Apparatus and methods for measuring permeability and conductivity in materials using multiple wavenumber magnetic interrogations

Abstract

Devices and analytical techniques are disclosed for measuring spatial profiles of complex permeability and conductivity of a material by multiple wavenumber interrogations. Coil array structures are disclosed which define a number of different fundamental wavelengths (or wavenumbers). Spatially periodic interrogation signals (of temporal frequency "ω") from the coil array structures are attenuated by varying degrees in the material undergoing analysis, depending on the wavenumber ("k"), thereby permitting the derivation of composite complex permeability/conductivity profile.

What is claimed is: 
     
       1. An apparatus for measuring a property of a material, the apparatus comprising: an array of electromagnetic elements capable of imposing a magnetic field in a material and sensing a resulting electromagnetic response;   a wavenumber controller connected to said array for applying a current to subsets of said electromagnetic elements and thereby defining fundamental wavenumbers associated with said imposed magnetic field in the material; and   an analyzer connected to said wavenumber controller, including means for comparing the resulting responses sensed by said electromagnetic array at different wavenumbers to predictions in order to derive a spatial profile of complex permeability or conductivity in the material.   
     
     
       2. The apparatus of claim 1 wherein the electromagnetic array comprises a plurality of electrically-driven layered coils. 
     
     
       3. The apparatus of claim 1 wherein the electromagnetic array comprises a plurality of layered coil arrays having conduction paths distributed so as to approximate a sinusoidal distribution. 
     
     
       4. The apparatus of claim 1 wherein the electromagnetic array further comprises a plurality of driver coil arrays and at least one sensor coil array, the driver coil arrays each having a different wavelength and each disposed at a different location on the surface of the material. 
     
     
       5. The apparatus of claim 4 wherein the sensing array of coils comprises a plurality of layered coil arrays capable of sensing the flux induced at the surface of the material at a plurality of wavelengths. 
     
     
       6. The apparatus of claim 4 wherein the electromagnetic array further comprises a plurality of coil elements formed as intermeandering conductors. 
     
     
       7. The apparatus of claim 4 wherein the wavenumber controller further comprises electronic switches for applying one current to the appropriate driver coil array. 
     
     
       8. The apparatus of claim 4 wherein the wavenumber controller further comprises means for connecting the terminals of the sensor coil array or arrays so that a signal is detected at the wavelength that is the same as that of a corresponding .[.the.]. driver coil array. 
     
     
       9. The apparatus of claim 1 wherein the permeability/conductivity analyzer is a microcomputer programmed to measure the permeability/conductivity of the material at multiple wavelengths based upon a parametric model of the material and further includes means for performing interactive analyses to obtain a refined spatial profile. 
     
     
       10. A method for measuring the properties of a material, the method comprising: disposing an array of electromagnetic elements in proximity to a material, the electromagnetic elements being adapted to impose a magnetic field in the material and sense a resulting electromagnetic response;   interrogating the material with a magnetic field at multiple wavelengths .[.defines.]. .Iadd.defined .Iaddend.by subsets of said electromagnetic elements;   sensing the resulting electromagnetic response at multiple wavelengths; and   deriving a spatial profile of at least one property of the material from said electromagnetic response.   
     
     
       11. The method of claim 10 wherein the step of interrogating the material further includes sequentially interrogating the material at multiple wavelengths. 
     
     
       12. The method of claim 10 wherein the step of interrogating the material further includes simultaneously interrogating the material at multiple wavelengths. 
     
     
       13. A method for measuring the properties of a material, the method comprising: disposing an array of electromagnetic elements in proximity to a material, the electromagnetic elements being adapted to impose a magnetic field in the material and sense a resulting electromagnetic response;   interrogating the material with a magnetic field at multiple wavelengths defined by subsets of said electromagnetic elements;   sensing the resulting electromagnetic response at multiple wavelengths to derive a spatial profile of at least one property of the material;   measuring the gain at the shortest wavelength to derive a profile estimation for a first material layer; and   measuring the gain with at least one longer wavelength to derive a profile estimation for a subsequent material layer.   
     
     
       14. The method of claim 13 wherein the method further includes employing a parametric model to obtain said estimations for the first and subsequent material layers. 
     
     
       15. The method of claim 13 wherein a linear model is employed to represent the layers. 
     
     
       16. The method of claim 13 wherein an exponential model is employed to represent the layers. 
     
     
       17. The method of claim 13 wherein a continuous model is employed to represent the layers. 
     
     
       18. The method of claim 13 wherein a step function model is employed to represent the layers. 
     
     
       19. The method of claim 13 wherein the method further includes refining the estimate of each layer based on a prior estimation of all the layers other than the one being refined. 
     
     
       20. The method of claim 18 wherein the step of refining the estimate is iteratively repeated for each layer until a best fit to a parametric model is obtained. .Iadd. 
     
     
       21.  An apparatus for measuring a property of a material, the apparatus comprising: an array of electromagnetic elements which impose a magnetic field with a dominant spatial wavenumber, and approximating a sinusoidal distribution in the material to be measured and sense a resulting electromagnetic response;   a signal source which applies an electromagnetic signal to the electromagnetic elements to impose a magnetic field in the material; and   an analyzer connected to the array of electromagnetic elements to sense the resulting electromagnetic response of the material to the imposed magnetic field..Iaddend..Iadd.22. An apparatus for measuring a property of a material, the apparatus comprising:   an array of electromagnetic elements capable of imposing a magnetic field with a dominant spatial wavenumber, the electromagnetic array having conduction paths distributed so as to approximate a sinusoidal distribution and sensing a resulting electromagnetic response;   a signal source which applies an electromagnetic signal to the electromagnetic elements to impose a magnetic field in the material; and   an analyzer connected to the array of electromagnetic elements to sense the resulting electromagnetic response of the material to the imposed magnetic field..Iaddend..Iadd.23. The apparatus of claim 22 wherein the electromagnetic array comprises a driver conductor and at least one sensor coil array..Iaddend..Iadd.24. The apparatus of claim 23 comprising at least two sensor coil arrays on opposite sides of a driver coil array in a   
     
     
        common plane therewith..Iaddend..Iadd.25.  The apparatus of claim 23 wherein the electromagnetic array further comprises a plurality of coil elements formed as meandering conductors with current flow in alternating directions..Iaddend..Iadd.26. An apparatus for measuring a property of a material, the apparatus comprising: an array of electromagnetic elements capable of imposing a magnetic field with a dominant spatial wavenumber, and sensing a resulting electromagnetic response;   a signal source which applies an electromagnetic signal to the electromagnetic elements to impose a magnetic field in the material;   an analyzer connected to the array of electromagnetic elements to sense the resulting electromagnetic response of the material to the imposed magnetic field; and   the analyzer derives complex permeability or conductivity of the material based upon a parametric model of the material..Iaddend..Iadd.27. A method for measuring the properties of a material, the method comprising:   disposing electromagnetic elements in proximity to the material;   through the electromagnetic elements imposing a magnetic field in the material with a dominant spatial wavenumber and additional spatial shorter wavenumbers; and   sensing a resulting electromagnetic response of the material to both dominant and shorter wavenumbers of the imposed magnetic field..Iaddend..Iadd.28. The method of claim 27 wherein the step of sensing is performed by a sensor coil array..Iaddend..Iadd.29. A method for measuring the properties of a material, the method comprising:   disposing electromagnetic elements in proximity to the material;   through the electromagnetic elements imposing a magnetic field in the material with a dominant spatial wavenumber and sensing the electromagnetic response of the material with a sensor coil array of elements disposed on opposite sides of and with the same spatial wavenumber as the electromagnetic elements adapted to impose the magnetic field..Iaddend..Iadd.30. The method of claim 27 wherein the measured properties of the material are complex permeability or conductivity of the material in a layer defined by the spatial wavenumber..Iaddend..Iadd.31. The method of claim 27 wherein the method further includes employing a parametric model to obtain an estimation of the material property from the electromagnetic response of the material..Iaddend..Iadd.32. The method of claim 27 wherein a linear model is employed to represent the electromagnetic response in layers..Iaddend..Iadd.33. The method of claim 27 wherein an exponential model is employed to represent the electromagnetic response in layers..Iaddend..Iadd.34. The method of claim 27 wherein a continuous model is employed to represent the electromagnetic   
     
     
        response in layers..Iaddend..Iadd.35.  The method of claim 27 wherein a step function model is employed to represent the electromagnetic response in layers..Iaddend..Iadd.36. The method of claim 27 wherein in the electromagnetic response is sensed by a sensor coil array of elements disposed parallel to the electromagnetic elements imposing the magnetic field..Iaddend..Iadd.37. The method of claim 36 wherein the elements of the sensor coil array are of the same spatial wavenumber as the array of electromagnetic elements imposing the magnetic field..Iaddend..Iadd.38. The method of claim 36 wherein the sensor coil array and the electromagnetic elements imposing the magnetic field are disposed in a common plane..Iaddend..Iadd.39. The method of claim 36 wherein the sensor coil array and the electromagnetic elements imposing the magnetic field are disposed in parallel but offset planes..Iaddend..Iadd.40. The method of claim 27 wherein the electromagnetic elements are formed as meandering conductors with current flow in alternating directions..Iaddend..Iadd.41. The apparatus of claim 21 wherein the analyzer comprises a sensor coil array parallel to the array of electromagnetic elements capable of 
     
     
        imposing the magnetic field..Iaddend..Iadd.42.  The apparatus of claim 41 wherein the sensor coil array is of the same spatial wavenumber as the array of electromagnetic elements capable of imposing the magnetic field..Iaddend..Iadd.43. The apparatus of claim 41 wherein the sensor coil array and the electromagnetic elements imposing the magnetic field are disposed in a common plane..Iaddend..Iadd.44. The apparatus of claim 41 wherein the sensor coil array and the electromagnetic elements imposing the magnetic field are disposed in parallel but offset 
     
     
        planes..Iaddend..Iadd.45.  An apparatus for measuring a property of a material, the apparatus comprising: electromagnetic elements comprising a driver coil and a separate sensor coil, both driver and sensor coils to be positioned to only one side of a material to be measured, which impose a magnetic field with a dominant spatial wavenumber and shorter spatial wavenumber in the material to be measured and sense a resulting electromagnetic response;   a signal source which applies an electromagnetic signal to the electromagnetic elements to impose a magnetic field in the material; and   an analyzer connected to the electromagnetic elements to sense the resulting electromagnetic response of the material to the imposed magnetic field..Iaddend..Iadd.46. The apparatus of claim 45 comprising at least two sensor coil elements on opposite sides of a driver coil element in a common plane therewith..Iaddend..Iadd.47. An apparatus for measuring a property of a material, the apparatus comprising:   electromagnetic elements comprising a driver coil formed as meandering conductors with current flow in alternating directions and a separate sensor coil, both driver and sensor coils to be positioned to only one side of a material to be measured, capable of imposing a magnetic field with a dominant spatial wavenumber in the material to be measured and sensing a resulting electromagnetic response;   a signal source which applies an electromagnetic signal to the electromagnetic elements to impose a magnetic field in the material; and   an analyzer connected to the electromagnetic elements to sense the resulting electromagnetic response of the material to the imposed magnetic field..Iaddend..Iadd.48. A method for measuring the properties of a material, the method comprising:   disposing electromagnetic elements in proximity to the material;   through the electromagnetic elements imposing a magnetic field in the material with a dominant spatial wavenumber; and   sensing a resulting electromagnetic response of the material to the imposed magnetic field by a sensor coil positioned to only a side of the material in common with the electromagnetic elements imposing the magnetic field..Iaddend..Iadd.49. A method for measuring the properties of a material, the method comprising:   disposing electromagnetic elements in proximity to the material;   through the electromagnetic elements imposing a magnetic field in the material with a dominant spatial wavenumber; and   sensing a resulting electromagnetic response of the material to the imposed magnetic field by a sensor coil positioned to only a side of the material in common with the electromagnetic elements imposing the magnetic field, and   the sensor coil elements disposed on opposite sides of and with the same spatial wavelength as the electromagnetic elements imposing the magnetic field..Iaddend..Iadd.50. An apparatus for measuring a property of a material, the apparatus comprising:   electromagnetic elements capable of imposing a magnetic field with a dominant spatial wavenumber in the material to be measured and sensing a resulting electromagnetic response;   a signal source which applies an electromagnetic signal to the electromagnetic elements to impose a magnetic field in the material; and   an analyzer connected to the electromagnetic elements to sense the resulting electromagnetic response of the material to the imposed magnetic field, wherein the analyzer derives complex permeability or conductivity of the material based upon a parametric model of the material..Iaddend..Iadd.51. An apparatus for measuring a property of a material, the apparatus comprising:   electromagnetic elements capable of imposing a magnetic field with a dominant spatial wavenumber in the material to be measured and sensing a resulting electromagnetic response;   a signal source which applies an electromagnetic signal to the electromagnetic elements to impose a magnetic field in the material; and   an analyzer connected to the electromagnetic elements to sense the resulting electromagnetic response of the material to the imposed magnetic field;   wherein the electromagnetic elements comprise a sensing coil elements of the same spatial wavenumber as driver electromagnetic elements capable of imposing the magnetic field..Iaddend..Iadd.52. An apparatus for measuring a property of a material, the apparatus comprising:   electromagnetic elements capable of imposing a magnetic field with a dominant spatial wavenumber in the material to be measured and sensing a resulting electromagnetic response;   a signal source for applying an electromagnetic signal to the electromagnetic elements to impose a magnetic field in the material; and   an analyzer connected to the electromagnetic elements to sense the resulting electromagnetic response of the material to the imposed magnetic field;   wherein the electromagnetic elements comprise driver coil elements and parallel sensor coil elements, the driver coil and the sensor coil elements being disposed in parallel but offset planes..Iaddend..Iadd.53. A method for measuring the properties of a material, the method comprising:   disposing electromagnetic elements in proximity to the material;   through the electromagnetic elements imposing a magnetic field in the material with a dominant spatial wavenumber; and   sensing a resulting electromagnetic response of the material to the imposed magnetic field;   wherein the electromagnetic response is sensed by sensor coil elements disposed on opposite sides of and with the same spatial wavelength as the electromagnetic elements adapted to impose the magnetic field..Iaddend..Iadd.54. A method for measuring the properties of a material, the method comprising:   disposing electromagnetic elements in proximity to the material;   through the electromagnetic elements imposing a magnetic field in the material with a dominant spatial wavenumber; and   sensing a resulting electromagnetic response of the material to the imposed magnetic field;   wherein the measured properties of the material are complex permeability or conductivity of the material in a layer defined by the spatial wavenumber..Iaddend..Iadd.55. A method for measuring the properties of a material, the method comprising:   disposing electromagnetic elements in proximity to the material;   through the electromagnetic elements imposing a magnetic field in the material with a dominant spatial wavenumber;   sensing a resulting electromagnetic response of the material to the imposed magnetic field; and   employing a parametric model to obtain an estimation of the material property from the electromagnetic response of the   
     
     
        material..Iaddend..Iadd. 6.  The method of claim 55 wherein a linear model is employed to represent the electromagnetic response in layers..Iaddend..Iadd.57. The method of claim 55 wherein an exponential model is employed to represent the electromagnetic response in layers..Iaddend..Iadd.58. The method of claim 55 wherein a continuous model is employed to represent the electromagnetic response in layers..Iaddend..Iadd.59. The method of claim 55 wherein a step function model is employed to represent the electromagnetic response in layers..Iaddend..Iadd.60. A method for measuring the properties of a material, the method comprising: disposing electromagnetic elements in proximity to the material;   through the electromagnetic elements imposing a magnetic field in the material with a dominant spatial wavenumber; and   sensing a resulting electromagnetic response of the material to the imposed magnetic field;   wherein the electromagnetic response is sensed by sensor coil elements disposed parallel to the electromagnet elements imposing the magnetic field and the sensor coil elements are of the same spatial wavenumber as the electromagnetic elements imposing the magnetic field..Iaddend..Iadd.61. A method for measuring the properties of a material, the method comprising:   disposing electromagnetic elements in proximity to the material;   through the electromagnetic elements imposing a magnetic field in the material with a dominant spatial wavenumber; and   sensing a resulting electromagnetic response of the material to the imposed magnetic field;   wherein the electromagnetic response is sensed by sensor coil elements disposed parallel to the electromagnetic elements imposing the magnetic field and the sensor coil elements and the electromagnetic elements imposing the magnetic field are disposed in a common   
     
     
        plane..Iaddend..Iadd. 2.  A method for measuring the properties of a material, the method comprising: disposing electromagnetic elements in proximity to the material;   through the electromagnetic elements imposing a magnetic field in the material with a dominant spatial wavenumber; and   sensing a resulting electromagnetic response of the material to the imposed magnetic field;   wherein the electromagnetic response is sensed by sensor coil elements disposed parallel to the electromagnetic elements imposing the magnetic field and the sensor coil elements and the electromagnetic elements imposing the magnetic field are disposed in parallel but offset planes..Iaddend..Iadd.63. A method for measuring the properties of a material, the method comprising:   disposing electromagnetic elements in proximity to the material;   through the electromagnetic elements imposing a magnetic field in the material with a dominant spatial wavenumber; and   sensing a resulting electromagnetic response of the material to the imposed magnetic field;   wherein the electromagnetic elements are formed as meandering conductors   
     
     
        with current flow in alternating directions..Iaddend..Iadd.64.  The apparatus of claim 21 wherein the electromagnetic elements comprise a driver coil and at least one separate sensor coil, both to be positioned to only one side of the material to be measured..Iaddend..Iadd.65. A method as claimed in claim 27 wherein the resulting electromagnetic response is sensed by a sensor coil positioned to only a side of the 
     
     
        material in common with the electromagnetic elements..Iaddend..Iadd.66. An apparatus for measuring a property of a material, the apparatus comprising: an array of electromagnetic elements capable of imposing a magnetic field using a continuous meandering drive winding conductor with a dominant spatial wavenumber in the material to be measured and sensing a resulting electromagnetic response;   a signal source which applies an electromagnetic signal to the electromagnetic elements to impose a magnetic field in the material; and   an analyzer connected to the array of electromagnetic elements to sense the resulting electromagnetic response of the material to the imposed magnetic field..Iaddend.