Traffic sensor having piezoelectric sensors which distinguish lanes

We claim:

1. A traffic sensor for sensing the number of vehicles travelling in each lane of a predetermined portion of a roadway, comprising:

a piezoelectric sensor stretched across a width of said predetermined portion of said roadway, said piezoelectric sensor generating an electrical signal when deflected by a vehicle, generated electrical signal having a first polarity when deflected by a vehicle in a first lane of said roadway and a second polarity when deflected by a vehicle in a second lane of said roadway; and

means for discriminating the polarity of said generated electrical signal and for determining from the respective polarities in which lane of said roadway said piezoelectric sensor has been deflected by one of the vehicles.

2. The traffic sensor as in claim 1, wherein said discriminating and determining means comprises first and second counters corresponding to said first and second lanes of said roadway, said first counter being incremented when said electrical signal has said first polarity and said second counter being incremented when said electrical signal has said second polarity.

3. The traffic sensor as in claim 1, wherein said discriminating and determining means comprises a microprocessor for determining the time of arrival of a received electrical signal, and the polarity of a received electrical signal and a memory for storing data indicating said time of arrival along with a designation of a lane from which said electrical signal was generated.

4. The traffic sensor as in claim 3, further comprising an inductive loop for detecting the passage of a vehicle, said microprocessor being responsive to an output of said inductive loop and determining from said output the number of the electrical signals generated in a particular lane correspond to a single vehicle.

5. A traffic sensor for sensing the number of vehicles travelling in each lane of a predetermined portion of a roadway, comprising:

a first piezoelectric sensor stretched across a width of a lane of said predetermined portion of said roadway, said first piezoelectric sensor outputting an electrical signal having a first polarity when deflected by a vehicle in said lane;

a second piezoelectric sensor stretched across said width of said lane and another lane of said predetermined portion of said roadway, said second piezoelectric sensor outputting an electrical signal having a second polarity when deflected by a vehicle in either said lane or said another lane; and

means responsive to said electrical signals from said first and second piezoelectric sensors for uniquely identifying from the polarities of said electrical signals whether a vehicle has passed through said lane or said another lane of said roadway.

6. A traffic sensor for sensing the number of vehicles traveling in each of L lanes of a predetermined portion of a roadway comprising:

n piezoelectric sensors stretched across a width of said predetermined portion of said roadway, each of said n piezoelectric sensors generating an electrical signal having one of s states of polarity when deflected by a vehicle in one of said L lanes of said roadway; at least one lane having a different polarity from an adjacent lane; and

a lane identifier responsive to respective polarities of generated electrical signals from said n piezoelectric sensors for uniquely identifying one of L=S.sup.n said lanes from at least one other of said lanes of said roadway in which at least one of said n piezoelectric sensors was deflected by a sensed vehicle.

7. The traffic sensor as recited in claim 6, wherein said lane identifier comprises, respective counters for said lanes of said roadway, a counter corresponding to a particular lane being incremented when one of the electrical signals generated by at least one of said n piezoelectric sensors is received which has a state of polarity uniquely identifying said particular lane from at least one other lane.

8. The traffic sensor as in claim 6, wherein said lane identifier comprises a microprocessor for determining the time of arrival of received electrical signals, and the state of the polarity of received electrical signals and a memory for storing data indicating said time of arrival along with a designation of a lane from which respective said electrical signals were generated.

9. The traffic sensor as in claim 8, further comprising an inductive loop for detecting the passage of a vehicle in a lane of said roadway, said microprocessor being responsive to an output of said inductive loop and determining from said output the number of the electrical signals generated in a particular lane corresponds to a single vehicle.

10. The traffic sensor as in claim 6, wherein said n piezoelectric sensors are disposed substantially parallel to each other over said L lanes of said predetermined portion of said roadway.

11. The traffic sensor as in claim 10, wherein said n piezoelectric sensors are disposed concentrically with respect to each other over said L lanes of said predetermined portion of said roadway.

12. The traffic sensor as recited in claim 6, wherein said L lanes is defined by (L=s.sup.n -1) when n includes a polarity of neutral.

13. The traffic sensor as recited in claim 12, wherein, said discriminator comprises, a microprocessor determining the time of arrival and polarity of a received electrical signal and a memory storing data indicating said time of arrival along with a designation of a lane from which said electrical signal was generated.

14. The traffic sensor as recited in claim 13, and further comprising: an inductive loop detecting a passage of a single vehicle, and said microprocessor being responsive to an output of said inductive loop and determining from said output the number of the electrical signals corresponds to said single vehicle.

15. A method of making a piezoelectric sensor having a first polarity for a first finite length in a first longitudinal section thereof and a second polarity, different from said first polarity, for a second finite length in a second longitudinal section which is adjacent said first longitudinal section in a longitudinal direction of said sensor, comprising the steps of:

extruding a piezoelectric material through an extruder at a predetermined rate;

applying an electric field having said first polarity to said piezoelectric material for a first predetermined amount of time in accordance with said predetermined rate until said first finite length is polarized with said first polarity;

switching said electric field to said second polarity; and

applying said electric field having said second polarity to said piezoelectric material for a second predetermined amount of time in accordance with said predetermined rate until said second finite length is polarized with said second polarity.

16. A traffic data acquisition method, comprising the steps of:

laying n piezoelectric sensors across L lanes of a predetermined portion of a roadway;

generating an electrical signal by each of said n piezoelectric sensors, said signals vehicle in one of said lanes L of said roadway at least one lane having a different polarity from an adjacent lane; and

determining from said electrical signals from said n piezoelectric sensors which one of L=s.sup.n said lanes of said roadway and at least one of said n piezoelectric sensors deflected by a vehicle.

17. The method as in claim 16, wherein said laying step comprises the further step of disposing said n piezoelectric sensors across said L lanes of said predetermined portion of said roadway such that the states of the polarity of the electrical signals generated in a particular lane for the respective piezoelectric sensors in said particular lane uniquely identify each of said L=s.sup.n lanes of said roadway.

18. The method as in claim 17, comprising the further steps of time stamping received electrical signals and storing time of receipt data with lane data identifying the lane from which said electrical signals were received.

19. A traffic sensor for sensing the number of vehicles travelling in each lane of a predetermined portion of a roadway, comprising: at least one piezoelectric sensor stretched across a width of said predetermined portion of said roadway, said piezoelectric sensor generating at least one electrical signal when deflected by a vehicle, each said electrical signal having a first polarity when deflected by a vehicle in a first lane of said roadway and a second polarity when deflected by another vehicle in another lane of said roadway, and a discriminator for determining the polarity of each said electrical signal that corresponds to the lane in which the piezoelectric sensor was deflected by each of the vehicles.

20. The traffic sensor as recited in claim 19, wherein, said discriminator comprises, first and second counters corresponding to said first and second lanes of said roadway, said first counter being incremented when said electrical signal has said first polarity, and said second counter being incremented when said electrical signal has said second polarity.

21. A traffic sensor for sensing vehicles traveling in different lanes of a roadway, comprising: piezoelectric sensors placed in the different lanes, a first piezoelectric sensor and a second piezoelectric sensor have polarities along their respective longitudinal sections corresponding to each lane so that a unique combination of electrical signals will be received, wherein said first piezoelectric sensor placed in a first land with a first polarity, said second piezoelectric sensor placed in an adjacent lane with a different polarity, and each piezoelectric sensor generating at least one electrical signal having one of said combinations of polarities when said each piezoelectric sensor is deflected by a vehicle in one of the different lanes, and a discriminator discriminating the combination of polarities of each said electrical signal to determine the corresponding lane in which said piezoelectric sensor was deflected by said vehicle.

22. The traffic sensor as recited in claim 21, wherein, said first piezoelectric sensor comprises piezoelectric material having a first combination of solely positive polarity adapted to be in a first of said lanes, and said second piezoelectric sensor comprises a second solely negative polarity adapted to be in a second of the lanes, and the discriminator comprises a bipolar discriminator.

23. The traffic sensor as recited in claim 21, wherein, said each piezoelectric sensor comprises at least two strips of piezoelectric material parallel with one another, and a series of multiple said parallel strips having respective polarities, the multiple said parallel strips being adapted to be in respective lanes of the roadway, and the electrical signal having a combination of said respective polarities unique to the corresponding one of the lanes in which said strips were deflected by said vehicle.

24. The traffic sensor as recited in claim 23, wherein, said strips are concentric.

25. The traffic sensor as recited in claim 21, and further comprising: a lane identifier responsive to said combinations and uniquely identifying one of said lanes from at least one other of said lanes.

26. The traffic sensor as recited in claim 25, wherein, said identifier comprises, a counter for each of said combinations, said counters being incremented individually by respective signals having said combinations.

27. The traffic sensor as recited in claim 26, wherein, said identifier comprises, an inductive loop detecting passage of a single vehicle, and a microprocessor responsive to an output of the inductive loop and the counter, and determining the number of electrical signals received during passage of said single vehicle.

28. The traffic sensor as recited in claim 25, wherein, said identifier comprises, a microprocessor determining the time of arrival and the combination of a received electrical signal, and a memory storing data indicating said time of arrival and a designation of a lane from which said electrical signal was generated.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a traffic sensor having bipolar or multi-polar piezoelectric sensing elements which uniquely identify a lane in which a vehicle is detected, and more particularly, to a piezoelectric cable or film which when stretched across a roadway generates electrical signals of different polarities or of different states in respective lanes of the roadway so that the lane from which one or more electrical signals are received may be readily discriminated by the polarity or state of the received electrical signal(s).

2. Description of the Prior Art

Traffic engineers typically collect data concerning traffic speed and density, vehicle size, loading and type, and vehicle condition as an aid in determining the design parameters for roads, highways, bridges and other structures. However, for multi-lane highways, acquiring the data required for complete evaluation and planning of these structures becomes quite difficult because of the need to monitor many lanes simultaneously. Indeed, the volume and complexity of the data required to make a complete evaluation of multi-lane roadways renders manual traffic counting impractical. As a result, automatic traffic recorders have been developed for recording data in a form which may be readily tabulated and evaluated.

Due to their electromechanical characteristics, piezoelectric materials such as piezoelectric polymer cables and films have been used as traffic sensors for acquiring traffic data. In the standard configuration, one piezoelectric sensor is disposed in each lane of traffic so that discrete electrical signals may be detected from each of the piezoelectric sensors. Unfortunately, while this technique works well for two-lane roadways, it becomes quite burdensome when traffic is to be monitored for more than two lanes of traffic. One major problem with such sensors is that they are difficult to install in the roadway and such installation requires substantial labor and creates major safety concerns. As a result, it is desired that as few easy to install sensors as possible be used to obtain the desired traffic data.

Such a standard traffic sensor is described by Myers in U.S. Pat. No. 3,911,390. Myers obtains traffic information by placing an elongated traffic sensor strip having a plurality of detector segments appropriately spaced along the sensor across a multi-lane roadway to monitor traffic in the lanes of the multi-lane roadway. The detector segments may each include a pair of parallel spaced conducting plates which generate an output signal when pushed together by the weight of a vehicle, or alternatively, the detector segments may each comprise a coaxial cable in place of the parallel spaced conducting plates. Generally, a separate detector segment is placed in each lane so that the lane may be discriminated; however, in an alternative embodiment, two or more coaxial cables are placed across the roadway to provide lane segregation. In the latter embodiment, the first coaxial cable extends completely across two lanes of traffic while the second coaxial cable extends only across one lane. The lane through which a vehicle passes is then discriminated by logically ANDing the positive outputs from each cable which are generated when the coaxial cables are deflected by the wheels of a vehicle. In this manner, the lane is discriminated in accordance with whether a positive pulse is received from just one or both cables.

The traffic sensor described by Myers typically has a low profile so that it is not readily visible by the motorists and has a gradually tapering profile so that it provides a smooth tire transition for a vehicle. The traffic sensor described by Myers is generally designed to be quite durable so that it can resist wear and damage from dirt or moisture. However, the durability of the sensor is improved by anchoring it in the roadway so that it will remain in position over a long period of time. Unfortunately, the sensors of Myers are difficult to install in the roadway, require the roadway to be closed for installation, and do not alleviate the above-mentioned safety concerns.

Traffic sensors have also been used to measure the dynamic loads exerted on a highway by traffic. For example, Siffert et al. describe in U.S. Pat. No. 4,712,423 a process for allegedly measuring the dynamic load exerted on a highway by the axles of vehicles by using the outputs of two piezoelectric cables installed in the roadway which are sensitive to the pressure and speed of vehicles passing thereover. In particular, the electrical pulses generated by the passage of vehicles over the sensors described by Siffert et al. are processed to extract weight information and speed information therefrom which is in turn used to calculate the dynamic load. However, such weigh-in-motion techniques, though relatively simple in theory, have proven difficult to implement in practice. Moreover, Siffert et al. do not disclose how to discriminate such information for different lanes of multi-lane roadways.

Similarly, Gebert et al. describe in U.S. Pat. No. 5,008,666 traffic measurement equipment including a pair of coaxial cables having piezoelectric materials and a vehicle presence detector embedded therein for detecting vehicle count, vehicle length, vehicle time of arrival, vehicle speed, the number of axles per vehicle, axle distance per vehicle, vehicle gap, headway and axle weights, and the like. This is accomplished by extending the coaxial cables including the piezoelectric materials across the roadway, measuring signals induced in the cable by passage of vehicle wheels thereover, and processing the signals to compute a total integrated spectral power of the measured signals so as to establish an empirical relationship between speed and weight of the vehicle wheels passing over the coaxial cables. However, as with Siffert et al., Gebert et al. install a separate detector in each lane and thus provide no means for collecting traffic data from multiple lanes using a minimum number of easy to install detectors.

It is desired to extend the traffic measurement techniques described by Myers, Siffert et al. and Gebert et al. to further include means for distinguishing traffic data collected from multiple lanes of a roadway using a minimum number of easy to install detectors. In particular, it is desired to develop a piezoelectric material which can generate pulses of different polarities or states in different longitudinal sections thereof so that, for example, if the piezoelectric material is extended across a multi-lane roadway, pulses of different polarities are generated in different lanes so as to uniquely identify those lanes. It is also desirable that the resulting traffic sensor be easy to install so that it can be placed across multi-lane roadways with minimum disruption of traffic. The present invention has been designed to meet these needs.

SUMMARY OF THE INVENTION

The present inventors have developed a bipolar or multi-polar elongated piezoelectric material which may be used in a traffic sensor to discriminate lanes by generating electrical signals having different polarities in different lanes of a multi-lane roadway. During manufacture of the piezoelectric sensor of the invention, the polarity of the poling field of the piezoelectric material is varied in different longitudinal sections of the piezoelectric material so that the piezoelectric material will generate pulses having different polarities in different longitudinal sections. When stretched across a roadway, the piezoelectric sensor will give, for example, a positive output when run over by a vehicle in one lane and a negative output when run over by a vehicle in another lane. Then, by using only two bipolar piezoelectric sensors in a single traffic sensor in accordance with the techniques of the invention, traffic data from up to eight lanes of traffic may be discriminated using only one simple to install traffic sensor.

In particular, the present invention relates to a piezoelectric sensor having a first polarity for a finite length in a first longitudinal section thereof and a second polarity, different from the first polarity, for a finite length in a second longitudinal section which is adjacent the first longitudinal section in a longitudinal direction of the sensor. When so configured, a deflection of the piezoelectric sensor in one of the longitudinal sections generates an electrical signal having a polarity unique to the deflected longitudinal section.

The piezoelectric sensor of the invention may be configured as a piezoelectric cable or a piezoelectric film formed by a variety of techniques. For example, the piezoelectric sensor may be formed from a first piezoelectric cable or film having the first polarity which is spliced to a second piezoelectric cable or film having the second polarity. The spliced piezoelectric cables also may be enclosed in a braided sheath and an outer jacket for protection from dirt and moisture and the like. The piezoelectric material also may comprise a piezoelectric cable or film which is polarized during manufacture to have the first polarity in the first longitudinal section and then is polarized to have the second polarity in the second longitudinal section. This may be accomplished, for example, by varying the applied electric field as the piezoelectric material is extracted through an extruder. Of course, the piezoelectric material may be polarized into more than two polarities as desired. In addition, the piezoelectric sensor may be formed by twisting the piezoelectric material such that it has different polarization states on either side of the twist. The same effect may also be achieved by placing conducting electrodes on either side of the longitudinal sections of the piezoelectric material and connecting electrodes on opposite sides of the piezoelectric material in different longitudinal sections by way of through holes so that electric fields of different polarities may be applied to adjacent longitudinal sections.

The invention further includes a traffic sensor incorporating such a piezoelectric sensor for sensing the number of vehicles travelling in each lane of a predetermined portion of a roadway. In particular, a traffic sensor in accordance with the invention preferably comprises a piezoelectric sensor stretched across a width of the predetermined portion of the roadway so as to generate an electrical signal when deflected by a vehicle. Preferably, the generated electrical signal has a first polarity when deflected by a vehicle in a first lane of the roadway and a second polarity when deflected by a vehicle in a second lane of the roadway. The polarity of the generated electrical signal is then discriminated by roadside electronics for determining from the polarity of the received electrical signal(s) in which lane of the roadway the piezoelectric sensor has been deflected by a vehicle.

The electronics may comprise, for example, first and second counters corresponding to the first and second lanes of the roadway, where the first counter is incremented when the electrical signal has the first polarity and the second counter is incremented when the electrical signal has the second polarity. The electronics may also include a microprocessor for determining the time of arrival and polarity of a received electrical signal and a memory for storing data indicating the time of arrival along with a designation of the lane from which the electrical signal was generated. The microprocessor may also be responsive to an inductive loop which detects the passage of a vehicle so as to determine how many electrical signals generated in a particular lane correspond to a single vehicle.

Generally, a traffic sensor in accordance with the invention may measure the number of vehicles travelling in each lane L of a predetermined portion of a multi-lane roadway by stretching n piezoelectric sensors in a substantially parallel manner across a width of the predetermined portion of the roadway and generating at each of the n piezoelectric sensors an electrical signal having one of s states when that piezoelectric sensor is deflected by a vehicle in one of the lanes L of the roadway. Up to L=s.sup.n lanes of the multilane roadway may be uniquely identified in this manner. In a preferred embodiment, the n piezoelectric sensors are disposed concentrically with respect to each other in the same cable housing and placed over at least two lanes of the predetermined portion of the roadway. Alternatively, one concentric piezoelectric sensor may be placed across two lanes while the other differently polarized concentric piezoelectric sensor is placed across only one lane in a manner analogous to that described by Myers.

An alternative embodiment of a traffic sensor is also described in which a separate piezoelectric sensor for each lane of the predetermined portion of the roadway is disposed in a rugged housing which is stretched across the predetermined portion of the roadway. A separate cable within the housing is connected to each of the piezoelectric sensors in each lane for relaying the electrical signals generated by the piezoelectric sensors to a measuring location where the lane from which the traffic data is measured is readily determined by the cable from the traffic data is received.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the invention will become more apparent and more readily appreciated to one of ordinary skill in the art from the following detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, of which:

FIG. 1 illustrates a bipolar piezoelectric cable sensor which is formed by splicing a positively polarized piezoelectric cable with a negatively polarized piezoelectric cable.

FIG. 2 illustrates a bipolar piezoelectric cable sensor which is formed by splicing a positively polarized piezoelectric cable with a negatively polarized piezoelectric cable and enclosing the spliced cables within a braided sheath and an outer jacket for protection from the elements.

FIG. 3 illustrates a multi-polar piezoelectric cable sensor which has different polarities in different longitudinal sections thereof which are formed during the manufacturing process by applying an electric field having a first polarity during extrusion of a first length of cable and applying an electric field having a second polarity during extrusion of a second length of cable.

FIG. 4 illustrates a piezoelectric film sensor comprising oppositely polarized piezoelectric films which are spliced together.

FIG. 5 illustrates a piezoelectric film sensor comprising a single twisted piezoelectric film which has different polarities on either side of the twist.

FIG. 6 illustrates a piezoelectric film sensor having opposite electrodes from adjacent longitudinal sections connected via through holes so that electric fields of opposite polarity may be applied to the adjacent longitudinal sections of the piezoelectric film.

FIG. 7 illustrates a multi-polar piezoelectric film sensor which is polarized during manufacture in the same manner as the piezoelectric cable sensor of FIG. 3.

FIG. 8 illustrates an implementation of the piezoelectric sensors of the invention as a traffic sensor which discriminates lanes of a roadway.

FIG. 9 illustrates an implementation of the piezoelectric sensors of the invention as a traffic sensor whereby two piezoelectric sensors having different polarities in different lanes discriminate 8 different lanes of a multi-lane roadway.

FIG. 10 illustrates an embodiment of a traffic sensor of the invention in which two piezoelectric sensors are concentrically disposed within the same cable.

FIG. 11 illustrates an alternative embodiment of the invention in which a separate piezoelectric sensor is used for each lane of the roadway and is connected to a measuring device at the side of the roadway by separate