Permeability-modulated carrier referencing

• US 6,380,868 B1
• Filed: 03/22/2000
• Issued: 04/30/2002
• Est. Priority Date: 03/22/1999
• Status: Expired due to Fees

• Current Assignee: Inductive Signature Technologies Incorporated
• Original Assignee: Inductive Signature Technologies Incorporated
• Inventors: Hilliard, Steven R., Hilliard, Geoffrey W.
• Primary Examiner(s): POPE, DARYL C
• Application Number:
  US09/532,590
• Time in Patent Office: 769 Days
• Field of Search: 340/933, 340/941, 340/934, 377/9
• US Class Current: 340/941
• CPC Class Codes: E01F 11/00 Road engineering aspects of Embedding pads or other sensitive devices in paving or other road surfaces , e.g. traffic detectors, vehicle-operated pressure-sensitive actuators, devices for monitoring atmospheric or road conditionspressure-sensitive elements G01L; traffic control systems G08G
• CPC Class Codes: G01P 3/50 for measuring linear speed G01P3/56 takes precedence
• CPC Class Codes: G01P 3/66 using electric or magnetic means G01P3/80 takes precedence; measuring short time intervals G04F8/00, G04F10/00
• CPC Class Codes: G01V 3/101 by measuring the impedance of the search coil; by measuring features of a resonant circuit comprising the search coil measuring impedance or characteristics derived therefrom G01R27/00, e.g. quality factor G01R27/26
• CPC Class Codes: G08G 1/042 using inductive or magnetic detectors

Abstract

An apparatus and method for measuring the inductance of a wire-loop without direct reference to any particular time-constant or frequency. Low-frequency noise induced into the wire-loop is canceled within the detector circuit and crosstalk between a plurality of adjacent wire-loops is nullified using passive transformers. New wire-loop configurations utilize the permeability-modulated carrier referencing to provide repeatable inductive signatures with less intrusive, and non-intrusive installations.

Claims

1. An inductive sensor for use in a roadway having a surface and a width, the roadway defining at least one traffic lane, said inductive sensor comprising:

at least one wire-loop defining a first leg, a second leg, and a third leg, each of said first leg and said second leg having a length so as to be substantially deployed across the roadway width, said first leg deployed across the roadway at a first angle relative to an edge of the roadway, said second leg deployed across the roadway at a second angle relative to an edge of the roadway, and said third leg connecting said first leg and said second leg; and

at least one power source in communication with said at least one wire-loop.

2. The inductive sensor of claim 1 wherein said at least one wire-loop is oriented substantially parallel to the roadway surface.

3. The inductive sensor of claim 1 wherein said first angle and said second angle are not equal.

4. The inductive sensor of claim 1 further comprising a controller in communication with said at least one power source for selectively energizing each of said at least one wire-loop.

5. The inductive sensor of claim 1 wherein said at least one power source produces an output having a polarity, said power source being capable of reversing the polarity of the output.

6. The inductive sensor of claim 1 having a pair of wire-loops.

7. The inductive sensor of claim 6 wherein said first legs and said third legs form opposing sides of a quadrilateral.

8. The inductive sensor of claim 6 wherein said first legs and said third legs form opposing sides of a parallelogram.

9. The inductive sensor of claim 6 further comprising a second pair of wire-loops disposed substantially parallel to a first said pair of wire-loops, said first pair of wire-loops and said second pair of wire-loops being substantially concentric.

10. The inductive sensor of claim 1 wherein each of said at least one wire-loop defines an independent current path.

11. The inductive sensor of claim 1 further comprising at least one switch selectively connecting one of said at least one wire-loop to another said at least one wire-loop in series to define a single current path.

12. The inductive sensor of claim 1 wherein said at least one wire-loop is disposed substantially parallel to the roadway surface.

13. The inductive sensor of claim 1 wherein said first angle is not equal to said second angle.

14. An inductive sensor for use in a roadway having a surface and a width, the roadway defining at least one traffic lane, said inductive sensor comprising:

a first wire-loop deployed within the roadway;

a second wire-loop deployed within the roadway, wherein said second wire-loop is located proximate to said first wire-loop such that a mutual inductive coupling exists between said first wire-loop and said second wire-loop; and

a passive transformer having a first coil in communication with said first wire-loop and a second coil in communication with said second wire-loop, said passive transformer inductively coupling said first wire-loop with said second wire-loop, said passive transformer selected to substantially nullify the mutual inductive coupling.

15. An apparatus for detecting a vehicle travelling on a roadway having a width and defining at least one traffic lane and a path of vehicular travel, said apparatus comprising:

a first wire-loop substantially spanning the width of the roadway and defining a current path;

a second wire-loop substantially spanning the width of the roadway and defining a current path, said second wire-loop current path being independent of said first wire-loop current path;

a first oscillator circuit in communication with said first wire-loop;

a second oscillator circuit in communication with said second wire-loop; and

a controller in communication with at least one of said first oscillator circuit and said second oscillator circuit, said controller for selectively energizing at least one of said first oscillator circuit and said second oscillator circuit.

16. The apparatus of claim 15 wherein said controller is in communication with each of said first oscillator and said second oscillator.

17. The apparatus of claim 15 further comprising a vehicle detection circuit in communication with said controller.

18. The apparatus of claim 17 wherein a plurality of vehicles travelling on said roadway are detected simultaneously by said vehicle detection circuit which produces a composite inductive signature, said composite inductive signature being separated into individual vehicle inductive signatures through subsequent processing.

19. The apparatus of claim 15 wherein said controller is in communication with said first oscillator circuit and a further controller is in communication with said second oscillator circuit.

20. The apparatus of claim 15 wherein a current flows through each of said first wire-loop and said second wire-loop in a selected direction when each of said first wire-loop and said second wire-loop is energized, said controller being capable of switching said direction of current flow through at least one of said first wire-loop and said second wire-loop.

21. The apparatus of claim 15 further comprising a switch in communication between said first wire-loop and said second wire-loop, said switch connecting said first wire-loop and said second wire-loop in a single current path when closed and separating said first wire-loop current path from said second wire-loop current path when open.

22. The apparatus of claim 15 wherein said first wire-loop and said second wire-loop are substantially coplanar and each of said first wire-loop and said second wire-loop define at least one leg, one of said at least one leg of said first wire-loop abutting said one of said at least one leg of second wire-loop to define a common leg.

23. The apparatus of claim 22 wherein a first current flows through said first wire-loop in a first direction thereby producing a first magnetic field and a second current flows through said second wire-loop in a second direction thereby producing a second magnetic field, said second direction being opposite said first direction such that said first magnetic field destructively interferes with said second magnetic field along said common leg.

24. The apparatus of claim 23 wherein said first current is substantially equal in magnitude to said second current.

25. The apparatus of claim 22 wherein a first current flows in said first wire-loop in a first direction thereby producing a first magnetic field and a second current flows in said second wire-loop in a second direction thereby producing a second magnetic field, said second direction being substantially identical to said first direction such that said first magnetic field constructively interferes with said second magnetic field along said common leg.

26. The apparatus of claim 25 wherein said first current is substantially equal in magnitude to said second current.

27. The apparatus of claim 15 further comprising:

a third oscillator circuit;

a third wire-loop that is substantially concentric with said first wire-loop, said third wire-loop disposed at a greater depth than said first wire-loop with respect to the roadway surface, said third wire-loop being in communication with said third oscillator circuit;

a fourth oscillator circuit in communication with said controller; and

a fourth wire-loop that is substantially concentric with said second wire-loop, said fourth wire-loop disposed at a greater depth than said second wire-loop with respect to the roadway surface, said fourth wire-loop being in communication with said fourth oscillator circuit;

wherein said controller is in communication with at least one of said first oscillator circuit, said second oscillator circuit, said third oscillator circuit and said fourth oscillator circuit, said controller for selectively energizing at least one of said first oscillator circuit, said second oscillator circuit, said third oscillator circuit and said fourth oscillator circuit.

28. The apparatus of claim 27 wherein said first wire-loop and said second wire-loop are substantially coplanar and said third wire-loop and said fourth wire-loop are substantially coplanar, said first wire-loop abutting said second wire-loop to define a first common leg and said third wire-loop abutting said fourth wire-loop to define a second common leg.

29. The apparatus of claim 28 wherein a third current flows in said third wire-loop in a third direction thereby producing a third magnetic field and a fourth current flows in said fourth wire-loop in a fourth direction thereby producing a fourth magnetic field, said third direction being opposite said fourth direction such that said third magnetic field destructively interferes with said fourth magnetic field along said common leg.

30. The apparatus of claim 29 wherein said third current is substantially equal in magnitude to said fourth current.

31. The apparatus of claim 28 wherein a third current flows in said third wire-loop in a third direction thereby producing a third magnetic field and a fourth current flows in said fourth wire-loop in a fourth direction thereby producing a fourth magnetic field, said third direction being substantially identical to said fourth direction such that said third magnetic field constructively interferes with said fourth magnetic field along said common leg.

32. The apparatus of claim 31 wherein said third current is substantially equal in magnitude to said fourth current.

33. A wire-loop sensor for sensing changes in inductance caused by vehicular traffic on a roadway, the roadway having a width and a surface and defining at least one traffic lane and a path of vehicular travel, said wire-loop sensor comprising:

a first leg;

a second leg substantially spanning the width of the roadway at a first angle relative to said first leg; and

a third leg substantially spanning the width of the roadway, said third leg being not parallel to said second leg.

34. The wire-loop sensor of claim 33 wherein said second leg and said third leg are disposed substantially parallel to the roadway surface.

35. The wire-loop sensor of claim 33 wherein said first leg electrically connects said second leg and said third leg.

36. The wire-loop sensor of claim 33 wherein said first leg is located outside of each at least one traffic lane.

37. The wire-loop sensor of claim 33 wherein said first leg is located proximate to an edge of at least one traffic lane.

38. An inductive sensor deployed in a roadway for use with a vehicle detector, the roadway having a width and defining at least one traffic lane, a path of vehicular travel and a reference line having a major component substantially perpendicular to the path of vehicular travel, said inductive sensor comprising:

a first leg substantially spanning the width of the roadway, said first leg deployed at a first angle relative to the reference line;

a second leg substantially spanning the width of the roadway, said second leg deployed at a second angle relative to the reference line; and

a third leg in communication with said second leg;

wherein said first leg is in communication with one of said second leg and said third leg.

39. The inductive sensor of claim 38 wherein said first angle is not equal to said second angle.

40. The inductive sensor of claim 38 wherein each of said first angle and said second angle is between the angle perpendicular to the reference line and the angle parallel to the reference line.

41. The inductive sensor of claim 38 wherein each of said first angle and said second angle is an acute angle.

42. The wire-loop sensor of claim 38 wherein said first leg and said second leg are disposed substantially parallel to the roadway surface.

43. An inductive sensor deployed in a roadway for use with a vehicle detector, the roadway having a width and defining a path of vehicular travel, said inductive sensor comprising:

a first wire-loop including at least a first leg, a second leg and a third leg in communication so as to define a current path, said first leg and said second leg each having one end located proximate to a first reference point located on a side of the roadway, said first wire-loop first leg deployed at a first angle relative to the path of vehicular travel, said first wire-loop second leg deployed at a second angle relative to the path of vehicular travel; and

a second wire-loop including at least a first leg, a second leg and third leg in communication so as to define a current path, said first leg and said second leg each having one end located proximate to a second reference point located on the opposite side of the roadway from said first reference point, said second wire-loop first leg deployed at a third angle relative to the path of vehicular travel, said second wire-loop second leg deployed at a fourth angle relative to the path of vehicular travel.

44. The inductive sensor of claim 43 wherein said first angle is not equal to said second angle and said third angle is not equal to said fourth angle.

45. The inductive sensor of claim 43 wherein said first angle is substantially equal to said third angle and said second angle is substantially equal to said fourth angle.

46. The inductive sensor of claim 43 wherein said first wire-loop first leg is substantially parallel to said second wire-loop first leg and said first wire-loop second leg is substantially parallel to said second wire-loop second leg.

47. The inductive sensor of claim 43 wherein said first wire-loop second leg is substantially parallel to and substantially abuts said second wire-loop second leg.

48. The inductive sensor of claim 43 wherein said first wire-loop current path is independent of said second wire-loop current path.

49. The inductive sensor of claim 43 wherein said first wire-loop current path and said second wire-loop current path form a single current path.

50. The inductive sensor of claim 43 further comprising a switch in communication between said first wire-loop and said second wire-loop, said switch electrically connecting said first wire-loop and said second wire-loop to form a single current path when closed and electrically isolating said first wire-loop and said second wire-loop to form separate current paths when open.

51. The inductive sensor of claim 43 further comprising a controller in communication each of said first wire-loop and said second wire-loop, said controller for selectively energizing each of said first wire-loop and said second wire-loop.

52. The inductive sensor of claim 43 further comprising a controller in communication each of said first wire-loop and said second wire-loop, said controller for controlling a phase relationship between a first current flowing in said first wire-loop and a second current flowing in said second wire-loop.

53. The inductive sensor of claim 43 wherein the roadway further defines a surface, said first wire-loop and said second wire-loop being disposed substantially parallel to the roadway surface.

54. The inductive sensor of claim 43 wherein said first wire-loop is substantially coplanar with said second wire-loop.

55. The inductive sensor of claim 43 wherein said first wire-loop is driven by a first current to produce a first magnetic field and said second wire-loop is driven by a second current to produce a second magnetic field.

56. The inductive sensor of claim 55 wherein one of said first wire-loop first leg and said first wire-loop second leg substantially abuts one of said second wire-loop first leg and said second wire-loop second leg to define a common leg.

57. The inductive sensor of claim 56 wherein said first current and said second current travels through said common leg in opposite directions such that said first magnetic field destructively interferes with said second magnetic field along said common leg.

58. The inductive sensor of claim 56 wherein said first current and said second current travels through said common leg in identical directions such that said first magnetic field constructively interferes with said second magnetic field along said common leg.

59. The inductive sensor of claim 43 further comprising:

a third wire-loop including at least a first leg, a second leg and a third leg in communication so as to define a current path, said first leg and said second leg each having one end located proximate to a third reference point located on a side of the roadway, said third wire-loop first leg deployed at a fifth angle relative the path of vehicular travel, said third wire-loop second leg deployed at a sixth angle relative to the path of vehicular travel, said third wire-loop being disposed below and substantially concentric with said first wire-loop; and

a fourth wire-loop including at least a first leg, a second leg and a third leg in communication so as to define a current path, said first leg and said second leg each having one end located at a fourth reference point located on the opposite side of the roadway from said third reference point, said fourth wire-loop first leg deployed at a seventh angle relative to the path of vehicular travel, said fourth wire-loop second leg deployed at a eighth angle relative to the path of vehicular travel, said fourth wire-loop being disposed below and substantially concentric with said second wire-loop.

60. The inductive sensor of claim 59 wherein said first wire-loop is substantially coplanar with said second wire-loop and said third wire-loop is substantially coplanar with said fourth wire-loop.

61. The inductive sensor of claim 59 wherein said first wire-loop is driven by a first current to produce a first magnetic field, said second wire-loop is driven by a second current to produce a second magnetic field, said third wire-loop is driven by a third current to produce a third magnetic field and said fourth wire-loop is driven by a fourth current to produce a fourth magnetic field.

62. The inductive sensor of claim 61 wherein said first current, said second current, said third current and said fourth current are alternating currents, said first current and said third current being phase-locked at a first frequency and said second current and said fourth current being phase-locked at a second frequency.

63. The inductive sensor of claim 61 wherein one of said first wire-loop first leg and said first wire-loop second leg substantially abuts one of said second wire-loop first leg and said second wire-loop second leg to define a first common leg and one of said third wire-loop first leg and said third wire-loop second leg substantially abuts one of said fourth wire-loop first leg and said fourth wire-loop second leg to define a second common leg.

64. The inductive sensor of claim 63 wherein said first current and said second current flow through said first common leg in opposite directions and said third current and said fourth current flow through said second common leg in opposite directions such that said first magnetic field destructively interferes with said second magnetic field and said third magnetic field destructively interferes with said fourth magnetic field.

65. The inductive sensor of claim 63 wherein said first current and said second current flow through said first common leg in substantially identical directions and said third current and said fourth current flow through said second common leg in substantially identical directions such that said first magnetic field constructively interferes with said second magnetic field and said third magnetic field constructively interferes with said fourth magnetic field.

66. An apparatus for detecting a vehicle travelling on a roadway having a width and defining a path of vehicular travel, said apparatus comprising:

a vehicle detection circuit;

a first sensor configured in a substantially triangular arrangement including a first wire segment, a second wire segment substantially spanning the width of the roadway at a first angle relative to said first wire segment and a third wire segment substantially spanning the width of the roadway at second angle relative to said first wire segment wherein said first wire segment, said second wire segment and said third wire segment form a current path, said first sensor in communication with said vehicle detection circuit; and

a second sensor configured in a substantially triangular arrangement including a fourth wire segment substantially spanning the width of the roadway, a fifth wire segment and a sixth wire segment substantially spanning the width of the roadway at a third angle relative to said fifth wire segment, said fourth wire segment disposed at a fourth angle relative to said fifth wire segment wherein said fourth wire segment, said fifth wire segment and said sixth wire segment form a current path, said second sensor in communication with said vehicle detection circuit.

67. The apparatus of claim 66 wherein said first angle is not equal to said second angle.

68. The apparatus of claim 66 wherein said third angle is not equal to said fourth angle.

69. The apparatus of claim 66 wherein said first angle is substantially equal to one of said third angle and said fourth angle and said second angle is substantially equal to the other of said third angle and said fourth angle.

70. The apparatus of claim 66 wherein said second wire segment is substantially parallel to said fourth wire segment and said third wire segment is substantially parallel to said sixth wire segment.

71. The apparatus of claim 66 wherein said third wire segment substantially abuts said sixth wire segment.

72. The apparatus of claim 66 wherein said third wire segment is substantially parallel to and substantially abuts said sixth wire segment.

73. The apparatus of claim 66 wherein said first sensor and said second sensor define independent current paths.

74. The apparatus of claim 66 wherein said first sensor and said second sensor define a single current path.

75. The apparatus of claim 66 further comprising a switch in communication between said first sensor and said second sensor, said switch electrically connecting said first sensor and said second sensor to form a single current path when closed and electrically isolating said first sensor and said second sensor to form separate current paths when open.

76. The apparatus of claim 66 further comprising a controller in communication with said first sensor, said second sensor and said vehicle detection circuit, said controller selectively energizing said first sensor and said second sensor such that a lateral position within the roadway of a vehicle passing over each of said first sensor and said second sensor while energized is determined.

77. The apparatus of claim 66 further comprising a controller in communication with each of said first sensor and said second sensor, said controller for selectively energizing each of said first sensor and said second sensor.

78. The apparatus of claim 66 wherein the roadway further defines a surface, said first sensor and said second sensor being disposed substantially parallel to the roadway surface.

79. The apparatus of claim 66 wherein one of said second wire segment and said third wire segment substantially abuts one of said fourth wire segment and said sixth wire segment to define a common leg.

80. The apparatus of claim 79 wherein said vehicle detection circuit produces a first magnetic field in said first sensor that destructively interferes with a second magnetic field produced by said vehicle detection circuit in said second sensor along said common leg.

81. The apparatus of claim 79 wherein said vehicle detection circuit produces a first magnetic field in said first sensor that constructively interferes with a second magnetic field produced by said vehicle detection circuit in said second sensor along said common leg.

82. The apparatus of claim 66 further comprising:

a third sensor configured in a substantially triangular arrangement including a seventh wire segment, an eighth wire segment substantially spanning the width of the roadway at a fifth angle relative to said seventh wire segment and a ninth wire segment substantially spanning the width of the roadway at sixth angle relative to said seventh wire segment wherein said seventh wire segment, said eighth wire segment and said ninth wire segment form a current path, said third sensor in communication with said vehicle detection circuit, said third sensor being disposed below and substantially concentric with said first sensor; and

a fourth sensor configured in a substantially triangular arrangement including a tenth wire segment substantially spanning the width of the roadway, an eleventh wire segment and a twelfth wire segment substantially spanning the width of the roadway at a seventh angle relative to said eleventh wire segment, said tenth wire segment disposed at an eighth angle relative to said eleventh wire segment wherein said tenth wire segment, said eleventh wire segment and said twelfth wire segment form a current path, said fourth sensor in communication with said vehicle detection circuit, said fourth sensor being disposed below and substantially concentric with said second sensor.

83. The apparatus of claim 82 wherein one of said second wire segment and said third wire segment substantially abuts one of said fourth wire segment and said sixth wire segment to define a first common leg and one of said eighth wire segment and said ninth wire segment substantially abuts one of said tenth wire segment and said twelfth wire segment to define a second common leg.

84. The apparatus of claim 82 wherein said first sensor is substantially coplanar with said second sensor and said third sensor is substantially coplanar with said fourth sensor.

85. The apparatus of claim 83 wherein said vehicle detection circuit produces a first magnetic field in said first sensor that destructively interferes with, along said first common leg, a second magnetic field produced by said vehicle detection circuit in said second sensor.

86. The apparatus of claim 85 wherein said vehicle detection circuit produces a third magnetic field in said third sensor that destructively interferes with, along said second common leg, a fourth magnetic field produced by said vehicle detection circuit in said fourth sensor.

87. The apparatus of claim 85 wherein said vehicle detection circuit produces a third magnetic field in said third sensor that constructively interferes, along said second common leg, with a fourth magnetic field produced by said vehicle detection circuit in s,aid fourth sensor.

88. The apparatus of claim 83 wherein said vehicle detection circuit produces a first magnetic field in said first sensor that constructively interferes, along said first common leg, with a second magnetic field produced by said vehicle detection circuit in said second sensor.

89. The apparatus of claim 88 wherein said vehicle detection circuit produces a third magnetic field in said third sensor that destructively interferes with, along said second common leg, a fourth magnetic field produced by said vehicle detection circuit in said fourth sensor.

90. The apparatus of claim 88 wherein said vehicle detection circuit produces a third magnetic field in said third sensor that constructively interferes, along said second common leg, with a fourth magnetic field produced by said vehicle detection circuit in said fourth sensor.

91. An apparatus for detecting a vehicle travelling on a roadway having a width and defining a path of vehicular travel, said apparatus comprising:

a vehicle detection circuit;

a first sensor configured in a substantially triangular arrangement including at least a first wire segment, a second wire segment substantially spanning the width of the roadway at a first angle relative to the first wire segment and a third wire segment substantially spanning the width of the roadway at second angle relative to the first wire segment, a first end of said first wire segment electrically connected to said vehicle detection circuit, a second end of said first wire segment electrically connected to a first end of said second wire segment, a second end of said second wire segment electrically connected to a first end of said third wire segment, and a second end of said third wire segment electrically connected to said vehicle detection circuit; and

a second sensor configured in a substantially triangular arrangement including at least a first wire segment substantially spanning the width of the roadway, a second wire segment and a third wire segment substantially spanning the width of the roadway at a third angle relative to said second wire segment, said second sensor first wire segment disposed at a fourth angle relative to said second wire segment, a first end of said first wire segment electrically connected to said vehicle detection circuit, a second end of said first wire segment electrically connected to a first end of said second wire segment, a second end of said second wire segment electrically connected to a first end of said third wire segment, and a second end of said third wire segment electrically connected to said vehicle detection circuit.

92. The apparatus of claim 91 wherein said first angle is not equal to said second angle.

93. The apparatus of claim 91 wherein said third angle is not equal to said fourth angle.

94. The apparatus of claim 91 wherein said first angle is substantially equal to one of said third angle and said fourth angle and said second angle is substantially equal to the other of said third angle and said fourth angle.

95. The apparatus of claim 91 wherein said first sensor second wire segment is substantially parallel to said second sensor first wire segment and said first sensor third wire segment is substantially parallel to said second sensor third wire segment.

96. The apparatus of claim 91 wherein said first sensor third wire segment substantially abuts said second sensor third wire segment.

97. The apparatus of claim 91 wherein said first sensor third wire segment is substantially parallel to and substantially abuts said second sensor third wire segment.

98. The apparatus of claim 91 wherein said first sensor and said second sensor define independent current paths.

99. The apparatus of claim 91 wherein said first sensor and said second sensor define a single current path.

100. The apparatus of claim 91 further comprising a switch in communication between said first sensor and said second sensor, said switch electrically connecting said first sensor and said second sensor to form a single current path when closed and electrically isolating said first sensor and said second sensor to form separate current paths when open.

101. The apparatus of claim 91 further comprising a controller in communication each of said first sensor and said second sensor, said controller for selectively energizing each of said first sensor and said second sensor.

102. The apparatus of claim 91 wherein the roadway further defines a surface, said first sensor and said second sensor being disposed substantially parallel to the roadway surface.

103. An apparatus for detecting a vehicle travelling on a roadway having a width and defining a path of vehicular travel, said apparatus comprising:

means for generating an inductive field;

means for sensing changes in said inductive field when a vehicle is present, said means for sensing changes in said inductive field adapted to detect each wheel of a vehicle independently; and

means for recording said changes in said inductive field due to each wheel of a vehicle as a wheel spike;

means for recording an occurrence time for each said wheel spike;

means for producing an inductive signature based on each said wheel spike and said occurrence time for each said wheel spike.

104. The apparatus of claim 103 further comprising means for selectively energizing said means for generating said inductive field and said means for sensing changes in said inductive field.

105. The apparatus of claim 103 further comprising means for determining a lateral position of a vehicle.

106. The apparatus of claim 103 further comprising means for preventing breakage of said means for sensing changes due to expansion of said roadway.

107. The apparatus of claim 103 further comprising means for classifying the vehicle represented by said inductive signature.

108. The apparatus of claim 103 further comprising means for separating a composite inductive signature that includes a combined plurality of inductive signatures simultaneously obtained into a plurality of individual vehicle inductive signatures.

109. A method for installing an inductive sensor, said method comprising the steps of:

(a) cutting a channel for receiving a wire, said channel defining a depth and a length; and

(b) varying said channel depth along said channel length.

110. The method of claim 109 wherein said step of varying said channel depth along said channel length includes the step of varying said channel depth in an undulating pattern.

111. An inductive sensor deployed in a roadway for use with a vehicle detector the roadway having a width and a surface, said inductive sensor comprising:

at least one channel cut into the roadway surface, said at least one channel defining a bottom located at a depth and having a length, said channel depth varying over said channel length; and

a wire disposed within said channel along said channel bottom, said wire defining a loop for detecting variations in an inductive field caused by a metallic object located proximate to said loop;

wherein said varying channel depth necessitates a length of wire in excess of said length of any said at least one channel such that said wire is not broken by expansion of the roadway.

112. The inductive sensor of claim 111 wherein said channel depth oscillates over said channel length.

113. The inductive sensor of claim 111 wherein said channel depth undulates over said channel length.

114. The inductive sensor of claim 111 wherein said channel bottom defines a wavy surface.

115. The inductive sensor of claim 111 wherein said channel bottom defines an undulating surface.

116. A method of determining vehicle parameters comprising the steps of:

(a) installing at least one inductive sensor in a known geometric configuration;

(b) connecting said at least one inductive sensor to at least one vehicle detection circuit;

(c) connecting each said at least one vehicle detection circuit to a processing device;

(d) detecting each wheel of a vehicle crossing over said at least one inductive sensor as a wheel spike;

(e) recording an occurrence time for each said wheel spike; and

(f) correlating said occurrence time for each said wheel spike with said known geometric configuration of said at least one inductive sensor to determine at least one vehicle parameter.

117. A method of re-identifying a vehicle travelling along a roadway containing a plurality of inductive sensors in communication with a common processing device, said method comprising the steps of:

(a) detecting at least one wheel spike of a vehicle crossing over one of said plurality of inductive sensors as a first inductive signature;

(b) detecting at least one wheel spike of a vehicle subsequently crossing over another of said plurality of inductive sensors as a second inductive signature;

(c) comparing at least one wheel spike of said first inductive signature with a corresponding wheel spike of said second inductive signature;

(d) identifying the vehicle represented by said first inductive signature as being identical to the vehicle represented by said second inductive signature when said corresponding wheel spike substantially matches.

118. The method of claim 117 wherein said step of identifying the vehicle represented by said first inductive signature as being identical to the vehicle represented by said second inductive signature further comprises the step of correlating said first inductive signature with said second inductive signature using a wheelbase measurement.

119. The method of claim 117 wherein said step of identifying the vehicle represented by said first inductive signature as being identical to the vehicle represented by said second inductive signature further comprises the step of correlating said first inductive signature with said second inductive signature using a tire width measurement.

120. The method of claim 117 wherein at least one of said step of detecting at least one wheel spike of a vehicle crossing over one of said plurality of inductive sensors as a first inductive signature and said step of detecting at least one wheel spike of a vehicle subsequently crossing over another of said plurality of inductive sensors as a second inductive signature further comprises the step of recording an occurrence time for said at least one wheel spike.

121. The method of claim 117 further comprising the step of assuming the vehicle defines a wheelbase having a substantially rectangular geometry.

122. The method of claim 117 further comprising the step of normalizing each of said first inductive signature and said second inductive signature with respect to speed.

123. The method of claim 117 wherein said step of detecting at least one wheel spike of a vehicle crossing over one of said plurality of inductive sensors as a first inductive signature includes the steps of

(i) detecting a first wheel spike for a wheel of a vehicle crossing over a first road-traversing leg of one of said plurality of inductive sensors; and

(ii) detecting a second wheel spike for a wheel of a vehicle crossing over a second road-traversing leg of one of said plurality of inductive sensors.