Not Applicable.
1. Field of Invention
This invention relates to a system for measuring changes in an inductive field due to the presence of a vehicle. More specifically, this invention relates to the measurement by inductive sensors of an inductive signature corresponding to a particular vehicle.
2. Description of the Related Art
Metal detectors are widely used to locate metallic objects that are buried or otherwise hidden from view in military, forensic, geological prospecting, archaeological exploration, and recreational treasure-hunting applications. They have many industrial uses including proximity and position sensing and the automated inspection of manufacturing, assembly, and shipping processes. They are the active component in pedestrian screening devices used at airports and other high-security areas to detect the presence of concealed weapons. Inductive vehicle detectors are widely deployed on highways and at intersections for traffic-flow monitoring and control and at parking facilities for revenue and access control.
The measurable inductance of a wire-loop is directly proportional to the magnetic permeability of the space surrounding the loop. Non-metallic matter typically has no measurable effect on the magnetic permeability of the space it occupies, while metallic matter can measurably increase or decrease the magnetic permeability of the space it occupies depending upon its composition. It is well known in the prior-art to measure the inductance of a wire-loop to detect the presence or absence of metal near the loop. The presence of iron tends to increase the inductance of a wire-loop, while the presence of non-ferrous metal tends to decrease the inductance of a wire-loop.
The variation of inductance typically observed by vehicle detectors of the prior art is on the order of five-percent (5%) of the nominal inductance of a conventional wire-loop. Such variation is approximately the same order of magnitude as the electromagnetic noise and thermal drift which affect the wire-loop inductance. Major identifiable sources of electromagnetic noise include electrical power lines, computing and communications equipment, automotive ignition systems, and cross-talk between wire-loops when two or more sensors are deployed in close proximity to one another.
Although variations exist, conventional wire-loops are generally deployed in a rectangular geometry on a plane which is roughly parallel to the surface of the roadway into which they are embedded. For such a conventional wire-loop, the magnetic field generated by the flowing current is described by the Biot-Savart law of physics. The magnetic field forms a generally cylindrical magnetic field around each leg of the wire-loop. The intensity of the magnetic field diminishes linearly as the radial distance from the wire-loop increases. The magnetic fields generated by opposing legs of a wire-loop tend to cancel each other out. By increasing the distance between the opposing legs, a stronger composite magnetic field is created allowing better detection of vehicles. However, the vulnerability of the wire-loop to electromagnetic noise also increases as the separation between the opposing legs of the wire-loop increases. Accordingly, large dimension wire-loops suffer from poor signal-to-noise ratios.
It is well known in the art that the signal strength is strongest when all four legs of a wire-loop inductive sensor simultaneously interact with a vehicle. Increasing the area of the roadway loop so that a vehicle passes only over part of the wire-loop decreases the signal strength and the resulting poor signal-to-noise ratio makes it difficult to reliably detect the presence of differing classes of vehicles. Accordingly, a conventional wire-loop intended for vehicle presence detection is generally centrally positioned within a traffic lane and dimensioned smaller than a typical vehicle such that the variation in the inductance due to a vehicle crossing over the wire-loop is maximized while uncertainties due to electromagnetic noise are minimized. The increased signal strength and improved signal-to-noise ratio obtained using techniques common to conventional wire-loops does not come without cost. A narrowly dimensioned conventional wire-loop is not suitable for providing inductive signature data for vehicle classification and identification as it forfeits the strong signals produced by the wheels. Additionally, the free-running oscillators of the prior art requires the wire-loops in adjacent lanes to be separated by a fair distance to avoid crosstalk. This virtually eliminates the possibility of deploying conventional wire-loops in a manner to present a uniform presence over the entire width of a traffic lane. Because conventional wire-loops do not present a uniform presence, a vehicle may cross at different angles and different lateral offsets. This results in varying inductance measurements for the same vehicle. Therefore, the inductive signature measurements obtained from conventional wire-loops are not repeatable, making accurate classification and identification difficult.
Finally, with regard to the dimensions of conventional wire-loops, it should be noted that their length is limited because a conventional wire-loop is formed within slots cut into the roadway surface. For larger conventional wire-loops, the thermal expansion of the roadway surface tends to destroy those loops which reduces reliability and increases maintenance costs.
Conventional detectors measure inductance by making the wire-loop part of a free running oscillator circuit which has a frequency determined by the inductance of the wire-loop and the capacitance of the circuit. A frequency-counter then counts the number of charge-discharge cycles of the oscillator over a pre-determined period of time. This count is partially a function of the varying inductance of the wire-loop, but also varies with the electromagnetic noise and thermal drift. A temperature change in the wire-loop of only 6-degrees Centigrade would typically cause a baseline drift equal to the full-scale of the inductance variations being measure because the resistance of the wire in the wire-loop is temperature dependent.
Conventional detectors which are able to reliably detect passenger cars are unable to reliably detect vehicles with high ground clearance, such as motorcycles, snow plows, and other large trucks, because of the uncertainty imposed by ambient electromagnetic noise and temperature drift. In addition to reducing traffic flow efficiency, this can lead to property damage and personal injury caused by automated parking gates which prematurely close on vehicles having high ground clearance.
Other devices for measuring changes in an inductive field due to the presence of a vehicle have been disclosed. Typical of the prior art are the following U.S. Patents.
U.S. Pat. No. 1,992,214 (the ""214 patent) issued to David Katz on Feb. 26, 1935 discloses a traffic detector which operates by detecting changes in a magnetic field induced by the iron in a vehicle. Katz teaches using a coil of wire for measuring disturbances in the earth""s magnetic field. With regard to the position and orientation of the coil, Katz teaches a variety of horizontal and vertical arrangements, both above and below ground. To achieve a usable measurement, the dimensions of the coil are selected to produce sufficient separation between the legs of the coil. Katz teaches a separation of approximately three to five feet for vertical coil orientations and five to twelve feet for horizontal coil orientations. The leg separation requirement for vertical coil orientations makes these arrangements impracticable to install, especially in pre-existing roadways, as they require the cutting of at least a three-foot deep slot. Further, Katz teaches encasing the coil in a casing or pipe.
U.S. Pat. No. 3,641,569 (the ""569 patent) issued to David Bushnell on Feb. 8, 1972 discloses a highway vehicle sensor system. Bushnell teaches a system including three inductive loops. A first, or main, loop is disposed horizontally within the roadway. Along each of the transverse legs of the main loop, with respect to the roadway axis, a probe loop which is vertically oriented is disposed closely proximate to and coaxial with the transverse leg. The main loop is energized to produce a magnetic field which is sensed by the probe loops. When no vehicles are present, the fields sensed by the probe loops are equivalent. However, when a vehicle crosses one of the transverse legs of the main loop, the field, and therefore the outputs of the probe loops, becomes unbalanced. This imbalance allows the detection of various data such as vehicle presence, direction of travel, vehicle speed, vehicle length, and separation between vehicles.
U.S. Pat. No. 3,827,389 (the ""389 patent) issued to Victor Neeloff on Dec. 16, 1975 discloses an apparatus for determining, during operation, the category of a vehicle according to a pre-established group of categories. Specifically, the ""338 patent is directed to an arrangement for multiple pressure sensitive cables disposed in a roadway which allows the counting and classification of vehicles based upon wheel base. The cable arrangements taught are selected to reduce the propagation of counting errors in conjunction with the associated wheel height sensor and vehicle separation sensor.
U.S. Pat. No. 3,984,764 (the ""764 patent) issued to Steve Koerner on Oct. 5, 1976 discloses an inductive loop structure for detecting the presence of vehicles over a roadway. Koerner teaches a pair of side-by-side loops disposed horizontally within the roadway. The loop pair form a single current path having two terminals which are connected to the detector circuitry. Alternating current is applied to the terminals to produce a commonly oriented magnetic field around the adjacent sides of the loops and an oppositely oriented magnetic field of lesser magnitude around each of the outer sides of the loops.
U.S. Pat. No. 4,276,539 (the ""539 patent) issued to Kamran Eshraghian, et al., on Jun. 30, 1981 discloses a vehicle detection system which distinguishes between changes in the measured signal resulting from changes in the environmental conditions and changes caused by an approaching vehicle. The apparatus taught in the ""539 patent employs a discrete logic feedback circuit which operates by periodically sampling the measured signal and storing the sample. The sample is then compared to an envelope level equivalent to the amplitude of the previous sample plus a predetermined fixed increment. Should the new sample exceed the envelope level, i.e., the increase per sample time increment exceeds the predetermined maximum rate of change, the xe2x80x9crapidxe2x80x9d change in the measured signal is determined to result from the passage of a vehicle as opposed to a xe2x80x9cslowerxe2x80x9d environmental change.
U.S. Pat. No. 5,198,811 (the ""811 patent) issued to Thomas Potter, et al., on Mar. 30, 1993 discloses a vehicle communication system using existing roadway loops wherein the physical integrity of the loop is kept intact. Specifically, Potter et al., disclose a vehicle communication system which uses a vehicle mounted transmitting antenna to communicate with a stationary receiver. The stationary receiver is taught to be an existing, conventional roadway loop to receive signals from the vehicle. Potter et al., do not teach anything about the configuration of a conventional wire-loop sensor. Additionally, Potter et al., in the various figures, illustrate a existing roadway loop which is a single loop providing a single current path which is overlapped to form two coils occupying the same general area of the roadway.
U.S. Pat. No. 5,245,334 (the ""334 patent) issued to Franz J. Gerbert, et al., on Sep. 14, 1993 discloses a traffic detection cable installation. Specifically, the installation method taught in the ""334 patent applies to pressure sensitive cables, e.g., piezoelectric, triboelectric, or stress/strain gauges. The cables are encased in an elastic material in either a side-by-side or an over-under configuration. The elastic material is selected such that mechanical/physical impulses from the weight of the vehicle crossing over the cable enclosure are transmitted to the cables.
U.S. Pat. No. 5,481,475 (the ""475 patent) issued to Gordon F. Rouse, et al., on Feb. 13, 1996 discloses a magnetometer vehicle detector. Rouse, et al., teach the use of magneto-resistive sensors having the capability of distinguishing different magnetic signatures of basic vehicle types. The disclosed magnetometers do not constrain vehicles crossing the wire-loop sensors to present repeatable signatures which renders them impractical for precise vehicle classification and identification applications. Additionally, the sensors are sensitive to vehicles in adjacent traffic lanes which introduces an added element of uncertainty to any signature recorded.
U.S. Pat. No. 5,523,753 (the ""753 patent) issued to Mickiel P. Fedde, et al., on Jun. 4, 1996 discloses a vehicle detector system with periodic source filtering. Fedde, et al., teach the cancellation of some low-frequency components of the electromagnetic noise which is predictably generated by power-lines and which have a basically periodic nature. Low-frequency noise is amplified, high-frequency noise is unaffected, and only approximately 120 inductance measurements per second may be made using this technique. The time-aperture of the detector is open for an entire 16.7 milliseconds of each sample. The period is undesirable for making precision measurements of rapidly varying inductance. The time-aperture of the detector is time during which a change in the inductance being measured will cause a change in the inductance measurement.
U.S. Pat. No. 5,614,894 (the ""894 patent) issued to Daniel Stanczyk on Mar. 25, 1997 discloses a device to detect particularly one or several wheels of a vehicle or of a wheeled mobile engine and the process for using this device. Stanczyk teaches inductive loop detectors disposed horizontally on a roadway. The ""894 patent teaches the installation of a number of inductive loops defining a single current path. At least one of the loops has a roadway axis dimension being smaller than the diameter of a vehicle wheel.
U.S. Pat. No. 5,861,820 (the ""820 patent) issued to Boris Kerner, et al., on Jan. 19, 1999 discloses a method for the automatic monitoring of traffic including the analysis of back-up dynamics. Kerner, et al., teach the periodic measurement of traffic data, such as vehicle speed and traffic flow, at multiple points within a region of interest along a roadway using inductive loops. By monitoring the relative speeds of the vehicles within the region of interest, areas of traffic congestion are identified.
Accordingly, there is a need for a system and method for the measurement of the inductance corresponding to a vehicle crossing an inductive sensor. There is need for a system that presents a uniform sensor geometry to a vehicle crossing the sensors regardless of the lateral position of the vehicle within the traffic lane. Further, there is a need for a system and method that removes variations due to incident noise and thermal drift from the measured inductance. Finally, a system and method that is capable of sampling changes in the inductive field at a higher sampling frequency than is available with the prior art is needed.
Therefore, it is an object of the present invention to provide an inductive sensor for vehicle detection that constrains a vehicle crossing over the inductive sensor to present a substantially repeatable inductive signature.
It is another object of the present invention to provide an inductive sensor for vehicle detection that substantially overcomes the practical limitations on the length of wire-loops deployed within roadway surfaces.
It is yet another object of the present invention to provide an inductive signature sampling element which is simpler to install and maintain within existing roadway surfaces.
It is a further object of the present invention to maximize the identifying information contained within the inductive signature.
It is a still further object of the present invention to increase the signal-to-noise ratio of the inductive sensor.
It is also an object of the present invention to measure the inductance of an inductive sensor in a relatively short time with relatively high precision by a method that is serially repeatable and substantially independent of preceding and succeeding measurements.
It is another object of the present invention to record an inductive signature of an automotive vehicle by making a plurality of successive measurements of the inductance as a vehicle crosses over an inductive sensor.
It is an additional object of the present invention to use velocity and acceleration profiles of a vehicle crossing over the inductive sensor to compensate, or normalize, for distortions of the inductive signatures recorded for those vehicles.
It is a further additional object of the present invention to classify an unknown vehicle through correlation of the unknown vehicle inductive signature with a known inductive signature.
It is still further additional object of the present invention to classify an unknown vehicle through correlation of a sequence of characteristic point magnitudes from the unknown vehicle inductive signature with sequence of characteristic point magnitudes from a known inductive signature.
It is another object of the present invention to identify whether a newly measured vehicle is a vehicle previously measured through correlation of the unknown vehicle inductive signature with a known inductive signature.
A system and method for measuring a plurality of successive induction measurements, collectively known as the xe2x80x9cinductive signaturexe2x80x9d of a vehicle, and classifying the vehicle described by the measured inductive signature is described. The system includes a blade-type wire-loop configuration, or blade sensor and a corresponding measurement circuit employing a discrete measurement technique, as opposed to the frequency counting technique of the prior art.
The blade sensor is a conductor that is formed around a loop forming member, or form. It is preferred that the blade sensor is installed in a substantially vertical orientation with respect to the roadway surface. A substantially vertical orientation provides the sharpest longitudinal aperture and produces higher quality and more repeatable inductive signatures. Generally, the blade sensor is designed to achieve four goals not simultaneously available with conventional wire loops for measuring inductive signatures. First, the blade sensor is designed to produce repeatable inductive signatures regardless of the lateral position of the vehicle within a traffic lane. Second, the blade sensor is designed to cancel most of the incident electromagnetic differential noise within the blade sensor and to produce a high signal-to-noise ratio. Third, the blade sensor is designed to be easy to install within an existing roadway surface. Finally, the blade sensor is designed to be reliable and generally immune to the effects of thermal expansion of the roadway surface.
In order to achieve repeatability of inductive signatures, the blade sensor is configured to present a uniform loop geometry across the entire width of the portion of the roadway where the measurement of inductive signatures is desired. Accordingly, the form is dimensioned such that the length of the form is sufficient to extend across the entire area of interest, adjusting for the angle of installation. Orienting the blade sensor at an angle with respect to the direction of traffic such that each wheel is detected separately maximizes the information available from the blade sensor. Using such an angular orientation, each wheel produces an identifiable peak in the measured inductance as it crosses the blade sensor thereby improving the quality of the signature. These peaks vary with the differences in the wheels and tires of a vehicle. The character of these peaks is useful for distinguishing between vehicles of the same make and model.
A vehicle crossing the blade sensor has greater influence over the magnetic field generated by the nearer upper leg than over the magnetic field of the lower leg. The difference in magnetic field interaction strength due to the differing vertical depth of each leg from the vehicle results in an improved signal-to-noise ratio when compared to a conventional sensor. It is desirable to maximize the repeatable difference between these two interactions; detectable differences that are not repeatable are considered to be noise and are undesirable.
Both the goals of easy installation and improvement in noise reduction are achieved from shallow depth blade sensors. Although the blade sensor of the present invention accommodates virtually any desired depth, a shallow depth on the order of a few centimeters is convenient to install. The blade sensor of the present invention does not enclose any pavement material. Accordingly, the risk of breakage due to thermal expansion of the pavement is minimized regardless of the dimensions of the blade sensor and reliability is increased while maintenance costs are reduced.
To compensate for noise and thermal drift, one embodiment of the blade sensor includes a secondary loop positioned below the primary loop. The addition of the secondary loop provides for the common-mode rejection of incident noise. To maximize the signal-to-noise ratio, the secondary loop is disposed coplanar with and dimensioned similarly to the primary loop. Because current-carrying wires in close proximity naturally exert significant forces on one another, the primary loop lower leg and the secondary loop upper leg are firmly anchored with respect to one another.
By attaching a capacitor to each of the primary loop and the secondary loop, inductance-capacitance-resistance (LCR) oscillator circuits are formed. These LCR oscillator circuits offer significant improvement over simple inductance-resistance (LR) circuits. Further, closely matching the LCR oscillator circuits such that they have equivalent impedance at the frequency of interest or equivalent LCR values produces the benefits of common-mode rejection, noise cancellation, and thermal-drift compensation. When using the primary wire-loop and the secondary loop, the net magnetic field interaction is substantially that of the primary loop upper leg and the secondary loop lower leg.
The present invention measures the inductance of the wire-loops by a discrete method rather than the frequency-counter method of the prior art. To implement discrete measurement, both LCR circuits are charged for a period of time and then discharged. The output of the LCR circuits is sampled during the discharge cycle. This discrete method of inductance measurement requires only one charge and discharge cycle of the LCR circuits per measurement. Thus, the discrete method provides a result in a relatively short period of time with relatively high precision. The relatively short period of time between the charging of the LCR circuits and the sampling of the output sinusoid defines a favorably narrow time-aperture for the detector. Further, the discrete method is serially repeatable and substantially independent of preceding and subsequent measurements.
The directly observable inductive signature, referred to as the inductive time-signature, represents the inductive profile of the vehicle as a function of time as it crosses the blade sensor. Because the instantaneous velocity of a vehicle crossing the blade sensor can vary over a wide range, the inductive time-signature of a vehicle is generally not as useful for classification or identification of the vehicle as the inductive length-signature. Therefore, it is desirable to transform, or normalize, the inductive time-signature of the vehicle into the more useful inductive length-signature. The inductive length-signature is a normalized set of inductive measurements that can be compared to other inductive length-signatures through statistical methods to classify and identify the vehicle crossing the sensor. Once a correlation is determined to exist between two inductive length-signatures with an acceptable degree of confidence, either the classification or identity of the vehicle is known within a finite degree of confidence. This information is available to be used as intended for a wide variety of applications that include Advanced Transportation Management Systems (ATMS), parking-lot revenue control, car-bomb detection, traffic-law enforcement, and community security among many others.