Patent Publication Number: US-2022236224-A1

Title: Inductive sensor with a magnetic biased coil for eddy current testing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from the U.S. Provisional Patent Application Ser. No. 63/141,467, filed on Jan. 25, 2021, and entitled “ Gain Configurable Inductive Sensor Biased Actively by a Magnetic Field For High Sensitivity and Linearity ”, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF INVENTION 
     The present invention relates to sensors for eddy current testing to detect and characterize surface and sub-surface flaws in conductive materials, and more particularly, the present invention relates to an inductive sensor with improved measurement sensitivity and/or reduce the nonlinearity. 
     BACKGROUND OF THE INVENTION 
     In general, Eddy current (EC) testing makes use of electromagnetic inductive principle to detect and characterize surface and sub-surface flaws in conductive materials. A standard pulse eddy current (PEC) method induces circular eddy currents into the surface layer of the metal target through the sudden change of the equilibrium magnetic field by fast switching off the stable charging current in the transmitter coil. The EC generates the magnetic field that can be detected by the receiver coil as an EC “echo signal”. The EC on the surface layer rapidly decreases in strength due to the thermal dissipation of the resistance from the target metal body. The changes of EC on the surface layer cause the magnetic field strength change that induces the secondary EC further into the deeper layer where the secondary EC starts decaying as well due to resistance. The process repeats and keeps going until all energy is burned out along time in the depth inside the metal body. This process is well known as EC diffusion and damping. During the process, EC goes deeper and becomes weaker along the depth. As the result, the associated magnetic field strength reduces over time. The detection signal from the magnetic strength change on the receiver coil is decayed accordingly as well. The received time transient signal can be analyzed to identify resistance changes along the time corresponding to the depth from the surface, which can then be used for detecting and locating the defects on the surface and under the surface of the metal target. The deeper the EC penetrates, the smaller the signal is received on the receiver coil. In principle, PEC has a very large signal dynamic range for deep detection applications, normally around 120 dB or more, from the target. In addition, the sensitivity of received signal depends on the sensor core ferromagnetic permeability which decreases gradually along the magnetic field strength decay corresponding to the Eddy Current decay. As the result, the signal along time for the depth from the target becomes lower and less sensible by the core as part of the sensor, resulting in a low Signal-to-Noise Ratio (SNR). In addition, the range of sensitivity changes (up to 40 dB in difference) corresponding to the core permeability decrease over the magnetic field strength shows a strong nonlinearity of the signal from the receiver coil measurements. 
     Thus, an industry need exists for an apparatus and method that is devoice of the drawbacks and limitations of the existing eddy current testing methods. 
     Hereinafter, the abbreviation “EC” refers to “eddy current(s)”, SNR refers to signal to noise ratio, and PEC refers to “Pulsed Eddy Current”, all are known in the art. 
     SUMMARY OF THE INVENTION 
     The following presents a simplified summary of one or more embodiments of the present invention in order to provide a basic understanding of such embodiments. This summary is not an extensive overview of all contemplated embodiments and is intended to neither identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later. 
     The principal object of the present invention is therefore directed to an inductive sensor apparatus that statically or dynamically adjusts a magnetic bias level to achieve an optimal sensitivity response. 
     It is another object of the present invention that the inductive sensor apparatus has a stronger signal response with high sensitivity. 
     It is still another object of the present invention that the inductive sensor apparatus has higher linearity for the response signal. 
     It is yet another object of the present invention that the inductive sensor apparatus has a wider signal response dynamic range for higher measurement resolution. 
     It is a further object of the present invention that the inductive sensor apparatus has a higher signal-to-noise ratio (SNR). 
     In one aspect, disclosed is an inductive sensor apparatus for nondestructive testing of metallic objects. This inductive sensor apparatus has a ferromagnetic core, a transmitter coil and a receiver coil wound on the ferromagnetic core, and a magnetic bias coil positioned around the ferromagnetic coil. The receiver coil is separate from the transmitter coil. The magnetic bias coil is adapted to apply an electric current to build up a bias magnetic field inside the ferromagnetic core to shift permeability of the ferromagnetic core to a desired level. The magnetic bias coil is separated, normally, from the transmitter coil and the receiver coil. However, it can be shared entirely and partially with the transmitter coil by designs. 
     In one aspect, a network with a current source supported by a power supply and a controllable switch is connected to the transmitter coil such that a current is applied to and can be switched off on the ferromagnetic core in order to induce an eddy current on the surface layer of the metallic object. A separate current source can be connected to the magnetic bias coil. The electric current can be applied to the magnetic bias coil to shift the permeability of the ferromagnetic core when receiving the EC echo signal from the metal target. 
     In one implementation of the inductive sensor apparatus, the receiver coil, the transmitter coil, and magnetic bias coil, all can be positioned into separate sections along the core or overlapped with each other in layers over the core as dictated by designs for various applications. The ferromagnetic core can be a single ferromagnetic core. 
     These and other objects and advantages of the present invention will become apparent from reading attached specifications and appended claims. Also, the foregoing Section is intended to describe, with particularity, the preferred embodiments of the present invention. It is understood that modifications to these preferred embodiments can be made within the scope of the present claims. As such, this section should not be construed, in any way, as limiting of the broad scope of the present invention. The present invention should only be limited by the following claims and their legal equivalents. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, which are incorporated herein, form part of the specification and illustrate embodiments of the present invention. Together with the description, the figures further explain the principles of the present invention and to enable a person skilled in the relevant arts to make and use the invention. 
         FIG. 1  is a diagrammatic illustration of an inductive sensor apparatus of the prior art. 
         FIG. 2  is a graph showing the PEC based on sensor current charging and EC echo signal decaying curve of the prior art. 
         FIG. 3  is a graph showing an example of measurement logs in B-mode scan (VDL plot) for the defects or desired characteristics of EC decays with respect to different SNRs. 
         FIG. 4  is a graph showing the core permeability characteristics in terms of sensitivity and nonlinearity. 
         FIG. 5  is a diagrammatic illustration of the disclosed and its symbolic network model, according to an exemplary embodiment of the present invention. 
         FIG. 6  illustrates the method, network connections, and the output corresponding signals to control the shift of the signal working zone from low in sensitivity and high in nonlinearity to considerably high in sensitivity and low in nonlinearity, according to an exemplary embodiment of the present invention. 
         FIG. 7  is a graph showing the lab verification result as a proof of concept (POC) of the disclosed inductive sensor apparatus, according to an exemplary embodiment of the present invention. 
         FIG. 8  is a diagrammatic illustration of choosing different control currents for the use of different biased magnetic fields to shift the signal working zones for adapting various signal dynamic ranges and characteristics associated with the inductive sensor apparatus, according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Subject matter will now be described more fully hereinafter with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific exemplary embodiments. Subject matter may, however, be embodied in a variety of different forms and, therefore, covered or claimed subject matter is intended to be construed as not being limited to any exemplary embodiments set forth herein; exemplary embodiments are provided merely to be illustrative. Likewise, a reasonably broad scope for claimed or covered subject matter is intended. Among other things, for example, the subject matter may be embodied as methods, devices, components, or systems. The following detailed description is, therefore, not intended to be taken in a limiting sense. 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Likewise, the term “embodiments of the present invention” does not require that all embodiments of the invention include the discussed feature, advantage, or mode of operation. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of embodiments of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The following detailed description includes the best currently contemplated mode or modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention will be best defined by the allowed claims of any resulting patent. 
     Disclosed is an inductive sensor apparatus and method for nondestructively evaluating metallic surfaces using a PEC principle with enhanced signal acquisition topology. The disclosed inductive sensor apparatus may comprise a transmitter coil to generate a static magnetic field by the means of exciting the coil with a certain amount of current, and then switching off the current. The initial EC is induced on the surface of the metal object and gradually decays inside the target due to diffusion and damping processes. Those skilled in the art will recognize that the eddy currents can be induced by any other means without departing from the scope of the present invention. The inductive sensor apparatus may also include a separate receiver coil which can detect decaying magnetic field due to EC decaying. The receiver coil generates a voltage signal in response to the EC magnetic field change which, when analyzed further, reveals the target&#39;s features, such as thickness or defects that alter the metal resistances. The inductive sensor apparatus also includes an adaptive bias coil that is utilized to boost the signal working range to an optimal working region which results in a higher SNR, sensitivity, dynamic range, and linearity. The following description of the illustrations will further provide a thorough understanding of the invention. 
     Referring to  FIG. 1  illustrates, in general, an inspection apparatus in accordance with the prior art. Such an apparatus consists of a transducer  101 , a core  102 , a transmitter coil  103 , and a receiver coil  104 . Both the transmitter coil  103  and the receiver coil  104  are wound on the core  102 . The material of the core  102  is normally a ferromagnetic material with high magnetic permeability value for high signal sensitivity. However, it can also be non-ferromagnetic. The transducer  101  can be used to measure the features, properties, and/or flaws  110  of the metallic object  109 . The test object  109  can be any metallic object which includes, but not limited to pipes, plates, sheets, structures, and so on. The transmitter coil  103  produces magnetic field  105  with the flux direction shown by arrows  106 , also known as magnetic flux density field distribution function, when a DC current is applied across the transmitter coil  103 . The magnetic field  105  is stabilized after some time duration. When the current is removed by switching off from the transmitter  103  and the magnetic field  105  collapses, a circular EC  107  is induced due to the changing magnetic field  105  on the surface of the metallic object  109 . The circularly flowing EC  107  on metallic object  109  generate EC associated magnetic field flux  112  getting into the core  102 . Due to thermal power dissipation from resistance of the metallic object  109 , the EC  107  continuously decays, which results in the subsequent magnetic field  112  decay. Changes of the magnetic field  112 , induce the secondary EC  108  in nearby conductive layers under the surface of the metallic object  109 . Due to the same reason of thermal power dissipation as for EC  107 , EC  108  decays along the time. The inductive-while-decaying process of EC  108  keeps repeating as EC penetrating deeper into the metallic object  109  and becoming weaker and weaker over time. The behavior of EC decaying process is called diffusion and damping. Eventually, the EC get dissipated out along time and depth inside the metallic object  109 . The corresponding magnetic field  112  inside the core  102  keeps decreasing along with the EC  108  decaying. The changes of the magnetic field  112  associated with EC decaying are sensed by the receiver coil  104 , which develops a voltage signal in response. The resultant voltage signal can be measured and then further analyzed in the post-processing domain to extract properties of the metallic object  109  such as thickness changes  113  and/or flaws  110 . 
     As EC diffuses and dampens inside the body of the metallic object  109 , the strength of the EC is reduced as illustrated above. In principle, the time and the depth can be mapped with respect to each other. As the result, the signal decay along time measured from receiver coil  104  can be translated into the signal decay along the depth. Eventually the measured signal from receiver coil  104  decreases along the depth to an unmeasurable level and the signal acquisition time is over, which corresponds to a one-round measurement process of PEC detection sequence. The measurement process may be repeated at a stationary position to measure the same location multiple times to acquire multiple data frames. This combined data would then be stacked and averaged to obtain higher Signal-to-Noise Ratio (SNR) than a single frame of acquired data. The measurement process may also be repeated while in motion, shown by an arrow  111  in  FIG. 1 , for scanning while measuring the target area of the metallic object  109 . The scanning while measuring motion can be achieved by moving either the transducer  101  or the target  109  in a scanning way against the transducer. The scanning signals can be built and presented in 2D or 3D B-Mode scan image. The example of a 2D B-Mode scan image is shown in  FIG. 3 . 
       FIG. 2  shows typical profiles of an excitation current pulse  203  when applied to the transmitter coil  103  and the received time-transient measurement voltage signal  207  corresponding to the EC  108  decaying, along the time, sensed in the receiver coil  104 , as illustrated in the embodiment shown in  FIG. 1 . For the TX charging current pulse  203 , the stable state of the magnetic field B 0    105  is required while the core  102  is charged by the charging current  203  until it reaches the stable level as shown as I 0    204 . The charging process  203  takes time denoted in  FIG. 2  as the TX charging window and as the TX duration 201. At the beginning of charging, the current is applied to the transmitter coil  103  and gradually builds up as shown in  203  until it plateaus at I 0    204 . The stable magnetic B 0    105  field in the core  102  is established. As a result, the portion of the static magnetic field B 0    105  through the flux distributions in the metallic object  109  is also established. The sudden removal of the charging current I 0    204  from the transmitter coil  103  produces the corresponding changes in magnetic field B 0    105  both in the core  102  and in the metallic object  109 . The portion of the B 0  flux  105  in the metallic object  109  induces the initial EC  107  as I ECO  in 
     
       
         
           
             
               
                 
                   
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     on the surface of the metallic object  109 . Then the process of EC  107 / 108  decaying starts as illustrated in  FIG. 1 . As the result, the EC  108  decaying along the time can be shown in the following: 
     
       
         
           
             
               
                 
                   
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     where, μ m  is the magnetic permeability and ρ m  is the resistivity of the metallic object  109 . 
     The equation (2) shows that when the resistivity of the target region increases, the τ decreases, and the EC  108  decay is faster along the time. In this case, EC  108  shall be smaller than in the region of the defect  110  after the certain time point when EC reaches the depth where the defect  110  is located, compared to the region without any defects. The EC  108  generates the secondary magnetic field B EC    112  that can be sensed by the receiver coil  104  along the time as the received voltage signal ν EC  shown as curve  207  following in 
     
       
         
           
             
               
                 
                   
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     where, N is the number of turns of the receiver coil  104 ; A is the section area of the core  102 ; B EC  is the magnetic field inside the core  102 ; μ C  is the magnetic permeability of the core  102 ; and H EC  is the magnetic strength distribution generated by EC  108 . From the equation (3), the changes of EC  108  decay are sensed by the receiver coil  104  as the received signal ν EC  that represents the EC  108  decaying process. From the equation (2), the EC  108  decaying along time is linked to the local properties, such as the resistivity, of the measurement target, such as the metallic object  109 . There is the time gap  208  right after the charging window and before the acquisition window for measuring the EC decaying. Within the time gap  208 , the sudden change of the magnetic field B 0  inside the core generates the high voltage on the transmitter coil  103  through self-inductive process as well as the high voltage on the receiver coil  104  through mutual-inductive process. Both high voltages, normally called “switching interferences”, can be sensed on the receiver coil  104  as the received signal that is not from the target EC  107 / 108  decaying process measured as voltage signal  207 . As a result, the initial portion of EC  107 / 108  decay in received signal  207  within the time gap  208  is heavily contaminated by the switching interferences. Normally, those interference signal voltages are damped close to zero in short time within the time gap  208  by using active and/or passive damping networks. After the time gap  208 , the EC  108  decay from the target without the switching interferences can be detected reliably as the voltage signal  207  received from the receiver coil  104 . The RX duration 202 is for the signal  207  acquisition duration in which EC  108  decaying is measurable. After  202 , the magnetic B EC  change in the core  102  along the time, 
     
       
         
           
             
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     can no longer be sensible and measurable. 
       FIG. 3  illustrates the analysis and the presentation of the decaying voltage curve  207 , which is used for extracting the defect  110 , which, as one of the examples of features of the metallic object  109 , could be a small fracture inside the body  113  of metallic target  109 . The main challenges of using the PEC testing method are the measurement sensitivity, signal dynamic range, and SNR. In  FIG. 3 , the received voltage signal  207  may have a high dynamic range of up to −100 dB to −140 dB decaying along the time and the depth. When diffusing EC  108  reaches the defect region underneath the surface, the circular current I EC  flows through the defect  110  region. The resistance R EC  along the circular EC path increases due to the changes of metal conductivity properties caused by the defect  110 , resulting in the thermal power dissipation increases proportionally to I EC   2 R EC  based on the Ohm&#39;s Law. Therefore, the received signal  301  of the decaying EC  108  after the time point  303  when EC reaches the defect  110  region is faster and lower than the received signal  302  in the region without defect  110 . When defining the received signal  302  as the nominal signal ν n  and the rest of measurement signals including the received signal across the defect  110  area as the test signal ν r , can be obtained the relative measurement signal ν VDL , as a Variable Depth Log (VDL) signal as 
     
       
         
           
             
               
                 
                   
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     When scanning across the defect  110  area, the ν VDL , can be presented in gray scales as 2D B-Mode Scan Image coordinated in distance and depth mapped from the time of the EC decaying process. Without system noises, the B-Mode Scan Image is shown in  305 . The depth and width of the defect  110  can be viewed and estimated. The time point  303  is mapped in depth in the B-Mode Scan Image  305 . When the system noise  304  is present, which is always the case in the real-world environment, the SNR of the measurement signal  302  may change from high in positive to low or even negative along the time after the point of SNR=0 dB while EC decays continuously. As the decaying signals  302  and  301  cross the noise floor  304 , they suffer from noise interference as shown on the B-Mode Scan Image  306 , resulting in difficulties for viewing and estimating, both in accuracy and precision, the depth and width of the defect  110 . In addition, the defects may be buried very deep inside the metallic object  109  and the decaying signal responses may be very small, which requires exceptionally high sensitivity as well as wide signal dynamic range from the transducer  101  for measurements. 
       FIG. 4  depicts sensitivity and linearity challenges that this invention addresses. The equation (3) shows that the permeability μ C  of the core  102  is proportional to the received signal ν EC  given the EC decaying magnetic strength H EC  changes in the core  102 . To increase the sensitivity of the received signal ν EC  for H EC  changes, ferromagnetic materials with high magnetic permeability μ C  are normally selected to construct the core  102 . However, the permeability μ C  of a ferromagnetic material is not constant or linear but is instead a nonlinear function of magnetic field  404  vs magnetic strength  403 , typically, shown in the permeability B-H curve  401 . In this case, the magnetic permeability of a sensor core is defined as 
     
       
         
           
             
               
                 
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     On the B-H curve  401 , the μ 2  at the point  405  is larger than the μ 1  at the point  406 , given the same amount of EC decaying magnetic  112  strength change in ΔH 1  at the point  406  and ΔH 2  at the point  405  where ΔH 1 =ΔH 2 . It then follows that the corresponding ΔB 1  at the point  406  and ΔB 2  at the point  405  are different where ΔB 2 &gt;ΔB 1  due to μ 2 &gt;μ 1 . From Equation (3), ν 2 &gt;ν 1  for the same EC decaying changes in ΔH depending upon the magnetic field B level inside the core. It is clear that B 2 &gt;B 1  at points  405  and  406  on the B-H curve  401 . Because of the B-H curve of ferromagnetic cores, the received voltage signal ν EC  may have reduced sensitivity and exhibit high nonlinearity due to core permeability behavior against a wide dynamic range of the decaying EC  108  along time.  FIG. 4  depicts four regions of operation—the saturation region  402 , the high sensitivity around the point  405 , the low sensitivity zones around the point  406 , and the nonlinear transition where the magnetic field spans a wide range from the point  405  to the point  406 . When the EC decays to very low level where the magnetic field in the core is far below the point  406 , the permeability from Equation (5) is near zero on the B-H curve  401 . As the result, the received voltage signal ν EC  will also appear close to zero and become almost unsensible, according to Equation (3), even though ΔH remains none-zero and measurable. As can be seen, the ideal signal working region of operation is that with high sensitivity and low nonlinearity where the μ 2  value is positioned. If the permeability could stay within a narrow region relatively constant over time, then the received voltage signal measurement response ν EC  would have a strong sensitivity and less nonlinearity. 
       FIG. 5  illustrates the conversion of the physical transducer-target measurement system into a transformer-based symbolic-network model diagram for ease of description and understanding of the invention in the following figures.  FIG. 5  shows a transducer  501 , the core  502 , the transmitter coil  503 , and the receiver coil  504  are converted directly into symbols. A bias coil  510  is added as the preferred embodiment of the invention that presents a solution to the challenges described above. The metallic object  509  is represented as a L-R load loop  512  (network) coupled through the core  502 . Within the L-R load loop  512 , the inductance L is related to the loop path of circular EC  108  decaying over the permeability μ m  of the metal material, and R is related to the resistivity ρ m  of the loop path of the circular EC  108  decaying. When defects are present, the loop resistivity would change, and response would change as well. The transformer-based symbolic-network model  511  is also shown. 
       FIG. 6  illustrates the principle of transformer-based symbolic-network model  511  and the resultant effect of using such a bias coil embodiment 510 in PEC measurements. To operate the PEC measurement sequence as illustrated in  FIG. 2 , the transmitter coil  503  may be connected to an excitation current source  601  through the switch  604  for I 0  charging for the TX duration 201 to provide the initial magnetic field B 0    505  in the metallic object  509  under inspection. The receiver coil  504  senses the received voltage ν EC    207  that is developed during the acquisition period in response to the EC  108  decaying. The buffer stage  603  provides impedance isolation for the receiver coil  504  from the receiver voltage measurement system. It can also provide a linear gain if needed to map the signal dynamic range to match the dynamic range of the signal channel for measurement systems. The switch  604  is turned off after the TX duration to remove B 0    505  and the current source  602  is connected to provide current I B  to the bias coil to generate the biased magnetic field B B  inside the core  502 . As mentioned, the bias current I 8  is meant to establish the magnetic bias field B B  inside the transducer core  502  in order to shift its permeability to provide higher sensitivity of the receiver coil  504  during the acquisition duration 202, as opposed to the high-level initial charging current I 0  applied to the transmitter coil  503  during the charging duration 201 in order to establish high initial magnetic field B 0  to charge the surrounding metallic object  509 . After the time gap  208 , the buffer stage  603  is connected through  604  for the period of RX duration 202 for the measurement of received voltage signal ν EC . When the EC  108  generates B EC  and the decaying EC  108  produces the magnetic strength range as ΔH, the corresponding magnetic field regions and the magnetic permeability inside the core  502  are different from the work zone  606  without the biased magnetic field B B  to the work zone  605  with the biased magnetic field B B . According to Equation (3), the received voltage signal ν EC  in the work zone  605  with high permeability is much higher than the voltage signal in the work zone  606  with low permeability, resulting in the measurement signal sensitivity increase due to the addition of the biased magnetic field B B . Furthermore, the differences of the signal responses to the region with the defect on the target  109  against the region without defects are depicted in from  301  to  302 . 
     The comparison of the received signal with different measurement sensitivity and dynamic range with and without adding the biased magnetic field B B  is shown in  301  in the region with defect  110  embedded in and  302  in the region without any defects, respectively. With the corresponding noise floor, the SNR of the received signal  303  in the work zone  605  is higher than the received signal  304  in the work zone of  606 . 
     Also, illustrated in  FIG. 6  is an exemplary embodiment of sharing the bias coil with the transmitter coil, wherein the transmitter coil  503  may deliver both the charging current I 0  for the magnetic field B 0  generation and the biased current I B  to provide biased magnetic field B B  in the transducer&#39;s core  502 . Such an arrangement allows a single transmitter coil  503  to generate both fields sequentially by connecting to the charging current source  601  to the transmitter coil  503  during the TX duration 201 and connecting to the bias current source  602  to the transmitter coil  503  during the RX duration 202 by operating an electronically controlled switch  604  in a pre-programmed sequence for each respective cycle. 
     As illustrated from the graph in  FIG. 6 , as the EC magnetic field strength reduces and EC penetrate deeper into the surface of the inspection object  509 , the permeability value exhibits a non-linear downwards drift. Along time, the permeability value decreases so much that the system sensitivity becomes very weak. As shown in  606 , a portion of the signal which corresponds to the zone of material flaw detection at greater depths is well below the noise floor level, which, when combined with low sensitivity in that area, makes the quantitative analysis of the material defects in that region of depth or distance very difficult if not impossible due to dominant noise sources and almost no sensitivity. 
     In summary,  FIG. 6  illustrates that by changing the work zone using the biased magnetic field inside the core, the permeability value increases, resulting in higher measurement signal sensitivity and SNR. Additionally, the measurement signal dynamic range can be changed and/or increased for better signal mapping or less nonlinearity. 
       FIG. 7  illustrates the lab experiment results that confirms the differences with or without addition of the biased magnetic field inside the core for the embodiments of the art described above. Both signals show the decaying behavior measuring the same metal target. The “biased signal” with the biased magnetic field in the core is higher than the “unbiased signal” as expected and described in  FIG. 6 , which proves that the signal sensitivity is higher when the biased magnetic field is added in the core. The noises from the lab environment are shown as the fluctuations around the trend line. The SNR of the biased signal is higher than that of the unbiased signal given the situation of the similar noise conditions for both test cases. 
       FIG. 8  illustrates that a multiple magnetic field bias control schemes including static, dynamic, and adaptive can be employed. The static method is illustrated in  FIG. 6 . The biased current I B  is a constant I 8 =C as in  805 , which generates static (DC) biased magnetic field. The received signal ν R  is boosted from  801  to  802 . As described, the received signal ν r  from the receiver coil  104  is determined by Equation (3). The static biased magnetic field B B  yields 
     
       
         
           
             
               
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                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   B 
                   B 
                 
               
               
                 d 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 t 
               
             
             = 
             0 
           
         
       
     
     and the measurement signal ν EC  directly related to EC  108  decaying will be ν EC =ν r , shown in  804 . When the biased magnetic field B B  is linearly incremented as shown in  807  which corresponds to the linear current increase I B =Ct along the acquisition time as in  815 , the received signal ν R  is boosted from  808  to  809 , and measurement signal ν EC  directly related to the EC  108  decaying will be ν EC =ν r −C since 
     
       
         
           
             
               
                 
                   d 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     B 
                     B 
                   
                 
                 
                   d 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   t 
                 
               
               = 
               C 
             
             , 
           
         
       
     
     shown in  806 . A further control function can be chosen to adapt and compress the EC decaying signal dynamic range for the received signal working within a relatively small permeability region to achieve both high sensitivity and high linearity. For example, the biased magnetic field B B  may be generated by I B  according to a special predefined quadratic function as in  811 . The change of B B  is known and controlled in  813 , where the received signal ν r  is pushed from  816  to  810 , and the measurement signal ν EC  directly related to the EC  108  decaying will be ν EC =ν r −Ct for 
     
       
         
           
             
               
                 
                   d 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     B 
                     B 
                   
                 
                 
                   d 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   t 
                 
               
               = 
               
                 C 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 t 
               
             
             , 
           
         
       
     
     shown in  812 . In that case, the received signal ν, has higher voltage level due to the high signal sensitivity, less dynamic range in a better linear permeability region that has further less nonlinearity, and high improvement in SNR. Point  814  serves as an example, when the system noise floor  803  is present compared to the original received signal  816  at the same point of 814. A method for removing the biased field can be employed in the case of linear and functional bias current I B  applications so as to remove the artifacts of the changing biased magnetic field from the output signal  207 . As the function of the biased magnetic field B B  can be mapped in a controlled environment, the artifacts of this field appearing on the output signal can be cancelled out in the signal post-processing domain if needed. The constant bias current case where I B =C and 
     
       
         
           
             
               
                 d 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   B 
                   B 
                 
               
               
                 d 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 t 
               
             
             = 
             0 
           
         
       
     
     does not require this step as the receiver coil  104  is only sensitive to a changing magnetic field where 
     
       
         
           
             
               
                 d 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   B 
                   B 
                 
               
               
                 d 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 t 
               
             
             ≠ 
             0. 
           
         
       
     
     In one exemplary embodiment, I B  and I 0  are different, I 0  is the charging current to build up the initial magnetic field B 0  in the target, working in the TX duration. Once the charging current I B  is switched off, dB 0 /dt generates the high Eddy Current I EC,0  in the target. Then the EC decays due to the diffusion and damping processes inside the target. Normally, the higher the I 0 , the higher the I ECO , the higher the measurement signal ν EC , the higher the SNR for measurement signal. As a result, I 0  can be high in the level of amperes to several hundreds of amperes. I B  is the bias current, working in the RX duration when transducer measures 
     
       
         
           
             
               v 
               
                 E 
                 ⁢ 
                 C 
               
             
             = 
             
               μ 
               ⁢ 
               
                   
               
               ⁢ 
               N 
               ⁢ 
               
                   
               
               ⁢ 
               A 
               ⁢ 
               
                 
                   
                     d 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       H 
                       
                         E 
                         ⁢ 
                         C 
                       
                     
                   
                   
                     d 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     t 
                   
                 
                 . 
               
             
           
         
       
     
     According to ferromagnetic core B-H curve shown in  FIG. 4 , the permeability μ decreases along the H EC  which is corresponding to the EC decaying inside the target. However, adding the bias current I B  to the bias coil to generate the bias B field inside the core can push the working point of permeability μ toward the higher region along the B-H curve during RX acquisition time window to increase the measurement signal 
     
       
         
           
             
               v 
               
                 E 
                 ⁢ 
                 C 
               
             
             = 
             
               μ 
               ⁢ 
               N 
               ⁢ 
               A 
               ⁢ 
               
                 
                   d 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   B 
                 
                 
                   d 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   H 
                 
               
             
           
         
       
     
     in order to boost SNR. It can also be referred to as boosting the sensitivity of the inductive transducer to make the outside signal much higher given the same EC decaying 
     
       
         
           
             
               d 
               ⁢ 
               
                   
               
               ⁢ 
               
                 H 
                 
                   E 
                   ⁢ 
                   C 
                 
               
             
             
               d 
               ⁢ 
               
                   
               
               ⁢ 
               t 
             
           
         
       
     
     input corresponding to the same level of EC decaying inside the target. The bias current I B  may not be high in value that can charge the target but the bias current I B  can be much smaller in the range of small fraction of ampere. 
     In one exemplary embodiment, the transmitter coil also acts as the magnetic bias coil, wherein the inductive sensor apparatus further includes a switching mechanism configured to alternately connect the transmitter coil to the first current source and the second current source.  FIG. 6  shows an exemplary embodiment of the switching mechanism as the switch  604 . A circuit network can control the operation of different components and includes the switching mechanism to connect and disconnect different components including the transmitter coil, the receiver coil, and the magnetic bias coils based on a predefined set of rules. The circuit network operates the transmitter coil, by actuating the switching mechanism to connect the transmitter coil to the first current source, for a predetermined charging duration (TX duration), to generate the magnetic field B 0 . The circuit network disconnects the transmitter coil, by actuating the switching mechanism to disconnect the transmitter coil from the first current source, after the predetermined charging duration. The circuit network operates the receiver coil for a predetermined acquisition duration to detect the magnetic field generated by induced eddy currents to generate an eddy current voltage signal. The circuit network operates the magnetic bias coil, by actuating the switching mechanism to connect the magnetic bias coil to the second current source, during the predetermined acquisition duration to manipulate the permeability of the ferromagnetic core. 
     While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above-described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the invention as claimed.