A flow measurement method and system based on a mixed time difference method improved from an ultrasonic time difference method

Abstract

This invention claims protection for a mixed-frequency time-difference amplification flow measurement method based on an improved ultrasonic time-difference method, belonging to the field of ultrasonic fluid velocity measurement technology. Addressing the problem of insufficient time-difference resolution in low-frequency ultrasonic flow measurement, this invention constructs a flow measurement architecture that deeply coordinates fiber optic sensing and heterodyne mixing: 1) An interferometric fiber optic sensor is used to acquire upstream and downstream ultrasonic signals, whose high signal-to-noise ratio and phase fidelity provide a distortion-free signal basis for subsequent time-difference amplification; 2) The two signals are digitally mixed with a reference signal and low-pass filtered. Utilizing the complete phase information retained by the fiber optic sensor, the microsecond-level original time difference is losslessly converted into a significantly amplified apparent time difference on the difference frequency signal; 3) The apparent time difference is extracted by cross-correlation and then the amplification factor is used to inversely deduce the actual propagation time difference; 4) The flow velocity and flow rate are calculated. In this invention, fiber optic sensing ensures the measurability and phase accuracy of weak low-frequency signals, while heterodyne mixing breaks through the traditional time-domain resolution limit. The two complement each other and work synergistically to significantly improve the accuracy and stability of flow measurement under complex river conditions.

Description

Technical Field

[0001] This invention belongs to the field of ultrasonic fluid velocity measurement technology, and relates to a flow measurement method based on an improved ultrasonic time-of-flight amplification method. Specifically, it involves an ultrasonic time-of-flight flow measurement method and system that utilizes an interferometric fiber optic sensor to receive ultrasonic signals and achieves high-precision flow measurement through frequency mixing and time-of-flight amplification. This invention is mainly applicable to various scenarios for accurate detection of liquid velocity and flow rate, such as pipeline fluids, open channels in rivers, industrial circulating water systems, water conservancy and hydrological monitoring, chemical fluid transportation, and municipal water supply and drainage. It is also adaptable to complex industrial and natural fluid detection conditions such as high sediment content, wide rivers, and low signal-to-noise ratios. It can be widely applied in the fields of water conservancy engineering monitoring, industrial automation measurement and control, environmental hydrological monitoring, and energy fluid transportation metering. Background Technology

[0002] The closest existing technology to this solution is the traditional single-frequency ultrasonic time-difference flow measurement technology. This technology mainly consists of an ultrasonic transducer, a driving and transmitting circuit, a receiving, amplifying, and filtering circuit, a timing unit, and a main control unit. Two sets of transducers are symmetrically arranged on both sides of the fluid cross-section to achieve alternating transmission and reception in both directions. Its hardware only handles the generation, transmission, basic amplification, and time-domain acquisition of the ultrasonic signal; it lacks frequency conversion and time-difference amplification processing circuits and corresponding algorithms. In actual operation, the main control unit controls the transmission of ultrasonic pulses at a fixed frequency. After propagation through the fluid, the pulses are received on the opposite side. After noise reduction and amplification by analog circuitry, the total propagation time in both directions is directly obtained through hardware timing, and the original time difference is calculated. This time difference is then substituted into the sound velocity and distance parameters to calculate the flow velocity. Some optimized versions use basic time-domain cross-correlation for delay correction at the back end, but the entire process is based on the original ultrasonic baseband signal processing. This technology is only suitable for general flow velocities and low-interference, coarse-grained monitoring scenarios, such as ordinary water supply and drainage pipelines and water conservancy channels. However, this existing technology has the following inherent drawbacks when applied to harsh hydrological environments such as rivers with high sediment content and long spans:

[0003] (1) No active amplification mechanism for time difference, resulting in insufficient resolution of minute time difference. Existing technologies directly extract the time difference based on the original ultrasonic carrier signal without introducing any frequency conversion or time difference amplification. In wide river scenarios, the ultrasonic propagation path is long and the time difference between upstream and downstream is only at the microsecond or even nanosecond level. However, traditional timing methods are limited by the signal edge triggering accuracy and cannot effectively resolve this minute quantity, resulting in significant measurement errors in low-velocity regions.

[0004] (2) Low-frequency operation results in low phase resolution, making it impossible to extract weak flow velocity characteristics. To ensure the long-distance propagation of ultrasonic signals in sediment-laden water, lower frequency ultrasonic waves are often used in engineering to reduce scattering attenuation caused by sediment particles. However, low-frequency ultrasonic signals have large pulse widths and smooth waveform edges, resulting in extremely small carrier phase offsets for the same flow velocity. Existing technologies do not amplify the phase information, making it difficult to effectively extract phase changes from the original waveform. This leads to poor repeatability and large data fluctuations in slow-flowing and shallow-flowing regions.

[0005] (3) The transducer has insufficient anti-interference capability, and it is impossible to balance penetration and measurement accuracy. Existing technologies generally use piezoelectric ceramic transducers, which have a significantly reduced signal-to-noise ratio under strong electromagnetic interference and high humidity environments, affecting the reliable acquisition of weak echo signals. At the same time, once the operating frequency is determined, it remains fixed: increasing the frequency can improve the time resolution accuracy, but it suffers severe attenuation and insufficient propagation distance in water bodies with high sediment content; while decreasing the frequency can meet the penetration distance, it sacrifices measurement accuracy. It is difficult to balance the two and cannot adapt to the changing water flow and sediment concentration conditions of natural river channels.

[0006] Therefore, how to effectively amplify small propagation time differences, improve measurement accuracy at low flow rates, and enhance the anti-interference capability of the receiving system while ensuring the long-distance propagation capability of ultrasonic signals are technical problems that urgently need to be solved in this field.

[0007] Statement of Opinions

[0008] A search revealed application publication number CN118226075A, which discloses a parameter adjustment method and apparatus for an ultrasonic time-of-flight flow measurement device. Compared with existing technologies, this invention considers the problem of varying ultrasonic signal attenuation under different environments (different amounts of bubbles, suspended particles, etc.). By comparing and determining the attenuation coefficient, a frequency sweep is performed to find the optimal frequency, thereby reducing signal propagation attenuation and stabilizing the received ultrasonic signal, which is beneficial for achieving stable and reliable continuous measurement. It can effectively mitigate the impact of environmental changes on ultrasonic signal attenuation and improve the stability of time-of-flight flow measurement data. Furthermore, for environments with severe signal attenuation, the received ultrasonic signal strength can be further enhanced by adjusting the driving voltage and amplification stage, mitigating the impact of environmental changes on ultrasonic signal attenuation and further improving the stability of time-of-flight flow measurement data.

[0009] This patent possesses the following technical features: It determines the ultrasonic signal attenuation coefficient based on differences in the content of bubbles and suspended particles in the water, and uses a frequency sweep method to select the optimal operating frequency suitable for the current environment, reducing energy loss during ultrasonic propagation and ensuring a stable and continuous received signal waveform. Simultaneously, it can adjust the ultrasonic transmission drive voltage and signal amplification stage according to the actual degree of signal attenuation, increasing the amplitude of the ultrasonic signal at the receiving end and weakening the interference of environmental factors on signal transmission, thereby improving the stability of the measurement data obtained by the time-of-flight flow measurement method.

[0010] However, it has obvious limitations: First, it only optimizes from the perspective of hardware frequency selection and signal amplitude enhancement, and only solves the problems of signal attenuation and waveform instability. It does not address the core pain points of insufficient resolution of minute time differences and low time delay extraction accuracy at low operating frequencies, and cannot improve the inherent defects of poor timing resolution in low-frequency conditions. Second, this solution focuses on adapting to environmental attenuation and ensuring signal receptivity, without amplifying the weak time differences generated by fluid flow. When using low-frequency operating mode in wide rivers with high sediment content, the problem of small effective time difference ratio, easy noise submersion, and large measurement error at low speeds still exists. Third, relying on hardware parameter adjustment and optimization can easily increase equipment power consumption, and can only optimize signal integrity, but cannot improve the accuracy of time delay calculation from the algorithm level, so the room for improvement in measurement accuracy is limited.

[0011] This invention complements and upgrades the optimization direction of the comparative patent, going beyond frequency optimization and hardware gain adjustment to directly address the core shortcoming of insufficient time difference measurement accuracy under low-frequency conditions. The core innovation lies in the deep synergy between heterodyne mixing time difference amplification technology and interferometric fiber optic sensing technology, producing a significant synergistic effect. The fiber optic sensor output signal possesses phase characteristics linearly correlated with sound pressure, high signal-to-noise ratio, and amplitude stability, providing an ideal "fidelity input" for heterodyne mixing; while heterodyne mixing linearly amplifies the minute propagation time difference into a significant phase difference in the low-frequency difference signal. The two are seamlessly integrated and mutually supportive in the signal chain—fiber optic sensing ensures the phase accuracy and low noise of the original signal, making the time difference characteristics after mixing and amplification more clearly discernible; mixing and amplification compensate for the limitation of fiber optic sensing itself in not being able to actively amplify the time difference. On the one hand, under the premise of using low-frequency ultrasound to ensure propagation capability in water bodies with high sediment content, the mixing technology effectively amplifies the difficult-to-identify microsecond-level propagation time difference into an explicit time difference in the difference frequency signal, thus overcoming the problem of weak time domain resolution of low-frequency ultrasound in principle. On the other hand, the high-sensitivity phase detection capability of fiber optic sensing further reduces the noise floor of the difference frequency signal after mixing, enabling the cross-correlation algorithm to more accurately lock the peak time delay coordinates, thereby accurately capturing the time difference information corresponding to weak flow velocities without significantly increasing the transmission power. The two work together to transform the tiny phase changes "buried" in the original signal into a time delay that can be measured with high confidence through a three-stage gain chain of "fiber optic high-fidelity pickup → mixing linear amplification → cross-correlation precise extraction". This collaborative mechanism breaks through the traditional contradiction of "low-frequency ultrasound has good penetration but poor accuracy", and solves multiple problems such as rapid signal attenuation, low signal-to-noise ratio, and difficulty in time difference extraction in complex waters. Compared to the patented approach that improves signal strength through single means such as frequency stabilization and hardware gain adjustment without fundamentally improving the accuracy of time difference measurement, this invention achieves a balance between environmental adaptability (low-frequency long-distance propagation) and high measurement accuracy (equivalent time difference amplification). In field hydrological scenarios with high sediment content, slow flow, and strong electromagnetic interference, it can significantly reduce measurement errors in slow-flow and shallow-flow areas, achieving stable and accurate flow measurement under all operating conditions. Summary of the Invention

[0012] This invention aims to solve the problems of the prior art. It proposes a mixing time-difference amplification current measurement method based on an improved ultrasonic time-difference method. The technical solution of this invention is as follows:

[0013] A mixed-frequency time-difference amplification current measurement method based on ultrasonic time-difference method includes the following steps:

[0014] 1) An interferometric fiber optic sensor is used to receive ultrasonic signals propagating through the fluid, convert the ultrasonic signals into optical signals, and then perform photoelectric conversion to obtain two original ultrasonic signals, one for forward flow and one for reverse flow. The fiber optic sensor has high frequency response sensitivity at the ultrasonic transmission frequency, and exhibits high sensitivity, flat amplitude-frequency response, and linear phase response at the ultrasonic operating frequency. It can convert sound pressure signals into linearly phase-modulated optical signals, and after photoelectric conversion, obtain two original ultrasonic signals, one for forward flow and one for reverse flow. Its high signal-to-noise ratio and amplitude stability can meet the input conditions for subsequent mixing operations.

[0015] 2) For the downstream direction: The downstream transmitted signal and the downstream received signal are acquired synchronously. These two signals are digitally mixed with the same local oscillator reference signal. After low-pass filtering, two difference frequency signals are obtained: the downstream transmitted difference frequency signal and the downstream received difference frequency signal. The same operation is performed for the upstream direction to obtain the upstream transmitted difference frequency signal and the upstream received difference frequency signal. Due to the high sensitivity and phase linearity of the fiber optic sensor at the operating frequency, the original phase difference information can be completely preserved, so that the phase difference of the difference frequency signal and the original propagation time difference maintain a strict linear ratio, realizing lossless equivalent amplification of the microsecond-level time difference.

[0016] 3) Perform cross-correlation calculations on the downstream transmitted difference frequency signal and the downstream received difference frequency signal to obtain the apparent time difference after the signal is mixed and amplified; define the time difference amplification factor as the ratio of the ultrasonic transmission main frequency to the difference frequency signal frequency, which represents the equivalent amplification factor of the small propagation time difference after frequency domain transformation; perform reverse conversion based on the predetermined time difference amplification factor to reverse the apparent time difference and restore it, remove the time difference amplification effect brought about by the frequency domain transformation, and restore the real time difference of the downstream ultrasonic wave in the fluid.

[0017] 4) Calculate the fluid velocity and flow rate based on the actual propagation time in both directions.

[0018] Furthermore, in step 1), the original ultrasonic signal is detected by an optical fiber sensor and converted into an electrical signal via a photodetector.

[0019] Furthermore, step 2) specifically includes: selecting a local oscillation reference signal with a fixed frequency; performing digital multiplication and mixing of the acquired downstream transmission signal and downstream reception signal with the local oscillation reference signal, and then filtering out the high-frequency and low-frequency components through a low-pass filter to obtain two difference frequency signals; performing the same operation on the upstream transmission signal and upstream reception signal.

[0020] Furthermore, cross-correlation is performed on the two difference frequency signals of the forward and reverse flows respectively, and the apparent time difference between the forward and reverse flow difference frequency signals is extracted by determining the coordinates corresponding to the peak value of the cross-correlation function.

[0021] Furthermore, step 2) involves mixing and filtering the transmitted and received signals, specifically including:

[0022] Let the amplitude of the signal driving voltage be A. tx The signal frequency is The transmitted signal generated by electric drive is then represented as:

[0023]

[0024] Let P be the acoustic pressure amplitude of the ultrasonic signal propagating in the fluid to the fiber optic sensor under this driving signal. Then the ultrasonic signal can be expressed as:

[0025]

[0026] The phase shift sensitivity of the interferometric fiber optic sensor is M ϕ (Unit: rad / μPa), then the phase change of the light output by the sensor is:

[0027]

[0028] After the interfering light signal passes through the photodetector, the output voltage and phase change are linearly related. Let the phase-to-voltage conversion coefficient of the photodetector be K. PD (Unit: V / rad), then the output voltage signal is:

[0029]

[0030] make The received signal is then represented as:

[0031]

[0032] Let the frequency of the reference signal be... The reference signal is:

[0033]

[0034] The transmitted and received signals are mixed with a reference signal respectively, and then low-pass filtered to obtain two differences.

[0035]

[0036] The actual time difference Δt1 between the transmitted and received signals is:

[0037]

[0038] The apparent time difference Δt2 between the difference frequency signals is:

[0039]

[0040] Heterodyne mixing can amplify the actual time difference between the original signals into the apparent time difference between the difference frequency signals. The time amplification factor is the ratio of the transmitted signal frequency to the difference frequency signal frequency, denoted as k.

[0041] k

[0042] The apparent time difference Δt2 between the difference frequency signals is obtained by cross-correlation calculation, therefore its true propagation time difference is:

[0043] = 。

[0044] Furthermore, step 4) of calculating the fluid velocity and flow rate based on the actual propagation time difference specifically includes:

[0045] An ultrasonic transmitter and receiver are arranged on both sides of the receiver, with a distance L between them. Due to the influence of water flow, the propagation speed of the sound wave differs along the upstream and downstream paths. The propagation speed v1 of the downstream ultrasonic wave is slightly higher than that of the upstream ultrasonic wave v2. Therefore, the propagation time of the ultrasonic wave along the upstream and downstream paths will also differ, as follows:

[0046]

[0047]

[0048] Using the formula, the actual propagation time difference between upstream and downstream can be derived as t. Theoretically, the propagation speed of ultrasound in water is approximately 1500 m / s, therefore c 2 ≫v 2 From cosθ, the formula for calculating the average flow velocity v of the water layer can be derived:

[0049] .

[0050] A flow measurement system implementing any of the methods described herein includes an ultrasonic signal transmitting module, comprising a signal generator and an ultrasonic transducer, for generating an electrically driven signal to drive the ultrasonic transducer to transmit an ultrasonic detection signal; an ultrasonic signal receiving module, comprising an optical fiber sensor and a photodetector, for receiving the ultrasonic echo signal after propagation through the fluid, and sequentially converting the acoustic signal into an optical signal and an electrical signal; the ultrasonic signal transmitting module and the ultrasonic signal receiving module form a corresponding link for acoustic signal propagation. An ultrasonic signal acquisition module includes a signal acquisition board, for electrically connecting the ultrasonic signal transmitting module and the ultrasonic signal receiving module respectively, synchronously acquiring the original driving electrical signal from the transmitting end and the echo analog electrical signal from the receiving end, and converting the two analog signals into digital signals for buffering; a host computer signal processing module, communicatively connected to the ultrasonic signal acquisition module, for reading the digital signal, and performing mixing, filtering, and cross-correlation delay calculation operations, using an amplification factor k to calculate the actual propagation delay, and then obtaining the fluid velocity and flow rate according to the velocity and flow rate calculation formula.

[0051] The advantages and beneficial effects of this invention are as follows:

[0052] Compared to traditional ultrasonic time-of-flight flow measurement technology, this invention achieves comprehensive performance improvements across three levels—signal acquisition, frequency domain transformation, and time delay inversion—through deep synergy between fiber optic sensing technology and heterodyne mixing time-of-flight amplification mechanism, without altering the core structure of existing mainstream flow measurement hardware. The two technologies form a closed-loop gain chain of "high-fidelity fiber optic pickup → linear mixing amplification → precise cross-correlation extraction," resulting in the following synergistic benefits that surpass those of any single technology:

[0053] I. Synergistically Improve Measurement Accuracy: Breaking Through the Limit of Time Domain Resolution in Low-Frequency Ultrasonic Measurements

[0054] Traditional techniques are limited by the large pulse width and smooth edges of low-frequency ultrasound, making it difficult to distinguish microsecond-level propagation time differences. In this invention, the high signal-to-noise ratio, low distortion, and excellent phase linearity of the original signal provided by the fiber optic sensor lay a "clean" data foundation for mixing operations; heterodyne mixing linearly amplifies the tiny phase differences buried in the original signal into significant apparent time differences in the difference frequency signal. The two work together to make the time difference characteristics that were previously undetectable under low-frequency conditions clearly measurable. The cross-correlation algorithm can sharpen peaks and suppress spurious peaks on this foundation, avoiding time delay drift caused by noise. A single technology cannot achieve this effect: mixing alone without high-fidelity fiber optic acquisition will result in phase distortion of the mixed difference frequency signal due to noise interference; fiber optic sensing alone without mixing amplification cannot actively amplify the time difference due to its high sensitivity. The combination of the two significantly improves the measurement accuracy and repeatability across the entire flow velocity range (especially the low-velocity range), meeting the requirements for precise flow measurement in environments with high sediment content and wide river widths.

[0055] II. Synergistic Enhancement of Detection Capabilities: Constructing an Integrated Link of "Anti-interference + Fidelity + Amplification"

[0056] Traditional piezoelectric sensors suffer from a sharp drop in signal-to-noise ratio (SNR) under strong electromagnetic interference and high humidity conditions in the field, and cannot achieve gain linkage with mixing algorithms. The fiber optic sensor used in this invention employs a passive optical structure, effectively isolating electromagnetic interference and power frequency noise, while possessing high SNR, large dynamic range, and phase fidelity. This characteristic is not merely isolated "interference resistance," but directly serves the subsequent mixing stage: the low-noise, high-fidelity original signal results in minimal phase distortion and sharp correlation peaks in the mixed difference frequency signal, significantly reducing time difference calculation errors. Conversely, the mixing amplification mechanism allows the fiber optic sensor to receive weak ultrasonic echoes after high attenuation and long-distance propagation without overly relying on its own sensitivity limits. Even with low original signal amplitude, effective information can still be extracted through time difference amplification after mixing. The two work together to form a dual gain: "fiber optic anti-interference pickup of weak signals → active amplification of time difference characteristics through mixing," overcoming the limitation of traditional solutions where "receiving sensitivity and time difference resolution can only be chosen one." Furthermore, the frequency response characteristics of the fiber optic sensor are deeply coupled with the heterodyne mixing time-difference amplification mechanism, forming an integrated collaborative link of frequency optimization, sensitivity enhancement, phase fidelity preservation, and time-difference amplification. First, the frequency response sensitivity of the fiber optic sensor is matched to the ultrasonic transmission frequency, ensuring the fiber exhibits the highest response sensitivity, flat amplitude-frequency characteristics, and linear phase characteristics at the target ultrasonic frequency. This achieves selective enhancement and noise suppression of the effective ultrasonic signal, providing a high signal-to-noise ratio and distortion-free phase input for subsequent mixing processing. Second, the fiber-optimized and phase-fidelity-preserved signal is fed into the heterodyne mixing unit. Frequency transformation converts the original minute time difference into a significant apparent time difference of the difference frequency signal, achieving equivalent amplification. The fiber's frequency response matching ensures a strictly linear phase difference before and after mixing, providing a prerequisite for precise time-difference amplification; heterodyne mixing further amplifies the already enhanced weak time difference, breaking through the low-frequency time-domain resolution limit. Attached Figure Description

[0057] Figure 1 This invention provides a preferred embodiment of the ultrasonic time-of-flight method, including its principle and module schematic diagram.

[0058] Figure 2 This is a schematic diagram of a heterodyne mixer.

[0059] Figure 3 This is a flowchart of a signal processing algorithm;

[0060] Figure 4 This is a schematic diagram of a fiber optic sensor. Detailed Implementation

[0061] The technical solutions of the embodiments of the present invention will be clearly and thoroughly described below with reference to the accompanying drawings. The described embodiments are merely some embodiments of the present invention.

[0062] The technical solution of the present invention to solve the above-mentioned technical problems is:

[0063] This invention discloses an improved ultrasonic time-difference amplification flow measurement method based on the ultrasonic time-difference method. The overall technical solution retains the conventional ultrasonic transceiver hardware architecture, employing fiber optic sensing at the signal receiving end and adding a signal mixing time-difference amplification processing link at the signal back-end digital processing layer. This is combined with a cross-correlation time delay calculation algorithm to calculate fluid velocity and flow rate. This approach abandons the existing methods of directly calculating the time delay of the original ultrasonic signal and the inherent mode of traditional piezoelectric sensing. Its core working principle is as follows: utilizing the fundamental physical law that ultrasonic waves propagate with and against the current in flowing fluids, two ultrasonic signals in opposite directions are first received using an interferometric fiber optic sensor. The output signal of the fiber optic sensor has characteristics such as phase linearly correlated with sound pressure, high signal-to-noise ratio, amplitude stability, and resistance to electromagnetic interference. It can completely preserve the phase time difference information in the weak ultrasonic echoes after long-distance propagation through high-sediment-content water bodies, providing a "low-distortion, low-noise, high-fidelity" original data foundation for subsequent signal processing. Based on this, the two original signals are digitally mixed with the same local oscillation reference signal, and the difference frequency signal is obtained through low-pass filtering. Due to the excellent phase linearity and extremely low noise floor of the fiber optic sensor output, a strict linear mapping relationship is maintained between the original phase difference and the difference frequency phase difference during the mixing process. This effectively amplifies the microsecond-level propagation time difference caused by fluid flow into a clearly discernible apparent phase difference in the difference frequency signal, effectively differentiating the effective time delay characteristics from the noise signal. In this process, the fiber optic sensor provides "clean" input conditions for the mixing amplification, while the mixing amplification compensates for the limitation of the fiber optic sensor itself not being able to actively amplify the time difference. The two form a front-to-back gain coupling of "high-fidelity acquisition → linear amplification" in the signal link. Subsequently, a cross-correlation algorithm is used on the amplified and denoised difference frequency signal. Leveraging the sharp correlation peak brought by the high signal-to-noise ratio of the fiber optic sensor, the apparent time difference is accurately locked, and combined with a predefined time difference amplification coefficient, the actual time difference of ultrasonic wave propagation in the fluid is inversely reconstructed. Finally, the flow velocity and flow rate are converted by substituting into the classic fluid velocity measurement formula.

[0064] II. Implementation steps, technical means used, and corresponding functions of the technical solution:

[0065] The first step is the bidirectional acquisition and reception of ultrasonic signals. This step follows the existing ultrasonic flow measurement hardware deployment method, symmetrically deploying ultrasonic transducers and fiber optic sensors on both sides of the fluid being measured. The transmitter is driven by a signal generator to output a fixed rated frequency ultrasonic detection signal, completing the transmission and reception of ultrasonic waves in the downstream and upstream directions in a time-division manner, and completing the complete acquisition and storage of two channels of raw time-domain ultrasonic signals. The purpose of this step is to obtain the basic raw signal containing fluid flow velocity and time delay information, providing a data source for subsequent algorithm processing. The hardware deployment and signal transmission and acquisition methods in this step are consistent with existing technologies.

[0066] The second step, ultrasonic signal mixing and time difference amplification, is the core differentiating technique of this invention from existing technologies. A local oscillation reference signal with a fixed frequency is selected. The downstream ultrasonic transmitted and received signals are digitally multiplied and mixed with the local oscillation signal, respectively. The same operation is performed for the upstream direction. Then, a low-pass filter is used to remove the high-frequency and differential frequency components generated after mixing, retaining the low-frequency difference signal. Frequency transformation is then used to convert the original microsecond-level time difference into a difference frequency signal time difference, completing the linear amplification of the time difference characteristic. The same operation is performed for the upstream direction.

[0067] The third step involves cross-correlation time delay calculation. A global correlation traversal operation is performed on the two difference-frequency amplified signals. By locking the peak coordinates of the cross-correlation function, the time difference between the two difference-frequency signals is accurately extracted.

[0068] The fourth step is to calculate the flow velocity and flow rate parameters and output the data. The time delay of the difference frequency signal obtained by the solution is used to back-calculate the true time delay based on the amplification factor k. The true time delay is then substituted into the ultrasonic time difference velocity derivation formula to obtain the external output measurement result, thus completing the entire flow measurement process.

[0069] III. Functional Module Division, Module Functions, and Internal Data Flow

[0070] The entire flow measurement system of this invention can be divided into four modules: an ultrasonic signal transmission module, an ultrasonic signal receiving module, an ultrasonic signal acquisition module, and a signal processing module, as follows: Figure 1 The specific module functions are as follows:

[0071] 1. Ultrasonic signal transmission module: The signal generator generates a signal and drives the ultrasonic transducer to generate and transmit ultrasonic signals.

[0072] 2. Ultrasonic signal receiving module: such as Figure 4 It uses a fiber optic sensor to receive ultrasonic signals.

[0073] 3. Ultrasonic signal acquisition module: The acquisition board acquires data from both the transmitted and received signals.

[0074] 4. Signal Processing Module: Uploads data to the host computer for processing. For example... Figure 3 As shown, the ultrasonic transceiver signal is mixed, filtered, and cross-correlated to calculate the time delay. The actual time delay is obtained through the amplification factor K, and the flow velocity is inversely calculated based on the flow velocity formula to complete the flow measurement work.

[0075] IV. Principle of Ultrasonic Time-of-Flight Flow Measurement and Principle of Mixing Time-of-Flight Amplification:

[0076] like Figure 1 As shown, an ultrasonic transmitter and receiver are arranged on both sides of the receiver, with a distance L between them. The angle between the ultrasonic wave propagation direction and the water flow direction is θ. Due to the influence of the water flow, the propagation speed of the sound wave differs along the upstream and downstream paths. The downstream ultrasonic wave propagation speed v1 is slightly higher than the upstream ultrasonic wave propagation speed v2. Therefore, the propagation time of the ultrasonic wave along the upstream and downstream paths will also differ, as follows:

[0077]

[0078]

[0079] The time difference between upstream and downstream flow can be derived using the formula. Theoretically, the propagation speed of ultrasound in water is approximately 1500 m / s, therefore c 2 ≫v 2 From cosθ, the formula for calculating the average flow velocity v of the water layer can be derived:

[0080]

[0081] As can be seen from the above formula, the river flow velocity can be obtained simply by obtaining the upstream and downstream transmission times t1 and t2 of the ultrasonic waves, without needing to consider the influence of temperature and salinity on the transmission speed of ultrasonic waves in the water.

[0082] Heterodyne mixing, through the core logic of "frequency shifting + phase invariance," transforms the minute time difference of high-frequency ultrasound signals into a significant apparent time difference of low-frequency signals, achieving a time difference amplification effect, such as... Figure 2 The core of the heterodyne principle is to introduce a local oscillation signal at a reference frequency, mix it with the transmitted and received signals to generate sum and difference frequency components, and then extract the target difference frequency component through filtering. This process can completely preserve the phase information of the high-frequency signal in the low-frequency difference frequency signal, and because the frequency is reduced, the apparent time difference is amplified. Thus, the problem of measuring the time difference between the original transmitted and received signals is transformed into measuring the apparent time difference between two difference frequency signals. By utilizing the amplification effect of the time difference, the accuracy of the time difference measurement is improved.

[0083] Let the amplitude of the signal driving voltage be A. tx The signal frequency is The transmitted signal generated by electric drive is then represented as:

[0084]

[0085] Let P be the acoustic pressure amplitude of the ultrasonic signal propagating in the fluid to the fiber optic sensor under this driving signal. Then the ultrasonic signal can be expressed as:

[0086]

[0087] The phase shift sensitivity of the interferometric fiber optic sensor is M ϕ (Unit: rad / μPa), then the phase change of the light output by the sensor is:

[0088]

[0089] After the interfering light signal passes through the photodetector, the output voltage and phase change are linearly related. Let the phase-to-voltage conversion coefficient of the photodetector be K. PD (Unit: V / rad), then the output voltage signal is:

[0090]

[0091] make The received signal is then represented as:

[0092]

[0093] Let the frequency of the reference signal be... The reference signal is:

[0094]

[0095] The transmitted and received signals are mixed with a reference signal respectively, and then low-pass filtered to obtain two differences.

[0096]

[0097] The actual time difference Δt1 between the transmitted and received signals is:

[0098]

[0099] The apparent time difference Δt2 between the difference frequency signals is:

[0100]

[0101] Heterodyne mixing can amplify the actual time difference between the original signals into the apparent time difference between the difference frequency signals. The time amplification factor is the ratio of the transmitted signal frequency to the difference frequency signal frequency, denoted as k.

[0102] k

[0103] One specific implementation of the fiber optic sensor: As a specific embodiment of the present invention, the fiber optic sensor may adopt a structure based on the Michelson interferometry principle. For example... Figure 4 As shown, the laser emits 1550 nm light, which is input at port 1 and output at port 2 of a three-port circulator, and then enters the fiber optic ultrasonic transducer. The optical signal is split into two paths by a 1×2 fiber coupler. One path is a reference arm, which is transmitted through a reference fiber and then returns via a mirror, largely unaffected by external disturbances. The other path is a sensing arm, where the phase of the transmitted light is modulated under the influence of sound pressure. The two reflected beams return to the fiber coupler and interfere, forming an interference optical signal carrying sound pressure information. This signal is output at port 3 of the three-port circulator to the APD photodetector, completing the conversion from optical to electrical signal. This structure is only an example; other interference structures (such as Mach-Zehnder and Fabry-Perot) can achieve the same function. Fiber optic sensors exhibit differentiated response sensitivities to different ultrasonic frequencies, selectively amplifying the target frequency band signal and suppressing noise in non-target frequency bands. This provides an input foundation for heterodyne mixing, offering "frequency optimization and pre-enhanced signal-to-noise ratio." Heterodyne mixing further linearly amplifies the already enhanced weak time difference from the fiber optic cable, forming a closed-loop gain of "frequency filtering → sensitivity enhancement → time difference amplification." This frequency matching design ensures that the fiber optic sensor can capture the original ultrasonic signal with high sensitivity without introducing additional amplitude and phase distortion due to frequency response fluctuations, guaranteeing a strictly linear phase mapping relationship before and after mixing. The interferometric fiber optic sensor selected in this scheme achieves a phase shift sensitivity of -140dB re 1rad / μPa at an operating frequency of 25kHz, which translates to an acoustic pressure sensitivity of approximately -177dB re 1V / μPa. This is nearly 30dB higher than that of a piezoelectric sensor in the same frequency band (-206dB re 1V / μPa). The fiber optic sensor has a higher phase shift sensitivity at the operating frequency, which is superior to that of a piezoelectric sensor in the same frequency band. It can effectively enhance the weak ultrasonic signal after attenuation by high sediment content water to above the dynamic range threshold required by the mixer.

[0104] The core technical approach that distinguishes this invention from existing technologies is the deep integration of an interferometric fiber optic sensor with a heterodyne mixing time-difference amplification algorithm, constructing an integrated measurement link of "high-fidelity acquisition—linear amplification—precise inversion." The technical principle is as follows: the fiber optic sensor utilizes the principle of optical interference to linearly map sound pressure into phase change, outputting a raw ultrasonic signal with high signal-to-noise ratio, excellent phase linearity, electromagnetic interference resistance, and a large dynamic range, providing ideal input conditions of "low distortion and low noise" for mixing operations. Heterodyne mixing, by multiplying the two raw signals with the same local oscillation reference signal and low-pass filtering, linearly maps the microsecond-level propagation time difference into a significant apparent phase difference in the low-frequency difference signal, achieving equivalent amplification of the time difference. The phase linearity of the fiber optic sensor output ensures a constant ratio of phase difference before and after mixing; its high signal-to-noise ratio results in a sharp peak and minimal variance in the cross-correlation function of the difference signal; and its electromagnetic interference resistance prevents the low-frequency difference band from being contaminated by power frequency noise. In the conventional thinking of those skilled in the art, fiber optic sensing is mostly used for deformation and vibration monitoring, while time difference amplification (TDA) falls under the category of radio frequency signal processing. These two technologies belong to different branches and are generally not considered for combined application in river flow measurement. However, this invention, by combining the two, not only solves the traditional contradiction of "good penetration of low-frequency ultrasound but inaccurate time difference measurement," but also addresses the limitations of single fiber optic sensing in actively amplifying time differences, and the phase distortion caused by high noise in the input signal of a single mixing algorithm. The fusion of the two technologies allows the high-fidelity characteristics of fiber optic sensing to fully utilize the mixing amplification, which in turn compensates for the shortcomings of fiber optic sensing in low-frequency time difference resolution. Thus, without increasing transmission power or altering the mainstream hardware structure, it significantly improves the accuracy and stability of flow measurement under complex conditions such as low flow velocity, high sediment content, and strong interference.

[0105] To achieve the high-precision flow measurement objective of this invention, in addition to the above-mentioned core preferred embodiments, there are several equivalent alternative embodiments that can achieve the same technical effect. The first alternative is to replace the digital domain mixing processing with an analog circuit hardware mixing scheme, adding an analog mixing circuit and a filtering circuit at the ultrasonic signal receiving backend, and completing the time difference amplification processing at the analog signal level in advance. The second alternative, while keeping the core processing flow of mixing time difference amplification unchanged, replaces the optimized cross-correlation algorithm with a phase difference method time delay solution algorithm and a wavelet transform time delay extraction algorithm. The third alternative is to adjust the signal transmission form, replacing the traditional continuous ultrasonic detection signal with a pulse-coded ultrasonic signal, and combining it with the mixing amplification processing logic of this invention to further improve the signal anti-interference capability. The systems, devices, modules or units described in the above embodiments can be implemented by computer chips or physical entities, or by products with certain functions.

[0106] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0107] The above embodiments should be understood as illustrative only and not as limiting the scope of protection of the present invention. After reading the description of the present invention, those skilled in the art can make various alterations or modifications to the present invention, and these equivalent changes and modifications also fall within the scope defined by the claims of the present invention.

Claims

1. A mixed-frequency time-difference amplification current measurement method based on ultrasonic time-difference method, characterized in that, Includes the following steps: 1) An interferometric fiber optic sensor is used to receive the ultrasonic signal after it has been propagated through the fluid, and the ultrasonic signal is converted into an optical signal. After photoelectric conversion, the original ultrasonic signals in both the forward and reverse directions are obtained. 2) The driving electrical signal of the downstream transmitter and the downstream receiving signal are synchronously acquired with the same sampling clock and digitally mixed with the same local oscillator reference signal. After low-pass filtering, two difference frequency signals are obtained. The same operation is performed in the reverse direction. 3) Perform cross-correlation calculation on the transmitted and received difference frequency signals in the downstream direction to extract the apparent time difference after mixing and amplification in the downstream direction; perform the same operation on the upstream direction; define the time difference amplification factor as the ratio of the ultrasonic transmission main frequency to the difference frequency signal frequency, which represents the equivalent amplification factor of the small propagation time difference after frequency domain transformation; perform reverse conversion based on the predetermined time difference amplification factor to reverse the apparent time difference and restore it, stripping away the time difference amplification effect brought about by frequency domain transformation, and restoring the real time difference of ultrasonic waves in the fluid. 4) Calculate the fluid velocity and flow rate based on the actual propagation time in both directions.

2. The flow measurement method according to claim 1, characterized in that, The fiber optic sensor is an interferometric fiber optic sensor, which converts the sound pressure signal into an optical phase change based on the principle of optical phase modulation, and the output signal has a phase characteristic that is linearly related to the sound pressure.

3. The flow measurement method according to claim 1, characterized in that, Step 2) specifically includes: selecting a local oscillation reference signal with a fixed frequency; performing digital multiplication and mixing of the acquired downstream transmission signal and downstream reception signal with the local oscillation reference signal, and then filtering out the high-frequency and low-frequency components through a low-pass filter to obtain two difference frequency signals; performing the same operation on the upstream transmission signal and upstream reception signal.

4. The flow measurement method according to claim 3, characterized in that, The digital multiplication mixing employs heterodyne frequency transformation to preserve the phase information of the original signal; the time difference amplification factor k is defined as the ratio of the ultrasonic transmission frequency f1 to the difference frequency (f1−f0), where f0 is the frequency of the local oscillation reference signal.

5. The flow measurement method according to claim 1, characterized in that, The cross-correlation operation extracts the apparent time difference between the downstream and upstream transmit / receive frequency difference signals by determining the coordinates corresponding to the peak value of the cross-correlation function.

6. The flow measurement method according to claim 1, characterized in that, Step 2) involves mixing and filtering the transmitted and received signals, specifically including: Let the amplitude of the signal driving voltage be A. tx The signal frequency is The transmitted signal generated by electric drive is then represented as: Let P be the acoustic pressure amplitude of the ultrasonic signal propagating in the fluid to the fiber optic sensor under this driving signal. Then the ultrasonic signal can be expressed as: The phase shift sensitivity of the interferometric fiber optic sensor is M ϕ (Unit: rad / μPa), then the phase change of the light output by the sensor is: After the interfering light signal passes through the photodetector, the output voltage and phase change are linearly related; let the phase-to-voltage conversion coefficient of the photodetector be K. PD (Unit: V / rad), then the output voltage signal is: make The received signal is represented as: Let the frequency of the reference signal be... The reference signal is: The transmitted and received signals are mixed with a reference signal respectively, and then low-pass filtered to obtain two differences. The actual time difference Δt1 between the transmitted and received signals is: The apparent time difference Δt2 between the difference frequency signals is: Heterodyne mixing can amplify the actual time difference between the original signals into the apparent time difference between the difference frequency signals. The time amplification factor is the ratio of the transmitted signal frequency to the difference frequency signal frequency, denoted as k. k The apparent time difference Δt2 between the difference frequency signals is obtained by cross-correlation calculation, therefore its true propagation time difference is: = 。 7. The flow measurement method according to claim 1, characterized in that, Step 4) calculates the fluid velocity and flow rate based on the actual propagation time difference. Specifically, this includes: arranging an ultrasonic transmitter and receiver on either side of the receiver, where L is the distance between the transmitter and receiver, θ is the angle between the ultrasonic wave propagation direction and the water flow direction, and c is the speed of sound in the fluid. Due to the influence of the water flow, the propagation speed of the sound wave differs along the upstream and downstream propagation paths. The downstream ultrasonic wave propagation speed v1 is slightly higher than the upstream ultrasonic wave propagation speed v2. Therefore, the propagation time of the ultrasonic wave along the upstream and downstream paths will also differ, and they are as follows: Using the formula, the actual propagation time difference between upstream and downstream can be derived as t. Theoretically, the propagation speed of ultrasound in water is approximately 1500 m / s, therefore c 2 ≫v 2 From cosθ, the formula for calculating the average flow velocity v of the water layer can be derived: 。 8. A flow measurement system for implementing the method of any one of claims 1-7, characterized in that, include: An ultrasonic signal transmitting module, including a signal generator and an ultrasonic transducer, is used to generate an electrical drive signal to drive the ultrasonic transducer to transmit ultrasonic detection signals. An ultrasonic signal receiving module, including a fiber optic sensor and a photodetector, is used to receive the ultrasonic echo signal after propagation through the fluid and sequentially convert the acoustic signal into an optical signal and an electrical signal. An acoustic signal propagation link is formed between the ultrasonic signal transmitting module and the ultrasonic signal receiving module through the fluid being measured. An ultrasonic signal acquisition module, including a signal acquisition board, is used to electrically connect the ultrasonic signal transmitting module and the ultrasonic signal receiving module respectively, synchronously acquiring the original drive electrical signal from the transmitting end and the echo analog electrical signal from the receiving end, and converting the two analog signals into digital signals for buffering. A host computer signal processing module, communicatively connected to the ultrasonic signal acquisition module, is used to read the digital signals and perform mixing, filtering, and cross-correlation delay calculation operations. The actual propagation delay is calculated using the amplification factor k, and the fluid velocity and flow rate are obtained according to the velocity and flow rate calculation formula.

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