Single-channel high-speed transient field monitoring system and method based on variable-reflectivity grating array

Abstract

The invention discloses a single-channel high-speed transient field monitoring system and method based on a variable-reflectivity grating array. A single-channel architecture composed of a pulse light source, an optical circulator, a series fiber grating array and a single high-speed photoelectric detector is adopted. The core feature is that a plurality of fiber bragg gratings with the same initial central wavelength are distributed on the optical fiber in series and are weak reflection gratings; by presetting different grating inscribing numbers, the reflectivity of each grating is gradually increased in a stepped manner along the light transmission direction. According to the method, time sequence addressing is carried out through light flight time, light energy balance and amplitude identity verification are achieved through the reflectivity gradient, edge filtering demodulation is matched, and the rate bottleneck of traditional spectrum scanning is broken through. The system has the advantages of being extremely simple in structure, low in cost, high in interference resistance and the like, is particularly suitable for thermal insulation transient working conditions such as rocket projectile cold launching and explosion impact, can achieve MHz-level high-speed distributed monitoring, and can synchronously solve the motion trail of loads and pressure field distribution.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing and high-speed dynamic testing technology, and in particular, it is a single-channel high-speed transient field monitoring system and method based on a variable reflectivity grating array. Background Technology

[0002] Currently, mainstream fiber optic grating sensor networks mainly rely on wavelength division multiplexing (WDM) technology, which involves writing gratings with different center wavelengths on the same optical fiber and demodulating the wavelength using an optical spectrum analyzer (OSA) or a tunable laser.

[0003] However, traditional spectroscopic equipment typically has sampling rates limited to the kHz level. For transient physical processes with extremely short durations, such as milliseconds, and extremely rapid changes, such as nanoseconds, the demodulation rate is too low to monitor them. Currently, high-speed spectroscopic analysis modules are complex, bulky, and expensive, making them unsuitable as disposable consumables for high-risk destructive experiments. However, the development of high-speed and somewhat dangerous products requires such explosive-like experiments to provide accurate research data. If traditional same-wavelength tandem (TDM-FBG), i.e., same-wavelength distributed FBG sensing, is adopted, if the front-end sensor has too high reflectivity, or if the cumulative effect of tandem light energy causes a severe "shadowing effect" that prevents light energy from being transmitted to the back-end sensor, it will cause long-distance multi-point monitoring failure. Summary of the Invention

[0004] The purpose of this invention is to provide a single-channel high-speed transient field monitoring system and method based on a variable reflectivity grating array, so as to realize target quantity monitoring of the target under test, target risk monitoring, and self-diagnosis of the sensor system structure, thereby improving the demodulation rate in the traditional sense.

[0005] The technical solution to achieve the purpose of this invention is as follows:

[0006] A single-channel high-speed transient field monitoring system based on a variable reflectivity grating array includes:

[0007] A pulsed light source is used to generate pulsed laser signals. Its center wavelength is anchored at the midpoint of the linear region of the reflection spectrum of the FBG sensor, and its optical pulse width is less than twice the minimum physical distance between two adjacent FGB sensors.

[0008] An optical circulator is used to guide the pulsed laser signal from the pulsed light source into the FBG sensor array and guide the light signal reflected from the FBG sensor array into a photodetector, thereby separating the transmission and reception channels.

[0009] A photodetector is used to convert received optical signals into electrical signals.

[0010] A data acquisition system is used to collect analog electrical signals from a photodetector, convert them into digital signals, and record them.

[0011] An FBG sensor array consists of multiple FBG sensors connected in series on a single-mode grating. All FBG sensors have the same center wavelength and reflectivity of less than 10%. The reflectivity increases sequentially according to the direction of optical signal transmission.

[0012] The data processing unit stores the physical coordinates of each FBG sensor on the structure under test. Based on the speed of light in the optical fiber, it uses the absolute time of the pulse occurrence to distinguish which physical sensor the current signal comes from. It calculates the normalized strain value of each FBG sensor at each physical coordinate position and maps the normalized strain values ​​of each FBG sensor at the same moment onto the spatial coordinates to generate a real-time pressure distribution map along the axis of the structure under test.

[0013] A single-channel high-speed transient field monitoring system based on a variable reflectivity grating array, the monitoring method of which includes:

[0014] Set the parameters of the pulse light source, perform static calibration and reference acquisition on the FBG sensor array and perform self-test. Under static conditions without load, start the system to obtain a time-domain waveform containing multiple pulses, which is the acquired reference waveform. At the same time, perform identity verification to check whether the amplitude of these pulses conforms to the preset step-like increasing law, so as to confirm the sensor status and position sequence and record the reference voltage value of each FBG sensor.

[0015] Dynamic transient capture: During the occurrence of a transient event, the data acquisition system is triggered to continuously record and acquire a dynamic time-domain waveform data stream containing information about the entire load process;

[0016] Based on the speed of light in the optical fiber, the absolute time point of the pulse is used to distinguish which physical location the current signal comes from. The normalized strain value of the FBG sensor at each physical coordinate position is calculated. The normalized strain values ​​of each FBG sensor at the same moment are mapped onto the spatial coordinates to generate a real-time pressure distribution map along the axis of the structure under test.

[0017] The significant advantages of this invention compared to existing technologies are:

[0018] (1) The "light energy equalization" mechanism solves the "shadowing effect" of long-distance serial multiplexing: The "variable reflectivity weak grating array" design is adopted, that is, along the light transmission direction, the number of gratings (or reflectivity) of the FBG sensor is configured to increase in a stepwise manner with weaker reflectivity at the front end and stronger reflectivity at the back end. This solves the technical bias in the same-wavelength serial distributed structure, where the high reflectivity of the front-end sensor leads to weak light energy received by the back-end sensor and a sharp deterioration in the signal-to-noise ratio (i.e., shadowing effect or spectral overlap loss). This allows the sensor at the end of the optical fiber to use its high reflectivity to compensate for long-distance transmission loss and front-end splitting loss, ensuring that the signal amplitude of all nodes received by the detector is on the same order of magnitude. This not only increases the number of sensors that can be multiplexed, but also ensures the consistency of the measurement signal-to-noise ratio over the entire length.

[0019] (2) Possesses the ability of "spatiotemporal dual identity verification" under strong vibration environment and has extremely strong anti-interference ability: By using grating number encoding, a specific "amplitude fingerprint" (such as the preset V1 < V2 < V3 step feature) is given to each physical location in the time domain waveform. This solves the problem that a single time-division multiplexed signal is easily misjudged by stray light or link micro-bending interference in strong vibration and strong noise environments such as rocket launch. When demodulating, the system not only confirms the location through "time" but also performs secondary verification through "amplitude feature". If the detected signal does not conform to the preset amplitude gradient law, the system can automatically identify it as environmental noise and remove it, which greatly improves the confidence of the data under harsh working conditions.

[0020] 3) The system architecture is extremely simple and low-cost, making it suitable for large-scale applications in high-risk and destructive experiments: It adopts a simple architecture with a single light source, a single detector, and a single optical fiber, and uses a general-purpose high-speed ADC chip to replace the expensive spectral analysis module; it solves the problem that traditional optical fiber sensing equipment is expensive (usually in the tens of thousands of yuan range) and difficult to use as a "consumable" in explosion, collision, or live-fire weapon tests that may damage the equipment; the core hardware cost of this invention is greatly reduced, so that even if the sensor or even the demodulation terminal is destroyed in rocket launch tests and destructive structure tests, the critical "last second" data can be obtained at a very low cost.

[0021] 4) Achieved synchronous decoupled measurement of "motion trajectory" and "pressure distribution": Combining nanosecond-level time division multiplexing mechanism and incremental normalization algorithm, it solves the limitation of traditional single-point sensors that can only measure local strain and cannot simultaneously obtain the load motion state. The system uses a single optical fiber channel to calculate the load's motion velocity and acceleration through the time difference of signal peaks, and to invert the pressure gradient inside the pipe caused by friction and gas flow through the relative distribution of voltage increments at each node, thus achieving "one fiber, multiple measurements". Attached Figure Description

[0022] Figure 1This is a schematic diagram of the optical path principle and installation of the system of the present invention.

[0023] Figure 2 This is a system block diagram of the present invention.

[0024] Figure 3 This is a schematic diagram of the time-domain voltage waveform output by the photodetector of the present invention.

[0025] Figure 4 This is a schematic diagram of the data reconstruction process of the present invention. Detailed Implementation

[0026] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0027] The following is for reference only. Figure 1-4 The following description further illustrates specific technical embodiments of the present invention to enable those skilled in the art to further understand the present invention, without constituting a limitation on its rights.

[0028] This invention discloses a single-channel high-speed transient field monitoring system based on a variable reflectivity grating array. Please refer to the following for details regarding the system. Figure 1 As shown, in one example, the system controls a pulsed light source 1 to generate an optical signal A0, which is then guided by an optical circulator 2 into a sensing optical fiber 6 laid on the outer wall of the rocket launch tube 5. The optical signal is reflected at a series of fiber Bragg grating (FBG) arrays, during which the pressure or strain generated by the movement of the rocket 7 modulates the reflected light An. The reflected light An, carrying sensing information, is guided by the optical circulator 2 to a photodetector 3, where it is converted into an electrical signal. Finally, the data acquisition system 4 processes the signal to demodulate the dynamic strain or pressure distribution during the launch process.

[0029] The system consists of a pulsed light source 1, which serves as the detection light source to generate short-pulse laser signals; an optical circulator 2, a three-port optical device, guides the direction of the optical signal. It guides the light from the pulsed light source 1 into the sensing fiber 6 and directs the reflected light signal from the sensing fiber 6 into the photodetector 3, thus separating the transmission and reception channels; the photodetector 3 converts the received optical signal into an electrical signal for subsequent processing. A high-speed detector is typically used here to monitor high-speed transient processes; the data acquisition system 4 collects the analog electrical signal from the photodetector 3, converts it into a digital signal for recording and transmission to the data post-processing system for analysis; the rocket launch tube 5 is the monitored structure; and the sensing fiber 6, laid on the outer wall of the rocket launch tube 5, transmits the optical signal and connects to the FBG sensor. The fiber Bragg gratings (FBG1, FBG2, FBG3) are sensor arrays distributed in series along the sensing fiber. The sensing fiber is attached to the outer wall of the launch tube to sense strain or pressure changes in the tube wall during rocket launch. In the diagram, to increase reflectivity along the path and balance the echo signal intensity, the number of grating lines for FBG sensor 1, FBG sensor 2, and FBG sensor 3 increases sequentially. Rocket 7 is the target signal source that the system needs to monitor. When the rocket is launched from the launch tube (especially during high-speed transient launch), high pressure and impact are generated inside the tube. The resulting transient pressure causes deformation of the launch tube, which is then sensed. A0 is the initial light signal emitted by the pulsed light source, and An and An... ’ Let A2 and A3 be the emitted and transmitted light of FBG sensor n, respectively, and the reflected light of FBG sensor 2 and FBG sensor 3, respectively. ’ This is the transmitted light from FBG sensor 2. The data processing unit performs post-processing on the data acquired by the data acquisition system.

[0030] The present invention provides a single-channel high-speed transient field monitoring method based on a variable reflectivity grating array, comprising the following steps:

[0031] Step 1: Set up the pulse light source module; including the following steps:

[0032] Step 1) The pulsed light source adopts a nanosecond-level pulsed laser: a narrow linewidth laser is configured with a high-speed pulse modulator, and the center wavelength is anchored at the linear edge of the FBG sensor's reflection spectrum, that is, the middle of the linear region of the falling edge (or rising edge) of the FBG sensor's reflection spectrum reflectance, specifically within the range of 30% to 70% of the peak reflectance, in order to achieve edge filtering intensity demodulation.

[0033] Step 2) Set the time parameter: optical pulse width The spatial length of the optical pulse in the optical fiber must be less than twice the minimum physical distance between two adjacent FBG sensors. This ensures that the echo signal of the next FBG sensor arrives only after the echo signal of the previous FBG sensor has finished, preventing time-domain signal overlap.

[0034] Step 3) Setting the laser's operating wavelength (i.e., center wavelength anchoring): Based on the edge filtering demodulation principle, a spectrum analyzer or wavelength meter is used as an auxiliary monitoring device to precisely adjust the center wavelength of the pulsed laser. The wavelength drift of the FBG sensor array is locked at the midpoint of the linear region of the reflectance spectrum (i.e., the position where the slope of reflectance changes with wavelength is the largest), so that the wavelength drift of the FBG caused by external disturbances can be converted into a linear change in the intensity of reflected light in subsequent monitoring.

[0035] Step 2: Deploy the variable reflectivity FBG sensor array; including the following steps:

[0036] Step 1) Prepare the fiber Bragg grating: Set up N cascaded FBG sensors (denoted as S1, S2, ... Sn, ... S) on a single-mode grating. N ), and the center wavelength of all FBG sensors same;

[0037] Step 2) Low Reflection Design: All FBG sensors are low-reflectivity gratings (reflectivity R < 10%) to ensure that light energy can be transmitted to the end;

[0038] Step 3) While satisfying Step 2), the reflectivity of the FBG sensor should strictly increase sequentially according to the direction of light signal transmission and be gradient-encoded: S1 (near end) has a small number of gratings (extremely low reflectivity, such as 0.1%); S N The large number of (far-end) gratings (relatively high reflectivity, such as 5%) causes the reflectivity to increase in a stepwise manner. The reflectivity R satisfies R1 < R2 < ... < R N N is the number of sensors.

[0039] Step 4) Sensor Installation and Deployment: The prepared optical fibers are tightly attached to the surface of the structure to be measured, and a sensor position mapping table is established. Taking the measurement of rocket launch as an example, the bottom (or any fixed end) of the rocket launch tube is taken as the origin. A ranging tool is used to accurately measure and record the position of each FBG sensor (S1~S2). N The longitudinal physical coordinates of the launch tube (denoted as x1, x2, ..., x...) N The location information is pre-stored in the data processing unit for subsequent spatial positioning; high-rigidity epoxy adhesive or other special coupling agents are used for full-length or point-fixation to ensure that the transient strain of the structure under test is effectively transferred to the fiber Bragg grating. The precise physical coordinates of each sensor are recorded.

[0040] Step 3: Configure the high-speed single-channel demodulation module; including the following steps:

[0041] Step 1) The core of this single-channel demodulation module is to construct an optical path loop using an "optical circulator + a single high-speed photodetector + a high-speed acquisition card": A three-port optical circulator is connected. Port 1 connects to a pulsed light source, port 2 connects to a sensing fiber array, and port 3 connects to a photodetector. The emitted light pulses are guided into the optical fiber, while the weak reflected signals are separated and guided into the detector, achieving bidirectional transmission without interference on a single fiber.

[0042] Step 2) Use a single high-speed photodetector (PD): The response bandwidth of the PD must be greater than the frequency characteristics of the optical pulse signal, usually reaching 500MHz or 1GHz or more, to ensure that the rising edge of the nanosecond pulse is not "flattened" and to accurately capture transient characteristics.

[0043] Step 3) High-speed data acquisition: The weak voltage signal output by the detector is amplified by a high-speed transimpedance amplifier and then connected to a high-speed acquisition card or oscilloscope.

[0044] Step 4: Implement data monitoring and acquisition for the target quantity, including the following steps:

[0045] Step 1) Static calibration and reference acquisition self-check: Under static conditions without load, start the system to acquire data. The system should obtain a time-domain waveform containing multiple pulses, which is the acquired reference waveform; at the same time, perform identity verification to check whether the amplitude of these pulses conforms to the preset "step-like increasing" rule (i.e., V). 基准1 < V 基准2 < V 基准3 < ... <V 基准N This step is used to confirm that the sensors are in good condition and in the correct position sequence, and to record these reference voltage values;

[0046] Step 2) Dynamic transient capture: During a transient event (such as launch or explosion), the high-speed acquisition card is triggered to continuously record and acquire a dynamic time-domain waveform data stream containing information about the entire payload process;

[0047] Step 3) Signal demodulation and parameter inversion: Post-processing of the acquired dynamic data.

[0048] A. Timing-based localization: Based on the speed of light in an optical fiber, the sensor can distinguish the physical location from which the current signal originates by using the absolute time point of the pulse occurrence.

[0049] B. Incremental normalization demodulation of measured values: For each sensor location, extract its real-time voltage peak value. The initial voltage value is V0. Calculate the voltage increment. :

[0050]

[0051] Execution sensitivity normalization: Calculate the relative rate of change S.

[0052]

[0053] S is directly proportional to the ratio of transient strain to pressure at that location, thus eliminating the sensitivity difference caused by different reflectivities.

[0054] Step 4) Physical map reconstruction: First, reconstruct the motion trajectory and extract the times when the maximum signal change occurs at each sensor (t1, t2, t3, ..., t...). n ..., t N ), combined with the sensor's physical location (x1, x2, x3, ..., x n ..., x N The process involves plotting the xt (position-time) curve of the load motion and calculating the velocity v and acceleration a. Next, the pressure field distribution is reconstructed by mapping the normalized strain values ​​S of each sensor at the same moment onto spatial coordinates (physical coordinates). A relationship function between the relative rate of change of voltage and the fiber strain value is established, which is calculated by dividing by the system sensitivity coefficient (i.e., the percentage change in voltage per unit of strain produced by the entire optoelectronic system). The fiber strain value is equal to the surface strain value of the structure to which the fiber is bonded. Through the relationship between the strain value and the relative rate of change, the pressure change state of the structure under test is inferred, generating a real-time pressure distribution map along the axis of the structure under test.

[0055] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A single-channel high-speed transient field monitoring system based on a variable reflectivity grating array, characterized in that, include: A pulsed light source is used to generate pulsed laser signals. Its center wavelength is anchored at the midpoint of the linear region of the reflection spectrum of the FBG sensor, and its optical pulse width is less than twice the minimum physical distance between two adjacent FGB sensors. An optical circulator is used to guide the pulsed laser signal from the pulsed light source into the FBG sensor array and guide the light signal reflected from the FBG sensor array into a photodetector, thereby separating the transmission and reception channels. A photodetector is used to convert received optical signals into electrical signals. A data acquisition system is used to collect analog electrical signals from a photodetector, convert them into digital signals, and record them. An FBG sensor array consists of multiple FBG sensors connected in series on a single-mode grating. All FBG sensors have the same center wavelength and reflectivity of less than 10%. The reflectivity increases sequentially according to the direction of optical signal transmission. The data processing unit stores the physical coordinates of each FBG sensor on the structure under test. Based on the speed of light in the optical fiber, it uses the absolute time of the pulse occurrence to distinguish which physical sensor the current signal comes from. It calculates the normalized strain value of each FBG sensor at each physical coordinate position and maps the normalized strain values ​​of each FBG sensor at the same moment onto the spatial coordinates to generate a real-time pressure distribution map along the axis of the structure under test.

2. The single-channel high-speed transient field monitoring system based on a variable reflectivity grating array according to claim 1, characterized in that, Normalized strain value: in Vn is the real-time peak voltage of the FBG sensor at the nth position, and V0 is the initial voltage value of the FBG sensor at the nth position.

3. The single-channel high-speed transient field monitoring system based on a variable reflectivity grating array according to claim 1, characterized in that, The pulsed light source is a nanosecond-level pulsed laser.

4. The single-channel high-speed transient field monitoring system based on a variable reflectivity grating array according to claim 1, characterized in that, The FBG sensor array is fixed to the structure under test either along its entire length or at points using a coupling agent.

5. The single-channel high-speed transient field monitoring system based on a variable reflectivity grating array according to claim 1, characterized in that, The optical circulator is a three-port optical circulator, with port 1 connected to a pulsed light source, port 2 connected to an FBG sensor array, and port 3 connected to a photodetector.

6. The single-channel high-speed transient field monitoring system based on a variable reflectivity grating array according to claim 1, characterized in that, The response bandwidth of the photodetector must be greater than the frequency characteristics of the optical pulse signal.

7. The single-channel high-speed transient field monitoring system based on a variable reflectivity grating array according to claims 1-6, characterized in that, Its monitoring methods include: Set the parameters of the pulse light source, perform static calibration and reference acquisition on the FBG sensor array and perform self-test. Under static conditions without load, start the system to obtain a time-domain waveform containing multiple pulses, which is the acquired reference waveform. At the same time, perform identity verification to check whether the amplitude of these pulses conforms to the preset step-like increasing law, so as to confirm the sensor status and position sequence and record the reference voltage value of each FBG sensor. Dynamic transient capture: During the occurrence of a transient event, the data acquisition system is triggered to continuously record and acquire a dynamic time-domain waveform data stream containing information about the entire load process; Based on the speed of light in the optical fiber, the absolute time point of the pulse is used to distinguish which physical location the current signal comes from. The normalized strain value of the FBG sensor at each physical coordinate position is calculated. The normalized strain values ​​of each FBG sensor at the same moment are mapped onto the spatial coordinates to generate a real-time pressure distribution map along the axis of the structure under test.

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