Electromagnetic method of liquid level monitoring

• US 20030019291A1
• Filed: 09/24/2002
• Published: 01/30/2003
• Est. Priority Date: 08/14/1998
• Status: Active Grant

• Current Assignee: MTS Systems Corporation (Illinois Tool Works Inc.)
• Original Assignee: MTS Systems Corporation (Illinois Tool Works Inc.)
• Inventors: Pchelnikov, Yuriy N., Nyce, David S.
• Granted Patent: US 6,564,658 B2
• Time in Patent Office: Days
• US Class Current: 73/290V
• CPC Class Codes: G01F 23/284 Electromagnetic waves

Abstract

A method and apparatus for monitoring one or more parameters of a variable physical structure, such as liquid level, is disclosed. The method and apparatus includes an electrodynamic element placed in proximity to a monitored structure and exciting within said element an alternating electromagnetic field. The electromagnetic field should be at a frequency at which the electromagnetic field penetrates into the monitored structure and then variations of the electromagnetic field parameters are measured for the element caused by a variation in the structure. The exciting of the electrodynamic element is by an electromagnetic field in the form of at least one slowed electromagnetic wave having suitable energy distribution of the electric and magnetic fields for the measuring of the electromagnetic field parameters.

Claims

1. A method for monitoring one or more parameters of a variable physical structure, comprising:

placing an electrodynamic element in proximity to the monitored structure (3, 8);

exciting in said element an alternating electromagnetic field at a frequency at which the electromagnetic field penetrates into the monitored structure (3, 8);

measuring of the variation of at least one of the electromagnetic field parameters of said element caused by variation of the structure;

said exciting of said element is by an electromagnetic field in the form of at least one slowed electromagnetic wave having suitable energy distribution of the electric and magnetic fields for the measuring of the variation of the propagation constant of the electromagnetic field, as said electromagnetic field parameter.

2. The method of claim 1, wherein the variable physical structure is a liquid level and said parameter is a liquid level measurement.

3. The method of claim 2, wherein said electrodynamic element is placed in parallel with surface of the liquid.

4. The method of claim 2, wherein said electrodynamic element is positioned at an angle to the surface of the liquid.

5. The method of claim 2, wherein said electrodynamic element is placed perpendicular to the surface of the liquid.

6. The method of claim 2, wherein

said electromagnetic field is excited in the form of at least two slowed electromagnetic waves at different frequencies;

variation of the propagation constants of both being converted into a level and a monitored permittivity.

7. The method according to claim 6, wherein

the field of one said slowed electromagnetic wave in the monitored structure (3, 8) is set as the plus and minus space harmonics while the field of another said slowed electromagnetic wave is set as zero space harmonic.

8. The method according to claim 2, wherein

said electromagnetic field is excited in the form of at least one slowed electromagnetic wave with electric field shifted in the monitored volume.

9. The method according to claim 8, wherein

said slowed electromagnetic wave is excited as an in-phase type wave.

10. The method according to claim 2, wherein

said electromagnetic field is excited in the form of at least one slowed electromagnetic wave with magnetic field shifted in the monitored volume;

11. The method according to claim 10, wherein

said slowed electromagnetic wave is excited as an anti-phase type wave.

12. The method according to claim 2 wherein

said electromagnetic field is excited in the form of two slowed electromagnetic waves with one in-phase type wave, the other anti-phase type wave.

13. The method according to claim 2, wherein

the field of said slowed electromagnetic wave in the monitored structure is set as a zero space harmonic.

14. The method according to claim 2, wherein

the field of said slowed electromagnetic wave in the monitored structure is set as the plus and minus space harmonics.

15. The method according to claim 2, wherein

the variation of the real part of the propagation constant is measured.

16. The method according to claim 15, wherein

the variation of the real part of the propagation constant is converted into the variation of the slowed wave attenuation in the electrodynamic element.

17. The method according to claim 2, wherein

the variation of the imaginary part of the propagation constant is measured.

18. The method according to claim 17, wherein

the variation of the imaginary part of the propagation constant is converted into a phase delay variation in the electrodynamic element.

19. The method according to claim 17, wherein

the variation of the imaginary part of the propagation constant is converted into variation frequency of a generator.

20. The method according to claim 17, wherein

the variation of the imaginary part of the propagation constant is converted into a resonant frequency of the electrodynamic element variation.

21. The method according to claim 2, wherein

the distance δ

′

between the electrodynamic element and monitored fluid is chosen in accordance to condition;

Δ

l/l=(ε

₁/ε

)(th δ

′

τ

−

cth δ

′

τ

)Δ

ε

/ε

where τ

is the transverse constant, ε

is a relative permittivity of the liquid, Δ

ε

/ε

is a permittivity ε

relative alteration, ε

₁ is a relative permittivity of a medium between the electrodynamic element and fluid, Δ

l/l is the allowable relative measuring error of the level l measurement.

22. The method according to claim 2, wherein

a slowing n of said surface electromagnetic wave is approximately the square root of a relative permittivity ε

of the monitored fluid.

23. The method according to claim 1, wherein

said electromagnetic field is excited in the form of two slowed waves with one inphase type wave, the other anti-phase type wave.

24. The method according to claim 1, wherein

the field of said slowed electromagnetic wave in the monitored structure is set as a zero space harmonic.

25. The method according to claim 1, wherein

the field of said slowed electromagnetic wave in the monitored structure is set as the plus and minus one space harmonics.

26. The method according to claim 1, wherein

the variation of the real part of the propagation constant is measured.

27. The method according to claim 1, wherein

the variation of the imaginary part of the propagation constant is measured.

28. The method according to claim 1, wherein there is further included the step of converting said variation into a representation of the parameter.

29. An apparatus monitoring one or more parameters of variable physical structure, comprising:

an electrodynamic element 1, a measuring circuit, including at least radio frequency generator 5, a primary transducer 6 and a converter converting an electric signal into the parameter;

said electrodynamic element 1 includes an input 12, an output 13 of RF signal and a section of a slow-wave structure 11 set to distribute in a given ratio the components of an electric and magnetic fields in the monitored structure (“

a multipole”

);

said slow-wave structure further including at least two conductors 14, 16, at least one of them being an impedance conductor 14 facing the monitored structure and fashioned as row of conducting members 50 arranged in series in the direction of the slowed wave propagation and connected to one another with spacing 52.

30. An apparatus according to claim 29, wherein

said multipole includes an additional section of a two-conductor slow-wave structure.

31. An apparatus according to claim 29, wherein

said multipole includes at least one inductance.

32. An apparatus according to claim 29, wherein

said multipole includes at least one capacitance.

33. An apparatus according to claim 29, wherein

said slow-wave structure comprising at least one screen conductor.

34. An apparatus according to claim 29, wherein

said period T of said row of conducting members alters along the slow-wave structure.

35. An apparatus according to claim 29, wherein

the distance between said two conductors of said slow-wave structure alters along its length.

36. An apparatus according to claim 29, wherein

the width of at least one of said conductors of said slow-wave structure alters along its length.

37. An apparatus according to claim 29, wherein

said impedance conductor includes at least one-wire cylindrical helix.

38. An apparatus according to claim 37, wherein

said screen conductor includes a rod.

39. An apparatus according to claim 37, wherein

said screen conductor includes a cylinder.

40. The apparatus according to claim 37, wherein

said screen conductor includes at least one tape.

41. The apparatus according to claim 29, wherein

said impedance conductor includes at least one-wire radial spiral, the screen conductor including at least one radial beam.

42. The apparatus according to claim 29, wherein

said slow-wave structure comprising at least two impedance conductors.

43. The apparatus according to claim 42, wherein

said impedance conductors facing each other and are configured as mirror images of one another turned through 180°

.

44. The apparatus according to claim 43, wherein

said impedance conductors are made as radial spirals with opposite directions of winding.

45. The apparatus according to claim 29, wherein

said impedance conductors being identical and placed on the same surface one in the other.

46. The apparatus according to claim 45, wherein

the structure includes a cylinder having a dielectric wall;

and

said impedance conductors form a symmetrical slow-wave structure and are installed on the dielectric wall of the container, splitting said impedance conductors from the monitored volume, the period T of impedance conductors being chosen in accordance with the condition;

T>

π

δ

where δ

is a dielectric wall thickness.

47. The apparatus according to claim 45, wherein

the structure includes a cylinder having a dielectric wall;

and

said impedance conductors form a two-stage slow-wave structure and are installed on the dielectric wall of the container, splitting said impedance conductors from the monitored volume; and

the period T of impedance conductors is chosen in accordance with the condition;

T>

2π

δ

,

where δ

is a dielectric wall thickness.

48. The apparatus according to claim 47, wherein

said impedance conductors are made as zigzag-lines.

49. The apparatus according to claim 47, wherein

said impedance conductors are made as vertically placed interdigital combs with fingers inclined so that the bases of the each finger of both combs are on the same horizontal plane.

50. The apparatus according to claim 47, wherein

said impedance conductors are made as interdigital combs and are installed on the dielectric wall of the container with angle ζ

to a vertical in accordance to condition;

Sinζ

=T/2a

where T is period of combs, α

is the fingers length.

51. The apparatus according to claim 46, wherein

said impedance conductors are made as cylindrical helices, mounted on a dielectric base and forming a bifilar helix.

52. The apparatus according to claim 29, wherein said multipole is a dipole.

53. The apparatus according to claim 29, wherein said multipole is a quadripole.

54. The apparatus according to claim 29, wherein said multipole is a hexipole.