WO1996005584A1 - Linear pressure sensor - Google Patents

Abstract

A roadway sensor(10) is comprised of an elastomeric carrier having conductive properties. A sensor passage (11) formed in the carrier and a plurality of polytetrafluorethylene coated wire conductors (15') tightly fill the sensor passage (11). Signal output terminals connected to said carrier and said plurality of wires. The coating on said wire is selected from FEP or TFE polytetrafluorethylene. In a preferrred embodiment, the passage (11) is waterproof and in another embodiment, the sensor passage (11) has walls and means on the walls for preventing moisture from affecting signals induced in the coated wire conductors (15').

Description

LINEAR PRESSURE SENSOR REFERENCE TO RELATED APPLICATIONS:

This application is a continuation-in-part of application Serial No. 08/288,755 filed August 11, 1994, which is related to U.S. application Serial No. 08/038,999 filed March 29, 1993, which is a continuation of application Serial No. 07/406,345 filed September 13, 1989. This application is also related to application Serial No. 07/925,694, which is a continuation-in- part of application Serial Nos: 07/880,410 filed May 8, 1992 and 07/346,685 filed May 3, 1989.

The present invention relates to linear pressure sensors and, more particularly to linear roadway vehicle sensors for traffic monitoring and detection of roadway traffic and other wheeled vehicles.

It is known that coaxial electrical cables produce an electrical effect by generating a voltage between a conductor and a shield to the cable with the amplitude of the voltage being proportional to pressure on the cable. Such a system is disclosed in Meyers U.S. Patent 3,911,390 wherein a coaxial cable is sandwiched between a pair of polyethylene strips along with metallic plates positioned in the envelope along the length of the strip for attaching the strip to the surface of the roadway. In Gebert et al. U.S. Patent 5,008,666 (European Patent 0387093) subsurface mounted or top surface mounted sensors or traffic detection cables are comprised of a molding of prismatic shape and piezoelectric cables are carried in the moldings, the cables being either piezoelectric or triboelectric or exhibit any other effect including capacitance base effects upon the receipt of mechanical impulse and/or physical strain associated with the impulse or stress field generated by the impulse. Merryman et al. U.S. Patent 2,787,784 discloses a triboelectric detection system in which a pair of conductors have a loose friction producing fit with a dielectric insulator and produces signals of from 1/2 to 500 millivolts.

Reliable signal voltages induced in such systems typically have values under 1 volt and some commercial units have approximately 35-200 millivolts signal voltage.

The object of the present invention is to provide an improved linear pressure sensor, particularly linear pressure sensors adapted for sensing vehicular traffic on roadways which have reliable high signal voltages at relatively low cost and are easy to fabricate.

Another object of the invention is to provide improved structures and methods for installation of such sensors in subsurface or roadway grooves and on top or surface mounting of such units.

According to the invention, the roadway sensor is comprised of an elastomeric carrier having conductive properties, preferably an elastomeric carrier that has been extruded to have a predefined cross-sectional area, a sensor passage formed in the carrier, preferably the leading edge of the carrier (the leading edge being defined as the edge first engaged and traversed by the wheel of a vehicle). A plurality of Teflon™ coated wire conductors fill the sensor passage and a signal output terminal is connected to the conductive elastomeric carrier and to the plurality of wires to provide an output signal voltage to a utilization device, such as a counting circuit, memory circuit, etc.

In a preferred embodiment of the invention, the sensor passage is made water-proof. The plurality of Teflon™ coated wire conductors filling the sensor passage are connected at one end as one output terminal to the center conductor in a coaxial cable which then is sealingly connected to a water-proofed lead out structure. In a further embodiment, the sensor passage is coated with an insulating compound so that in this case, the sensor passage need not be water-proof. This prevents moisture from affecting signals induced in the Teflon™ coated wire conductors.

In one installation embodiment, for in-pavement roadway sensor insertion in a groove formed in a roadway is substantially normal to traffic flow and the mounting of the sensor in the groove includes an epoxy resin having a predetermined shore hardness. In the in-surface or sub-surface mounting arrangement, the conductive elastomeric carrier includes a rigid support strip and means securing the elastomeric material to the rigid support strip. In a further embodiment, the rigid carrier support strip is metal and in conductive contact with the elastomeric carrier and one end thereof can serve as a grounded output terminal for the signal .

For this in-pavement roadway sensor, the elastomeric carrier has an upper surface and a lower surface with a flange integrally formed on and laterally extending from the lower surface and rivet means are utilized for securing the flange means to the metallic carrier strip and to provide opposition to any vertical movement, which may try to dislodge sensor assembly.

In still a further embodiment, for an on-pavement or surface mounted roadway sensor construction, the elastomeric carrier has a lower surface and means for causing the roadway sensor to hug the roadway surface. In a further embodiment, means for causing the roadway sensor to hug the roadway surface is either a lead weight having a weight per unit length sufficient to cause the carrier to hug the roadway surface or the on-pavement roadway sensor is caused to adhere to the roadway surface by an adhesive, or tape, or mechanical clamps.

For multiple lanes of traffic, the sensors are all carried in a common sensor passage extending transversely to the flow of roadway traffic, there being curb-side lane sensors and interior lane sensor with all of said sensors being separate and commonly carried in a common sensor groove, preferably on an edge of the elastomeric carrier. In this embodiment, the plural sets of Teflon™ coated wires, one for each roadway, are connected to their own respective coaxial signal carrying cables (coaxial cable does not have to be used until you exit the carrier) which are further carried to the edge of the roadway and beyond by coaxial cables which are fitted-in oversized coaxial signal carrying coaxial cable passages. By being oversized (e.g. not fitting tight, and not having a Teflon™ jacket), there is less inducement of ambiguous signals in the coaxial cables per se.

Sensors constructed according to the invention provide extremely large voltage output signals compared to the prior art. In some instances, some test models produced signal voltages in the range of upwards of 70 volts for heavy truck traffic traveling at high speeds.

According to the invention, the conductive elastomeric carrier:

1. Shields the assembly from extraneous fields,

2. Absorbs road vibrations to prevent adjacent lane pick¬ up, e.g., automatically buffers from the roadway. (The prior art uses aluminum channels filled with an epoxy and ceramic piezoelectric powders.

3. Conveys pressure to the sensor wires.

4. Keeps water and moisture out.

5. Protects the sensor wires from damage.

6. Has flexibility which aids in installation (conforms to ruts and crowns). Avoids the need for aluminum channels which is commonly used in the art.

7. Can be buried flush with the roadway surface so snowplows and dragging tail pipes do not snag.

8. Can be buried below roadway so long as the epoxy has the same durometer of the EPDM conductive carrier. 9. EPDM conductive carrier is resistant to high and low temperatures.

Since electronic pressure sensors constructed according to the present invention has significantly higher voltage outputs than the prior art electronic pressure sensors (200 millivolts vs. at least 1 volt (1000 millivolts)), upwards of 70 volts, the high output voltage allows fusing for lightning protection. In addition, the higher output voltage means that a longer lead line can be used. That is to say, the cabinet housing electronic monitoring and data storage circuits and devices can be 2-300 feet away from the sensor per se.

Since the ground parts carry the same potentials as the road, it eliminates charge that may be carried by the tire of a vehicle and also can eliminate effects of lightning. DESCRIPTION OF THE DRAWINGS:

The above and other objects, advantages and features of the invention will become more apparent when considered with the following specification and accompanying drawings wherein:

FIG. la is a sectional isometric view of a sensor incorporating the invention, and FIG. lb is a graph illustrating the waveform of the output voltage wherein a vehicle is driven thereover,

FIG. 2 is a second embodiment of a linear roadway sensor incorporating the invention,

FIG. 3a is a sectional isometric view of a linear roadway sensor with in-pavement-type construction, and FIG. 3b shows the sensor embedded in a groove cut in the roadway,

FIG. 4 is a sectional isometric view of an on-pavement embodiment of the invention,

FIG. 5 is a sectional view of an embodiment of the invention showing the connections of a coaxial cable and the manner of maintaining a substantially water-proof sensor passage for the sensor wires,

FIG. 6 is a sectional isometric view of a further embodiment of the invention,

FIG. 7 is a sectional isometric view of a still further embodiment of the invention for use with multiple lane traffic with on-pavement mounting,

FIG. 8a is a sectional isometric view of a further embodiment of the invention for measuring speed with dual sensor passages and, FIG. 8b is a waveform diagram showing the spacing between the induced pulses for speed measurement purposes. DETAILED DESCRIPTION OF THE INVENTION:

In its most basic form shown in Figs. 1 and 2, the invention comprises a linear elastomeric member 10 which is preferably an extrusion of a conductive rubber or rubber-like material such as EPDM (a polymer made rubber-like material which has conductive carbon particles therein so that the resistivity is low, 2 killohms per inch or less, the more conductive the elastomeric member 10 is the better the signal) with good abrasion resistance. Elastomeric conductive member 10 has formed therein a senso passage or chamber 11 which, in the embodiment shown in Fig. 1, is waterproof. Filling the space of the sensor passage 11 are a plurality of sensor wires which are commercial, high temperature hook-up wires coated with a polytetrafluorethylene (Teflon™), TFE Teflon™ and Teflon™ TFE insulation, all have provided very good results. In a typical embodiment, the diameter of sensor passage 11 is .200" (200 mils) and was filled with a plurality of TFE Teflon™ coated wire conductors filling the chamber. In this embodiment, with a 200 mil diameter sensor passage 11, approximately 16 26 AWG silver-plate copper (stranded) wires with FEP Teflon™ insulation of about 10 mils thickness and nominal outside diameter of about .036" fill the chamber. It is important that the wires snugly or tightly fill the sensor passage and not be loose in the sensor passage or chamber and that they be a tight fit so as to assure sharp clean pulses. Any air spaces means that the pressure from tires as they travel over the sensor strip is isolated from the sensor wires 12 and the output signal is diminished significantly.

Moreover, while each sensor wire has two ends, in one preferred embodiment, the wires are doubled so that although there may be sixteen wires filling the sensor passage, the ends of each wire is connected together with the other wires so that the eight wires folded over have sixteen ends which are all connected together. It will be appreciated that there may be significantly more or significantly fewer wires in a given embodiment and that the numbers given are purely exemplary.

The embodiment shown in Fig. 2 is similar to the embodiment in Fig. 1 except with significant difference: in Fig. 2, the sensor passage 15 in conductive extrusion 16 is filled with sensor wires 17 in the same way as described earlier. However, in this case, the sensor passage is coated with a insulating layer by using an EPDM solvent (459X-EPDM), which is an adhesive primer for rubber and EPDM. By treating the interior wall surfaces of the sensor passage 15, an insulating coating layer 18 is formed so that substantially the same high voltage signal output is produced even in the presence of moisture in the sensor passage 15. Again, the Teflon™ coated wires have to be tightly or snugly fitted inside the sensor passage 15 so as to assure sharp, clean high output voltage pulses when vehicles traverse or pressure is applied to the outer surface of the sensor.

As indicated, the output signal is coupled by a coaxial cable CC, to a utilization device such as a roadway counter, or may be communicated by radiowaves, infrared or other wireless communication means to a central data collection facility (not shown) .

The extrusion is a conductive rubber-type elastomeric compound, which in embodiment is EPDM-type material which has been made conductive and having conductive properties discussed earlier. For on-surface mounting, the conductive extrusion should have a shore hardness or durometer of between 75 and 85, plus or minus 5. (Lower duro eters decrease life, higher decrease output and ability to conform to road surface.)

Referring now to Figs. 3a and 3b, an in-pavement installation is illustrated wherein the roadway surface RS has a groove 20 cut therein to a depth sufficient to receive a sensor assembly as shown in Fig. 3b. As shown in Figs. 3a and 3b, the sensor is comprised of an conductive extrusion 30 having a senso passage 31 filled with Teflon™ coated sensor wires 32. Extrusion 30 has a upper surface which is slightly domed or convex, and a lower surface which has a pair of outwardly projecting flange members 33, 34 which are secured by rivets 35, 36 to an aluminum mounting plate 37. The mass of wires 32 filling sensor groove 31 have been placed by slitting extrusion as at 39 and spreading the walls apart so as to in place the sensor wires 32. In this embodiment, the sensor passage 31 has had its walls treated with an adhesive primer for rubber or EPDM and dried so that an insulating layer is provided over the unit. The assembly shown in Fig. 3b is installed in the groove 20 in Fig. 3 by suspending the assembly of Fig. 3b in the slot 20 b carrier 40 and ties 41.

In the assembly shown in Fig. 3b, EPDM primer is applied to the underside of the carrier so that a layer of bituthene tape B adheres better and also facilities adhering the metal base 37 in addition to the mechanical bond. The rivets provide good electrical coupling along the length of the sensor extrusion 30 to the metal base 37 in addition to providing the mechanical securement. In this embodiment the EPDM extrusion adhesive primer is Chemlock 459X and is manufactured by Lord Corporation, Chemical Division)

As noted above, the tight fit between the Teflon™ coated wires and the walls of the sensor passage is important in assuring that a sharp, clean voltage pulses (Fig. lb) are provided at the output terminals.

For asphalt roadway it is preferred that the epoxy 25 filling the groove have a durometer approximately the same (about 85 +.5) as the durometer of the extrusion 30 and that the epoxy not be hard and brittle.

Referring now to Fig. 4, which shows an on-pavement or surface mounted assembly, the conductive elastomeric extrusion 50 is provided with a sensor passage 51 filled with Teflon™ coated wires 52. The upper surface 53 is ramped as at 53R and 53R2, these surfaces having ramp surfaces of approximately 30° to the road surface in the preferred embodiment. These ramp surfaces are preferred because they substantially reduce a horizontal component of tire force striking the sensor assembly.

The sensor assembly itself is maintained in a fixed position by an adhesive layer, tape, or clamps, or a lead weight 55. The lead weight is as disclosed in the above-identified Tyburski applications, which is incorporated herein by reference. Both the adhesive or the lead weight function to cause the sensor assembly to hug the roadway and not be dislodged by heavy trucks and the like. The lead weight is preferred in the portable reusable-type assembly which can be easily lifted up and removed and reinstalled from place to place.

Referring now to Fig. 5, the preferred manner of coupling to the sensor wires is illustrated as well as the preferred technique for assuring water and moisture-proofness of the sensor chamber. In Fig. 5, the left end of the sensor chamber is shown as receiving a threaded plug 60 which is threadably engaged with the walls of the sensor passage 60. End plug 60 is coated with a "GOOP™" sealant S and threaded into the sensor passageway and when the sealant cures, it forms a perfect water-tight seal in conjunction with the plug 60. Sensor wires 62 are tightly packed in the sensor package 61 substantially filling same so that there is no air gaps or spaces, assuring direct transference of pressure from vehicles wheels to Teflon™ coated sensor wires. In the embodiment shown in Fig. 5, the sensor wires 62 are folded at their ends 63 so that the terminal ends 64 of all the wires are twisted together to form the output lead. These wires are then connected to the center wire 65 of coaxial cable 66. A heat shrink double insulating sheath 67 is preferably fitted over the juncture of the sensor wire terminal ends 64 with the central signalling carrying wire 65 of the coax cable 66. This assures that the ends of the wires do not short out to the conductive elastomeric member forming the carrier. The coaxial cable 66 passes through a conductive copper, stainless steel sleeve 70 which makes good electrical contact with the braided outer covering 71 of coaxial cable 66. The left-most end of the conductive sleeve is free from sealant SI and makes a good electrical contact with the conductive extrusion. When the conductive metal sleeve is secured to the extrusion, it is externally threaded for only a portion of its length. The first portion is screwed into the sensor passageway 61 for approximately 1/2" and then the threads are coated with a sealant ("GOOP™") and it is threaded in the rest of the way. The sealant or Goop™ bonds and forms a bead 72 on the exterior. This material in the threads and the bead together prevent water from entering between the conductive sleeve and the extrusion. The conductive metal sleeve 70 is crimped as at 73 to tightly engage the outer metal braided conductive metal conductive portion of the coaxial cable.

Right end 74 of metal sleeve 70 relatively snugly embraces the insulating sheath 75 of the coaxial cable. A shrink-wrap sleeve 76 which has an internal coating 77 is fitted over the end of copper sleeve 70. When shrink-wrap sleeve 76 is heated, the internal coating 77 melts as the sleeve contracts in diameter on heating and the internal coating 77 seals to all surfaces and some is squoze out at the ends. This prevents moisture or water from entering between the conductive sleeve 70 and the external insulating surface 75 of the coaxial cable jacket.

Thus, the signal voltage induced in the wires by the pressure of vehicles traversing thereover is coupled to the central wires of the coaxial cable and the conductive elastomeric sheath surrounding the central passageway 61 is coupled to the external sheath or ground of the coaxial cable. When the base of the extrusion is formed in the manner shown in Fig. 3, a grounding conductor may be connected directly to the aluminum plate and nailed to the roadway to provide a good ground for static and for lightning protecting purposes.

Referring now to Fig. 6, the conductive extrusion 80 is provided with a pair of sensor passages 81, 82, each of which has an end fitment similar to that shown in Fig. 5 and connected to corresponding coaxial cables.

In the embodiment shown in Fig. 7, the conductive elastomeric extrusion 90 is provided with a leading edge 91 which is ramped or sloped at approximately a 30° angle and is provided with a sensor passage 92 which extends for multiple lanes of roadway (in this case, four lanes), and a plurality of coax cable grooves or passages 93, 94 and 95 are provided. These grooves are larger than the external diameters of the coaxial or lead-in line cables so that minimum ambiguous signals are induced therein. In other words, they are buffered from the pressure by an air space and loose fitting of the grooves onto the coaxial cables. The sensor groove 92 is long enough to accommodate four lanes of roadway and each lane of roadway is provided with their own separate grouping of sensor wires, all of which are conventional Teflon™ coated hook-up wires. In this case, for the three inside lanes, the portion of the extrusion between the corresponding coaxial cable portion 93, 94 and 95 is cut-out and the coaxial cable turned in that portion and extended to make connection with the sensor wire with a shrink fit thereon . corresponding to the shrink fit insulation tube 67 (shown in Fig. 5). This portable unit is provided with a lead weight chamber 96 which is formed in the base of extrusion 90. Lead weight 97 is secured in place by a double-faced adhesive layer 98 which, in addition, seals the ends or edges of the slits cut in the extrusion so that the coaxial cables may be fitted into coax cable grooves 93, 94 and 95, respectively, thereby precluding the instances of water and moisture. In addition, the grooves may be coated with the 459X EPDM adhesive primer for rubber and EPDM materials to provide an insulating coating.

Referring now to Figs. 8a and 8b, the extrusion is shown as having two spaced sensor passages, sensor passage 101 and sensor passage 102 which are spaced a distance D apart, as disclosed in Dixon application Serial No. 07/904,623. The sensor passages 101 and 102 are filled with Teflon™ coated sensor wires as described earlier herein. In this case, since the distance between the sensors is fixed and known, and maintained constant by the carrier material between the two sensor passages, the time between pulses induced in the sensor wire assemblies 103, 104 respectively, can be used to measure the speed and classification of vehicles traversing thereover.

While preferred embodiments of the invention have been shown and illustrated, it will be appreciated many other embodiments, adaptations and modifications of the basic invention will be readily apparent to those skilled in the art.

WHAT IS CLAIMED IS:

Claims

1. A roadway sensor, comprising: an elastomeric carrier having conductive properties, a sensor passage formed in said carrier, a plurality of polytetrafluorethylene coated wire conductors tightly filling said sensor passage, and signal output terminals connected to said carrier and said plurality of wires.

2. The roadway sensor defined in claim 1 wherein the coating on said wire is selected from FEP or TFE polytetrafluorethylene.

3. The roadway sensor defined in claim 1 wherein said passage is water-proof.

4. The roadway sensor defined in claim 1 wherein said passage has walls and means on said walls for preventing moisture from affecting signals induced in said coated wire conductors.

5. An in-pavement roadway sensor for insertion in a groove formed substantially normal to the direction of traffic flow on said roadway, means mounting the roadway sensor defined in claim 1 in said groove, including an epoxy resin having a predetermined Shore hardness.

6. The in-pavement roadway sensor defined in claim 5 wherein said roadway sensor includes a rigid carrier support strip and means securing said elastomeric carrier to said rigid carrier support strip.

7. The in-pavement roadway sensor defined in claim 6 wherein said rigid carrier support strip is metal and in conductive contact with said elastomeric carrier.

8. The in-pavement roadway sensor defined in claim 6 wherein said elastomeric carrier has an upper surface and a lower surface, flange means integrally formed on and laterally extending from said lower surface, and conductive rivet means securing said flange means to said metallic carrier strip.

9. An on-pavement roadway sensor for mounting on a surface of a roadway, comprising, the roadway sensor defined in claim 1, said elastomeric carrier having a lower surface and means for causing said lower surface to hug said surface of a roadway.

10. The on-pavement roadway sensor defined in claim 9 wherein said means for causing said lower surface to hug said surface of said roadway is a linear weight member having a predetermined weight per unit length sufficient to cause said carrier to hug the roadway surface.

11. The on-pavement sensor defined in claim 9 wherein said means for causing said lower surface to hug said roadway surface is selected from a linear lead weight, adhesive, tape, clamps, or a combination thereof.

12. The roadway sensor defined in claim 1 wherein said elastomeric carrier is flat and has a centerline and the edge of said carrier first engaged by vehicle tires is a leading edge relative to said centerline, and said sensor passage is in said leading edge.

13. The roadway sensor defined in claim 12 wherein said carrier has a length sufficient to span plural lanes of roadway, and there are sets of said plurality of Teflon™ coated wires, one set for each roadway, a corresponding set of coaxial signal carrying cables, each coaxial cable being connected to each respective set of said plurality of Teflon™ coated wires, said set being located in the roadway portion of said sensor passage of an associated lane, and further non-sensitive passage means in said carrier for each coaxial cable of said sets above one and a coaxial cable in said further passage and connected to respective ones of said sets of Teflon™ coated wires.

14. A linear sensor for a roadway for sensing vehicles, comprising, an extrusion of a conductive elastomeric carrier, a waterproof linear sensor passage formed in said carrier, linear electrical pressure sensitive wire means filling said waterproof passage and conductor means connecting electrical pulses induced in said electrical pressure sensitive wire means to a utilization device, and means for mounting said carrier strip to said roadway.

15. The invention defined in claim 14 wherein said carrier has a flat lower surface, and a double-faced adhesive layer on said flat lower surface.

16. The invention defined in claim 15 including a detachable protector strip on the outer surface of said double- faced adhesive layer.

17. The invention defined in claim 14 wherein said carrier when laid transversely of the roadway has an edge which is first contacted by a vehicle's wheels traversing said roadway and said sensor passage is adjacent said edge.

18. The invention defined in claim 17 wherein said roadway has at least two lanes and said carrier and waterproof sensor passage has length sufficient to cross all of said lanes, a sensor in said waterproof sensor passage for each of said lanes, respectively, a separate passage for a signal carrying coaxial cable and a coaxial cable connected to the center-most of said pressure sensitive wire means, said carrier having an upper surface, said upper surface having a groove therein so as to prevent vehicle tire pressure from being applied to said coaxial cable and induce false signals therein.

19. The invention defined in claim 18 wherein a lead weigh is mounted in said carrier so that the carrier has sufficient weight as to avoid lifting of the carrier due to air flows cause by vehicle traffic.

20. The invention defined in claim 14 wherein said linear sensor adapted to be mounted in a groove cut in said roadway and embedded in a curable epoxy matrix.

21. The invention defined in claim 20 wherein said epoxy matrix when cured and said elastomeric carrier have approximatel the same durometer hardness.

22. The invention defined in claim 21 wherein 21 wherein said durometer hardness is between 75 and 85.

23. The invention defined in claim 20 including an conductive metal strip carrying said elastomeric extrusion, and means for adjusting the depth of said metal strip in said roadwa groove.

24. The invention defined in claim 1 wherein said waterproof passage has ends which are sealed by a curable elastomeric sealant.