Installation of optical sensor for use in traffic monitoring

Abstract

To provide optical sensors for use in traffic monitoring.SOLUTION: One or more spacers 320 for installing an optical cable 330 are disposed in a trench 310 that extends along an axis. The optical cable 330 includes one or more optical sensors. Each spacer 320 includes a base 322 configured to rest in a bottom of the trench 310. A first arm 324 extends from the base 322. The first arm 324 is adjacent to a first wall of the trench 310. An opposing second arm 326 extends from the base 322. The second arm 326 is adjacent to an opposing second wall of the trench 310. The optical cable 330 is configured to extend along the axis.SELECTED DRAWING: Figure 3A

Description

[Technical Field]

[0001] This application relates generally to technology for traffic monitoring. This application also relates to components, devices, systems, and methods relating to such technology. [Background technology]

[0002] Fiber optic (FO) sensors can be used to detect parameters such as strain, temperature, pressure, current, voltage, chemical composition, and vibration. FO sensors are attractive components because they are thin, lightweight, highly sensitive, robust to harsh environments, and immune to electromagnetic interference (EMI) and electrostatic discharge. FO sensors can be configured to simultaneously measure multiple spatially distributed parameters with high sensitivity in multiplexed configurations over long fiber optic cables. One example of how this can be achieved is the fiber Bragg grating (FBG) sensor. FBG sensors are formed by periodic modulation of the refractive index along a finite length (typically a few millimeters) of the core of an optical fiber. This pattern reflects a wavelength (called the Bragg wavelength) determined by the periodicity of the refractive index profile. The Bragg wavelength is sensitive to external stimuli (such as strain and / or temperature) that change the periodicity of the grating and / or the refractive index of the fiber. Therefore, FBG sensors rely on detecting small wavelength changes that occur in response to a stimulus of interest. In some implementations, FO sensors may be installed, for example, on and / or under the roadway and may operate to detect parameters associated with vehicles traveling on the roadway, such as strain, temperature, and vibration. Summary of the Invention

[0003] Embodiments described herein include an apparatus comprising one or more spacers for installing an optical cable within a trench extending along an axis. The optical cable includes one or more optical sensors. Each spacer includes a base configured to rest on the bottom of the trench. A first arm extends from the base. The first arm is adjacent to a first wall of the trench. An opposing second arm extends from the base. The second arm is adjacent to a second, opposing wall of the trench. The optical cable is configured to extend along the axis.

[0004] The system includes a plurality of spacer groups disposed within a plurality of trenches. Each spacer group is configured to support an optical cable within a respective trench extending along an axis. The optical cable includes one or more optical sensors. Each spacer includes a base configured to be placed at the bottom of the trench. A first arm extends from the base. The first arm is adjacent to a first wall of the trench. An opposing second arm extends from the base. The second arm is adjacent to an opposing second wall of the trench. The optical cable is configured to extend along the axis. [Brief explanation of the drawings]

[0005] Throughout this specification, reference is made to the accompanying drawings. [Figure 1A] 1 shows a diagram of a FO traffic monitoring system according to embodiments described herein. [Figure 1B] 1 illustrates a wavelength multiplexed system that can use a compensated sensor array that includes multiple FBG sensors disposed on a single optical fiber, according to embodiments described herein. [Figure 2] 1 shows a more detailed view of a sensing fiber deployment according to embodiments described herein. [Figure 3A] 1 shows a diagram of an optical fiber installed in a trench using one or more spacers according to embodiments described herein. [Figure 3B] 1 shows a diagram of an optical fiber installed in a trench using one or more spacers according to embodiments described herein. [Figure 3C] 1 shows a more detailed view of an exemplary spacer according to embodiments described herein. [Figure 4A] 1 illustrates a diagram of an optical fiber installed in a trench using one or more spacers and support materials, according to embodiments described herein. [Figure 4B] 1 illustrates a diagram of an optical fiber installed in a trench using one or more spacers and support materials, according to embodiments described herein. [Figure 5A] 1 illustrates a stackable spacer configured to support two optical fibers at different heights within a trench, according to embodiments described herein. [Figure 5B] 1 illustrates a stackable spacer configured to support two optical fibers at different heights within a trench, according to embodiments described herein. [Figure 6] 10 illustrates the alignment of two or more fibers placed at different heights within a trench, according to embodiments described herein. [Figure 7A] 10 illustrates an output signal showing temporal characteristics that can be used to determine various aspects of a vehicle as it travels on a road above an optical fiber, according to embodiments described herein. [Figure 7B] 10 illustrates an output signal showing temporal characteristics that can be used to determine various aspects of a vehicle as it travels on a road above an optical fiber, according to embodiments described herein. [Figure 8A] 10 illustrates vehicle speed determination based on output signals according to embodiments described herein. [Figure 8B] 10 illustrates vehicle speed determination based on output signals according to embodiments described herein. [Figure 9A] 1 illustrates vehicle classification based on spatial and temporal features in sensed signals according to embodiments described herein. [Figure 9B] 1 illustrates vehicle classification based on spatial and temporal features in sensed signals according to embodiments described herein.

[0006] The drawings are not necessarily to scale. Like numbers used in the drawings refer to like components. However, it will be understood that the use of a number to refer to a component in a given drawing is not intended to limit the component in another drawing labeled with the same number. DETAILED DESCRIPTION OF THE INVENTION

[0007] Embodiments described herein may include traffic monitoring systems capable of extracting traffic parameters, including vehicle characteristics and vehicle movement on roadways, which are essential to enable better traffic management and pavement maintenance / design, helping to mitigate traffic congestion problems, prevent catastrophic accidents due to poor road conditions, and improve the quality of life for residents.

[0008] Embodiments described herein may include one or more of: 1) the ability to monitor multiple parameters; 2) high accuracy; 3) robustness under various site and / or weather conditions; and 4) low installation and / or maintenance costs and low downtime. Embodiments herein may include traffic monitoring system hardware based on optical sensors. According to various configurations, the sensors may be fiber Bragg grating (FBG) strain sensors, Fabry-Perot sensors, and / or other interferometric optical sensors. In some cases, the sensors may include one or more of electrical and / or resistive sensors, mechanical sensors, and / or other types of strain gauges. In some cases, a combination of different types of sensors may be used.

[0009] The sensor described herein is generally described as a fiber inscribed with an FBG array as a sensing element for traffic monitoring. The FBG is a wavelength-specific, narrowband reflector formed in the core of a standard fiber by introducing periodic variations in the refractive index (RI) of the fiber core. Several factors, including temperature and strain, that alter the RI variations will shift the FBG's reflected wavelength and thus be sensed by the FBG. While many of the embodiments described herein use FBGs as an example, it should be understood that any suitable type of sensor may be used. Detailed considerations for FBG array design for specific cases are discussed. The proposed optical fiber (FO)-based sensing system has several unique features. For example, the sensing system may be substantially immune to electromagnetic interference. This may reduce the frequency of system maintenance and / or calibration, which may aid in reliable long-term deployment in the field. The proposed system may be independent of visibility conditions in the field. The proposed system may be capable of temperature self-calibration.

[0010] The proposed scheme may be capable of monitoring multiple parameters, including one or more of weight-in-motion, speed, axle count, and vehicle class, with high accuracy and high dynamic range. The proposed scheme may provide higher spatial resolution of vehicles on lanes and may detect lane change and / or lane crossing events. According to various embodiments, the high spatial resolution may enable the systems described herein to detect unbalanced loads and / or missing and / or deflated tires.

[0011] Various embodiments present an installation strategy for permanently incorporating fibers into the pavement. While this is an invasive installation that introduces a certain amount of material into the pavement, the proposed FBG-based FO sensing system is envisioned to facilitate a standardized installation procedure, has the potential for high levels of multiplexing, and is compatible with the mature mass production of FBG FO sensors, making the present invention suitable and cost-effective for large-scale deployment for multi-parameter traffic monitoring.

[0012] The embodiments described herein include a fiber etched with an FBG array embedded in the pavement to sense vehicle movement. FIG. 1A shows a diagram of a FO traffic monitoring system according to the embodiments described herein. Vehicles traveling through a sensing area 105 induce pavement deformation. This deformation causes strain on the sensors 120 embedded in the pavement, generating an FBG wavelength-shifted signal. The fiber is connected at one end to an FBG interrogator, which tracks the center wavelength of each FBG on the fiber at a desired frequency. The center wavelengths of the FBGs can be streamed to a processor 130 having a data collection module 132 and an analysis module 134, which converts this information into traffic parameters. The traffic parameters may include, for example, one or more of vehicle speed, traffic volume, the number of axles of at least one vehicle on the road, the vehicle classification of at least one vehicle on the road, the vehicle position within the lane, vehicle weight, and the weight per axle of at least one vehicle on the road. The extracted information can then be forwarded to the cloud 140, allowing a remote control center to use the information for traffic and / or road condition management. In some embodiments, the information transformation can occur after the raw sensory data is transferred to the cloud.

[0013] Typically, there are multiple FBG sensors on a single fiber. The central wavelength of each FBG's reflection band is distributed over a specific wavelength range. For example, the wavelength range can be 1510 nm to 1590 nm. In one embodiment, the reflection wavelengths of each FBG on the same fiber are spectrally spaced. For example, the spectral spacing of FBGs on the same fiber can be approximately 2 to 3 nm. In the wavelength range of 1510 to 1590 nm, a 3 nm spacing allows approximately 26 FBGs on a single fiber to be interrogated simultaneously. In another embodiment, FBGs on the same fiber can have overlapping reflection bands, and signals from different FBGs are distinguished by additional time-domain features (e.g., reflection times). In general, the sensing fiber design of this application must consider the required multiplexing level and the tradeoff between system performance (e.g., sampling rate, wavelength accuracy) and overall cost (e.g., hardware, installation, maintenance).

[0014] FO sensors can measure multiple spatially distributed parameters simultaneously with high sensitivity in a multiplexed configuration over a long FO cable. One example of how this can be achieved is with fiber Bragg grating (FBG) sensors. Figure 1B shows a wavelength-multiplexed system 100 that can use a compensated sensor array including multiple FBG sensors 121, 122, and 123 arranged on a single optical fiber 111. The sensors 121-123 can be configured to sense parameters including, for example, one or more of temperature, strain, and / or vibration. As shown in Figure 1B, input light is provided by a light source 110, which can include, or be, for example, a light-emitting diode (LED) or a superluminescent laser diode (SLD). The spectral characteristics (intensity vs. wavelength) of the broadband light are shown by inset graph 191. The intensity is highest near the center of the spectrum and drops off at the edges of the spectrum. The sensors 121, 122, and 123 include compensation, e.g., one or more of different reflectivities and different attenuations, to reduce differences in the intensity of the output signal light reflected by the sensors and compensate for input light that is non-uniform in intensity due, for example, to spectral non-uniformity of the light source and / or scattering losses in the optical fiber. The input light is transmitted to the first FBG sensor 121 via the optical fiber (FO) cable 111. The first FBG sensor 121 reflects a portion of light in a first wavelength band having a center wavelength λ1. Light having wavelengths other than λ1 within the first wavelength band is transmitted through the first FBG sensor 121 and transmitted to the second FBG sensor 122. The spectral characteristics of the light transmitted to the second FBG sensor 122 are shown in inset graph 192, which shows a notch 181 in the first wavelength band centered at λ1, indicating that light in this wavelength band is reflected by the first sensor 121.

[0015] The second FBG sensor 122 reflects a portion of the light in a second wavelength band having a center wavelength λ2. Light that is not reflected by the second FBG sensor 122 is transmitted through the second FBG sensor 122 to the third FBG sensor 123. The spectral characteristics of the light transmitted to the third FBG sensor 123 are shown in inset graph 193 and include notches 181, 182 centered at λ1 and λ2.

[0016] The third FBG sensor 123 reflects a portion of light in a third wavelength band having a center or peak wavelength λ3. Light that is not reflected by the third FBG sensor 123 is transmitted through the third FBG sensor 123. The spectral characteristics of the light transmitted through the third FBG sensor 123 are shown in inset graph 194 and include notches 181, 182, 183 centered at λ1, λ2, and λ3.

[0017] Light in wavelength bands 161, 162, 163 having center wavelengths λ1, λ2, and λ3 (shown in inset graph 195) is reflected along FO cables 111 and 111′ by first, second, or third FBG sensors 121, 122, 123, respectively, to optical wavelength demultiplexer 150. The compensated input characteristics of sensors 121, 122, 123 reduce the difference in intensity peaks of light 161, 162, 163 when compared to the intensity peaks from an uncompensated sensor array.

[0018] From the wavelength demultiplexer 150, the sensor lights 161, 162, 163 may be sent to a wavelength shift detector 155, which generates an electrical signal responsive to a shift in the center wavelengths λ1, λ2, λ3 and / or wavelength bands of the sensor lights. The wavelength shift detector 155 receives the reflected light from each of the sensors and generates a corresponding electrical signal responsive to a shift in the center wavelengths λ1, λ2, and λ3 or wavelength bands of the light reflected by the sensors 121-123. The analyzer 156 may compare the shift to a characteristic fundamental wavelength (known wavelength) to determine whether a change has occurred in the value of a parameter sensed by the sensors 121-123. The analyzer 156 may determine that the value of one or more of the sensed parameters has changed based on the wavelength shift analysis and may calculate a relative or absolute measurement of the change.

[0019] In some cases, instead of emitting broadband light, the light source can scan a wavelength range, emitting light in narrow wavelength bands to which various sensors located on the FO cable are sensitive. Reflected light is sensed during several sensing periods timed relative to the emission of the narrowband light. For example, consider a scenario in which sensors 1, 2, and 3 are located on the FO cable. Sensor 1 is sensitive to wavelength band (WB1), sensor 2 is sensitive to wavelength band WB2, and sensor 3 is sensitive to WB3. The light source may be controlled to emit light having WB1 during period 1 and sense reflected light during period 1a, which overlaps with period 1. Following period 1a, the light source may emit light having WB2 during period 2 and sense reflected light during period 2a, which overlaps with period 2. Following period 2a, the light source may emit light having WB3 during period 3 and sense reflected light during period 3a, which overlaps with period 3. Using this version of time-domain multiplexing, each of the sensors can be interrogated during a separate period. If the intensity of the narrowband light source varies, a compensated sensor array as described herein can be useful to compensate for the intensity fluctuations of the light source.

[0020] FO cables may include single-mode (SM) FO cables or multimode (MM) FO cables. While single-mode fiber optic cables provide signals that are easier to interpret, multimode fibers may also be used to achieve wider applicability and lower manufacturing costs. MM fibers may be made of plastic rather than the silica typically used in SM fibers. Plastic fibers may have a smaller turning radius compared to that of silica fibers. This can offer the potential for curved or flexible configurations, for example. Furthermore, MM fibers can operate with less expensive light sources (e.g., LEDs) as opposed to SM fibers, which may require more precise alignment with superluminescent diodes (SLDs). Therefore, sensing systems based on optical sensors in MM fibers can yield lower-cost systems.

[0021] FIG. 2 shows a more detailed view of a sensing fiber deployment according to some embodiments. In this example, there are two lanes 230, 240 with a direction indicated by arrow 250. Two optical fibers 210, 220 with multiple FO sensors 215, 225 are installed perpendicular to the direction of traffic 250. For example, the second optical fiber 220 may be installed a predetermined distance from the first optical fiber 210. According to various embodiments, there may be more than two optical fibers, and / or the optical fibers may be installed in a configuration other than perpendicular to the direction of traffic. While FIG. 2 shows optical fibers installed above and / or below two lanes, it should be understood that optical fibers may be installed above and / or below any number of lanes.

[0022] According to various embodiments described herein, an optical fiber is placed in two trenches that are a predetermined distance from each other. For example, the distance between the two trenches can range from about 0.5 m to about 5 m. In some cases, the distance between the two tranches ranges from about 1 m to about 3 m. For example, as shown in Figures 3A and 3B, each trench 310 can have cross-sectional dimensions (W x H) of about 20 mm x 25 mm.

[0023] According to various embodiments, each fiber 330 can be held at a desired height within the trench 310 by one or more spacers 320. For example, the sensing fiber 330 can be held approximately 5 mm below and / or at the same height as the road surface. Each spacer can include a base 322 and two arms 324, 326. At least one of the first arm 324 and the opposing second arm 326 can extend from the base 322 at an angle greater than 90°, for example. One or both of the first arm 324 and the second arm 326 can be configured to clamp to a respective wall of the trench. The clamping mechanism can be configured to be permanent and / or temporary. The first arm 324 and the second arm 326 are shown extending substantially linearly from the base 322. It should be understood that one or both of the first arm 324 and the second arm 326 can extend non-linearly from the base.

[0024] The base 322 can be configured to rest at the bottom of the trench 310. The spacers 320 are designed to act as support posts for the optical fibers 330. According to various configurations, the spacers 320 are configured to stay in place when they are deployed for a certain amount of shock during installation. The spacers 320 can be made of one or more of plastic, acrylic, metal, and / or epoxy. According to various embodiments, the spacers 320 can be molded and / or cured in place.

[0025] 3B shows a top view of an optical fiber 330 with one or more sensors 350, 352 installed within a trench 310. The optical fiber 330 is supported by one or more spacers 350, 352. The spacers 350, 352 are spaced apart at a predetermined distance S to adequately support the optical fiber 330. D For example, S D can range from about 1 cm to about 50 cm. D can range from about 10 cm to about 35 cm.

[0026] 3C shows a more detailed view of an exemplary spacer according to embodiments described herein. The spacer has a height S ranging from about 5 mm to about 20 mm. H The spacer holds the optical fiber at a predetermined distance S from the bottom of the trench. BH For example, the SBH may be in the range of about 2 mm to about 20 mm. The spacer may have a bottom width S in the range of about 5 mm to about 15 mm. BW The spacer has a width S on the surface facing the fiber in the range of about 2 mm to about 10 mm. FW may have:

[0027] Some support materials, such as metal bars, can be used in conjunction with one or more spacers to further stabilize the fiber within the trench, as shown in FIGS. 4A and 4B. One or more of the dimensions, material, and / or structure of the support material 440 are selected to: 1) prevent significant deflection under its own weight when held within the trench 410 by the spacers 420, optimize its overall mass, and thereby eliminate potential hazards to the pavement; and 3) be compatible with the filler resin (e.g., matched thermal expansion coefficients). In some cases, the filler material may include epoxy. The use of additional support materials is expected to improve the mechanical robustness of the system during and / or after installation and make the installation more controllable. For example, a single rigid bar (e.g., a metal bar and / or composite bar) can be initially positioned inside the trench 410 and held in place by the spacers 420. The optical fiber 430 carrying the FBG 435 can then be attached to the surface of the spacers 440, and a desired prestrain can be applied to the sensing point. The support material 440 may be configured to prevent the fiber from becoming strained by a subsequent resin filling step when the trench is at least partially filled with epoxy. In some embodiments, the sensing fiber 430 may be pre-mounted on the support bar 440 with a desired pre-strain applied to a sensing point 435 outside the trench 410. The functionalized support bar may then be positioned within the trench 410 and held in place by the spacers 420.

[0028] The ability to control the amount of prestrain applied to the sensor and prevent the fiber from being obstructed by the resin can be useful in multiplexing applications, where FBGs on one sensing fiber can be densely packed in a specific spectral range with a small spectral distance between them. Unwanted stretching or compression of the sensing fiber during installation can significantly change the spectral position of the affected FBG sensor, thus interfering with the spectral spacing between sensing points. According to various embodiments, if two sensing points overlap in the spectrum, the performance of the sensing system can significantly deteriorate. Therefore, introducing support materials, such as metal bars, can make the proposed system less susceptible to errors, improve the success rate of deployment, and / or improve the reliability of the overall system.

[0029] According to various embodiments, the height of the sensing fiber within the trench affects the signal strength / characteristics of the signal picked up by the FBG sensor. The closer the fiber is to the road surface, the stronger the wavelength shift of the FBG may be when a vehicle passes. However, when the fiber is closer to the surface, the signal pattern may be more complex as there are multiple factors that may contribute to deformation of the shallow layer of pavement, including friction between the tire and road, and the weight and / or load of the vehicle.

[0030] In one embodiment, the sensing fiber can be attached close to the bottom of the first asphalt layer (e.g., about 20 mm to about 30 mm from the road surface). In this configuration, replacing the first pavement layer does not necessarily destroy the sensing system. At this height, the FBG sensor can provide a cleaner signal for vehicle feature extraction because it is less affected by frictional forces and is more specific to vertical strain, which correlates better with vehicle weight.

[0031] After the fiber is in place, resins can be used to seal the trench and connect the sensing element to the pavement, enabling traffic monitoring. Polyurethane-based PU200, acrylic-based AS475, P5G, and epoxy-based G100 are some examples of filler materials.

[0032] In some cases, fibers with a reinforcing coating are used as the sensing element. The coating can be configured to mechanically strengthen the optical cable. In some cases, the coating is configured to protect the optical cable from chemicals and / or moisture. For example, polyetheretherketone (PEEK™) coatings and / or glass fiber reinforced polymer (GFRP) coatings can be used. The coated fiber allows the system to maintain its mechanical robustness while further simplifying the installation steps (e.g., no additional supporting material is required, and the fiber can be oriented and attached to the spacer while maintaining a specific pre-strain), significantly reducing on-site installation time. This can be beneficial for system deployment and make this technology more acceptable in applications (less time required for traffic control, reduced labor costs).

[0033] According to various embodiments, local temperature can affect strain transmission between the tire and the pavement, and between the pavement and the sensing element. FBGs can sense both temperature- and vehicle-induced strain, and by looking at this signal at different time scales (e.g., temperature-related strain: <1 Hz, vehicle-related strain >1 Hz), these two factors can be separated. Thus, FBG-based systems can perform self-calibration with respect to temperature factors.

[0034] According to various configurations, two or more fibers can be placed at different heights within the trench, as shown in FIGS. 5A and 5B . In this example, one or more first stackable spacers 540 are configured to contact the bottom of the trench 510 in accordance with embodiments described herein. The first stackable spacers 540 are configured to support the first optical fiber 530. One or more second stackable spacers 542 are positioned such that the base of the second stackable spacer 542 is opposite the base of the first stackable spacer 540. The base of the third stackable spacer 544 is configured to substantially contact the base of the second stackable spacer 542. The third stackable spacer 544 is configured to support the second optical fiber 532. The second stackable spacer 542 and the third stackable spacer 544 space the second optical fiber 532 a predetermined distance F from the first optical fiber 530. H For example, F H can range from about 10 mm to about 20 mm.

[0035] According to various embodiments, the two fibers 530, 532 can be used to target different sensing parameters. The fiber 532 located closer to the pavement surface may be more sensitive and generate a larger response when a vehicle passes. These sensors 532 may be useful for event detection and axle counting, and may improve system detection accuracy for small and / or light vehicles. The fiber 530 deeper in the trench may be less affected by frictional forces and provide a cleaner signal for vehicle weight analysis.

[0036] According to various embodiments, the second and third stackable spacers are integrated into one stackable spacer, as shown in FIG. 5B . In this example, one or more first stackable spacers 546 are configured to contact the bottom of trench 512. First stackable spacer 546 is configured to support first optical fiber 534. One or more second stackable spacers 548 are positioned such that the base of second stackable spacer 548 is opposite the base of first stackable spacer 546. Second stackable spacer 548 is configured to support second optical fiber 536. While FIGS. 5A and 5B show an example in which two optical fibers are placed at different heights within the trench, it should be understood that any number of optical fibers may be placed at different heights within the trench. For example, more spacers may be used to accommodate additional optical fibers.

[0037] In some embodiments, the sensing points 650, 652 on the two fibers 630, 632 have substantially identical positions along their respective fibers, and each sensing point is substantially aligned with its corresponding sensing point on the other fiber during installation, as shown in FIG. 6 . In this case, a vehicle traveling perpendicular to the fiber pair may activate the same set of sensors on the two fibers in a similar manner. Given the complex nature of tire-pavement interactions, this configuration may provide the sensing system with self-referencing and / or self-calibrating capabilities, which may help achieve highly accurate performance in real-world situations. In some cases, the sensing points 650, 652 are not aligned with each other. The sensing point alignment shown in FIG. 6 may be applicable to two fibers installed in different trenches along a single lane.

[0038] As a vehicle travels down a roadway above the optical fiber, temporal signatures can be collected. For example, Figures 7A and 7B show an example in which a first axle 710 of the vehicle crosses the optical fiber, creating a first signal peak 715. A second axle 720 of the vehicle crosses the optical fiber, creating a second signal peak 725 at a later time.

[0039] The vehicle speed can be determined by the temporal characteristics of the signal. For example, as shown in the example of Figures 8A and 8B, the time delay between the signals of two fibers 820, 830 corresponding to the same axle and / or other identifiable feature of the vehicle 810 can be utilized. The spacing ΔL between the two sensing fibers 820, 830 and the sampling rate of the detection system determine the speed detection resolution of the system in this case. For example, in Figure 8B, the vehicle speed can be determined by

[0040]

number

[0041] Vehicle axle counting and classification is determined by features (e.g., number of peaks) extracted from the dynamic response of the sensors during an event. The physical spacing of the sensing points (lateral spatial resolution of the sensing system), the system sampling rate, and the depth of the sensing elements in the pavement all affect the event features and therefore the accuracy of axle counting and classification.

[0042] Vehicle weight can be derived from the signal strength of the FBG wavelength shift. The high spatial resolution of the FBG array provides details about the tire-pavement interaction. The actual depth of the sensing element, the local stiffness of the pavement, and the temperature affect the amplitude of the wavelength shift for a given weight, so calibration of the local response can be used to achieve accurate running weight using the system. Figures 9A and 9B show vehicle classification based on spatial and temporal features in the sensing signal.

[0043] Unless otherwise indicated, all numbers expressing shapes, sizes, quantities, and physical properties used in the specification and claims are to be understood in all instances as modified by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and appended claims are approximations that may vary depending upon the desired properties sought to be obtained by one of ordinary skill in the art utilizing the teachings disclosed herein. The use of numerical ranges by endpoints includes all numbers within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5), and any range within that range.

[0044] The various embodiments described above may be implemented using circuits and / or software modules that interact to provide particular results. Those skilled in the computing arts can readily implement such described functionality, either at a modular level or as a whole, using knowledge commonly known in the art. For example, the flowcharts illustrated herein may be used to create computer-readable instructions / code for execution by a processor. Such instructions may be stored on a computer-readable medium and transferred to a processor for execution as known in the art.

[0045] The foregoing description of exemplary embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the inventive concepts to the precise form disclosed. Many modifications and variations are possible in light of the above teachings. Any or all features of the disclosed embodiments may be applied individually or in any combination and are not intended to be limiting, but are purely illustrative. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.

Claims

**Claim 1** An apparatus, two or more spacers for installing an optical cable in a trench extending along an axis, wherein the optical cable includes one or more optical sensors, and each spacer includes a base configured to be placed at the bottom of the trench, a first arm extending from the base, the first arm being adjacent to a first wall of the trench, a second opposite arm extending from the base, the second arm being adjacent to a second opposite wall of the trench, the optical cable being configured to extend along the axis, and the optical sensors being located away from the two or more spacers such that light input to the one or more optical sensors experiences a wavelength shift due to traffic moving along the optical cable, a support member configured to extend between the first arm and the second arm along the axis, An apparatus comprising one or more spacers. **Claim 2** The apparatus according to claim 1, wherein the one or more optical sensors include fiber Bragg grating (FBG) sensors, and pavement deflection due to traffic causes strain in the FBG sensors, and the strain generates the wavelength shift. **Claim 3** The apparatus according to claim 1, wherein the FBG sensors include two or more FBG sensors having reflection wavelengths spaced apart in a spectrum and being capable of being interrogated simultaneously. **Claim 4** The apparatus according to claim 1, wherein at least one of the first arm and the second opposite arm extends from the base at an angle greater than 90°. **Claim 5** The apparatus according to claim 1, wherein the optical cable is configured to be in substantial contact with the base, and the base holds the optical cable at a predetermined distance from the trench. **Claim 6** The apparatus according to claim 1, wherein the optical cable is configured to be in substantial contact with the support material. **Claim 7** The apparatus according to claim 1, wherein the support material is a rigid bar. **Claim 8** The apparatus according to claim 1, wherein the optical cable has a coating that mechanically strengthens the optical cable and protects the optical cable from one or both of chemicals and moisture. Each spacer of the one or more spacers is a first stackable spacer and further includes a second stackable spacer, and the second stackable spacer is arranged such that a base portion of the second stackable spacer becomes an opposing base portion of the base portion of the first stackable spacer. The apparatus according to claim 1.

9. The apparatus according to claim 1, wherein the trench is disposed in one or more of on and under the road.

10. The apparatus according to claim 9, wherein the one or more optical sensors are configured to measure at least one parameter of traffic on the road in response to the wavelength shift.

11. The apparatus according to claim 10, wherein the at least one parameter includes one or more of vehicle speed, traffic volume, number of axles of at least one vehicle on the road, vehicle classification of the at least one vehicle on the road, and weight per axle of the at least one vehicle on the road.

12. The apparatus according to claim 1, further comprising a resin configured to seal the trench and connect the one or more sensors to a road pavement.

13. The apparatus according to claim 1, wherein the first arm portion and the second arm portion are configured to clamp to respective wall portions of the trench.

14. An apparatus, One or more spacers for installing an optical cable in a trench extending along an axis, wherein the optical cable includes one or more optical sensors, and each spacer includes A base configured to be placed at the bottom of the trench, A first arm portion extending from the base, the first arm portion being adjacent to a first wall portion of the trench, the first arm portion, An opposing second arm portion extending from the base, the second arm portion being adjacent to an opposing second wall portion of the trench, and the optical cable being configured to extend along the axis. Each spacer of the one or more spacers includes an opposing second arm portion, and each spacer of the one or more spacers is a first stackable spacer and further includes a second stackable spacer, and the second stackable spacer is arranged such that the base portion of the second stackable spacer opposes the base portion of the first stackable spacer. An apparatus comprising one or more spacers. **Claim 15**: The apparatus according to claim 14, further comprising a third stackable spacer, wherein the base of the third stackable spacer is configured to substantially contact the base of the second stackable spacer, and a second optical cable is configured to extend along the trench on the base of the third stackable spacer. **Claim 16**: The apparatus according to claim 15, wherein the second stackable spacer and the third stackable spacer are combined to create one continuous spacer. **Claim 17**: A system comprising: A plurality of spacer groups installed in a plurality of trenches, each spacer group being configured to support an optical cable in a respective trench extending along an axis, the optical cable including one or more optical sensors, and each spacer in each spacer group comprising: A base configured to be placed at the bottom of the trench; A first arm extending from the base, the first arm being adjacent to a first wall of the trench; An opposing second arm extending from the base, the second arm being adjacent to an opposing second wall of the trench, the optical cable being configured to extend along the axis, and the optical sensors being positioned away from the two or more spacers such that an optical input to the one or more optical sensors experiences a wavelength shift due to traffic moving along the optical cable. A support member configured to extend between the first arm and the second arm along the axis; A system comprising a plurality of spacer groups. **Claim 18**: The system according to claim 17, wherein the one or more optical sensors include fiber Bragg grating (FBG) sensors, and pavement deflection due to traffic causes strain in the FBG sensors, and the strain generates the wavelength shift. **Claim 19**: The system according to claim 18, wherein the FBG sensors include two or more FBG sensors having reflection wavelengths spaced apart in a spectrum and being capable of being interrogated simultaneously. **Claim 20**: The system according to claim 17, wherein the plurality of trenches are disposed under a road. **Claim 21**: The system according to claim 20, wherein the one or more optical sensors are configured to measure at least one parameter of traffic on the road in response to the wavelength shift. **Claim 22** The system according to claim 21, wherein the at least one parameter includes one or more of vehicle speed, traffic volume, the number of axles of at least one vehicle on the road, the vehicle classification of the at least one vehicle on the road, and the weight per axle of the at least one vehicle on the road. **Claim 23** The system according to claim 17, wherein each of the plurality of trenches is disposed at a predetermined distance from at least one adjacent trench of the plurality of trenches. **Claim 24** The system according to claim 23, wherein the predetermined distance ranges from about 1 m to about 3 m.

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