Reliable Beacon and Time Synchronization Method Using 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 generally relates to network time synchronization and distributed fiber optic sensing (DFOS) systems, methods, and architectures, and related technologies. More specifically, this application relates to a time synchronization method using reliable beacons and distributed fiber optic sensing.

Background Art

[0002] Time synchronization is a very important consideration for emerging sensing network operations that require accurately mapping and understanding the temporal order of events. For example, in wireless sensing networks, accurate time synchronization of sensor nodes is required to determine the order of messages, control collaborative activities, and serialize concurrent access to shared objects.

[0003] With the development of modern wireless sensor networks and the Internet of Things (IoT) technology, especially in large-scale applications such as smart cities and smart factories, the number of deployed sensor nodes is increasing rapidly. Therefore, as the density of such nodes increases, time synchronization becomes even more important and difficult.

[0004] Conventional synchronization methods such as the Network Time Protocol (NTP) and the Global Positioning System (GPS) are not suitable for such emerging sensor networks for various reasons. In NTP, the sender and receiver need to exchange messages multiple times, which is inefficient for sensor nodes due to energy consumption constraints and computing capabilities of the sensor nodes. GPS devices are often too expensive to install on each sensor node, and GPS signals may be blocked or unavailable indoors or in urban valleys.

[0005] To overcome these problems, several synchronization methods designed for wireless sensor networks have been proposed, such as Reference Broadcast Synchronization (RBS), Timing-sync Protocol for Sensor Networks (TPSN), Flooding Time Synchronization Protocol (FTSP), etc. These methods broadcast synchronization messages from reliable beacons to sensor groups within the transmission range or network layer. All sensors can "locally" synchronize to the time of the beacons within the broadcast range. However, due to power constraints, the communication range between beacons and sensors is severely limited. Therefore, each time beacon can cover only a limited area. In the case of large-scale operation, multiple beacons need to be deployed to provide services across the entire effective area. As a result, time / clock synchronization between multiple beacons under various environmental conditions is required.

Summary of the Invention

[0006] Aspects of the present disclosure relate to a novel method for accurately synchronizing all sensor nodes across a large effective area by utilizing a new type of time beacon incorporated into Distributed Fiber Optic Sensing (DFOS) technology, thereby advancing the art.

[0007] In contrast to the prior art, from a first aspect, the approach of the present invention employs several reliable time beacons attached to a DFOS sensing fiber, which is connected to a DFOS interrogator. The beacons transmit signals via two different media, namely, (1) wirelessly transmit signals to sensor nodes within the effective area, and (2) transmit signals via vibrations on the fiber to a DFOS / DAS system in a reliable 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 along with the corresponding beacon index.

[0008] The beacon transmits a timestamp (or clock) by generating vibrations in the fiber via a built-in vibrator or speaker. The fiber sensing interrogator detects the timestamp signal from the beacon that includes a unique position that functions as a unique beacon ID (BID).

[0009] Since each beacon is placed at a known fixed unique position along the fiber, the DFOS / DAS system automatically differentiates the timestamps of each beacon without additional beacon ID transmission.

[0010] Advantageously, by adjusting the timestamp, data streams from different sensors can be centrally synchronized. Even more advantageously, the method of the present invention can be used in a wireless sensor network with an optical fiber cable as a backbone, or a hybrid fiber and wireless sensor network.

[0011] As will be understood and recognized by those skilled in the art, particularly prominent 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 onto a nearby optical fiber sensor cable; 2) a distributed fiber sensing system that detects synchronization messages from beacons placed at multiple positions; 3) the innovative technology of the present invention that decodes the timestamp received from the optical fiber sensing signal and synchronizes the sensors; and 4) since the detection of the acoustic / vibration signal of the beacon by the DFOS / DAS system is spatio-temporal (i.e., the position of the beacon is also detected), the position of each beacon is indicated by their unique ID, and thus, compared to wireless solutions, the beacon does not need to transmit an additional unique ID to the central station.

Brief Description of the Drawings

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DETAILED DESCRIPTION OF THE INVENTION

[0019] The following merely illustrates the principles of the present disclosure. Therefore, it will be understood that those skilled in the art can devise various configurations that embody the principles of the present disclosure and are within its spirit and scope, although not explicitly described or illustrated herein.

[0020] Furthermore, all examples and conditional terms described herein are intended solely for the educational purpose of assisting the reader in understanding the concepts contributed by the inventors to facilitate the principles and techniques of the present disclosure, and should not be construed as being limited to such specifically recited examples and conditions.

[0021] Furthermore, all descriptions in this specification that describe the principles, aspects, and embodiments of the present disclosure, as well as specific examples thereof, are intended to encompass both their structural and functional equivalents. Furthermore, such equivalents are intended to include both currently known equivalents and equivalents developed in the future, i.e., developed elements that perform the same function regardless of structure.

[0022] Thus, for example, it will be understood by those skilled in the art that any block diagram in this specification represents a conceptual diagram of an exemplary circuit implementing the principles of the present disclosure.

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

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

[0025] As will be understood and appreciated by those skilled in the art, DFOS technology can be deployed to continuously monitor vehicle movement, human traffic, excavation activities, seismic activity, temperature, structural integrity, leakage of liquids and gases, and many other conditions and activities. This is used worldwide to monitor power plants, communication networks, railways, roads, bridges, borders, critical infrastructure, onshore and offshore power lines and pipelines, and downhole applications in oil, gas, and enhanced geothermal power. Advantageously, distributed fiber optic sensing is not restricted by line of sight or remote power access and, depending on the system configuration, can be deployed in continuous lengths exceeding 30 miles, with sensing / detection possible at all points along that length. Thus, the cost per sensing point over long distances is usually not comparable to competing technologies.

[0026] Distributed fiber optic sensing measures changes in the "backscattering" of light that occur within an optical sensing fiber when the optical sensing fiber encounters environmental changes, including events of vibration, strain, or temperature change. As described above, the optical sensing fiber functions as a sensor over its entire length, providing real-time information regarding the physical / environmental surroundings and the integrity / security of the fiber. Further, distributed fiber optic sensing data identifies the exact location of events and conditions occurring at or near the sensing fiber.

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

[0028] As is well known, modern interrogators are systems that generate input signals to an optical sensing fiber, reflect / scatter them, and then detect / analyze the received signals. The received signals are analyzed to generate an output indicating the environmental conditions encountered along the fiber. The received backscattered signals may be due to reflections within the fiber, such as Raman backscattering, Rayleigh backscattering, Brillouin backscattering, etc.

[0029] As will be appreciated, modern DFOS systems include an interrogator that periodically generates optical pulses (or any encoded signal) and directs them into an optical sensing fiber. The incident optical pulse signals are transmitted along the optical fiber.

[0030] At positions along the fiber, a small portion of the signal is backscattered / reflected and returned to the interrogator where it is received. The backscattered / reflected signal conveys information that the interrogator uses to detect, for example, changes in power levels indicative of mechanical vibrations.

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

[0032] FIG. 2 is a schematic diagram showing an exemplary network including a plurality of beacons and a distributed fiber optic sensing system (DFOS) according to aspects of the present disclosure. As described above, this document discloses a method for accurately synchronizing all sensor nodes over a large effective range by utilizing a new time beacon in combination with distributed fiber optic sensing technology.

[0033] As exemplarily shown in FIG. 2, several reliable time beacons (a total of M beacons in this exemplary example) are connected to the sensing fiber, and the sensing fiber is connected to a DFOS interrogator (in this example, a distributed acoustic sensing (DAS) system). 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.

[0034] The wireless broadcast to nearby sensors includes a time stamp 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.

[0035] Also, the beacon transmits a time stamp (or clock) by generating vibrations on the fiber via a built-in vibrator or speaker. The fiber sensing interrogator detects the time stamp signal from the beacon that includes a unique position that functions as a unique beacon ID (BID).

[0036] Since each beacon is arranged at a known fixed unique position along the fiber, the DFOS / DAS system can automatically distinguish the time stamps of each beacon without additional beacon ID transmission.

[0037] Advantageously, by adjusting the time stamp, data streams from different sensors can be intensively synchronized. More advantageously, the method of the present invention can be used in a wireless sensor network with a fiber cable as a backbone or a hybrid fiber and wireless sensor network.

[0038] As will be understood and recognized by those skilled in the art, particularly prominent 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 onto a nearby fiber optic sensor cable; 2) a distributed fiber sensing system that detects synchronization messages from beacons placed at multiple locations; 3) an innovative technique of the present invention that decodes time stamps received from fiber optic sensing signals to synchronize sensors; and 4) since the detection of the acoustic / vibration signals of the beacon by the DFOS / DAS system is spatio-temporal (i.e., the position of the beacon is also detected), the position of each beacon is indicated by its unique ID, and thus, compared to wireless solutions, the beacon does not need to transmit an additional unique ID to the central station.

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

[0040] The components of the interrogator include a laser light source, a modulator, an amplifier, an optical fiber circulator device, a receiver, and an acquisition device. The light from the laser light source is shaped into optical pulses by the modulator. An amplifier (e.g., an erbium-doped optical fiber amplifier or a semiconductor optical amplifier) is optionally provided to adjust the optical pulse power to a desired level. The optical fiber circulator device (e.g., a circulator or a coupler) transmits the optical pulses to the sensing fiber and collects the backscattered signals 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 an optical filter can also be arranged in front of 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 reliable time beacon (hereinafter referred to as "beacon") in the present invention is called a reliable device equipped with an internal clock, a wireless module, a vibration module, a processing board, and any other sensors. The internal clock is used to generate time stamps in a pre-defined time format. Each reliable time beacon has a unique beacon ID (hereinafter referred to as "BID") mapped to a unique position along the sensing fiber cable.

[0042] Figure 3 is a schematic flow chart showing an exemplary operation according to an aspect of the present disclosure.

[0043] Figure 4 is a schematic diagram showing an exemplary structure of a wireless synchronization message broadcast from a beacon that does not require an explicit beacon identifier (ID) according to an aspect of the present invention. As shown in this figure, the structure of the synchronization message is shown, and it includes a preamble, format information, a time stamp, a BID, and any sensor information (temperature, pressure, CO2, humidity, snow depth / water level, geographical information, etc.) as required.

[0044] The wireless circuit of the beacon broadcasts such a synchronization message to all nearby sensors within its communication range. For example, FIG. 5 is a schematic diagram showing an exemplary local synchronization model of a wireless synchronization message broadcast from a beacon that does not require an explicit beacon identifier (ID) according to an aspect of the present disclosure. Note that at the time of data transmission, the BID used for synchronization is included in the sensor node, and the sensor nodes (♯1-1, ♯2-1,..., ♯N1-1) within the effective range of beacon ♯1 receive the synchronization message from beacon ♯1. Next, the sensor node decodes the synchronization message to obtain a timestamp, BID, and other information. The timestamp is used to adjust the local time of the sensor node and realizes a local synchronization time scale as shown in the figure.

[0045] The built-in vibrator arranged in each beacon mechanically generates a vibration signal (or an acoustic signal). In this method, by installing the beacon near an existing optical fiber sensor cable, the vibration from the beacon can be detected by the sensing fiber. Alternatively, a dedicated sensing optical fiber or cable can also 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 directly attached to the fiber, but the signal-to-noise ratio generally improves when it is installed close to the optical fiber sensor. In actual applications, the beacon may be installed on a utility pole suspending an aerial optical fiber cable, a manhole where the optical fiber cable is buried, or an LTE / 5G tower equipped with an optical fiber connection, as exemplarily shown in FIG. 6. FIG. 6 is a schematic diagram showing an exemplary installation method in the application according to an aspect of the present invention.

[0047] The vibration module encodes the synchronization message as an encoded 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 be different from the message transmitted from the wireless module. Since the position of each beacon on the sensing fiber is known, fixed, and unique, BID information for the vibration message is not required. The interrogator detects the vibration signals from all beacons and records the synchronization messages transmitted from each beacon. The interrogator decodes the timestamp, BID (from the position), and any sensor information from the message and stores them as a timestamp list. Any sensor information can be used not only for the sensing data from all sensor nodes but also to correct the environmental effects on the synchronization data.

[0048] The timestamp list from the interrogator is shared with a processing device that processes the sensor node data. As shown in FIG. 4, the locally synchronized sensor data already contains the corresponding BID. Thereafter, through the timestamp list, the timestamp corresponding to each group of data can be determined. By adjusting the difference between timestamps, as shown in FIG. 7, multiple groups of locally synchronized sensor data can be globally synchronized. FIG. 7 is a schematic diagram showing an exemplary global synchronization model according to an aspect of the present disclosure. Once all the data is synchronized, the BID information becomes unnecessary and can be deleted to reduce the data size.

[0049] So far, the present disclosure has been presented using several specific examples, but those skilled in the art will recognize that the present teachings are not so limited. Therefore, the present disclosure should be limited only by the claims appended hereto.

Claims

1. A method for a distributed fiber optic sensing (DFOS) system, comprising: An optical sensor fiber; An optical interrogator configured to generate an optical pulse, inject the generated optical pulse into the optical sensor fiber, and receive a backscattered optical signal in response to the injected optical pulse; An analyzer configured to analyze the backscattered optical signal to determine vibration 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 wireless transceiver are respectively provided at a plurality of positions along the optical sensor fiber; A method in which the plurality of beacons generate mechanical vibrations and the vibrations are detected by the DFOS system.

2. The method of claim 1, further comprising providing a plurality of sensor nodes located within a 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, and the plurality of sensor nodes being 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 transmits a broadcast synchronization message including a time stamp and beacon ID information to the plurality of sensor nodes.

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

5. The method according to claim 4, wherein the plurality of sensor nodes are configured to adjust a clock delay of sensing data according to a received timestamp and synchronize the plurality of sensor nodes within an entire effective range area.

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

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