Time Synchronization Method Using Reliable Beacons and Distributed Fiber Sensing

Abstract

A time synchronization method using a DFOS sensing fiber that employs several reliable time beacons attached to the DFOS sensing fiber connected to a DFOS interrogator. The beacons transmit signals via two different media, namely, (1) wirelessly transmit signals to sensor nodes within the effective range area, and (2) transmit signals via vibrations on the fiber to a DFOS / DAS system in a reliable area such as a central office. The wireless broadcast to nearby sensors includes a timestamp and a beacon ID. All sensors in the field use one of the nearby beacons (the one with the strongest signal) as a time reference and send back data along with the corresponding beacon index.

Description

[Technical Field]

[0001] This application relates generally to network time synchronization and distributed fiber optic sensing (DFOS) systems, methods, and architectures and related techniques. More specifically, this application relates to a time synchronization method using reliable beacons and distributed fiber optic sensing. [Background technology]

[0002] Time synchronization is a critical consideration for emerging sensing network operations, which require accurate mapping and understanding of the temporal ordering of events. For example, wireless sensing networks require precise time synchronization of sensor nodes to determine message ordering, control collaborative activities, and serialize concurrent access to shared objects.

[0003] With the development of modern wireless sensor networks and Internet of Things (IoT) technologies, the number of deployed sensor nodes is rapidly increasing, especially in large-scale operations such as smart cities and smart factories. As such, time synchronization becomes even more important and challenging as these nodes become more densely packed.

[0004] Traditional synchronization methods, such as the Network Time Protocol (NTP) and Global Positioning Satellite (GPS) systems, are not suitable for these emerging sensor networks for a variety of reasons. NTP requires senders and receivers to exchange messages multiple times, which is inefficient for sensor nodes due to their energy consumption constraints and computational capabilities. GPS equipment is often too expensive to install on each sensor node, and GPS signals can be obstructed or unavailable inside buildings and urban canyons.

[0005] To overcome these problems, several synchronization methods designed for wireless sensor networks have been proposed, including Reference Broadcast Synchronization (RBS), Time Synchronization Protocol for Sensor Networks (TPSN), and Flooding Time Synchronization Protocol (FTSP). These methods broadcast synchronization messages from a trusted beacon to a group of sensors within its transmission range or network layer. All sensors can "locally" synchronize to the time of a beacon within its broadcast range. However, due to power constraints, the communication range between the beacon and the sensor is severely limited. Therefore, each time beacon can only cover a limited area. For large-scale operation, multiple beacons must be deployed to serve the entire coverage area. As a result, time / clock synchronization among multiple beacons under various environmental conditions is required. Summary of the Invention

[0006] An advancement in the art is made in accordance with aspects of the present disclosure regarding a novel method for precisely synchronizing all sensor nodes over a large coverage area by utilizing a new type of time beacon incorporated into distributed fiber sensing (DFOS) technology.

[0007] In contrast to the prior art, from a first perspective, our approach employs several reliable time beacons attached to DFOS sensing fibers, which are connected to a DFOS interrogator. The beacons transmit signals via two different media: (1) wirelessly to sensor nodes within their coverage area, and (2) via vibration on the fiber to a DFOS / DAS system in a trusted area, such as a central station. The wireless broadcast to nearby sensors includes a timestamp and a beacon ID. All sensors in the field use one of the nearby beacons (the one with the strongest signal) as a time reference and send back data with the corresponding beacon index.

[0008] The beacon transmits a timestamp (or clock) by vibrating the fiber via a built-in vibrator or speaker. The fiber sensing interrogator detects the timestamp signal from the beacon, which includes its unique location, which serves as a unique beacon ID (BID).

[0009] Because each beacon is located at a known, fixed, and unique position along the fiber, the DFOS / DAS system automatically distinguishes the timestamp of each beacon without any additional beacon ID transmission.

[0010] Advantageously, by coordinating the timestamps, data streams from different sensors can be centrally synchronized.Further advantageously, the method of the present invention can be used in wireless sensor networks with a fiber cable backbone or in hybrid fiber-wireless sensor networks.

[0011] As those skilled in the art will understand and appreciate, particularly distinguishing features of the present disclosure include at least: 1) a beacon that wirelessly broadcasts a synchronization message to nearby sensor nodes and acoustically encodes the message into nearby fiber optic sensor cables; 2) a distributed fiber sensing system that detects synchronization messages from beacons located in multiple locations; 3) the present invention's innovative technique for decoding timestamps received from fiber optic sensing signals and synchronizing the sensors; and 4) because the DFOS / DAS system's detection of beacon acoustic / vibration signals is spatiotemporal (i.e., the beacon's location is also detected), the location of each beacon is indicated by its unique ID, and therefore, compared to wireless solutions, beacons do not need to transmit an additional unique ID to a central station. [Brief explanation of the drawings]

[0012] [Figure 1(A)] FIG. 1 is a schematic diagram illustrating an exemplary prior art uncoded DFOS system. [Figure 1(B)] FIG. 1 is a schematic diagram illustrating an exemplary prior art coded DFOS system.

[0013] [Figure 2] 1 is a schematic diagram illustrating an example network including multiple beacons and a distributed fiber optic sensing system (DFOS), according to aspects of the present disclosure.

[0014] [Figure 3] FIG. 1 is a schematic flow diagram illustrating an exemplary operation according to aspects of the present disclosure.

[0015] [Figure 4] 1 is a schematic diagram illustrating an example structure of a wireless synchronization message broadcast from a beacon that does not require an explicit beacon identifier (ID), according to an embodiment of the present invention.

[0016] [Figure 5] FIG. 1 is a schematic diagram illustrating an example local synchronization model according to aspects of the present disclosure.

[0017] [Figure 6] 1 is a schematic diagram illustrating an exemplary installation scheme in an application according to an aspect of the present invention.

[0018] [Figure 7] FIG. 2 is a schematic diagram illustrating an exemplary global synchronization model according to an aspect of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] The following is merely illustrative of the principles of the present disclosure, and it will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the present disclosure and are included within its spirit and scope.

[0020] Furthermore, all examples and conditional language set forth herein are intended to be for educational purposes only to aid the reader in understanding the concepts contributed by the inventors to further the principles and techniques of the present disclosure, and should not be construed as being limited to such specifically recited examples and conditions.

[0021] Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, such equivalents are intended to include both currently known equivalents as well as equivalents developed in the future, i.e., elements developed that perform the same function, regardless of structure.

[0022] Thus, for example, it will be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the disclosure.

[0023] Unless otherwise specified herein, the figures comprising the drawings are not drawn to scale.

[0024] As some additional background, note that a distributed fiber optic sensing system interconnects an optoelectronic integrator to an optical fiber (or cable), transforming the fiber into an array of sensors distributed along the fiber. In effect, the fiber becomes the sensor, and the interrogator generates / injects laser light energy into the fiber to sense / detect events along the fiber.

[0025] As those skilled in the art will understand and appreciate, DFOS technology can be deployed to continuously monitor vehicle movement, human traffic, drilling activity, seismic activity, temperature, structural integrity, liquid and gas leaks, and many other conditions and activities. It is used worldwide to monitor power plants, communication networks, railroads, roads, bridges, borders, critical infrastructure, onshore and offshore power lines and pipelines, and downhole applications in oil, gas, and enhanced geothermal power generation. Advantageously, distributed fiber optic sensing is not constrained by line of sight or remote power access and, depending on the system configuration, can be deployed over continuous lengths of more than 30 miles, with sensing / detection possible at every point along that length. Therefore, the cost per sensing point over long distances is typically incomparable to competing technologies.

[0026] Distributed fiber optic sensing measures changes in the "backscatter" of light that occurs within an optical sensing fiber when the fiber encounters environmental changes, including vibration, strain, or temperature change events. As previously mentioned, the optical sensing fiber acts as a sensor along its entire length, providing real-time information about the physical and environmental surroundings and the integrity and security of the fiber. Additionally, distributed fiber optic sensing data pinpoints the precise location of events and conditions occurring on or near the sensing fiber.

[0027] A schematic diagram illustrating the generalized arrangement and operation of a distributed optical fiber sensing system that may advantageously include artificial intelligence / machine learning (AI / ML) analysis is illustratively shown in Figure 1(A). Referring to Figure 1(A), it can be seen that the optical sensing fiber is connected to an interrogator. Although not shown in detail, the interrogator can include a coded DFOS system that can employ a coherent receiver arrangement known in the art, such as that shown in Figure 1(B).

[0028] As is well known, a modern interrogator is a system that generates an input signal into an optical sensing fiber and detects and analyzes the reflected / backscattered signal that is then received. The received signal is analyzed and an output is generated that is indicative of the environmental conditions encountered along the fiber. The received backscattered signal may be due to reflections within the fiber, such as Raman backscatter, Rayleigh backscatter, or Bullion backscatter.

[0029] As will be appreciated, modern DFOS systems include an interrogator that periodically generates optical pulses (or any coded signal) and launches them into an optical sensing fiber, which then transmits the optical pulse signal along the optical fiber.

[0030] At certain locations along the fiber, a small portion of the signal is backscattered / reflected back to the interrogator where it is received. The backscattered / reflected signal carries information that the interrogator uses to detect, such as changes in power level that indicate mechanical vibrations.

[0031] The received backscattered signal is converted to the electrical domain and processed within the interrogator. Based on the time of pulse incidence and the time the received signal is detected, the interrogator can determine from which location along the optical sensing fiber the received signal returned, thereby sensing activity at each location along the optical sensing fiber. Classification methods may also be used to detect and locate events or other environmental conditions, including acoustic and / or vibration and / or heat, along the optical sensing fiber.

[0032] 2 is a schematic diagram illustrating an exemplary network including multiple beacons and a distributed fiber optic sensing system (DFOS) according to an embodiment of the present disclosure. As previously mentioned, disclosed herein is a method for precisely synchronizing all sensor nodes over a large coverage area by utilizing a novel time beacon in combination with distributed fiber sensing technology.

[0033] As exemplarily shown in Figure 2, several reliable time beacons (a total of M beacons in this illustrative example) are connected to a sensing fiber, which is in turn connected to a DFOS interrogator (in this example, a distributed acoustic sensing (DAS) system). The beacons transmit signals via two different media: (1) wirelessly to sensor nodes within their coverage area, and (2) via vibrations on the fiber to a DFOS / DAS system located in a trusted area, such as a central station.

[0034] The radio broadcast to nearby sensors contains a timestamp and a beacon ID. All sensors in the field use one of the nearby beacons (the one with the strongest signal) as a time reference and send back data with the corresponding beacon index.

[0035] The beacon also transmits a timestamp (or clock) by vibrating the fiber via a built-in vibrator or speaker. The fiber sensing interrogator detects the timestamp signal from the beacon, which includes its unique location, which serves as a unique beacon ID (BID).

[0036] Because each beacon is located at a known, fixed, and unique position along the fiber, the DFOS / DAS system automatically distinguishes the timestamp of each beacon without any additional beacon ID transmission.

[0037] Advantageously, by coordinating the timestamps, data streams from different sensors can be centrally synchronized.Further advantageously, the method of the present invention can be used in wireless sensor networks with a fiber cable backbone or in hybrid fiber-wireless sensor networks.

[0038] As those skilled in the art will understand and appreciate, particularly distinguishing features of the present disclosure include at least: 1) a beacon that wirelessly broadcasts a synchronization message to nearby sensor nodes and acoustically encodes the message into nearby fiber optic sensor cables; 2) a distributed fiber sensing system that detects synchronization messages from beacons located in multiple locations; 3) the present invention's innovative technique for decoding timestamps received from fiber optic sensing signals and synchronizing the sensors; and 4) because the DFOS / DAS system's detection of beacon acoustic / vibration signals is spatiotemporal (i.e., the beacon's location is also detected), the location of each beacon is indicated by its unique ID, and therefore, compared to wireless solutions, beacons do not need to transmit an additional unique ID to a central station.

[0039] As shown in Figure 2, the overall system includes a fiber sensing interrogator, a sensing fiber (or cable), a reliable time beacon, and a sensor node. A fiber sensing interrogator (also known as an "interrogator") is generally referred to as a distributed optical fiber sensing system based on the scattering effect of optical fibers. As mentioned above, the interrogator can acquire changes in physical parameters (such as phase, intensity, and spectral characteristics) due to external perturbations such as vibrations and sound waves.

[0040] The interrogator components include a laser source, a modulator, an amplifier, a fiber optic circulator, a receiver, and an acquisition device. Light from the laser source is shaped into optical pulses by the modulator. An amplifier (e.g., an erbium-doped fiber amplifier or a semiconductor optical amplifier) ​​is optionally provided to adjust the optical pulse power to the desired level. The fiber optic circulator (e.g., a circulator or coupler) transmits the optical pulses into the sensing fiber and collects the backscattered signal from the fiber. The optical receiver (direct detection, coherent detection, or phase demodulator configuration) detects the returned optical signal and converts it into an electrical signal. Another amplifier and optical filter can be placed before the receiver to increase the signal-to-noise ratio (SNR) of the backscattered signal. The acquisition device (e.g., an analog-to-digital converter) digitizes the electrical signals from the receiver and transfers them to a processor / computer for further processing.

[0041] A trusted time beacon (hereinafter referred to as "beacon") in this invention is a trusted device equipped with an internal clock, a radio module, a vibration module, a processing board, and any other sensors. The internal clock is used to generate timestamps in a predefined time format. Each trusted time beacon has a unique beacon ID (hereinafter referred to as "BID") that maps to a unique location along the sensing fiber cable.

[0042] FIG. 3 is a schematic flow diagram illustrating an exemplary operation according to an aspect of the present disclosure.

[0043] 4 is a schematic diagram illustrating an exemplary structure of a wireless synchronization message broadcast from a beacon without requiring an explicit beacon identifier (ID), according to an embodiment of the present invention. As shown in this figure, the structure of the synchronization message is shown and includes a preamble, format information, a timestamp, a BID, and optional sensor information (temperature, pressure, CO2, humidity, snow / water level, geographic information, etc.) as needed.

[0044] The beacon's radio circuitry broadcasts such synchronization messages to all nearby sensors within its communication range. For example, FIG. 5 is a schematic diagram illustrating an example local synchronization model for wireless synchronization messages broadcast from beacons without requiring explicit beacon identifiers (IDs) according to aspects of the present disclosure. Note that when transmitting data, the sensor nodes include the BID used for synchronization. Sensor nodes (#1-1, #2-1, ..., #N1-1) within the effective range of beacon #1 receive the synchronization message from beacon #1. The sensor nodes then decode the synchronization message to obtain the timestamp, BID, and other information. The timestamp is used to adjust the sensor node's local time, achieving the local synchronization timescale shown in the figure.

[0045] A built-in vibrator located in each beacon generates a mechanical vibration signal (or acoustic signal). In this method, the beacon is installed near an existing optical fiber sensor cable, so that vibrations from the beacon can be detected by the sensing fiber. Alternatively, a dedicated sensing optical fiber or cable can be installed near the beacon. The detailed installation method varies depending on the application.

[0046] It should be noted that the beacon does not necessarily need to be attached directly to the fiber, but locating it closer to the optical fiber sensor generally improves the signal-to-noise ratio. In practical applications, the beacon may be installed on a utility pole hanging an overhead optical fiber cable, in a manhole with a buried optical fiber cable, or on an LTE / 5G tower with an optical fiber connection, as exemplarily shown in Figure 6. Figure 6 is a schematic diagram illustrating an exemplary installation scheme in an application according to an embodiment of the present invention.

[0047] The vibration module encodes the synchronization message as a coded vibration pattern and transmits it to a nearby fiber via a predefined modulation format, such as frequency modulation (FM). Note that the message transmitted from the vibration module may differ from the message transmitted from the wireless module. Because the position of each beacon on the sensing fiber is known, fixed, and unique, BID information in the vibration message is not required. The interrogator detects the vibration signals from all beacons and records the synchronization message transmitted from each beacon. The interrogator decodes the timestamp, BID (from the location), and any sensor information from the message and stores them as a timestamp list. The optional sensor information can be used to compensate for environmental effects on the synchronization data as well as the sensing data from all sensor nodes.

[0048] The timestamp list from the interrogator is shared with the processing unit that processes the sensor node data. As shown in FIG. 4, locally synchronized sensor data already includes a corresponding BID. The timestamp corresponding to each group of data can then be determined through the timestamp list. By adjusting the difference between the timestamps, multiple groups of locally synchronized sensor data can be globally synchronized, as shown in FIG. 7. FIG. 7 is a schematic diagram illustrating an exemplary global synchronization model according to an embodiment of the present disclosure. Once all data is synchronized, the BID information is no longer necessary and can be removed to reduce data size.

[0049] While the present disclosure has been presented above using some specific examples, those skilled in the art will recognize that the present teachings are not so limited. Accordingly, the present disclosure should be limited only by the scope of the claims appended hereto.

Claims

1. 1. A method for a distributed fiber optic sensing (DFOS) system, comprising: an optical sensor fiber; an optical interrogator configured to generate optical pulses, couple the generated optical pulses into the optical sensor fiber, and receive backscattered optical signals in response to the coupled optical pulses; an analyzer that analyzes the backscattered light signals to determine vibrational activity occurring at positions along the optical sensor fiber; a DFOS system including: a vibrator configured to generate mechanical vibrations; a wireless transceiver configured to transmit and receive wireless signals; a plurality of beacons including: The method wherein the plurality of beacons generate mechanical vibrations that are detected by the DFOS system.

2. 10. The method of claim 1, further comprising: providing a plurality of sensor nodes located within wireless range of at least one of the plurality of beacons, the plurality of sensor nodes including one or more sensors configured to sense one or more environmental conditions, the plurality of sensor nodes configured to wirelessly provide the sensed environmental conditions to at least one of the plurality of beacons.

3. The method of claim 2 , wherein at least one of the plurality of beacons wirelessly communicates a broadcast synchronization message to the plurality of sensor nodes, the broadcast synchronization message including a timestamp and beacon ID information.

4. The method of claim 3 , wherein the plurality of sensor nodes receiving the timestamp and beacon ID information are configured to locally synchronize in response to receiving the synchronization message including the timestamp and beacon ID information.

5. The method of claim 4 , wherein the plurality of sensor nodes are configured to adjust clock delays of sensing data according to received timestamps to synchronize the plurality of sensor nodes within an entire coverage area.

6. 4. The method of claim 3, wherein at least one of the plurality of beacons encodes the synchronization message including the timestamp and beacon ID with a vibration code and mechanically transmits the vibration-encoded synchronization message to the DFOS system.

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