherein said piston drives a linear alternator.

6. The sensor of claim 4 wherein said piston drives a compressor.

7. The sensor of claim 3 wherein said engine is a free piston internal combustion engine.

8. The sensor of claim 1 wherein said movable electrode is structurally and electrically in contact with a displacer of a Stirling engine.

9. The sensor of claim 1 wherein said movable electrode comprises a sleeve of an alternator.

10. The sensor of claim 1 wherein said fixed electrode is formed from plastic and further comprises at least one area of conductive plating.

11. The sensor of claim 1 wherein said variable capacitance results from a variance in the amount of overlap between said fixed electrode and said movable electrode.

12. The sensor of claim 1 wherein said fixed electrode and said movable electrode are cylindrical in shape.

13. The sensor of claim 12 wherein said movable electrode and said fixed electrode are positioned co-axially and wherein said movable electrode selectively slides over said fixed electrode.

14. The sensor of claim 1 further comprising a phase lead compensation network for correcting low-pass filter phase lag.

15. A free piston Stirling engine including a power piston, said free piston Stirling engine capable of determining the instantaneous position of said power piston via a capacitive position sensor, said capacitive position sensor comprising:

a fixed electrode support structure comprising a fixed electrode and a guard electrode, said fixed electrode being electrically isolated from said guard electrode; and

a movable electrode, said movable electrode positioned to move in direct proportion to the movement of said power piston;

wherein said fixed electrode and said movable electrode form a capacitive structure, said capacitive structure having a variable capacitance which is directly proportional to the instantaneous position of said power piston.

16. The free piston Stirling engine of claim 15 wherein said fixed electrode and said movable electrode are cylindrical in shape, wherein said movable electrode and said fixed electrode are positioned co-axially, and wherein said movable electrode selectively slides over said fixed electrode.

17. A capacitive position sensor comprising:

a fixed electrode support structure comprising a fixed electrode and a guard electrode, said fixed electrode being electrically isolated from said guard electrode; and

a movable electrode, said movable electrode positioned to move in direct proportion to the movement of a reciprocating member;

wherein said guard electrode is driven at a potential which is the same as that of said fixed electrode.

18. The position sensor of claim 17 wherein said position sensor is embodied in a free piston Stirling engine.

19. The position sensor of claim 17 wherein said position sensor is embodied in a free piston internal combustion engine.

20. The position sensor of claim 18 wherein said free piston Stirling engine drives a linear alternator.

21. The position sensor of claim 20 wherein said linear alternator drives a free piston Stirling machine as a compressor.

22. The position sensor of claim 17 wherein said guard electrode and said fixed electrode are driven by a constant voltage source generating a constant amplitude triangle modulating wave.

23. The position sensor of claim 17 wherein said guard electrode and said fixed electrode are driven by a voltage source generating a constant amplitude sine modulating wave.

24. The position sensor of claim 17 further comprising an AC current sensor said AC current sensor providing an input signal to a signal amplifier and said signal amplifier providing an output signal which is indicative of the capacitance between said fixed electrode and said movable electrode.

25. The position sensor of claim 22 further comprising a second constant voltage source and a ground referenced current sensing shunt resistor wherein said second constant voltage source drives said guard electrode rather than said first constant voltage source.

26. The position sensor of claim 23 further comprising a second constant voltage source and a ground referenced current sensing shunt resistor wherein said second constant voltage source drives said guard electrode rather than said first constant voltage source.

27. The position sensor of claim 17 further comprising a transimpedance amplifier module to monitor current.

28. The position sensor of claim 17 further comprising a constant amplitude current source and a voltage follower, said voltage follower adjusting voltage to ensure that the potential at said guard electrode is essentially the same as the potential at said current source.

29. The position sensor of claim 17 further comprising a phase lead compensation network for correcting low-pass filter phase lag.

30. A method for determining the position of a reciprocating member, said method comprising the steps of:

(i) providing a fixed electrode and a guard electrode, said fixed electrode being electrically isolated from said guard electrode;

(ii) providing a movable electrode, said movable electrode positioned to move in direct proportion to the movement of said reciprocating member; and

(iii) periodically measuring the capacitance between said fixed electrode and said movable electrode in order to determine the instantaneous position of said reciprocating member.

31. The method of claim 30 wherein said capacitance between said fixed electrode and said movable electrode is directly proportional to the instantaneous position of said reciprocating member.

32. The method of claim 30 wherein said guard electrode is driven at a potential which is the same as the potential at said fixed electrode.

33. The method of claim 30 wherein said reciprocating member comprises a piston in a free piston Stirling engine.

34. The method of claim 30 wherein said reciprocating member comprises a displacer in a free piston Stirling engine.

35. The method of claim 33 wherein said piston drives a linear alternator.

36. The method of claim 30 wherein said movable electrode is structurally and electrically in contact with said reciprocating member.

37. The method of claim 30 wherein said reciprocating member is a piston in a free piston internal combustion engine.

38. The method of claim 30 wherein said capacitance varies over time in response to a variation in the amount of overlap between said fixed electrode and said movable electrode.

39. The method of claim 30 wherein said step of periodically measuring the capacitance further comprises the step of:

providing an AC current sensor, said AC current sensor measuring the current flowing through the capacitor formed by said fixed electrode and said movable electrode;

wherein said output of said AC current sensor is indicative of the capacitance of said capacitor and the position of said reciprocating member.

40. A method for controlling the movement of a reciprocating member, said method comprising the steps of:

(i) providing a fixed electrode and a guard electrode, said fixed electrode being electrically isolated from said guard electrode;

(ii) providing a movable electrode, said movable electrode positioned to move in direct proportion to the movement of said reciprocating member;

(iii) periodically measuring the capacitance between said fixed electrode and said movable electrode in order to determine the instantaneous position of said reciprocating member; and

(iv) controlling the movement of said reciprocating member based upon said determined instantaneous position of said reciprocating member.

41. The method of claim 40 wherein said reciprocating member is a piston.

42. The method of claim 41 wherein said piston is a component of a Stirling engine.

43. The method of claim 40 wherein said reciprocating member is a displacer in a Stirling engine.

44. The method of claim 40 wherein said step of controlling the movement comprises adjusting fuel flow rate.