Communications: electrical – Vehicle detectors – Inductive
Reexamination Certificate
2001-09-04
2003-10-28
Pope, Daryl (Department: 2632)
Communications: electrical
Vehicle detectors
Inductive
C340S933000, C340S934000, C340S935000, C377S009000
Reexamination Certificate
active
06639521
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
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.
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.
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 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, C, are counted. Because the frequency of the LCR oscillator is inversely proportional to the square-root of the inductance, L, 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'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, &dgr;I, 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 p
Hilliard Geoffrey W.
Hilliard Steven R.
Inductive Signature Technologies
Pitts & Brittian P.C.
Pope Daryl
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