WO2004059292A2 - Probes for measurements of complex dielectric permittivity of porous and other materials and methods of use thereof - Google Patents

Abstract

A probe and a method for use in a time domain reflectometer (TDR) having a transmision conductor (106) and a ground conductor (104) configured to permit measurement of a dielectric permittivity of materials (108) having small surface areas or small volumes.

Description

PROBES FOR MEASUREMENTS OF COMPLEX DIELECTRIC PERMITTIVITY OF POROUS AND OTHER MATERIALS AND METHODS OF

USE THEREOF

CROSS-REFERENCE TORELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 60/436,336 filed December 23, 2002, which is incorporated herein by reference.

TECHNICAL FIELD

The invention relates to dielectric probes for measuring the dielectric permittivity of a range of materials using time domain reflectometry (TDR), time domain transmission (TDT), or frequency domain techniques and methods of use thereof. More specifically, the invention relates to probes for measuring water content or dielectric content in thin films or irregular samples and for measuring the thickness of materials.

BACKGROUND

Measurement of water content is important in many laboratory, field and industry situations. The moisture content of grains, seeds and food products such as wheat, rice, coffee is an important factor for the storage of grain, determination of the time of harvesting, marketing and processing (S. O. Nelson, V. K. Chari Kandala and Kurt C. Lowrence. "Moisture determination in single grain kernels and Nuts by RF Impedance Measurements". IEEE Trans, on Instrumentation and Measurements, Vol 41, No. 6, December 1992 and Andrezej W. Kraszevaski, "Microwave Aquametry—Needs and Perspective," IEEE Trans, on Microwave theory and techniques, Vol 39, No. 5, May 1991). Measurement of moisture of materials is important in several industrial processes such as production of soaps. Similarly moisture measurement for liquids such as fruit products is essential. Measurement of moisture of materials is important in several industrial processes such as production of soaps, powders, biscuits, sugar syrups and for oil based products such as margarine.

Frequently there is a need for measuring water content under field conditions. This requires that the instrumentation be small, easily transported and rugged. The prior art is generally not very well suited to field situations, and if damaged, is expensive to repair or replace. For example, resonant-cavity-based techniques and other free-air transmission methods are generally poorly suited for field work.

The simplest technique for measuring water content is gravimetrically. Standard gravimetric laboratory tests are tedious. They often requiring several hours for completion and are inherently inaccurate, especially when dealing with small samples sizes. They are also destructive in nature. Further, they are often not practical in field situations.

A number of techniques have been developed for non-destructive determination of water content. For example, there are conductivity-based techniques and capacitance-based techniques that measure the average moisture content in bulk materials. These require a large enough sample size in order to generate meaningful and reproduceable data. In general, these are affected by air gaps and non-uniform shapes in particulate samples.

Waveguide resonant cavity techniques have also been widely used to perform non- destructive moisture measurements as well as complex permittivities of semiconductor materials. While these techniques are considered to be accurate, it is difficult and time consuming to load and unload the samples. Another U.S. Patent, Serial No. 5,420,517 defines a time domain reflectometry waveguide assembly for measuring the moisture content in a medium.

Microwave moisture sensors (U.S. Patent Serial No. 4,991,915) work on the principle of absorption of the microwave energy into the material. These sensors are based on power measurement and must be calibrated for use with each type of material. US Patent Serial No. 6,407,555 discloses a nondestructive microwave instrument for moisture measurement by means of electromagnetic field probe in the vicinity of a stripline, microstripline or coplanar waveguide as an external probing instrument connected to a Time Domain reflectometer.

Another U.S. Pat. No. 5,646,537 describes a time domain reflectometry waveguide assembly for measuring the moisture content in a medium and comprises calculating the apparent dielectric constant value of the medium based on a time delay measured in response to the detectable characteristic reference reflection, and correlating the apparent dielectric constant value with data reflecting the moisture content of the medium. Measurement of dielectric properties can also be important in the industrial processes involving polymers, rubbers, ceramics and plastics for the quality control as well as correct chemical compositions of the product in the liquid, semisolid or solid state. In certain applications there is a need to measure the thickness of a material and this too can be done by measuring dielectric permitttivity. The methods employed are destructive. One non-destructive method for measuring involves the use of a conventional single transmission line conductor. If a conventional single transmission line conductor is used to measure the thickness of a surface, the surface area to be measured must be flat over a reasonably large distance.

Time domain techniques are a powerful measurement tool for determining dielectric permittivity of samples. Conventional TDR measurement techniques typically require that the transmission line be at least 10 cm in length. If straight, parallel conductors are used, the sample under test must be at least as long as the transmission line. As a result, conventional TDR methods are not suitable for small samples, nor are they suitable for samples with an irregular surface. Sample size can be reduced using two spiral conductors, so that circular sample areas of diameters of about 3 cm can be measured.

It is an object of the present invention to overcome the deficiencies in the prior art.

SUMMARY

The present invention provides dielectric probes that allow the measurement of the complex dielectric permittivity of a range of materials using time domain reflectometry (TDR), time domain transmission (TDT), or frequency domain techniques. Such probes can be used to measure, for example:

(i) water content or dielectric constant of thin films, sheets, or layers, including paper and leaf tissues in situ;

(ii) water content or dielectric constant of irregular samples such as grains, food products, pharmaceuticals, and soils; and (iii) paint or oxide thickness on electrically conductive or other surfaces.

Such probes can have a number of physical configurations and a particular probe configuration can be selected for convenience in a particular application. In one embodiment of the invention, a dielectric probe is provided that is comprised of at least one non-linear conductor.

Incident, reflected, and transmitted signal components are delivered to a receiver/detector that is in communication with a processor/display. Based on the detected signal components, amplitude and phase data associated with signal propagation at the dielectric probe are obtained and dielectric constant calculated. Typically the signal source provides a single frequency electrical signal having a frequency that can be adjusted or swept over a selected range of frequencies.

DETAILED DESCRIPTION OF THE DRAWINGS

With reference to FIG. 1, a probe includes a transmission line 102 comprising a ground or reference conductor 104 and a transmission line conductor 106. A dielectric layer 108 can be situated with respect to the transmission line 102 so that electrical signal propagation on the conductors 104, 106 includes contributions from the dielectric layer 108. Spacings, shapes, and other properties of the conductors 104, 106 can be configured based on, for example, properties or geometries of dielectric or other samples.

A representative example is shown in FIG. 2. A dielectric probe includes a transmission line comprising a spiral conductor 202 formed on, or mounted to a circuit board 204 or other substrate. As shown in FIG. 2, a second spiral conductor 203 is also provided. The transmission line includes a planar ground conductor 206 formed as a conductive layer on a circuit board or other substrate, or a conductive sheet. A center conductor 214 and a ground conductor 212 of a coaxial cable 210 are connected to the spiral conductor 202 and the ground conductor 206, respectively by a connector 211. A material under test (not shown in FIG. 2) is typically situated between the ground conductor 206 and the spiral conductor 202 and a dielectric constant of the material under test is determined by measuring a travel time (time delay) of a high frequency pulse applied via the coaxial cable 210. The spiral conductor 202 defines the impedance of the transmission line. As shown in FIG. 2, the spiral conductor 202 is formed on a substrate and in other examples can be formed as a conductive rod or conductor of other cross section, and a distance between the ground conductor 206 and the spiral conductor 202 can be adjustable to accommodate various sample thicknesses, or can be fixed. The time delay can be measured using TDR, TDT, or frequency domain techniques. In other examples, a ground conductor can be configured as rigid, continuous metal sheet, a flexible metallic screen, a porous metal film, or other electrically conductive material. In some examples, the ground conductor is planar, but in other examples the ground conductor can be cylindrical, spherical, conic, or other regular or irregular shape. In some applications (for example, determination of paint thickness on a conductive surface), a sample surface serves as a ground conductor and the other element of the transmission line is held at a fixed distance from this sample surface. Using a ground conductor instead of a second spiral conductor halves the sample surface area needed for a transmission line of the same length.

A spiral conductor/ground plane configuration has several additional advantages compared to dual spiral conductor probes: i) propagating fields are constrained so that fields do not extend beyond the ground plane so that the dielectric permittivity is measured within a fixed, known sample volume; and ii) a spatial sensitivity of the probe is largely constant so that there is a consistent relationship between measured travel times, time delays, or other quantities, and such measurements reflect a volume-weighted average dielectric permittivity of a material situated between the conductors.

In some examples, large sample areas are preferred. In such cases, the spiral conductor/ground plane probe can be configured to measure over a larger area by increasing the distance between traces of the spiral conductor and/or by increasing the length of the spiral component of the transmission line. The maximum useable length of a transmission line can be selected based on an electrical conductivity of a sample, and typically decreases with increasing electrical conductivity.

Probes such as that shown in FIG. 2 have several additional advantages over conventional methods of dielectric permittivity measurement. The instruments with which this probe can be used are inexpensive, small, portable, and rugged enough for field use. The probes themselves are constructed of very low cost materials and can be easily fabricated. As a result, probes can be custom designed for specific applications at low cost or can be used in applications that require single-use, disposable probes.

Example 1. Probe configured for irregular or fluid samples With reference to FIG. 3, a probe 300 is configured for measurements of dielectric permittivity of small, irregular, fluid, or other samples. A spiral conductor 302 and a conductive cup 304 serve as a transmission line conductor, and a ground conductor, respectively. The spiral conductor 302 is configured to serve as a lid to enclose or partially enclose a sample volume defined by the cup 304. A conductor 306 electrically connects a ground plane portion of a circuit board 307 to the cup 304. A second spiral conductor 303 is also provided, and the spiral conductors can be electrically connected to an electrical signal source such as a pulse generator at a connection portion 308. A sample to be measured can be placed in the cup 304, and the conductor 302 situated to form a lid for the container 304. The cup can be configured to define a small volume, so that multiple measurements can be performed to ensure sample uniformity for samples such as pharmaceuticals. Because measurements can be made rapidly with small sample volumes, probes and measurement systems can be configured for rapid measurement of "grab" samples for materials having large variabilities such as food products, grains, or soils. The container permits measurement of liquid samples.

Example 2. Measurements of thin sheets.

Conventional TDR probes are not well suited to the measurement of the dielectric permittivity of thin materials because the sample volume of the probe cannot be confined to the material alone. The properties of a spiral conductor/ground conductor probe permit measurements of thin samples of rigid or flexible materials. In addition, because the ground conductor can be constructed using a flexible, electrically conductive sheet, samples may have either smooth or irregular surfaces. Representative samples include paper, leaves, skin grafts, wood, canvas, ceramics, glass, cloth, printed circuit boards, and other materials.

Referring to FIG. 4, a probe 400 includes a spiral conductor 402 formed on a circuit board 404. Electrical connection to the spiral conductor is provided with a coaxial cable 406 and a connector 408. A sample to be measured can be placed adjacent or in contact with a surface 410 of the circuit board 404.

Example 3. Measurements of dielectric permittivity of "breathing" surfaces.

With reference to FIG. 5A, a probe 500 includes a spiral conductor assembly 502 and a mesh ground conductor 504 that permits liquid and gas transfer to the material under test. This configuration allows for long term and/or continuous measurements without interfering with natural processes occurring within the materials under study. Materials for which this may be important include documents, paintings, cloth, leaves, and skin. The ground conductor 504 can be configured as a conductor on a perforated plate or as a partial conductor coating on a porous substrate. Alternatively, the spiral conductor assembly can be configured with perforations between portions of the spiral conductor, or as a rigid spiral conductor so that the spiral conductor does not require a substrate. FIG. 5B shows another example of a breathable probe.

Example 4. Combined instrument applications The flexibility of design of the probes describe herein allows for probe use in combination with measurement elements including, for example, load cells for sample weighing. Such a combination permits direct determination of relationships between dielectric permittivity and water content of samples during drying. Other combinations include instruments to determine temperature, water potential, salinity, relative humidity, and many others.

Example 5. Additional illustrative probe geometries

Additional probe examples are illustrated in FIGS. 6-8. With reference to FIG. 6, a probe includes one or more serpentine or S-shaped conductors 602, 603. A series of such serpentine conductors can be arranged in a rectangular area. The probe can also be based on three dimensional configurations such as those of FIGS. 7-8. With reference to FIGS. 7A- 7B, a cylindrical spiral conductor 702 is situated along an axis 704. In other examples, a cylindrical spiral extends along a circular arc or other curve. A cylindrical configuration allows for dielectric measurements of non-planar samples. The configurations of FIG. 6 and FIGS. 7A-7B are geometries are representative of many probe shapes and it will be obvious to someone skilled in the art, that these configurations can be modified in arrangement and detail. In addition, a ground conductor can be configured to be two dimensional or three dimensional as well.

FIGS. 8A-8B illustrate example probes having rectangular or square conductor configurations.

Example 6. Network analyzer measurement system

With reference to FIG. 9, a network analyzer based measurement system includes a signal source that delivers an electrical signal to a dielectric probe. Incident, reflected, and transmitted signal components are delivered to a receiver/detector that is in communication with a processor/display. Based on the detected signal components, amplitude and phase data associated with signal propagation at the dielectric probe are obtained and dielectric constant calculated. Typically the signal source provides a single frequency electrical signal having a frequency that can be adjusted or swept over a selected range of frequencies.

Example 7. TDR-based measurement systems

With reference to FIG. 10, a TDR-based measurement system includes a pulse generator that delivers an electrical pulse to a dielectric probe. Reflected portions of the electrical pulse are delivered to a sample and hold circuit that is in communication with a signal processor. A display is configured to display a time delay, or to display a signal portion reflected at an input to the dielectric probe and a signal portion reflected after propagation through the dielectric probe. Based on a time delay between such signal portions, a dielectric constant of a sample is estimated in using a personal computer, or other processing system.

The foregoing description of describes the preferred embodiments and is not meant to be limiting. As would be apparent to one skilled in the art, the ground conductor, for example, can be of a shape other than those described above. Non- planar conductors can also be used. The conductive strip can be straight or a spiral or some other shape. Further, the spacing between the ground conductor and the conductive strip can be fixed or variable. An adjustment mechanism can be provided for selecting a desired separation. In general, probes can be configured for specific applications and probe cost can be low.

The probes of the present invention exhibit several advantages. The volume or surface area of the material under test can be much smaller than that required using conventional TDR probes. The material under test can form one element of the transmission line. Measurements can be made on a volume of material defined by the probe. Material can be enclosed in a "breathable enclosure" that allows dielectric measurements as a function of water content. Mass measurements of samples can be performed in conjunction with dielectric measurements. Measurements can be made on materials with minimal interference with gas exchange using breathable conductors for either a ground conductor or a transmission line conductor.

Claims

What is claimed is:

1. A probe for attachment to a cable for a dielectric permittivity meter, such as a time domain reflectometer (TDR), time domain transmitter (TDT), or a frequency domain monitor, to measure dielectric permittivity of a material, said probe comprising: a transmission conductor comprising a transmission line of a suitable length configured to provide a high ratio of length to surface area to allow production of a sustainable electromagnetic field; a ground conductor, said ground conductor for juxtapositioning with said transmission conductor and spaced from said transmission conductor to accept a material and to allow an electromagnetic field to be generated between said ground conductor and said transmission conductor; and a suitably selected connector for electronic coupling of said transmission conductor and said ground conductor to a cable for a dielectric permittivity meter; such that in use, an electrical signal propagated on the transmission conductor and ground conductor may be modified by a material.

2. The probe of claim 1 wherein said transmission line is in side-by side relation with itself.

3. The probe of claim 2 wherein said transmission line is arranged in essentially parallel relation with itself.

4. The probe of claim 3, wherein said transmission conductor is comprised of at least one transmission line.

5. The probe of claim 3 wherein said transmission conductor is comprised of two transmission lines.

6. The probe of any one of claims 1 to 6 wherein said transmission conductor comprises transmission line that is spirally arranged.

7. The probe of any one of claims 1 to 6 wherein said transmission conductor comprises transmission line that is sigmoidally arranged.

8. The probe of any one of claims 1 to 7, wherein said transmission conductor is essentially planar.

9. The probe of any one of claims 1 to 8 wherein said transmission conductor is mounted on a substrate.

10. The probe of claim 9 wherein said substrate is a circuit board.

11. The probe of any one of claims 1 to 10, wherein said ground conductor is reticulate.

12. The probe of claim 11 wherein said ground conductor comprises mesh.

13. The probe of any one of claims 1 to 12, wherein said ground conductor is essentially planar.

14. The probe of any one of claims 1 to 12 wherein said ground conductor is cup-shaped.

15. The probe of any one of claims 1 to 7 wherein said transmission conductor is coiled around a rod-like ground conductor.

16. A device for measuring dielectric permittivity of a material, said device comprising; a meter selected from the group comprising a time domain reflectometer (TDR), time domain transmitter (TDT), or a frequency domain monitor; a transmission conductor comprising a transmission line of a suitable length configured to provide a high ratio of length to surface area to allow production of a sustainable electromagnetic field; a ground conductor, said ground conductor for juxtapositioning with transmission conductor and spaced from said transmission conductor to accept a material and to allow an electromagnetic field to be generated between said ground conductor and said transmission conductor; and a cable for coupling to said meter; and a suitably selected connector for electronic coupling of said transmission conductor and said ground conductor to said cable, such that in use, the dielectric permittivity of a material can be measured.

17. The device of claim 16 wherein said transmission line is in side-by side relationship with itself.

18. The device of claim 17 wherein said transmission line is arranged in essentially parallel relation with itself.

19. The device of any one of claims 16 to 18, wherein said transmission conductor is comprised of at least one transmission line.

20. The device of any one of claims 16 to 19 wherein said transmission conductor is comprised of two transmission lines.

21. The device of any one of claims 16 to 20 wherein said transmission conductor comprises transmission line that is spirally arranged.

22. The device of any one of claims 16 to 20 wherein said transmission conductor comprises transmission line that is sigmoidally arranged.

23. The device of any one of claims 16 to 22 wherein said transmission conductor is mounted on a substrate.

24. The device of claim 23 wherein said substrate is a circuit board.

25. The device of any one of claims 16 to 24, wherein said ground conductor is reticulate.

26. The device of claim 25 wherein said ground conductor comprises mesh.

27. The device of any one of claims 16 to 26, wherein said transmission conductor is essentially planar.

28. The device of any one of claims 16 to 27, wherein said ground conductor is essentially planar.

29. The device of any one of claims 16 to 27 wherein said ground conductor is cup-shaped.

30. The device of any one of claims 16 to 26 wherein said transmission conductor is coiled around a rod-like ground conductor.

31. A method of measuring dielectric permittivity of a material comprising the steps of; locating a material between a conducting member and a ground member; inducing an electrical signal in said conducting member and said ground member; and measuring a time delay in an electrical signal with a method selected from the group consisting of time domain reflectometery (TDR), time domain transmittance (TDT), or a frequency domain techniques.

32. The method of claim 31 , wherein time domain reflectometery is employed.

33. The method of claim 31 wherein time domain transmittance is employed.

34. The method of claim 31 wherein a frequency domain technique is employed.

35. The probe of any one of claims 1 to 12 wherein said ground conductor is cylindrical.

36. The probe of any one of claims 1 to 12 wherein said ground conductor is spherical.

37. The probe of any one of claims 1 to 13, wherein said ground conductor comprises a porous conductive film.

38. The device of any one of claims 16 to 27, wherein said ground conductor is cylindrical.

39. The device of any one of claims 16 to 27, wherein said ground conductor is spherical.

40. The device of any one of claims 16 to 29, wherein said ground conductor comprises a porous conductive film.

41. The probe of any one of claims 1 to 15, and 35 to 37, wherein said transmission line is a conductive strip.

42. The device of any one of claims 16 to 30, and 38 to 40, wherein said transmission line is a conductive strip.

43. The probe of any one of claims 1 to 15, 35 to 37 and 41, wherein said transmission line is at least approximately 10 cm long.

44. The device of any one of claims 16 to 30, 38 to 40 and 42, wherein said transmission line is at least approximately 10 cm long.

45. The method of claim 31 , further characterized in that the method comprises delivering said electrical signal to a sample and hold circuit that is in communication with a signal processor, said signal processor displaying a signal portion reflected at an input to a dielectric probe and a signal portion reflected after propagation through said dielectric probe, and estimating, based in a time delay between said signal portions, a dielectric permittivity of a material.

46. A network analyzer-based measurement system comprising a signal source that delivers an electrical signal to a dielectric probe.