Abstract:
A method and apparatus for measuring changes induced in an inductive field on a wire-loop of an oscillator circuit based upon variations in a current function without the need to measure frequency changes. Induced noise has independent effects upon the current function and voltage function of an inductance measurement circuit. By inductively coupling one input and directly coupling the second input of a comparator circuit to the inductance measurement circuit, the phase of the current function can be adjusted to coincide with the phase of the voltage function. By combining the voltage function with the current function, an output isolating the induced noise from the measured inductance is obtained.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a division of application Ser. No. 09/532,590, filed Mar. 22, 2000, now U.S. Pat. No. 6,380,868, which claims the benefit of U.S. Provisional Application No. 60/125,660, filed Mar. 22, 1999. 

   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates to an apparatus and method for the measurement of inductance. More precisely the present invention relates to an apparatus and method for the measurement of inductance of vehicles moving in a traffic lane using permeability-modulated carrier referencing. 
   2. Description of the Related Art 
   It is well known in the prior art to measure the inductance of a wire-loop, which is part of the frequency determining circuit of an inductance-capacitance-resistance (“LCR”) oscillator, using frequency-counting techniques. Typically, the number of zero-crossings per time increment of the voltage across the terminals of the LCR capacitor is counted. Because the frequency of the LCR oscillator is inversely proportional to the square root of the inductance of the LCR circuit, changes in the inductance of the wire-loop are reflected in changes of the number of zero-crossings counted per time increment. 
   The Class-C wire-loop oscillator described in U.S. Pat. No. 3,873,964 issued to Thomas R. Potter on Mar. 25, 1975 is typical of LCR oscillators used in the prior-art. When a vehicle passes over a wire-loop connected to a running LCR oscillator, the metal of the vehicle changes the permeability of some of the space surrounding the wire-loop causing modulation of the carrier wave generated by the LCR oscillator. Changes in the inductance of the wire-loop caused by the vehicle are thus superimposed onto the LCR oscillator&#39;s carrier wave, yielding a permeability-modulated carrier. Next, the inductive signature is retrieved from the permeability-modulated carrier. One method of demodulating the carrier is the use of frequency counting techniques, such as with “signature cards” which are commercially available from 3M Corporation and Peek Traffic. The signature cards offer approximately a 100 Hz-sample rate, which is not fully adequate for demodulating the inductive signatures of vehicles moving at highway speeds. 
   Another problem associated with the measurement of inductance in a wire-loop is crosstalk. Crosstalk between two or more wire-loops is a result of inductive coupling between the wire-loops, which results in energy transfer between the wire-loops when a changing current is flowing through them. If two wire-loops are operating at nearly the same frequency, then the energy transfer can result in an exaggerated buildup, or stagnation, of transferred energy in one LCR circuit, and a corresponding exaggerated energy depression in the other. This can cause the carrier waves of the two circuits to become entrained with each other in a more-or-less fixed phase differential and effectively eliminates the ability of the wire-loops to detect vehicles independently of each other. Typically, an inductive coupling coefficient of only a few percent is sufficient to cause complete entrainment. In prior-art vehicle detectors, carrier wave entrainment due to crosstalk is partially avoided by operating the oscillator circuits associated with the wire-loops at different frequencies, typically by varying the value of the capacitance, C, of the LCR circuit. This can prevent stagnation and entrainment, but does not address the underlying errors induced into each detector by the energy transfer due to mutual inductive coupling. 
   Accordingly, there is a need for an apparatus and method for measuring the changes in the inductance of a wire-loop caused by a vehicle traveling along a monitored roadway. The apparatus and method need be capable of measuring changes in the inductance of a wire-loop caused by a vehicle traveling at highway speeds. Further, the apparatus and method should be capable of measuring inductance without attempting to identify frequency changes. Finally, there is a need for an apparatus and method capable of measuring inductance using multiple inductive sensors without significant errors resulting from crosstalk. 
   Therefore, it is an object of the present invention to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle traveling along a monitored roadway. 
   It is another object of the present invention to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle traveling at highway speeds. 
   It is a further object of the present invention to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle and producing an inductive signature for that vehicle. 
   It is yet another object of the present invention to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle and producing an inductive signature of that vehicle by referencing a measured voltage to a permeability-modulated current carrier wave. 
   A still further object of the present invention is to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle using multiple inductive sensors without significant errors resulting from crosstalk. 
   Another object is to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle which does not need to be installed in the driving surface of a roadway. 
   BRIEF SUMMARY OF THE INVENTION 
   In a typical LCR circuit, a number of factors are related to the value of the inductance. For example, the frequency is inversely proportional to the square root of the inductance, L. This relationship is a consequence of the direct dependence of the instantaneous rate of change in current flow, δI, upon the value of the inductance. Accordingly, frequency is only an indirect indication of this more general relationship because the circuit voltage, V, is in turn a function of current, I, and capacitance, C. A more direct indication of inductance in an LCR oscillator is the amplitude of the current function, I(t), which is inversely proportional to the inductance of the LCR circuit. The changing current function, I(t), in the LCR circuit of an inductive vehicle detector is a permeability-modulated carrier. This carrier is modulated in both frequency and amplitude by the changing apparent permeability of the space surrounding a wire-loop caused by the motion of a nearby metallic object, typically an automotive vehicle. It should be noted that induced electromagnetic noise, such as from high voltage power lines, also effectively modulates the current function carrier wave. However, the induced noise modulates the voltage function, V(t), in an asymmetric manner by shifting the voltage function on the magnitude axis. Because the modulation resulting from the induced noise affects the current flow and the voltage function differently, the permeability-modulated current carrier function, I(t), can be cross-referenced with the voltage function, V(t), to isolate the desired inductance from the induced noise. This method of isolating the inductance is known as permeability-modulated carrier referencing (PMCR). PMCR is particularly effective at removing low-frequency induced noise from an inductance measuring circuit. Those skilled in the art will recognize that although PMCR is described herein with reference to an LCR oscillator, the principles are equally applicable to other forms of carrier functions including, but not limited to, pulsed-type discrete cycle inductance measurement techniques. 
   Another factor affecting the performance of the present invention is crosstalk wherein the direction of current flow in an inductor determines the direction of the induced differential current flow in inductors that are inductively coupled to it. One method of reducing crosstalk is to nullify the underlying mutual inductive coupling of a plurality of wire-loops using passive transformers. The passive transformer inductively couples the inductors in precisely the opposite polarity and magnitude in which they were originally coupled nullifying the original coupling and eliminating the potential for crosstalk at the source. in addition to removing the gross errors introduced by crosstalk, nullification of the inductive coupling also removes the more subtle transient errors in the detectors, which appear as non-repeatable errors in recorded inductive signatures. 
   A single-turn wire-loop spanning the width of one or more traffic lanes is sufficient to detect the speed, the direction, the lane position, and the wheelbase dimensions for any vehicle passing over the wire-loop. The speed and the lateral lane position of a vehicle are unambiguously determined if the two active legs of the wire-loop span the traffic lanes at different skew angles. Symmetric skew angles also produce useful data, but are ambiguous in resolving the vehicle direction. Similar skew angles are unable to resolve the lane position; however, this is not as important for single traffic lanes as it is for multiple traffic lanes. Finally, zero skew angles can produce speed and axle-count data, but are ambiguous in resolving vehicle direction, can not resolve the lane position or the width of the wheelbase, and are ambiguous with respect to vehicle continuity when multiple traffic lanes are involved. 
   For multi-lane traffic, a pair of single-turn wire-loops in a complimentary wedge-shaped configuration is ideal for collecting the maximum unambiguos traffic-flow data. This configuration is a hybrid of rectangular wire-loops and blades which gives repeatability of signatures that is characteristic of the blades along with the less-intrusive installation that is characteristic of simple wire-loops. shallow saw-cuts are desirable for a traffic sensor spanning long distances in a pavement surface to prevent the formation of a shear-plane and slot faulting. 
   A large-aperture wire-loop can detect metallic objects at great distances. The magnetic field generated by a wire-loop is highly directional at a significant distance from the wire-loop. More precisely, a wire-loop is most sensitive to distant objects in the same plane as the wire-loop and sensitivity decreases as the object moves away from the plane of the wire-loop. At a significant distance, objects approaching the plane perpendicular to that of the wire-loop are virtually invisible to the wire-loop. This directional sensitivity of the wire-loop is useful in determining the relative direction to detected objects in a similar way as a radar antenna is directional. Large-aperture wire-loops are used in non-intrusive vehicle-detecting applications because they do not need to be embedded in or laid on the pavement to detect passing vehicles. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The above-mentioned features of the invention will become more clearly understood from the following detailed description of the invention read together with the drawings in which: 
       FIG. 1  is a schematic diagram of an inductance measuring circuit of the preferred embodiment of the present invention; 
       FIG. 2  is a plot of the raw waveform data produced by a circuit simulation of the inductance measuring circuit of  FIG. 1 ; 
       FIG. 3  is a plot of the mixed waveform data produced by a circuit simulation of the inductance measuring circuit of  FIG. 1 ; 
       FIG. 4  is an inductive signature recorded for a vehicle using one embodiment of the present invention; 
       FIG. 5  is a schematic diagram of a passive transformer wired for crosstalk nullification between two wire-loops; 
       FIG. 6  depicts a single wire-loop deployed in the preferred wedge-shape configuration across three lanes of traffic for maximum data resolution; 
       FIG. 7  depicts a pair of complimentary wedge-shaped configuration wire-loops deployed across three lanes of traffic with opposing current flow to nullify the magnetic field of the two common legs; 
       FIG. 8  depicts a pair of complimentary wedge-shaped configuration wire-loops deployed across three lanes of traffic with complimentary current flow to accentuate the magnetic field of the two common legs; 
       FIG. 9  illustrates a cross-section of the wire-loop pair illustrated in  FIGS. 7 and 8 ; 
       FIG. 10  illustrates a cross-section of an alternate embodiment of the wire-loop vehicle detector having two pair of substantially parallel and concentric wire-loops which are vertically separated from one another configured as illustrated in  FIGS. 7 and 8 ; 
       FIG. 11  depicts a non-intrusive wire-loop vehicle detector deployed in a horizontal configuration; 
       FIG. 12  depicts one embodiment of a non-intrusive wire-loop vehicle detector deployed in a vertical configuration; 
       FIG. 13  depicts an alternate embodiment of a non-intrusive wire-loop vehicle detector having a pair of horizontally oriented wire-loops; 
       FIG. 14  illustrates a pass-through wire-loop configuration; and 
       FIG. 15  illustrates a block diagram of one embodiment of a control box of the wire-loop vehicle detector of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An apparatus and method for measuring the inductance of a wire-loop without direct reference to any particular time-constant or frequency is illustrated generally at  10  in the figures. The apparatus  10  utilizes permeability-modulated carrier referencing to identify the inductive signature of a vehicle passing over a wire-loop. 
   In a typical LCR circuit, a number of factors are related to the value of the inductance. For example, the frequency is inversely proportional to the square root of the inductance, L. This relationship is a consequence of the direct dependence of the instantaneous rate of change in current flow, δI, upon the value of the inductance as defined in the following equation: 
               δ   ⁢           ⁢   I     =     V   L             (   1   )             
 
   Accordingly, frequency is only an indirect indication of this more general relationship because the circuit voltage, V, is in turn a function of current, I, and capacitance, C, as defined in the following equation: 
               δ   ⁢           ⁢   V     =     I   C             (   1   )             
 
   A more direct indication of inductance in an LCR oscillator is the amplitude of the current function, I(t), which is inversely proportional to the inductance of the LCR circuit. The changing current function, I(t), in the LCR circuit of an inductive vehicle detector is a permeability-modulated carrier. This carrier is modulated in both frequency and amplitude by the changing apparent permeability of the space surrounding a wire-loop caused by the motion of a nearby metallic object, typically an automotive vehicle. It should be noted that induced electromagnetic noise, such as from high voltage power lines, also effectively modulates the current function carrier wave. However, the induced noise modulates the voltage function, V(t), in an asymmetric manner by shifting the voltage function on the magnitude axis. Because the modulation resulting from the induced noise affects the current flow and the voltage function differently, the permeability-modulated current carrier function, I(t), can be cross-referenced with the voltage function, V(t), to isolate the desired inductance signature from the induced noise. This method of isolating the inductance signature is known as permeability-modulated carrier referencing (PMCR). PMCR is particularly effective at removing low-frequency induced noise from an inductance measuring circuit. Those skilled in the art will recognize that although PMCR is described herein with reference to an LCR oscillator, the principles are equally applicable to other forms of carrier functions including, but not limited to, pulsed-type discrete cycle inductance measurement techniques. 
     FIG. 1  illustrates a circuit diagram of one embodiment of the present invention for PMCR. The illustrated embodiment includes a class-C oscillator  11  connected to signal conditioning electronics. Using the signal conditioning electronics, a high sample rate relative to prior-art vehicle detectors is achieved by measuring the amplitude modulation of the voltage, V(t), and changing current, I(t), rather than measuring frequency changes. Those skilled in the art will recognize that other oscillators could be substituted for the class-C oscillator  11  shown without interfering with the objects and advantages of the present invention. 
     FIGS. 2 and 3  represent the output obtained from the circuit of FIG.  1 . Cancellation of the noise induced into the wire-loop is achieved by mixing two signals generated by the circuit of FIG.  1 : V 18  and V 19 . Specifically,  FIG. 2  illustrates the raw voltage of V 18  in a first plot  12  and V 19  in a second plot  14 . V 13  represents the voltage function output from the oscillator  11  while V 14  represents the current function. At the output of the oscillator  11 , the current function and the voltage function are 90° out of phase. However, by inductively coupling the inductor L 3  with the inductor L 1 , the phase of the current function V 14  is altered such that it coincides with that of the voltage function V 13 .  FIG. 3  illustrates a first plot  16  of V 19 subtracted from V 18  overlaid with a second plot  18  of V 19  added to V 18 . By subtracting V 19  from V 18  the noise is canceled and the inductance signal is increased showing the changing inductance. Conversely, by adding these two functions as in the second plot  18 , the inductance signal is canceled and only the noise remains which may be useful for analysis or for referencing in additional noise-cancellation steps downstream. Those skilled in the art will recognize that although PMCR is shown here using an oscillator, the principles taught herein can be applied to an alternate embodiment of the present invention such as where the wire-loop is driven by an impulse carrier signal rather than a free-running oscillator. Finally, by referencing the permeability-modulated current carrier to the voltage and removing the induced noise, an inductive signature, such as that illustrated in  FIG. 4 , representative of a vehicle is produced. 
   Another factor affecting the performance of the present invention is crosstalk. One method of reducing crosstalk is to nullify the underlying mutual inductive coupling of a plurality of wire-loops using passive transformers  22 . Because the direction of current flow in an inductor determines the direction of the induced differential current flow in inductors which are inductively coupled to it, a passive transformer  22  inductively linking the wire-loops  20   a ,  20   b  which are connected to inductive signature detection circuits  24   a ,  24   b  is used, as illustrated in FIG.  5 . The passive transformer  22  inductively couples the inductors in precisely the opposite polarity and magnitude in which they were originally coupled. This has the effect of nullifying the original coupling and eliminating the potential for crosstalk at the source. In addition to removing the gross errors introduced by crosstalk, nullification of the inductive coupling also removes the more subtle transient errors in the detectors which appear as non-repeatable errors in recorded inductive signatures. 
   A single-turn wire-loop spanning the width of one or more traffic lanes  30  of a roadway  28  is sufficient to detect the speed, the direction, the lane position, and the wheelbase dimensions for any vehicle  32  passing over the wire-loop. The speed and the lateral lane position of a vehicle  32  are unambiguously determined if the two active legs  26   a ,  26   b  of the wire-loops  20  span the traffic lanes  30  at different skew angles  27   a ,  27   b  as illustrated in FIG  6 . In the preferred embodiment, the configuration of each wire-loop  20  is deployed in a wedge-shaped configuration defining an acute triangle. Other embodiments of the present invention which include symmetrically skewed legs, and parallel skewed legs of the single wire-loop are contemplated but are not preferred. Symmetric skew angles also produce useful data, but are ambiguous in resolving the vehicle direction. Similar skew angles are unable to resolve the lane position; however, this is not as important for single traffic lanes as it is for multiple traffic lanes. Finally, zero skew angles can produce speed and axle-count data, but are ambiguous in resolving vehicle direction, can not resolve the lane position or the width of the wheelbase, and are ambiguous with respect to vehicle continuity when multiple traffic lanes are involved. 
   For multi-lane traffic, a pair of single-turn wire-loops in the preferred embodiment of  FIGS. 7 and 8  is ideal for collecting the maximum unambiguous traffic-flow data. Specifically, the wire-loop wedges  20  are deployed in a complimentary configuration such that the outer legs  34   a ,  34   b  are approximately parallel and the inner legs  36   a ,  36   b  are coincidental. In the wedge-shaped configuration, two legs  34 ,  36  of each wedge traverse the width of the roadway  28 , preferably at two different skew angles  27   a ,  27   b . The depth to which the wire is embedded in the pavement is the minimum depth consistent with reliable performance over time. Because thermal expansion of the pavement applies tensile stress to long wires which are embedded in it, it is preferred that the depth of the saw-cuts oscillate slightly in order to allow the pavement to expand without breaking the wires embedded therein. This configuration is a hybrid of rectangular wire-loops and blades which gives repeatability of signatures that is characteristic of the blades along with the less-intrusive installation that is characteristic of simple wire-loops. Shallow saw-cuts are desirable for a traffic sensor spanning long distances in a pavement surface to prevent the formation of a shear-plane and slot faulting if the two wire-loops  20  are energized so that current flows as illustrated in  FIG. 7 , then the magnetic fields generated by the common center legs  36  of the wire-loops  20  cancel and the signatures recorded are the difference between the perturbations of the magnetic fields from the outside legs  34 . Conversely, if the two wire-loops  20  are energized so that current flows as illustrated in  FIG. 8 , then the magnetic fields generated by the common center  36  legs of the wire-loops  20  combine making the common center legs  36  the dominant detection zone for this configuration. 
     FIG. 9  clearly illustrates the relative position of the legs  34 ,  36  of the wire-loops  20  as shown in  FIGS. 7 and 8 . Specifically,  FIG. 9  illustrates a cross-section of the roadway  28  showing the slots  60  cut into the pavement for receiving the wire-loops  20 . A filler material  62  is used to seal the slots and hold the wire-loops  20  in position. 
     FIG. 10  illustrates an alternate embodiment of a wire-loop configuration using two pair of wire-loops  20 ,  20 ′ represented by legs  34 ,  36  and  34 ′,  36 ′. The first pair  20  and second pair  20 ′ of wire-loops are concentric and disposed in parallel, vertically separated planes. To maintain adequate vertical separation, a spacing member  64  is disposed between the first pair  20  and second pair  20 ′ of wire-loops. Those skilled in the art will recognize that the spacing member  64  can be a separate member which is insertable between the wire-loop pairs or integrally formed with the wire-loop pairs to provide the desired separation without interfering with the objects and advantages of the present invention. 
   Separately energizing the wire-loops  20  produces still different results with the inductance depending upon the vehicle&#39;s lane of travel. For example, in  FIG. 7 , if the first wire-loop  20   a  is energized, but the second wire-loop  20   b  is not, then the measured inductance strength is dependent upon the vehicle&#39;s lane of travel, increasing from the first lane  30   a  to the third lane  30   c . Conversely, if the second wire-loop  20   b  is energized and the first wire-loop  20   a  is not, then the measured inductance strength decreases from the first lane  30   a  to the third lane  30   c . By selectively energizing two or more wire-loops  20  in these various sensing configurations, a number of useful data points are produced. Multiple vehicle signatures recorded when more than one vehicle is being simultaneously sensed by the wire-loop are separated using deduction and linear algebra to solve the multiple simultaneous equations generated. 
   In the preferred embodiment of the present invention, each wheel of a vehicle  32  is detected as it rolls over each of the two road-traversing legs of the wedge-shaped wire-loop  20 . For example, a four-wheel passenger vehicle  32  will produce eight distinct wheel spikes as it rolls over the wire-loop. Knowledge of the timing of the wheels spike events combined with the known geometry of the wire-loop  20  and the assumed rectangular geometry of the vehicle&#39;s wheelbase allows for the unambiguous derivation of the traffic parameters sought. The magnitude of the wheel spikes or other parameters of the vehicle are used for vehicle classification or for re-identification downstream and may be desirable in alternate embodiments. 
   In the preferred embodiment of the present invention, two time-stamps are detected for each wheel that rolls over the sensor; one for each leg of the wire-loop which spans the traffic lane. A four-wheel passenger vehicle for example, generates eight time-stamps as it passes over the wire-loop. In one embodiment, each time-stamp is expressed as a 16-bit integer resulting in a 16-byte description of the vehicle passing over the wire-loop. More information which would be useful for downstream re-identification of the vehicle could be collected if desired, such as the wheel-spike amplitude, the body profile, etc. For large traffic flows, it is highly desirable for a traffic sensor to produce compact records on a per-vehicle basis to minimize the data storage requirements. 
   Referring now to  FIGS. 11 through 13 , a large-aperture wire-loop can detect metallic objects at great distances. The magnetic field generated by a wire-loop is highly directional at a significant distance from the wire-loop. More precisely, a wire-loop is most sensitive to distant objects in the same plane as the wire-loop and sensitivity decreases as the object moves away from the plane of the wire-loop. At a significant distance, objects approaching the plane perpendicular to that of the wire-loop are virtually invisible to the wire-loop. This directional sensitivity of the wire-loop is useful in determining the relative direction to detected objects in a similar way as a radar antenna is directional. Accordingly, large-aperture wire-loops are useful in non-intrusive vehicle-detecting applications because they do not need to be embedded in or laid on the pavement to detect passing vehicles. 
   The large-aperture wire-loops may be deployed in a number of configurations.  FIG. 11  illustrates a single large-aperture wire-loop  40  deployed in a horizontal orientation on each side of the roadway. Using a single loop  40 , the presence of a vehicle is detected when proximate the wire-loop but A additional information such as the lane of travel is unavailable. However, by employing two loops  40   a ,  40   b , the relative inductance measured at each loop  40  is used to identify the lane of travel.  FIG. 12  illustrates a single large-aperture wire-loop  42  deployed in a vertical orientation adjacent a roadway. The wire-loop is shaped around a loop-forming member  44 .  FIG. 13  illustrates a pair of large-aperture wire-loops  48   a ,  46   b  deployed on one side of a roadway. The centers of the wire-loops  46   a ,  46   b  are offset such that one wire-loop  46   a  is closer to the roadway  28  than the other  46   b . In the illustrated embodiment, the wire-loops are offset in the direction of travel along the roadway to clearly show that two separate wire-loops exist; however, such offset is not necessary for operation. The illustrated configuration produces a differential between the relative inductance measured by the inductive sensor  10  allowing additional information such as the lane of travel to be identified. Those skilled in the art will recognize that principles of the teachings of the present invention can be applied to the non-intrusive large aperture wire-loop configurations without interfering with the objects of the present invention. 
     FIG. 14  illustrates a pass-through wire-loop configuration  70 . The pass-through configuration  70  includes a central wire-loop  72  deployed such that the vehicle passes through the center of the open wire-loop. At least one additional outer wire-loop  74  is deployed in conjunction with the central wire-loop  72 . Each outer wire-loop  74  has a first dimension which is substantially equal to the corresponding dimension of the central wire-loop  72  and a second dimension which is larger than the corresponding dimension of the central wire-loop  72 . Each outer wire-loop  74  is disposed so that it coincides with the central wire-loop  72  on three sides. The fourth side of the outer wire-loop  74  and the central wire-loop  72  are offset by the difference in the dimensions. In the illustrated embodiment, four outer wire-loops  74   a - 74   d  are used. By selectively energizing one of the outer wire-loops  74   a - 74   d  and the central wire-loop  72 , the changing inductance is measured from any one of the four orientations allowing additional information to be obtained about the vehicle. Such a pass through detector  70  is also useful for walk-through metal detectors to identify not only the presence of a metal object, but also the relative location of the object. Although not shown in  FIG. 14 , those skilled in the art will recognize various methods and apparatuses for maintaining the shape and position of the wire-loops used in the pass-through configuration. Further, those skilled in the art will recognize that any of the teachings of the present invention can be applied to the pass through configuration without interfering with the objects of present invention. 
     FIG. 15  illustrates a block diagram of the control box  24  configured for use with a pair of wire-loops  20 . In the illustrated embodiment, the control box  24  contains a power supply  48  for each of the pair of wire-loops  20 . Each power supply  48  is responsive to a controller  54 . The controller  54  directs the operation of the power supplies  48  to selectively energize the corresponding wire-loop  20 . Each power supply  48  is capable of reversing the polarity of the outputs  50 ,  52  thereby allowing the controller  54  to dictate the direction of current flow through the wire-loops. Further, the control box  24  includes a switch  56  that serially connects the wire-loops  20  to present a single current path. This allows the wire-loops  20  to be joined thereby equalizing the apparent inductance and preventing any divergence in phase angle. In the illustrated embodiment, the switch  56  is responsive to the controller  54 ; however, those skilled in the art will recognize that the linking of the wire-loops can be accomplished in a number of ways without interfering with the objects and advantages of the present invention. 
   Accordingly, an apparatus and method for measuring the inductance of a wire-loop without direct reference to any particular time-constant or frequency has been disclosed. By comparing the permeability-modulated current carrier function, I(t), with the voltage function, V(t), changes in the inductance of a wire-loop caused by a vehicle passing over a wire-loop are isolated. Crosstalk is nullified using passive transformers. For inductance detection, a pair of single-turn wire-loops are deployed in a complimentary wedge-shaped configuration for collecting the maximum unambiguous traffic-flow data. By selectively energizing each wire-loop, a variety of traffic-flow data can be measured. Finally, the apparatus and method disclosed herein permits the use of large-aperture wire-loops in non-intrusive vehicle-detecting applications. 
   While a preferred embodiment has been shown and described, it will be understood that it is not intended to limit the disclosure, but rather it is intended to cover all modifications and alternate methods falling within the spirit and the scope of the invention as defined in the appended claims.