Note: Descriptions are shown in the official language in which they were submitted.
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1
INDUCTIVE LOOP SENSOR AND METHOD
OF MANUFACTURING SAME
BACKGROUND OF THE INVENTION
This invention relates generally to an inductive loop sensor for
determining the movement or presence of an object, in particular to an
inductive
loop sensor having improved strength, and more particular to an inductive loop
sensor that is used in a roadway for sensing movement or presence of
automobiles.
Due to their proven performance characteristics, reliability, and low cost,
inductive loop sensors are widely employed for traffic monitoring and control
systems. For example, many stoplights have inductive loop sensors embedded
within the asphalt in order to determine when to change the stoplight based,
for
example, on a detected number of automobiles waiting for the stoplight to
change.
Generally, the inductive loop sensor is permanently installed either under new
roadway during construction or into a saw-cut trench cut into the roadway
surface.
In any installation, the inductive sensor loop may be exposed to very high
temperatures. The inductive loop sensor embedded in the roadway must survive
repeated compression forces due to traffic crossings, because replacing an
inductive loop embedded within the asphalt or concrete is difficult at best
and
interrupts traffic on the road while the repair occurs.
During new roadway construction, the inductive loop sensor is usually
positioned on the road bed, then hot asphalt is laid over the inductive loop
sensor,
covering the inductive loop sensor. As the roadway deteriorates, the roadway
may
be repaved over the inductive loop sensor. Therefore, it is necessary for the
inductive loop sensor to survive the paving and repaying process intact. Most
conventional preformed inductive loops are sufficiently fragile so that they
are
frequently broken and rendered inoperative by normal traffic moving over the
inductive loop or by the paving/repaving of the road during the installation
of the
inductive loop sensor or during repair operations of the road.
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In existing roadways, the inductive loop sensors are placed in a saw-cut
trench that is cut into the roadway. Once in the trench, the inductive loop
sensor is
covered and sealed in the roadway with hot asphalt or sealant chemicals. As
cars
travel over the trench, the sealant or asphalt becomes cracked, allowing
exposure
of the inductive loop sensor to changes in environmental conditions, such as
temperature and humidity. Most conventional prefonmed loop sensors are rigid
and not adjustable in size or circumference, therefore, the saw-cut trench
must be
larger than required to allow for manufacturing tolerances. Ideally, the saw-
cut
trench should be as narrow as possible.
The permanent installation of an inductive loop sensor under new
roadway, or the temporary installation of an inductive loop sensor on the top
of an
existing roadway, does not impose any of the specific dimensional requirements
on the inductive loop sensor as the saw-cut installation process does.
However,
they all must survive the installation process. Most inductive loop sensor
failures
occur during the installation process due to the exposure to high temperature
from
the molten asphalt which is laid down on top of the inductive loop sensor or
due to
the repaying of the road. In addition, the inductive loop sensor must permit
strict
control of the loop's cross-section geometry and electrical properties, since
these
properties affect the accuracy of the signal generated by the inductive loop
sensor.
Therefore, it is desirable to provide an inductive loop sensor that can
survive repeated compression forces due to traffic crossings, be unaffected by
changes in environmental conditions, such as temperature or humidity, be able
to
withstand abrasive materials and high temperature encountered by the loop
during
the paving or sealing process, and to be resistant to the sealant chemicals
often
used in paving and to the petroleum-based contaminants generated by vehicles
traveling over the inductive loop sensor. Finally for the saw-cut
installation, it is
desirable that the inductive loop sensor have a narrow cross-section and be
adjustable to fit into the trench formed by the saw-cut.
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SUMMARY OF THE INVENTION
The invention provides a, rugged preformed inductive traffic loop sensor
based on the use of cables that are readily available in mass production
quantities
which reduces the overall cost of the inductive loop sensors. The inductive
loop
sensors are installed either in trenches formed by saw-cuts in the road
surface or
under new roadway and paved over with material such as asphalt or concrete.
The
inductive loop sensor may be exposed to very high temperatures during
installation, so high-temperature insulation materials are used to protect the
internal conductors and wires. In addition, during the repaying of a road when
the
road surface is scrapped to form a rough surface for the new asphalt, the
inductive
loop sensor has a sufficiently low profile so that the scraping process does
not
destroy the inductive loop sensor.
For the saw-cut installation, a loop with a narrow (e.g., < 0.25 inches)
cross-section is constructed, with either a flat cable design or a thin round
cable
that is flexible enough to fit into a trench formed by the saw-cut. Because
most
inductive loop sensor failures occur during the installation process, the
inductive
loop sensor is rugged. The inductive loop is also resistant to environment
conditions, such as temperature or humidity, due to the selection of the
insulation
and jacketing materials and the unique construction. In addition, the
inductive
loop sensor uses a pre-fabricated cable for the inductive loop cable so that
the
cross-section and the electrical properties of the loop may be tightly
controlled. In
addition, the cost of the sensor is reduced by utilizing materials and
manufacturing
processes that are normally used in production of cables and cable assemblies.
For
example, the cable components of the preformed loop are produced using
standard
extrusion methods and may be inexpensively produced in large quantities even
when high-performance insulation/jacketing materials are used.
In summary, the inductive loop sensors in accordance with the invention
are relatively inexpensive, easily manufactured, easily used in any type of
loop
sensor installation, use high-performance materials that have proven track
records
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4
and are readily available, are designred to survive roadway regaving
operations,
and are dosig~aed to have a long service life,
Grognet in Ele~ctronzq~ae hsatustrielle (1969, 03104), 142, 207!211 describes
a. vehaclc prcsenco dGte~ctor l~v~ a loop sensor with gluial sets of windings
which is connectod by a three conductor cable to an electronic unit for
detoctioa of
the presence or passage of automobiles.
pt~pEND~D SHE~~
Cr$r Ceryt~DU3s.~7sG.1
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a perspective view of a flat cable inductive loop sensor that
embodies an illustrative example of the invention for saw-cut installation;
Figures 2A - 2C are detailed diagrams showing a flat cable inductive loop
5 sensor for saw-cut installation in accordance with one embodiment of the
invention;
Figure 3 is an enlarged view of a round cable inductive loop sensor that
embodies another illustrative example of the invention for saw-cut
installation;
Figures 4A - 4C are detailed diagrams showing a round cable inductive
loop sensor for saw-cut installation in accordance with another embodiment of
the
invention;
Figure 5 is a perspective view of an inductive loop sensor that embodies an
illustrative example of the invention for installation on top or underneath a
new
road surface;
Figures bA - 6D are detailed diagrams showing a flat bottom cable
inductive loop sensor for new roadway installation in which the internal
conductors are stacked vertically;
Figures 7A- 7D are detailed diagrams showing a round cable inductive
loop sensor for new roadway installation in which the internal conductors are
in a
triangle shape;
Figure 8 is another embodiment similar to Figure 7 showing a flat bottom
cable inductive loop sensor in which the internal conductors are in a triangle
shape.
Figure 9 is an enlarged view illustrating the adjustable section of the
inductive loop sensor for saw-cut installation;
Figure 10 is a cross-sectional view of a round inductive cable loop in
accordance with the invention;
Figure 11 is a chart illustrating the inductance from flat and round cables
with 1 to N loop turns of cable;
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Figure 12 is a chart illustrating the sensitivity of the inductive loop sensor
in accordance with the invention with one to N loop turns of cable;
Figure 13 shows examples of saw-cut shapes in which the inductive loop
sensor may be installed; and
Figures 14A-14D show various cross-sections of a roadway with various
embodiments of the inductive loop sensor installed.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention is particularly applicable to an inductive loop sensor and
method of manufacturing the same for installation in a saw-cut trench in an
existing roadway or underneath a new roadway used to measure the presence or
movement of traffic flow. Figure 14 shows various roadways with the inductive
loop sensor installed. Figure 14A shows an existing roadway with a saw-cut
trench with the inductive loop sensor installed. Figure 14B shows a new
roadway
where the inductive loop sensor is placed on a roadbed and then paved over.
These are the two types of inductive loop sensors that will be detailed below.
In
addition, there are other roadway installations for which the present
invention may
be used. Figure 14C shows the inductive loop sensor placed on an existing
roadway with new paving covering the sensor (this would be similar to Figure
14
B). Figure 14D shows the inductive loop sensor placed on top of an existing
road
without being paved over. This is envisioned to be used for short periods of
time
due to the sensor being in direct contact with vehicles and the environment.
It will
be appreciated, however, that the sensor and method or manufacture in
accordance
with the invention has greater utility. For example, the inductive loop sensor
may
also be used to detect the presence or movement of an aircraft or ground
vehicle at
an airport.
Figure 1 is an illustrative embodiment of an inductive loop sensor 30 that
is designed to fit within a plurality of saw-cuts 32 made within a surface of
a
roadway 33. In this first embodiment, the inductive loop cable is a flat loop
cable
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36 that fits into the saw-cuts 32. The flat loop cable 36 may be made from pre-
fabricated cable and have any number of different loops or turns of internal
conductors, but three turns are shown in the Figures. In the preferred
embodiment
of the inductive loop sensor 30, the loop cable 36 has three internal
conductors
connected together to form the three turns within the loop. In this
embodiment, as
described in more detail below, the inductive loop sensor 30 consists of the
flat
loop cable 36 and a lead-in cable 40, each having a protective covering, with
a
loop/lead-in connector 38 surrounding the conductors of the loop cable 36 and
wires of the lead-in cable 40 where they are joined together. As shown, the
connector 38 is narrow and fits into the saw-cut 32, which is typically 1/4"
wide.
The connector 38 is not a "T" connector, as is used with most conventional
preformed loops, but instead is a linear junction in which the wires of the
lead-in
cable 40 enter one end of the connector 38 and the ends of the conductors in
the
loop cable 36 enter the other end of the connector 38. This unique design of
the
inductive loop sensor permits the adjustment of the loop circumference to
match
the saw-cut. Shown in Figure 9, the first end 36a and second end 36b of the
loop
cable 36 enter the connector 38 from the same end. The first end 36a is
positioned
above the second end 36b in a substantially vertically oriented plane. Because
of
this positioning, when the first end 36a and second end 36b are pushed
together to
fit in the saw-cut, they are aligned in a vertical plane. This makes the loop
cable
36 fully adjustable to fit a variety of saw-cut configurations, shown in
Figure 13.
In use, the loop cable 36 is placed in the saw-cut 32. Any excess loop
cable 36 is the placed in lead-in saw-cut 31. Because the first end 36a and
second
end 36b are stacked vertically, there is no need to make the lead-in saw-cut
31 any
wider than the loop saw-cut 32.
Figures 2A, 2B, 2C and 2D show various detailed views of the conductor
connection area of the inductive loop sensor 30. The sensor 30 includes a flat
cable 36 and a lead-in cable 40. The inductive loop sensor 30 also includes
the
connector 38, made of a molded plastic housing, that protects the connection
area
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for the flat loop cable 36 and lead-in cable 40 from damage. Shown in the
preferred embodiment, the flat loop cable 36 has three conductors 42, 44 and
46
within the cable which, when connected, form three turns. As described above,
the invention is not limited to any particular number of conductor turns. As
with
S the embodiment described above, to form the inductive sensor 30 having three
turns, the flat loop cable 36 is used having the two ends 36a and 36b. Each of
the
internal conductors also has two ends. To form the turns, each of the internal
conductors are attached to each other and to lead-in wires. To do this, a
first end
42a of conductor 42 is attached to a first wire 48 of the lead-in cable 40 at
a first
connection 52 and a second end 42b is attached to a first end 44a of the
conductor
44 at 54, forming the first turn. A second end 44b is attached to a first end
46a of
conductor 46 at 56, forming the second turn. And finally, a second end 46b is
attached to a second wire 50 of the lead-in cable 40 at a second connection
58,
forming the third turn.
By attaching the conductors in the flat cable and the wires in the lead-in
cable this way, a continuous path is formed from the lead-in wire 48 through
the
first conductor 42 ("first turn"), through the second conductor 44 ("second
turn"),
through the third conductor 46 ("third turn") and finally to the lead-in wire
50.
The various conductors and wires may preferably be soldered or spliced
together.
Figure 2B illustrates a cross-sectional view along line A-A, as shown in
Figure 2A. As shown and described above, the flat loop cable 36 has three
conductors 42, 44 and 46 within the cable. In this embodiment, the conductors
are
aligned to form a flat cable. As shown, cable 36 has a jacket layer 60 that
protects
the cable from damage. The material choices that may be used for this jacket
layer will be described below.
Figure 2C is a cross-sectional view along a line B-B as shown in
Figure 2A. As shown, the lead-in cable 40 has a first wire 48 and a second
wire
50. In addition, the lead-in cable 40 has a jacket layer 62 which protects
both of
the lead-in wires. The jacket layer 62 and the materials used for the
insulating
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layer will be described in more detail below. This flat cable embodiment has a
high sensitivity to object presence, which will be described below, and fits
into a
1 /4" saw-cut.
Figure 3 is another illustrative embodiment of the invention showing an
inductive loop sensor 130 that may be inserted into a saw-cut insulation.
Inductive loop sensor 130 is similar to the inductive loop sensor 30,
described
above in relation to figures 1 and 2, the difference being that a round loop
cable
136 is used instead of the flat loop cable 36. In particular, as shown in
Figure 3, a
plurality of saw-cuts 132, typically 1/4" wide, house the round loop cable 136
of
the loop and a lead-in saw-cut 131_ which may ales hp , ~n» ...:a.. w__.___ _
portion of the round loop cable 136, a connector 138 and lead-in wire 140. The
connector 138 is made of a molded plastic which surrounds the conductors of
the
loop cable 136 and the wires of the lead-in cable 140 where they are soldered
or
spliced together. The connector 138 also isolates the internal conductors in
the
loop cable 136 and the wires of the lead-in cable 140 from external
environmental
stresses. In this embodiment, loop cable 136 is constructed out of round cable
with a protective covering. The ends 136a and 136b of the round loop cable 136
are stacked on top of each other as they enter the lead-in saw-cut 131, as
shown in
Figure 3. As with the first embodiment shown in Figure 1, lead-in cable 140
exits
the connector 138 opposite the loop cables 136a and 136b. Additional details
about this round inductive loop cable 136 in accordance with the invention
will be
described in more detail below.
Figures 4A, 4B and 4C show various views of one embodiment of the
inductive loop sensor 130. As shown in Figure 4A, the inductive loop sensor
130
includes a molded connector 138 which surrounds and seals round loop cable 136
and lead-in cable 140 which are connected together. As described above, the
round loop cable 136 foams the inductive loop which measures the presence or
movement of an object near the inductive loop while the lead-in cable 140
transmits the signals generated by the inductive loops to a detector. In this
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embodiment, the inductive loop sensor 130 has three conductor turns. However,
it
should be apparent that the inductive loop sensor is not limited to any
particular
number of conductor turns and can be increased or decreased. The structure of
the
round loop cable 136 and the lead-in cable 140 will be described below in more
5 detail with reference to Figures 4B and 4C.
The connector 138 houses the connections of the various conductors in the
round loop cable 136 and the wires in the lead-in cable 140. As shown, the
round
loop cable 136 has three conductors 142, 144 and 146 within the cable. Thus,
to
form the three tum sensor, each of the internal conductors are attached to
each
10 other and the lead-in cable. To do this, a first end 142a of conductor 142
is
attached to a first lead-in wire 148 of the lead-in cable 140 at 152 and
second end
142b is attached to a first end 144a of conductor 144 at 154, forming a first
turn.
A second end 144b is attached to a first end 146a of conductor 146 at 156,
forming a second turn. And finally, a second end 146b is attached to a second
lead-in wire 1 SO of the lead-in cable 140 at 158, forming a third turn. By
attaching the conductors in the round cable 136 and lead-in cable 140 in this
way,
a continuous path is formed from the first lead-in wire 148 through the first
conductor 142 ("first turn"), the second conductor 144 ("second turn"), the
third
conductor 146 ("third turn") and finally the second lead-in wire 150. Thus,
with
this configuration, a three turn inductive sensor is formed. As described
above,
the invention is not limited to a three turn conductor and a different number
of
turns may be also used. The conductors and wires may be soldered or spliced
together. The connector 138 may be a molded plastic housing which may
surrounds and protects the connected conductors and wires from damage during
installation or use. The connector 138 also isolates the conductors and wires
from
the environment.
Figure 4B is a diagram illustrating a cross-sectional view along the line A-
A, as shown in Figure 4A. As described above, each round loop cable 136 has
three conductors 142, 144 and 146 within the cable. In this embodiment, the
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conductors may be twisted about each other and form a triangular type of shape
which, when surrounded by a jacket layer 160, form a round cable. The details
of
the jacket layer 160 of the cable will be described below.
Figure 4C is a cross-sectional view along a line B-B as shown in
Figure 4A showing the connector 138 with the lead-in cable 140. As described
above, the lead-in cable 140 has a first wire 148 and second wire 150. To
protect
the lead-in cable 140 from damage, the lead-in cable 140 also has a jacket
layer
162. The materials for the jacket layer 162 will be described below. As
described
above, the connector 138 may be thin enough to fit in a 1/4" wide saw-cut
trench
along with the loop 136 and lead-in cable 140 because the ends of the round
loop
cable 136a and 136b are stacked on top of each other as they enter the
connector
138.
Figure 5 is an illustrative embodiment of the invention showing a cable
inductive loop sensor 230 that may be installed on top of a new road bed,
which
will then be paved over with some sort of paving material such as asphalt.
With
this embodiment of the invention, the adjustment of the circumference of the
loop
is not as critical as the saw-cut embodiments discussed previously since the
loop
can be adjusted prior to laying down of the asphalt. In addition, the narrow
width
of the saw-cut, as required above, is not necessary. In this embodiment, the
inductive loop sensor 230 has a loop cable 236 which is secured down to the
road
bed surface prior to paving. The loop sensor 230 may be made of a loop cable
236
with any number of turns formed by conductors within the cable. The loop cable
236 may be made from a pre-fabricated cable. In the preferred embodiment, the
loop cable 236 has three conductors which form three turns. The conductors may
be insulated and jacketed for protection. In this embodiment, a connector 238
may be used forming a "T" junction in which a lead-in cable 240 exits the
connector 238 from an end of the T while one end of the round cable 236 enters
from one end while the other end of the round cable 236 enters an opposite end
of
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the connector 238 into each leg of the T. Additional details about this and
other
embodiments of the present invention will be described in more detail below.
Figures 6A through 6D are diagrams illustrating a first embodiment of the
pave over inductive loop sensor 330. In particular, with reference to Figure
6A, a
"T" shaped, molded connector 338 houses the attachment of the conductors of
the
loop cable 336 with the lead-in wires of the lead-in cable 340. The connector
338
protects the conductors of the loop cable and lead-in wires from damage during
installation and/or during use, and isolate the conductors and wires from the
environment. The loop cable 336 includes a first conductor 342, a second
conductor 344 and a third conductor 346 which are interconnected with each
other
to form a three turn inductive loop sensor. The loop cable 336 has a first end
336a
and a second end 336b with each internal conductor also having first and
second
ends. To form the turns, the internal conductors are attached to each other
and
with the lead-in wires. To do this, a first end 342a of conductor 342 is
attached to
1 S a first lead-in wire 348 of lead-in cable 340 at a first connection 352
and a second
end 342b is attached to a first end 344a of conductor 344 at 354, forming a
first
turn. A second end 344b is attached to a first end 346a of conductor 346 at
356,
forming a second turn. And finally, a second end 346b is attached to a second
lead-in wire 350 of the lead-in cable 340 at a second connection 358, forming
a
third turn. As seen in the figure, each end of the cable 336 enters opposite
sides of
the connector 338 with the first connection 352 opposite the second connection
358. As described above, in the event that additional or fewer turn of
conductors
is required, the connections between the various conductors will have to be
modified. The conductors may be soldered or spliced together. In this
embodiment for the pave over installation, the configuration of the "T"
connector
338 used has each end 336a and 336b of cable 336 entering the connector 338 on
opposite sides and the lead-in cable 340 exiting the connector from a side
direction.
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Figure 6B is a cross-sectional view along line A-A, as shown in Figure 6A.
As shown, the loop cable 336 includes the three conductors 342, 344, 346. In
addition, as shown in Figure 6D, the flat loop cable 336 also includes a
jacket
layer 360, which protects the conductors from damage during installation or
use.
In this particular embodiment, the loop cable 336 is formed in a domed shape
having a flat bottom 337. However, the shape is not critical to the invention.
Figure 6C is a cross-sectional view along line B-B, as shown in Figure 6A.
As shown in Figure 6C, the lead-in cable 340 has a first lead-in wire 348 and
second lead-in wire 350 which connect to the various conductors of the loop
cable
to form the various turns of the sensor. In addition, the lead-in cable 340
also
includes a jacket layer 362 which protects the lead-in wires from damage
during
use or installation. The details of the jacket layer and its materials will be
described in more detail below. Now, a second embodiment of the inductive loop
sensor in accordance with the invention that may be paved over during
installation
will be described.
Figures 7A through 7D illustrate a second embodiment of a pave over
inductive loop sensor 430, including a molded connector 438, a loop cable 436
and a lead-in cable 440, as shown in Figure 7A. As shown in Figure 7C, the
loop
cable 436 includes three conductors 442, 444 and 446 and a jacket layer 460,
which protects the conductors from damage during the insulation and/or during
operation. In this embodiment, the triangular layout of the three conductors
and
the jacket layer 460 forms a round cable. The details of the jacket layer 460
will
be described below. Figure 7D is a cross-sectional view along line B-B, as
shown
in 7B, of the connector 438 and the lead-in cable 440. As shown, the lead-in
cable 440 includes a first lead-in wire 448 and a second lead-in wire 450 and
a
jacket layer 462. The details of the jacket layer 462 will be described below
in
more detail. The loop cable 436 has a first end 436a and a second end 436b
with
each internal conductor having a first end (442a, 444a, 446a) and a second end
(442b, 444b, 446b). The attachments of the conductors and lead-in wires to
form
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14
the turns are the same as that described for the previous embodiment shown in
Figure 6A.
Figure 8 illustrates another embodiment of the inductive loop sensor 430
shown in Figures 7A-7D. The difference is that the triangular layout of the
three
conductors (442, 444, 446) form a loop cable having a flat bottom 437, similar
in
cross-section to Figure 6D.
Now, the construction of the inductive loop sensors, including the potential
jacket layer materials, details of the loop sensor (coil) design, the
specification of
the lead-in cable, the connector design, and the overall design of loop
assembly
will be described. First, the possible materials for cable insulation and
jacket layer
will be described.
Materials
There are a large and increasing variety of synthetic materials that are used
for wire insulation and jacketing. Taking into account the large range of
material
properties that may be obtained for each of the basic plastics through changes
in
formulation, the resulting list of potentially acceptable materials for
inductive loop
sensors may be quite extensive. Some of the key parameters for material
selection
include tensile strength, Shore hardness, temperature range in use, water
absorption resistance, abrasion resistance, weather resistance, chemical
resistance
and price. Temperature variations, other environmental factors, and aging may
also lead to large variations in the properties of plastics. Because of the
wide
variation in material properties for each of the materials, the more common
names
will be used.
Most of the candidate materials considered for the wire insulation and
protective jacket layer are either thermoplastics (i.e., these materials do
not set or
cure under heat), or thermoset material. Thermoplastic wire insulation may be
easily produced through the standard extrusion process, giving the
thermoplastics
a cost advantage over thermoset materials. Thermoplastics may be remelted and
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are thus very suitable for making molded sealed connections between parts made
of the same or similar material. In general, thermoplastics tend to be tougher
and
less brittle than thermosets but are much less dimensionally and thermally
stable.
It is a common practice in electrical cable manufacturing to use different
5 materials for electrical insulation of the conductors and wires, and for
jacketing
(environmental protection), thus combining the benefits of two different
materials
and often reducing the cost. In the inductive loop sensor for a traffic
application,
the electrical insulation requirements are not very strict, as the loop
operates under
low current and low voltage, so that usually an insulation rated for 500 V is
used.
10 It is, however, important that the electrical properties of the material do
not
change significantly with time.
Materials which may be used for jacketing/insulation include polyvinyl
chloride (PVC), polyurethane, polyolefin, polyethylene, polypropylene,
polyester,
cross-linked polyolefin, fluoroplastic, ETFE (brand name Tefzel~, elastomers,
15 silicone, neoprene, hypalon and thermoplastic elastomers.
A large number of insulation and cable jacket materials are suitable for
construction of a prefonmed inductive loop in accordance with the invention.
Continuous improvements in the formulation and processing of plastics almost
assures that better and less expensive materials will become available in the
future. It appears that almost all of the materials listed above may be
suitable for
at least some inductive loop sensor applications. For low-temperature
applications
(i.e., for installation under concrete) or in a saw-cut, higher hardness grade
polyurethanes seem to be preferable due to their mechanical properties and
previously successful underground applications. To allow the same material to
be
used in a high-temperature application, polyurethane may be cross-linked to
raise
its working temperature. Also, for high-temperature installation (i.e.,
installed
under hot asphalt), Tefzel~ appears to be another material of choice.
Preferred Designs
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Cross-linked polyurethane seems to be the preferred material for
construction of the jacket layers due to its superior properties and
relatively low
cost. The cross-linking process improves the physical properties of
polyurethane
and in particular its heat resistance. Thus, cross-linked polyurethane is
suitable for
S the high-temperature installation under asphalt.
Making the connectors from the same material as the insulation/jackets is
preferred as it assures an integrated, monolithic construction. Also, the
irradiation
process adds only little to the price of the assembly.
The cross-linked polyurethane jacket may be used for both cable and
connector construction. The preferred wire insulation is cross-linked
polyethylene
due to its superior insulation properties and water resistance.
Inductive Loop Cable Specification
Due to wide differences in installation procedures and associated stresses
that the loop cable has to withstand for saw-cut and pave over installations,
we
consider two different specifications for the loop cable: one for a saw-cut;
and
one for pave over.
Installation in a Saw-cut
Traditional saw-cut installed loop sensors are generally made from a single
wire wound several times around the saw-cut to form a mufti-turn loop to match
the saw-cut circumference. Preformed loops are rarely installed in saw-cuts
because of the large loop cross-section, and the large size of the loop/lead-
wire
junction. Therefore, there is a need for a narrow-profile preformed loop in
accordance with the present invention that can be installed in a saw-cut.
Overall, the saw-cut preformed loop in accordance with the invention
should fit into a 1/4-inch saw-cut. This limitation dictates either a flat
cable
(shown in Figure 2) or thin round cable (shown in Figure 4) construction of
the
mufti-turn loop. Stacking the ends of the loop cables at the connector allows
widths less than 0.25 inch, for example, as shown in Figure 2, which shows the
flat cable cross-section, and Figure 4, which shows the thin round cable cross
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17
section, both with a thin, tough layer of insulating jacket. The loop cable
conductors are typically 16 or 18 AWG and multistranded for increased
flexibility.
The materials chosen for wire insulation and cable jacketing also needs to be
sufficiently flexible to permit installing the cable into sharp bends of the
saw-cut.
Molding a connector housing around the conductor and wire connections also
helps minimize the width of the sensor by eliminating the need for a separate
housing and sealant.
The saw-cut preformed loop has an adjustable perimeter that is used to
accommodate variations in the saw-cut length. To make the loop adjustable and
minimize the road damage during installation, the loop cable ends are stacked
on
top of each other at the connector and installed in a short section of the
lead-in
saw-cut before they separate to go around the loop saw-cut circumference,
shown
in Figure 1. The saw-cut embodiments, as described above with reference to the
figures, allow adjustability without a wider lead-in saw-cut.
Installation Under New Road Surface
Presently, preformed inductive loops are installed predominantly under
new roadway surface. As in the case of the saw-cut loop, one of the main
challenges for the inductive loop sensor is to survive the installation
process. For
the pave over installation, the size of the loop sensor cable is not a
constraint.
Therefore, standard round cross-section cable may be used for example, as
shown
in Figure 10. The loop cable is constructed of a round cable 536 with three
individually insulated copper wire conductors 542, 544, 546. The wires in this
case may be multistranded or solid to increase the stiffness of the loop
assembly.
To simplify construction of the junction, each wire has insulation 561 of a
different color. A jacketing material 560 covers and protects the conductors.
The preformed inductive loop installed under new roadway surfaces
should also have a low profile so that when resurfacing operations are
preformed,
the loop does not get damaged. This requirement stems from one common
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problem, after the loop has been under the road for many years, cold planing
of the
road for resurfacing often damages existing preformed loops. Presently
available
designs currently in the field have large loop/lead-wire junctions which do
not
have enough road material covering it to protect it during cold planing. In
accordance with the invention, the Ioop/lead-wire junction should also be as
low
profile as possible.
The inductive loop must withstand the high temperature of heated asphalt
300° to 350°F (or 150° to 180°C). This requirement
necessitates the use of high-
temperature materials in the construction of the loop.
The loop must not interfere with asphalt paving equipment. When asphalt
is laid, the machine that spreads the asphalt often rips up presently
available loops
that have been laid out to be covered. If the loop is low profile, then the
asphalt
machinery will not snag it and ruin the installation.
The loop must withstand being run over by construction vehicles before
being covered with new road material. When preformed loops are installed under
new road surfaces, they are laid out in their position prior to the final
covering of
the road. Between the time they are laid out and the time they get covered, it
is
expected that they will get run over many times by heavy equipment. In a
preferred embodiment, a cross-linked polyurethane cable will be best able to
withstand loads associated with construction traffic.
Lead-in Cable Specification
The lead-in cable is a simple twisted pair cable, preferably, insulated and
jacketed with the same material as that used for the sensor loop and must meet
the
same ruggedness requirements as the loop cable. The jacket thickness will be
the
same as that used for the loop cable to assure the same mechanical properties.
The
lead-in cable will consist of a twisted pair of wires. The wires may also be
shielded if required, the shield made of aluminum foil wrap.
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Additionally, for the saw-cut installation, the lead-in cable must fit into
the
1/4-inch saw-cut. A twisted pair cable that meets this specification is easily
manufactured with an outside diameter less than 1/4 inch.
The lead-in cable must have low impedance and must not be a noisy
transmission line. Twisted pair conductors give the design low impedance, and
optional shielding adds noise rejection to the design.
The lead-in cable insulation must bond with the connector housing to
maintain a sealed unit. Manufacture of the lead-in cable from the same
material as
the loop cable and the connector housing fulfills this requirement.
Connector Specification
The connection between the loop cable and the lead-in cable may
potentially be the weakest point of the prefonmed loop assembly. To assure the
necessary mechanical integrity, environmental protection, and small size of
the
connector, a molded enclosure around the soldered or spliced connectors of the
conductors and lead-in wires, as described above, may be used. The mold will
be made of the same material as the coil loop cable and lead-in cable
insulation or
jacket. This will permit the molten housing material to fuse with the cable
insulation and totally isolate the conductor and wire connections from the
outside
environment. As well as being tough, this design has the advantage of being
easily manufacturable and has the best chance of withstanding many years of
service under the road surface. The combination of the connector housing and
sealant in one mold reduces the number of parts and the time required to
assemble
the loop. This simplification actually reduces the cost of the loop sensor,
while at
the same time making it more robust.
Manufacture
The inductive loop sensor in accordance with the invention may include a
lead-in cable, a loop cable and a connector that electrically connects the
loop cable
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and lead-in cable into a mufti-turn loop surrounded by a molded housing
material.
To manufacture the inductive loop sensor, a lead-in cable is cut to length and
the
loop cable is cut to length. The conductor and lead-in wires are stripped and
soldered or spliced together to form the turns, as described previously. The
5 conneciton of the sires are covered for protection and then a final housing
is
formed around the entire area, forming the connector. The connector may be
formed using injection molding, which is a process where molten insulation
material is injected into a metal mold that has the cables passing into it
through
channels from the outside.
Geometry of the Inductive Loop Sensor
The loop design developed requires a strict control of the loop conductors'
geometry and thus of the loop inductance. This, in general, is not true for
the
presently available preformed loops, where the mutual position of the loop
1 S conductors is controlled only to the extent that all turns must fit into a
conduit.
A simplified expression for the loop inductance may be determined to
obtain approximate scalings of the loop sensitivity with geometrical
parameters.
These scalings provide better insight into the loop properties and are
supported by
calculations. We show that from the point of view of loop sensitivity, the
preferred design is a loop with multiple conductors. In the following, we use
the
expressions for inductance of a circular loop, but the same general
conclusions are
obtained for other loop shapes.
For this discussion, three coil geometries are considered: a ribbon cable
with N conductors; a round cable with N conductors, and a single ribbon
conductor. The loop inductance is calculated as the self inductance of a short
coil,
which is expressed in terms of the inductance value for an infinite solenoid
multiplied by a factor that gives a measure of the end effects.
To calculate the inductance of a loop made of a ribbon cable with N
conductors, we use the formula for a circular, single-layer round coil as
given by
Grover:
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L f = 0.002 N'' ~(2rlb)K(bllr). ( 1 )
where N is the number of turns, r is the radius of the loop, b is the height
of the loop (i.e., of the current sheet). For short coils, [3 = bl2r « l, and
K = 2(i / n {[In(4/(3) - 1/2J + [32/8[ln(4/~i) + 1/8J -
$ (i4 /64[ln(4/~i) - 2/3J + $36/1024[ln(4/[3) - 109/120) +...} (2)
.: (blpr)In(8rlb)
Thus, for (3=b/2r « 1,
Lf~0.0047~rNz ln(8r/b) (3)
For the cable,
b f = Nd",,
where d", is the spacing between wires (or, approximately, the diameter of
a single wire).
The inductance of a loop made of a round cable with N conductors may be
1$ calculated using the formula for a round loop with square cross-section:
Lr = O.OOIrNZPo (4)
where
Po = 4n{0.$[1+[i2/6Jln(88/(32) - 0.84834 + 0.2041[32) . (s)
In this case, b is the height (equal to the width) of the coil, and is given
approximately as
b = Nii2
r
For [3=b/2r < < 1:
Po = 4nln(8r/b), (6)
and the same approximate expression for inductance is obtained as above
2$ for the flat cable.
Thus, a loop made of round cable has higher inductance for a given
number of turns due to smaller value of b (effective coil height). This result
is
illustrated in Figure 11, which gives the results of exact calculations of
Equations 1 and 4.
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The parameter that should be taken into account in determining the number
of turns is the detection sensitivity that depends on the mutual inductance
between
the loop and the vehicle. The apparent change in the loop inductance due to
the
passing vehicle is given by
oLi = _(M~2)2/I-2 , (7)
where L2 is the effective inductance of a closed loop that represents the
vehicle body, and M12 is the mutual inductance between the loop (L~) and the
vehicle loop. The loop sensitivity is thus equal to
S = oL~/L, _ -(M12)2/(L2L1) ~ (8)
The mutual inductance between the loop and a vehicle is a sum of mutual
inductances between the single turns of the traffic loop and the vehicle and,
thus, it
is proportional to N:
Miz =Em~2 ~ Nm~2 ,
where m,z is the mutual inductance between a single loop and the vehicle.
The sensitivity is thus approximately equal to
S ~ (m12)2 /[L20.004~trln(8r/b)] . (10)
Thus, in the first-order approximation, the sensitivity depends only on the
height of the loop (current sheet) and not on the number of turns. Figure 12
shows
the exact relative sensitivity of loops made of flat and round cables. The
sensitivity increases as the loop wires are spread apart, and is larger for a
loop
made of vertical ribbon (slot) cable than for a loop made of round cable.
There is
also a weak increase of sensitivity with the number of turns that has been
omitted
from the approximate expression.
As an alternative design for a multiple conductor loop, one may consider a
single conductor ribbon cable (i.e., with a ribbon shape single conductor).
Although a single conductor has lower inductance (by a factor of N2) than a
multiple conductor, there is no gain in sensitivity because the mutual
inductance
between the loop and the vehicle is reduced by factor of N. Thus, a single
conductor ribbon has the same sensitivity as a multiconductor ribbon cable.
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The inductance of lead-in cable also needs to be taken into account in
evaluation of the detector sensitivity. This inductance reduces the
sensitivity by a
factor of (1+L/I,l)~ where L is the lead-in cable inductance and Ll is the
loop
inductance. It is a common practice to make the number of turns large enough
such that inductance of the loop cable is larger than the inductance of the
lead-in
cable (L~»L), which, for a mufti-turn construction, is achieved by increasing
the
number of turns. As discussed above, a larger number of turns also results in
an
improved sensitivity of the "primary" loop. The substantially lower inductance
of
a single conductor would require the use of an additional transformer to
reduce the
effect of lead-in cable inductance on the signal magnitude and sensitivity.
Therefore, it appears that increasing loop inductance by increasing the number
of
turns, as in the preferred embodiment, will be more cost effective.
Noise Considerations
Traffic loop detectors are sometimes reported to suffer from
electromagnetic noise produced by power lines and electrical installations.
Cables
shielded with stainless steel or copper mesh may provide increased immunity to
the sources of noise.
To determine if shielded cables are required, two loops were built, each six
feet in diameter and with four turns. One was made with a shielded four-
conductor cable of 20 AWG wire, the other with an unshielded four-conductor
cable of 20 AWG wire. Each had the same length of shielded, twisted pair lead-
in
lead. The loop shield was grounded through the lead-in cable shield and the
detector in such a way that it did not form a closed loop. The loops were
connected to a unit which gives a digital measurement of inductance.
A location was found where there was a high level of ambient
electromagnetic noise. The location was in a parking garage where electric
motors
and power lines created significant noise. The two test loops were then
brought to
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that location and data was collected in the noisy region. No significant
difference
in the signal was observed between the shielded and unshielded loops.
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Preferred Designs
Cross-linked polyurethane seems to be the preferred material for
construction of the jacket layer due to its superior properties and relatively
low
cost. The cross-linking process improves the physical properties of
polyurethane
5 and in particular its heat resistance. Thus, cross-linked polyurethane is
suitable for
the high-temperature installation under asphalt.
Making the connectors from the same material is preferred as it assures an
integrated, monolithic construction. Also, the irradiation process adds only
little
to the price of the assembly.
10 The cross-linked polyurethane jacket may be used for both cable and
connector construction. The preferred wire insulation is cross-linked
polyethylene
due to its superior insulation properties and water resistance.
While the foregoing has been with reference to a particular embodiment of
the invention, it will be appreciated by those skilled in the art that changes
in this
1 S embodiment may be made without departing from the principles and spirit of
the
invention.
