Abstract:
An inventively enhanced near-field sensor includes circuitry which removes variation in standoff distance (of the sensor from the inspected object) as a factor in the inspection system readings. An original output voltage which varies linearly according to standoff distance is, modified and added to a counterbalancing output voltage which equivalently but oppositely varies linearly according to standoff distance, resulting in a constant output voltage regardless of standoff distance. For calibration purposes, a third output voltage can also be summed along with the modified output voltage and the counterbalancing output voltage. Since the effect of surface variation is nullified, the practitioner can more truly assess the interior physical condition of the inspected object, knowing that the object&#39;s surface roughess is rendered irrelevant.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to the nondestructive testing or evaluation of physical properties of materials, more particularly to methods and apparatuses for accomplishing near-field inspection of materials, such as involving utilization of microwave radiation in association with materials such as metallic or composite materials. 
     Various kinds of near-field microwave inspection have been conducted with respect to various kinds of structures (e.g., composite or metallic structures) having an extended surface area. Generally, a near-field probe (for example, a open-ended rectangular waveguide probe or an open-ended coaxial probe) is used in conventional practice of near-field microwave inspection. Typically, the microwave inspection inherently incorporates or assumes a “standoff distance” or “liftoff” of the near-field probe in relation to the surface area of the material being inspected. 
     The measurement results are usually sensitive to the changes in this standoff distance. Sometimes a change in the standoff distance is related to variation in surface roughness (or, synonymously expressed, surface height). For instance, in the case of glass reinforced epoxy composites, the change in the standoff distance can be caused by surface roughness/height variations in the composite skin. 
     It is generally important to distinguish between or among various types of defects. For instance, in the case of a composite laminate, it may be desirable that an internal defect such as a layer-layer disbond be distinguished from a defect on the surface such as related to impact damage. In order to differentiate between or among internal and external defects, the influence of standoff distance variation must somehow be accounted for. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide method and apparatus for effectuating near-field microwave nondestructive testing of an object in such a way as to more capably distinguish between internal physical characteristics and external physical characteristics. 
     The present invention features the neutralization of the effect of surface variation of the object in the context of near-field sensing. Provided by the present invention is a circuit which accounts for standoff distance variation and eliminates its influence from the final inspection system output. The inventive compensatory and equalizing circuit has been designed and successfully tested by the U.S. Navy and Colorado State University in association with open-ended rectangular waveguide probes. 
     According to typical embodiments of the present invention, the inventive apparatus is used in association with a sensing device which is capable of producing a nonconstant device signal for inspecting an object. The sensing device&#39;s nonconstant device signal varies in accordance with the distance of the sensing device from the object. The inventive apparatus comprises means for rendering the sensing device capable of producing a constant device signal at least until reaching said object, wherein the constant device signal is constant regardless of the distance. 
     According to many inventive embodiments, the inventive means for rendering includes: means for producing a nonconstant counteractive output signal, the nonconstant counteractive signal varying in accordance with the distance; means for modifying the nonconstant device signal so as to become a nonconstant modified device signal which is commensurate with the nonconstant counteractive signal; and, means for combining signals, the means for combining signals including means for combining the nonconstant counteractive signal and the nonconstant modified device signal. The constant device signal is based on the combining of the nonconstant counteractive signal and the nonconstant modified device signal. 
     According to frequently preferred inventive practice, the inventive means for rendering also includes means for producing a constant offset signal. Thus, the means for combining signals includes means for combining the nonconstant counteractive signal, the nonconstant modified device signal and the constant offset signal. The constant device signal is based on the combining of the nonconstant modified device signal, the nonconstant counteractive signal and the constant offset signal. 
     In typical inventive practice, the nonconstant device signal, the nonconstant modified device signal and the nonconstant counteractive each vary linearly according to distance. Frequent inventive practice prescribes such linear variation in terms of voltage value. The “constancy” characteristic of both the constant device signal and the the constant offset signal presupposes a nonvarying linearity of each constant signal, freqently manifested in inventive practice as a constancy (i.e., single-valued linearity or invariability) in voltage value. In contrast, the “nonconstancy” characteristic of the nonconstant device signal, the nonconstant modified device signal and the nonconstant counteractive signal entails a varying linearity of each nonconstant signal, frequently manifested in inventive practice as a nonconstancy (i.e., plural-valued linearity or linear variability) in voltage value. 
     Usually, the nonconstant device signal varies linearly in accordance with the standoff distance; however, the inventive principles are still applicable whether the nonconstant device signal varies linearly or nonlinearly in accordance with the standoff distance. In fact, the present invention can be practiced regardless of whether the nonconstant device signal, the nonconstant modified device signal and the nonconstant counteractive signal vary linearly or nonlinearly according to distance. If, for instance, the initial voltage output varies as a nonlinear function of standoff distance, according to this invention a counterbalancing voltage output can be effected which equally but oppositely varies as a nonlinear function of standoff distance. Similarly, if the initial voltage output varies as a linear function of standoff distance, according to this invention a counterbalancing voltage output can be effected which equally but oppositely varies as a linear function of standoff distance. 
     Featured by the present invention is the provision of a voltage commensurate with the inspected material&#39;s surface roughness, and the addition of such provided voltage to, or the subtraction of such provided voltage from, the voltage detected by the microwave detector. In other words, according to this invention, a voltage is provided which is proportional to the surface roughness and is then added to or subtracted from the voltage detected by the microwave detector; such proportionality of voltage with respect to surface roughness can equivalently be considered to be a proportionality of voltage with respect to standoff distance. In this way, the present invention renders the final output voltage independent of surface roughness variations, which are typically slight but which can manifest diverse degrees and kinds of irregularity. 
     A near-field microwave device typically produces a voltage output signal which is a linear function of standoff distance. According to the present invention, potentiometer circuitry is provided to produce a voltage output signal which is a linear function of standoff distance, but which is oppositely sloped in comparison with the voltage output signal of the microwave device. Thus, if the microwave device&#39;s voltage output linearly increases in accordance with standoff distance, the inventive potentiometer circuitry&#39;s voltage output linearly decreases in accordance with standoff distance; on the other hand, if the microwave device&#39;s voltage output linearly decreases in accordance with standoff distance, the inventive potentiometer circuitry&#39;s voltage output linearly increases in accordance with standoff distance. 
     Further, according to the present invention, the slope of the microwave device&#39;s voltage output is rendered not only opposite to but also equal in magnitude to that of the potentiometer circuitry&#39;s voltage output. In this regard, the microwave device&#39;s voltage output is multiplied by an appropriate multiplication factor, thereby yielding a slope which is not only oppositely signed but which also has a magnitude which is equal to that of the potentiometer circuitry&#39;s output voltage. 
     Therefore, in accordance with this invention, when the microwave device&#39;s voltage output is multiplicatively modified and then counteractively (e.g., additively or subtractively) associated with the potentiometer circuitry&#39;s voltage output, the result is a constant voltage output irrespective of standoff distance. If graphically visualized as voltage output (y-axis) as a function of standoff distance (x-axis), the voltage output of the near-field microwave device, as inventively modified, is zero-sloped (i.e., horizontal). 
     The present invention thus enhances or improves the inspection capability of near-field microwave nondestructive testing techniques (such as those which implement open-ended rectangular waveguide sensors or open-ended coaxial sensors) for detection of interior flaws (e.g., manufactured or in-service produced flaws) in materials (such as multi-layered dielectric composites) in which a certain degree of surface roughness is present. 
     Although the present invention is applicable to diverse types of materials such as generally categorized as composite materials, it is especially beneficial when practiced with respect to composite laminates, wherein it is desirable to distinguish internal anomalies (such as associated with bonding of lamina) from external (surface) irregularities. Currently, there is no known efficient technique for continuously correcting for standoff distance variations caused by surface roughness in structures (such as dielectric composite structures) as a near-field probe (such as an open-ended rectangular waveguide aperture probe) scans over a material (such as a composite material). The new circuitry according to this invention monitors standoff distance variations (e.g., due to surface roughness) and electronically corrects the microwave detector output voltage for this variation. 
     Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein like numbers indicate the same or similar components, and wherein: 
     FIG. 1 is a diagrammatic representation of implementation of a near-field microwave sensor with respect to an object having a surface characterized by a degree of irregularity. 
     FIG. 2 is a block diagram of an embodiment of a standoff distance variation compensator/equalizer in accordance with the present invention. 
     FIG. 3 is a circuit diagram of the inventive embodiment shown in FIG.  2 . 
     FIG. 4 is a more detailed version of the circuit diagram shown in FIG.  3 . 
     FIG. 5 is a graph, obtained using a computer simulation program for electronic circuitry, of voltage (y-axis) versus time (x-axis), wherein time is representative of standoff distance. FIG. 5 is illustrative of how standoff dependency may be removed in accordance with the present invention. 
     FIG. 6 is a conceptual graph of voltage (y-axis) versus standoff distance (x-axis), further illustrative of how standoff dependency may be removed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, conventional near-field sensor  50  is a microwave detector such as the kind which includes an oscillator and a crystal diode detector. Near-field sensor includes an open-ended probe (such as a rectangular waveguide probe or a coaxial probe) having a probe open end  51  for scanning material  60  having material surface  61 . Near-field scanning of inspected material  60  by sensor  50  is typically performed while sensor  50  is caused to move in a bidirection p which is generally parallel to material surface  61 , probe open end  51  thereby generally maintaining a standoff distance d from material surface  61 . 
     The electromagnetic radiation (microwave radiation, in this example) is caused by near-field sensor  50  to be transmitted to material surface  61  and at least partially through the thickness of material  60 , and then return to near-field sensor  50 . Output voltage V out  is the signal which reachs material surface  61 . Upon penetration of material  60 , output voltage V out  is altered by the internal physical characteristics of material  60 . Thus, input voltage V in , the signal which returns to near-field sensor  50 , is altered as compared with output voltage V out . The problem with conventional near-field sensor  50  is that output voltage V out  is variable due to variability of material surface  61 , thus rendering readings based on input voltage V in  less meaningful, less accurate or more problematical. 
     Material surface  61  is not perfectly smooth or flat, and is in fact characterized by surface irregularities (“surface roughness”) which are significant enough to appreciably change output voltage V out  (the voltage emanating from near-field sensor  50 ), since such voltage varies in accordance with distance d. The output voltage V out  of near-field sensor  50  is variable in a manner commensurate with the variability of standoff distance d at different points along material surface  61 , e.g., unequal standoff distances d 1 , d 2 , d 3  and d 4  as shown in FIG.  1 . 
     For illustrative purposes, material  60  is shown in FIG. 1 to have an internal defect  62 , such as a localized disengagement (disbonding) between two layers of material  60  if material  60  is a composite laminate. Because of the variability of standoff distance d and hence of the output voltage V out , of near-field sensor (e.g., microwave detector)  50 , when utilizing conventional near-field sensor  50  it will be difficult or impossible to distinguish between internal aberrations of material  60 , such as defect  62 , from external aberrations and irregularities in general of material  60  which are manifested at material surface  61 . This is because, when input voltage V in  is reflected from material  60  and returned to near-field sensor  50  for detection, it will be difficult or impossible to determine to what extent the change in V in  vis-a-vis V out  is attributable to the internal physical characteristics of material  60 , and to what extent such change is attributable to the roughness of material surface  61 . 
     However, let us consider the near-field sensor shown in FIG. 1 to be inventively enhanced near-field sensor  5000 , rather than conventional near-field sensor  50 . Inventively enhanced near-field sensor  5000  produces the same output voltage V out  regardless of which location on material surface  61  output voltage V out  has reached—equivalently expressed, regardless of distance d. In other words, at least until output voltage V out  has reached material surface  61 , output voltage V out  will be constant. At the point at which output voltage V out  begins to penetrate inspected material  60 , output voltage V out  is subject to change. Hence, according to the present invention, the value difference of input voltage V in  in comparison with output voltage V out  is assured to be entirely attributable to internal physical characteristics of inspected material  60 , and not the least bit attributable to surface variation of material surface  61 . In contrast to readings based on input voltages V in  for near-field sensor  50 , readings based on input voltages V in  for inventively enhanced near-field sensor  5000  are free of distortions associated with nonconstancy of output voltage V out . 
     Still referring to FIG.  1  and also referring to FIG. 2, near-field sensor includes a crystal diode detector  51  and an oscillator  52 . The objective of inventive circuitry  500  is to inventively enhance the near-field sensor (e.g., waveguide probe or other microwave detector or device)  50  circuitry so as to effectuate compensation of the voltage output V out0  as measured from microwave device  50  with respect to changes in the standoff distance d from the (waveguide) opening  51  to the surface  61  of material  60 . 
     In the absence of the electronic apparatus  500  in accordance of the present invention, near-field sensor  50  produces an output voltage V out0  (such as would be received at terminal or point  22  shown in FIG. 4) which is proportional to standoff distance d. When inventively enhanced through connection with inventive circuitry  500 , conventional near-field sensor  50  becomes inventively enhanced near-field sensor  5000  which produces an inventively corrected output voltage V out  which is the same regardless of standoff distance d. Inventively enhanced microwave detector  5000  has an overall circuitry which comprises the combination of the original microwave detector  50  circuitry and the inventive circuitry  500 . 
     In this regard, the present invention takes advantage of the fact that the output voltage V out0  of microwave device  50  changes as a linear function of standoff distance d. Thus, according to this invention, the output voltage V out0  of microwave device  50  can be corrected by incorporating in or connecting to the circuitry of microwave device  50  a spring-loaded, piston potentiometer  100 , such as shown in FIG.  3  and FIG. 4, which produces a linear output voltage based on the distance from microwave device  50  to the surface  61  of the inspection material  60 . 
     Still with reference to FIG.  1  and FIG.  2  and particularly with reference to FIG.  3  and FIG. 4, microwave device  50  includes crystal (diode) detector  51 . Crystal detector  51  is akin to a voltmeter, and is an integral part of microwave device  50 . The model of crystal detector  51  shown in FIG.  3  and FIG. 4 is for electromagnetic radiation having a particular frequency; however, the ordinarily skilled artisan understands in the light of this disclosure that such a model depends on the frequency (or frequencies) of the system, and that, more generally, the physical characteristics of the inventive circuitry  500  are a function of frequency. Various aspects of inventive circuit  500  are modeled based on the involved frequency or frequencies, and the models should be changed accordingly. 
     Inventive circuit  500  includes proportionality subcircuit  1 , compensation subcircuit  2 , offset subcircuit  3  and summation subcircuit  4 . Proportionality subcircuit  1  includes potentiometer  100 , operational amplifier (op-amp)  10  and resistor R 13 . Compensation subcircuit  2  includes operational amplifier (op-amp)  20  and resistor R 9 . Offset subcircuit  3  includes operational amplifier (op-amp  30 ) and resistor R 17 . Operational amplifier  30  includes includes offset element  300 , which includes resistor R 14 , variable resistor R offset  and resistor R 15 . Summation subcircuit  4  includes operational amplifier (op-amp)  40 . 
     Potentiometer  100  includes resistor R 1  and resistor Rpot. Operational amplifier  10  includes point (electrical contact)  11 , point  12 , point  13 , point  14  and point  15 . Operational amplifier  20  includes resistor R 7 , resistor R 8 , variable resistor R vary , point  21 , point  22 , point  23 , point  24  and point  25 . Operational amplifier  30  includes resistor R 14 , variable resistor R offset , resistor R 15 , point  31 , point  32 , point  33 , point  34  and point  35 . Operational amplifier  40  includes resistor R 4 , point  41 , point  42 , point  43 , point  44  and point  45 . 
     Operational amplifier  10  acts as a buffer circuit isolating the potentiometer  100  portion from the rest of inventive circuit  500 . Resistor R pot  models the variable resistance of potentiometer  100  as the standoff distance d changes. 
     Operational amplifier  20  performs two functions. First, it amplifies the signal from crystal detector  51 . Secondly, as further explained herein with reference to FIG. 5, by adjusting the variable resistor R vary , the overall circuitry can be “balanced” to remove the effect of standoff variation. This represents a first type of “calibration” which is effectuated according to the present invention. According to typical inventive practice, variable resistor R vary  is inventively set in accordance with a particular inspected material  60 . 
     Operational amplifier  30  acts as a buffer circuit for the offset voltage resistor network, viz., offset subcircuit  3 . Resistor R offset , can be used to set the overall output voltage V out  to zero or any other selected value. In other words, resistor R offset  is selected so as to yield an output voltage V out  having a desired value which represents a reference voltage for inventively enhanced near-field sensor  5000 . This represents a second type of “calibration” which is effectuated according to the present invention. Variable resistor R offset  can be inventively set to yield a desired value of output voltage V out . 
     Reference voltage V out  is the constant voltage existing until the electromagnetic (e.g., microwave) radiation emanating from inventively enhanced near-field sensor  5000  reaches surface  61  of inspected material  60 , at which point the voltage, as it proceeds internally in material  60 , may be affected (altered) in accordance with the physical characteristics of material  60 . The electromagnetic radiation in its altered form (e.g., altered voltage) returns to near-field sensor in feedback loop fashion. This change in the properties of the signal (e.g., change in voltage) is detected by crystal detector  51  and is measured by inventively enhanced near-field sensor  5000  with reference to the reference voltage V out . Operational amplifier  30 , by producing output voltage V out3 , thus serves the purpose of setting voltage V out  at a desired value for use as a reference voltage, with which the voltage returned to and detected by inventively enhanced near-field sensor  5000  is compared. Setting voltage V out  as a zero-valued reference voltage may be preferable for many inventive embodiments. 
     Summation subcircuit  4  includes operational amplifier  40  which adds the voltages V out1 , V out2  and V out3  together to give the final output voltage V out . Voltage V out1  is the output voltage from proportionality subcircuit  1  (which includes operational amplifier  10  and potentiometer  100 ). Voltage V out2  is the output voltage from compensation subcircuit  2  (which includes operational amplifier  20 ). 
     As further explained hereinbelow, voltage V out2  is equal to the product of an inventively determined factor [1+(R vary +R 7 )/R 8 ] and the output voltage V out0  from crystal detector  51 . Voltage V out3  is the output voltage from the offset voltage resistor network, viz., offset subcircuit  3  (which includes operational amplifier  30 ). In inventive practice, resistor R 4  can be replaced with a non-linear component, if variable gain is required. 
     The inventive circuit  500  for correcting the output voltage V out0  of the microwave device&#39;s crystal detector  51  is shown in summary form in FIG.  3  and in greater detail in FIG.  4 . Basically, inventive standoff compensating/equalizing circuit  500  involves four input/output subcircuits, as follows: (i) the voltage V pot , which is output from potentiometer  100  and which reflects changes in standoff distance d, is input into operational amplifier  10 , with a resultant voltage V out1  output from operational amplifier  10 ; (ii) the voltage V out0 , which is output from microwave device  50 , is input into operational amplifier  20 , with a resultant voltage V out2  output from operational amplifier  20 ; (iii) the voltage V off , which is output from offset element  300  (between resistor R off  and resistor R 15 ) and which is used to produce a zero reference (or other selected value reference) output voltage V out , is input into operational amplifier  30 , with a resultant voltage V out3  output from operational amplifier  30 ; (iv) the voltages V out1 , V out2  and V out3 , which are input from operational amplifiers  10 ,  20  and  30 , respectively, are input into operational amplifier  40 , with a resultant voltage V out  output from operational amplifier  40 , which is the voltage output by inventively enhanced near-field sensor  5000  during operation thereof with respect to material  60  while fronting material surface  61 . 
     In other words, inventive circuit  500  includes three operational amplifiers (i.e., operational amplifiers  10 ,  20  and  30 ) which are used to condition the corresponding input signals (i.e., voltages V pot , voltage V out0 , and voltage V off , respectively). Additionally, inventive circuit  500  includes a final stage operational amplifier, viz., operational amplifier  40 , to add the three output signals (i.e., voltages V out1 , V out2  and V out3 ) together and thereby produce a final output voltage V out . 
     More specifically, operational amplifier  10  is a unity gain or voltage follower of the input voltage V pot  from potentiometer  100 . Operational amplifier  10  acts as a buffer to isolate potentiometer  100  from the rest of inventive circuit  500 . Potentiometer  100  touches or contacts material surface  61  and thereby tracks the surface roughness of material surface  61 . The spring-loaded potentiometer  100  is modeled at a 78 kΩ variable resistor, R pot , in series with a 1 kΩ resistor R 1  connected to a +5 Volt DC power supply. The 1 kΩ resistor R 1  acts in conjunction with the R pot  as a voltage divider. The voltage V pot  to the noninverting input terminal of operation amplifier  10 , indicated at point (location or terminal)  12 , is taken from the point at the potentiometer, V pot , as shown in FIG.  4 . The output of operational amplifier  10 , point  11 , follows the input voltage. 
     Operational amplifier  20  handles the input V out0 , output from microwave crystal detector  51  of microwave device  50 . The model for the microwave crystal detector  51  is a 100 kΩ resistor R cd  in series with a variable voltage input V cd  and a −82 mVolt DC power supply. Crystal detector  51  is connected tov the noninverting input of operational amplifier  20  at input  22  as shown in FIG.  4 . The negative feedback loop of operation amplifier  20  contains a 10.96 kΩ resister R vary  in series with a 10 kΩ resistor R 7  connected to the inverting terminal at point  23 . The inverting terminal of operational amplifier  20  is connected to ground through a 1 kΩ resistor R 8 . 
     Operational amplifier  20  acts a linear multiplier for the voltage V out0 , seen from the crystal detector  51  circuit. The output voltage V out2  for operational amplifier  20  at point  21  is equal to [1+(R vary +R 7 )/R 8 ] times the output voltage input V out0  of crystal detector  51 . By selecting the appropriate value for R vary , the voltage V out0  from crystal detector  51  can be multiplied by the appropriate factor [1+(R vary +R 7 )/R 8 ] so as to obtain a voltage V out2  which compensates (offsets or counterbalances) the output voltage V out1 , which is derived from the voltage V pot  input from potentiometer  100 , which in effect measures the standoff distance d. That is: 
     
       
           V   out2   =V   out0 ×[1+( R   vary   +R   7 )/ R   8 ]. 
       
     
     The output voltage V out2  from the operational amplifier  20  is equal but opposite in slope with respect to the output voltage V out1  from operational amplifier  10 . That is, the absolute value of the slope described by V out1  equals the absolute value of the slope described by V out2 . In the absence of an offset output voltage V out3 , the sum of V out1  and V out2  will be a constant k. That is, V out +V out2 =k. The equal and opposite slopes of V out1  and V out2  cancel each other, leaving a net voltage k. 
     Operational amplifier  30 , like operational amplifier  10 , is also a unity gain follower. Operational amplifier  30  is a unity gain follower for the offset voltage V offset  taken at the point between R offset  and R 15  as input to the noninverting input terminal at point  32 . The 11.55 kΩ variable resistor R offset  is in series with a 10 kΩ resister R 14  and a 20 kΩ resister R 15  connected to a +5 Volt DC power supply. The variable resistor R offset  allows for adjusting of the final output voltage of the operational amplifier  40 , viz., output voltage V out , to 0 Volts when the inventively enhanced microwave circuit  5000  is calibrated. During calibration, inventively enhanced microwave circuit  5000  can be at any acceptable fixed calibration standoff distance d 0  from the inspection material surface  61  in the near field; inventively enhanced microwave circuit  5000  can be in contact with material surface  61  (i.e., wherein the standoff distance d 0  equals zero) or at a fixed distance d 0  greater than zero. 
     In other words, according to typical inventive practice, offset subcircuit  3 , which includes operational amplifier  30 , outputs a voltage V out3  which serves as a calibrational “zeroing” offset with respect to the sum of output voltages V out1 , V out2  and V out3  because the sum of output voltages V out  and V out2  is k. That is, since V out1 +V out2 =k, the value of V out3  will determine the value of V out  in the equation V out =V out1 +V out2 +V out3 . Otherwise expressed, V out =k +V out3 . If V out3  equals −k, then V out  equals zero. It is thus seen that, according to this invention, V out3  can be selectively set during calibration to obtain a value of “zero” or practically any other desired value of the overall output voltage V out . Since |V out |=V out1 +V out2 +V out3 , and V out1 +V out2 =k, if V out3 =−k, then V out =0. 
     According to many inventive embodiments, operational amplifier  40  is simply a voltage adder. The 10 kΩ resistor R 4  in the negative feedback loop has a 10 kΩ resistance, equal to the 10 kΩ resistance for each of resistors R 13 , R 9  and R 17 , which look at the corresponding outputs (V out1 , V out2  and V out3 , respectively) from the three previous amplifiers (operational amplifiers  10 ,  20  and  30 , respectively). Hence, the last stage of inventive circuit  500 , viz., summation subcircuit  4  (which includes operational amplifier  40 ), simply adds together the three output voltages V out1 , V out2  and V out3 , as follows: 
     
       
           V   out =−( V   out1   +V   out2   +V   out3 ) 
       
     
     The above equation states that, according to the inventive embodiment described herein and to some other inventive embodiments, V out  equals negative the quantity V out1  plus V out2  plus V out3 . It is noted that, according to some embodiments of the present invention, V out  equals positive the quantity V out1  plus V out2  plus V out3 ; that V out =V out1 +V out2 +V out3 . 
     Output voltage V out  represents the overall output voltage which reaches material surface  61 . When the microwave radiation having voltage V out  penetrates material  60 , it will be affected by irregular internal physical manifestations therein such as internal defect  62  shown in FIG.  1 . Defect  62  will cause a change in voltage, ΔV. Thus, input voltage V in  (which returns to near-field sensor  50 ) will deviate from output voltage V out  (which emits from near-field sensor  50 ) by voltage change ΔV (which is attributable to an internal physical characteristic such as internal defect  62 ). That is, V in =V out +ΔV. In the absence of an internal aberration or irregular physicality (e.g., internal defect  61 ) which affects the voltage by a factor of ΔV, ΔV=0, and hence V in =V out . If material  60  is physically homogeneous, then ΔV=0; hence, V in =V out +ΔV=V out +0=V out . If material  60  is physically nonhomogeneous in some respect, then, in relation to such nonhomogeneity, ΔV&gt;0 or ΔV&lt;0; hence, since V in =V out +ΔV, it follows that V in &gt;V out  or V in &lt;V out . 
     To summarize, by correctly selecting the multiplier value, R vary , the output voltage V out1  derived from the potentiometer  100  measuring standoff can be completely compensated. Thus, as the standoff distance d from the material surface  61  to the waveguide opening  51  changes, the output voltage. V out  of the inventive circuit  500  (and hence, of the inventively enhanced near-field sensor  5000 ) will remain constant. By selecting R offset  appropriately, the constant output voltage V out  can be set to 0 Volts (or to another desired voltage value). Once the two variable resistors R vary  and R offset  are set in an inventive calibration procedure, then the near-field sensor  5000  device can be used for inspection of defects in material  60 . At this point, any changes in the output voltage V out  are resultant of changes in material properties of material  60 , not of standoff distance d. 
     Reference is now made to FIG.  5  and FIG. 6, which pertain to inventive circuitry  500  having operational amplifiers characterized by polarities which are opposite those shown for operational amplifiers  10 ,  20 ,  30  and  40  in FIG.  3  and FIG.  4 . With reference to. FIG. 5, the inventive removal of standoff dependency can perhaps be better understood by considering the depicted set of curves obtained in a PSPICE simulation of an embodiment of an inventive circuit  500 . PSPICE is a computer program which permits performance of computer simulations of electronic circuits. The program supports schematic entry and provides graphical output, and can do several types of circuit analyses. “SPICE” stands for “Simulation Program for Integrated Circuits Emphasis.” PSpice® is a commercially available PC version of Spice, made by MicroSim Corp., which in recent years merged with OrCAD, Inc. 
     In the plot shown in FIG. 5, changing time models changing standoff distance d; the x-axis of time directly corresponds to standoff distance d. In this plot, the signals have been shifted to show them on the same plot. The voltage V out1 , derived from the potentiometer  100  portion of inventive circuit  500 , was modeled as a piecewise linear voltage source that corresponded to experimental values measured with proportionality subcircuit  1  (which includes potentiometer  100 ) in the lab at several points of standoff distance d. A shifted version of voltage V out1  is shown in the linear curve indicated as curve “V out1 .” The experimental values of the crystal detector  51  voltage V out0  at each standoff distance d were also modeled Using another piecewise linear voltage, as shown in the linear curve indicated as curve “V out0 .” 
     It is noted that, as shown in FIG. 5, these two voltages have slopes with opposite signs; that is, voltage V out1  is negatively sloped, whereas V out0  is positively sloped. Therefore, by multiplying the crystal detector  51  voltage V out0  by an appropriate factor and adding it to the potentiometer-related voltage V out1 , the effect of standoff distance d can be eliminated. Appropriately changing or adjusting R vary  in inventive circuit  500  sets this multiplication factor, which is the mathematical expression [1+(R vary +R 7 )/R 8 ]. By doing this, the output voltage V out0  (represented by curve “V out0 ”) can be transformed into another output voltage (viz., output voltage V out2 ) in order that the final output voltage (viz., output voltage V out ) be made independent of standoff distance d. 
     With reference to FIG. 6, output voltage V out0  has become output voltage V out2 , which defines a positively sloped line having angle θ 2  with respect to the x-axis. In FIG. 6, which is generally conceptually illustrative of neutralization of standoff distance d in accordance with the present invention, output voltage V out2  can be considered to be based on an original output voltage V out0  such as output voltage V out0  shown in FIG.  5 . Output voltage V out0  defines a positively sloped line having an angle θ 0  with respect to the x-axis, wherein angle θ 0  is smaller than angle θ 2 . Output voltage V out1  defines a negatively sloped line having angle θ 1  with respect to the x-axis. Angle θ 1  equals angle θ 2 . 
     Therefore, when output voltage V out2  is added to Output voltage V out1 , the net result is a zero-sloped (horizontal) line corresponding to offset-exclusive final output voltage V out  and having an output voltage value k. The absolute value of offset-exclusive final output voltage V out  equates as follows: 
     
       
           |V   out   |=V   out1   +V   out0 [1+( R   vary   +R   7 )÷ R   8 ]= V   out1   +V   out2   =k.   
       
     
     If an offset output voltage V out3  is entered into the equation whereby V out3  has an output voltage value −k and whereby output voltage V out1  and output voltage V out2  and output voltage V out3  are added together, the net result is a zero-sloped (horizontal) line corresponding to offset-inclusive final output voltage V out . The absolute value of offset-inclusive final output voltage V out  equates as follows: 
     
       
           |V   out   |=V   out1   +V   out0 [1+( R   vary   +R   7 )÷ R   8 ]+ V   out3   =V   out1   +V   out2   +V   out3   =k +(− k )=0 
       
     
     As shown in FIG. 6, offset output voltage V out3 , offset-inclusive final output voltage V out  and offset-exclusive final output voltage V out  are horizontal (parallel to each other and to the x-axis), but are characterized by different constant voltages (y-axis values). Accordingly, regardless of whether or not offset output voltage V out3  has been introduced, the original output voltage V out0  has been rendered independent of standoff distance d. The output voltage V out  will be insensitive to standoff distance d, but will be sensitive to changes (e.g., defects) in material  60 . However, it will generally facilitate inventive practice to calibrate inventive circuit  500  via an output voltage V out3 , so that the readings are referenced to a particular voltage (e.g., zero voltage) and thereby rendered more meaningful to the practitioner. 
     Offset-inclusive final output voltage V out  is seen to lie directly and equidistantly between offset-exclusive final output voltage V out  and offset voltage V out3 . Voltage value k represents the difference between offset-exclusive final output voltage V out  (which equates to k voltage) and offset-inclusive final output voltage V out  (which equates to zero voltage). Voltage value k also represents the difference between offset-inclusive final output voltage V out  (which equates to zero voltage) and output voltage V out3  (which equates to −k voltage). Voltage value 2k represents the difference beween offset-exclusive final output voltage V out3  (which equates to k voltage) and output voltage V out3  (which equates to −k voltage). 
     In the light of this disclosure, it is readily understood by the ordinarily skilled artisan that the present invention may be practiced in association with any and all types of near-field sensing devices. Although an inventive embodiment is described herein in relation to a near-field sensor employing microwave radiation, it is emphasized that the present invention is applicable or adaptable to near-field sensing or near-field sensors which employ practically any kind of electromagnetic radiation (waves), including but not limited to microwave radiation (waves). 
     Other embodiments of this invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. Various omissions, modifications and changes to the principles described may be made by one skilled in the art without departing from the true scope and spirit of the invention which is indicated by the following claims.