A non-contact liquid level monitoring method based on coda wave interference

Abstract

The application aims to provide a non-contact liquid level monitoring method based on coda wave interference, comprising the following steps: (1) obtaining a measured signal; (2) obtaining a liquid level expression; (3) obtaining an unknown liquid level signal; (4) calculating an unknown liquid level decorrelation coefficient; and (5) liquid level estimation. Compared with the commonly used contact liquid level monitoring method, the application reduces the influence of the liquid on the collection device, improves the service life, reduces the maintenance cost, and increases the applicable scenarios. Compared with the commonly used non-contact liquid level monitoring method, the application does not need to be installed inside the container and does not need to move the collection equipment, thereby improving the installation and disassembly efficiency and reducing the maintenance cost. Moreover, by virtue of the high sensitivity of the coda wave to the medium, only a single sensor is needed to realize high-precision monitoring of the weak liquid level change.

Description

Technical Field

[0001] The present invention relates to a liquid level detection method, specifically a non-contact liquid level monitoring method. Background Technology

[0002] Liquid level measurement refers to measuring the position of a liquid surface. Accurate measurement of liquid levels is crucial for accurate detection and real-time control in production processes. Traditional liquid level detection is contact-based, meaning that part or all of the experimental device comes into contact with the liquid during the detection process. This method is unsuitable if the liquid is corrosive, toxic, or highly viscous, or if a high level of hygiene and safety is required during the test. Furthermore, manual operation is extremely difficult in high-temperature environments, resulting in very low efficiency and low accuracy of monitoring results. Therefore, non-contact liquid level detection has been researched and widely applied. It avoids equipment malfunctions caused by contact between the equipment and the liquid, reduces equipment maintenance costs, and extends equipment lifespan.

[0003] Ultrasonic level monitoring is a non-contact method for detecting liquid levels, offering advantages such as safety, low cost, high flexibility, and applicability to various sound transmission media. However, ultrasonic level detection devices are typically installed inside containers, making installation and maintenance difficult in complex container environments. If the testing process requires a closed or sterile environment, changes in equipment or inspections will inevitably increase monitoring errors. Installing sensors at the top or bottom of the container, rather than inside, can lead to significant energy loss during ultrasonic propagation in environments containing vapor, powder, or other absorbing substances, resulting in a significantly shortened detection distance and potential failure to detect level changes. Installing sensors on the side of the container typically requires moving the acquisition device or using multiple sensors, leading to low detection efficiency, a small detection range, and high equipment costs. Ultrasonic wake waves, due to multiple sampling within the medium, have a longer propagation path than direct waves, making them highly sensitive to subtle changes in the medium. Wake wave interferometry is a method that utilizes the sensitivity of wake waves to the physical properties of the medium to detect minute changes. The Stretching method, a high-precision signal processing technique, is widely used in seismic and ultrasonic non-destructive testing fields. Thanks to the high sensitivity of wake waves to changes in the medium, a small number of ultrasonic sensors can be fixedly installed on the outside of the container to achieve continuous monitoring of the liquid level. This avoids the inconvenience of installation and disassembly caused by moving the sensors, as well as measurement errors, and enables true non-contact measurement in complex environments.

[0004] In summary, existing liquid level monitoring methods suffer from problems such as inconvenient installation, high maintenance costs, and low accuracy, failing to meet the needs for high-precision, low-cost, and efficient safety monitoring. Summary of the Invention

[0005] The purpose of this invention is to provide a non-contact liquid level monitoring method based on wake wave interferometry that can achieve non-contact, high precision, low cost, intuitive and efficient operation.

[0006] The objective of this invention is achieved as follows:

[0007] This invention discloses a non-contact liquid level monitoring method based on wake wave interferometry, characterized in that:

[0008] (1) Obtain the measured signal;

[0009] (2) Obtain the liquid level expression;

[0010] (3) Obtain unknown liquid level signals;

[0011] (4) Calculate the decorrelation coefficient for the unknown liquid level;

[0012] (5) Liquid level estimation.

[0013] The present invention may also include:

[0014] 1. The process of acquiring the measured signal is as follows: At least one ultrasonic sensor is fixedly installed outside the container, and the transmitter and receiver are combined to transmit ultrasonic signals and acquire the time-domain signal u0(t) under the initial liquid-free state and the time-domain signal u under different liquid levels x. i (t), i≥1.

[0015] 2. The process of obtaining the liquid level expression is as follows: Based on the signal energy and phase difference, the time window, grid range, and grid accuracy are set, and then the decorrelation coefficient DC for different liquid levels is calculated within the set parameter range using the wake interferometry method. i Finally, by fitting the correlation coefficient to the liquid level, the functional relationship between the two was determined:

[0016] a. Setting parameters: Set the start position t1 and the length t of the time window according to the signal energy. len Estimate the time-domain signals u0(t) and u i (t) at the center of the time window t center The phase difference τ at that point, and then based on the phase difference τ and time t center The ratio sets the grid range ε range and grid accuracy ε grid ;

[0017] b. Calculate the decorrelation coefficient: Within the set time window, calculate the time-domain signals u0(t) and u based on the wake interferometry method. i Cross-correlation coefficient CC between (t) i , by 1-CC i Calculate the decorrelation coefficient DC i ;

[0018] c. Determine the function expression: Fit the decorrelation coefficient DC at different liquid levels i The optimal functional relationship DC is determined based on the R-squared maximization principle and the root mean square (RMSE) minimization principle. i =F(x), where F represents DC i The mapping relationship with x.

[0019] 3. The process of acquiring the unknown liquid level signal is as follows: Based on the signal u of the arbitrary unknown liquid level height acquired by the device in step (1). x (t).

[0020] 4. Step (4) Calculation of the correlation coefficient of the unknown liquid level: The unknown liquid level signal u is calculated using the same parameters and processing method as in step (2). x The decorrelation coefficient DC between (t) and the initial liquid-free state time-domain signal u0(t).

[0021] 5. The process of liquid level estimation is as follows: Substitute the DC obtained in step (4) into the function expression determined in step (2) to solve for x, which is the liquid level to be estimated.

[0022] The advantages of this invention are:

[0023] 1. Compared with commonly used contact-type liquid level monitoring methods, it reduces the impact of liquid on the acquisition device, extends its service life, reduces maintenance costs, and expands the applicable scenarios.

[0024] 2. Compared with commonly used non-contact liquid level monitoring methods, it does not require installation inside the container or relocation of the acquisition equipment, improving installation and disassembly efficiency and reducing maintenance costs. Furthermore, thanks to the high sensitivity of wake waves to the medium, a single sensor is sufficient to achieve high-precision monitoring of minute liquid level changes. Attached Figure Description

[0025] Figure 1 This is a flowchart of the present invention;

[0026] Figure 2 This is a diagram of the experimental equipment and apparatus of the present invention;

[0027] Figure 3 This is a diagram of the measured signal acquisition device of the present invention;

[0028] Figure 4 This is a graph showing the change in liquid level.

[0029] Figure 5 To visualize the relationship between the correlation coefficient and the liquid level;

[0030] Figure 6 This is a diagram showing the initial state and the unknown liquid level signal. Detailed Implementation

[0031] The invention will now be described in more detail with reference to the accompanying drawings:

[0032] Combination Figure 1-6 The specific steps of the non-contact liquid level monitoring method based on wake wave interferometry of the present invention are as follows:

[0033] Step 1: Acquisition of measured signals. The detailed process is as follows: One or more ultrasonic sensors are fixedly installed on the outside of the container, and the transmitter and receiver are combined to emit ultrasonic signals, such as... Figure 2 As shown. Liquid is dripped into or extracted from the container to reach the set liquid level. The acquisition system receives and records the time-domain signal u0(t) in the initial liquid-free state and the time-domain signal u at different liquid levels x. i (t), i≥1. It should be noted that in single-transmitter, single-receiver mode, the transmitted signal energy needs to be strong enough to allow the wave to penetrate the container and return to be received.

[0034] Step 2, obtaining the liquid level expression, the detailed process includes:

[0035] 2.1: Parameter Settings. The start time t1 of the window should be sufficiently late to ensure that multiple scattering of the signal occurs within the window (t1 ≥ 10t). * ), t * This represents the mean free time of transmission. The time window length is t. len It needs to be long enough to contain enough signals (t) len ≥10t * However, it cannot be too long; the signal-to-noise ratio must be at least 40dB. Grid range ε range Determined by the phase difference between signals, the center position t within the time window center The time delay τ between the two signals is estimated manually, and then divided by t. center The relative velocity change δv / v is roughly estimated. This estimate is then expanded by at least one to two orders of magnitude to avoid underestimating the actual velocity change. The absolute value of the expanded relative velocity change, rounded to the nearest whole number, is then used to determine the grid range. To detect velocity changes caused by minute liquid level variations while maintaining computational complexity, ε... range The range is usually set at 10 -1 ~10 -4 Between. And the grid accuracy ε grid This represents the minimum detectable change in the medium, i.e., the computational precision. It involves reducing the estimated relative velocity change δv / v by at least one order of magnitude, and then taking the absolute value of this reduced relative velocity change. This absolute value is typically set to 10. -5 ~10 -8 between.

[0036] 2.2: Correlation coefficient calculation. The Stretching method first calculates the waveform u under different liquid levels. i Perform a time-domain scaling transformation on u(t), and then calculate its cross-correlation coefficient with the initial liquid-free state u0(t) using the following formula. Adjust the scaling factor ε, ε∈[-ε range :ε grid :ε range ], when R i When the cross-correlation coefficient (ε) reaches its maximum value, remove the correlation coefficient DC. i It is 1 minus the maximum cross-correlation coefficient, i.e., DC. i =1-R i (ε), i≥1, and DC i Its value is between 0 and 1;

[0037]

[0038] 2.3: Determining the Function Expression. The decorrelation coefficient DC obtained in step 2.2... i By fitting the equation DC to different liquid levels x, the functional relationship is determined. i =F(x), where F represents DC i The mapping relationship with x. The goodness of fit is evaluated based on the magnitudes of R-squared and root mean square (RMSE). R-squared is a metric for measuring correlation; the closer to 1, the better, with an R-squared greater than 0.9 as the standard. The optimal fitting function is determined based on the principle of maximizing R-squared combined with the principle of minimizing RMSE.

[0039] Step 3: Acquisition of the unknown liquid level signal. The detailed process involves acquiring the time-domain signal u from the liquid container with the unknown liquid level using the same experimental equipment and acquisition system. x (t);

[0040] Step 4: Calculate the correlation coefficient for the unknown liquid level. This process uses the parameter settings from Step 2, referring to Step 2.2 for calculating the unknown liquid level signal u. x The decorrelation coefficient DC between (t) and the time-domain signal u0(t) in the initial liquid-free state;

[0041] Step 5, liquid level estimation, involves using the decorrelation coefficient obtained in step 2.3 as prior information and substituting the decorrelation coefficient DC obtained in step 4 into the functional relationship DC. i = Replace DC in F(x) i Solve for x, and x is the liquid level that needs to be estimated.

[0042] The following is a specific experimental example to further illustrate this point.

[0043] In a specific example of this invention, three grooves, L, M, and R, serve as test containers for observing changes in liquid level. Figure 3 As shown in the figure. In the experiment, to ensure the transmitted signal passes through the entire container, two ultrasonic sensors were used. Ultrasonic sensor A, acting as the ultrasonic transmitter, was installed on the upper surface of the container, and ultrasonic sensor B, acting as the ultrasonic receiver, was installed on the outer wall of the container. Taking the addition of water to an anhydrous groove M as an example, 2 mL of water was added each time, up to 40 mL. The liquid level change curve is shown in the figure. Figure 4 As shown in the figure. First, the time-domain signal of the initial waterless state is collected and denoted as u0(t). Then, measurements are taken starting from the empty liquid state, with each drop of 2mL of water constituting one experiment, for a total of 21 experiments, thus obtaining 21 time-domain signals u at different liquid levels. i (t), i = 1, 2, ..., 21.

[0044] Choose an appropriate time window, grid range, and grid precision. Specific parameters will be described below. Calculate u using the Stretching calculation formula from step 2.2. i The cross-correlation coefficients of u0(t) and u0(t) under different scaling factors are calculated, and then the maximum value is obtained. The maximum cross-correlation coefficient is then subtracted from 1 to obtain the decorrelation coefficient DC. i Plot the decorrelation coefficient as a function of liquid level, as shown below. Figure 5 As shown in the measured signal, the trend of the decorrelation coefficient change is almost linear with the liquid level change. Therefore, a one-dimensional linear fitting of the decorrelation coefficient and the liquid level is used to estimate the fitting coefficients a1 and a0. In this experiment, a1 and a0 are calculated as follows: a1 = 4.731 × 10⁻⁶. -3 With a0 = 0.1644, the R-squared value of 0.9903 under this fitting coefficient meets the requirements. The optimal decorrelation coefficient is the functional relationship between the liquid level and the liquid level. i =4.731×10 -3 x+0.1644.

[0045] Randomly draw any amount of water (any milliliters) again and obtain the time-domain signal of the unknown liquid level. This invention uses the example of drawing 32 mL of water again, leaving 8 mL of liquid in the container. The time-domain signal u at this liquid level is then obtained. x The time-domain waveforms of u0(t) and u0(t) in step 1 have strong tail wave energy and high signal-to-noise ratio when the time window is [0.4s, 1.2s]. Therefore, t1 = 0.4s is set. len =0.8s, plot the two signals within this time window as follows Figure 6 As shown. (Enlarged) Figure 6 The time window is located in the center, i.e., t centerAt 0.8s, a phase difference of approximately 0.0001s between the two signals is estimated, meaning the relative velocity change is 1.25 × 10⁻⁶. -5 Therefore, ε is set in this invention. range =0.001, ε grid =10 -7 That is, the range of the scaling factor ε is -0.001:10. -7 :0.001, a total of 20001 variations. Substitute different scaling factors ε into the formula in step 2.2 to calculate the cross-correlation coefficients under different scaling factors, and then take the maximum value, u x The maximum cross-correlation coefficient R(ε) between u0(t) and u0(t) is 0.7968, so the removal correlation coefficient DC = 0.2032. Substituting DC into DC... i =a1x + a0, that is, 0.2032 = 4.731 × 10 -3 x + 0.1644, solving for x gives x = 8.2012, which is very close to the theoretical value of 8 mL of liquid, with a relative error of 2.515%. This shows that the wake wave interferometry method can achieve non-contact liquid level monitoring efficiently and with high precision.

Claims

1. A non-contact liquid level monitoring method based on wake wave interferometry, characterized in that: (1) Obtain the measured signal; The process of acquiring the measured signal is as follows: At least one ultrasonic sensor is fixedly installed outside the container, and the transmitter and receiver are combined to transmit ultrasonic signals and acquire the time-domain signal in the initial liquid-free state. and different liquid levels Lower time domain signal ; (2) Obtain the liquid level expression; The process of obtaining the liquid level expression is as follows: based on the signal energy and phase difference, the time window, grid range, and grid accuracy are set, and then the decorrelation coefficients for different liquid levels are calculated within the set parameter range using the wake interferometry method. Finally, by fitting the correlation coefficient to the liquid level, the functional relationship between the two was determined: a. Setting parameters: Set the start position of the time window according to the signal energy. and time window length Estimate the time-domain signal and At the center of the time window Phase difference at Then based on the phase difference Center position of the time window The ratio sets the grid range. and grid precision ; b. Calculate the decorrelation coefficient: Calculate the time-domain signal based on the wake interferometry within the set time window. and cross-relationships between ,Depend on Calculate the decorrelation coefficient ; c. Determine the function expression: Fit the decorrelation coefficients for different liquid levels. The optimal functional relationship is determined based on the R-squared maximization principle and the root mean square (RMSE) minimization principle. F represents The mapping relationship with x; (3) Obtain the unknown liquid level signal; (4) Calculate the correlation coefficient for the unknown liquid level; (5) Liquid level estimation.

2. The non-contact liquid level monitoring method based on wake wave interferometry according to claim 1, characterized in that: The process of acquiring the unknown liquid level signal is as follows: based on the signal of any unknown liquid level height acquired by the device in step (1). .

3. The non-contact liquid level monitoring method based on wake wave interferometry according to claim 1, characterized in that: Step (4) Calculation of the correlation coefficient for the unknown liquid level: The unknown liquid level signal is calculated using the same parameters and processing methods as in step (2). Time-domain signal of the initial liquid-free state The decorrelation coefficient DC between them.

4. The non-contact liquid level monitoring method based on wake wave interferometry according to claim 1, characterized in that: liquid level... The estimation process is as follows: Substitute the DC obtained in step (4) into the function expression determined in step (2) to solve for x, which is the liquid level to be estimated.

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