WO2003044453A1

WO2003044453A1 - Measurement of the thickness of metals using a spectrum of electromagnetic frequencies and magnetic saturation

Info

Publication number: WO2003044453A1
Application number: PCT/US2001/046119
Authority: WO
Inventors: K. Amini Bijan
Original assignee: Em-Tech Llc
Priority date: 2001-11-20
Filing date: 2001-11-20
Publication date: 2003-05-30
Also published as: AU2002220161A1

Abstract

A method is provided for creating a spectral EM frequency to calculate the thickness of a material with unknown permeability and conductivity using metallic transparencies. The method comprises the steps of testing empirically to approximate the conductivity, testing empirically to approximate the permeability, creating a first set of electromagnetic waves adjacent to the material to be measured of a relatively low frequency, impinging the first set of electromagnetic waves on the material for saturating the material, creating a second set of electromagnetic waves having specific constant amplitude of a higher frequency than the first set of electromagnetic waves, the second set of electromagnetic waves for engaging the material and generating a transmitter signal having modified characteristics, and receiving the transmitter signal through the saturated material such that the modified characteristics of the transmitter signal are processed to determine the thickness of the material.

Description

TITLE

MEASUREMENT OF THE THICKNESS OF METALS USING A SPECTRUM OF ELECTROMAGNETIC FREQUENCIES AND MAGNETIC SATURATION

FIELD OF THE INVENTION

The present invention relates generally to utilizing controlled transmissions of electromagnetic (EM) energy through or across ferromagnetic materials, including their alloys to determine the thickness and EM characteristics of the materials.

BACKGROUND OF THE INVENTION

It has long been possible to measure metallic thickness variations by electromagnetics. Prior methods have typically excited the metal by eddy currents or dc fields. After excitation, the known methods looked for variations in amplitude of the signal corresponding to variations in metallic thickness. Ferromagnetic metals such as iron and carbon steel have, however, been considered barriers to electromagnetic energy, preventing EM waves or signals from be transmitted across or through such materials.

SUMMARY OF THE INVENTION To achieve the foregoing objects, features, and advantages and in accordance with the purpose of the invention as embodied and broadly described herein, a method is provided for creating a spectral EM frequency to calculate the thickness of a material with unknown permeability and conductivity using metallic transparencies. The method comprises the steps of (a) testing empirically to approximate the conductivity, (b) testing empirically to approximate the permeability, (c) creating a first set of electromagnetic waves adjacent to the material to be measured of a relatively low frequency, (d) impinging the first set of electromagnetic waves on the material for saturating the material, (e) creating a second set of electromagnetic waves having specific constant amplitude of a higher frequency than the first set of electromagnetic waves, the second set of electromagnetic waves for engaging the material and generating a transmitter signal having modified characteristics, and (f) receiving the transmitter signal through the saturated material such that the modified characteristics of the transmitter signal are processed to determine the thickness of the material.

In another embodiment, a method to calculate the thickness of a material of unknown permeability (μ) and conductivity (σ) is provided utilizing a spectrum of EM frequencies and amplitude. The method incorporates the relationship between skin depth of EM energy penetration and the frequency of the EM energy oscillations, material permeability and material conductivity. This relationship is commonly depicted wherein the skin depth (δ) is the depth into the material where the strength of the impinging EM energy is attenuated to 1/e of the original signal strength.

and L is a unit of length, e.g., meters or centimeters. The invention also utilizes the relationship between the depth of penetration into a material and the conductivity and permeability of the material, as shown in the relationship

where

-5= penetration depth σ = conductivity

/ = frequency, μ_r = relative permeability, and μo = absolute permeability, The invention determines the frequency (/') within a frequency spectrum of constant amplitude and having the relationship

where /' is the frequency of the EM wave at which the penetration depth is equal to the thickness of the material. This relationship can be express as

O/ ^— ^(material thickness).

The frequency /' is used to determine the conductivity and permeability of the material and, using the values of permeability and conductivity, to then determine the thickness of the material.

Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent from the description, or may be learned by practice of the invention. The features and advantages of the invention may be realized by means of the combinations and steps particularly pointed out in the appended claims

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings which are incorporated in and constitute a part of the specification, illustrate a preferred embodiment of the invention and together with the general description of the invention given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention. Figure. 1 is a block diagram of one embodiment of a magnetic saturation flux generator (saturation flux generator) of the present invention.

Figure 1A is a block diagram of another embodiment of the saturation flux generator used in the present invention.

Figure. 2 illustrates a graph of transmitter current and saturation current versus time with respect to the present invention.

Figure. 3 illustrates the relationship between transmitter signal frequency and penetration depth for a cross-section of a piece of ferromagnetic material with constant conductivity and permeability and several imposed frequencies, f_x, for the present invention. Figure 4 illustrates the flux circuit core of one embodiment of the magnetic saturation flux generator used to generate the magnetic saturation current required in practicing the present invention. Figure 4A illustrates one embodiment of a flux circuit core for use with the present invention.

Figure 4B illustrates one embodiment of a magnetic circuit for use with the present invention. Figures 4C, 4D and 4E illustrate the relationship between the transmitter current amplitude (Figure 4C), the saturation current amplitude (Figure 4D), and the receiver current amplitude (Figure 4E) with respect to the magnetic circuit illustrated in Figure 4B.

Figure 5 illustrates one embodiment of a bistatic magnetic saturation flux generator of the present invention.

Figure 6 illustrates one embodiment of a saturation flux generator of the present invention in operative association with a culminator.

Figure 7 illustrates the relationship between the flux density β and the change in magnetic field intensity H (ΔH) in turns-amp/meters. Figure 8 illustrates the relationship between the receiver signal amplitude

AR_X and magnetic field intensity H in turns-amp/meters.

Figure 9 is a graph of amplitude versus time for a bistatic configured magnetic saturation flux generator of the present invention.

Figure 10 illustrates another embodiment of a magnetic saturation flux generator used to create a near saturated or saturated area through the thickness of a ferromagnetic material in practicing the present invention.

Figure 10A illustrates an embodiment of a saturation flux generator used to generate a magnetically saturated area (transparency) with respect to a material for practicing the present invention as could be adapted in Figure 10. Figure 10B illustrates another embodiment of a saturation flux generator used to generate a saturated area with respect to a material for practicing the present invention as could be adapted in Figure 10.

The above general description and the following detailed description are merely illustrative of the generic invention, and additional modes, advantages, and particulars of this invention will be readily suggested to those skilled in the art without departing from the spirit and scope of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferred embodiments of the invention as described in the accompanying drawings. It is possible to greatly improve existing methods of measuring electrical conductivity of a ferromagnetic material or alloy containing ferromagnetic material by using a spectrum of frequencies rather than one frequency. In addition, the metallic permeability must be measured to accurately depict the effects of the metallic barrier for measuring through the barrier. Using a range of frequencies allows a single device to function where metallic thickness may vary from zero (no metal) to inches thick. Using the same frequency over such a wide range results in a large loss of resolution and accuracy. Therefore, for a given range of material thickness, a particular group of frequencies will provide improved resolution and better accuracy. To eliminate the effects of a varying metallic permeability, it is necessary to create a local metallic transparency with the permeability as close to unity as possible while the frequency is being varied. Then, while the frequency is held constant, varying the strength (amplitude) of the current creating the greatly reduced permeability within the material, i.e., relative permeability of near unity. Monitoring the resulting change in received signal induced within the material allows the conductivity to be determined.

One embodiment of the present invention as embodied and broadly described herein is a method for creating a spectral EM frequency metallic thickness measurement and greatly reduced magnetic permeability (metallic transparencies). In order to calculate the thickness of a material with unknown permeability and conductivity, empirical testing is used to first approximate the conductivity and permeability. Conductivity and permeability can be determined in any order using techniques herein discussed.

The first step to calculate the thickness of a material is to create a first set of electromagnetic waves ("saturation flux" or "saturation signal") adjacent to or near the material to be measured. The first set of waves may be generated by a permanent magnet, an electromagnet powered by dc current or ac current. If ac current is used to create the EM energy for magnetic saturation flux, it is preferred that the frequency of the ac current be relatively low. The first waves saturate (or nearly saturate) the barrier material with the magnetic component. A second set of electromagnetic waves ("transmitter flux" or "transmitter signal") is generated with specific constant amplitude (and higher frequency) and is monitored using a receiver. The receiver is located adjacent to or near the material to be measured and maybe a distance from the transmitter or alternately co-located with the transmitter. The transmitter frequency is varied incrementally while the amplitude is held constant and the received signal is monitored. As required by skin depth theory, for a given wave of constant amplitude and varying frequency, the lower frequencies penetrate deeper into a piece of metal than the higher frequencies. The higher the frequency the more loss of signal, or the more attenuation occurs. See Figure 3. Therefore, a transmitted wave of single frequency can be generated, then incrementally increased, for example by stepping, while monitoring the received frequency. See Figure 4D. If the strength of the wave penetrating through the saturated material is monitored, the signal strength will attenuate each time the frequency of the transmitter wave is increased transmitted. Once the received signal is not detectable by the receiver, the maximum frequency of a transmitter wave that can penetrating the entire thickness of the magnetically saturated material can be determined. The last frequency to generate a received signal is the maximum penetration frequency for a material of constant (but unknown) thickness and relative permeability of near one. See Figure 4D.

It will be appreciated that in most applications, it will not be possible to access the opposite side of the material to measure the signal penetrating through the material. However, the retro-flux signal can be measured and the utilized to determine the frequency of the transmitter flux where the penetration through the material thickness is achieved. It will be appreciated that at higher frequencies, most of the energy will be reflected back from the first surface of the material. As the frequency is decreased, but signal amplitude remains constant, more of the transmitter signal or wave will penetrate into the material. The retro- flux signal will decrease. The oscillating transmitter flux will induce eddy currents within the electrically conductive material. As the transmitter signal penetrates through to the opposite side of the material, the total retro-flux signal will again increase. This signal increase will result from transmitter flux reflecting back from the second side of the barrier. As the transmitter flux frequency is further diminished, part of the flux will penetrate through the opposite side of the material, causing the retro-flux signal to again diminish. As the frequency is lowered further, a greater portion of the signal penetrates thorough and the retro-flux signal continues to diminish.

In this technique, the maximum penetration frequency will be the apex of measured retro-flux signal against the spectrum of transmitter frequency. The maximum penetration frequency of a signal of constant amplitude and material of constant relative permeability (near unity) is used in the present invention to determine material thickness. It will be appreciated that the full or partial saturation of the material is achieved by the first wave.

The second step in calculating the thickness of a material with unknown permeability and conductivity is the approximation of permeability. Using the same transmitter, receiver, and saturation procedures described in the first step, a saturation wave is generated near or close to the barrier material to be measured. The saturation wave has a known yet variable current. A transmitter wave of known and constant frequency and amplitude is generated at or near the barrier material within a zone to be effected by the saturation current. A receiver monitors the resulting electromagnetic response generated by the transmitted signal. While monitoring the received response and holding the transmitted wave frequency and amplitude constant, the saturation current is varied incrementally. Thus, the received signal will generally mirror the steps of the saturation current steps but at different amplitudes than the transmitted signal. As the saturation current increases, the barrier material becomes more and more transparent to a wave of constant transmitted signal amplitude, thus, generating for an increased received signal proportional to the increases in saturation current. The saturation current is further increased, preferably in incremental steps, and the received signal is monitored while again holding the amplitude of the transmitter signal constant until the received signal registers no change. The point at which the received signal registers no change is called total saturation. See Figure 4E. Once total (or near total) saturation occurs, additional saturation current has no affect on the magnitude of the received signal. Thus, the transmitter signal is coupled with the material and the material is now transparent to the transmitter signal. The current history and the associated received signal, as illustrated in Figures 4C, 4D and 4E, provide for full or partial saturation of a localized area. Further, the current history and the receiver signal information can be used to mathematically determine the permeability. Once determinations are achieved of the permeability and conductivity, the material thickness can be calculated.

In one embodiment of the invention, the maximum penetration frequency (/') is first determined and this frequency is then utilized as the fixed or constant frequency of transmitter signal in the second step wherein the saturation current is varied and resulting changes in the receiver signal monitored. In another embodiment, the sequence of changes in saturation can begin at a high value and sequentially decreased, preferably in an incremental or linear manner. In another embodiment, the change in retro-flux signal is monitored as the permeability of the material is varied by change in the saturation current.

It will also be readily appreciated by persons skilled in the art after reading the specification of this invention that the sequential monitoring of EM signals in response to changes in separate single will allow the information to be evaluated in relation to the differing steps. It will also be appreciated that the controlled and monitored variation of the signal allows application of Ohm's law establishing the fundamental relationship among electromotive force, current and conductivity. The technique of the present invention for calculating the thickness of a material with unknown permeability and conductivity can be used to further classify various materials and thickness such that a general lookup table can be created. The general lookup table can contain known results from numerous test samples allowing for quick lookup and display of thickness based on known samples meeting the test criteria. The test criteria can be for a range of thickness for specified materials having the same permeability and conductivity. General look up tables can also be created for the identification of unknown materials, but of known thickness based upon monitoring changes in receiver signals as the saturation current and transmitter frequency are varied. It order to obtain an accurate measurement of permeability and/or conductivity, electronic and geometric nulling are required. Geometric nulling positions the transmitter, receiver and saturation coils in the optimum locations for the particular system designed. Various designs are provided yielding excellent results. Also, an electronic nulling circuit can simultaneously null all of the frequencies at once. Pursuant to practicing the present invention as described herein, one skilled in the art will know and appreciate how to arrange the transmitter, receiver and saturation coils in optimum locations for the particular system being used, and will know and appreciate how to simultaneously null all of the frequencies at once to provide electronic nulling.

Figure 1 is a block diagram of one embodiment of a magnetic saturation flux generator 500 of the present invention. The magnetic saturation flux generator 500 comprises a large coil 551 , a small coil 300, and a receiver coil 580. The large coil 551 generates the saturation flux. The small coil 300 generates the transmitter signal. The receiver coil 580 accepts the returning transmitter signal. The large coil 551 for generating the saturation flux is engaged with a pulser 566, one or more capacitors 561 and a power source 560. The small coil 300 of the transmitter and the receiver coil 580 are engaged with a switch 562, a frequency generator 563, a low noise amplifier (LNA) 564, an electrical nulling circuit 581 for digital signal processing and an output means 582.

Figure 1A illustrates schematically one embodiment of the components of the sensor tool 500 subject of the invention. The components of the tool 500 are contained within a tool housing 572. The tool 500 includes (a) a magnetic saturation generator 501 for creating a transparency within the material and including one or more saturation coils 551 , (b) a magnetic flux transmitter component 300, comprising the transmitter coil 301 , a switch 562, and a low noise amplifier (LNA) 564, (c) a receiver component 580 for the detection and measurement of magnetic flux penetrating through the railroad track and comprising a receiver coil 581 , (d) a frequency generator 563, (e) a pulser 566, (f) one or more capacitors 561 and (g) a nulling device 582. The magnetic saturation generator 501 includes the saturation coil 551 , saturation core or magnetic culminator (not shown). The magnetic saturation generator 501 , saturation coil 551 , the transmitter 300, transmitter coil 301 and any associated core (not shown), the receiver 580, including the receiver coil 581; and the associated components described above and depicted within the sensor tool housing 572, can be maintained apart from the material to minimize possible damage. The output display 583, operator controls (not shown) and power source 560 may be located away from the material surface and linked to the tool housing 572 by means of standard cables and connectors 568 and 588. The operator's console or display 583 may also record and display historical trends of material properties and be used to monitor erosion or ablation of the material over time.

The saturation coil 551 is a principle element of the magnetic saturation generator 501. It may be utilized in conjunction with one or more transmitter components, receiver components, or combinations of both. The saturation coil 551 generates a magnetic flux that engages (or couples) with and saturates a portion of the material. The transmitter coil 301 is a principle element of the transmitter component ("transmitter") 300. The transmitter 300 creates the oscillating magnetic flux ("transmitter flux") that engages with and is transmitted through a magnetically saturated portion of the material (not shown). The spectrum of oscillating magnetic flux of the transmitter 300 will preferably be at a higher frequency, e.g., higher by a multiple of 10, than the frequency of the saturation flux. It will be appreciated that the transmitter has the capability to generate a plurality of separate magnetic flux, each having distinct frequencies. The receiver 580 may be combined with a separate saturation coil 551 , thereby allowing the receiver 580 to be placed away from the transmitter 300. This has a number of advantages, including facilitating nulling between the transmitter 300 and receiver 580. An embodiment of the tool 500 of the present invention in which the transmitter 300 and receiver 580 are located proximate to separate magnetic saturation generators is termed a "bistatic arrangement" or "bistatic configuration."

The saturation coil 551 and saturation core 552, the transmitter coil 301 and the receiver coil 581 , are often depicted separately from the other components described above and depicted within the "electronics component" 570 in Figure 1A. For clarity, many of the drawings contained within this specification do not depict the electronics component. Further, the drawings may show an illustration of a coil only, but may be variously labeled as a magnetic saturation generator, transmitter or receiver. It is understood that the other components or sub-components are deemed to be included as necessary. In addition, the components of the invention, including but not limited to the saturation coil 551 , transmitter coil 301 and receiver coil 581 are not placed in physical contact with the material.

Illustrated schematically as an apparatus in Figure 1 and 1A and conceptually in Figure 2, the saturation coil 551 generates the saturation flux 401 , which in turn creates the transparency in the material 100. The saturation coil is comprised of conductive material preferably wrapped around a highly permeable core (saturation core or flux circuit core) and powered either by dc current or a current oscillating at a low frequency. The transmitter flux 411 may be generated by the transmitter 300, comprised of the coil 301 of conductive material, powered by alternating current, preferably at a controlled frequency, wrapped upon or near the saturation coil 551. Preferably, the spectrum of transmitter flux 411 is at a higher frequency than the saturation flux 401. It is preferred that the frequency of the transmitter flux 411 be at least a multiple of 10 greater than the frequency of the saturation (also termed saturation flux 401). As discussed above, the higher frequency of the transmitter flux 411 relative to the saturation flux 401 allows, for example, 10 wavelengths of the transmitter flux 411 to be emitted, and thereby couple with the material. The detection and measurement of this separate oscillating flux may be dependant upon or occur only within the time period that the material is within (or above) the required partial saturation level 421. It will be appreciated that the constant saturation signal may be pulsed or otherwise varied in the first step of the invention, but that at each variation or change in monitored transmitter flux, 401 , the material is in the same state of full or near full (partial) saturation, i.e., at or above the required saturation level 421.

As discussed above, the higher transmitter flux frequency allows, for example, 10 wavelengths of measurement before the transparency is closed.

In Figure 2, the high frequency transmitter signal 411 is illustrated being pulsed at less than 0.5 millisecond rates. If the lower frequency saturation flux 401 generated by the larger coil 551 , is pulsed or activated "on" for 10 milliseconds 430, there is sufficient time for twenty transmitter signals (e.g., with a wavelength of only 0.5 millisecond) to go out to a near object and take 10 wavelengths of measurements during the "on" pulse of the saturation flux. During this 10-millisecond pulse, the saturation flux will exceed the saturation energy level 420. The higher frequency transmitter signal 411 from the transmitter coil 300 is being pulsed at a 0.5 millisecond rate so that 20 transmitter signals will be available during a 10 millisecond pulse of the saturation flux signal 401 that creates the transparency through the material.

For most applications, a power source of 300 watts or less is sufficient to create the transmitter and saturation flux. For thicker material, strong pulses and signals may be generated by utilizing the charge storing capacitors 561. The capacitors 561 are slowly charged then quickly discharged through a switch contact and then through the low impedance large coil 551. At the same time, the higher frequency small signal coil 300 is pulsed.

Figure 3 illustrates the relationship between signal frequency, f_Xι and penetration depth for a cross-section of a ferromagnetic material with constant electrical conductivity and magnetic permeability utilized in the present invention. For a oscillating EM wave, i.e., a transmitter signal, of constant amplitude and varying frequency, and a material of constant permeability and conductivity, it is known by skin depth theory that a lower frequency penetrates deeper than a higher frequency EM wave. Therefore, one can find the transmitter signal frequency within a spectrum or range or frequencies that, at a given and constant saturation current, will have a penetration depth approximate the depth of the material thickness. This measured frequency can be used to characterize the electrical conductivity of the material. Skin depth is the designation for the depth into the material at which the signal has reached a percentage attenuation from original signal strength equal to 1/e. Therefore, skin depth (δ) will have a unit dimension of depth (L) as shown by the relationship.

Of course, for a given sample each frequency of transmitter signal will have a different value or magnitude of skin depth. It will also be appreciated that the procedure of the present invention will allow determination of the oscillating transmitter flux frequency (/'), where the transmitter flux penetration depth (δ ) is equivalent to the thickness of the material. The familiar relationship

1 δ =

<Jσμ_rμ₀f where δ = penetration depth, σ = conductivity

/ = frequency, μ_r = relative permeability, and μ₀ = absolute permeability, can be utilized where 0 ⁼ 0/ ⁼ ^material thickness)

/ = /' and

/' is within the range of frequencies having a relationship comparable to /β > Is > U > > h > U- See Figure 4B.

In Figure 3, f_x is a set of frequencies wherein the elements of the set have a relationship comparable to the relationship of frequencies

Figure 4 illustrates one embodiment of a magnetic saturation flux generator

500 used to generate the saturation flux required in practicing the present invention. The magnetic saturation flux generator 500 provides for containing flux lines to completely saturate the intended barrier material 100 volume region. Also, Figure 4 illustrates one embodiment of the flux circuit core 501 for use with the present invention. The flux circuit core 501 comprises a top flange 504, a bottom flange 505 and a core 552. The core 552, upon which the coils of the electromagnet are wrapped, is located between the top flange 504 and bottom flange 505. The tank wall comprises the barrier material 100. The complete magnetic saturation flux generator 500 incorporates the flux circuit core 501 for providing a transparent volume region that is illustrated having a width W 920, a height H 930 and a thickness L 960. The barrier volume region may be termed the target material. It is appreciated that the transmitter coils 300, the receiver coils 580 and the saturation coil 551 to the magnetic saturation flux generator 500 are each geometrically nulled to the other. Figure 4A illustrates one embodiment of the flux circuit core 501 for use with the present invention. The flux circuit core 501 comprises a top flange 504, a bottom flange 505 and a core 552.

Figure 4B illustrates one embodiment of the magnetic circuit 502 for use with the present invention. The magnetic circuit 502 comprises the saturation coil 551, the transmitter coil 300, the receiver coil 580 and the barrier material 100. The magnetic saturation flux generator 500 is disposed from the barrier material 100 by a gap G. The barrier material 100 has a thickness L 560. The magnetic circuit 502 operates by energizing the saturation coil 551 for saturating the barrier material 100, transmitting a transmitter signal from the transmitter coil 300, and receiving a response via the receiving coil 580. The change in relative penetration of a transmitter signal of constant frequency and amplitude is achieved by the change in the saturation current. Thus, as the saturation current increases from i-i, to i₂, to i₃, to i₄, the penetration depth increases from δι, to δ₂, to δ₃, to δ , respectively. Figure 4B illustrates the incremental increase in penetration by the field lines F-i, F₂, F₃ and F₄.

Note again that at this step, the frequency of the transmitter signal is not changed. Also, consideration of the cross-sectional area of each component of the magnetic circuit 502 is required to assure that no component goes into total saturation for a specific power requirement necessary to drive the EM wave across the air gap G.

Figures 4C, 4D and 4E illustrate the relationship between the transmitter current amplitude, the saturation current amplitude, and the receiver current amplitude with respect to the magnetic circuit 502 illustrated in Figure 4B. Figure 4C illustrates that the transmitter current amplitude 411 maybe constant over time. It will be appreciated that the transmitter frequency is also maintained constant. Figure 4D shows that the saturation current amplitude is increased as a step function over time 401 A, 401 B, 401 C and 401 D. With the transmitter current amplitude 411 held constant over time and the saturation current amplitude 401 increased as a step function over time, Figure 4E shows the receiver current amplitude 451 will increase 451 A and 451 B as a step function congruent with the saturation current amplitude up to and until the barrier material approaches the state of total saturation. When the barrier material is in a state of total or near total saturation, the receiver current amplitude is at a maximum 452 and cannot be increased by further increase of saturation current because maximum reduction of permeability has been achieved.

Figure 5 illustrates one embodiment of a bistatic magnetic saturation flux generator 590 of the present invention. Using the bistatic magnetic saturation flux generator 590 shown in Figure 5, the permeability is driven to unity. Electromagnetic waves are transmitted by the transmitter 592 at different frequencies and monitored with the receiver 594. A metallic transparency is created by generating a saturation of the barrier material 599. An electromagnetic wave is generated using the transmitter 592 at a preset frequency and constant amplitude. Assuming the first frequency is within the detectable frequency range, the frequency is increased incrementally until the received signal is lost. See Figures 4C-E. The last frequency detected prior to losing the received signal determines the maximum frequency detectable in a certain piece of barrier material 599 of constant thickness, permeability, and conductivity. See Figure 4E. Using the data and information received in empirical testing for permeability, the material properties and thickness can be very precisely calculated.

In Figure 5, the bistatic magnetic saturation flux generator 590 comprises a housing 591 , a transmitter 592 and a receiver 594. The transmitter 592 and the receiver 594 are displaced by a distance D. The transmitter 592 includes a transmitter coil 592C and a saturation magnet 593. The receiver 594 includes a receiver coil 594C and a saturation magnet 595. The bistatic magnetic saturation flux generator 590 is in operative association with a barrier material 599 having a defect 599D. It can be appreciated by those skilled in the art that in the bistatic configuration illustrated in Figure 5 the distance D must be sufficiently small to detect the defect 599A. Such that the accuracy is limited by the mass to be evaluated and the displacement distance D.

Figure 6 illustrates one embodiment of a magnetic saturation flux generator 600 of the present invention in operative association with a culminator 602. The transmitter 606 and the receiver 608 are both on the same culminator 602. The displacement distance D between the transmitter 606 and the receiver 608 is essentially zero. The displacement distance D is essentially zero because of the close configuration of the transmitter 606 and the receiver 608. The intensity of the frequencies received will show the metal thickness. For example, if all the higher frequencies are attenuated, the metal is thick. If all the high frequencies are detected with little attenuation of the low frequencies, the metal is thin. For a given power, the displacement distance D between the transmitter 606 and the receiver 608 determines the resolution of the thickness measurement. The resolution effects the size of the defect measurable.

Figure 7 illustrates the relationship between the flux density β and the change in magnetic field intensity H (ΔH) in turns-amp/meters. The permeability μ is plotted. For the relationship between the flux density β and changes in field intensity ΔH, the function defining the permeability μ remains constant for a given material. Although the function defining the permeability μ remains constant, the change in relative permeability for a thinner sample of a given material is greater for a given value of ΔH. Thus, incremental changes in H create a faster advancement up the permeability curve toward saturation. For example, a given Hi_., corresponds to the value of βu and a corresponding Hι_2 corresponds to the value of βι_₂- Thus, the value for L2 moves faster up the permeability μ curve than the value for L1. Figure 8 illustrates the relationship between the receiver amplitude ARX and

H in turns-amp/meters. As in Figure 7, the slope of the curve in Figure 8 is related to the permeability μ. However, the receiver amplitude AR_X reaches a different maximum value depending on the thickness of the material. For thinner materials, the receiver amplitude AR_X reaches its maximum value at a lower amplitude ARX. For thicker materials, the receiver amplitude AR_X reaches its maximum value at a higher amplitude AR_X. Figure 8 illustrates a thinner material having a maximum at A_R1, a thicker material having a maximum at A_R3, and an intermediate thickness material having a maximum at A_R2.

Figure 9 is a graph of amplitude versus time for a bistatic configured magnetic saturation flux generator of the present invention. The frequency is held constant (fixed) and the material is varied. The bistatic magnetic saturation flux generator was nulled using copper 902. Thereafter, the copper was replaced with brass causing the amplitude to vary from the original nulled position 904 to a new position 904. Since brass and copper have related properties, the dislocation 904 from the copper nulled position 902 is small. However, when the brass is replaced with aluminum the amplitude 906 varies significantly from the original nulled position 902. Aluminum and copper have significantly different physical characteristics.

Figure 10 illustrates another embodiment of a magnetic saturation flux generator 200 used to generate a transparency in practicing the present invention. The magnetic saturation flux generator 200 comprises an outer cylindrical portion 202 and an inner cylindrical portion 204. The transmitter, receiver and saturation coils are disposed on, in or around the outer cylindrical portion 202 and the inner cylindrical portion 204.

Figure 10A illustrates an embodiment of a magnetic saturation flux generator 200 used to generate a transparency with respect to a material 100 for practicing the present invention as could be adapted in Figure 10. A transmitter coil 210 is disposed at the remote end of the outside diameter of the inner cylindrical portion 204 of the magnetic saturation flux generator 200. A saturation coil 220 is disposed at the inner end of the outside diameter of the inner cylindrical portion 204 of the magnetic saturation flux generator 200. A receiver coil 230 is disposed within the inside diameter of the inner cylindrical portion 204 of the magnetic saturation flux generator 200. The receiver coil 230 can be located at different positions using a shaft 232 which telescopes within the inside diameter of the inner cylindrical portion 204 of the magnetic saturation flux generator 200. The telescoping shaft 232 can also rotate using a set-screw adjustment 206 and a sets-crew housing 208. Also, wiring 234 can be channeled through the shaft 232. FigurelOB illustrates another embodiment of a magnetic saturation flux generator 200 used to generate a transparency with respect to a material 100 for practicing the present invention as could be adapted in Figure 10. A transmitter coil 210 is disposed at the remote end of the outside diameter of the outer cylindrical portion 202 of the magnetic saturation flux generator 200. A saturation coil 220 is disposed along the outside diameter of the inner cylindrical portion 204 of the magnetic saturation flux generator 200. A receiver coil 230 is disposed within the inside diameter of the inner cylindrical portion 204 of the magnetic saturation flux generator 200. The receiver coil 230 can be located at different positions using a shaft 232 which telescopes within the inside diameter of the inner cylindrical portion 204. The telescoping shaft 232 can also rotate using a set-screw adjustment 206 and a set-screw housing 208. Also, wiring 234 can be channeled through the shaft 232.

Additional advantages and modification will readily occur to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus, and the illustrative examples shown and described herein. Accordingly, the departures may be made from the details without departing from the spirit or scope of the disclosed general inventive concept.

Claims

WHAT IS CLAIMED IS: 1. A method for creating a spectral EM frequency to calculate the thickness of a material with unknown permeability and conductivity by varying EM frequencies and amplitudes comprising the steps of: (a) impinging the material with a first EM wave to reduce the relative permeability to near unity, (b) impinging the material with a second EM wave having varied frequency and monitoring the change in resulting signal from the material, (c) evaluating for the frequency of the second wave where the depth of penetration is related to the thickness of the material, (d) impinging the material with a third EM wave of varied current and a fourth EM wave of constant amplitude and frequency generating a resulting signal, (e) varying the current of the third EM wave and monitoring the resulting signal, (f) evaluating the resulting signal and current of the third wave for a relationship between the third wave and saturation of the material, (g) evaluating the monitored results of the first and second waves with the monitored results of the third and fourth wave to obtain the conductivity and permeability of the material, (h) evaluating the thickness of the material using the relationship of

where δ = penetration depth, σ = conductivity / = frequency, μ_r = relative permeability, and μo = absolute permeability.

2. The method of claim 1 wherein the evaluated depth of penetration of the second wave in terms of the monitored signal is equal to the thickness of the material.

3. The method of claim 1 wherein the evaluated current of the third EM wave in terms of the monitored signal is equal to the EM current needed to reduce the permeability of the material through the thickness to near unity.

4. The method of claim 1 further comprising a. at least two additional values of frequency are measured and a curve plotted among measured values, b. at least two additional values of current are measured and a curve is plotted among measured values, and c. the curve of plotted frequency and the curve of plotted current are compared with corresponding curves of know material to identify the unknown material.

5. A method for creating a spectral EM frequency to calculate the thickness of a material of unknown permeability and conductivity comprising the steps of: (a) calculating the penetration depth δ using

where δ = penetration depth, / = frequency, σ = conductivity, μ_r = relative permeability, and μ₀ = absolute permeability. (b) impinging the material with at least one electromagnetic wave selected from a first set including constant and relatively low frequency waves having constant amplitude sufficient to reduce the relative permeability of the material to near unity, (c) impinging the material with at least one second electromagnetic wave of variable frequency and constant amplitude selected from a group having a minimum frequency higher than the first set and generating a signal within the material, (d) varying the frequency of the second electromagnetic wave at a constant amplitude such that /₆ > s > /-. > ₃ > H > /ι, where /₆ is a higher frequency, -i is a lower frequency and monitoring the generated signal, (e) evaluating the monitored signal in terms of the varying frequencies for a frequency having a penetration depth related to the thickness of the material, (f) impinging the material with at least one electromagnetic wave of known and variable current selected from a third group including constant and relatively low frequency waves, (g) impinging the material with a fourth electromagnetic wave of constant frequency and amplitude and monitoring the responsive signal as the current of the third electromagnetic wave is varied, (h) measuring the current of the third set of waves when the responsive signal does not change with an increase in the current and does change with a decrease in the current, and (i) using the measured amplitude, current and frequency of the electromagnetic waves to calculate the permeability and conductivity of the material and the penetration depth of the measured frequency where the penetration depth is related to the thickness of the material.

6. The method of Claim 5 wherein the penetration depth of the measured frequency is equal to the thickness of the material.

7 The method of claim 5 wherein the evaluated current of the third EM wave in terms of the monitored signal is equal to the EM current needed to reduce the permeability of the material through the thickness to near unity.

8. The method of claim 5 further comprising a. at least two additional values of frequency are measured and a curve plotted among measured values, b. at least two additional values of current are measured and a curve is plotted among measured values, and c. the curve of plotted frequency and the curve of plotted current are compared with corresponding curves of know material to identify the unknown material.

9. A method for creating a spectral EM frequency to calculate the thickness of a material of unknown permeability and conductivity comprising the steps of: (a) impinging the material with at least one first electromagnetic wave of relatively low frequency having a constant amplitude, (b) impinging the material with at least one second electromagnetic wave of constant amplitude and adjustable higher frequency than the first electromagnetic wave and generating a transmitter signal , (c) increasing the frequency of the second wave and monitoring the transmitter signal ; (d) measuring the frequency of the second wave when the transmitter signal does not change with an increase in frequency and does change with a decrease in the frequency of the second wave; (e) impinging the material with one or more third constant low frequency electromagnetic wave of known and variable current, (f) impinging the material with a fourth electromagnetic wave of constant frequency and amplitude and monitoring the transmitter signal generated by the fourth wave; (g) varying the current of the third wave, (h) measuring the current of the third wave when the transmitter signal does not change with increases in the current of the third wave and changes with a decrease in the current, and (i) using the measured values of amplitude, current, and frequency of the electromagnetic waves, calculating the permeability and conductivity of the material and penetration depth of the measured frequency where the thickness of the material is related to the penetration depth.

10. The method of claim 9 wherein the depth of penetration of the measured frequency is equal to the thickness of the material.

11. The method of claim 9 further comprising a. at least two additional values of frequency are measured and a curve plotted among measured values, b. at least two additional values of current are measured and a curve is plotted among measured values, and c. the curve of plotted frequency and the curve of plotted current are compared with corresponding curves of know material to identify the unknown material.

12. The method of claim 8 wherein a. the frequency of the second wave is changed in equal amounts, b. the frequency is measured when the magnitude of a corresponding change in the receiver signal is the greatest, c. the current of the third wave is changed in equal uniform amounts, and d. the value of the current of the third wave is measured when the corresponding change in receiver signal is the greatest.

13. A method for evaluating the thickness of a magnetically permeable and electrically conductive material comprising a. One step comprising (1 ) substantially lowering the magnetic permeability of the material by impinging it with an electromagnetic signal, (2) impinging the material with a plurality of electromagnetic signals of differing frequencies, (3) monitoring any responsive signal, (4) evaluating the differing frequencies in terms of the monitored responsive signal, b. an additional step comprising (1 ) impinging the material with an EM signal selected from a group comprising constant low oscillating frequency signals and constant non oscillating signals, (2) varying the current amplitude of the EM signal, (3) impinging the material with a second EM signal of higher constant frequency and constant amplitude, (4) monitoring the responding signal as the current EM signal is varied, (5) evaluating the variable current amplitude in terms of the responding signal, c. evaluating permeability and conductivity of the material in terms of the varying frequency of the higher frequency signal and the varying current of the constant or low frequency signal, and d. utilize the relationship of signal penetration to conductivity, permeability, and frequency to evaluate the material thickness.

14. The method of claim 13 further comprising evaluating the monitored changes in the responding signals in terms of Ohm's law of electromotive force.

15. An apparatus for measuring the thickness of a material of unknown permeability and conductivity a magnetic saturation flux generator comprising components selected from a group including one or more saturation coils, saturation cores, transmitter coils, receiver coils, power supplies, frequency generator, pulser, switches, amplifiers, low noise amplifiers and capacitors.

16 The apparatus of claim 15 wherein at least two of the components within the group of saturation coils, transmitter coils and receiver coils are in a nulled relationship.

17. The apparatus of claim 16 wherein the nulled relationship includes geometric nulling.

18. The apparatus of claim 17 further comprising a nulled relationship wherein the longitudinal axis of at least two coils are in a substantially orthogonal relationship.

19. The apparatus of claim 16 wherein the nulled relationship includes electronic nulling.

20. The apparatus of claim 16 wherein the nulled relationship includes separation nulling

21. The apparatus of claim 15 further comprising all the components located on the same side of the material to be evaluated.

22. The apparatus of claim 15 wherein the components are not in electrical contact with the material to be evaluated.

23. The apparatus of claim 15 wherein at least some of the components are contained in a housing.

24. The apparatus of claim 23 wherein at least some portion of the housing is comprised of non magnetically permeable material.

25. The apparatus of claim 23 wherein at least some portion of the housing is comprised of non electrically conductive material.

26. The apparatus of claim wherein the magnetic flux is coupled with at least one magnetically permeable and electrically conductive object to allow the flux field be altered.

27. An apparatus for evaluating the thickness of a material with unknown magnetic permeability and electrical conductivity comprising a) an outer portion, b) an inner portion, c) at least one transmitter, receiver, and saturation coils disposed upon and in orientation with the outer portion and inner portion.

28. The apparatus of claim 27 further comprising at least one receiver coil having a longitudinal axis orthogonal to the longitudinal axis of the transmitter coil and the receiver coil located within a cylindrical area having a circumference formed by the circumference of the transmitter coil 29. The apparatus of claim 28 wherein the receiver coil can be moved in relationship to the transmitter coil..