U.S. patent application number 09/755343 was filed with the patent office on 2001-11-22 for method and apparatus for active isolation in inductive loop detectors.
Invention is credited to Hilliard, Geoffrey W., Hilliard, Steven R..
Application Number | 20010043124 09/755343 |
Document ID | / |
Family ID | 22636873 |
Filed Date | 2001-11-22 |
United States Patent
Application |
20010043124 |
Kind Code |
A1 |
Hilliard, Steven R. ; et
al. |
November 22, 2001 |
Method and apparatus for active isolation in inductive loop
detectors
Abstract
An oscillator circuit for use with a wire-loop inductive sensor
and method for use. The oscillator circuit highly attenuates
common-mode noise detected by the wire-loop and differential noise
from both ambient and crosstalk sources are filtered by active
isolation.
Inventors: |
Hilliard, Steven R.;
(Knoxville, TN) ; Hilliard, Geoffrey W.; (Signal
Mountain, TN) |
Correspondence
Address: |
J. Kenneth Hoffmeister
Pitts & Brittian, P.C.
P.O. Box 51295
Knoxville
TN
37950-1295
US
|
Family ID: |
22636873 |
Appl. No.: |
09/755343 |
Filed: |
January 5, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60174627 |
Jan 5, 2000 |
|
|
|
Current U.S.
Class: |
331/65 ;
324/207.16; 331/167; 340/941 |
Current CPC
Class: |
G01R 27/2611 20130101;
G01D 5/2013 20130101; G08G 1/042 20130101 |
Class at
Publication: |
331/65 ; 331/167;
324/207.16; 340/941 |
International
Class: |
G08G 001/042; H03B
005/08 |
Goverment Interests
[0002] Not Applicable.
Claims
Having thus described the aforementioned invention, we claim:
1. An oscillator circuit for use in an inductive sensor, said
oscillator circuit comprising: a first capacitor; a second
capacitor in electrical communication with said first capacitor; a
wire-loop coupled to each of said first capacitor and said second
capacitor; a voltage supply; an excitation circuit connectable with
said first capacitor and said second capacitor to connect said
voltage supply to said first capacitor and said second capacitor at
a selected polarity; and wherein either of said first capacitor and
said second capacitor is charged to a preselected voltage by said
excitation circuit and the other of said first capacitor and said
second capacitor is discharged when said excitation circuit is
connected and each of said first capacitor and said second
capacitor is discharged when said excitation circuit is
disconnected to produce a pair of decaying oscillations having a
caduceus-shaped output.
2. The oscillator circuit of claim 1 wherein said excitation
circuit is a plurality of switches.
3. The oscillator circuit of claim 2 wherein said oscillator
circuit includes a ground such that when a first of said plurality
of switches is closed, one of said first capacitor and said second
capacitor is connected to said voltage source and when said a
second of said plurality of switches is closed, the other of said
first capacitor and said second capacitor is connected to a
ground.
4. The oscillator circuit of claim 1 further comprising a
transformer having a first coil and a second coil, said first coil
electrically connected to said wire loop and said second coil in
electrical communication with said first capacitor and said second
capacitor whereby said wire loop is inductively coupled to each of
said first capacitor and said second capacitor.
5. An oscillator circuit for use in an inductive sensor, said
oscillator circuit comprising: a wire-loop; a
resistance-capacitance network coupled to said wire-loop, said
resistance-capacitance network including at least two capacitors; a
voltage supply; and an excitation circuit connectable to said
resistance-capacitance network to connect said
resistance-capacitance network to said voltage source at a selected
polarity; wherein said resistance-capacitance network is charged to
a preselected voltage by said excitation circuit when said
excitation circuit is connected and said resistance-capacitance
network is discharged when said excitation circuit is disconnected
to produce a pair of decaying oscillations having a caduceus-shaped
output.
6. The oscillator circuit of claim 5 wherein said excitation
circuit is a plurality of switches.
7. The oscillator circuit of claim 6 wherein said oscillator
circuit includes a ground such that when a first of said plurality
of switches is closed, one of said at least two capacitors is
connected to said voltage source through said
resistance-capacitance network and when said a second of said
plurality of switches is closed, the other of said at least two
capacitors is connected to said ground through said
resistance-capacitance network.
8. The oscillator circuit of claim 5 further comprising a
transformer having a first coil and a second coil, said first coil
in electrical communication with said wire-loop and said second
coil in electrical communication with said resistance-capacitance
network whereby said transformer inductively couples said wire loop
to said resistance-capacitance network.
9. A method for actively isolating noise in an inductive vehicle
detector, said method comprising the steps of: (a) selecting a
polarity of a voltage source to produce a polarized voltage output;
(b) exciting a first capacitor using said polarized voltage output;
(c) discharging a second capacitor; (d) isolating said first
capacitor and said second capacitor from said voltage source; (e)
oscillating said first capacitor and said second capacitor; (f)
measuring a voltage of each of said first capacitor and said second
capacitor; (g) producing an output signal related to a voltage
differential between said first capacitor voltage and said second
capacitor voltage; (h) terminating a measurement cycle when said
first capacitor voltage and said second capacitor voltage are
substantially equal and stable; and (i) repeating said steps of
selecting a polarity of the voltage source to said step of
terminating a measurement cycle.
10. The method of claim 11 wherein said step of producing an output
signal includes the step of generating a pulse when said voltage
differential alternates polarity.
11. The method of claim 11 further comprising the step of canceling
noise by summing any two of said output signal produced in a
successive pair of said measurement cycles.
12. The method of claim 11 further comprising the step of
quantifying noise by subtracting any two of said signal produced in
a successive pair of said measurement cycles.
13. An oscillator circuit for use in an inductive sensor, said
oscillator circuit comprising: means for measuring changes in an
inductive field; means for storing a charge; means for exciting
said means for storing the charge; means for interruptably
connecting said means for charging to said means for storing the
charge; means for alternating a polarity of said means for exciting
said means for storing the charge; means for discharging said means
for storing the charge; means for measuring a voltage differential
of said means for storing the charge; means for producing an output
signal related to said voltage differential; and means for
determining a frequency of said output signal.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/174,627, filed Jan. 5, 2000.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention generally relates to the measurement
of inductance, and more particularly to inductive vehicle
detectors.
[0005] 2. Description of the Related Art
[0006] Inductive wire-loop vehicle detectors of the prior art are
typically based on the Colpitts oscillator using a wire-loop
sensor. A simple wire-loop has two terminals that are typically
connected to the rest of the detector circuit through a pair of
lead wires. The lead wires connected to the primary coil of a
transformer serving as a common-mode choke typically having 40 dB
common-mode noise attenuation. The secondary coil of the
transformer is connected to a capacitor effectively forming an
inductance-capacitance-resistance (LCR) circuit with the
wire-loop.
[0007] In the typical Colpitts oscillator-based detector, one leg
of the LCR circuit is connected to a positive direct current (DC)
power supply terminal. Because of this arrangement, the common-mode
noise appearing at the secondary coil of the transformer is
converted to differential noise as the common-mode current flowing
through the leg of the secondary coil tied to the positive
power-supply terminal is drained away. Consequently, the
common-mode current flowing through the other leg of the secondary
coil charges the capacitor of the LCR circuit. These current flows
create a differential noise voltage, which is added to the existing
differential noise on the circuit. The largest component of
common-mode noise is typically power-line noise around 60 Hz. For a
typical two-meter loop having three turns of wire, the differential
noise induced by a 60 Hz power line is at 60 Hz and its harmonics.
The primary method for canceling ambient noise in prior-art
detectors is to integrate the sampling period of the detector over
a time chosen to coincide with the local power-line voltage
period.
[0008] Additionally, where multiple wire-loop sensors are placed in
close proximity, crosstalk is a concern. Crosstalk between
detectors is a function of the inductive coupling between the
wire-loops and the transformers as well as the relative phase and
amplitudes of the oscillating signals on the loops. The primary
method for mitigating the effects of crosstalk in prior-art
detectors is to use different capacitance values in the LCR
circuits. This tends to randomize the relative phase of the
oscillating signals on adjacent loops over time.
[0009] The primary methods for minimizing crosstalk and canceling
ambient noise described above tend to limit the sampling rate of
prior-art wire-loop sensors to approximately 60 Hz, which is well
below what is desirable for recording repeatable inductive
signatures on vehicles traveling at highway speeds.
BRIEF SUMMARY OF THE INVENTION
[0010] It is desirable to isolate signal from noise in an inductive
vehicle detector. Inductance is typically measured indirectly as a
function of the resonant frequency of an LCR circuit in which the
oscillation frequency is approximately inversely proportional to
the square root of the product of inductance and capacitance. In
practice, significant errors in the measurement of this oscillation
frequency are typical.
[0011] In the absence of noise errors, the measured inductance of a
wire-loop is independent of the polarity of the excitation current
used to make the LCR circuit oscillate. However, when random and
non-random differential noise is induced into the circuit,
typically through the wire-loop, lead-wire, and transformer, the
resulting inductance measurement errors strongly depend on the
polarity of the excitation current. By employing an oscillator
circuit having two balanced capacitors and by controlling the
polarity of the excitation current, the effects of common-mode and
differential noise can be greatly reduced with minimal effect on
the inductance measured at the wire-loop sensor.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0012] The above-mentioned features of the present invention will
become more clearly understood from the following detailed
description of the invention read together with the drawings in
which:
[0013] FIG. 1 illustrates a block diagram of one embodiment of the
oscillator circuit of the present invention;
[0014] FIG. 2 illustrates a block diagram of an alternate
embodiment of the oscillator circuit of the present invention;
[0015] FIG. 3 is a schematic diagram of the oscillator circuit
embodied in FIG. 1;
[0016] FIG. 4 is a simplified schematic diagram of the oscillator
circuit embodied in FIG. 3;
[0017] FIG. 5a is a graphic representation of one output of the
oscillator circuit of the present invention using the circuit of
FIG. 3;
[0018] FIG. 5b is a graphic representation of the other output of
the oscillator circuit of the present invention using the circuit
of FIG. 3;
[0019] FIG. 5c is a graphic representation of the output of FIG. 5a
overlain with the output of FIG. 5b;
[0020] FIG. 5d is a graphic representation of the pulse-train
output of the differential comparator of the circuit of FIG. 3;
[0021] FIG. 6 is a graphic representation of inductive signature
data obtained using the oscillator circuit of the present
invention;
[0022] FIG. 7 is a graphic representation of an array of inductive
loop sensors disposed in a speed-trap configuration; and
[0023] FIG. 8 is a graphic representation of inductive signature
data obtained using a prior art oscillator circuit.
DETAILED DESCRIPTION OF THE INVENTION
[0024] An oscillator circuit for use in an inductive vehicle
detector is shown generally at 10 in the Figures. Inductance is
typically measured indirectly as a function of the frequency of an
LCR oscillator in which the oscillation frequency is approximately
inversely proportional to the square root of the product of
inductance and capacitance. In practice, significant errors in the
measurement of this oscillation frequency are typical. In the
absence of noise errors, the measured inductance of a wire-loop is
independent of the polarity of the excitation current driving the
oscillation of the LCR circuit. However, both random and non-random
differential noise induced into the circuit cause inductance
measurement errors, which have a polarity that is strongly
dependent on the polarity of the excitation current.
[0025] FIG. 1 illustrates a block diagram of one embodiment of an
inductive vehicle detector including the oscillator circuit 10 of
the present invention. The vehicle detector includes an inductive
loop sensor 100. Those skilled in the art will recognize the
various configurations that may be used for the inductive loop
sensor 100. A simple inductive loop sensor 100 is a wire-loop
having two terminals connected to the rest of the detector circuit
through a pair of lead wires. Alternatively, the wire-loop is
connected to the primary coil of a transformer serving as a
common-mode choke 102, as illustrated in FIG. 1. The secondary coil
of the transformer is connected to a resistance-capacitance (RC)
circuit 104 forming an inductance-capacitance-resistance (LCR)
circuit with the wire-loop 100. Typically, connecting the wire-loop
through a common-mode choke 102 provides approximately 40 dB of
common-mode noise attenuation. An excitation circuit 108
interruptably connects a voltage supply 110 to the secondary coil
of the common-mode choke 102. In the illustrated embodiment, the
oscillating output of the oscillator circuit 10 is passed to a
measuring device 112 that samples the frequency of the oscillation
at any given time. The frequency of the oscillation in turn relates
to the inductance of the wire-loop 100. The measuring device 112
includes a differential comparator 106 whose output switches
whenever the differential output of the oscillator circuit 10
changes polarity. The measuring device 112 also includes a counter
114 that counts the pulses generated by a high-speed,
fixed-frequency clock 116 gated by the differential comparator 106.
These measurements are then processed by a processing device 118 to
extract useful information from the data.
[0026] The excitation circuit 108 is typically implemented using a
plurality of switches. These switches are gated by precisely timed
digital signals that are generated by a timing source 120. In one
embodiment, the timing source 120 is implemented using a binary
counter that counts the high-speed clock source 116. Typically, the
binary counter 120 is initialized to zero at the start of a
measurement cycle and the counter proceeds to count each successive
pulse from the clock source. When a predetermined count is reached,
a gate signal is generated to turn on one pair of switches and
counting continues until a second predetermined count is reached at
which time the gate signal is terminated to turn off the pair of
switches. The measurement cycle is complete when a third
predetermined count is reached, and a new cycle begins.
[0027] FIG. 2 illustrates a block diagram of an alternate
embodiment of the oscillator circuit 10' of the present invention.
In the illustrated embodiment, the measuring device 112 is replaced
by a differential sample-and-hold amplifier 200 feeding an
analog-to-digital converter (ADC) 202. The differential
sample-and-hold amplifier 200 combined with the ADC 202 provide
greater flexibility over the frequency measuring device 112 in the
aforementioned embodiment by sampling the entire differential
output of the oscillator circuit 10, as opposed to only sampling
the zero crossings. To develop a baseline, the decaying sinusoids
are sampled at multiple points which allows a determination of the
base resistance and inductance of the inductive sensor. This
baseline sampling need only occur once, but can be repeated to
monitor the oscillator circuit 10 for baseline drift, which, for
example, can be caused by temperature variations. Thereafter, the
decay sinusoids are sampled to determine the change in the
inductance of the wire-loop 100 due to the presence of a vehicle or
other metal object. Again, the measurements are processed by a
processing device 118 to extract useful information from the
data.
[0028] FIG. 3 is a schematic diagram of the oscillator circuit 10
embodied in FIG. 1. The oscillator circuit 10 is an
inductance-capacitance-resista- nce (LCR) oscillator having two
legs. In the illustrated embodiment, each leg including a
resistance R.sub.1, R.sub.2 in series with a capacitor C.sub.1,
C.sub.2. Those skilled in the art will recognize that R.sub.1,
R.sub.2 can represent the inherent resistance of the circuit or
discrete resistors selected to balance the circuit. The RC circuit
104 is coupled to a wire-loop sensor 100 used as a sensor. In the
illustrated embodiment, the wire-loop sensor 100 includes an
inherent inductance L.sub.L and a resistance R.sub.L connected via
the common-mode choke 102. Again, those skilled in the art will
recognize that R.sub.L and L.sub.L typically represent the inherent
inductance and resistance of the wire loop and that discrete
components are not typically used. Further, those skilled in the
art will recognize that the wire-loop sensor 100 may be directly
connected to the RC circuit if desired, without departing from the
scope and spirit of the present invention. Each leg of the
oscillator 10 is connected to an input of the differential
comparator 106. Because two capacitors C.sub.1, C.sub.2 are used in
this LCR oscillator 10, rather than the single capacitor that is
typical of the prior-art, the oscillator output resembles the
Caduceus, hence it is useful to refer to the circuit of the present
invention as a Caduceus oscillator 10.
[0029] The two capacitors C.sub.1, C.sub.2 prevent the common-mode
noise passed through the common-mode choke 102 from being converted
into differential noise, as occurs in conventional oscillator
circuits used for inductive sensors. In the illustrated embodiment,
the common-mode noise from the common-mode choke 102 appears as a
common-mode voltage at the Caduceus oscillator outputs 304a, 304b.
This common-mode voltage is rejected with a high attenuation by
either the differential comparator 106 or the differential
sample-and-hold circuit 200. Those skilled in the art will
recognize that while the common terminal 306 of the two capacitors
C.sub.1, C.sub.2 is depicted as connected to a biasing voltage
V.sub.HS at one-half of the power supply voltage in FIG. 3, the
common terminal 306 can be fixed at a different voltage potential,
if desired. By using a biasing voltage V.sub.HS, it is possible to
use a single positive supply voltage supply without the need for a
negative voltage supply and, further, prevents the voltage between
the capacitors C.sub.1, C.sub.2 from floating.
[0030] The excitation circuit 108 includes two pairs of switches
302a, 302b arranged in a bipolar charging arrangement with one
switch S.sub.1, S.sub.3 of each switch pair 302 connected to a
voltage supply V.sub.S and the other switch S.sub.2, S.sub.4
connected to the ground. The switch pairs 302 are arranged in a
totem-pole configuration. In one embodiment, the switches
S.sub.1-S.sub.4 are implemented using power metal-oxide
semiconductor field effect transistors (MOSFETs); however, those
skilled in the art will recognize that other switches may be used
without departing from the scope and spirit of the present
invention. The voltage supply V.sub.S is a direct current (DC)
power supply producing a positive voltage and, typically, the
supply voltage to each switch pair 302 is the same.
[0031] There are several allowable switching configurations of the
switches S.sub.1-S.sub.4 In FIG. 3. First, all switches
S.sub.1-S.sub.4 are opened to allow the Caduceus oscillator 10 to
oscillate freely. Second, the ground-connected switches S.sub.2,
S.sub.4 are closed to quench the Caduceus oscillator 10 to ground.
Third, the voltage supply connected switches S.sub.1, S.sub.3 are
closed to quench the Caduceus oscillator 10 to the positive supply
voltage. Finally, a voltage-supply-connected switch S.sub.1,
S.sub.3 from one switch pair 302 and ground-connected switch
S.sub.2, S.sub.4 from the other switch pair 302 are momentarily
closed to charge the Caduceus oscillator 10 to a given polarity
depending on the desired direction of current flow through the
excitation circuit 108. For ease of discussion, the set of switches
represented by switches S.sub.1 and S.sub.4 and the set of switches
represented by switches S.sub.2 and S.sub.3 will be referred to as
the first set and the second set, respectively. It is undesirable
for both switches of either switch pair 302 to be closed
simultaneously, i.e., both S.sub.1 and S.sub.2 closed or both
S.sub.3 and S.sub.4 closed.
[0032] When the Caduceus oscillator 10 is charged, the voltage on
the capacitors C.sub.1, C.sub.2 is initialized very close to the
power supply voltage and practically all of the accumulated noise
voltage from the previous cycle is eliminated. When the Caduceus
oscillator 10 is decoupled from the power supply and begins to
oscillate, any common-mode or differential noise currents present
in the circuit, integrated over observed time, produce an
accumulating noise voltage on the capacitors. Accordingly, it is
desirable to sample the Caduceus oscillator outputs 304 soon after
the power supply is decoupled so that less accumulated noise will
be present [i.e., the accumulated noise increases while the
sinusoid decays].
[0033] FIG. 4 illustrates a simplified schematic diagram of the
Caduceus oscillator 400 shown in FIG. 3. The equation describing
the Caduceus oscillator 400 in the time domain is 1 v c ( t ) = [ v
1 ( 0 + ) + v 2 ( 0 + ) ] ( R 2 L t 1 + ( R 2 L 1 LC s - ( R 2 L )
2 ) 2 cos [ 1 LC s - ( R 2 L ) 2 t - tan - 1 ( R 2 L 1 LC s - ( R 2
L ) 2 ) ] ) u ( t ) ( 1 )
[0034] where 2 C s = C 1 C 2 C 1 + C 2
[0035] If the circuit is very underdamped, the differential voltage
input to the comparator can be rewritten as 3 v c ( t ) = [ v 1 ( 0
+ ) + v 2 ( 0 + ) ] ( R 2 L t cos ( t LC s ) ) u ( t ) ( 2 )
[0036] The inputs to the differential comparator are illustrated
graphically in FIGS. 5a-5c. FIGS. 5a and 5b represent the
respective inputs to the positive and negative terminals of the
differential comparator. FIG. 5c illustrates the input of FIG. 5a
overlain with the input of FIG. 5b. The sinusoidal decay produced
by momentarily closing the first set of switches S.sub.1, S.sub.4
is depicted at t=0.3 milliseconds and t=0.9 milliseconds and the
opposite polarity sinusoidal decay produced by momentarily closing
the second set of switches S.sub.2, S.sub.3 is depicted at t=0.0
milliseconds, and t=0.6 milliseconds in FIGS. 5a-5c.
[0037] When a differential comparator 106 is used, the frequency of
the Caduceus oscillator 10 is measured by timing the comparator
output pulse-train, as illustrated in FIG. 5d. One method of timing
the pulse-train is counting a high-speed clock 116 with a counter
114 gated by the pulse-train, as illustrated in FIG. 1. The counter
114 can be gated on the leading edge, the trailing edge, or both
the leading and trailing edges of the comparator output
pulse-train. In one embodiment the counter 114 is gated on both
edges. In the illustrated embodiment, two or more gated counts are
added together to produce a summed count having reduced
quantization error relative to any single gated count. However, it
should be noted that, for a pulse-train having a given number of
zero-crossings per cycle, only the second half of the counts
captured by the counter 114 are summed because discontinuities
arise in the inductive signatures when more than the last half of
the captured counts are double integrated.
[0038] Bipolar charging is useful for active isolation of the
Caduceus oscillator frequency measurements from ambient noise 606
and crosstalk errors 608, as in FIG. 6, which illustrates inductive
signature data obtained using an array of wire-loop sensors and the
Caduceus oscillator of the present invention. Because induced
currents are a significant cause of most ambient noise 606 and
crosstalk errors 608, these errors have a distinct polarity. This
is especially true for differential errors, which are the most
difficult errors to deal with. On the other hand, the frequency of
the oscillation is independent of its polarity.
[0039] Ambient noise 606 is actively isolated from the oscillator
frequency 602a, 602b, 604a, 604b measurement by alternating the
polarity of the excitation voltages on successive frequency
measurement cycles. By alternating the excitation polarity, errors
due to low-frequency ambient noise 606 become largely equal and
opposite on alternating measurement cycles. Low-frequency ambient
noise 606 is effectively canceled by adding any two adjacent and
oppositely polarized inductive measurements together.
Alternatively, low-frequency ambient noise is quantified by
subtracting any two adjacent and oppositely polarized inductive
measurements from each other.
[0040] Crosstalk 608 between two or more inductively coupled
wire-loop sensors induces a non-random error into the frequency
measurement of each detection cycle. This non-random error has a
polarity attribute similar to that of ambient noise. Crosstalk
errors are cancelled by alternating the relative polarity of the
excitation circuits of inductively coupled detectors. By adding any
two or more adjacent samples obtained from inductively coupled
wire-loop sensors having oppositely polarized excitation circuits,
crosstalk is substantially cancelled and by subtracting any two or
more adjacent samples together the effect of the crosstalk error is
quantified. Those skilled in the art will recognize that other
mathematical functions for combining successive samples derived
using different phase-permutations of the excitation circuit 108
are within the scope of the present invention.
[0041] The bipolar excitation circuit 108 produces two polarity, or
phase, permutations for the wire-loop sensor. For an array having
any number, n, of inductively coupled wire-loop sensors, each
having a bipolar excitation circuit, it is possible to actively
isolate the frequency measurement error due to any crosstalk path
within the array. By way of example, consider an array of two
inductively coupled wire-loops, as shown in FIG. 7. FIG. 7
illustrates the wire-loop sensor array having two 2-meter
wire-loops A, B separated by a distance of approximately 2-meters
and a vehicle C. If wire-loop A and wire-loop B are simultaneously
excited and allowed to oscillate with any arbitrary set of
polarities (e.g., loop A, negative polarity and loop B, negative
polarity, there would be non-random errors, e[.function.(A)],
e[.function.(B)], in the frequency measurements, .function.(A),
.function.(B), due to crosstalk on the inductively coupled
path.
[0042] Referring now to FIG. 6, a set of four inductive signatures
602a, 602b, 604a, 604b were recorded from the automobile C passing
over the wire-loop sensor array A, B. The present invention
actively isolates the non-random frequency measurement errors due
to crosstalk for the inductively coupled wire-loops by proper
selection of the excitation phases of each wire-loop sensor A, B in
the array on successive samples. In the inductive signature data
shown in FIG. 6, the non-random frequency measurement errors are
actively isolated by exciting the array on successive cycles with a
different set of phase permutations, e.g., wire-loop A, positive
polarity; wire-loop B, negative polarity and wire-loop A, negative
polarity; wire-loop B, negative polarity. Providing inductively
coupled wire-loop sensors A, B within an array with differing
polarities inverts the non-random frequency measurement errors due
to crosstalk and the non-random frequency errors are inverted
relative to the original phase permutation. The central traces
602c, 604c represent the estimated inductive signatures after
crosstalk errors 608 have been accounted for through active
isolation by averaging each pair of traces 602a, 602b; 604a, 604b.
The trace pairs 602a, 602b; 604a, 604b represent the raw inductive
signature data recorded by the wire-loop sensor circuits as each of
the possible excitation circuit phase permutations were sequenced
through on successive samples and exemplifies crosstalk
quantification through active isolation. The sinusoid lines
overlying the trace pairs represent the ambient noise 606. Those
skilled in the art will recognize that crosstalk may be
substantially canceled or quantified by active isolation in an
array having any number of inductively coupled wire-loops through
the use of suitably chosen phase-permutation sequences similar to
that described for a two-loop array.
[0043] The vertical scale for these signatures represents
frequency. In the absence of a vehicle, these two wire-loop sensor
circuits have significantly different oscillation frequencies. As
the test vehicle passes over the loops, the frequencies converge
indicating that the crosstalk errors 608 are partially a function
of the relative frequencies of the two oscillations.
[0044] Contrast the inductive signature data obtained using the
present invention with the inductive signature data obtained with a
prior art LCR oscillator circuit, as shown in FIG. 8. In the prior
art, the inability to vary the polarity of the oscillations between
measurement cycles removes the ability to eliminate crosstalk
errors between inductively coupled loops as only one set of raw
data 900 at a single polarity is obtained. Further, the sampling
rate of the prior art circuit is limited to the power line
frequency and ambient noise above the power line frequency can not
be eliminated. Trace 902 represents the true inductive signature
for comparison with the raw data 900.
[0045] While one 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 scope of the invention as
defined in the appended claims.
* * * * *