WO1999047894A1

WO1999047894A1 - Inductive loop sensor and method of manufacturing same

Info

Publication number: WO1999047894A1
Application number: PCT/US1999/005160
Authority: WO
Inventors: Charles Tweedy; Dariusz Wroblewski; Donald K. Owen
Original assignee: Orincon Technologies, Inc.
Priority date: 1998-03-17
Filing date: 1999-03-08
Publication date: 1999-09-23
Also published as: CA2318621A1; AU3074299A

Abstract

A flat or round cable used in an inductive loop sensor. The inductive loop sensor may be installed in a saw-cut groove made in a roadway or positioned in a new roadway. The inductive loop sensor includes an inductive loop having a conductor with one or more turns covered with a protective jacket. First and second ends of the loop conductor are connected in a connection area to a pair of wires in a lead-in cable. The connection area is sealingly encased in a molded connector. This inductive loop may be made from round cable.

Description

1

INDUCTIVE LOOP SENSORAND 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 repaving 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. 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 preformed 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 repaving 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. 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 repaving 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 and are readily available, are designed to survive roadway repaving operations, and are designed to have a long service life.

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 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 6A - 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; 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 36 that fits into the saw-cuts 32. The flat loop cable 36 may be made from prefabricated 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 2 A, 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 8

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 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 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 also be 1/4" wide, houses a 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 forms 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 10

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 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 turn sensor, each of the internal conductors are attached to each 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 150 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 11

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 4 A 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 12

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 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. 13

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 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 15

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 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. 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, silicone, neoprene, hypalon and thermoplastic elastomers.

A large number of insulation and cable jacket materials are suitable for construction of a preformed 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 16

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 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 multi-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 multi-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- 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 18

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 loop/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 prefeπed 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. 19

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 preformed 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 20

and lead-in cable into a multi-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 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 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: 21

L_f = 0.002 N- y(2r/b)K(b/2r), ( 1 ) where Ν is the number of turns, r is the radius of the loop, b is the height of the loop (i.e., of the cmxent sheet). For short coils, β = b/2r « 1, and K = 2β / π{[ln(4/β) - 1/2] + β²/8[ln(4/β) + 1/8] — β4 /64[ln(4/β) - 2/3] + 5β⁶/1024[ln(4/β) - 109/120] +...} (2)

« (b/pr)lnβr/b) Thus, for β=b/2r « l,

E «0.00477crΝ² ln(8r/b) (3)

For the cable, b/ = N where d_w is the spacing between wires (or, approximately, the diameter of a single wire).

The inductance of a loop made of a round cable with Ν conductors may be calculated using the formula for a round loop with square cross-section:

L_r = 0.001 rN²Po (4) where

Po = 4π{0.5[l+β²/6]ln(88/β²) - 0.84834 + 0.2041 β²) . (5)

In this case, b is the height (equal to the width) of the coil, and is given approximately as b_r = Ν^,/2d_w For β=b/2r « 1 :

Po = 4πln(8r/b), (6) and the same approximate expression for inductance is obtained as above 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. 22

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 ΔL, = -(M₁₂)²/L₂ , (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_j) and the vehicle loop. The loop sensitivity is thus equal to

S = ΔL₁/L₁ = -(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: ₁₂ =∑m₁₂ « Nm₁₂ , (9) where m₁₂ is the mutual inductance between a single loop and the vehicle. The sensitivity is thus approximately equal to

S * (m₁₂)² /[L₂0.004πrln(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 N ) 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. 23

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 (l+L L--_!), where L is the lead-in cable inductance and L_j 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 multi -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 24

that location and data was collected in the noisy region. No significant difference in the signal was observed between the shielded and unshielded loops.

25

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 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. 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 embodiment may be made without departing from the principles and spirit of the invention.

Claims

26CLAIMS What is claimed is:

1. An inductive loop sensor, comprising: an inductive loop cable having a loop conductor with one or more turns terminating in first and second conductor ends, the turns of the loop conductor being surrounded by a first protective jacket with the first and second conductor ends extending from the first protective jacket; a lead-in cable having first and second lead-in wires surrounded by a second protective jacket, an end of the first and second lead-in wires extending beyond the second protective jacket, the first lead-in wire being connected at a first connection to the first conductor end of the loop conductor and the second lead-in wire being connected at a second connection to the second conductor end of the loop conductor; and a molded connector sealingly covering the first and second connections.

2. The inductive loop sensor of Claim 1 wherein the first and second connections are in a substantially vertically oriented plane with the first connection disposed above the second connection.

3. The inductive loop sensor of Claim 2 wherein the one or more turns of the conductor are stacked in a vertical plane.

4. The inductive loop sensor of Claim 1 wherein the first and second connections are in a substantially horizontally oriented plane with the first connection disposed opposite the second connection at the molded connector.

5. The inductive loop sensor of Claim 1 wherein the first protective jacket has a height greater than a width. 27

6. An inductive loop sensor comprising: an inductive loop made from a first cable having internal conductors in two or more turns terminating in first and second conductor ends; a first protective jacket enclosing the turns; a lead-in cable made from a second cable with first and second lead-in wires, the first lead-in wire being connected at a first connection to the first conductor end and the second lead-in wire being connected at a second connection to the second conductor end; a second protective jacket enclosing the first and second lead-in wires; and a molded connector sealingly covering the first and second connections.

7. The inductive loop sensor of Claim 6 wherein the first and second connections are in a substantially vertically oriented plane with the first connection disposed above the second connection.

8. The inductive loop sensor of Claim 6 wherein the first and second connections are in a substantially horizontally oriented plane with the first connection disposed opposite the second connection at the molded connector.

9. The inductive loop sensor of Claim 6 wherein the first cable is a flat cable.

10. The inductive loop sensor of Claim 6 wherein the first cable is a round cable.

11. The inductive loop sensor of Claim 6 wherein the first cable is a pre-fabricated cable. 28

12. A method of making an inductive loop sensor comprising the steps of:

(a) providing a loop cable with internal loop conductors in one or more turns terminating in first and second conductor ends; (b) covering the loop conductors with a first protective jacket;

(c) providing a lead-in cable having first and second lead-in wires surrounded by a second protective jacket;

(d) connecting the first lead-in wire at a first connection to the first conductor end; (e) connecting the second lead-in wire at a second connection to the second conductor end; and

(f) forming a molded connector over the first and second connection.

13. The method of Claim 12 wherein the first and second connections are in a substantially vertically oriented plane with the first connection disposed above the second connection.

14. The method of Claim 12 wherein the first and second connections are in a substantially horizontally oriented plane with the first connection disposed opposite the second connection at the molded connector.

15. The method of Claim 12 wherein the loop cable is a flat cable.

16. The method of Claim 12 wherein the loop cable is a round cable. 29

17. A method of making an inductive loop sensor comprising the steps of:

(a) providing a first cable with multiple internal conductors, attaching the multiple internal conductors in a loop conductor configuration forming a multiple turn inductive loop terminating in first and second conductor ends;

(b) covering the multiple turn inductive loop with a first protective jacket;

(c) providing a second cable having first and second lead-in forming a lead-in cable;

(d) covering the lead-in cable with a second protective jacket; (e) connecting the first lead-in wire to the first conductor end at a first connection;

(f) connecting the second lead-in wire to the second conductor end at a second connection; and

(g) forming a molded connector over the first and second connections.

18. The method of Claim 17 wherein the first and second connections are in a substantially vertically oriented plane with the first connection disposed above the second connection.

19. The method of Claim 17 wherein the first and second connections are in a substantially horizontally oriented plane with the first connection disposed opposite the second connection at the molded connector.

20. The method of Claim 17 wherein the first cable is a flat cable.

21. The method of Claim 17 wherein the first cable is a round cable.

22. The method of Claim 17 wherein the first cable is a pre-fabricated cable.