Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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; FIELD OF THE INVENTION
This invention is directed toward improved road
vehicle sensing apparatus.
BACKGROUND OF THE INVENTION
Vehicle sensing apparatus is employed to provide
a count of vehicles using a road or highway, usually within
a specific time interval. These vehicle counts can be employed ;~
to provide vehicle traffic pattern data and/or to control the
flow of vehicle traffic, locally or regionally, via traffic
signals. The sensing apparatus can also be employed to provide
data regarding the type and/or speed of each vehicle sensed if
~ desired.
j Many vehicle sensing systems, for carrying out the
above, are known. Some systems employ buried magnetic loops
or pneumatic tubes and plates. Others employ various sonic or
~ optical means. Still others employ electromagnetic waves
-1 reguiring both a transmitter and a receiver. Thçse known
systems are however, generally relatively complicated, expensive
and bulky. In addition, the known systems are n~t always
reliable in use under all operating conditions.
STATEMENT OF THE INVENTION
It is the purpose of the present invention to provide
an improved vehicle sensing apparatus which is relatively simple
and compact. It is a further purpose to provide an improved
vehicle sensing apparatus which operates reliably under all
weather conditions, day or night.
The improved road vehicle sensing apparatus of the
present invention is particularly charac~erized in that it
employs the principle of microwave radiometry.
jl~ 30 Microwave radiometry can be defined as the passive
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detection of signals of thermal origin having wavelengths
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between one hundred centimeters and one millimetl~r. An ultra
sensitive receiver is used to detect the signals picked up by
a directional antenna, the beam of which is directed at the
area to be observed. The magnitude of signal received can be
generally said to be proportional to the temperature of, and
reflected by, the area, or object in the area, observed.
As applied to vehicle sensing, the temperature signal
received from an empty area of roadway would be different from
the temperature signal received from a vehicle in the same
roadway area since the vehicle reflects the sky temperature
to a greater degree than the roadway. It is this principle
in general which is employed in the present invention to provide
an effective, reliable, vehicle sensing apparatus.
Broadly, the vehicle sensing apparatus of the present
invention comprises an antenna for collecting microwave tempera-
ture signals from an area of roadway, and means connected to
the antenna for processing the collected signals to indicate
, the presence of a vehicle in the roadway area due tG a change
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q in the magnitude of the temperature signals collected which
change is caused by the presence of the vehicle in the roadway ~-
area. ;
The processing means preferably includes means ~or
detecting the amplitude of the signal to provide an indication
of the size of vehicle sensed.
.
The processing means also includes means for providing
a count of the vehicles sensed.
BRIEF DESCRIPTION OF THE INVENTION
The invention will now be described in detail having
reference ta the accompanying drawings in which:
Figure l is a schematic view of a simple sensing
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~ apparatus of the present invention;
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Figure 2 is a schematic view of a more sensitive
sensing apparatus;
Figure 3 is a schematic view of a preferred sensing
apparatus;
, 5 Figure 4, appearing on the same sheet as Fig. 1, is
a graph illustrating the nature of the signals received by the `~apparatus;
Figure 5~ appearing on the same sheet as Fig. 1, is
a schematic view of one form of sensing apparatus for sensing
vehicles in a plurality of lanes; and
Figure 6, appearing on the same sheet as Fig. 1, is
a schematic view of another form of sensing apparatus for
sensing vehicles in a plurality of lanes.
DESCRIPTION OF A PREFERRED EMBODIMENT
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1~ The vehicle sensing apparatus 1 of the present
invention, can, in its simplest form as shown in Fig. 1,
comprise a directional antenna 3 mounted above a road or ;~
' highway 5 for picking up temperature signals fro~ an area 7
of the highway 5, and means 9 for processing the received
signals. The processing means 9 essentially comprises, a
signal detector 11, connected to the antenna 3, ~or detecting
the temperature signals, an amplifier 13 for amplifying the
detected signals, a threshold circuit 15 for passing signals
of a predetermined magnitude indicating the detection of a
vehicle, and a counter 17 for providing a count of the detected
vehicles. ~ -The area 7 of the highway, covered by the beam 19
of the antenna 3, is substantially equal to the area of a car
21 so that maximum utilization is made of the detected signal.
In this arrangement a car 21 gives the same signal as a truck `~
23 which is larger than the car. The apparatus 1 can of course,
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in this arrangement, provide a total co~lnt only of the vehi-
cles sensed, without distinguishing between cars and trucks.
The temperature signal, T received by the antenna 3
from area 7 is in general a combination of three temperature
signals. More specifically the temperat~re signal T is given
by the equation:
T = aTa + sTs ~ bTb
where a, s and b are emissive, reflective and transmissive
coefficients respectively, and Ta~ Ts and Tb are the ambient
temperature of the area, the reflected sky temperature from
the area and the background temperature of the area, re-
spectively. In viewing area 7 alone, being a part of the
roadway 5, the ambient and transmissive values are relatively ~
high and the reflective value is low. The temperature signal ~ `
in this case might be representative of a temperature in the
range between 200K and 300K. When a car or truck is viewed -
in this area however, the signal received, because the car or
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truck is reflective is primarily the reflected sky temperature
which is relatively low and might be representative of a
temperature in the range between 10K and 50K. The differ-
ence in the values of the temperature signals received oper-
ates signal detector 11 in a manner to generate a signal in
the system indicative of the presence of a car or truck.
To obtain sharper signal definition, the processing
i 25 means 9 shown in Fig. 1 can instead comprise a radiometer 109,
l such as the one commonly known as a Dicke T.M. radiometer shown
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in Fig. 2. The radiometer 109 operates by comparing the signal
at the antenna 103 with that from a reference road 125 alterna-
tiyely by operation o~ a switch 127. Both signals are detected
by a detector 129 and either directly, or after conversion to
an IF frequency, are ampli~ied by an amplifier 131 to a
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convenien~ level to assure that only desired signals are
observed. The amplifier 131 is followed by a synchronous ~
detector 133, which is sensitive only at the switching rates
of switch 127, and by an integrator 135, a threshold circuit
137 and a counter 139 as before.
While the time sharing of the antenna 103 reduces
the signals by one half, the detection process greatly enhances
the detection of weak signals and reduces unwanted noise and
gain fluctuations. In many instances a second switch, not
shown, is placed between the reference load 125 and the first
switch 127, such that the load may be compared to a second
I load of a known controlled temperature. In this manner signals
can be derived from a second synchronous detector to be used
as a calibration value by placing the second switch in the
port containing the load. The degradation in the sensitivity
, of the system is minimized as ~he signal path length still
~¦ contains but one switch.
The temperature sensitivity of a miorowave radiometer
used for ground mapping is normally defined by the following
¦ 20 expression9 (which relates to a signal-to-noise valu of unity)
, aT = K NF To
~t
where l<K<2. For most Dicke radiometers K is approximately
equal to 2 . NF relates to the noise figure of the radiometer9
To is its ambient temperature (usually 290K), B is the pre-
detection bandwidth, and t is the integration time in seconds.
l As can be seen from the expression, sensitivity is proportional
¦ to the noise figure of the system but inversely proportional to
I the square root of the predetection bandwidth and integration
time,
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Atmospheric attenuation plays a large role in the
radiometric temperature seen by a microwave radil~meter. Thus,
it is exceedingly important that the proper operating frequency
be selected within one of the so-called window fnequencies in
the upper centimeter or lower,millimeter portion of the
electromagnetic spectrum. Window frequencies are character-
istics of an attenuation curve as a function of frequency for
electromagnetic waves in the atmosphere. Window frequencies '~
below 10 GHz are not suitable as the aperture si~es necessary
'lO for good resolution are too large. Above 90 GHz, the
sensitivity of the system is so low and the cost so great
that all-weather capabilities cannot be fully utilized.
Frequency bands at 10 GHz and 35 GHz are a good compromise
~ because they are within a window frequency. ',
3 15 A preferred system 201, utilizing a 10 GHz radiometer
'I is shown in Fig.3. This system is also designed to distinguish
, types of vehicles as will be described. The system has an '~
antenna 203 of a size and beamwidth commensurate with the
, height of the system and the vehicle size to be detected. A
horn antenna with a beamwidth of 32 at the 3db points can be
' used for example. The traffic system is suspended over the
highway 205 at a distance which allows a truck 207 to fill the
area 209 on the highway illuminated by the beam 211 when '
`'I passing under the sensor. This is the area encompassed on the
' 25 highway by the 3db beamwidth of the antenna. When the traffic
I sensor is observing the highway 205 in the absence of a vehicle,
it views the radiometric temperature of the highway which is
~! Tr = Tke ~ ''
Where Tr ~ radiometry temperature '~
Tk = kinetic temperature
e - emissivity of the highway
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This temperature on a warm summer day is approxi-
mately 200~ Kelvin. When a truck 207 enters the beam 211, the
truck 207 reflects the sky temperature which is approximately
10K. This gives rise to a temperature signal oF 190K. This
signal (centered at the band around 10 GHz in the present
system~ is collected by the antenna 203 and is ct~upled to a
pin diode switch 213, toggled by a square wave of plus and
minus one volt at a rate of 1 KHz. In the closed position it
has an attenuation of 2db and in the open position its
attenuation is 60db. In one position of the switch 213 it
connects the antenna 203 to the isolator 215. In the other
position it connects a reference load 217, which it is at
ambient temperature, to the isolator 215. Appearing at the
isoiator 215 is a square wave signal of an amplitude pro-
1 15 portional to the difference between the antenna signal (antennabrightness temperature) and the reference load. The isolator
215 couples the signal to the mixer-preamplifier 219. The
isolator has a forward attenuation of 0.3db and la reverse
attenuation (isolation) of 20db, resulting in a low attenuation
for a signal passing from the antenna to the mixler-preamplifier.
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1 However, a s;gnal passing from the mixer to the ,~ntenna is ~ ;~
attenuated heavily. Since the receiver is a supler-heterodyne
l type, the local oscillator 221 supplies energy at or near the
-l incoming signal frequency. Some of this energy used to down-
convert the incoming signal is coupled through the mixer to
the antenna. If there is a mismatch between the antenna and
mixer, energy is reflected back to the mixer and is down
converted as a signal. By placing the isolator 215 between
the switch 213 and mixer 219, the energy coupled out of the
mixer is absorbed by the isolator preventing the energy from
getting to the mismatch and re~lecting back to the mixer as
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a signal. The attenuator 223 reduces the local qscillator
signal to the proper power level for the mixer. It also
provides isolation between the mixer 219 and local oscillator
221 and prevents local oscillator pulling with s1gnal amplitude.
The down converted signal (10 MHz to 4~0 MHz) is
amplified by the preamplifier 219, having an overall gain of
23db. This signal is coupled to a main IF amplifier 225, with
a gain of 43db and a bandwidth of over 500 MHz. The output of
the IF amplifier 225 is coupled to second detect~r 227 which
j10 envelope detects the double side band IF signal, producing DC
and a noisy 1 KHz square wave. The second detector 227 has a
tangential sensitivity of -52 dbm when coupled t~ an amplifier
with a noise figure (NF) of 2db and a bandwidth of 2 MHz. The
output is AC coupled to a low noise (NF 2db) witb a bandwidth
¦15 of 30 KHz. Its output feeds a synchronous detector and
JI integrating amplifier. The overall gain of this amplifier
chain 229 is 60db. After synchronous detection and integration,
I the output signal is DC with an AC signal (noise). The AC
; signal represents an rms variation about the DC signal.
¦ 20 The output of the synchronous detector and integrator
229 is coupled to a dual threshold circuit block 231. As a car
or a truck enters the beam, the resulting temperature seen by
~' the antenna is the average of the ground and sky temperatures
weighed by the rdtlo of the surface of the vehicle to the total
area. Total temperature is given by the expression
Ttot - Te Av ~Tv - Te)
Where Te = temperature of earth or highway
v ~ temperature of vehicle or sky temperature
Av = area of vehicle
Atot = area of beam on highway
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For any given time Te can be considered a constant and Ttot
changes by the ratio Av Since a truck and a car have
tot
different areas, the amplitude of the s-igna1 is proportional
to the areas.
~ 5 This amplitude difference is detected by the dual
; threshold circuits 231 which consist of parallel comparators
having their thresholds set for car and truck amplitude
respectively. The output of each comparator is ooupled to
their respective counter. Since a car has a smaller area than
a truck~ its signal will only exceed the first threshold and
one counter 233 will count. A truck whose signal is larger
than a car will exceed both thresholds and will cause both
counters 233, 235 to count. The low threshold counter counts
total vehicles and the high threshold counter only trucks.
By subtracting the count for trucks from total v,ehicle count,
the total car count can be obtained.
~, The counter outputs are fed to seven segment decoders
:;! which drive seven-segment pin lights for a visual display 237.
A clock circuit drives a seven segment pin light to show
elapsed time as well in the display 237.
A multivibrator 239 generates a square wave signal,
used to toggle the switch 213 and the synchronous detector 229.
Fig. 4 illustrates the type of signals received by `~
the system 201 just described when sensing vehicles in a time
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interval. The signals illustrate graphically, small vehicles,
~ small trucks and large trucks and trailers. The amplitude of
j the signals are proportional to the size of the vehicles.
If it is desired to measure the velocity of each
vehicle sensed3 the system shown in Fig. 3 can be modified to
do so. The system can include an operational amplifier in a
differentiating mode to measure the slope of the vehicle signal
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received. The slope in the signal is due to the fact that the
signal varies in amplitude at a rate proportionall to the
velocity at which a vehicle enters the antenna beam. In
addition, means are provided for measuring t-he amplitude of
the signal since the slope is also prop~rtional to this
amplitude. The combination of these two measurements gives
the velocity of the vehicle sensed.
The system of Fig. 3 can be used as we`ll to identify
emergency vehicles. This is useful if the system is controlling
traffic lights so that upon sensing an emergency vehicle, the
system will set the traffic lights to give the vehicle a non-
stop route. To identify emergency vehicles the ~ystem is
frequency multiplexed and equipped with a PCM demodulator.
Approximately four megahertz of the system bandwidth
IS will be used to receive radiated PCM signals transmitted by an
emergency vehicle or vehicle to be identified. The PCM
demodulator will decode the PCM signal and operate the traffic
light or interrogate a control computer. To interrogate an
emergency vehicle the local oscillator can be coupled to the ;
antenna through a power split~er and perform as a low power
PCM transmitter.
It should be noted that the vehicle count operation
observes "cold" signals about an average background. The
e~ergency signal is a "hot" radiated signal which is easily
separated and distinguished ~rom the cold vehicle signal.
Conversely, the hot signal does not interfere with or cause
a count in ~he vehicle count mode.
The systems have been described as sensing vehicles
in one lane of traffic. The antenna could however be used to
` 30 sense traffic in two or more lanes simultaneously. As shown
in Fig. 5 the antenna 401 has a beam width to cover three
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traffic lanes 403, 405 and 407. A vehicle in any lane is
sensed by signal processing means 409 and counted by a counter
411. This system gives an average vehicle count for each
lane (by dividing the total counter by the number of lanes).
I~ a traffic count for each lane is desired, the
system shown in Fig. 6 can be used. Here an antenna 501, 503,
- 505 is provided for each lane 507, 509, 511 respectively. A
counter 513~ 515, 517 is also provided for each lane S07, 509,
511 respectively. A single processing means 519 is provided
connected by switches 521, 523 to the antenna and counters
respectively. The switches are operated in a time-sharing
mode to connect each antenna to its reSpeCtiYe counter.
Hence, a microwave radiometric traffic sensor made
in accordance with the present invention is a simple passive
all weather device. It does not radiate energy to generate
a signal. It also operates day or night and in every type
of weather. It does not require modification to the road bed ~;
nor does it require surface sensors. It has the capability
to distinguish various catagories of vehicles by the amplitude
and shape of the signal. It can measure velocity. Road
wetness can be determined by monitoring the road temperature.
It does not have the disadvantage of pressure sensors or
treadles which become inoperative in snow or ice. It will not
wear out by constant vehicle passage. Optical and infrared -;
devices become inoperative in fog since the attenuation of the
radiation becomes quite high. Doppler radar devices are all-
weather but exhibit problems in resolution both in distance
and amplitude when se t-up for appropriate speed measurement
and resolution.
While specific types of antennas and signal
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processing means have been described in this application, other
suitable antennas and signal processing means ma\~ be used as
well, without departing from the scope of the invent~on.
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