Communications: electrical – Vehicle detectors – Inductive
Reexamination Certificate
2000-10-10
2002-01-29
Pope, Daryl (Department: 2632)
Communications: electrical
Vehicle detectors
Inductive
C340S933000, C340S934000
Reexamination Certificate
active
06342845
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
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.
Inventor(s)
Issue Date
1,992,214
D. Katz
02/26/1935
3,641,569
D. Bushnell
02/08/1972
3,827,389
V. Neeloff
12/16/1975
3,984,764
S. Koerner
10/05/1976
4,276,539
K. Eshraghian, et al.
06/30/1981
5,198,811
T. Potter, et al.
03/30/1993
5,245,334
F. Gerbert, et al.
09/14/1993
5,481,475
G. Rouse, et al.
02/13/1996
5,523,753
M. Fedde, et al.
06/04/1996
5,614,894
D. Stanczyk
03/25/1997
5,861,820
B. Kerner, et al.
01/19/1999
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 co
Hilliard Geoffrey W.
Hilliard Steven R.
Inductive Signature Technologies
Pitts & Brittian P.C.
Pope Daryl
LandOfFree
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