Abstract:
The present device relates to a sensor capable of detecting changes in the electromagnetic field it generates when in proximity to either conductive or nonconductive materials. This occurs by way of oscillating a transmit coil with an electro motive force at a resonant frequency thus creating an electromagnetic field. The magnetic field passes through a target of either conductive or nonconductive material and is then intercepted by a receive coil which likewise oscillates at a resonant frequency, which when in proximity to the transmit coil and transmit coils resonant frequency produces an enhanced signal by way of the interaction of the respective resonant frequencies and receive coil output.

Description:
RELATED APPLICATION DATA 
       [0001]    This application claims the priority date of provisional application No. 61/442,742 filed on Feb. 14, 2011. 
     
    
     BACKGROUND 
       [0002]    The present device relates to a sensor capable of detecting changes in the electromagnetic field it generates when in proximity to either conductive or nonconductive materials. 
         [0003]    There has been a persistent need to inspect both conductive and nonconductive items for consistency and for the presence of flaws with a single technology capable of overcoming deficiencies associated with traditional x-ray, eddy current, ultrasonic and other nondestructive inspection methods currently employed. The problem with x-ray has been the dangerous nature of the high energy electromagnetic wave and the hazards to biological organisms are well understood, given this and the need for elaborate shielding, x-ray can be very undesirable. Also, while x-ray is useful for detecting volumetric anomalies such as voids or the presence of foreign objects, flaws such as cracks where the adjoining faces of the cracks may be in intimate contact and having no appreciable volume, are very difficult to detect. 
         [0004]    Standard eddy current inspection is useful in detecting discontinuities in metal and other conductive materials, but do not work well when inspecting nonconductive materials. The inability to inspect nonconductive materials has limited eddy current applications. Eddy current inspection may also employ design features which allow the effects of signal output due to changes in liftoff (the distance between the sensor and the item) to be inspected to be mitigated. These design features are permanent and may not be changed on the fly during inspection, thus limiting its ability to instantaneously determine liftoff. 
         [0005]    Ultrasonic inspection can be difficult to employ, given the need to provide a coupling fluid or gel to transmit the ultrasonic frequency from a transducer to a target being inspected. It is often impractical to use such coupling fluids and gels on many structures as well as completed structures such as can be expected in the air frame of a finished aircraft, especially when constructed of composite. Also, it is not possible to use ultrasonic inspection technologies when there is an air gap separating otherwise inspectable walls, as air lacks the necessary transmissive qualities associated with a coupling fluid. 
         [0006]    Accordingly, there is a need for a sensor which does not produce harmful radiation, which can inspect conductors and nonconductors alike and can inspect through walls of various materials and air gap transitions. Such a sensor should be very compact to allow easy access to confined spaces and should also allow for inspection of small features and anomalies which may be critical to the performance of the item or system being inspected. The sensor should provide an output that has signal variation relative to varying features or anomalies of a target and which may be located in the item being inspected. The sensor should have the ability to control for variables such as liftoff or material changes without the need to make permanent physical changes to the sensor. 
       SUMMARY 
       [0007]    The above mentioned need is met by the present resonant electromagnetic sensor, which provides for an enhanced signal output by utilizing a transmit coil which resonates at a fixed or series of resonant frequencies. When an electro motive force (EMF) at resonant frequency or frequencies is induced to the transmit coil, it generates an electromagnetic field which oscillates relative to the frequency applied. This electromagnetic field passes through a target of either conductive or nonconductive material; and is then intercepted by a receive coil which also resonates at a frequency or series of frequencies in strategic proximity to the resonant frequency or frequencies of the transmit coil. The receive coil, by way of Lenz&#39;s Law converts the intercepted oscillating magnetic field and converts it to a signal which can be analyzed to reveal subtle and gross changes in the material being inspected. The proximity of the frequencies of the transmit and receive coils is meant to maximize sensor output by way of high ‘Q’ or quality factor and of high output signal which occurs when the transmit and receive coils have been tuned and brought into proximity to one another. 
         [0008]    The present sensor also provides frequencies at which the effects of liftoff and/or target material change may be mitigated if the transmit and receive coils have been appropriately tuned. Because of its high ‘Q’ and output signal, the present sensor is very sensitive to not only the subtle changes that may exist in a target of conductive material, but nonconductive material as well, so that it may scan from one type of material to the next without the need for sensor changes. Because of its unique “tuning” ability by way of adjusting resonant frequencies of transmit and receive coils, the present sensor may neglect the effects of liftoff and or changing materials under the sensor in order to generate a more complete image of the material being inspected. The present sensor is also capable of scanning through multiple walls of materials, with air and other materials at the transition boundary between the walls, and resolve characteristics not only of the intermediate walls but of the wall on the far side as well. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of the resonant electromagnetic sensor constructed in accordance with this specification; 
           [0010]      FIG. 2  is an orthographic end view of the sensor; 
           [0011]      FIG. 3  is an orthographic side view of the sensor; 
           [0012]      FIG. 4  is a perspective view of the sensor with a target material positioned in proximal to the sensor; 
           [0013]      FIG. 5  is a schematic of transmit coil; 
           [0014]      FIG. 6  is a frequency response graph of the transmit coil; 
           [0015]      FIG. 7  is a schematic of the transmit coil and receive coil; 
           [0016]      FIG. 8  is a frequency response graph of the transmit and receive coil; 
           [0017]      FIG. 9  is a frequency response graph showing sympathetic resonance; 
           [0018]      FIG. 10  is a schematic of the transmit and receive coils where the transmit capacitance is variable; 
           [0019]      FIG. 11  is a schematic of the transmit and receive coils where the receive capacitance is variable; 
           [0020]      FIG. 12  is a schematic of the transmit and receive coils where both receive and transmit capacitance are variable; 
           [0021]      FIG. 13  is a frequency response graph showing an air gap control frequency; 
           [0022]      FIG. 14  is a frequency response graph showing a wall control frequency; and 
           [0023]      FIG. 15  is a schematic showing rectification and amplification of the receive coil output. 
       
    
    
       [0024]      
         [0000]    
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 LISTING OF REFERENCE NUMERALS of 
               
               
                 FIRST-PREFERRED EMBODIMENT 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Sensor Assembly 
                 20 
               
               
                   
                 First Lead of the Transmit Coil 
                 22 
               
               
                   
                 First Lead of the Receive Coil 
                 24 
               
               
                   
                 Receive Coil 
                 26 
               
               
                   
                 Transmit Coil 
                 28 
               
               
                   
                 Core 
                 30 
               
               
                   
                 Second Lead of the Receive Coil 
                 32 
               
               
                   
                 Second Lead of the Transmit Coil 
                 34 
               
               
                   
                 Oscillating Magnetic Field 
                 36 
               
               
                   
                 Discontinuity in Target Material 
                 38 
               
               
                   
                 Target Material 
                 40 
               
               
                   
                 Transmit Coil Circuit 
                 41 
               
               
                   
                 Source of Oscillating EMF 
                 42 
               
               
                   
                 Receive Coil Circuit 
                 43 
               
               
                   
                 Transmit Coil Capacitor 
                 44 
               
               
                   
                 Transmit Coil Resistor 
                 46 
               
               
                   
                 Resonant Peak 
                 48 
               
               
                   
                 Voltage Level at −3 dB 
                 50 
               
               
                   
                 Upslope Side of Curve 
                 52 
               
               
                   
                 Frequency 1 
                 54 
               
               
                   
                 Resonant Frequency 
                 56 
               
               
                   
                 Frequency 2 
                 58 
               
               
                   
                 Bandwidth 
                 59 
               
               
                   
                 Downslope Side of Curve 
                 60 
               
               
                   
                 Peak Voltage at Resonant Frequency 
                 62 
               
               
                   
                 Receive Coil Resistor 
                 64 
               
               
                   
                 Signal Monitoring and/or Conditioning Device 
                 66 
               
               
                   
                 Receive Coil Capacitor 
                 68 
               
               
                   
                 Transmit Coil Resonant Peak 
                 70 
               
               
                   
                 Trough 
                 72 
               
               
                   
                 Receive Coil Resonant Peak 
                 74 
               
               
                   
                 Transmit Coil Variable Capacitor 
                 76 
               
               
                   
                 Transmit Coil First Resonant Peak 
                 78 
               
               
                   
                 Transmit Coil Second Resonant Peak 
                 80 
               
               
                   
                 Sympathetic Resonant Peak 
                 82 
               
               
                   
                 Transmit Coil Fourth Resonant Peak 
                 84 
               
               
                   
                 Transmit Coil Fifth Resonant Peak 
                 88 
               
               
                   
                 Transmit Coil Sixth Resonant Peak 
                 90 
               
               
                   
                 Receive Coil Variable Capacitor 
                 92 
               
               
                   
                 Wall Control Frequency 
                 94 
               
               
                   
                 Resonant Frequency Shift for Air Gap 
                 96 
               
               
                   
                 Air Gap Control Frequency 
                 98 
               
               
                   
                 Resonant Frequency Shift for Wall 
                 100 
               
               
                   
                 Rectifier Portion of Circuit 
                 102 
               
               
                   
                 Amplifier First Stage 
                 104 
               
               
                   
                 Amplifier Second Stage 
                 106 
               
               
                   
                 Signal Output 
                 108 
               
               
                   
                 Offset Input 
                 110 
               
               
                   
                 Gain Resistor 
                 112 
               
               
                   
                   
               
             
          
         
       
     
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views. The following description of the resonant electromagnetic sensor is the preferred embodiment when said system is reduced to practice however, it is not intended to be the only embodiment as features and practices may be altered while still remaining within the intent and scope of this specification. 
         [0026]      FIG. 1  is a preferred embodiment of the sensor assembly  20 , comprised of a transmit coil  28  and a receive coil  26  concentrically arranged and with the receive coil  26  within the transmit coil  28 . Within the receive coil is an optional core  30  made of material with high magnetic permeability and suitable for concentrating a magnetic field. This core serves to direct a greater amount of magnetic field to be generated by the transmit coil  28  into the area within the receive coil  26  so as to provide greater magnetic field to the receive coil  26 . This magnetic field once concentrated within the receive coil  26  by way of the core  30  can be converted to an oscillating electromotive force or EMF in accordance with Lenz&#39;s Law. Also shown in this figure are the leads of the coils. The first lead of the transmit coil  22  and the second lead of the transmit coil  34  are to be energized with an oscillating electromotive force or EMF. The first lead of the receive coil  24  and the second lead of the receive coil  32  provide a signal output by converting an induced magnetic field to an EMF. 
         [0027]      FIG. 2  is an end view of the sensor assembly showing the transmit coil  28  wound outside and concentric to the receive coil  26 . There is a gap shown between the two coils as illustrated, but this gap can be very small or the two coils may be in contact with one another. There may even be materials used to separate the coils or a bobbin used to wind the transmit coil, which then becomes interposed between the two coils. Also visible in this figure is the core  30  of high permeability material meant to concentrate the magnetic field to be generated by the transmit coil  28 . 
         [0028]      FIG. 3  shows the side view of the sensor and how the various components may be arranged within it. While the coils and the core are all of equal length, these lengths may be varied for ease of construction or to enhance performance. Also the number of turns on the transmit  28  and receive coil  26  may vary greatly. The number of turns selected for each will depend on several factors, such as the desired operating frequency, the desired energy transfer, and the desired amount of parasitic characteristics, or characteristics such as resistance, capacitance and inductance inherent in the winding itself. 
         [0029]      FIG. 4  shows the oscillating magnetic field  36  which has been generated by providing and oscillating EMF to the transmit coil  28 . This magnetic field oscillates at a frequency which matches the oscillation applied to the leads  22  and  34  of the transmit coil  28 . Placed in front of the sensor assembly  20 , or in sensing proximity, is the target material  40 , which may be made of conductive or nonconductive matter or a compound of materials. This matter or compound may be solid, liquid or gas as the sensor assembly  20  is capable of discerning characteristics for all of these states. For the sake of this explanation however, we will assume that this target material  40  is solid. Within or on the target material  40  is a discontinuity  38 , which may be a flaw or a desired feature of either the same material of the target or different material. This discontinuity may be present on the surface closest to the sensor, within the target or on the side of the target farthest from the sensor assembly  20 . 
         [0030]      FIG. 5  is a schematic of the basic transmit coil circuit  41  and is shown to better understand the details of the sensor assembly  20 . In this schematic, the source of oscillating EMF  42  can be seen as well as a classic LRC circuit taught in basic electronics. In this circuit there is a resistor  46 , an inductor or transmit coil  26  and a capacitor  44 . Transmit coil  26  having leads  24  and  32  connecting it to the circuit. It is well understood that in such a circuit the resonant frequency can be known by the formula f=1/2π(LC) 1/2 . Where f is the resonant frequency of the transmit coil circuit  41  and L is the inductance of the transmit coil  28  and C is the transmit coil capacitor  44 . It is important to note that while there is a resistor and capacitor shown, a contributing resistance and capacitance in the circuit can also be by way parasitic resistance and capacitance in the transmit coil  26 . Also, while the resistance, inductance and capacitance in this circuit is shown in series, one or more of these elements could be in parallel arrangement. It is also useful to recognize that resonance is reached when inductive reactance X L  is equal to and opposite capacitive reactance X C  and since XL=2πfL and XC=½πfC, it is easy to see how the formula for resonant frequency is derived. 
         [0031]    While resistance is not shown in these formulas, it is an important component in the overall amplitude of the magnetic field  36  being created by the transmit coil  28 . Altering either capacitance by way of changing the transmit coil capacitor  44  or the inductance of the transmit coil  28  has a dramatic effect on the resonant frequency of the circuit. Although it is not shown, inductance can be varied by adding an additional inductor or a variable inductor. However, the preferred embodiment is to vary the transmit coil capacitor  44  to tune resonant frequency as you might a radio receiver. 
         [0032]      FIG. 6  shows the frequency response of a simple LRC circuit as with the transmit coil circuit  41  where there is a clear resonant peak  48  where X L  is equal to X C . It is clear that at frequencies below and above resonant frequency  56  the reactance increases and efficiency drops as is shown by the upslope side of the curve  52  as well as the downslope side of the curve  60 . An important way to measure the quality of a resonating circuit or ‘Q’ is to divide the resonant frequency  56  by the bandwidth  59 . Bandwidth  59  is given by measuring 3 dB down from the peak voltage at resonant frequency  62  to arrive at the voltage level at −3 dB  50 . At that voltage level a horizontal line can be drawn  50  and where it intersects the frequency response curve two vertical lines can be drawn  54  and  58  where 54 is frequency  1  and  58  is frequency  2 . By subtracting frequency  2 ,  58  from frequency  1 ,  54  bandwidth  59  can be known, or bandwidth=f 2 −f 1 . To calculate ‘Q’ the resonant frequency  56  is divided by the bandwidth  59 . ‘Q’ will be used later in describing preferred operating frequencies of the sensor assembly  20 . 
         [0033]      FIG. 7  shows a schematic of the transmit coil circuit  41  and the receive coil circuit  43 . The receive coil  26 , as mentioned, is collocated concentrically with and inside the transmit coil  28 . Its purpose is to intercept the magnetic field  36  generated by the transmit coil  28  after having passed through the target material  40 . It is preferred not to simply intercept the magnetic field  36 , but rather to first tune the resonant frequency of the receive coil  26  to in some cases exactly match or have parity with the resonant frequency  56  of the transmit coil  26  and in other cases to be close to, or have approximate parity to the resonant frequency  56  of the transmit coil  26 . This is done by again tuning receive coil circuit  43  by varying either inductance or the receive coil capacitor  68 . In the preferred embodiment it is desirable to adjust or tune capacitance by varying the receive coil capacitor  68 . As before variations in the receive coil resistor  64  serves to affect amplitude of the signal output. By tuning both the transmit circuit  41  and the receive coil circuit  43  to either parity or approximate parity, depending on the particulars of the circuit, an enhanced transmission of power can be realized from the transmit coil circuit  41  to the receive coil circuit  43 . 
         [0034]    The energy transferred to the receive coil circuit  43  is monitored with signal monitoring and or conditioning device  66 . This device may monitor the oscillating signal from the receive coil circuit with a display, commonly referred to as an impedance plane display, where impedance is given on an oscilloscope type device, where one axis of the display represents resistance of the circuit and the other axis represents inductive reactance. The preferred method of conditioning and monitoring in this embodiment which will be explained in  FIG. 15  is rectification and then amplification of the DC signal. It is this preferred method that was used in the collecting of data for the frequency response curves in this specification. 
         [0035]      FIG. 8  shows a frequency response of the circuit in  FIG. 7  where the transmit coil circuit  41  has a resonant peak  70  which is at approximately 99 KHz and the receive coil circuit  43  has a receive coil resonant peak  74  which is approximately at 195 KHZ. While each of these peaks are at resonance and each is capable of detecting variations in material  40 , this circuit has not been optimized. It can be seen that there is a trough  72  between the transmit coil resonant peak  70  and the receive coil resonant peak  74 . This trough  72  is indicative of poor energy transfer from transmit coil circuit  41  and receive coil circuit  43  by way of transmit coil  26  and receive coil  28 . It is desirable to minimize this trough  72  to enhance performance of the circuit of  FIG. 7  and of the sensor assembly  20 . This trough  72  can be minimized by proper tuning of the circuit of  FIG. 7 . 
         [0036]      FIG. 9  shows the frequency response of multiple variations of the circuit of  FIG. 7 , where the receive coil capacitor  68  has been set and held at  519  pfd (pico farads) giving a receive coil resonant peak  74  of about 195 KHz. It can be seen that as the transmit coil capacitor  44  of the transmit coil circuit  41  is changed to different values there is a dramatic effect on frequency response. It can be seen that a transmit coil first resonant peak  78  with a transmit coil capacitor  44  of 1052 pfd is far removed from the receive coil resonant peak  74  and transfers a low amount of energy from the transmit coil circuit  41  to the receive coil circuit  43  and that the trough  72  is quite wide. The transmit coil second resonant peak  80  has greatly improved in amplitude by using a transmit coil capacitor  44  of 519 pfd. This has brought its resonant peak  80  closer to the receive coil resonant peak  74  and in so doing has boosted energy transfer by improving “sympathetic resonance”, where the resonant frequency of the transmit coil is either in parity with or in approximate parity to the resonant frequency of the receive coil such that output is increased beyond the output of the constituent resonant peaks. Maximum output of this particular circuit of  FIG. 7  reaches its maximum when the transmit coil capacitor  44  is set at 237 pfd, yielding sympathetic resonant peak  82 . At this frequency of about 142 KHz, the circuit will be most sensitive to changes in target material  40  and will be most able to detect variations such as discontinuities in target material  38 . In this case, this peak occurred at an approximate parity frequency which does not match the receive coil resonant peak  74 . This is due to a wide variety of reasons from the construction of the sensor assembly  20  to the particular tuning of the circuit of  FIG. 7 . Depending on construction and tuning, the sympathetic resonant peak could be at frequencies lower than, greater than or equal to the receive coil resonant peak  74 . Transmit coil fourth, fifth and sixth resonant peaks  84 ,  88  and  90 , respectively, occur at different frequencies but are not optimized. 
         [0037]      FIGS. 10 ,  11  and  12  show the addition of variable capacitors to either the transmit coil circuit  41  or the receive coil circuit  43  or both.  FIG. 10  shows transmit coil capacitor  44  being replace with transmit coil variable capacitor  76 .  FIG. 11  shows receive coil capacitor  68  being replaced by receive coil variable capacitor  92  and  FIG. 12  shows both the transmit coil capacitor  44  and the receive coil capacitor  68  being replace by transmit coil variable capacitor  76  and receive coil variable capacitor  92  respectively. These aforementioned variable capacitors may be manually variable or variable by electronic signal. The purpose of these variable capacitors is to allow rapid switching to other desired resonant peaks or sympathetic resonant peaks in order to more thoroughly inspect the target material  40 . 
         [0038]      FIG. 13  shows a circuit tuned to a resonant frequency which may or may not be the sympathetic resonant frequency, where desirable characteristics other than maximum power transfer or maximum output occur. This tuning may be achieved by adjusting one or more variable capacitors such as in the circuits of  FIG. 10 ,  11  or  12 . 
         [0039]    It is often a desirable feature of a sensor to be able to control for variables such as liftoff, the gap or distance from the sensor assembly  20  to the target material  40 , or changes in material configuration such as the wall thickness of that material.  FIG. 13  shows how the control of gap may be accomplished by monitoring the output of the circuit at the air gap control frequency  98  of 75 KHz as opposed to the resonant peak. In doing this, it can be seen that the effects of gap are greatly mitigated relative to other frequencies. 
         [0040]    The same circuit is shown in  FIG. 14 , but instead of varying gap, the wall thickness of the material is varied. It can be seen that the air gap control frequency  98 , which mitigates changes in gap, is sensitive to changes in wall. This means that even though there are changes in the distance from the sensor to the target, those changes are mitigated while the effects of varying wall can be clearly seen. 
         [0041]    Similarly, at the wall control frequency  94  of 63 KHz, as wall is varied the signal is mitigated, but as gap is varied, the signal output changes appreciably. In this manner the sensor assembly  20  may be tuned to control variables and or tuned to provide maximum output and frequencies may be switched as desired to achieve maximum signal or mitigated signal. While the control signals for wall and gap have been shown, other control frequencies exist to mitigate change in material or change in temperature which are found by similar tuning methods. 
         [0042]    Further studying the frequency response curve of  FIG. 13 , it can be appreciated that the compression of curves at and about the air gap control frequency  98  and the subsequent expansion of curves at the wall control frequency  94  occurs as a result of a resonant frequency shift for air gap  96 . It can be seen that as air gap increase the signal amplitude rises while the resonant frequencies shift lower. This is true of this particular tuning setting and the phenomena may be reversed if tuned differently where the resonant frequency shift for air gap may be to higher frequencies, causing a reversal in the compression and expansion of the curves and or causing a reduction in signal due to increased air gap. 
         [0043]    Conversely, in  FIG. 14  as wall thickness changes the resonant frequency shift for wall  100  is to higher frequencies as wall thickness increases and signal increases as wall increases. This causes a compression of the curves at the wall control frequency  94  and an expansion of the curve at the air gap control frequency  98 . Again, depending on tuning, these compression and expansion areas may be reversed and signal may diminish relative to wall. 
         [0044]      FIG. 15  shows a preferred embodiment of the signal monitoring and or conditioning device  66 , where the output of the receive coil circuit  43  is fed into a rectifier circuit  102  to convert the oscillating signal to a DC or direct current output. The DC signal is then fed into an amplifier first stage  104  where the signal is amplified. The amplified signal is then sent to the amplifier second stage  106 , where additional amplification may be accomplished by setting or adjusting gain resistor  112 . Often, there is a computer which will receive the output  108  of the signal monitoring and or conditioning device  66  and  FIG. 15 , as many computers can tolerate a relatively narrow voltage input of perhaps +/−10 volts. Should the signal become too large due to amplification, resonant tuning or high voltage being delivered by source of oscillating EMF  42 , an offset input  110  may be applied. In so doing the output voltage is shifted to a lower voltage which can be received by the computer while preserving any effects that may have come about by monitoring variations in target material  40 .